HARVARD UNIVERSITY.

LIBRARY

MUSEUM OF COMPARATIVE ZOOLOGY

Library of
SAMUEL GARMAN

OJbUMyy^tc^^ 1^, 192^6-

^Oyf-Vyx^^jui^

SEP 1 5 1928

/

A MONOGRAPH

ON THE

DEVELOPMENT OF
ELASMOBEANCH FISHES.

BY

F. M. BALFOUE, M.A.,

FELLOW AND LECTURER OF TRINITY COLLEGE, OAMBHIDGE.

Uonlion :

MACMILLAN AND CO.
1878

[All Rights reserved.]

CatnbrttiSE:

PRINTED BY C. J. CLAY, M.A.
AT THE UNIVEESITY PRESS.

PEEFACE.

The present Monograph is a reprint of a series of
papers published in the Journal of Anatomy and
Physiology during the years 1876, 1877 and 1878.
The successive parts were struck off as they appeared,
so that the earher pages of the work were in print
fully two years ago. I trust the reader will find in
this fact a sufficient excuse for a certain want of
coherence, which is I fear observable, as well as for
the omission of references to several recent publica-
tions. The first and second chapters would not have
appeared in their present form had I been acquainted,
at the time of writing them, with the researches which
have since been published, on the behaviour of the
germinal vesicle and on the division of nuclei. I may
also call attention to the valuable papers of Prof. His^
on the formation of the layers in Elasmobranchs, and
of Prof Kowalevsky^ on the development of Am-
phioxus, to both of which I would certainly have
referred, had it been possible for me to do so.

Professor His deals mainly with the subjects
treated of in Chapter III., and gives a description
very similar to my own of the early stages of develop-

^ Zeitsckriftf. Anat. u. Entwicklungsgeschichte, Bd. n.
2 Archivf. Micr. Anat. Bd. xiii.

IV PREFACE.

ment. His interpretations of the observed changes
are, however, very different from those at which I
have arrived. Although this is not the place for
a discussion of Prof His's views, I may perhaps state
that, in spite of the arguments he has brought for-
ward in support of his position, I am still inclined
to maintain the accuracy of my original account. The
very striking paper on Amphioxus by Kowalevsky
(the substance of which I understand to have been
published in Russia at an earlier period) contains
a confirmation of the views expressed in Chapter VI.
on the development of the mesoblast, and must be
regarded as affording a conclusive demonstration, that
in the case of Vertebrata the mesoblast has primi-
tively the form of a pair of diverticula from the walls
of the arch enter on.

The present Memoir, while differing essentially in
scope and object from the two important treatises
by Professors His^ and Gotte^, which have recently
appeared in Germany, has this much in common with
them, that it deals monographically with the develop-
ment of a single type : but here the resemblance ends.
Both of these authors seek to establish, by a careful
investigation of the development of a single species,
the general plan of development of Vertebrates in
general, if not of the whole animal kingdom. Both
reject the theory of descent, as propounded by Mr
Darwin, and offer completely fresh explanations of the
phenomena of Embryology. Accepting, as I do, the
principle of natural selection, I have had before me, in
writing the Monograph, no such ambitious aim as the

^ Erste Anlage des Wirbelthierleibes.
2 Entwicklungsgeschichte dcr Unke.

PREFACE. V

establishment of a completely new system of Mor-
phology. My object will have been fully attained if
I have succeeded in adding a few stones to the edifice,
the foundations of which were laid by Mr Darwin in
his work on the Origin of S'pecies.

1 may perhaps call attention to one or two special
points in this work which seem to give promise of
further results. The chapter on the Development of
the Spinal and Cranial Nerves contains a modification
of the previously accepted views on this subject, which
may perhaps lead to a more satisfactory conception
of the origin of nerves than has before been possible,
and a more accurate account of the origin of the
muscle-plates and vertebral column. The attempt to
employ the embryological relations of the cephalic
prolongations of the body-cavity, and of the cranial
nerves, in the solution of the difficult problems of the
Morphology of the head, may prove of use in the line
of study so successfully cultivated by our great English
Anatomist, Professor Huxley. Lastly, I venture to
hope that my conclusions in reference to the relations
of the sympathetic system and the suprarenal body,
and to the development of the mesoblast, the noto-
chord, the limbs, the heart, the venous system, and
the excretory organs, are not unworthy of the attention
of Morphologists.

The masterly manner in which the systematic
position of Elasmobranchs is discussed by Professor
Gegenbaur, in the introduction to his Monograph on
the Cranial Skeleton of the group, relieves me from
the necessity of entering upon this complicated question.
It is sufficient for my purpose that the Elasmobranch
Fishes be regarded as forming one of the most primi-
B. h

VI PREFACE.

tive groups among Vertebrates, a view wliicli finds
ample confirmation in the importance of the results
to which Prof. Gegenbaur and his pupils have been
led in this branch of their investigations.

Though I trust that the necessary references to
previous contributions in the same department of en-
quiry have not been omitted, the * literature of the
subject' will nevertheless be found to occupy a far
smaller share of space than is usual in works of a similar
character. This is an intentional protest on my part
against, what appears to me, the unreasonable amount
of space so frequently occupied in this way. The pages
devoted to the ^ previous literature ' only weary the
reader, who is not wise enough to skip them, and
involve a great and useless expenditure of time on
the part of any writer, who is capable of something
better than the compilation of abstracts.

In conclusion, my best thanksare due to DrsDohrn
and Eisig for the uniformly kind manner in which they
have forwarded my researches both at the Zoological
Station in Naples, and after my return to England;
and also to Mr Henry Lee and to the Manager and
Directors of the Brighton Aquarium, who have
always been ready to respond to my numerous de-
mands on their liberality.

To my friend and former teacher Dr Michael
Foster I tender my sincerest thanks for the never-
failing advice and assistance which he has given
throu<2:hout the whole course of the work.

TABLE OF CONTENTS.

CHAPTER I.

THE RIPE OVARIAN OVUM, pp. 1 9.

Structure of ripe ovum. Atrophy of germinal vesicle. The extrusion of its
membrane and absorption of its contents, Oellaeher's observations on the
germinal vesicle. Gotte's observations. Kleineuberg's observations. General
conclusions on the fate of the germinal vesicle. Germinal disc.

CHAPTER IL

THE SEGMENTATION, pp. 10—32.

Appearance of impregnated germinal disc. Stage with two furrows. Stage
with twenty-one segments. Structure of the sides of the fun-ows. Later stages
of segmentation. Spindle-shaped nuclei. Their presence outside the blastoderm.
Knobbed nuclei. Division of nuclei. Conclusion of segmentation. Nuclei of
the yolk. Asymmetry of the segmented blastoderm. Comparison of Elasmo-
branch segmentation with that of other meroblastic ova. Literature of
Elasmobranch segmentation.

CHAPTER III.

FORMATION OF THE LAYERS, pp. 33 — 70.

Division of blastoderm into two layers. Formation of segmentation cavity.
Disappearance of cells from floor of se.^mentation cavity. Nuclei of yolk and of
blastoderm. Formation of embryonic rim. Appearance of a layer of cells on
the floor of the segmentation cavity. Formation of mesoblast. Formation
of medullary groove. Disappearance of segmentation cavity. Comparison of
segmentation cavity of Elasmobranchs with that of other types. Alimentary
cavity. Formation of mesoblast in two lateral plates. Protoplasmic network
of yolk. Summary. Nature of meroblastic ova. Comparison of Elasmobianch
development with that of other types. Its relation to the Gastrula. Haeckel's
views on vertebrate Gastrula. Their untenable natiu'e. Comparison of primitive
streak with blastopore. Literature.

viii TABLE OF CONTENTS.

CHAPTER IV.

GENERAL FEATURES OF THE ELASMOBRANCH EMBRYO AT SUCCESSIVE
STAGES, pp. 71 — 81.

Description of Stages A-Q. Enclosure of yolk by blastoderm. Relation of
the anus of Eusconi to the blastopore.

CHAPTER Y.

STAGES B — G, pp. 82 — 96.

General features of the eplblast. — Original uniform constitution. Separation
into lateral and central portions. The medullary groove. — Its conversion into
the medullary canal. The mesohlast. — Its division into somatic and splanchnic
layers. Formation of protovertebroe. The lateral plates. The caudal swellings.
The formation of the body-cavity in the head. The alimentary canal. — Its
primitive constitution. The anus of Eusconi. Floor formed by yolk. Forma-
tion of cellular floor from cells formed around nuclei of the yolk. Communica-
tion behind of neural and ahmentary canals. Its discovery by Kowalevsky. Its
occurrence in other instances. General features of the hypoblast. The notochord.
— Its formation as a median thickening of the hypoblast. Possible interpreta-
tions to be put on this. Its occurrence in other instances.

CHAPTER VI.

DEVELOPMENT OF THE TRUNK DURING STAGES G TO K, pp. 97 — 139.

Order of treatment. External eplblast. — Characters of epiblast. Its late
division into horny and epidermic layers. Comparison of with Amphibian
epiblast. The unpaired fins. The paired fins. — Their formation as lateral ridges
of epiblast. Hypothesis that the limbs are remnants of contiuuous lateral fins.
Mesoblast. — Constitution of lateral plates of mesoblast. Their splanchnic and
Bomatic layers. Body-cavity constituting space between them. Their division
into lateral and vertebral plates. Continuation of body-cavity into vertebral
plates. Protovertfcbrffi. Division into muscle-plates and vertebral bodies.
Development of muscle-plates. Disappearance of segmentation in tissue to form
vertebral bodies. Body-cavity and parietal plates. Primitive independent
halves of body-cavity. Their ventral fusion, Sepai'ation of anterior part of body-
ca\dty as pericardial cavity. Communication of pericardial and peritoneal
ca\'ities. Somatopleure and splanchnopleure. Resume. General considerations
on development of mesoblast. Probability of lateral plates of mesoblast in
Elasmobranchs representing alimentary diverticula. Meaning of secondary
segmentation of vertebral column. The urinoycnital system. — Development of
segmental duct and segmental tubes as sohd bodies. Formation of a lumen
in them, and their opening into body-cavity. Comparison of segmental duct
and segmental tubes. Primitive ova. Their position. Their structure. The
notochord. — The formation of its sheath. The changes in its cells.

TABLE OF CONTEXTS. ix

CHAPTER VII.

GENERAL DEVELOPMENT OF THE TRUNK FROM STAGE K TO THE
CLOSE OF EMBRYONIC LIFE, pp. 140 — 155.

External epiblast. — Division into separate layers. Placoid scales. Formation
of their enamel. Lateral line. — Previous investigations. Distinctness of lateral
line and lateral nerve. Lateral nerve a branch of vagus. Lateral line a thick-
ening of epiblast. Its greater width behind. Its conversion into a canal by its
cells assuming a tubular arrangement. The formation of its segmental apertures.
Mucous canals of the head. Their nerve-supply. Eeasons for dissenting from
Semper's and Gotte's view of lateral nerve. Muscle -plates. — Their growth. Con-
version of both layers into muscles. Division into dorso-lateral and ventro-
lateral sections. Derivation of limb-muscles from muscle-plates. Vertebral
column and notochord. — Previous investigations. Formation of arches. Forma-
tion of cartilaginous sheath of notochord and membrana elastica externa. Dif-
ferentiation of neiu'al arches. Differentiation of haemal arches. Segmentation
of cartilaginous sheath of notochord. Vertebral and intervertebral regions.
Notochord.

CHAPTER YIII.

DEVELOPMENT OF THE SPINAL NERVES AND OF THE SYMPATHETIC
NERVOUS SYSTEM, pp. 156 173.

The spinal nerves. — Formation of posterior roots. Later formation of
anterior roots. Development of commissure uniting posterior roots. Sub-
sequent development of posterior roots. Their change in position. Develop-
ment of ganglion. Further changes in anterior roots. Junction of anterior
and posterior roots. Summary. General considerations. — Origin of nerves.
Hypothesis explaining peripheral growth. Hensen's views. Later investigations.
Gotte. Calberla. Relations between Annelidan and Vertebrate nervous systems.
Spinal canal. Dr Dohrn's views. Their difficulties. Hypothesis of dorsal
coalescence of lateral nerve cords. Sympathetic nervous system. — Development of
sympathetic ganglia on branches of spinal nerves. Formation of sympathetic
commissure.

CHAPTER IX.

DEVELOPMENT OF THE ORGANS IN THE HEAD, pp. 174 216.

Development of the Brain, pp. 174 — 183. General history. Fcre-brain.
— Optic vesicles. Infundibulum. Pineal gland. Olfactory lobes. Lateral
ventricles. Mid-brain. Hind-brain. — Cerebellum. Medulla. — Previous in-
vestigations. Huxley. Miklucho-Maclay. Wilder. Organs of sense.

X TABLE OF CONTENTS.

pp. 184—189. Olfactory organ.— OliactoTy pit. Sclineielerian folds. Eye.

General developmeut. Hyaloid membrane. Lens capsule. Processus

falciformis. Auditory Organs. — Auditory pit. Semicircular canals. Mouth
INVOLUTION and Pituitary body, pp. 189 — 191. Outgrowth of pituitary involution.
Separation of pituitary sack. Junction with infundibulum. Development
OF CRANIAL NERVES, pp. 191- -205. Early development of oth, 7th, 8th, 9th and
10th cranial nerves. Distribution of the nerves in the adult. The fifth nerve.
— Its division into ophthalamic and mandibular branches. Later formation of
superior maxillary branch. Seventh and auditor// nerves. — Separation of single
rudiment into seventh and auditory. Forking of seventh nerve over hyomandibular
cleft. Formation of anterior branch to form ramus ophthalmicus superficialis
of adult. General view of morphology of branches of seventh nerve. Glosso-
pharyngeal and vagus nerves. — General distribution at stage L. Their connection
by a commissure. Junction of the commissure with commissure connecting
posterior roots of spinal nerves. Absence of anterior roots. HjT)oglossal nerve.
Mesoblast of Head, pp. 206 — 209. Body-cavity and, myotomes of head, — Con-
tinuation of body-cavity into head. Its division into segments. Development
of muscles from their walls. General mesoblast of head. Notochord in Head,
pp. 209—210. Hypoblast of the Head, pp. 210 — 211. The formation of the
gill-shts. Layer from which gills are derived. Segmentation of the Head,
pp. 211 — 216. Indication of segmentation afforded by (1) cranial nerves,
(2) visceral clefts, (3) head-cavities. Comparison of results obtained.

CHAPTER X.

THE ALIMENTARY CANAL, pp. 217 229.

The solid (p,sophagus. — ffisophagus originally hollow. Becomes solid during
Stage K. The postanal section of the alimentary fj-aci. — Continuity of neural and
ahmentary canals. Its discovery by Kowalevsky. The postanal section of gut.
Its history in Scyllium. Its disappearance. Tlie cloaca and anus. — The forma-
tion of the cloaca. Its junction with segmental ducts. Abdominal pockets.
Anus. The thyroid body. — Its formation in region of mandibular arch. It becomes
solid. Previous investigations, llie pancreas. — Arises as diverticulum from
dorsal side of duodenum. Its further growth. Formation of duct. The liver. —
Arises as ventral diverticulum of duodenum. Hepatic cylinders. Comparison
with other types. 21ie suhnotochordal rod. — Its separation from dorsal wall of
alimentary canal. The section of it in the irunk. In the head. Its disap-
pearance. Views as to its meaning.

CHAPTER XL

THE VASCULAR SYSTEM AND VASCULAR GLANDS, pp. 230 217.

T'he heart. — Its development. Comparison with other types. Meaning of
double formation of heart. The general circulation. The venous system. The
primitive condition of. Comparison of, with Amphioxus and Annelids. The

TABLE OF CONTENTS. xi

cardinal veins. Eelations of caudal vein. The circulation of the yolk-sacTc.

Previous observations. Various stages. Difference of type in amniotic Verte-
brates. The vascular glands. -Supra-renal and inter-renal bodies. Previous
investigations. The snpra-renal bodies. — Their structure in the adult. Their
development from the sympathetic ganglia. The inter-renal body. — Its struc-
ture in the adult. Its independence of supra-renal bodies. Its development.

CHAPTER XII.

THE ORGAXS OF EXCRETION, pp. 249 — 287.

Previous investigations. Excretory organs and genital ducts in adult. In
male. — Kidney and Wolffian body. Wolffian duct. Ureters. Cloaca. Seminal
bladders. Kudimentary oviduct. In female.— ■'Wol&.Sin dnct. Ureters. Cloaca.
— Segmental openings. Glandular tubuli of kidney. Malpighian bodies. Ac-
cessory Malpighian bodies. Eelations of to segmental tubes. Vasa efferentia.
Comparison of ScylHum with other Elasmobranchs. Development of segmental
tubes. Their junction with segmental duct. Their division into four segments.
Formation of Malpighian bodies. Connection between successive segments.
Morphological interest of. Development of Miillerian and Wolffian ducts. In
female — General account. Formation of oviduct as nearly solid cord. Hymen.
In male — Kudimentary Miillerian duct. — Comparison of development of Miil-
lerian duct in Birds and Elasmobranchs. Own researches. Urinal cloaca. For-
mation of Woljian body and kidney proper. — General account. Details of forma-
tion of ureters. Vasa efferentia. — Views of Semper and Spengel. Difficulties
of Semper's views. Unsatisfactory result of own researches. General homo-
logies. Kesume. Postscript.

CHAPTER I.
The Ripe Ovarian Ovum.

The ripe ovum is nearly spherical, and, after the removal
of its capsule, is found to be unprovided with any form of
protecting membrane.

My investigations on the histology of the ripe ovarian ovum
have been made with the ova of the Gray Skate (Raja hatis)
only, and owing to a deficiency of material are somewhat
imperfect.

The bulk of the ovum is composed of yolk spherules,
imbedded in a protoplasmic matrix. Dr Alexander Schult^S
who has studied with great care the constitution of the yolk,
finds, near the centre of the ovum, a kernel of small yolk sphe-
rules, which is succeeded by a zone of spherules which gradually
increase in size as they approach the surface. But, near the
surface, he finds a layer in which they again diminish in size
and exhibit numerous transitional forms on the way to molecular
yolk-granules. These Dr Schultz regards as in a retrogressive
condition.

Another interesting feature about the yolk is the presence
in it of a protoplasmic network. Dr Schultz has completely
confirmed, and on some points enlarged, my previous observa-
tions on this subjects Dr Schultz's confirmation is the more
important, since he appears to be unacquainted with my pre-
vious investigations. In my paper (loc. cit), after giving a
description of the network I make the following statement as
to its distribution.

" A specimen of this kind is represented in Plate xiii. Fig. 2, n.
2/, where the meshes of the network are seen to be finer immediately
around the nuclei, and coarser in the intervals. The specimen
further shews, in the clearest manner, that this network is not
divided into areas, each representing a cell and each containing a
nucleus. I do not know to what extent this network extends into

1 ArcJiiv fiir Micro. Anat. Vol. xi. 1875.

2 Quart. Journ. Micro. Science, Oct. 1874.

B.

2 RIPE OVARIAN OVUM.

the yolk. I have never yet seen the limits of it, though it is very
common to see the coarsest yolk-gramiles lying in its meshes. Some
of these are shewn in Plate xiii. Fig. 2, y. ^."

Dr Schultz, by employing special methods of hardening and
cutting sections of the whole ^gg, has been able to shew that
this network extends, in the form of fine radial lines, from the
centre to the circumference ; and he rightly states, that it
exhibits no cell-like structures. I have detected this network
extending throughout the whole yolk in young eggs, but
have failed to see it with the distinctness which Dr Schultz
attributes to it in the ripe ovum. Since it is my intention to
enter fully both into the structure and meaning of this net-
work in my account of a later stage, I say no more about it
here.

At one pole of the ripe ovum a slight examination demon-
strates the presence of a small circular spot, sharply distin-
guished from the remainder of the yolk by its lighter colour.
Around this spot is an area which is also of a lighter colour
than the yolk, and the outer border of which gradually shades
into the normal tint of the yolk. If a section be made
through this part (vide PL I. fig. 1) the circular spot will be
found to be the germinal vesicle, and the area around it a
disc of yolk containing smaller spherules than the surround-
ing parts. The germinal vesicle possessed the same structure
in both the ripe eggs examined by me ; and, in both, it was
situated quite on the external surface of the yolk.

lu one of my specimens it was flat above, but convex
below ; in the other and, on the whole, the better preserved of
the two, it had the somewhat quadrangular but rather irregular
section represented ia PL I. fig. 1. It consisted of a thick-
ish membrane and its primitive contents. The membrane
surrounded the upper part of the contents and exhibited
numerous folds and creases (vide fig. 1). As it extended down-
wards it became thinner, and completely disappeared at some
little distance from the lower end of the contents. These,
therefore, rested below on the yolk. At its circumference the
membrane of the disc was produced into a kind of fold, forming
a rim which rested on the surface of the yolk.

In neither of my specimens is the cavity in the upper part

DEVELOPMENT OF ELASMOBRANCH FISHES. 3

of the membrane filled by the contents ; and the upper part of
the membrane is so folded and creased that sections through
almost any portion of it pass through the folds. The regularity
of the surface of the yolk is not broken by the germinal
vesicle, and the yolk around exhibits not the slightest signs of
displacement. In the germinal vesicle figured the contents
are somewhat irregular in shape ; but in my other specimen
they form a regular mass concave above and convex below. In
both cases they rest on the yolk, and the floor of the yolk is
exactly moulded to suit the surface of the contents of the
germinal vesicle. The contents have a granular aspect, but
differ in constitution from the surrounding yolk. Each germi-
nal vesicle measured about one-fiftieth of an inch in diameter.

It does not appear to me possible to suppose that the pecu-
liar, appearances which I have drawm and described are to be
looked upon as artificial products either of the chromic acid, in
which the ova were hardened, or of the instrument with which
sections of them were made. It is hardly conceivable that
chromic acid could cause a rupture of the membrane and the
ejection of the contents of the vesicle. At the same time the
uniformity of the appearances in the different sections, the regu-
larity of the whole outline of the egg, and the absence of any
signs of disturbance in the yolk, render it impossible to believe
that the structures described are due to faults of manipulation
during or before the cutting of the sections.

We can only therefore conclude that they represent the
real state of the germinal vesicle at this period. No doubt
they alone do not supply a sufficient basis for any firm con-
clusions as to the fate of the germinal vesicle. Still, if they
cannot sustain, they unquestionably support certain views. The
natural interpretation of them is that the membrane of the
germinal vesicle is in the act of commencing to atrophy, pre-
paratory to being extruded from the egg, while the contents of
the germinal vesicle are about to be absorbed.

In favour of the extmsion of the membrane rather than its
absorption are the following features,

(1) The thickness of its upper surface. (2) The extension
of its edge over the yolk. (3) Its position external to the yolk.

In favour of the view that the contents will be left behind

1—2

4 EIPE OVAEIAN OVUM.

and absorbed when the membrane is pushed out, are the follow-
ing features of my sections :

(1) The rupture of the membrane of the germinal vesicle
on its lower surface. (2) The position of the contents almost
completely below the membrane of the vesicle and surrounded
by yolk.

In connection with this subject, Oellacher's valuable observa-
tions upon the behaviour of the germinal vesicle in Osseous
Fishes and in Birds at once suoffj^est themselves \ Oellacher
sums up his results upon the behaviour of the germinal vesicle
in Osseous Fishes in the following way (p. 12) :

" The germinal vesicle of the Trout's egg, at a period when the
egg is very nearly ripe, lies near the surface of the germinal

disc which is aggregated together in a hollow of the yolk

After this a hole appears in tlie membrane of the germinal vesicle,
which opens into the space between the egg-membrane and the
germinal disc. The hole widens more and more, and tlie membrane
frees itself little by little from the contents of the germinal vesicle,
which remain behind in the form of a ball on the floor of the cavity
formed in this way. The cavity becomes flatter and flatter and the
contents are pushed up further and further from tlie germinal disc.
"When the hollow, in which lie the contents of the original germinal
vesicle, completely vanishes, the covering membrane becomes inverted

and the membrane is spread out on the convex surface of the

germinal disc as a circular, investing structure. It is clear that by
the removal of the membrane the contents of the germinal vesicle
become lost."

These very definite statements of Oellacher tell strongly
against my interpretation of the appearance presented by the
germinal vesicle of the ripe Skate's egg. Oellacher's account is
so precise, and his drawings so fully bear out his interpretations,
that it is very difficult to see where any error can have crept in.

On the other hand, with the exception of those which
Oellacher has made, there cannot be said to be any satisfactory
observations demonstratins^ the extrusion of the sferminal vesi-
cle from the ovum. Oellacher has observed this definitely for
the Trout, but his observations upon the same point in the
Bird would quite as well bear the interpretation that the mem-
brane alone became pushed out, as that this occurred to the
germiual vesicle, contents and all.

1 Arcliiv fur Micr. Anat. Vol. viii. p. 1.

DEVELOPMENT OF ELASMOBRANCH FISHES. 5

While, then, there are on the one hand Oellacher's observa-
tions on a single animal, hitherto unconfirmed, there are on the
other very definite observations tending to shew that the ger-
minal vesicle has in many cases an altogether different fate.
Gotte^ not to mention other observers before him, has in the
case of Batrachian's eggs traced out with great precision the
gradual atrophy of the germinal vesicle, and its final absorption
into the matter of the ovum.

Gotte distinguishes three stages in the degeneration of the
germinal vesicle of Bombinator's egg. In the first stage the ger-
minal vesicle has begun to travel up towards the surface of the
egg. It retains nearly its primitive condition, but its contents
have become more opaque and have partly withdrawn them-
selves from the thin membrane. The germinal spots are still
circular, but in some cases have increased in size. The most
important feature of this stage is the smaller size of the germi-
nal vesicle than that of the cavity of the yolk in which it lies,
a condition which appears to demonstrate the commencing
atrophy of the vesicle.

In the next stage the cavity containing the germinal vesicle
has vanished without leaving a trace. The germinal vesicle
itself has assumed a lenslike form, and its borders are irreofular
and pressed in here and there by yolk. Of the membrane of
the germinal vesicle, and of the germinal spots, only scanty
remnants are to be seen, many of which lie in the immediately
adjoining yolk.

In the last stage no further trace of a distinct germinal
vesicle is present. In its place is a mass of very finely granular
matter, which is without a distinct border and graduates into
the surrounding yolk and is to be looked on as a remnant of
the germinal vesicle.

This careful investigation of Gotte proves beyond a doubt
that in Batrachians neither the membrane, nor the contents of
the germinal vesicle, are extruded from the egg.

In Mammalia, Van Beneden^ finds that the germinal vesicle
becomes invisible, though he does not consider that it abso-
lutely ceases to exist. He has not traced the steps of the process

^ Enhcicldiingsgeschichte dor Unke.

^ Efcherches sur la Composition et la Signification de VCEuf.

6 RIPE OVARIAN OVUM.

with the same care as Gotte, but it is difficult to believe that
an extrusion of the vesicle in the way described by Oellacher
would have escaped his notice.

Passing from Vertebrates to Invertebrates, we find that
almost every careful investigator has observed the disappear-
ance, apparent or otherwise, of the germinal vesicle, but that
very few have watched with care the steps of the process.

The so-called Richtungskorper has been supposed to be the
extruded remnant of the germinal vesicle. This view has been
especially adopted and supported by Oellacher (Jioc. cit), and
Flemminsf \

The latter author regards the constant presence of this body,
and the facility witli which it can be stained, as proofs of its
connection with the germinal vesicle, which has, however,
according to his observations, disappeared before the appear-
ance of the Richtungskorper.

Kleinenberg ''^j to whom we are indebted for the most pre-
cise observations we possess on the disappearance of the germ-
inal vesicle, gives the following account of it, pp. 41 and 42.

" We left the germinal vesicle as a vesicle with a distinct doubly
contoured membrane, and equally distributed granular contents, in

which the germinal spot had ai)peared The germinal vesicle

reaches O'OGmm. in diameter, and at the same time its contents under-
go a separation. Tlie greater part withdraws itself from the membrane
and collects as a dense mass around the gei'miiial spot, while closely
adjoining the membrane there remains only a very thin but unbroken
lining of the plasmoid material. The intermediate space is filled
with a clear fluid, but the layer which lines the membrane retains
its connection with the mass around the germinal vesicle by means
of numerous fine threads which traverse the space filled with fluid.

At about the time when tlie formation of the psendocells in the

eg^ is C(jmpleted the germinal spot undergoes a retrogressive meta-
morphosis, it loses its circular outline and it now appears as if
coagulated ; then it breaks up into small fragments, and I am fairly

confident that these become dissolved. The germinal vesicle

becomes, on the egg assuming a sj)herical form, drawn into an
eccentric position towards the pole of the egg directed outwards,
where it lies close to the surface and only covered by a very thin
layer of plasma. In this situation its degeneration now begins,
and ends in its complete disap})earance. The granular contents
become more and more fluid ; at the same time part of them pass

1 Studicn in dcr Entwicklungsgcschichtc der Najudcn, Sitz. d. k. Akad.
Wien, Bd. lxxi. 1875.

2 Hydra. Leipzig, 1872.

DEVELOPMENT OF ELASMOBRANCH FISHES. 7

out through the membrane. This, which so far was firmly stretched,
next collapses to a somewhat egg-like sac, whose wall is thickened
and in places folded.

" The inner mass which up to this time has remaiaed compact
now breaks up into separate highly refi-active bodies, of spherical
or angular form and of very different sizes ; between them, here and

there, are scattered drops of a fluid fat I am very much inclined

to regard the solid bodies in question as fat or as that peculiar
modification of albuminoid bodies which we recognise as the certain
forerunner of the formation of fat in so many pathologically altered
tissues; and therefore to refer the disappearance of the germinal
vesicle to a fatty degeneration. On one occasion I believe that I
observed an opening in the membrane at this stage ; if this is a
normal condition it would be possible to believe that its solid con-
tents passed out and were taken up in the surrounding plasma.
What becomes of the membrane I am unable to say ; in any case
the germinal vesicle has vanished to the very last trace before
impregnation occurs."

Kleinenberg clearly finds that the germinal vesicle disappears
completely before the appearance of the Richtungskorper, in
which he states a pseudocell or yolk-sphere is usually found.

The connection between the Richtungskorper and the germi-
nal vesicle is not a result of strict observation, and there can
be no question that the evidence in the case of invertebrates
tends to prove that the germinal vesicle in no case disappears
owing to its extrusion from the egg, but that if part of it is
extruded from the egg as Richtungskorper this occurs when its
constituents can no longer be distinguished from the remainder
of the yolk. This is clearly the case in Hydra, where, as stated
above, one of the pseudocells or yolk-spheres is usually found
imbedded in the Richtungskorper.

My observations on the Skate tend to shew that, in its case,
the membrane of the germinal vesicle is extruded from the egg^
though they do not certainly prove this. That conclusion is
however supported by the observations of Schenk\ He found
in the impregnated, but not yet segmented, germinal disc a
cavity which, as he suggests, might well have been occupied
by the germinal vesicle. It is not unreasonable to suppose that
the membrane, being composed of formed matter and able only
to take a passive share in vital functions, could, without thereby
influencing the constitution of the ovum, be ejected.

If we suppose, and this is not contradicted by observation,

1 Die Eier von Eaja qua<Irimaculata, Sitz. der k. Akad. Wien, Bd. lxviii.

8 RIPE OVARIAN OVUM.

that the Richtungskorper is either only the metamorphosed
membrane of the germinal vesicle with parts of the yolk, or part
of the yolk alone, and assume that in Oellacher's observations
only the membrane and not the contents were extruded from
the egg, it would be possible to frame a consistent account of
the behaviour of the germinal vesicle throughout the animal
kingdom, which may be stated in the following way.

The germinal vesicle usually before, but sometimes imme-
diately after impregnation undergoes atrophy and its contents
become indistinguishable from the remainder of the egg. In
those cases in which its membrane is very thick and resistent,
e. g. Osseous and Elasmobranch Fishes, Birds, etc., this may be
incapable of complete resorption, and be extruded bodily from
the egg. In the case of most ova, it is completely absorbed,
though at a subsequent period it may be extruded from the egg
as the Richtungskorper. In all cases the contents of the
germinal vesicle remain in the ovum.

In some cases the germinal vesicle is stated to persist and
to undergo division during the process of segmentation; but
the observations on this point stand in need of confirmation.

My investigations shew that the germinal vesicle atrophies
in the Skate before impregnation, and in this respect accord
Avith very many recent observations. Of these the following
may be mentioned.

(1) Oellacher (Bird, Osseous Fish). (2) Gotte (Bombina-
tor igneus). (8) Kupffer (Ascidia Canina). (4) Strasburger
(Phallusia Mamillata). (5) Kleinenberg (Hydra). (6) Metsch-
nikoff (Geryonia, Polyzenia leucostyla, Fpibulia aurantiaca, and
other Hydrozoa).

This list is sufficient to shew that the disappearance of the
germinal vesicle before impregnation is very common, and I
am unacquainted with any observations tending to shew that
its disappearance is due to impregnation.

In some cases, e. g. Asterocanthion^, the germinal vesicle
vanishes after the spermatozoa have begun to surround the egg ;
but I do not know that its disappearance in these cases has
been shewn to be due to impregnation. To do so it would be
necessary to prove that in ripe eggs let loose from the ovary, but
not fertilized, the germinal vesicle did not undergo the same
1 Agassiz, Embryology of the Star-Fish.

DEVELOPMENT OF ELASMOBRANCH FISHES. 9

changes as in the case of fertilized eggs ; and this, as far as I
know, has not been done. After the disappearance of the
germinal vesicle, and before the first act of division, a fresh
nucleus frequently appears [ — vide — Auerbach (Ascaris nigro-
venosa), Fol (Geryonia), Kupffer (Ascidia canina), Strasburger
(Phallusia mamillata), Fleming (Anodon), Gotte (Bombinator
igneus)], which is generally stated to vanish before the appear-
ance of the first furroAV ; but in some cases (Kupffer and Gotte,
and as studied with especial care Strasburger) it is stated to
divide. Upon the second nucleus, or upon its relation to the
germinal vesicle, I have no observations ; but it appears to me
of great importance to determine whether this fresh nucleus
arises absolutely de novo, or is formed out of the matter of the
germinal vesicle.

The germinal vesicle is situated in a bed of finely divided
yolk-particles. These graduate insensibly into the coarser yolk-
spherules around them, though the band of passage between the
coarse and the finer yolk-particles is rather narrow. The mass
of fine yolk-granules may be called the germinal disc. It is
not to be looked upon as diverging in any essential particular
from the remainder of the yolk, for the difference between the two
is one of degree only. It contains in fact a larger bulk of active
protoplasm, as compared with yolk-granules, than does the
remainder of the ovum. The existence of this agreement in
kind has been already strongly insisted on in my preliminary
paper ; and Schultz [loc. cit) has arrived at an entirely similar
conclusion, from his own independent observations.

One interesting feature about the germinal disc at this
period is its size.

My observations upon it have been made with the eggs of
the Skate (Haja) alone ; but I think that it is not probable that
its size in the Skate is greater than in Scy Ilium or Pristiurus.
If its size is the same in all these genera, then the germinal
disc of the unimpregnated ovum is very much greater than that
portion of the ovum which undergoes segmentation, and which
is usually spoken of as the germinal disc in impregnated ova.

I have no further observation on the ripe ovarian ovum;
and my next observations concern an ovum in which two furrows
have already appeared.

CHAPTER II.

The Segmentation.

I HAVE not been fortunate enough to obtain an absolutely
complete series of eggs during segmentation.

In the cases of Pristiurus and Scyllium only have I had
any considerable number of eggs in this condition, though one
or two eggs of Raja in which the process was not completed
have come into my hands.

In the youngest impregnated Pristiurus eggs, which I have
obtained, the germinal disc was already divided into four seg-
ments.

The external appearance of the blastoderm, which remains
nearly constant during segmentation, has been already well
described by Leydig\

The yolk has a pale greenish tinge which, on exposure to
the air, acquires a yellower hue. The true germinal disc ap-
pears as a circular spot of a bright orange colour, and is, accord-
ing to Leydig's measurements, IJm. in diameter. Its colour
renders it very conspicuous, a feature which is further increased
by its being surrounded by a narrow dark line (PI. I. fig. 2),
the indication of a shallow groove. Surrounding this line is a
concentric space which is lighter in colour than the remainder
of the yolk, but whose outer border passes by insensible gra-
dations into the yolk. As was mentioned in my preliminary
paper {loc. cit.), and as Leydig (loc. cit.) had ^^efore noticed, the
germinal disc is always situated at the pole of the yolk which
is near the rounded end of the Pristiurus e^g. It occupies a
corresponding position in the eggs of both species of Scyllium
(stellare and canicula) near the narrower end of the egg to
which the shorter pair of strings is attached. The germinal disc
in the youngest Qgg examined, exhibited two furrows which

1 Rochen und Ilaie.

DEVELOPMENT OF ELASMOBRANCH FISHES. 11

crossed each other at right angles in the centre of the disc, but
neither of which reached its edge. These furrows accordingly
divided the disc into four segments, completely separated from
each other at the centre of the disc, but united near its cir-
cumference.

I made sections, though not very satisfactorily, of this
germinal disc. The sections shewed that the disc was composed
of a protoplasmic basis, in which were imbedded innumerable
minute spherical yolk-globules so closely packed as to constitute
nearly the whole mass of the germinal disc.

In passing from the coarsest yolk-spheres to the fine sphe-
rules of the germinal disc, three bands of different-sized yolk-
particles have to be traversed. These bands graduate into one
another and are without sharp lines of demarcation. The outer
of the three is composed of the largest-sized yolk-spherules
which constitute the greater part of the ovum. The middle band
forms a concentric layer around the germinal disc, and is com-
posed of yolk-spheres considerably smaller than those outside it.
Where it cuts the surface it forms the zone of lighter colour
immediately surrounding the germinal disc. The innermost
band is formed by the germinal disc itself and is composed of
spherules of the smallest size. These features are shewn in
PI. I. fig. 6, which is the section of a germinal disc with twenty-
one segments ; in it however the outermost band of spherules is
not present.

From this description it is clear, as has already been men-
tioned in the description of the ripe unimpregnated ovum, that
the germinal disc is not to be looked upon as a body entirely
distinct from the remainder of the ovum, but merely as a part
of the ovum in which the protoplasm is more concentrated and
the yolk-spherules smaller than elsewhere. Sections shew that
the farrows visible on the surface end below, as indeed they do
on the surface, before they reach the external limit of the finely
granular matter of the germinal disc. There are therefore at
this stage no distinct segments: the otherwise intact germinal
disc is merely grooved by two furrows.

I failed to observe any nuclei in the germinal disc just
described, but it by no means follows that they were not
present.

12 SEGMENTATION.

In the next youngest of the eggs* examined the germinal
disc was already divided into twenty-one segments. AVhen
viewed from the surface (PI. I. fig. 3), the segments appeared
divided into two distinct groups — an inner group of eleven
smaller segments, and an outer group of segments surrounding
the former. The segments of both the inner and the outer
group were very irregular in shape and varied considerably in
size. The amount of irregularity is far from constant and many
germinal discs are more regular than the one figured.

In this case the situation of the germinal disc and its relations
to the yolk were precisely the same as in the earlier stage.

In sections of this germinal disc (PL I. fig. 6), the groove
which separates it from the yolk is well marked on one side,
but hardly visible at the other extremity of the section.

Passing from the external features of this stage to those
which are displayed by sections, the striking point to be noticed
is the persisting continuity of the segments, marked out on the
surface, with the floor of the germinal disc.

The furrows which are visible on the surface merely form
a pattern, but do not isolate a series of distinct segments.
They do not even extend to the limit of the finely granular
matter of the germinal disc.

The section represented, PI. I. fig. 6, bears out the state-
ments about the segments as seen on the surface. There are
three smaller segments in the middle of the section, and two
larger at the two ends. These latter are continuous with the
coarser yolk-spheres surrounding the germinal disc and are not
separated from them by a segmentation furrow.

In a slightly older embryo than the one figured I met with

a few completely isolated segments at the surface. These

segments were formed by the apparent bifurcation of furrows

as they neared the surface of the germinal disc. The segments

thus produced are triangular in form. They probably owe their

origin to the meeting of two oblique furrows. The last-formed

of these furrows apparently ceases to be prolonged after meeting

the first-formed furrow. I have not in any case observed an

example of two furrows crossing one another at this stage.

^ The germinal disc figured was from the egg of a Scy Ilium stellare and not
Pristiurup, but I have also sections of a Pristiurus egg of the same age, which
do not differ materially from the Scyllium sections.

DEVELOPMENT OF ELASSIOBRANCH FISHES. 13

The furrows themselves for the most part are by no means
simple slits with parallel sides. They exhibit a beaded structure,
shewn imperfectly in PL I. fig. 6, but better in PI. I. fig. 6 a,
■which is executed on a larger scale. They present intervals
of dilatations where the protoplasms of the segments on the
tv/o sides of the furrow are widely separated, alternating with
intervals where the protoplasms of the two segments are almost
in contact and are only separated from one another by a very
narrow space.

A closer study of the germinal disc at this period shews
that the cavities which cause the beaded structure of the
furrows are not only present along the lines of the furrows
but are also found scattered generally through the germinal disc,
though far more thickly in the neighbourhood of the furrows.
Their appearance is that of vacuoles, and with these they are
probably to be compared. There can be little question that
in the living germinal disc they are filled with fluid. In some
cases, they are collected in very large numbers in the region
of a furrow. Such a case as this is shewn in PI. I. fig. 6 h. In
numerous other cases they occur, roughly speaking, alternately
on each side of a furrow. Some furrows, though not many, are
entirely destitute of these structures. The character of their
distribution renders it impossible to overlook the fact that these
vacuole-like bodies have important relations with the formation
of the segmentation furrow\s.

Lining the two sides of the segmentation furrows there is
present in sections a layer which stains deeply with colouring
re-agents; and the surface of the blastoderm is stained in the
same manner. In neither case is it permissible to suppose
that any membrane-like structure is present. In many cases a
similar very delicate, but deeply-stained line, invests the vacuo-
lar cavities, but the fluid fiUing these remains quite unstained.
When distinct segments are formed, each of these is surrounded
by a similarly stained line.

The yolk-spherules are so numerous, and render even the
thinnest section so opaque, that I have failed to make satis-
factory observations on the behaviour of the nucleus. I find
nuclei in many of the segments, though it is very difficult even
to see them, and only in very favourable specimens can their

14 SEGMENTATION.

structure be studied. In some cases, two of them lie one on
each side of a furrow ; and in one case at the extreme end of a
furrow I could see two peculiar aggregations of yolk-spherules
united by a band through which the furrow, had it been con-
tinued, would have passed. The connection (if any exists) be-
tween this appearance and the formation of the fresh nuclei
in the segments, I have been unable to elucidate.

The peculiar appearances attending the formation of fresh
nuclei in connection with cell-division, which have recently
been described by so many observers, have hitherto escaped my
observation at this stage of the segmentation, though I shall
describe them in a later stage. A nucleus of this stage is
shewn on PL I. fig. 6 c. It is lobate in form and is divided by
lines into areas in each of which a deeply-stained granule is
situated.

The succeeding stages of segmentation present from the
surface no fresh features of great interest. The somewhat
irregular (PI. I. figs. 4 and 5) circular line, which divides the
peripheral larger from the central smaller segments, remains for
a long time conspicuous. It appears to be the representative of
the horizontal furrow which, in the Batrachian ovum, separates
the smaller pigmented spheres from the larger spheres of the
lower pole of the egg.

As the seofments become smaller and smaller, the distinction
between the peripheral and the central segments becomes less
and less marked ; but it has not disappeared by the time that
the segments become too small to be seen with the simple
lens. When the spheres become smaller than in the germinal
disc represented on PL I. fig. 5, the features of segmentation
can be more easily and more satisfactorily studied by means of
sections.

To the features presented in sections, both of the latter
and of the earlier blastoderms, I now return. A section of
one of the earlier germinal discs, of about the age of the one
represented on PL I. fig. 4, is shewn in PL ii. fig. 7.

It is clear at a glance that we are now dealing with true
segments completely circumscribed on all sides. The peri-
pheral segments are, as a rule, larger than the more central
ones, though in this respect there is considerable irregularity.

DEVELOPMENT OF ELA.SMOBRANCH FISHES. 15

The segments are becoming smaller by repeated division;
but, in addition to this mode of increase, there is now going
on outside the germinal disc a segmentation of the yolk, by
which fresh segments are being formed from the yolk and
added to those which already exist in the germinal disc. One
or two such segments are seen in the act of being formed
(PI. II. fig. 7 /) ; and it is to be noticed that the furrows
which will eventually mark out the segments, do so at first in a
partial manner only, and do not circumscribe the whole circum-
ference of the segment in the act of being formed. These
fresh furrows are thus repetitions on a small scale of the earliest
segmentation furrows.

It deserves to be noticed that the portion of the germinal disc
which has already undergone segmentation, is still surrounded
by a broad band of small-sized yolk-spherules. It appears to
me probable that owing to changes taking place in the spherules
of the yolk, which result in the formation of fresh spherules of
a small size, this band undergoes a continuous renovation.

The uppermost row of segmentation spheres is now com-
mencing to be distinguished from the remainder as a separate
layer which becomes progressively more distinct as segmenta-
tion proceeds.

The largest segments in this section measure about the
jl^th of an inch in diameter, and the smallest about s^o^h of
an inch.

The nuclei at this stage present points of rather a special in-
terest. In the first place, though visible in many, and certainly
present in all the segments ^ they are not confined to these :
they are also to be seen, in small numbers, in the band of
fine spherules which surrounds the already segmented part
of the germinal disc. Those found outside the germinal disc are
not confined to the spots where fresh segments are appearing,
but are also to be seen in places where there are no traces of
fresh segments.

This fact, especially when taken in connection with the forma-

^ In the figure of this stage, I have inserted nuclei in all the segments. In
the section from which the figure was taken, nuclei were not to be seen in many
of the segments, but I have not a question that they were present in all of them.
The difficulty of seeing them is, in part, due to the yolk-spherules and in part to
the thinness of the section as compared with the diameter of a segmentation
sphere.

16 SEGMENTATION.

tion of fresli segments outside the germinal disc and with other
facts which I shall mention hereafter, is of great morphological
interest as bearing upon the nature and homologies of the
food-yolk. It also throws light upon the behaviour and mode
of increase of the nuclei. All the nuclei, both those of the
segments and those of the yolk, have the peculiar structure I
described in the last stage.

In specimens of this stage I have been able to observe
certain points which have an important bearing upon the be-
haviour of the nucleus during cell-division.

Three figures, illustrating the behaviour of the nucleus, as
I have seen it in sections of blastoderms hardened in chromic
acid, are shewn in PL li. figs. 7a, 7 b and 7c.

In the place of the nucleus is to be seen a sharply defined
figure (Fig. 7 a) stained in the same way as the nucleus
or more deeply. It has the shape of two cones placed base
to base. From the apex of each cone there diverge to-
wards the base a series of excessively fine strioe. At the junc-
tion between the two cones is an irregular linear series of small
deeply stained granules which form an apparent break between
the two. The line of this break is continued very indistinctly
beyond the edge of the figure on each side.

From the apex of each cone there diverge outwards into the
protoplasm of the cell a series of indistinct markings. They are
rendered obscure by the presence of yolk-spherules, which
completely surround the body just described, but which are not
arranged with any reference to these markings. These latter
striae, diverging from the apex of the cone, are more distinctly
seen when the apex points to the observer (Fig. 7 b), than when
a side of the cone is in view.

The striae diverging outwards from the apices of the cones
must be carefully distinguished from the striae of the cones
themselves. The cones are bodies quite as distinctly dift'er-
entiated from the protoplasm of the cell as nuclei, while the
striae which diverge from their apices are merely structures in
the general protoplasm of the cell.

In some cells, which contain these bodies, no trace of a com-
mencing line of division is visible. In other cases (Fig. 7 c),
such a line of division does appear and passes through the

DEVELOPMENT OF ELASMOBRANCH FISHES, 17

junction of the two cones. In one case of this kind I fancied
I could see (and have represented) a coloured circular body
in each cone. I do not feel any confidence that these two bodies
are constantly present; and even where visible they are very
indistinct.

Instead of an ordinary nucleus a very indistinctly marked
vesicular body sometimes appears in a segment ; but whether
it is to be looked on as a nucleus not satisfactorily stained,
or as a nucleus in the act of being formed, I cannot decide.

With reference to the situation of the cone-hke bodies I have
described I have made an observation which appears to me
to be of some interest. I find that bodies of this kind are
found in the yolk completely outside the germinal disc. I have
made this observation, in at least two cases which admitted of
no doubt (vide Fig. 7 nx).

We have therefore the remarkable fact, that whatever
connection these bodies may have with cell-division, they can
occur in cases where this is altogether out of the question and
where an increase in the number of nuclei can be their only
product.

These are the main facts which I have been able to de-
termine with reference to the nuclei of this stage; but it will
conduce to clearness if I now finish wdiat I have to say upon
this subject.

At a still later stage of segmentation the same peculiar
bodies are to be seen as during the stage just described, but
they are rarer; and, in addition to them, other bodies are to be
seen of a character intermediate between ordinary nuclei and
the former bodies.

Three such are represented in PL ii. figs. Sa, 8b, 8c. In all
of these there can be traced out the two cones, which are however
very irregular. The striation of the cones is still present, but
is not nearly so clear as it was in the earlier stage.

In addition to this, there are numerous deeply stained
granules scattered about the two figures which resemble exactly
the granules of typical nuclei.

All these bodies occupy the place of an ordinary nucleus,
they stain like an ordinary nucleus and are as sharply defined
as an ordinary nucleus.

13 SEGMENTATION.

There is present around some of these, especially those
situated in the yolk, the network of lines of the yolk de-
scribed by me in a preliminary paper ^ and I feel satisfied that
there is in some cases an actual connection between the net-
work and the nuclei. This network I shall describe more fully
hereafter.

Further points about these figures and the nuclei of this
stasre I should like to have been able to observe more com-
pletely than I have done, but they are so small that with the
highest powers I possess (Zeiss, Immersion No. 2 = -f-^ in.) their
complete and satisfactory investigation is not possible.

Most of the true nuclei of the cells of the germinal disc
are regularly rounded ; those however of the yolk are fre-
quently irregular in shape and often provided with knob-like
processes. The gradations are so complete between typical
nuclei and bodies like that shewn (PL ii. fig. 8 c) that it
is impossible to refuse the name of nucleus to the latter.

In many cases two nuclei are present in one cell.

In later stages knob-like nuclei of various sizes are scattered
in very great numbers in the yolk around the blastoderm (vide
PI. III. IV. v.). In some cases it appears to me that several of
these are in close juxta-position, as if they had been produced by
the division of one primitive nucleus. I do not feel absolutely
confident that this is the case, owing to the fact that in the
investigation of a knobbed body there is great difiiculty in
ascertaining that the knobs, which appear separate in one plane,
are not in reality united in another.

I have, in spite of careful search, hitherto failed to find
amongst these later nuclei cone-like figures, similar to those I
found in the yolk during segmentation. This is the more remark-
able since in the early stages of segmentation, when very few
nuclei are present in the yolk, the cone-like figures are not un-
common ; whereas, in the latter stages of development when
the nuclei of the yolk are very common and obviously increas-
ing rapidly, such figures are not to be met with.

In no case have I been able to see a distinct membrane
round any of the nuclei.

I have hitherto attempted to describe the appearances

^ Loc. cit.

DEVELOPMENT OF ELASMOBRANCH FISHES. 19

bearing on the behaviour of the nuclei in as objective a manner
as possible.

My observations are not as complete as could be desired;
but, taken in conjunction with those of other investigators, they
appear to me to point towards certain definite conclusions with
reference to the behaviour of the nucleus in cell-division.

The most important of these conclusions may be stated as
follows. In the act of cell-division the nuclei of the resulting
cells are formed from the nucleus of the primitive cell.

This may occur; —

(1) By the complete solution of the old nucleus within the
protoplasm of the mother cell and the subsequent reaggregation
of its matter to form the nuclei of the freshly formed daughter
cells,

(2) By the simple division of the nucleus,

(3) Or by a process intermediate between these two where
part of the old nucleus passes into the general protoplasm and
part remains always distinguishable and divides ; the fresh
nucleus being in this case formed from the divided parts as well
as from the dissolved parts of the old nucleus.

Included in this third process it is permissible to suppose
that we may have a series of all possible gradations between
the extreme processes 1 and 2. If it be admitted, and the
evidence we have is certainly in favour of it, that in some
cases, both in animal and vegetable cells, the nucleus itself
divides during cell-division, and in others the nucleus com-
pletely vanishes during the cell-division, it is more reasonable
to suspect the existence of some connection between the two
processes, than to suppose that they are entirely different in kind.
Such a connection is given by the hypothesis I have just proposed.

The evidence for this view, derived both from my own
observations and those of other investigators, may be put as
follows.

The absolute division of the nucleus has been stated to
occur in animal cells, but the number of instances where the
evidence is quite conclusive are not very numerous. Recently
F. E. Schultze ^ appears to have observed it in the case of an
Amoeba in an altogether satisfactor}^ manner. The instance is

1 ArcMv f. Micr. Anat. xi. p. 592.

2—2

20 • SEGMENTATION.

quoted by Flemming*. Schultze saw the nucleus assume a
duinb-bell shape, divide, and the two halves collect themselves
: together. The whole process occupied a minute and a half and
was shortly followed by the division of the Amoeba, which occu-
pied eight minutes. Amongst vegetable cells the division of the
nucleus seems to be still rarer than with animal cells. Sachs'
admits the division of the nucleus in the case of the paren-
chyma cells of certain Dicotyledons (Sambucus, Helianthus,
Lysimachia, Polygonum, Silene) on the authority of Hanstein.

The division of the nucleus during cell-division, though
seemingly not very common, must therefore be considered as
a thoroughly well authenticated occurrence.

The frequent disappearance of the nucleus during cell-
division is now so thoroughly recognised, both for animal and
vegetable cells, as to require no further mention.

In many cases the partial or complete disappearance of the
nucleus is accompanied by the formation of two peculiar star-
like figures. Appearances of the kind have been described by
Fol^ Flemming^ Auerbach^ and possibly also Oellacher^ as
well as other observers.

These figures' are possibly due to the streaming out of
the protoplasm of the nucleus into that of the cell I The

^ Entwicklungsgeschichte der Najaden, lxxi. Bd. der Sitz. der k. Akad. Wien,
1875.

5 Text-Booh of Botany, English trans, p. 19.

^ Entw. d. Geryonideneies. Jenaische Zeitschrift, Bd. vii.

* Loc. cit.

^ Organologisclie Studien, Zweites Heft.

* Beitrcige z.Eiitxoicklungsgeschichte der Knochenfischen: Zeit. fi'iT Wiss. Zoo-
logie. Bd. xxii. 1872.

y The meraohs of Anerbach and Strasbnrger (Zellbildung u. ZeUtheihing)
have unfortunately come into m}' hands too late for me to take advantage of
them. Especially in the magnificent monograph of Strasbm-ger I find drawings
precisely resembling those from my specimens already in the hands of the
engraver. Strasbnrger comes to the conclusion from his investigations that the
modified nucleus always divides and never vanishes as is usually stated. If his
views on this point are correct part of the hypothesis I have suggested above
is rendered unnecessary. The striae of the protoplasm, which in accordance
with Auerbach's view I have considered as being due to a strcammg out of the
matter of the nucleus, he regards as resulting from a polarity of the particles
in the cell and the attraction of the nucleus. My own investigations though,
as far as they go, quite in accordance with those of Strasbnrger, do not supply
any grounds for deciding on the meaning of these striae; and in some re-
spects they support Strasburger's views against those of other observers, since
they demonstrate that in Elasmobranchs the modified nucleus does actuaUy
divide.

8 This is the view which has been taken by Anerbach {Orgaiwlogische
Studien).

DEVELOPMENT OF ELASMOBEANCH FISHES. 21

appearance of striation may on this hypothesis be explained
as due to the presence of granules in the protoplasm. When
the streaming out of the protoplasm of a nucleus into that of
a cell takes place, any large granule which cannot be moved by
the stream will leave behind it a slack area where there is no
movement of the fluid. Any granules which are carried into
this area will remain there, and by the continuation of a pro-
cess of this kind a row of granules may be formed, and a series
of such rows would produce an appearance of striation. In
many cases, e.g. Anodon, vide Flemming^, even the larger yolk-
spherules are arranged in this fashion.

On the supposition that the striation of these figures is
due to the outflow from the nucleus, the appearances presented
in Elasmobranchs admit of the following explanation.

The central body consisting of two cones (Figs. 7a, 7c) is almost
without question the remnant of the primitive nucleus. This
is shewn by its occupying the same position as the primitive
nucleus, staining in the same way, and by there being a series
of insensible gradations between it and a typical nucleus. The
contents must be supposed to be streaming out from the two
apices of the cones, as appears from the striae in the body con-
verging on each side towards the apex, and then diverging
again from it. In my specimens the yolk-spherules are not
arranged with any reference to the radiating striation.

It is very likely that in the cases of the disappearance
of the nucleus, its protoplasm streams out in two directions,
towards the two parts of the cell which will eventually become
separated from each other; and probably, after the division, the
matter of the old nucleus is again collected to form two fresh
nuclei.

In some cases of cell-division a remnant of the old nucleus is
stated to be visible after the fresh nuclei have appeared. These
cases, of which I have not seen full accounts, are perhaps
analogous to what occasionally happens with the germinal
vesicle of an ovum. The whole of the contents of the germinal
vesicle become at its disappearance mingled with the proto-
plasm of the ovum, but the resistent membrane remains and
is eventually ejected from the Qgg, vide p. 3 et seq. If the

1 Loc. cit.

22 SEGMENTATION.

remnant of the old nucleus in the cases described is nothing
more than its membrane, no difficulty is offered to the view
that the constituents of the old nucleus may help to form tho
new ones.

In many cases the total bulk of the new nuclei is greater
than that of the old one; in such instances part of the proto-
plasm of the cell necessarily has a share in forming the new
nuclei.

Although, in instances where the nucleus vanishes, an abso-
lute demonstration of the formation of the fresh nuclei from the
matter of the old one is not possible; yet, if cases of the division
of the old nucleus to form the new ones be admitted to exist,
the derivation in the first process of the fresh nuclei from the
old ones must be postulated in order to maintain a continuity
between the two processes of formation; and, as I have at-
tempted to shew, all the circumstantial evidence is in favour of it.

Admitting the existence of the two extreme processes of nu-
clear formation, I wish to shew that my results in Elasmobranchs
tend to demonstrate the existence of intermediate steps be-
tween them. The first figures I described of two opposed cones,
appear to me almost certainly to represent nuclei in the act of
dissolution; but though a portion of the nucleus may stream
out into the yolk, I think it impossible that the whole of it
does\

I described these bodies in two states. An earlier one, in
which the two cones were separated by an irregular row of
deeply stained granules ; and a later one in which a furrow had
already appeared dividing the cones as well as the cell. In
neither of these conditions could I see any signs of the body
vanishing completely. It was as clearly defined and as deeply
stained as an ordinary nucleus, and in its later condition the
signs of the streaming out of material from its pointed extremi-
ties were less marked than in the earlier stage.

All these facts, to my mind, point to the view that
these cone-like bodies do not disappear, but form the basis
for the new nuclei. Possibly the body visible in each cone

1 After Strcasburger's obsei-vation it must be considered very doubtful whether
the streaming out of the contents of the inicleus, in the manner implied in the
text, really takes place.

DEVELOPMENT OF ELASMOBRANCH FISHES. 23

in the later stage, was the commencement of this new
nucleus. Gotte^ has figured structures somewhat similar to
these bodies, but I hardly understand either his figure or his
account sufficiently clearly to be able to pronounce upon the
identity of the two. In case they are identical, Gotte gives a
very different explanation of them from my own^

A second of my results, which points to a series of inter-
mediate steps between division and solution of the nucleus, is
the distribution in time of the peculiar cone-like bodies. These
are present in fair abundance at an early period of segmen-
tation, when there are but few nuclei either in the blastoderm
or the yolk. But at later periods, when there are both more
nuclei, especially in the yolk, and they are also increasing in
numbers more rapidly than before, no bodies of this kind are
to be seen. This fact becomes the more striking from the
lobate appearance of the later nuclei of the yolk, an appearance
which exactly suits the hypothesis of the rapid budding off of
fresh nuclei.

The observations of R. Hertwig' on the gemmation of Podo-
phrya Gemmipara, support my interpretation of the knobbed
condition of the nuclei. Hertwig finds (p. 47) that

The horse-shoe shaped nucleus grows out into numerous
anastomosing projections. Over the free ends of the projections little
knobs appear on the surface of the body, into which the lengthening
ends of the processes of the nucleus grow up. Here they bend
themselves into a horse-shoe form. The newly-formed nucleus then
separates from the original nucleus, and afterwards the bud contain-
ing it from the body.

From the peculiar arrangement of the net-work of lines of
the yolk around these knobbed nuclei, it is reasonable to con-
clude that interchange of material between the protoplasm of
the yolk and the nuclei is still taking place, even during the
later periods.

These facts about the distribution in time of the cone-like

1 Enticickhingsgeschichte der Llike, PI. i. fig, 18.

2 As I before mentioned, Strasburger (ZellUldang u. Z elWheilung) hag repre-
sented bodies precisely similar to those I have described, which api^ear during
the segmentation in the egg of Phallusia MammiUata as well as similar figures
observed by Butschli in eggs of Cucullanus elegans and Blatta Gcrmanica. The
figures in this monograph are the only ones I have seen, which are identical
with my own.

' Morphologisches Jahrbuch, Bd, i, pp. 46, 47.

24 SEGMENTATION.

bodies afford a strong presumptive evidence of a change in the
manner of nuclear increase.

The hist argument I propose urging on this head is derived
from the bodies (PI. ii. fig. 8 a, h, c) which I hav« described as
intermediate between the true cone-like bodies and typical
nuclei. They appear to afford evidence of less and less of the
matter of the nucleus streaming out into the yolk and of a
large proportion of it becoming divided.

The conclusion to be derived from all these facts is that
for Elasmobranchs in the earlier stages of segmentation, and
during the formation of fresh segments, a partial solution of
the old nucleus takes place, but all its constituents serve for
the reconstruction of the fresh nuclei.

In later periods of development a still smaller part of the
nucleus becomes dissolved, and the rest divides; but the two
fresh nuclei are still derived from the two sources. After the
close of segmentation the fresh nuclei are formed by a simple
division of the older ones.

The appearance of the cone-like bodies in the yolk outside
the germinal disc is a point of some interest. It demonstrates
in a conclusive manner that whatever influence (if any) the
nucleus may have in ordinary cases of cell division, yet it may
undergo changes of a precisely similar character to those which
it experiences during cell division, without exerting any influence
on the surrounding protoplasm \ If the lobate nuclei are also
nuclei undergoing division, we have in the Qgg of an Elasmo-
branch examples of all the known forms of nuclear increase
unaccompanied by cell division.

The next stage in the segmentation does not present so
many features of interest as the last one. The segments are
now so small, as to be barely visible from the surface with a
simple lens. A section of an embryo of this stage is repre-
sented in PI. II. fig. 8. The section, which is drawn on the

1 Strasbiirger's [loc. cit.) arguments about the influence of tbe nucleus in
cell tlivision are not to my mind conclusive; though not without importance.
It is diiUcult to reconcile his views with the facts of cell division observable
during the Elasmobranch segmentation; but even if their truth be admitted they
do not bring us much nearer to a satisfactory understanding of cell division, un-
lei^s accompinied (and at present they are not so) by a rational explanation of
the forces which produce the division of the nucleus.

DEVELOPMENT OF ELASMOBRANCH FISHES. 25

sarae scale as the section belonging to the last stage, serves
to shew the relative size of the segments in the two cases.

The epiblast is now more distinct than it was. The seg-
ments composing it are markedly smaller than the remainder
of the cells of the germinal disc, but possess nuclei of an abso-
lutely larger size than do the other cells. They are irregular
in shape, with a slight tendency to be columnar. An average
segment of this layer measures about y^^ inch.

The cells of the lower layer are more polygonal than
those of the epiblast, and are decidedly larger. An average
specimen of the larger cells of the lower layer measures about
■^ij^ in. in diameter, and is therefore considerably smaller than
one of the smallest cells of the last stage. The formation of
fresh segments from the yolk still continues with fair rapidity,
but nearly comes to an end shortly after this.

Of the nuclei of the lower layer cells, there is not much
to add to what has already been said. Not infrequently two
nuclei may be observed in a single cell.

The nuclei in the yolk which surrounds the germinal disc
are more numerous than in the earlier periods, and are now
to be met with in fair numbers in every section (Fig. 8 ?i').

These are the main features which characterise the present
stage, they are in all essential points similar to those of the
last stage, and the two germinal discs hardly differ except in
the size of the segments of which they are composed.

In the last stage which I consider as belonging to the
segmentation, the cells of the whole blastoderm have become
smaller (PL ir. %. 9).

The epiblast (ej)) now consists of a very marked layer of
columnar cells. It is, as far as I have been able to observe,
never more than one cell deep. The cells of the lower layer
are of an approximately uniform size, though a few of those at
the circumference of the blastoderm considerably exceed the
remainder in the bulk.

There are two fresh features of importance in germinal
discs of this age.

Instead of being but indistinctly separated from the sur-
rounding yolk, the blastoderm has now very clearly defined
limits.

26 SEGMENTATION.

This is an especially marked feature of preparations made
with osmic acid. In tliese there may frequently be seen a
deeply stained doubly contoured line, which forms the limit of
the yolk, where it surrounds the germinal disc. Lines of this
kind are often to be seen on the surface of the yolk, or even
of the blastoderm, but are probably to be regarded as products
of reagents, rather than as organised structures. The outline
of the germinal disc is well rounded, though it is occasionally
broken, from the presence of a larger cell in the act of being
formed from the yolk.

It is not probable that any great importance is to be at-
tached to the comparative distinctness of the outline of the
germinal disc at this stage, which is in a great measure due
to a cessation in the formation of fresh cells in the surround-
ing yolk, and in part to the small and comparatively uniform
size of the cells of the germinal disc.

The formation of fresh cells from the yolk nearly comes to
an end during this period, but it still continues on a small scale.
The number of the nuclei around the germinal disc has
increased.

Another feature of interest which first becomes apparent
during this stage is the asymmetry of the germinal disc. If a
section were made through the germinal disc, as it lay m situ in
the egg capsule, parallel or nearly so to the long axis of the
capsule, one end of the section would be found to be much
thicker than the other. There would in fact be a far larger
collection of cells at one extremity of the germinal disc than at
the other. The end at which this collection of cells is formed
points towards the end of the egg capsule opposite to that near
which the yolk is situated. This collection of cells is the first
trace of the embryo; and with its appearance the segmentation
may be supposed to terminate.

The section I have represented, though not quite parallel to
the long axis of the egg, is sufficiently nearly so to shew the
greater mass of cells at the embryonic end of the germinal disc.
This very early appearance of a distinction in the germinal
disc between the extremity at which the embryo appears and
the non-embryonic part of the disc, besides its inherent interest,
has a further importance from the fact that in Osseous Fishes

DEVELOPMENT OF ELASMOBRANCH FISHES. 27

a similar occurrence takes place. Oellacher* and Gotte^ both
agree as to the very early period at which a thickening of one
extremity of the blastoderm in Osseous Fishes is formed, which
serves to indicate the position at which the embryo will appear.
There are many details of development in which Osseous Fish
and Elasmobranchs agree, which, although if taken in"dividually
are without any great importance, yet serve to shew how long
even insignificant features in development may be retained.

The segmentation of the Elasmobranch egg presents in most
of its features great regularity, and exhibits in its mode of
occurrence the closest resemblance to that in other meroblastic
vertebrate ova.

There is, nevertheless, one point with reference to which a
slight irregularity may be observed. In almost all eggs seg-
mentation commences by, w^hat for convenience may be called,
a vertical furrow which is followed by a second vertical furrow
at right ano-les to the first. The third furrow however is a
horizontal one, and cuts the other two at right angles. This
method of segmentation must be looked on as the normal
one, in almost all the important groups of the animal king-
dom, both for the so-called holoblastic and meroblastic eggs,
and the gradations intermediate between the two. The Frog
amongst vertebrates exhibits a most typical instance of this
form of segmentation.

In Elasmobranchs the first two furrows are formed in a per-
fectly normal manner, but though I have not observed the
actual formation of the next furrow, yet from the later stages,
w^hich I have obseiwed, I conclude that it is parallel to one of
the first formed furrows ; and it is fairly certain that, not till a
considerably later period, is a furrow homologous with the hori-
zontal furrow of the Batrachian egg formed. This furrow
appears to be represented in the Elasmobranch segmentation
by the irregular circumscription of a body of central smaller
spheres from a ring of peripheral larger ones (vide PL I.
figs. 3, 4 and 5).

In the Bird the representative of the horizontal furrow

1 Zeitschrift fiir Wiss. Zoolog'ie, Bel. xxiii. 1873.
a Archiv fiir Micr. Anat. Bd.^x. 1873.

28 SEGMENTATION.

appears relatively much earlier. It is formed when there are
eight segments marked out on the surface of tlie germinal
disc\ From Oellacher's^ account of the segmentation in the
fowP it seems certain, as might be anticipated, that this furrow
is nearly parallel to the surface of the disc, so' that it cuts the
earlier formed vertical furrows and causes the seo-ments of the
germinal disc to be completely circumscribed below as well as
at the surface. In the Elasmobranch egg this is not the case;
so that, even after the smaller central segments have become
separated from the outer ring of larger ones, none of the seg-
ments of the disc are completely circumscribed, and only appear
to be so in surface views (vide PL i. fig. 6). Segmentation in
the Elasmobranch egg differs in the following particulars from
that in the Bird's egg :

(1) The equivalent of the horizontal furrow of the Batra-
chian egg appears much later than in the Bird.

(2) When it has appeared it travels inwards much more
slowly.

As a result of these differences, the segments of the
germinal disc of the Birds' eggs are much earlier circumscribed
on all sides than those of the Elasmobranch egg.

As might be expected, the segmentation of the Elasmobranch
egg resembles in many points that of Osseous Fishes (vide
Oellacher* and Klein^). It may be noticed, that with Osseous
as with Elasmobranch Fishes, the furrow corresponding with the
horizontal furrow of the Amphibian's egg does not appear at
as early a period as is normal. The third furrow of an Osseous
Fish egg is parallel to one of the first formed pair.

In Oellacher's^ figures, PL xxiii. fig. 19 — 21, peculiar
headings of the sides of the earlier formed farrows are dis-
tinctly shewn. No mention of these is made in the text, but
they are unquestionably similar to those I have described in
the Elasmobranch furrows. In the case of Elasmobranchs I

^ Vide Elements of Embryology, p, 23.

2 Strickefs Studien, 1869, Pt. i, PI. ir. fig. 4.

3 Unfortunately Professor Oellacher gives no account of the surface appear-
ances of the germinal discs of which he describes the sections. It is therefore
uncertain to what period his sections belong.

4 Zeitschriftflir Wiss. Zool. Bd. xxii. 1872.

s Monthly Microscopical Journal, March, 1872.
° Loc. cit.

DEVELOPMENT OF ELASMOBRANCH FISHES. 29

pointed out that not only were the sides of the furrow beaded,
but that there appeared in the protoplasm, close to the furrows,
peculiar vacuole-like cavities, precisely similar to the cavities
which were the cause of the headings of the furrows.

The presence of these seems to shew that the molecular
cohesion of the protoplasm becomes, as compared with other
parts, much diminished in the region where a furrow is about
to appear, so that before the protoplasm finally gives way along
a particular hne to form a furrow, its cohesion is broken at
numerous points in this region, and thus a series of vacuole-
like spaces is formed.

If this is the true explanation of the formation of these
spaces, their presence gives considerable support to the views
of Dr Kleinenberg upon the causes of segmentation, so clearly
and precisely stated in his monograph upon Hydra; and is
opposed to any view which regards the forces which come into
play during segmentation as resident in the nucleus.

I have not observed the peculiar threads of protoplasm
which Oellacher^ describes as crossing the commencing seg-
mentation furrows. I have also failed to discover any signs of
a concentration of the yolk-spherules, round one or two cen-
tres, in the segmentation spheres, similar to that observed by
Oellacher in the segmenting eggs of Osseous Fish. The ap-
pearances observed by him are probably connected with the
behaviour of the nucleus during segmentation, and are related
to the curious bodies I have already described.

With reference to the nuclei which Oellacher^ has described
as occurring in the eggs of Osseous Fish during segmentation,
there can, I think, be little doubt that they are identical with
the peculiar nuclei in the Elasmobranch eggs.

He^ says :

In an unsegmented germ there occurred at a certain point

in the section a small aggregation of round bodies. I do not

feel satisfied whether these aggregations represent one or more
nuclei.

Fig. 29 shews such aggregation; by focusing at its optical sec-
tion eleven unequally large rounded bodies measuring from 0.004

1 Loc. cit,
^ Loc. cit.
' Loc. cit. p. 410, 411, &c.

80 SEGMENTATION.

— 0.009 Mm. may be distinguislied. They lay as if in a mnlti-locular
gaj> in the germ mass, which however they did not quite fill. In
each of these bodies there appeared another but far smaller body.
These aggregations were distinguished from the germ by an especially
beautiful intense violet gold chloride colouration of their elements.
The smaller elements contained in the larger were still more in-
tensely coloured than the larger.

He further states that these aggregations equal the
segments in number, and that the small bodies within the
elements are not always to be seen with the same distinct-
ness.

Oellacher s description as well as his figures of these bodies
leaves no doubt in my mind that they are exactly similar bodies
to those which I have already spoken of as nuclei, and the
characteristic features of w^hich I have shortly mentioned, and
shall describe more fully at a later stage. A moderately full
description of them is to be found in my preliminary paper \

Their division into a series of separate areas each w4th a
deeply-stained body, as well as the staining of the whole of them,
exactly corresponds to what I have found. That each is a single
nucleus is quite certain, though their knobbed form might
occasionally lead to the view of their being divided. This
knobbed condition, observed by Oellacher as well as myself,
certainly supports the view, that they are in the act of budding
off fresh nuclei. Oellacher conceives, that the areas into which
these nuclei are divided represent a series of separate bodies —
this according to my observations is not the case. Nuclei of the
same form have already been described in Nephelis, and are
probably not very rare. They pass by insensible gradations into
ordinary nuclei with numerous granules.

One marked feature of the segmentation of the Elasmobranch
egg is the continuous advance of the process of segmentation
into the yolk and the assimilation of this into the germ by
the direct formation of fresh segments out of it. Into the
significance of this feature I intend to enter fully hereafter; but
it is interesting to notice that Oellacher's descriptions point to
a similar feature in the segmentation of Osseous Fish. This
however consists chiefly in the formation of fresh segments

1 Loc. cit. p. 415.

DEVELOPMENT OF ELASMOBRANCH FISHES. 31

from the lower parts of the gerrainal disc which in Osseous Fish
is more distinctly marked off from the food-yolk than in
Elasmobranchs.

I conclude my description of the segmentation by a short
account of what other investigators have written about its
features in these fishes. One of the earliest descriptions of
this process was given by Leydig\ To his description of the
germinal disc, I have already done full justice.

In the first stage of segmentation which he observed 20 — 30
segments were already visible on the surface. In each of these
he recognised a nucleus but no nucleolus.

He rightly states that the segments have no membrane, and
describes the yolk-spherules which fill them.

The next investigator is Gerbel I have unfortunately been
unable to refer to this elaborate paper, but I gather from an
abstract that M. Gerbe has given a careful description of the
external features of segmentation.

Schenk^ has also made important investigations on the sub-
ject. He considers that the ovum is invested with a very
delicate membrane. This membrane I have failed to find a
trace of, and agree with Leydig* in denying its existence.
Schenk further found that after impregnation, but before seg-
mentation, the germinal disc divided itself into two layers,
an upper and a lower. Between the two a cavity made its
appearance which Schenk looks upon as the segmentation
cavity. Segmentation commences in the upper of the two
layers, but Schenk does not give a precise account of the
fate of the lower. I have had no opportunity of investigating
the impregnated ovum before the commencement of segmen-
tation, but my observations upon the early stages of this
process render it clear that no division of the germinal disc
exists subsequently to the commencement of segmentation, and
that the cavity discovered by Schenk can have no connection

1 Rochen u. Haie. It is here mentioned that Coste observed the segmenta-
tion in these fishes.

2 Eecherches sur la segmentation des products adventifs de Vceuf des Plagxos-
tomes et particuUerement des Bates. Eohin, Journal de VAnatomie et de la
Physiologie, p. 609, ]872.

'^ Die Eier von Raja quadrimaculata innerhalb der Eileiter. Sitz. der k.
ATiad. Wien. Vol. lxxiii. 1873,

^ Loc. cit. My denial of the existence of this membrane naturally applies
only to the egg after impregnation, and to the genera Scylhum and Pristinrus.

S2 SEGMENTATION.

whatever with the segmentation cavity. I am indeed inclined
to look upon this cavity as an artificial product. I have myself
met with somewhat similar appearances, after the completion
of segmentation, which were caused by the non-penetration of
my hardening reagent beyond a certain point.

Without attempting absolutely to explain the appearances
described by Professor Schenk, I think that his observations
ought to be repeated, either by himself or some other compe-
tent observer.

Several further facts are recorded by Professor Schenk in
his interesting paper. He states that immediately after im-
pregnation, the germinal disc presents towards the yolk a
strongly convex surface, and that at a later period, but still be-
fore the commencement of segmentation, this becomes flattened
out. He has further detected amoeboid movements in the disc
at the same period. As to the changes of the germinal disc during
segmentation, his paper contains no facts of importance.

Next in point of time to the paper of Schenk, is my own
preliminary account of the development of the Elasmobranch
Fishes \ In this a large number of the facts here described in
full are briefly alluded to.

The last author who has investigated the segmentation in
Elasmobranchs, is Dr Alexander Schultz^. He merely states
that he has observed the segmentation, and confirms Professor
Schenk's statements about the amoeboid movements of the
germinal disc.

1 Loc. cit.

^ Die Embryonal Aniage der Selachier. Vorlanjlge Mittheilung, Centralhlattf.
Med. Wiss. No. 33, 1875.

CHAPTER III.

FOKMATIOX OF THE LaYEP.S.

In the last chapter the blastoderm was left as a solid lens-
shaped mass of cells, thicker at one end than at the other,
its uppermost row of cells forming a distinct layer. There
very soon appears in it a cavity, the well known segmenta-
tion cavity, or cavity of von Baer, which arises as a small space
in the midst of the blastoderm, near its non-embryonic end (PL
III. fig. 1).

This condition of the segmentation cavity, though already^
described, has nevertheless been met with in one case only.
The circumstance of my having so rarely met with this con-
dition is the more striking because I have cut sections of a
considerable number of blastoderms in the hope of encounterino-
specimens similar to the one figured, and it can only be explained
on one of the two following hypotheses. Either the stage is
very transitory, and has therefore escaped my notice except
in the one instance ; or else the cavity present in this instance
is not the true segmentation cavity, but merely some abnormal
structure. That this latter explanation is a possible one, appears
from the fact that such cavities do at times occur in other
parts of the bJastoderm. Dr Schultz" does not mention having
found any stage of this kind.

The position of the cavity in question, and its general ap-
pearance, incline me to the view that it is the segmentation
cavity^. If this is the true view of its nature the fact should be
noted that at first its floor is formed by the lower layer cells
and not by the yolk, and that its roof is constituted by both the

^ Qy. Journal of Microsc. Science, Oct. 1874.

2 Centr. f. Med. Wiss. No. 38, 1875.

3 Professor Bambeke {Poissons Osseux, Mem. Acad. BeJgique 1875) describes
a cavity in the blastoderm of Leuciscus mtilus, which he regards as the true
segroentation cavity, but not as identical with the segmentation cavity of
Osseous Fishes, usually so called. Its relations are the same as those of my
segmentation cavity at this stage. This paper came into my hands at too late
a period for me to be able to do more than refer to it in this place.

B. 3

34 FORMATION OF THE LAYERS.

lower layer cells and the epiblast cells. The relations of the
floor underofo considerable modifications in the course of de-
velopment.

The other features of the blastoderm at this stage are very
much those of the previous stage.

The embryonic swelling is very conspicuous. The cells of
the blastoderm are still disposed in two layers : an upper one
of slightly columnar cells one deep, which constitutes the epi-
blast, and a lower one consisting of the remaining cells of the
blastoderm.

An average cell of the lower layer has a diameter of about
■gi^ inch, but the cells at the periphery of the layer are in some
cases considerably larger than the more central ones. All the
cells of the blastoderm are still completely filled with yolk
spherules. In the yolk outside the peculiar nuclei, before
spoken of, are present in considerable numbers. The}^ seem to
have been mistaken by Dr Schultz^ for cells: there can
however be no question that they are true nuclei.

In the next stage the relations of "the segmentation cavity
undergo important modifications.

The cells which form its floor disappear almost com-
pletely from that position, and the floor becomes formed by
the yolk.

The stage, during which the yolk serves as the floor of the
segmentation cavity, extends over a considerable period of time,
but during it I have been unable to detect any important
change in the constitution of the blastoderm. It no doubt
gradually extends over the yolk, but even this growth is not
nearly so rapid as in the succeeding stage. Although therefore
the stage I proceed to describe is of long continuance, a blasto-
derm at the beginning of it exhibits, both in its external and
in its internal features, no important deviations from one at
the end of it.

Viewed from the surface (PI. VI. fig. A) the blastoderm
at this stage appears slightly oval, but the departure from
the circular form is not very considerable. The long axis of
the oval corresponds with what eventually becomes the
long axis of the embryo. From the yolk the blastoderm is

1 Luc. cit.

DEVELOPMENT OF ELASMOBRAXCH FISHES. 35

Still well distinguished by its darker colour; and it is sur-
rounded by a concentric ring of light-coloured yolk, the outer
border of which shades insensibly into the normal yolk.

At the embryonic portion of the blastoderm is a slight
swelling, clearly shown in Plate VI. fig. A, which can easily
be detected in fresh and in hardened embryos. This swelling is
to be looked upon as a local exaggeration of a slightly raised
rim present around the whole circumference of the blastoderm.
The roof of the segmentation cavity (fig. A, s. c.) forms a second
swelling; and in the fresh embryo this region appears of a
darker colour than other parts of the blastoderm.

It is difficult to determine the exact shape of the blasto-
derm, on account of the traction exercised upon it in opening
the egg ; and no reliance can be placed on the forms assumed
by hardened blastoderms. This remark also applies to the
sections of blastoderms of this stage. There can be no doubt
that the minor individual variations exhibited by almost
every specimen are produced in the course of manipulations
while the objects are fresh. These variations may affect even
the relative length of a particular region and certainly the
curvature of it. The roof of the segmentation cavity is es-
pecially apt to be raised into a dome-like form.

The main internal feature of this stage is the disappearance
of the layer of cells which, during the first stage, formed the
floor of the segmentation cavity. This disappearance is never-
theless not absolute, and it is doubtful whether there is any
period in which the floor of the cavity, is quite without cells.

Dr Schultz supposes' that the entire segmentation cavity
is, in the living animal, filled with a number of loose cells.
Though it is not in my power absolutely to deny this, the
point being one which cannot be satisfactorily investigated in
sections, yet no evidence has come under my notice which
would lead to the conclusion that more cells are present in the
segmentation cavity than are represented on PL xiii. fig. 1, of
my preliminary paper', an illustration which is repeated on PI.
III. fig. 2.

The number of cells on . the floor of the cavity differs
considerably in different cases, but these cases come under the

1 Loc. cit. 2 j^oc. cit.

3-2

36 FORMATION OF THE LAYERS.

category of individual variations, and are not to be looked upon
as indications of different states of development.

In many cases especially large cells are to be seen on the
floor of the cavity (PI. ill. fig. 2, h d). In my preliminary
paper ^ the view was expressed that these are probably
cells formed around the nuclei of the yolk. This view I am
inclined to abandon, and to substitute for it the suggestion
made by Dr Schultz, that they are remnants of the larger
segmentation cells which were to be seen in the previous stages.

Plate III. figs. 2, 3, 4 (all sections of this stage) show the
different appearances presented by the floor of the segmentation
cavity. In only one of these sections are there any large number
of cells upon the floor; and in no case have cells been observed
imbedded in the yolk forming this floor, as described by Dr
Schultz^, but in all cases the cells simply rested upon it.

Passing from the segmentation cavity to the blastoderm
itself, the first feature to be. noticed is the more decided differ-
entiation of the epiblast. This now forms a distinct layer
composed of a single row of columnar cells. These are slightly
more columuar in the region of the embryonic swelling than
elsewhere, and become less elongated at the edge of the blasto-
derm. In my specimens this layer was never more than one
cell deep, but Dr Schultz^ states that, in the Elasmobranch
embryos investigated by him, the epiblast was composed of
more than a single row of cells.

Each epiblast cell is filled with yolk spherules and contains
a nucleus. Yery frequently the nuclei in the layer are ar-
ranged in a regular row (vide PI. III. fig. 3). In the later
blastoderms of this stage there is a tendency in the cells to
assume a wedge-like form with their thin ends pointing alter-
nately in opposite directions. This arrangement is, however,
by no means strictly adhered to, and the regularity of it is
exas^orerated in Plate iii. fio^. 4.

The nuclei of the epiblast cells have the same characters as
those of the lower layer cells to be presently described, but
their intimate structure can only be successfully studied in

^ Qy. Journal of Micros. Science, Oct. 1874.

2 Loc. cit. Probably Dr Scbiiltz, here as in other cases, has mistaken nuclei
for cells.
^ Loc. cit.

DEVELOPMENT OF ELASMOBRANX'H FISHES. 87

certain exceptionally favourable sections. In most cases the
yolk spherules around them render the finer details invisible.

There is at this stage no such obvious continuity as in the
succeeding stage between the epiblast and the lower layer cells;
and this statement holds good more especially with the best
conserved specimens which have been hardened in osmic acid
(Plate III. fig. 4). In these it is very easy to see that the
epiblast simply thins out at the edge of the blastoderm without
exhibiting the slightest tendency to become continuous with the
lower layer cells \

The lower layer cells form a mass rather than a layer,
and constitute the whole of the blastoderm not included in the
epiblast. The shape of this mass in a longitudinal section may
be gathered from an examination of Plate ill. figs. 3 and 4.

It presents an especially thick portion forming the bulk of
the embryonic swelling, and frequently contains one or two
cavities, w^hich from their constancy I regard as normal and not
as artificial products.

In addition to the mass forming the embryonic swelling
there is seen in sections another mass of lower layer cells at
the opposite extremity of the blastoderm, connected with the
former by a bridge of cells, which constitutes the roof of the
segmentation cavity. The lower layer cells may thus be divided
into three distinct parts :

(1) The embryo swelling.

(2) The thick rim of cells round the edge of the remainder
of the blastoderm.

(3) The cells which form the roof of the segmentation
cavity.

1 Prof. Haeckel (Die Gastrula u. die Eifurchimg d. Thiere, Jenaische Zeit-
schrift, Yol. ix.) has unfortunately copied a figure from my preliminary paper
(loc. cit.) (repeated now), which I had carefully avoided using for the purpose of
describing the formation of the layers on account of the epiblast cells in the
original having been much altered by the chromic acid, as a result of which the
whole section gives a somewhat erroneous impression of the condition of the
blastoderm at this stage. I take this opportunity of pointing out that the
colouration employed by Professor Haeckel to distinguish the layers in this
section is not founded on my statements, but is, on the contrary, in entire
opposition to them. From the section as represented by Professor Haeckel
it might be gathered that I considered the lower layer cells to be divided into
two parts, one derived from the epiblast, while the other constituted the hypo-
blast. Not only is no such division present at this period, but no part of the
lower layer cells, or the mesoblast cells into which they become converted, can
in any sense whatever be said to be derived from the epiblast.

38 FORMATION OF THE LAYERS.

These three parts form a continuous whole, but in addi-
tion to these there exist the previously mentioned cells, which
rest on the floor of the segmentation cavity.

With the exception of these latter, the lower layer is com-
posed of cells having a fairly uniform size, and exhibits no trace
of a division into two layers.

The cells are for the most part irregularly polygonal from
mutual pressure; and in their shape and arrangement, exhibit a
marked contrast to the epiblast cells. A few of the lower
layer cells, highly magnified, are represented in PI. III. fig.
2 a. An average cell measures about gi^^ to ^i^^ of an inch,
but some of the larger ones on the floor attain to the ^j^ of an
inch.

Owing to my having had the good fortune to prepare some
especially favourable specimens of this stage, it has been possible
for me to make accurate observations both upon the nuclei
of the cells of the blastoderm, and upon the nuclei of the
yolk.

The nuclei of the blastoderm cells, both of the epiblast and
lower layer, liave a uniform structure. Those of the lower
layer cells are about j^L_ of an inch in diameter. Roughly
speaking each con.sists of a spherical mass of clear protoplasm
refracting more highly than the protoplasm of its cell. The
nucleus appears in sections to be divided by deeply stained
lines into a number of separate areas, and in each of these a
deeply stained granule is placed. In some cases two or more
of such granules may be seen in a single area. The whole of
the nucleus stains with the colouring reagents more deeply
than the protoplasm of the cells; but this is especially the case
with the granules and lines.

Though usually spherical the nuclei not infrequently have
a somewhat lobate form.

Very similar to these nuclei are the nuclei of the yolk.
One of the most important differences between the two is
that of size. The majority of the nuclei present in the yolk are
as large or larger than an ordinary blastoderm cell; while many
of them reach a size very much greater than this. The ex-
amples I have measured varied from ^J^ to 230 ^^ ^"^ i^<^^^ i^^
diameter.

DEVELOPMENT OF ELASMOBRANCH FISHES. 39

Though they are divided, like the nuclei of the blastoderm,
with more or less distinctness into separate areas by a network
of lines, their greater size frequently causes them to present an
aspect somewhat different from the nuclei of the blastoderm.
They are moreover much less regular in outline than these, and
very many of them have lobate projections (PL iii. figs. 2a and
2c and 3), which vary from simple knobs to projections of such a
size as to cause the nucleus to present an appearance of com-
mencing constriction into halves. When there are several
such projections the nucleus acquires a peculiar knobbed figure.
With bodies of this form it becomes in many cases a matter
of great difficulty to decide whether or no a particular series of
knobs, which appear separate in one plane, are united in a lower
plane, whether, in fact, there is present a single knobbed nucleus
or a number of nuclei in close apposition. A nucleus in this
condition is represented in PI. III. fig. 2 b.

The existence of a protoplasmic network in the yolk has
already been mentioned. This in favourable cases may be
observed to be in special connection wdth the nuclei just de-
scribed. Its meshes are finer in the vicinity of the nuclei, and
its fibres in some cases almost appear to start from them
(PI. V. fig. 12). For reasons which I am unable to explain
the nuclei of the yolk and the surrounding meshwork present
appearances which differ greatly according to the reagent
employed. In most specimens hardened in osmic acid the
protoplasm of the nuclei is apparently prolonged in the sur-
rounding meshwork (PL V. fig. 12). In other specimens har-
dened in osmic acid (PI. V. fig. 11), and in all hardened in
chromic acid (PL III. fig. 2 a and 2 c), the appearances are far
clearer than in the previous case, and the protoplasmic mesh-
work merely surrounds the nuclei, without showing any signs of
becoming continuous with them.

There is also around each nucleus a narrow space in which
the spherules of the yolk are either much smaller than else-
where or completely absent, vide PI. ill. fig. 2 b.

It has not been possible for me to satisfy myself as to the
exact meaning of the lines dividing these nuclei into a number
of distinct areas. My observations leave the question open as to
wdiether they are to be looked upon as lines of division, or as

40 FORMATION OF THE LAYERS.

protoplasmic lines such as have been described in nuclei by
Flemming^, Hertwig^ and Van Benedenl The latter view ap-
pears to me to be the more probable one.

Such are the chief structural features presented by these
nuclei, which are present during the whole of the earlier periods
of development and retain throughout the same appearance.
There can be little doubt that their knobbed condition implies
that they are undergoing a rapid division. The arguments for
this view I have already insisted on, and, in spite of the obser-
vations of Dr Kleinenberg showing that similar nuclei of
Nephelis do not undergo division, the case for their doing so in
the Elasmobranch eggs is to my mind a very strong one.

During^ this stao^e the distribution of these nuclei in the
yolk becomes somewhat altered from that in the earlier stages.
Although the nuclei are still scattered generally throughout
the finer yolk-matter around the blastoderm, yet they are
especially aggregated at one or two points. In the first place
a special collection of them may be noticed immediately below
the floor of the segmentation cavity. They here form a dis-
tinct row or even layer. If the presence of this layer is cou-
pled with the fact that at this period cells are beginning to
appear on the floor of the segmentation cavity, a strong argu-
ment is obtained for the supposition that around these nuclei
cells are being produced, which pass into the blastoderm to
form the floor. Of the actual formation of cells at this period
I have not been able to obtain any satisfactory example, so
that it remains a matter of deduction rather than of direct
observation.

Another special aggregation of nuclei is generally present
at the periphery of the blastoderm, and the same amount of
doubt hangs over the fate of these as over that of the previously
mentioned nuclei.

The next stage is the most important in the whole history
of the formation of the layers. Not only does it serve to show,
that the process by which the layers are formed in Elasmo-

^ EntwickluiigsgescLiclite der Najaden, Sitz. d. k. Akad. Wien, 1875.
2 Morphologlsche Jahrhuch, Vol. i. Heft 3.

^ Developpement des Mammiferes, Bid. de VAcad. de BeJgique, xl. No. 12»-
1875.

DEVELOPMENT OF ELASMOBRANCH FISHES. 41

branchs can easily be derived from a simple gastrula type like
that of Ampbioxus, but it also serves as the key by which
other meroblastic types of development may be explained. At
the very commencement of this stage the embryonic swelling
becomes more conspicuously visible than it was. It now pro-
jects above the level of the yolk in the form of a rim. At one
point, which eventually forms the termination of the axis of the
embryo, this projection is at its greatest ; w^hile on either side
of this it gradually diminishes and finally vanishes. This pro-
jection I propose calling, as in my preliminary paper^ the em-
bryonic rim.

The segmentation cavity can still be seen from the surface,
and a marked increase in the size of the blastoderm may be
noticed. During the stage last described, the growth was but
very slight ; hence the rather sudden and rapid growth which
now takes place becomes striking.

Longitudinal sections at this stage, as at the earlier stages,
are the most instructive. Such a section on the same scale as
PL III. fig. 4, is represented in PL iii. fig. 5. It passes parallel
to the long axis of the embryo, through the point of greatest
development of the embryonic rim.

The three fresh features of the most striking kind are (1)
the complete envelopment of the segmentation cavity within
the lower layer cells, (2) the formation of the embryonic rim,
(3) the increase in distance between the posterior end of the
blastoderm and the segmentation cavity. The segmentation
cavity has by no means relatively increased in size. The roof
has precisely its earlier constitution, being composed of an
internal lining of lower layer cells and an external one of
epiblast. The thin lining of lower layer cells is, in the course
of mounting the sections, very apt to fall off; but I am abso-
lutely satisfied that it is never absent.

The floor of the cavity has undergone an important change,
being now formed by a layer of cells instead of by the yolk. A
precisely similar but more partial change in the constitution
of the floor takes place in Osseous Fishes ^

1 Qy. Journal Microsc. Science, Oct. 1874.

- Gotte, Der Keim cl. Forelleneies, Arch. f. Mikr. Anat. Vol. is.; Haeckel,
Die Gastrula u. die Eifurcliung d. Thiere, Jenaische Zeitschrift, Bd.ix.

42 FORMATION OF THE LAYERS.

The mode in which the floor is formed is a question of
some importance. The nuclei, which during the last stage
formed a row beneath it, probably, as previously pointed
out, take some share in its formation. An additional argument
to those already brought forward in favour of this view may
be derived from the fact that during this stage such a row of
nuclei is no longer present.

This argument may be stated as follows :

Before the floor of cells for the segmentation cavity is formed
a number of nuclei are present in a suitable situation to supply
the cells for the floor ; as soon as the floor of cells makes its
appearance these nuclei are no longer to be seen. From this
it may be concluded that their disappearance arises from their
having become the nuclei of the cells which form the floor.

It appears to me most probable that there is a growth in-
wards from the whole peripheral wall of the cavity, and that this
ingrowth, as well as the cells derived from the yolk, assist in
forming the floor of the cavity. In Osseous Fish there appears
to be no doubt that the floor is largely formed by an ingrowth
of this kind.

A great increase is observable in the distance between the
posterior end of the segmentation cavity and the edge of the
blastoderm. This is due to the rapid growth of the latter com-
bined with the stationary condition of the former. The growth
of the blastoderm at this period is not uniform, but is more
rapid in the non-embryonic than in the embryonic parts.

The main features of the epiblast remain the same as during
the last stages. It is still composed of a very distinct layer
one cell deep. Over the segmentation cavity, and over the
whole embryonic end of the blastoderm, the cells are very thin,
columnar, and, roughly speaking, wedge-shaped with the thin
ends pointing alternately in different directions. For this
reason, the nuclei form two rows ; but both the rows are
situated near the upper surface of the layer (vide PL III.
fig. 5). Towards the posterior end of the blastoderm the cells
are flatter and broader ; and the layer terminates at the non-
embryonic end of the blastoderm without exhibiting the slight-
est tendency to become continuous with the lower layer cells.
At the embryonic end of the blastoderm the relations of the

DEVELOPMENT OF ELxVSMOERANX'H FISHES. 43

epiblast and lower layer cells are very different. At tins part,
throughout the whole extent of the embryonic rim, the epiblast
is reflected and becomes continuous with the lower layer cells.

The lower layer cells form, for the most part, a uniform
stratum in which no distinction into mesoblast and hypoblast
is to be seen.

Both the lower layer cells and the epiblast cells are still
filled with yolk spherules.

The structures at the embryonic rim, and the changes
which are there taking place, unquestionably form the chief
features of interest at this stage.

The general relations of these parts are very fairly shown
in PL III. fig. 5, which represents a section passing through the
median line of the embryonic region. They are however more
accurately represented in PL iv. fig. 5 a, taken from the same
embryo, but in a lateral part of the embryonic rim ; or in
PL IV. fig. 6, from a slightly older embryo. In all of these
figures the epiblast cells are reflected at the edge of the
embryonic rim, and become perfectly continuous with the hypo-
blast cells. A few of the cells, immediately beyond the line
of this reflection, precisely resemble in character the typical
epiblast cells; but the remainder exhibit a gradual transition
into typical lower layer cells. Adjoining these transitional cells,
or partly enclosed in the corner formed between them and the
epiblast, are a few unaltered low^er layer cells {in), which at this
stage are not distinctly separated from the transitional cells.
The transitional cells form the commencement of the hypoblast
(hy); and the cells (m) between them and the epiblast form
the commencement of the mesoblast. The gradual conversion
of lower layer cells into columnar hypoblast cells, is a very clear
and observable phenomenon in the best specimens. Where the
embryonic rim projects most, a larger number of cells have
assumed a columnar form. Where it projects less clearly, a
smaller number have done so. But in all cases there may be
observed a series of gradations betw^een the columnar cells and
the typical rounded lower layer cells \

1 When writing my earlier paper I did not feel so confident about the mode
of formation of the hypoblast as I now do, and even doubted the possibihty of
determining it from sections. The facts now brought forward are I hope suffi-
cient to remove all scepticism on this point.

4i FORMATION OF THE LAYERS.

In tlie last described embryo, although the embryonic rim
had attained to a considerable development, no trace of the
medullary groove had made its appearance. In an embryo in
the next stage of which I propose describing sections, this struc-
ture has become visible.

A surface view of a blastoderm of this age, with the embryo,
is represented on PI. VI. fig. B; and I shall, for the sake of
convenience, in future speak of embryos of this age as belong-
ing to period B.

The blastoderm is nearly circular. The embryonic rim is
represented by a darker shading at the edge. At one point
in this rim may be seen the embryo, consisting of a somewhat
raised area with an axial groove [mg). The head end of the
embryo is that which points towards the centre of the blasto-
derm, and its free peripheral extremity is at the edge of the
blastoderm.

A longitudinal section of an embryo of the same age as
the one figured^ is represented on PL IV. fig. 7. The general
growth has been very considerable, though as before explained,
it is mainly confined to that part of the blastoderm where the
embryonic rim is absent.

A fresh feature of great importance is the complete dis-
appearance of the segmentation cavity, the place which was
previously occupied by it being now filled up by an irregular
network of cells. There can be little question that the oblite-
ration of the segmentation cavity is in part due to the entrance
into the blastoderm of fresh cells formed around the nuclei of
the yolk. The formation of these is now taking place with
great rapidity and can be very easily followed.

Since the segmentation cavity ceases to play any further
part in the history of the blastoderm, it will be w^ell shortly to
review the main points in its history.

Its earliest appearance is involved in some obscurity, though
it probably arises as a simple cavity in the midst of the lower
layer cells (PL ill. fig. 1). In its second phase the floor ceases
to be formed of lower layer cells, and the place of these is
taken by the yolk, on which however a few scattered cells

1 Owing to the small size of the plates this section has been drawn on a
considerably smaller scale than that represented in fig. 5.

DEVELOPMENT OF ELASMOBRANCH FISHES. 45

still remain (PI. in. figs. 2, 3, 4). During the third period of
its history, a distinct cellular floor is again formed for it, so
that it comes a second time into the same relations with the
blastoderm as at its earliest appearance. The floor^ of cells
which it receives is in part due to a growth inwards from the
periphery of the blastoderm, and in part to the formation of
fresh cells from the yolk. Coincidently with the commencing
differentiation of hypoblast and mesoblast the segmentation
cavity grows smaller and vanishes.

One of the most important features of the segmentation
cavity in the Elasmobranchs which I have studied, is the fact
that throughout its whole existence its roof is formed of loiver
layer cells. There is not the smallest question that the seg-
mentation cavity of these fishes is the homologue of that of
Amphioxus, Batrachians, etc., yet in the case of all of these
animals, the roof of the segmentation cavity is formed of
epiblast only. How comes it then to be formed of lower layer
cells in Elasmobranchii ?

To this question an answer w^as attempted in my paper,
"Upon the Early Stages of the Development of Vertebrates^"
It was there pointed out, that as the food material in the ovum
increases, the bulk of the lower layer cells necessarily also in-
creases ; since these, as far as the blastoderm is concerned, are
the chief recipients of food material. This causes the lower layer
cells to encroach upon the segmentation cavity, and to close
it in not only on the sides, but also above; from the same cause
it results that the lower layer cells assume, from the first, a
position around the spot wdiere the future alimentary cavity
will be formed, and that this cavity becomes formed by a
simple split in the midst of the lower layer cells, and not by
an involution.

All the most recent observations" on Osseous Fishes tend
to show that in them, the roof of the segmentation cavity is
formed alone of epiblast ; but on account of the great difficulty
w^hich is experienced in distinguishing the layers in the blasto-
derms of these animals, I still hesitate to accept as conclusive
the testimony on this point.

1 Quart. Journ. of Microscop. Science, July, 1875.

2 OeUacher, Zeit. f. Wiss. Zoologie, Bd. xxiii. Gotte, Archiv f. Milr.
Anat. Vol. ix. Haeckel, loc. cit.

4G FOUMxVTlON OF THE LAYERS.

In the formation a second time of a cellular floor for the
segmentation cavity in the third stage, the Elasmobranch embryo
seems to resemble that of the Osseous Fish'. Upon this feature
great stress is laid both by Dr Gotte^ and Prof. IlaeckeP: but I
am unable to agree with the interpretation of it offered by them.
Both Dr Gotte and Prof. Haeckel regard the formation of this
floor as part of an involution to which the lower layer cells owe
their origin, and consider the involution an equivalent to the
alimentary involution of Batrachians, Amphioxus, &c. To this
question T hope to return, but it may be pointed out that my
observations prove that this view can only be true in a very
modified sense; since the invagination by which hypoblast and
alimentary canal are formed in Amphioxus is represented in
Elasmobranchs by a structure quite separate from the ingrowth
of cells to form the floor of the segmentation cavity.

The eventual obliteration of the segmentation cavity by
cells derived from the yolk is to be regarded as an inherited rem-
nant of the involution by which this obliteration was primitively
effected. The passage upwards of cells from the yolk, may
possibly be a real survival of the tendency of the hypoblast cells
to grow inwards during the process of involution.

The last feature of the segmentation cavity which deserves
notice is its excentric position. It is from the first situated in
much closer proximity to the non- embryonic than to the embry-
onic end of the blastoderm. This peculiarity in position is also
characteristic of the segmentation cavity of Osseous Fishes, as is
shown by the concordant observations of Oellacher^ and Gotte^
Its meaning becomes at once intelligible by referring to the
diagrams in my paper® on the Early Stages in the Development
of Vertebrates. It in fact arises from the asymmetrical charac-
ter of the primitive alimentary involution in all anamniotic
vertebrates with the exception of Amphioxus.

Leaving the segmentation cavity I pass on to the other
features of my sections.

There is still to be seen a considerable aggregation of cells
at the non-embryonic end of the blastoderm. The position of
this, and its relations with the portion of the blastoderm which

^ This floor appears in most Osseous Fish to he only partially formed. Vide
Gotte, Joe. cit.

' L(h:. fit. 3 I.oc. rit. * T.oc. cit. '' Lor. cit. '' Tj>c. cit.

DEVELOPMENT OF ELASMOEllA^'CH FISHES. 47

at an earlier period contained the segmentation cavity, indicate
that the growth of the blastoderm is not confined to its edge,
but that it proceeds at all points causing the peripheral parts to
glide over the yolk.

The main features of the cells of this blastoderm are the
same as they were in the one last described. In the non-
embryonic region the epiblast has thinned out, and is composed
of a single row of cells, which, in the succeeding stages, become
much flattened.

The lower layer cells over the greater part of their extent,
have not undergone any histological changes of importance.
Amongst them may frequently be seen a few exceptionally
large cells, which without doubt have been derived directly
from the yolk.

The embryonic rim is now a far more considerable structure
than it was. Vide PI. IV. fig. 7. Its elongation is mainly
effected by the continuous conversion of rounded lower layer
cells into columnar hypoblast cells at its central or anterior
extremity.

This conversion of the lower layer cells into hypoblast cells
is still easy to follow, and in every section cells intermediate
between the two are to be seen. The nature of the chanofes
which are taking place requires for its elucidation transverse
as well as longitudinal sections. Transverse sections of a slightly
older embryo than B are represented on PL iv. fig. 8 a, 8 b,
and 8 c.

Of these sections a is the most peripheral or posterior, and c
the most central or anterior. By a combination of transverse and
longitudinal sections, and by an inspection of a surface view,
it is rendered clear that, though the embryonic rim is a far more
considerable structure in the region of the embryo than else-
where (compare fig. 6 and fig. 7 and 7 a), yet that this gain
in size is not produced by an outgrowth of the embryo beyond
the rest of the germ, but by the conversion of the lower layer
cells into hypoblast having been carried far further towards the
centre of the oferm in the axial line than in the lateral res^ions
of the rim.

The most anterior of the series of transverse sections (PI. iv.
fig. 8c) I have represented, is especially instructive with reference

48 FORMATION OF THE LAYERS.

to this point. Though the embryonic rim is cut through at
the sides of the section, yet in these parts the rim consists
of hardly more than a continuity between epiblast and lower
hiyer cells, and the lower layer cells show no trace of a divi-
sion into mesoblast and hypoblast. In the axis of the embryo,
however, the columnar hypoblast is quite distinct; and on it a
small cap of mesoblast is seen on each side of the medullary
groove. Had the embryonic rim resulted from a projecting
growth of the blastoderm, such a condition could not have
existed. It might have been possible to find the hypoblast
formed at the sides of the section and not at the centre ; but
the reverse, as in these sections, could not have occurred.
Indeed it is scarcely necessary to have recourse to sections to
prove that the growth of the embryonic rim is towards the
centre of the blastoderm. The inspection of a surface view of a
blastoderm at this period demonstrates it beyond a doubt (PI.
VI. fig. B). The embryo, close to which the embryonic rim
is alone largely developed, does not project outwards beyond
the edge of the germ, but inwards towards its centre.

The space between the embryonic rim and the yolk (PI. iv.
fig. 7 al) is the alimentary cavity. The roof of this is therefore
primitively formed of hypoblast and the floor of yolk. The exter-
nal opening of this space at the edge of the blastoderm is the exact
morphological homologue of the anus of Rusconi, or blastopore of
Amphioxus, the Amphibians, &c. The importance of the mode
of growth in the embryonic rim depends upon the homology of
the cavity between it and the yolk, with the alimentary cavity
of Amphioxus and Amphibians. Since this homology exists,
the direction of the growth of this cavity ought to be, as it in
fact is, the same as in Amphioxus, etc., viz. towards the centre
of germ and original position of the segmentation cavity. Thus
though a true invagination is not present as in the other cases,
yet this is represented in Elasmobranchs by the continuous con-
version of lower layer cells into hypoblast along a line leading
towards the centre of the blastoderm.

In the parts of the rim adjoining the embryo, the lower layer-
cells, on becoming continuous with the epiblast cells, assume a
columnar form. At the sides of the rim this is not strictly the
case, and the lower layer cells retain their rounded form, though

DEVELOPMENT OF ELASMOBRANCH FISHES. 49

quite continuous with the epiblast cells. One curious feature
of the layer of epiblast in these lateral parts of the rim is the
great thickness it acquires before being reflected and becoming
continuous with the hypoblast (PL IV. fig. 8 c). In the vicinity
of the point of reflection there is often a rather large formation
of cells around the nuclei of the yolk. The cells formed here
no doubt pass into the blastoderm, and become converted into
columnar hypoblast cells. In some cases the formation of these
cells is very rapid, and they produce quite a projection on the
under side of the hypoblast. Such a case is represented in
PI. IV. fig. 8 b, n. al. The cells constituting this mass even-
tually become converted into the lateral and ventral walls of the
alimentary canal.

The formation of the mesoblast has progressed rapidly.
While many of the lower layer cells become columnar and form
the hypoblast, others, between these and the epiblast, remain
spherical. The latter do not at once become separated as a
layer distinct from the hypoblast, and, at first, are only to be
distinguished from them through their different character, vide
Plate IV. figs. 6 and 7. They nevertheless constitute the com-
mencing mesoblast.

Thus much of the mode of formation of the mesoblast can
be easily made out in longitudinal sections, but transverse sec-
tions throw still further light upon it.

From these it may at once be seen that the mesoblast is
not formed in one continuous sheet, but as two lateral masses,
one on each side of the axial line of the embryo \ In my
preliminary account ^ it was stated that this was a condi-
tion of the mesoblast at a very early period, and that it was
probably its condition from the beginning. Sections are now in
my possession which satisfy me that, from the very first, the
mesoblast arises as two distinct lateral masses, one on each side
of the axial line.

1 Professor Lieberkiilin {Gesellschaft zu Marburg, Jan. 1876) finds in Mam-
malia a bilateral arrangement of the mesoblast, which he compares with that
described by me in Elasmobranchs, In Mammalia, however, he finds the two
masses of mesoblast connected by a very thin layer of cells, and is apparently of
opinion that a similar thin layer exists in Elasmobranchs though overlooked by
me. I can definitely state that, whatever may be the condition of the mesoblast
in Mammalia, in Elasmobranchs at any rate no such layer exists.

^ Loc. cit.

B. 4

50 FORMATION OF THE LAYERS.

In the embryo from which the sections PL IV. fig. 8 a, 8 h,
8 c were taken, the mesoblast had, in most parts, not yet become
separated from the hypoblast. It still formed with this a con-
tinuous layer, though the mesoblast cells were distinguish-
able by their shape from the hypoblast. In only one section
(b) was any part of the mesoblast quite separated from the
hypoblast.

In the hindermost part of the embryo the mesoblast is at its
maximum, and forms, on each side, a continuous sheet extending
from the median line to the periphery (fig. 8 a). The rounder
form of the mesoblast cells renders the line of junction between
the layer constituted by them and the hypoblast fairly distinct ;
but towards the periphery, where the hypoblast cells have the
same rounded form as the mesoblast, the fusion between the
two layers is nearly complete.

In an anterior section the mesoblast is only present as a cap
on both sides of the medullary groove, and as a mass of cells
at the periphery of the section (fig. 8 b) ; but no continuous layer
of it is present. In the foremost of the three sections (fig. 8 c)
the mesoblast can scarcely be said to have become in any
way separated from the hypoblast except at the summit of the
medullary folds (m).

From these and similar sections it may be certainly con-
cluded, that the mesoblast becomes first separated from the
hypoblast as a distinct layer in the posterior region of the em-
bryo, and only at a later period in the region of the head.

In an embryo but slightly more developed than B, the
formation of the layer is quite completed in the region of the
embryo. To this embryo I now pass on.

In the non-embryonic parts of the blastoderm no fresh fea-
tures of interest have appeared. It still consists of two layers.
The epiblast is composed of flattened cells, and the lower layer
of a network of more rounded cells, elongated in a lateral
direction. The growth of the blastoderm has continued to be
very rapid.

In the region of the embryo (PI. IV. fig. 9) more important
changes have occurred. The epiblast still remains as a single
row of columnar cells. The hypoblast is no longer fused with
the mesoblast, and forms a distinct dorsal wall for the alimentary

DEVELOPMENT OF ELASMOBRANCH FISHES. 51

cavity. Though along the axis of the embryo the hypoblast is
composed of a single row of columnar cells, yet in the lateral
part of the embryo its cells are less columnar and are one or
two deep.

Owing to the manner in which the mesoblast became split
off from the hypoblast, a continuity is maintained between the
hypoblast and the lower layer cells of the blastoderm (PL iv.
fig. 9), while the two plates of mesoblast are isolated and dis-
connected from any other masses of cells.

The alimentary cavity is best studied in transverse sections.
( Vide PI. V. fig. 10 a, 10 6 and 10c, three sections from the same
embryo.) It is closed in above and at the sides by the hypoblast,
and below by the yolk. In its anterior part a floor is commencing
to be formed by a growth of cells from the walls of the two
sides. The cells for this growth are formed around the nuclei
of the yolk ; a feature which recalls the fact that in Amphibians
the ventral wall of the alimentary cavity is similarly formed in
part from the so-called yolk cells.

We left the mesoblast as two masses not completely sepa-
rated from the hypoblast. During this stage the separation
between the two becomes complete, and there are formed two
great lateral plates of mesoblast cells, one on each side of the
medullary groove. Each of these corresponds to a united
vertebral and lateral plate of the higher Vertebrates. The plates
are thickest in the middle and posterior regions (PL V. fig.
10 a and 10 6), but thin out and almost vanish in the region of
the head. The longitudinal section of this stage represented in
PL V. fig. 9, passes through one of the lateral masses of
mesoblast cells, and shows very distinctly its complete inde-
pendence of all the other cells in the blastoderm.

From what has been stated with reference to the develop-
ment of the mesoblast, it is clear that in Elasmobranchs this
layer is derived from the same mass of cells as the hypoblast,
and receives none of its elements from the epiblast. In connec-
tion with its development, as two independent lateral masses,
I may observe, as I have previously done\ that in this respect
it bears a close resemblance to mesoblast in Euaxes, as de-

1 Quart. Journ. of Microsc. Science, Oct.. 1874.

4—2

52 FORMATION OF THE LAYERS.

scribed by Kowalevsky \ This resemblance is of some interest,
as bearing on a probable Annelid origin of Vertebrata. Kow-
alevsky has also shown '^ that the mesoblast in Ascidians is
similarly formed as two independent masses, one on each side
of the middle line.

It ought, however, to be pointed out that a similar bilateral
origin of the mesoblast had been recently met with in Lymnceus
by Carl Eabl ^ A fact which somewhat diminishes the genea-
logical value of this feature in the mesoblast in Elasmobranchs.

During the course of this stage the spherules of food-yolk
immediately beneath the embryo are used up very rapidly.
As a result of this the protoplasmic network, so often spoken
of, comes very plainly into view. Considerable areas may some-
times be seen without any yolk spherule whatever.

On PI. IV. fig. la, and PI. V. 11 and 12, I have attempted
to reproduce the various appearances presented by this
network: and these figures give a better idea of it than any
description. My observations tend to show that it extends
through the whole yolk, and serves to hold it together. It has
not been possible for me to satisfy myself that it had any defi-
nite limits, but on the other hand, in many parts all my efforts
to demonstrate its presence have failed. When the yolk sphe-
rules are very thickly packed, it is difficult to make out for cer-
tain whether it is present or absent, and I have not succeeded in
removing the yolk spherules from the network in cases of this
kind. In medium-sized ovarian eggs this network is very easily
seen, and extends through the whole yolk. Part of such an
^gg is shown in PI. V. fig. 14. In full-sized ovarian eggs,
according to Schultz'*, it forms, as was mentioned in the first
chapter, radiating striae, extending from the centre to the peri-
phery of the ^^^. When examined with the highest powers,
the lines of this network appear to be composed of immea-
surably small granules arranged in a linear direction. These
granules are more distinct in chromic acid specimens than in

1 Embryologische Studien an Wurmen u. Ai'tbropoden. Memoir es d. VAcad.
S. Petersbourg. Vol. xiv. 1873.

2 Archiv far Mikr. Anat. Vol. vii.

3 Jenaisclie Zeitsckrift. Vol. ix. 1875. A bilateral development of meso-
blast, according to Professor Haeckel {loc. cit.), occurs in some Osseous Fish.
Hensen, Zeit. fur Anat. n. Entw. Vol. i., has recently described the mesoblast
in Mammalia as consisting of independent lateral masses.

* Archiv fiir Mikr. Anat. Vol. xi.

DEVELOPMENT OF ELASMOBRANCH FISHES. 53

those hardened in osmic acid, but are to be seen in both.
There can be little doubt that these granules are imbedded
in a thread or thin layer of protoplasm.

I have already (p. 39) touched upon the relation of this
network to the nuclei of the yolk\

During the stages which have just been described specially
favourable views are frequently to be obtained of the formation
of cells in the yolk and their entrance into the blastoderm.
Two representations of these are given, in PI. iv. fig. 7 or,
and PL v. fig. 13. In both of these distinctly circumscribed
cells are to be seen in the yolk (c), and in all cases are situated
near to the typical nuclei of the yolk. The cells in the yolk
have such a relation to the surrounding parts, that it is quite
certain that their presence is not due to artificial manipulation,
and in some cases it is even difficult to decide whether or no a
cell area is circumscribed round a nucleus (PI. v. fig. 13).
Although it would be possible for cells in the living state to
pass from the blastoderm into the yolk, yet the view that they
have done so in the cases under consideration has not much to
recommend it, if the following facts be taken into consideration.
(1) That the cells in the yolk are frequently larger than those
in the blastoderm. (2) That there are present a very large
number of nuclei in the yolk which precisely resemble the
nuclei of the cells under discussion. (3) That in some cases
(PL V. fig. 13) cells are seen indistinctly circumscribed as if in
the act of being formed.

Between the blastoderm and the yolk may frequently be
seen a membrane-like structure, which becomes stained with
hgematoxylrn, osmic acid etc. It appears to be a la3^er of
coagulated albumen and not a distinct membrane.

1 A protoplasmic network resembling in its essential features the one just
described has been noticed by many observers in other ova. Fol has figured
and described a network or sponge-like arrangement of the protoplasm in the
eggs of Geryonia. (Jenaische Zeitschrift, vol. vii.) Metschnikoff [Zcitschrift f.
Wiss. Zoologie, 1874) has demonstrated its presence in the ova of many Sipho-
nophoriae and Medusas. Flemming {Entwicklungsgeschichte der Najaden, Sitz.
der k. Akad. Wien, 1875) has found it in the ovarian ova of fresh-water mussels
(Anodonta and Unio), but regards it as due to the action of reagents, since he
fails to find it in the fresh condition. Amongst vertebrates it has been carefully
described by Eimer {Archiv fur Mikr. Anat., vol. viii.) in the ovarian ova of
Eeptiles. Eimer moreover finds that it is continuous with prolongations from
cells of the epithehum of the follicle in which the ovum is contained. Accord-
ing to him remnants of this network are to be met with in the ripe ovum, but
are no longer present in the ovum when taken from the oviduct.

54 formation of the layers.

Summary.

At the close of segmentation, the blastoderm forms a some-
what lens-shaped disc, thicker at one end than at the other ;
the thicker end being termed the embryonic end.

It is divided into two layers — an upper one, the epiblast,
formed by a single row of columnar cells; and a lower one, con-
sisting of the remaining cells of the blastoderm.

A cavity next appears in the lower layer cells, near the non-
embryonic end of the blastoderm, but the cells soon disappear
from the floor of this cavity which then comes to be constituted
by yolk alone.

The epiblast in the next stage is reflected for a small arc at
the embryonic end of the blastoderm, and becomes continuous
with the lower layer cells; at the same time some of the lower
layer cells of the embryonic end of the blastoderm assume a
columnar form, and constitute the commencing hypoblast. The
portion of the blastoderm, where epiblast and hypoblast are
continuous, forms a projecting structure which I have called the
embryonic rim. This rim increases rapidly by growing inwards
more and more towards the centre of the blastoderm, through
the continuous conversion of lower layer cells into columnar
liypoblast.

While the embryonic rim is being formed, the segmentation
cavity undergoes important changes. In the first place, it
receives a floor of lower layer cells, partly from an ingrowth
from the two sides, and partly from the formation of cells
around the nuclei of the yolk.

Shortly after the floor of cells has appeared, the whole seg-
mentation cavity becomes obliterated.

When the embryonic rim has attained to some importance,
the position of the embryo becomes marked out by the appear- ^
ance of the medullary groove at its most projecting part. The
embryo extends from the edge of the blastoderm inwards to-
Avards the centre.

At about the time of the formation of the medullary groove,
the mesoblast becomes definitely constituted. It arises as two
independent plates, one on each side of the medullary groove,
and is entirely derived from lower layer cells.

DEVELOPMENT OF ELASMOBRANCH FISHES. 55

The two plates of mesoblast are at first unconnected with any
other cells of the blastoderm, and, on their formation, the hypo-
blast remains in connection with all the remaining lower layer
cells. Between the embryonic rim and the yolk is a cavity, —
the primitive alimentary cavity. Its roof is formed of hypo-
blast, and its floor of yolk. Its external opening is homologous
with the anus of Rusconi, of Amphioxus and the Amphibians.
The ventral wall of the alimentary cavity is eventually derived
from cells formed in the yolk around the nuclei which are
there present.

Since the important researches of Gegenbaur^ upon the
meroblastic vertebrate eggs, it has been generally admitted
that the ovum of every vertebrate, however complicated may
be its apparent constitution, is nevertheless to be regarded as a
simple cell. This view is, indeed, opposed by His' and to a
very modified extent by Waldeyer^ and has recently been
attacked from an entirely new standpoint by Gotte"; but, to
my mind, the objections of these authors do not upset the
well founded conclusions of previous observations.

As soon as the fact is recognised that both meroblastic
and holoblastic eggs have the same fundamental constitution,
the admission follows, naturally, though not necessarily, that
the eggs belonging to these two classes differ solely in degree,
not only as regards their constitution, but also as regards the
manner in w^hich they become respectively converted into the
embryo. As might have been anticipated, this view has gained
a wide acceptance.

Amongst the observations, which have given a strong
objective support to this view, may be mentioned those of
Professor Lankester upon the development of Cephalopoda^
and of Dr Gotte^ upon the development of the Hen's egg. In
Loligo Professor Lankester showed that there appeared, in

1 Wirbelthiereier mit partieller Dottertheilung. MuUer^$ Arch. 1861.

2 Erste Anlage des Wirbelthierleibes.

3 Eierstock u. Ei.

^ EnUcicklungsgeschichte der Unke. The important researches of Gotte on
the development of the ovum, though meriting the most careful attention, do
not admit of discussion in this place.

5 Annals and Magaz. of Natural History, Vol. xi. 1873, p. 81.

6 Archiv f. Mikr. Anat. Vol. x.

oG FORMATION OF THE LAYERS.

the part of the egg usually considered as food-yolk, a num-
ber of bodies, which eventually developed a nucleus and be-
came cells, and that these cells entered into the blastoderm.
These observations demonstrate that in the eggs of Loligo the
so-called food-yolk is merely equivalent to a part of the egg
which in other cases undergoes segmentation.

The observations of Dr Gotte have a similar bearing. He
made out that in the eggs of the Hen no sharp line is to be
found separating the germinal disc from the yolk, and that,
independently of the normal segmentation, a number of cells
are derived from that part of the egg hitherto regarded as
exclusively food-yolk. This view of the nature of the food-yolk
was also advanced in my preliminary account of the develop-
ment of Elasmobranchs\ and it is now my intention to put
forward the positive evidence in favour of this view, which is
supplied from a knowledge of the phenomena of the develop-
ment of the Elasmobranch ovum; and then to discuss how far
the facts of the growth of the blastoderm in Elasmobranchs
accord with the view that their large food-yolk is exactly
equivalent to part of the ovum, which in Amphibians undergoes
segmentation, rather than some fresh addition, which has no
equivalent in the Amphibian or other holoblastic ovum.

Taking for granted that the ripe ovum is a single cell, the
question arises whether in the case of meroblastic ova the cell
is not constituted of tvvo parts completely separated from one
another.

Is the meroblastic ovum, before or after impregnation, com-
posed of a germinal disc in which all the protoplasm of the cell
is aggregated, and of a food-yolk in which 7io protoplasm is
present? or is the protoplasm present throughout, being simply
more concentrated at the germinal pole than elsewhere? If the
former alternative is accepted, we must suppose that the mass of
food-yolk is a something added which is not present in holoblas-
tic ova. If the latter alternative is accepted, it may then be
maintained that holoblastic and meroblastic ova are constituted
in the same way and differ only in the proportions of their con-
stituents.

^ Quart. Journ. of Micr. Science, Oct. 1874.

DEVELOPMENT OF ELASMOBRANCH FISHES. 57

My own observatioDS in conjunction with the specially inter-
esting observations of Dr Schultz^ justify the view which regards
the protoplasm as present throughout the whole ovum, and not
confined to the germinal disc. Our observations show that a
fine protoplasmic network, with ramifications extending through-
out the whole yolk, is present both before and after impregna-
tion.

The presence of this network is, in itself, only sufficient to
prove that the yolk may be equivalent to part of a holoblastic
ovum; to demonstrate that it is so requires something more,
and this link in the chain of evidence is supplied by the
nuclei of the yolk, which have been so often referred to.

These nuclei arise independently in the yolk, and become
the nuclei of cells which enter the germ and the bodies of which
are derived from the protoplasm of the yolk. Not only so, but
the cells formed around these nuclei play the same part in the
development of Elasmobranchs as do the largest so-called yolk
cells in the development of Amphibians. Like the homologous
cells in Amphibians, they mainly serve to form the ventral wall
of the alimentary canal and the blood-corpuscles. The identity
in the fate of the so-called yolk cells of Amphibians with the cells
derived from the yolk in Elasmobranchs, must be considered
as a proof of the homology of the yolk cells in the first case
with the yolk in the second ; the difference between the yolk in
the two cases arising from the fact that in the Elasmobranch
ovum the yolk spherules bear a larger proportion to the proto-
plasm than they do in the Amphibian ovum. As I have
suggested elsewhere'', the segmentation or non-segmentation of
a particular part of the ovum depends solely upon the proportion
borne by the protoplasm to the yolk particles; so that, when
the latter exceed the former in a certain fixed proportion,
segmentation is no longer possible; and, as this limit is ap-
proached, segmentation becomes slower, and the resulting
segments larger and larger.

The question how far the facts in the developmental history
of the various vertebrate blastoderms accord with the view of
the nature of the yolk just propounded, is one of considerable

1 Arcliiv f. Mikr. Anat. Vol. xxi.

- Comparison, &c., Quart. Journ. Micr. Science, July, 1875,

58

FORMATION OF THE LAYERS.

interest. An answer to it lias already been attempted from a
general point of view in my paper* entitled ' The Comparison of
the early stages of development in Vertebrates'; but the subject
may be conveniently treated here in a special manner for
Elasmobranch embryos.

In the wood-cut, fig. 1 A, B, C^ are represented three dia-
grammatic longitudinal sections of an Elasmobranch embryo.

Fig. 1.

Diagrammatic longitudinal sections of an Elasmobranch embryo.

Epihlcist withont shading. Mesoblast black with clear outlines to the cells.
Lower layer cells and hypoblast with simple shading.

ep. epiblast. m. mesoblast. al. alimentary cavity. scj. segmentation
cavity, nc. neural canal, ch. notochord. x. point where epiblast and hypo-
blast become continuous at the posterior end of the embryo, n. nuclei of yolk.

A. Section of young blastoderm, with segmentation cavity in the middle of
the lower layer cells.

B. Older blastoderm with embryo in which hypoblast and mesoblast are
distinctly formed, and in which the alimentary slit has appeared. The seg-
mentation cavity is still represented as being present, though by this stage
it has in reality disappeared.

C. Older blastoderm with embryo in which neural canal has become
formed, and is continuous posteriorly with alimentary canal. The notochord,
though shaded like mesoblast, belongs properly to the hypoblast.

1 Loc. cit.

2 This figure, together with fig. 2 and 3, are reproduced from my paper upon
the comparison of the early stages of development in vertebrates.

DEVELOPMENT OF ELASMOBRANCH FISHES.

59

A nearly corresponds with the longitudinal section represented
on PI. III. fig. 4, and B with PL IV. fig. 7. In PL iv. fig. 7,
the segmentation cavity has however completely disappeared,
while it is still represented as present in the diagTam of the
same period. If these diagrams, or better still, the wood-cuts
fig, 2 A, B, C (which only differ from those of the Elasmo-
branch fish in the smaller amount of food-yolk), be compared
with the corresponding ones of Bombinator, fig. 3 A, B, G,
they will be found to be in fundamental agreement with them.
First let fig. 1 A, or fig. 2 A, or PL ill. fig. 4, be compared

Fig. 2.

GO

FOKMATION OF THE LAYERS.

Diagrammatic longitudinal sections of embryo, wliicli develops in the same
manner as the Elasmobranch embryo, but in which the ovum contains far
less food-yolk than is the case with the Elasmobranch ovum.
Epiblast without shading. Mesohlast black with clear outHnes to the cells.

Loiver layer cells and hypoblast with simple shading.

ep. epiblast. m. mesoblast. hy. hypoblast. sg. segmentation cavity.

al. alimentary cavity, nc. neural canal, hf. head-fold. n. nuclei of the yolk.
The stages A, B and G are the same as in figure 1.

with fig. 3 A. In all there is present a segmentation cavity
situated not centrally but near the surface of the egg. The
roof of the cavity is thin in all, being composed in the
Amphibian of epiblast alone, and in the Elasmobranch of
epiblast and lower layer cells. The floor of the cavity is, in

Fig. 3.

'L

DEVELOPMENT OF ELASMOBRANCH FISHES.

Gl

Diagrammatic longitudinal sections of Bombinator igneiis.
modifications from Gotte,

Reproduced with

Epiblast without shading. Mesoblast black with clear outlines to the cells.
Lower layer cells and hypoblast with single shading.

ep. epiblast. LI. lower layer cells, y. smaller lower layer cells at the sides
of the segmentation cavity. 711. mesoblast. hy. hypoblast, al. alimentary cavity.
sg. segmentation cavity, nc. neural cavity, yk. yolk-cells.

A is the youngest stage in which the alimentary involution has not yet ap-
peared. X is the point from which the involution will start to form the dors'al wall
of the ahmentary tract. The line on each side of the segmentation cavity, which
separates the smaller lower layer cells from the epiblast cells, is not present in
Gotte's original figure. The two shadings employed in the diagram render it
necessary to have some line, but at this stage it is in reahty not possible to
assert which cells belong to the epiblast and which to the lower layer.

B. In this stage the alimentary cavity has become formed, but the segmen-
tation cavity is not yet obliterated.

X. point where epiblast and hypoblast become continuous.

C. The nem-al canal is already formed, and communicates posteriorly
with the alimentary.

X. point where epiblast and hypoblast become continuous.

62 FORMATION OF THE LAYERS.

all, formed of so-called yolk {Vide PL ill. fig. 4), which in
all forms the main mass of the egg. In the Amphibian the
yolk is segmented, and, though it is not segmented in the Elas-
mobranch, it contains in compensation the nuclei so often men-
tioned. In all the sides of the segmentation cavity are
formed by lower layer cells. In the Amphibian the sides
are enclosed by smaller cells (in the diagram) which corre-
spond exactly in function and position with the lower layer cells
of the Elasmobranch blastoderm.

The relation of the yolk to the blastoderm in the Elasmo-
branch embryo at this stage of development very well suits the
view of its homology with the large cells of the Amphibian
ovum. The only essential difference between the two ova
arises from the roof of the segmentation cavity being in the
Elasmobranch embryo formed of lower layer cells, which are
absent in the Amphibian embryo. This difference no doubt
depends upon the greater quantity of yolk particles present in
the Elasmobranch ovum. These increase the bulk of the lower
layer cells, which are thus compelled to creep up the sides of
the segmentation cavity till they close it in above.

In the next stage for the Elasmobranch, fig. 1 and 2 B and
PI. IV. fig. 7, and for the Amphibian, fig. 8 B, the agreement
between the two types is again very close. In both for a small
portion {x) of the edge of the blastoderm the epiblast and hypo-
blast become continuous, while at all other parts the epiblast,
accompanied by lower layer cells, grows round the yolk or
round the large cells which correspond to it. The yolk cells
of the Amphibian ovum form a comparatively small mass,
and are therefore rapidly enveloped ; while in the case of the
Elasmobranch ovum, owing to the greater mass of the yolk,
the same process occupies a long period. In both ova the
portion of the blastoderm, where epiblast and hypoblast become
continuous, forms the dorsal lip of an opening — the anus of
Rusconi — which leads into the alimentary cavity. This cavity
has the same relation in both ova. It is lined dorsally by lower
layer cells, and ventrally by yolk or what corresponds with yolk;
the ventral epithelium of the alimentary canal being in both
cases eventually supplied by the yolk cells.

As in the earlier stage, so in the present one, the anatomical

DEVELOPMENT OF ELASMOBRANCH FISHES. 63

relations of the yolk to the blastoderm in the one case (Elasmo-
branch) are nearly identical with those of the yolk cells to the
blastoderm in the other (Amphibian). The main features in
which the two embryos differ, during the stage under considera-
tion, arise from the same cause as the solitary point of differ-
ence during the preceding stage.

In Amphibians, the alimentary cavity is formed coincidently
with a true ingrowth of cells from the point where epiblast and
hypoblast become continuous, and from this ingrowth the dorsal
wall of the alimentary cavity is formed. The same ingrowth
causes the obliteration of the segmentation cavity.

In the Elasmobranchs, owing to the larger bulk of the lower
layer cells caused by the food-yolk, these have been compelled
to arrange themselves in their final position during segmenta-
tion, and no room is left for a true invagination ; but instead
of this there is formed a simple split between the blastoderm
and the yolk. The homology of this mth the primitive
invagination is nevertheless proved by the survival of a
number of features belonging to the ancestral condition in
which a true invagination was present. Amongst the more
important of these are the following : — (1) The continuity of
epiblast and hypoblast at the dorsal lip of the anus of Rus-
coni. (2) The continuous conversion of indifferent lower layer
cells into hypoblast, which gradually extends backwards towards
the segmentation cavity, and exactly represents the course of
the invagination whereby in Amphibians the dorsal wall of
the alimentary cavity is formed. (3) The obliteration of the
segmentation cavity during the period when the pseudo-invagi-
nation is occurring.

The asymmetry of the gastrula or pseudo-gastrula in Cyclo-
stomes. Amphibians, Elasmobranchs and, I beUeve, Osseous
Fishes, is to be explained by the form of the vertebrate body.
In Amphioxus, where the small amount of food-yolk present is
distributed uniformly, there is no reason why the invagination
and resulting gastrula should not be symmetrical. In other
vertebrates, where more food-yolk is present, the shape and
structure of the body render it necessary for the food-yolk to
be stored away on the ventral side of the alimentary canal.
This, combined with the unsymmetrical position of the anus.

64 FORMATION OF THE LAYERS.

which primitively corresponds in position with the blastopore
ur anus of Rusconi, causes the asymmetry of the gastrula invagi-
nation, since it is not possible for the part of ovum which will
become the ventral wall of the alimentary canal, and which is
loaded with food-yolk, to be invaginated in the same fashion as
the dorsal wall. From the asymmetry, so caused, follow a large
number of features in vertebrate development, which have been
worked out in some detail in my paper already quoted \

Prof. Haeckel, in a paper recently published ^ appears to
imply that because I do not find absolute invagination in
Elasmobranchs, I therefore look upon Elasmobranchs as mili-
tating against his Gastr^a theory. I cannot help thinking
that Prof. Haeckel must have somewhat misunderstood my
meaning. The importance of the Gastrsea theory has always
appeared to me to consist not in the fact that an actual in-
gi'owth of certain cells occurs — an ingrowth which might have
many different meanings^ — but in the fact that the types of
early development of all animals can be easily derived from
that of the typical gastrula. I am perfectly in accordance with
Professor Haeckel in regarding the type of Elasmobranch
development to be a simple derivative from that of the gastrula,
although believing it to be without any true ingrowth or inva-
gination of cells.

Professor Haeckel* in the paper just referred to published
his view upon the mutual relationships of the various vertebrate
blastoderms. In this paper, which appeared but shortly after
my own^ on the same subject, he has put forward views which
differ from mine in several important details. Some of these
bear upon the nature of food-yolk ; and it appears to me that
Professor Haeckel's scheme of development is incompatible
with the view that the food-yolk in meroblastic eggs is the
homologue of part of the hypoblast of the holoblastic eggs.

The following is Professor Haeckel's own statement of the

^ Quart. Joum. of Micr. Science, July, 1875.

'•^ Die Gastrula u. Eifurchurig d. Thiere, Jenaische Zeitschrift, Vol. ix.

2 For instance, in Crustaceans it does not in some cases appear certain
whether an invagination is the typical gastrula invagination, or only an invagi-
nation by which, at a period subsequent to the gastrula invagination, the hind
gut is frequently formed.

4 Loc. cit.

s Loc. cit.

DEVELOPMENT OF ELASMOBRA^X'H FISHES. G5

scheme or type, which he regards as characteristic of mero-
blastic eggs, pp. 98 and 99.

Jetzt folgt der hochst wichtige und interessaiite Yorgang, den ich
als Einstiilpung der Bias till a aiifFasse imd der zur Bildiing der
Gastrula fiihrt (Fig. 63, 64) i. Es schliigt sicli namlicli der verdickte
Saiim der Keimscheibe, der "RandwiiLst" oder das Projjeristom,
nach innen um imd eine diinne Zellenscliicht wachst als du-ecte Fort-
setzung desselben, wie ein immer enger werdendes Diaphragma, in
die Keimhohle hinein. Diese Zellenscliicht ist das entstehende En-
toderm (Fig. 64 i, 74 ?'). Die Zellen, welche dieselbe zusammensetzen
und aiis dem innern Theile des Randwiilstes liervorwachsen, sind viel
gri5sser aber flacher als die Zellen der Keimhohlendecke iind zeigen
ein dunkleres grobkorniges Protoplasma. Aiif dem Boden der Keim-
hohle, d. h. also anf der Eiweisskugel des Naliriingsdotters, liegen sie
iinmittelbar aiif imd riicken hier durch centripetale Wand era ng
gegen dessen Mitte vor, bis sie dieselbe zuletzt erreichen und nun-
mehr eine ziisammenhangende einscliichtige Zelleulage aiif dem ganzen
Keimhohlenboden bilden. Diese ist die erste vollstandioje Anlage
des Darmblatts, Entoderms oder "Hypoblasts", und von nun an
k(3nnen wir, im Gesfensatz dazii den jjesammten iibriijen Theil des
Blastoderms, namlich die mehrschichtige Wand der Keimhohlendecke
als Hautblatt, Exoderm oder " Epiblast " bezeichnen. Der ver-
dickte Randwulst (Fig, 64 il\ 74 ^^), in welchem beide primare Keim-
l)latter in einander iibei-gehen, besteht in seinem oberen und ausseren
Theile aiis Exodermzellen, in seinem unteren und innercn Theile aus
Entodermzellen.

In diesem Stadium entspricht iinser Fisclikeim einer Amphi-
blastula, welche mitten in der Invagination begriffen ist, und bei
welcher die entstehende Urdarmhohle eine grosse Dotterkngel auf-
genommen hat. Die Invagination wird nunmehr dadurch vervoll-
stancligt und die Gastrulabildiing dadurch abgeschlossen, dass die
Keimhijhle verschwindet. Das waehsende Entoderm, dem die Dot-
terkngel innig anhangt, wolbt sich in die letztere hinein und nahert
sich so dem Exoderm. Die klare Fliissigkeit in der Keimhohle wird
resorbirt und scliliesslich legt sich die obere convexe Flache des
Entoderms an die untere concave des Exoderms eiig an : die Gastrula
des discoblastischen Eies oder die " Discogastrula" ist fertig (Fig.
65, 76; Meridiandurchschnitt Fig 66, 75).

Die Discogastrula unsers Knochenfisches in diesem Stadium der
voUen Ausbildung stellt nunmehr eine kreisrunde Kappe dar, welche
wie ein gefiittertes Miitzchen fast die ganze obere Hemisphare der
hyalinen Dotterkugel eng anliegend bedeckt (Fig. Q>o). Der Ueber-
ziig des Miitzchens entspricht dem Exoderm (e), sein Flitter dem
Entoderm (?'). Ersteres besteht aus drei Schichten von kleineren
Zellen, letzteres aus einer einzigen Schicht von grosseren Zellen.
Die Exodermzellen (Fig. 77) me^^sen 0,006 — 0,009 Mm., und haben
ein klares, sehr feinkorniges Protoplasma. Die Entodermzellen (Fig.

^ The references in this quotation are to the figures in the original.
B. 5

6Q FORMATION OF THE LAYERS.

78) messen 0,02 — 0,03 Mm. und ihr Protoplasma ist mehr grobkornig
und triiber. Letztere bilden aiich den grossten Theil des Rand-
wulstes, den wir nunmehr als XJrmundrand der Gastrula, als
" Projoeristoma " oder auch als '' RuscoNi'schen After " bezeiclmen
konnen. Der letztere umfasst die Dotterkugel, welcbe die ganze
Urdarmhohle ausfiillt und weit aus der dadurch verstopften Urmimd-
Oeffnung vorragt.

My objections to the view so lucidly explained in the pas-
sage just quoted, fall under two heads.

(1) That the facts of development of the meroblastic eggs
of vertebrates, are not in accordance with the views here
advanced.

(2) That even if these views be accepted as representing
the actual facts of development, the explanation offered of these
facts would not be satisfactory.

Professor Haeckel's views are absolutely incompatible with
the facts of Elasmobranch development, if my investigations are
correct.

The grounds of the incompatibility may be summed up
under the following heads :

(1) In Elasmobranchs the hypoblast cells occupy, even
before the close of segmentation, the position which, on Pro-
fessor Haeckel's view, they ought only eventually to take up
after being involuted from the whole periphery of the blasto-
derm.

(2) There is no sign at any period of an invagination of
the periphery of the blastoderm, and the only structure (the
embryonic rim) which could be mistaken for such an invagina-
tion is confined to a very limited arc.

(3) The growth of cells to form the floor of the segmenta-
tion cavity, which ought to be part of this general invagination
from the periphery, is mainly due to a formation of cells from
the yolk.

It is this ingrowth of cells for the floor of the segmenta-
tion cavity which, I am inclined to think, Professor Haeckel
has mistaken for a general invagination in the Osseous Fish he
has investigated.

(4) Professor Haeckel fails to give an account of the asym-
metry of the blastoderm; an asymmetry which is unquestion-

DEVELOPMENT OF ELASMOBRANX'H FISHES. G7

ably also present in the blastoderm of most Osseous Fishes,
though not noticed by Professor Haeckel in the investigations
recorded in his paper.

The facts of development of Osseous Fishes, upon which Pro-
fessor Haeckel rests his views, are too much disputed, for their
discussion in this place to be profitable \ The eggs of Osseous
Fishes appear to me unsatisfactory objects for the study of this
question, partly on account of all the cells of the blastoderm
being so much alike, that it is a very difficult matter to
distinguish between the various layers, and, partly, because
there can be little question that the eggs of existing Osseous
Fishes are very much modified, throuc^^h havino^ lost a o-reat
part of the food-yolk possessed by the eggs of their ancestors".
This disappearance of the food-yolk must, without doubt,
have produced important changes in development, which would
be especially marked in a pelagic egg, like that investigated
by Professor Haeckel.

The Avian egg has been a still more disputed object than
even the egg of the Osseous Fishes. The results of my own
investigations on this subject do not accord with those of Dr
Gotte, or the views of Professor HaeckeP.

1 A short statement by Kowalevskv on this subject in a note to his account of
the development of Ascidians, would seem to indicate that the type of development
of Osseous Fishes is precisely the same as that of Elasmobranchs. Kowalevsky
says, Arch. f. Micr. Anat. Vol. vii. p. 114, note 5. " Accordmg to my observa-
tions on Osseous Fishes the germinal wall consists of two layers, an upper and
lower, which are continuous with one another at the border. From the upper
one develops skin and nervous system, from the lower hypoblast and mesoblast."
This statement, which leaves unanswered a number of important questions, is
too short to serve as a basis for supporting my views, but so far as it goes its
agreement with the facts of Elasmobranch development is undoubtedly striking.

2 The eggs of the Osseous Fishes have, I beheve, undergone changes of the
same character, but not to the same extent, as those of Mammaha, which,
according to the views expressed both by Professor Haeckel and myself, are
degenerated from an ovum with a large food-yolk. The grounds on which I
regard the eggs of Osseous Fishes as having undergone an analogous change, are
too foreign to the subject to be stated here.

3 I find myself imable without figures to understand Dr Eauber's {Central-
blatt fiir 2Ied. Wiss. 1874, No. 50; 1875, Nos. 4 and 17) views with sufficient
precision to accord to them either my assent or dissent. It is quite in accord-
ance with the view propounded in my paper {loc. cit.) to regard, with Dr Eauber
and Professor Haeckel, the thickened edge of the blastoderm as the homologue
of the lip of the blastopore in Amphioxus ; though an invagination, in the manner
imagined by Professor Haeckel, is no necessary consequence of this view. If Dr
Eauber regards the whole egg of the bird as the homologue of that of Amphioxus,
and the inclosure of the yolk by the blastoderm as the equivalent to the process
of invagination in Amphioxus, then his views are practically in accordance with
mv own.

G8 FORMATION OF THE LAYERS.

Apart from disputed points of development, it appears
to me that a comparative account of the development of the
meroblastic vertebrate ova ought to take into consideration the
essential differences which exist between the Avian and Piscian
blastoderms, in that the embryo is situated in the centre of the
blastoderm in the first case and at the edge in the second \

This difference entails important modifications in develop-
ment, and must necessarily affect the particular points under
discussion. As a result of the different positions of the embryo
in the two cases, there is present in Elasmobranchs and Osseous
Fishes a true anus of Rusconi, or primitive opening into the
alimentary canal, which is absent in Birds. Yet in neither
Elasmobranchs^ nor Osseous Fishes does the anus of Rusconi
correspond in position with the point where the final closing in
of the yolk takes place, but in them this point corresponds
rather with the blastopore of Birds ^

Owing also to the respective situations of the embryo in the
blastoderm, the alimentary and neural canals communicate

1 I have suggested in a previous paper {^^ Comparison,'^'' &Q,., Quart. Journal of
3Iicr. Science, July, 1875) that the position occupied by the embryo of Birds at
the centre, and not at the periphery, of the blastoderm may be due to an abbre-
viation of the process by which the Elasmobranch embrj^os cease to be situated
at the edge of the blastoderm {vide p. 81 and PI. viii. fig. 1, 2). Assuming this
to be the real exi^lanation of the position of the embryo in Lirds, I feel inclined
to repeat a speculation which I made some time ago with reference to the primi-
tive streak in Birds {Quart. Journ. of Micr. Science, 1873, p. 280). In Birds
there is, as is well known, a structure called the primitive streak, which has
been shown by the observations of Dursy, corroborated by my observations {loc.
clt.), to be situated behind the medullary groove, and to take no part in the
formation of the embryo. I further showed that the peculiar fusion of epiblast
and mesoblast, called by His the axis cord, was confined to this structure and did
not occur in other parts of the blastoderm. Nearly similar results have been
recently arrived at by Hensen with reference to the primitive streak in Mam-
mals, The position of the primitive streak immediately behind the embiyo
suggests the speculation that it may represent the line along which the edges of
the blastoderm coalesced, so as to give to the embryo the central position which
it has in the blastoderms of Birds and Mammals, and that the peculiar fusion
of epiblast and mesoblast at this point may represent the primitive continuity
of epiblast and lower layer cells at the dorsal lip of the anus of Kusconi in
Elasmobranchs. I put this speculation forwards as a mere suggestion, in the
hope of elucidating the peculiar structiu'e of the j)rimitive streak, which not
improbably may be found to be the keystone to the nature of the blastoderm of
the higher vertebrates.

2 Vide p. 81 and Plate viii. fig. 1 and 2, and Self, " Comparison,^'' &c., loc.cit.

3 Tiie relation of the anus of Rusconi and blastopore in Elasmobranchs was
fully explained in the paper above quoted. It was there clearly shown that
neither the one nor the other exactly corresponds with the blastopore of Amphi-
oxus, but that the two together do so. Professor Haeckcl states that in the
Osseous Fish investigated by him the anus of Eusconi and the blastoi)ore coin-
cide. This is not the case in the Salmon.

DEVELOPMENT OF ELASMOBRANCH FISHES. 69

posteriorly in Elasmobranchs and Osseous Fishes, but not in
Birds. Of all these points Professor Haeckel makes no menticn.

The support of his views which Prof. Haeckel attempts to
gain from Gotte's researches in Mammalia is completely cut
away by the recent discoveries of Van Beneden^ and Hensenl

It thus appears that Professor Haeckel's views but ill accord
with the facts of vertebrate development; but even if they were
to do so completely it would not in my opinion be easy to give
a rational explanation of them.

Professor Haeckel states that no sharp and fast line can be
drawn between the types of 'unequal' and 'discoidal' segmenta-
tion^ In the cases of unequal segmentation he admits, as is
certainly the case, that the larger yolk cells (hypoblast) are
simply enclosed by a growth of the epiblast around them; which
is to be looked on as a modification of the typical gastrula inva-
gination, necessitated by the large size of the yolk cells {vide
Professor Haeckel's paper, Taf. ii. fig. 30). In these instances
there is no commencement of an ingrowth in the manner
supposed for merohlastic ova.

When the food-yolk becomes more bulky, and the hypoblast
does not completely segment, it is not easy to understand why
an ingrowth^ which had no existence in the former case, should
occur; nor where it is to come from. Such an ingrowth as is
supposed to exist by Professor Haeckel would, in fact, break
the continuity of development between meroblastic and holo-
blastic ova, and thus destroy one of the most important results
of the Gastrsea theory.

It is quite easy to suppose, as I have done, that in the cases
of discoidal segmentation, the hypoblast (including the yolk)
becomes enclosed by the epiblast in precisely the same manner
as in the cases of unequal segmentation.

But even if Professor Haeckel supposes that in the unseg-
mented food-yolk a fresh element is added to the ovum, it
remains quite unintelligible to me how an ingrowth of cells from
a circumferential line, to form a layer which had no previous

^ Devdoppement Embnjonnaire des Mamm^ feres, Bulletin de VAcad. r. d.
BeJgique, 1875.

- Loc. cit.

3 For an explanation of tliese terms, inde Prof. Haeckel's original paper or
the abstract in Quart. Journ. of Micr. Science for January, 1876.

70 FORMATION OF THE LAYERS.

existence, can be equivalent to, or derived from, the invagina-
tion of a layer, which exists before the process of invagina-
tion beg^ins, and which remains continuous throuo^hout it.

If Professor Haeckel's views should eventually turn out to
be in accordance with the facts of vertebrate development, it
will, in my opinion, be very difficult to reduce them into
conformity with the Gastroea theory.

Although some space has been devoted to an attempt to
refute the views of Professor Haeckel on this question, I wish
it to be clearly understood that my disagreement from his
opinions concerns matters of detail only, and that I quite
accept the Gastrgea theory in its general bearings.

Observations upon the formation of the layers in Elas-
mobranchs have hitherto been very few in number. Those
published in my preliminary account of these fishes are, I
believe, the earliest \

Since then there has been published a short notice on the
subject by Dr Alex. Schultz^ His observations in the main
accord with my own. He apparently speaks of the nuclei of
the yolk as cells, and also of the epiblast being more than one
cell deep. In Torpedo alone, amongst the genera investigated
by me, is the layer of epiblast, at about the age of the last
described embryo, composed of more than a single row of cells.

1 I omit all reference to a paper published in Russian by Prof. Kowalevsky.
Being unable to translate it, and the illustrations being too meagre to be in
themselves of much assistance, it has not been possible for me to make any
use of it.

2 Centralblatt f. Med. Wiss. No. 33, 1875.

CHAPTER IV.

The General Features of the Elasmobranch Embryo

at successive stages.

No complete series of figures, representing the variuus
stages in development of an Elasmobranch Embryo, has
hitherto been published. With the view of supplying this
deficiency Plates vi. and vii. have been inserted. The
embryos represented in these two Plates form a fairly com-
plete series, but do not all belong to a single species. Those
on PI. VI., with the exception of G, are embryos of Pristi-
urus ; G being an embryo of Torpedo. Those on PL Vll.,
excepting K, which is a Pristiurus embryo, are embryos of
Scyllium canicula. All the embryos on PL vil. were very
accurately drawn from nature by my sister, Miss A. B. Balfour.
Unfortunately the exceptional beauty and clearness of the
originals is all but lost in the lithographs. To facilitate
future description, letters will be employed in the remainder
of these pages to signify that an embryo being described is
of the same age as the embryo on these Plates to which the
letter used refers. Thus an embryo of the same age as L will
be spoken of hereafter as belonging to stage L.

A.

This figure represents a hardened blastoderm at a stage
when the embryo-swelling {e. s.) has become obvious, but
before the appearance of the medullary groove. The posi-
tion of the segmentation cavity is indicated by a slight swell-
ing of the blastoderm (5. c). The shape of the blastoderm, in
hardened specimens, is not to be relied upon, owing to the
traction which the blastoderm undergoes during the process of
removing the yolk from the egg-shell.

B.

B is the view of a fresh blastoderm. The projecting part
of this, already mentioned as the ' embryonic rim', is indicated

72 GENERAL FEATURES.

by the shading. At the middle of the embryonic rim is to be
seen the rudiment of the embryo (in.g.). It consists of an
area of the blastoderm, circumscribed on its two sides and at
one end, by a slight fold, and whose other end forms part of
the edge of the blastoderm. The end of the embryo which
points towards the centre of the blastoderm is the head end,
and that which forms part of the edge of the blastoderm is
the tail end. To retain the nomenclature usually adopted
in treating of the development of the Bird, the fold at the
anterior end of the embryo may be called the head fold, and
those at the sides the side folds. There is in Elasmobranchs
no tail fold, owing to the position of the embryo at the peri-
phery of the blastoderm, and it is by the meeting of the three
above-mentioned folds only, that the embryo becomes pinched
off from the remainder of the blastoderm. Along the median
line of the embryo is a shallow groove {m.g.)y the well-known
medullary groove of vertebrate embryology. It flattens out
both anteriorly and posteriorly, and is deepest in the middle
part of its course.

C.

This embryo resembles in most of its features the embryo
last described. It is, however, considerably larger, and the
head-fold and side-folds have become more pronounced struc-
tures. The medullary groove is far deeper than in the earlier
stage, and widens out anteriorly. This anterior widening is the
first indication of a distinction between the brain and the
remainder of the central nervous system, a distinction which
arises long before the closure of the medullary canal.

D.

This embryo is far larger than the one last described, but
the increase in length does not cause it to project beyond the
edge of the blastoderm, but has been due to a growth inwards
towards the centre of the blastoderm. The head is now indi-
cated by an anterior enlargement, and the embryo also widens
out posteriorly. The posterior widening {t. s.) is formed by a
pair of rounded prominences, one on each side of the middle

DEVELOPMENT OF ELASMOBRANCH FISHES. 73

line. These are very conspicuous organs during the earlier
stages of development, and consist of two large aggi'egations of
mesoblast cells. In accordance with the nomenclature adopted
in my preliminary paper^ they may be called 'tail-swellings'.
Between the cephalic enlargements and the tail-swellings is
situated the rudimentary trunk of the embryo. It is more
completely pinched off from the blastoderm than in the last
described embryo. The medullary groove is of a fairly uniform
size throughout the trunk of the embryo, but flattens out and
vanishes completely in the region of the head. The blastoderm
in Pristiurus and Scyllium grows very rapidly, and has by this
stage attained a very considerable size ; but in Torpedo its
growth is very slow.

E and F.

These two embryos may be considered together, for, al-
though they differ in appearance, yet they are of an almost
identical age ; and the differences between the two are purely
external. E appears to be a little abnormal in not having the
cephalic region so distinctly marked off from the trunk as is
usual. The head is proportionally larger than in the last stage,
and the tail-swellings remain as conspicuous as before. The
folding off from the blastoderm has progressed rapidly, and the
head and tail are quite separated from it. -The medullary
groove has become closed posteriorly in both embryos, but the
closing has extended further forwards in F than in E. In F
the medullary folds have not only united posteriorly, but have
very nearly effected a fre&h junction in the region of the neck.
At this point a second junction of the two medullary folds is in
fact actually effected before the posterior closing has extended
forwards so far. The later junction in the region of the neck
corresponds in position with the point, where in the Bird the
medullary folds first unite. No trace of a medullary groove is
to be met with in the head, which simply consists of a wide
flattened plate. Between the two tail-swellings surface views
present the appearance of a groove, but this appearance is de-
ceptive, since in sections no groove, or at most a very slight
one, is perceptible.

1 Quart. Journ. Jlicr. Science, Oct. 1874.

74 GENERAL FEATURES.

G.

During the preceding stages growth in the embryo is very-
slow, and considerable intervals of time elapse before any
perceptible changes are effected. This state of things now
becomes altered, and the future changes succeed each other
with far greater rapidity. One of the most important of these,
and one w^hich first presents itself during this stage, is the dis-
appearance of the yolk spherules from the embryonic cells, and
the consequently increased transparency of the embryo. As a
result of this, a number of organs, which in the earlier stages
were only to be investigated by means of sections, now become
visible in the living embryo.

The tail-swellings (t. s.) are still conspicuous objects at the
posterior extremity of the embryo. The folding off of the
embryo from the yolk has progressed to such an extent that it
is now quite possible to place the embryo on its side and
examine it from that point of view.

The embryo may be said to be attached to the yolk by
a distinct stalk or cord, which in the succeeding stages gra-
dually narrows and elongates, and is known as the umbilical
cord (so.s.). The medullary canal has now become completely
closed, even in the region of the brain, where during the last
stage no trace of a medullary groove had appeared. Slight
constrictions, not perceptible in views of the embryo as a
transparent object, mark off three vesicles in the brain. These
vesicles are known as the fore, mid, and hind brain. From the
fore-brain there is an outgrowth on each side, the first rudi-
ment of the optic vesicle (oj).).

The mesoblast on each side of the body is divided into
a series of segments, known as protovertebrse or muscle-plates,
the first of which lies a little behind the head. The mesoblast
of the tail has not as yet undergone this segmentation. There
are present in all seventeen segments. These first appeared
at a much earlier date, but were not visible owing to the
opacity of the embryo.

Another structure which became developed in even a
younger embryo than C is now for the first time visible in
the living embryo. This is the notochord : it extends from

DEVELOPMENT OF ELASMOBRANCH FISHES. to

almost the extreme posterior to the anterior end of the embryo.
It lies between the ventral wall of the spinal canal and the
dorsal wall of the intestine ; and round its posterior end
these two w^alls become continuous with each other {vide fig.).
Anteriorly the termination of the notochord cannot be seen,
it can only be traced into a mass of mesoblast at the base
of the brain, which there separates the epiblast from the hypo-
blast. The alimentary canal (cd.) is completely closed anteriorly
and posteriorly, though still wddely open to the yolk-sac in the
middle part of its course. In the region of the head it exhi-
bits on each side a slight bulging outwards, the rudiment of
the first visceral cleft. This is represented in the figure by two
lines (i V. c). The visceral clefts at this stage consist of
a pair of simple diverticula from the alimentary canal, and
there is no communication betw^een the throat and the ex-
terior.

H.

The present embryo is far larger than the last, but it has
not been possible to represent this increase in size in the
drawings. Accompanying this increase in size, the folding off
of the embryo from the yolk has considerably progressed, and
the stalk which unites the embryo with the yolk is propor-
tionately narrower and longer than before.

The brain is now very distinctly divided into the three
lobes, whose rudiments appeared during the last stage. From
the foremost of these, the optic vesicles now present themselves
as well-marked lateral outgrowths, towards which there appears
a growing in, or involution, from the external skin (op.) to form
the lens. The opening of this involution is represented by the
dark spot in the centre.

A fresh organ of sense, the auditory sac, now^ for the
first time becomes visible as a shallow pit in the external skin
on each side of the hind-brain (au. v.). The epiblast w^hich is
involuted to form this pit becomes much thickened, and
thereby the opacity, indicated in the figure, is produced.

The muscle-plates have greatly increased in number by
the formation of fresh segments in the tail. Thirty-eight of
them were present in the embryo figured. The mesoblast at
the base of the brain has increased in quantity, and there is

76 GENERAL FEATURES.

still a certain mass of unsegmented mesoblast whicli forms
the tail-swellings. The first rudiment of the heart becomes
visible during this stage as a cavity between the mesoblast
of the splanchnopleure and the hypoblast (ht.).

The fore and hind guts are now longer than they were. A
slight pushing in from the exterior to form the mouth has
appeared (m.), and an indication of the future position of the
anus is afforded by a slight diverticulum of the hind gut
towards the exterior some little distance from the posterior
end of the embryo (an.). The portion of the alimxcntary canal
behind this point, though at this stage large, and even dilated
into a vesicle at its posterior end (alv.), becomes eventually
completely atrophied. In the region of the throat the rudi-
ment of a second visceral cleft has appeared behind the first ;
neither of them are as yet open to the exterior. The number
of visceral clefts present in any given Pristiurus embryo affords
a very easy and simple way of determining its age.

I.

A great increase in size is again to be noticed in the
embryo, but, as in the case of the last embryo, it has not
been possible to represent this in the figure. The stalk con-
necting the embryo with the yolk has become narrower and
more elongated, and the tail region of the embryo propor-
tionately far longer than in the last stage. During this stage
the first spontaneous movements of the embryo take place,
and consist in somewhat rapid excursions of the embryo from
side to side, produced by a serpentine motion of the body.

The cranial flexure, which commenced in stage G, has
now become very evident, and the mid-brain^ begins to project
in the same manner as in the embryo fowl on the third day,
and will soon form the anterior termination of the long axis
of the embryo. The fore-brain has increased in size and dis-
tinctness, and the anterior part of it may now be looked on
as the impaired rudiment of the cerebral hemispheres.

Further growths have taken place in the organs of sense,

1 The part of the brain which I have here called mid-brain, and which
unquestionably corresponds to the part called mid-brain in the embryos of higher
vertebrates, becomes in the adult what Miklucho-IMaclay and Gegenbaur called
the vesicle of the thii'd ventricle or thalamencephalon. I shall always speak
of it as the mid-brain.

DEYELOPMEXT OF ELAS^IOBP.AXCH FISHES. 77

especially in the eye, in which the involution for the lens
has made considerable progress. The number of the muscle-
plates has again increased, but there is still a region of un-
segmented mesoblast in the taiL The thickened portions of
mesoblast which caused the tail-swellings are still to be seen
and would seem to act as the reserve from which is drawn the
matter for the rapid growth of the tail, which occurs soon after
this. The mass of the mesoblast at the base of the brain has
asfain increased. No fresh features of interest are to be seen in
the notochord. The heart is now much more conspicuous than
before, and its commencing flexure is very apparent. It now
beats actively. The hind gut especially is much longer than
in the last specimen ; and the point where the anus Avill
appear is very easily detected by the bulging out of the gut
towards the external skin at that point {an.). The alimentary
vesicle, first observable during^ the last stasre, is now a more
conspicuous organ {al.v.). Three visceral clefts, none of which
are as yet open to the exterior, may now be seen.

K.

The figures G, H, I are representations of living and trans-
parent embryos, but the remainder of the figures are drawings
of opaque embryos which were hardened in chromic acid.

The stalk connecting the embryo with the yolk is now, com-
paratively speaking, quite narrow, and is of sufficient length to
permit the embryo to execute considerable movements.

The tail has grown immensely, but is still dilated terminally.
This terminal dilatation is mainly due to the alimentary vesicle,
but the tract of gut connecting this with the gut in front 'of the
anus is now a solid rod of cells and very soon becomes com-
pletely atrophied.

The two pairs of limbs have appeared as elongated ridges
of epiblast. The anterior pair is situated just at the front
end of the umbilical stalk ; and the posterior pair, wdiich is
the more conspicuous of the two, is situated some little distance
behind the stalk.

The cranial flexure has greatly increased, and the angle
between the long axis of the front part of the head and of the
body is less than a right angle. The conspicuous mid-brain

78 GENERAL FEATURES.

forms the anterior termination of the long axis of the body.
The thin roof of the fourth ventricle may in the figure be
noticed behind the mid-brain. The auditory sac is nearly
closed and its opening is not shown in the figure. In the eye
the lens is completely formed.

Owing to the opacity of the embryo, the muscle-plates are
only indistinctly indicated, and no other features of the meso-
blast are to be seen.

The mouth is now a deep pit, whose borders are almost com-
pletely formed by the thickening in front of the first visceral cleft,
which may be called the first visceral arch or mandibular arch.

Four visceral clefts are now visible, all of which are open to
the exterior, but in a transparent embryo one more, not open to
the exterior, w^ould have been visible behind the last of these.

L.

This embryo is considerably older than the one last de-
scribed, but growth is not quite so rapid as might be gathered
from the fact that L is nearly twice as long as K, since the two
embryos belong to different genera ; and the Scyllium embryos,
of which L is an example, are larger than Pristiurus embryos.
The umbilical stalk is now quite a narrow elongated structure,
whose subsequent external changes are very unimportant, and
consist for the most part merely in an increase in its length.

The tail has again grown greatly in length, and its terminal
dilatation together with the alimentary vesicle contained in it,
have both completely vanished. A dorsal and ventral fin are
now clearly visible ; they are continuous throughout their whole
length. The limbs have grown and are more easily seen than
in the previous stage.

Great changes have been effected in the head, resulting in a
diminution of the cranial flexure. This diminution is never-
theless apparent rather than real, and is chiefly due to the rapid
growth of the rudiment of the cerebral hemispheres. The three
main divisions of the brain may still be clearly seen from the
surface. Posteriorly is situated the hind-brain, now consisting
of the medulla oblongata and cerebellum. At the anterior
part of the medulla is to be seen the thin roof of the fourth
ventricle, and anteriorly to this again the roof becomes thickened

DEVELOPMENT OF ELASMOBRAXCH FISHES. 79

to form the rudiment of the cerebellum. In front of the hind-
brain lies the mid-brain, the roof of which is formed by the
optic lobes, which are still situated at the front end of the long
axis of the embr3^o.

Beyond the mid-brain is placed the fore-brain, whose growth
is rapidly rendering the cranial flexure imperceptible.

The rudiments of the nasal sacs are now clearly visible as a
pair of small pits. The pits are widely open to the exterior,
and are situated one on each side, near the front end of the
cerebral hemispheres. Five visceral clefts are open to the
exterior, and in them the external gills have commenced to
appear (L').

The first cleft is no longer similar to the rest, but has com-
menced to be metamorphosed into the spiracle.

Accompanying the change in position of the first cleft, the
mandibular arch has begun to bend round and enclose the front
as well as the side of the mouth. By this change in the mandi-
bular arch the mouth becomes narrowed in an antero-posterior
direction.

M.

Of this embryo the head alone has been represented. Two
views of it are given, one (M) from the side and the other (M')
from the under surface. The grow^th of the front part of the
head has considerably diminished the prominence of the cranial
flexure. The full complement of visceral clefts is now present —
six in all. But the first has already atrophied considerably, and
may easily be recognised as the Sj^iracle. In Scyllium, there
are present at no period more than six visceral clefts. The first
visceral arch on each side has become bent still further round,
to form the front border of the mouth. The opening of the
mouth has in consequence become still more narrowed in an
antero-posterior direction. The width of the mouth in this
direction, serves for the present and for some of the subsequent
stages as a very convenient indication of age.

N.
The limbs, or paired fins, have now acquired the general
features and form which they possess in the adult.

The unpaired fins have now also become divided in a

80 GENERAL FEATURES.

manner not only cliaraoteristic of the Elasmobranclis but even
of the genus Scjllium.

There is a tail fin, an anal fin and two dorsal fins, both
the latter being situated behind the posterior paired fins.

In the head may be noticed a continuation of the rapid
growth of the anterior part.

The mouth has become far more narrow and slit-like ; and
with many other of the organs of the period commences to
approach the form of the adult.

The present and the three preceding stages show the gradual
changes by which the first visceral arch becomes converted into
the rudiments of the upper and of the lower jaw. The fact
of the conversion was first made known throucrh the investi-
gations of Messrs Parker and Gegenbaur.

O.

In this stage the embryo is very rapidly approaching the
form of the adult.

This is especially noticeable in the fins, which project in a
manner quite characteristic of the adult fish. The mouth is
slit-like, and the openings of the nasal sacs no longer retain
their primitive circular outline. The external gills project
from all the gill-slits including the spiracle.

P.

The head is rapidly elongating by the growth of the snout,
and the divisions of the brain can no longer be seen with
distinctness from the exterior, and, with the exception of the
head and of the external gills, the embryo almost completely
resembles the adult.

Q.

The snout has grown to such an extent, that the head has
nearly acquired its adult shape. In the form of its mouth
the embryo now quite resembles the adult fish.

This part of the subject may be conveniently supplemented
by a short description of the manner in which the blastoderm
encloses the yolk. It has been already mentioned that the
growth of the blastoderm is not uniform. The part of it in the
immediate neighbourhood of the embryo remains compara-

DEVELOPMENT OF ELASMOBRANCH FISHES. 81

lively stationary, while the growth elsewhere is very rapid.
From this it results that that part of the edge of the blas-
toderm where the embrj^o is attached forms a bay in the other-
wise regular outline of the edge of the blastoderm. By the
time that one-half of the yolk is enclosed the bay is a very con-
spicuous feature (PL viii. fig. 1). In this figure bl. points to
the blastoderm, and yk. to the part of the yolk not yet enclosed
by the blastoderm.

Shortly subsequent to this the bay becomes obliterated by
its two sides coming together and coalescing, and the embryo
ceases to lie at the edge of the yolk.

This stage is represented on PI. viil. fig. 2. In this figure
there is only a small patch of yolk not yet enclosed {yk)^
which is situated at some little distance behind the embryo.
Throughout all this period the edge of the blastoderm has
remained thickened, a feature which persists till the complete
investment of the yolk, which takes places shortly after the stage
last figured. In this thickened edge a circular vein arises, which
brings back the blood from the yolk-sac to the embryo. The
opening in the blastoderm (PL Vlll. fig. 2 yk.), exposing the
portion of the yolk not yet enclosed, may be conveniently called
the blastopore, according to Professor Lankester's nomenclature.

The interesting feature which characterizes the blastopore
in Elasmobranchs is the fact of its not corresponding in position
with the opening of the anus of Rusconi. We thus have in
Elasmobranchs two structures, each of which corresponds in part
with the single structure in Amphioxus which may be called
either blastopore or anus of Rusconi, which yet do not in Elas-
mobranchs coincide in position. It is the blastopore of Elas-
mobranchs which has undergone a change of position, owing
to the unequal growth of the blastoderm ; while the anus of
Rusconi retains its normal situation. In Osseous Fishes the
blastopore undergoes a similar change of position. The possi-
bility of a change in position of this structure is peculiarly
interesting, in that it possibly serves to explain how the blasto-
pore of different animals corresponds in different cases with the
anus or the mouth, and has not always a fixed situation \

1 For a fuller discussion of this question vide Self ,' A comparison of the early
stages of development in vertebrates.' Quart. Journ. of Micr. Science, July, 187 j.

B. 6

CHAPTER V.
Stacres B to G.

"O-

The present cha23ter deals with tlie history of the development
of the Elasmobranch embryo from the period when the medul-
lary groove first arises till that in which it becomes completely
closed, and converted into the medullary canal. The majority
of the observations recorded were made on Pristiurus embryos,
a few on embryos of Torpedo. Where nothing is said to the
contrary the statements made apply to the embryos of Pristiurus
only.

The general external features for this period have already
been given in sufficient detail in the last chapter ; and I
proceed at once to describe consecutively the history of the
three layers.

General features of the Epihlast.

At the commencement of this period, during the stage inter-
mediate between B and C, the epiblast is composed of a single
layer of cells. (PI. ix. fig. 1.)

These are very much elongated in the region of the embryo,
but flattened in other parts of the blastoderm. Throughout
they contain numerous yolk spherules.

In a Torpedo embryo of this age (as determined by the con-
dition of the notochord) the epiblast presents a very different
structure. It is composed of small spindle-shaped cells several
rows deep. The nuclei of these are very large in proportion to
the cells containing them, and the yolk spherules are far less
numerous than in the cells of corresponding Pristiurus embryos.

During stage C the condition of the epiblast does not under-
go any important change, with the exception of the layer be-
coming much thickened, and its cells two or three deep in the
anterior parts of the embryo, (PI, ix. fig. 2.)

DEVELOPMENT OF ELASMOBRANCH FISHES. 83

In the succeeding stages that part of the epiblast, which will
form the spinal cord, gradually becomes two or three cells deep.
This change is effected by a decrease in the length of the cells
as compared with the thickness of the layer. In the earlier
stages the cells are wedge-shaped with an alternate arrange-
ment, so that a decrement in the length of the cells at once
causes the epiblast to be composed of two rows of interlocking
cells.

The lateral parts of the epiblast which form the epidermis of
the embryo are modified in quite a different manner to the
nervous parts of the layer, becoming very much diminished in
thickness and composed of a single row of flattened cells,
PL IX. fig. 3.

Till the end of stage F, the epiblast cells and indeed all the
cells of the blastoderm retain their yolk spherules, but the epi-
blast begins to lose them and consequently to become transpa-
rent in staofe G.

Medullary Groove.

During stage B the medullary groove is shallow posteriorly,
deeper in the middle part, and flattened out again at the
extreme anterior end of the embryo. PL v. fig. 10, ah c.

A similar condition obtains in the stage between B and C,
but the canal has now in part become deeper. Anteriorly no
trace of it is to be seen. In stage C it exhibits the same general
features (Plate ix. fig. 2 a 2 6 2 c).

By stage D we find important modifications of the canal.

It is still shallow behind and deep in the dorsal region,
Plate IX. fig. Sd Se 3/; but the anterior flattened area in the
last stage has grown into a round flat plate which may be called
the cephalic plate, Plate VI. D and Plate ix. fig. Sa Sb 3 c.
This plate becomes converted into the brain. Its size and
form give it a peculiar appearance, but the most remarkable
feature about it is the ventral curvature of its edges. Its edges
do not, as might be expected, bend dorsalwards towards each
other, but become sharply bent in a ventral direction. This
feature is for the first time apparent at this stage, but becomes
more conspicuous during the succeeding ones, and attains its

G— 2

84 MEDULLARY GROOVE.

maximum in stage F (Plate ix. fig. 5), in which it mightal most
be supposed that the edges of the cephalic plate were about to
grow downwards and meet on the ventral side of the embryo.

In the stages subsequent to D the posterior part of the
canal deepens much more rapidly than the rest (vide PL IX.
fig. 4, taken from the posterior end of an embryo but slightly
younger than F), and the medullary folds unite and convert
the posterior end of the medullary groove into a closed canal
(PL VI. fig. F), while the groove is still* widely open else-
where*. The medullary canal does not end blindly behind, but
simply forms a tube not closed at either extremity. The im-
portance of this fact will appear later.

In a stage but slightly subsequent to F nearly the whole of
the medullary canal becomes formed. Tliis occurs in the usual
way by the junction and coalescence of the medullary folds. In
the course of the closing of the medullary groove the edges of
the cephalic plate lose their ventral curvature and become bent
up in the normal manner (vide PL ix. fig. 6, a section taken
through the posterior part of the cephalic plate), and the en-
larged plate merely serves to enclose a dilated cephalic portion
of the medullary canal. The closing of the medullary canal
takes place earlier in the head and neck than in the back. The
anterior end of the canal becomes closed and does not remain
open like the posterior end.

Elasmobranch embryos resemble those of the Sturgeon
(Acipenser) and the Amphibians in the possession of a spatula-
like cephalic expansion: but so far as I am aware a ventral
flexure in the medullary plates of the head has not been ob-
served in other groups.

The medullary canal in Elasmobranchs is formed precisely on
the type so well recognised for all groups of vertebrates with
the exception of the Osseous Fishes. The only feature in any
respect peculiar to these fishes is the closing of their medullary
canal first commencing behind, and then at a second point
in the cervical region. In those vertebrates in which the
medullary folds do not unite at approximately the same time

^ Vide Preliminary Account, etc. Q. Jl. Micros. Science, Oct. 1874, PI. xiv.
8 a. This and the otlier section from the same embryo (stage F) may be
referred to. I have not thouglit it worth v/hilc repeating them here.

DEVELOPMENT OF ELASMOBRANCH FISHES. 85

tlirougliout their length, tliey appear usually to do so first in
the rea'ion of the neck.

'o

Mesohlast.

The separation from tlie hypoblast of two lateral masses of
mesoblast has already been described. Till the close of stage C
the mesoblast retains its primitive bilateral condition unaltered.
Throughout the whole length of the embryo, with the exception
of the extreme front part, there are present two plates of rounded
mesoblast cells, one on each side of the medullary groove. These
plates are in very close contact with the hypoblast, and also
follow with fair accuracy the outline of the epiblast. This
relation of the mesoblast plates to the epiblast must not how-
ever be supposed to indicate that the medullary groove is due
to growth in the mesoblast: a view which is absolutely nega-
tived by the manner of formation of the medullary groove in
the head. Anteriorly the mesoblast plates thin out and com-
pletely vanish.

In stage D, the plates of mesoblast in the trunk undergo
important changes. The cells composing them become arranged
in two layers (Plate IX. fig. 3), a splanchnic layer adjoining the
hypoblast {sp), and a somatic layer adjoining the epiblast^ (so).
Although these two layers are distinctly formed, they do not
become separated at this stage in the region of the trunk, and
in the trunk no true body-cavity is formed.

By stage D the plates of mesoblast have ceased to be quite
isolated, and are connected wdth the lower layer cells of the
general blastoderm.

Moreover the lower layer cells outside the embryo now
exhibit distinct traces of a separation into two layers, one con-
tinuous with the hypoblast, the other with the mesoblast. Both
layers are composed of very flattened cells, and the mesoblast
layer is often more than one cell deep, and sometimes exhibits
a mesh-like arrangement of its elements.

Coincidentally with the appearance of a differentiation into
a somatic and splanchnic layer the mesoblast plates become

1 I under-estimated tlie distinctness of this formation in my earlier paper,
loc. cit., although I recognized the fact that the mesohlast cells hecame arranged
in two distinct layers.

86 MESOBLAST.

partially split by a series of transverse lines of division into pro-
tovertebrae. Only the proximal regions of the plates become
split in this way, while their peripheral parts remain quite intact
As a result of this each plate becomes divided into a proximal
portion adjoining the medullary canal, which is divided into
protO'Vertebrce, and may be called the vertebral plate, and. a
peripheral portion not so divided, which may be called the
lateral jylate. These two parts are at this stage quite continu-
ous with each other; and, as will be seen in the sequel, the
body-cavity originally extends uninterruptedly to the summit
of the vertebral plates.

By stage D at the least ten protovertebrse have appeared.

In Torpedo the mesoblast commences to be divided into
two layers much earlier than in Pristiurus ; and even before
stage C this division is more or less clearly marked.

In the head and tail the condition of the mesoblast is by no
means the same as in the body.

In the tail the plates of mesoblast become considerably
thickened and give rise to two projections, one on each side,
which have already been alluded to as caudal or tail-swellings ;
vide PI. VI. figs. D, F, and PL ix. fig. 3 /and fig. 4 ts.

These masses of mesoblast are neither divided into proto-
vertebrse, nor do they exhibit any trace of a commencing dif-
ferentiation into somatopleure and splanchnopleure.

In the head, so far as I have yet been able to observe, the
mesoblastic plates do not at this stage become divided into
protovertebrse. The other changes exhibited in the cephalic
region are of interest, mainly from the fact that here appears a
cavity in the mesoblast directly continuous with the body-cavity
(when that cavity becomes formed), bat which appears at a
very much earlier date than the body-cavity. This cavity can
only be looked on in the light of a direct continuation of the
body or peritoneal cavity into the head. Theoretical con-
siderations with reference to it I propose reserving till I have
described the changes which it undergoes in the subsequent
periods.

PI. IX. figures 3a, 36 and 3 c exhibit very well the condition
of the mesoblast in the head at this period. In fig. 3 c, a sec-
tion taken through the back part of the head. The mesoblast

DEVELOPMENT OF ELASMOBRAXCH FISHES. 87

plates have nearly the same form as in the sections im-
mediately behind. The ventral continuation of the mesoblast
formed by the lateral plate has, however, become much thinner,
and the dorsal or vertebral portion has acquired a more tri-
angular form than in the sections through the trunk (fig. 3cZ
and Se).

In the section (fig. Sh) in front of this the ventral portion of
the plate is no longer present, and only that i3art exists which
corresponds with the vertebral division of the primitive plate
of mesoblast.

In this a distinct cavity, forming part of the body cavity,
has appeared.

In a still anterior section, fig. 3 a, no cavity is any longer
present in the mesoblast; whilst in sections taken from the
foremost part of the head no mesoblast is to be seen (vide
PL IX. fig. 5, taken from the front part of the head of the
embryo represented in PI. VI. fig. f).

A continuation of the body-cavity into the head has already
been described by Oellacher^ for the Trout : but he believes
that the cavity in this part is solely related to the formation
of the pericardial space.

The condition of the mesoblast undergoes no important
change till the end of the period treated of in this chapter.
The masses of mesoblast which form the tail-swellings become
more conspicuous (PI. ix. fig. 4) ; and indeed their convexity
is so great that the space between them has the appear-
ance of a median groove, even after the closure of the neural
canal in the caudal region.

In embryos of stage G, which may be considered to belong
to the close of this period, eighteen protovertebrse are present
both in Pristiurus and Torj^edo embryos.

The Alimentary Canal.

The alimentary canal at the commencement of this period
(stage B) forms a space between the embryo and the yolk,
ending blindly in front, but opening posteriorly by a widish

1 Zeitschrift f. iciss. Zoologie, 1873.

88 ALIMENTARY CANAL.

slit-like aperture, wliich corresponds to the anus of Rusconi
(PL IV. fig. 7).

The cavity anteriorly has a more or less definite form,
having lateral walls, as well as a roof and floor (PI. V. fig.
106 and 10c). Posteriorly it is not nearly so definitely en-
closed (PL V. fig. 10a). The ventral wall of the cavity is
formed by yolk. But even in stage B there are beginnings
of a cellular ventral wall derived from an ingrowth of cells from
the two sides.

By stage C considerable progress has been made in the
formation of the alimentary canal. Posteriorly it is as flattened
and indefinite as during stage B (PL ix. figs. 26 and 2c).
But in the anterior part of the embryo the cavity becomes
much deeper and narrower, and a floor of cells begins to be
formed for it (PL ix. fig. 2); and, finally3 in front, it forms a
definite space completely closed in on all sides by cells (PL
IX. fig. 2a). Two distinct processes are concerned in effect-
ing these changes in the condition of the alimentary cavity.
One of these is a process of folding off the embryo from the
blastoderm. The other is a simple growth of cells inde-
pendent of any folding. To the first of these processes the
depth and narrowness of the alimentary cavity is due; the
second is concerned in forming its ventral walL The combi-
nation of the two processes produces the peculiar triangular
section which characterises the anterior closed end of the
alimentary cavity at this stage. The process of the folding off
of the embryo from the blastoderm resembles exactly the
similar process in the embryo bird. The fold by which the
constricting off of the embryo is effected is a perfectly con-
tinuous one, but may be conveniently spoken of as composed
of a head-fold and two lateral folds.

Of far greater interest than the nature of these folds is the
formation of the ventral wall of the alimentary canal. This, as
has been said, is effected by a growth of cells from the two
sides to the middle line (PL [X. fig. 2). The cells for this
are however not derived from pre-existing hypoblast cells, but
are formed spontaneously around nuclei of the yolk. This fact
can be determined in a large number of sections, and is fairly
well shcAvn in PL ix. fig. 2?ia. The cells are formed in the

DEVELOPMENT OF ELASMOBRANCH FISHES. 89

yolk, as has been already mentioned, by a simple aggregation
of protoplasm around pre-existing nuclei.

The cells being described are in most cases formed close to
the pre-existing hypoblast cells, but often require to undergo a
coDsiderable change of position before attaining their final
situation in the wall of the alimentary canal.

I have already alluded to this feature in the formation of
the ventral wall of the alimentary cavity. Its interest, as bear-
ing on the homology of the yolk, is considerable, owing to the
fact that the so-called yolk -cells of Amphibians play a similar
part in supplying the ventral epithelium of the alimentary
cavity, as do the cells derived from the yolk in Elasmobranchs.

The fact of this feature being common to the yolk-cells of
Amphibians and the yolk of Elasmobranchs, supplies a strong
argument in favour of the homology of the yolk-cells in the one
case with the yolk in the other \

1 Nearly simultaneously with Chapter III. of the present monograph on the
Development of Elasmobranchs^ which dealt in a fairly complete manner with
the genesis of cells outside the bla?toderm, theie appeared two important papers
dealing with the same subject for Teleostei. One of these, by Professor Bambeke,
Embryologie des Poissons Osseux, Mem. Cour. Acad. Belgique, 1875, which
appeared some little time before my paper, and a second by Dr Klein, Quart. Jour,
of Micr. Sci. April, 1876. In both of these papers a development of nuclei and
of cells is described as occm-ring outside the blastoderm in a manner which
accords fairly well with my own observations.

The conclusions of both these investigators differ however from my own.
They regard the finely granular matter, in which the nuclei appear, as pertain-
ing to the blastoderm, and morphologically quite distinct from the yolk. From
their observations we can clearly recognize that the material in which the nuclei
appear is far more sharply separated off from the yolk in Osseous Fish than in
Elasmobranchs, and this sharp separation forms the main argument for the
view of these authors. Dr Klein admits, however, that this granular matter
(which he caUs parablast) graduates in the typical food-yolk, though he explains
this by supposing that the parablast takes up part of the yolk for the purpose of
growth.

It is clear that the argument from a sharp separation of yolk and parablast
cannot have much importance, when it is admitted (1) that in Osseous Fish there
is a gradation between the two substances, while (2) in Elasmobranchs the one
merges slowly and insensibly into the other.

The only other argument used by these authors is stated by Dr Klein in the
following way. " The fact that the parablast has, at the outset, been forming
one unit ^ith what represents the archiblast, and, ^chile increasing has spread
i.e. grown over the yolk which underlies the segmentation-cavity, is, I think,
the most absolute proof that the yolk is as much different from the parablast as
it is from the archiblast." This argument to me merely demonstrates that cer-
tain of the nutritive elements of the yolk become in the course of development
converted into protoplasm, a phenomenon which must necessarily be supposed
to take place on my own as well as on Dr Klein's view of the nature of the yolk.
My own views on the subject have already been fully stated. I regard the so-
called yolk as composed of a larger or smaller amount of food-material imbedded

90 ALIMENTARY CANAL.

The history of the alimentary canal during the remainder
of this period may be told briefly.

The folding off and closing of the alimentary canal in the
anterior part of the body proceeds rapidly, and by stage D not
only is a considerable tract of alimentary canal formed, but
a great part of the head is completely folded off from the yolk
(PI. IX. fig. 3 a). By stage F a still greater part is folded off.
The posterior part of the alimentary canal retains for a long
period its primitive condition. It is not until stage F that it
beo^ins to be folded off behind. After the foldino^ has once com-
menced it proceeds with great rapidity, and before stage G,
the "hinder part of the alimentary canal becomes completely
closed in.

The folding in of the gut is produced by two lateral folds,
and the gut is not closed posteriorly.

It may be remembered that the neural canal also remained
open behind. Thus both the neural and alimentary canals are
open behind ; and, since both of them extend to the posterior
end of the body, they meet there, their walls coalesce, and a
direct communication from the neural to the alimentary canal

in protoplasm, and the meroLlastic ovnm as a body constituted of the same es-
sential parts as a holoblastic ovum, though divided into regions which differ in
the proportion of protoplasm they contain. J do not proi:)Ose to repeat the
positive arguments used by me in favour of this view, but content myself with
alluding to the protoplasmic network found by Schultz and myself extending
through the whole yolk, and to the similar network described by Bambeke as being
present in the eggs of Osseous Fish after deposition but before impregnation.
The existence of these networks is to me a conclusive proof of the correctness of
my views. I admit that in Teleostei the ' parablast ' contains more protoplasm
than the homologous material in the Elasmobranch ovum, while it is probable
that after impregnation the true yolk of Teleostei contains little or no proto-
plasm ; but these facts do not appear to me to militate against my views.

I agree with Prof. Bambeke in regarding the cells derived from the sub-
germinal matter as homologous with the so-called yolk-cells of the Amphibian
embryo.

I have recently, in some of the later stages of development, met with very
peculiar nuclei of the yolk immediately beneath the blastoderm at some little
distance from the embryo, PL ix. fig. 8. They were situated not in finely sub-
germinal matter, but amongst large yolk spherules. They were very large,
and presented still more peculiar forms than those already described by me,
being produced into numerous long filiform processes. The processes from the
various nuclei were sometimes united together, forming a regular network of
nuclei quite unlike anything that I have previously seen described.

The sub-germinal matter, in which the nuclei are usually formed, becomes
during the later stages of development far richer in protoplasm than during the
earlier. It continually arises at fresh points, and often attains to considerable
dimensions, no doubt by feeding on yolk-spherules. Its development appears to
be determined by the necessities of growth in the blastoderm or embryo.

DEVELOPMENT OF ELASMOBRAXCH FISHES. 91

is instituted. The process may be described in another way
by saying that the medullary folds are continuous round
the end of the tail with the lateral walls of the alimentary
canal ; so that, when the medullary folds unite to form a canal,
this canal becomes continuous with the alimentary canal, which
is closed in at the same time. In whatever way this arrange-
ment is produced, the result of it is that it becomes possible
to pass in a continuously closed passage along the neural canal
round the end of the tail and into the alimentary canal. A
longitudinal section shewing this feature is represented on
Plate IX. fig. 7.

This communication between the neural and alimentary
canals, which is coupled, as will be seen in the sequel, with the
atrophy of a posterior segment of the alimentary canal, is a
feature of great interest which ought to throw considerable
light upon the meaning of the neural canal. So far as I know,
no suggestion as to the origin of it has yet been made. It
is by no means confiaed to Elasmobranchs, but is present in
all the vertebrates whose embryos are situated at the centre
and not at the periphery of the blastoderm. It has been
described by Goette^ in Amphibians and by Kowalevsky, Ows-
jannikow and Wagner'^ in the Sturgeon (Acipenser). The same
arrangement is also stated by Kowalevsky^ to exist in Osseous
Fishes and Amphioxus. The same investigator has shewn that
the alimentary and neural canals communicate in larval Asci-
dians, and we may feel almost sure that they do so in the
Marsipobranchii.

The Reptilia, Aves, and Mammalia have usually been dis-
tinguished from other vertebrates by the possession of a well-
developed allantois and amnion. I think that we may further
say that the lower vertebrates, Pisces and Amphibia, are to
be distinguished from the three above-mentioned groups of
higher vertebrates, by the positive embryonic character that
their neural and alimentary canals at first communicate pos-

1 EiitwicJclungsgeschichte der JJiike.

2 Melanges Biologiques de VAcademie Petersbourg, Tome vii.

3 Archiv f. mikros. Anat. Vol. vii. p. 114. In the passage on this point
Kowalevsky states that in Elasmobranchs the neural and alimentary canals
communicate. This I believe to be the first notice published of this peculiar
arranpremeut.

92 THE NOTOCHORD.

teriorly. The presence or absence of this arrangement depends
on the different positions of the embryo in the blastoderm.
In Reptiles, Birds and Mammals, the embryo occupies a central
position in the blastoderm, and not, as in Pisces and Amphibia,
a peripheral one at its edge. We can, in fact, only compare
the blastoderm of the Bird and the Elasmobranch, by sup-
posing that in the blastoderm of the Bird there has occurred
an abbreviation of the processes, by which the embryo Elasmo-
branch is eventually placed in the centre of the blastoderm : as
a result of this abbreviation the embryo Bird occupies from
the first a central position in the blastoderm\

The peculiar relations of the blastoderm and embr3^o, and
the resulting relations of the neural and alimentary canal,
appear to me to be features of quite as great an importance
for classification as the presence or absence of an amnion
and allantois.

General features of the hypohlast.

There are but few points to be noticed with reference to the
histology of the hypoblast cells. The cells of the dorsal wall of
the alimentary cavity are columnar and form a single row.
Those derived from the yolk to form the ventral wall are at
first roundish, but subsequently assume a more columnar form.

The Notochord.

One of the most interesting features in the Elasmobranch
development is the formation of the notochord from the hypo-

1 Vide Note on p. 68, also p. 81, and PI. viii. Fig. 1 and 2, and Comparison,
&c., Qy. Jour, of Mic7'os. ScL July, 1875, p. 219. These passages give an
account of the cliauge of position of the Elasmobranch embryo, and the Note on
p. 08 contains a speculation about the nature of the primitive streak with its
contained primitive groove. I have suggested that the primitive streak is pro-
bably to be regarded as a rudiment at the position where the edges of the
blastoderm coalesced to give to the embryos of Birds and Mammals the central
position which they occupy.

If my hypothesis should turn out to be correct, various, now unintelligible,
features about the primitive streak would be explained : such as its position
behind the embryo, the fusion of the epiblast and mesoblast in it, the groove it
contains, &c.

The possibility of the primitive streak representing the blastopore, as it in
fact does according to my hypothesis, ought also to throw light on E. Van
lieneden's recent researches on the development of the Mammalian ovum.

In order clearly to understand the view here expressed, the reader ought to
refer to the passages above quoted.

DEVELOPMENT OF ELASMOBRAXCH FISHES. 93

blast. All the steps in ttie process by which this takes place
can be followed with great ease and certainty.

Up to stage B the hypoblast is in contact with the epiblast
immediately below the medullary groove, but exhibits no trace
of a thickening or any other formation at that point.

Between stage B and C the notochord first arises.

In the hindermost sections of this stage the h}^oblast retains
a perfectly normal structure and uniform thickness throughout.
In next few sections, PL ix. fig. 1 c, c/i', a slight thickening is
to be observed in the hypoblast, immediately below the medul-
lary canal. The layer, w^hich elsewhere is composed of a single
row of cells, here becomes two cells deep, but no sign of a
division into two layers exhibited.

In the next few sections the thickening of the hypoblast
becomes much more pronounced ; we have, in fact, a ridge
projecting from the hypoblast towards the epiblast (PL IX.
fig. Ih, cli).

This ridge is pressed firmly against the epiblast, and causes
in it a slight indentation. The hypoblast in the region of the
ridge is formed of two layers of cells, the ridge being entirely
due to the uppermost of the two.

In sections in front of this a cylindrical rod, which can at
once be recognised as the notochord and is continuous with the
ridge just described, begins to be split off from the hypoblast.
It is difficult to say at what point the separation of this rod
from the hypoblast is completed, since all intermediate gra-
dations betw^een complete separation and complete attachment
are to be seen.

Where the separation first appears, a fairly thick bridge of
hypoblast is left connectiug the two lateral halves of the layer,
but anteriorly this bridge becomes excessively delicate and thin
(PL IX. fig. 1 c), and in some cases is barely visible except
with high powers.

From the series of sections represented, it is clear that the
notochord commences to be separated from the hypoblast an-
teriorly, and that the separation gradually extends backwards.

The posterior extremity of the notochord remains for a long
time attached to the hypoblast ; and it is not till the end of the
period treated of in this chapter that it becomes com>pletely free.

94 THE NOTOCHORD.

A slieath is formed around the notocbord, very soon after its
formation, at a stage intermediate between stages C and D.
This sbeath is very delicate, though it stains with both osmic
acid and hsematoxylin. I conclude from its subsequent his-
tory, that it is to be regarded as a product of the cells of the
notocbord, but at the same time it should be stated that it
precisely resembles membrane-like structures, which I have
already described as being probably artificial.

Towards the end of this period the cells of the notocbord
become very much flattened vertically, and cause the well-
known stratified appearance which characterises the notocbord
in lonoitudinal sections. In transverse sections the outlines of

o

the cells of the notocbord appear rounded.

Throughout this period the notocbord cells are filled with
yolk spherules, and near its close small vacuoles make their
appearance in them.

An account of the development of the notocbord, substan-
tially similar to that I have just given, appeared in my prelimi-
nary paper^ on the development of the Elasmobranch fishes.

To the remarks which were there made, I have little to add.
There are two possible views, which can be held with reference
to the development of the notocbord from the hypoblast.

We may suppose that this is the primitive mode of develop-
ment of the notocbord, or we may suppose that the separation
of the notocbord from the hypoblast is due to a secondary
process.

If the latter view is accepted, it will be necessary to main-
tain that the mesoblast becomes separated from the hypoblast
as three separate masses, two lateral, and one median, and that
the latter become separated much later than the two former.

We have, I think, no right to assume the truth of this view
without further proof The general admission of assumptions
of this kind is apt to lead to an injurious form of speculation, in
which every fact presenting a difficulty in the way of some
general theory is explained away by an arbitrary assumption,
while all the facts in favour of it are taken for granted. It is
however clear that no theory can ever be fairly tested so long
as logic of this kind is permitted. If, in the present instance,

1 Loc. cit.

DEVELOPMENT OF ELASMOBRANCH FISHES. 95

the view is adopted that the notochord has in reality a meso-
blastic origin, it will be possible to apply the same view to
every other organ derived from the hypoblast, and to say that
it is really mesoblastic, but has become separated at rather a
late period from the hypoblast.

If, however, we provisionally reject this explanation, and
accept the other alternative, that the notochord is derived from
the h}^oblast, we must be prepared to adopt one of two
views with reference to the development of the notochord in
other vertebrates. We must either suppose that the current
statements as to the development of the notochord in other
vertebrates are inaccurate, or that the notochord has only be-
come secondarily mesoblastic.

The second of these alternatives is open to the same ob-
jections as the view that the notochord has only apparently a
hypoblastic source in Elasmobranchs, and, provisionally at least,
the first of them ought to be accepted. The reasons for ac-
cepting this alternative fall under two heads. In the first
place, the existing accounts and figures of the development of
the notochord exhibit in almost all cases a deficiency of clear-
ness and precision. The exact stage necessary to complete the
series never appears. It cannot, therefore, at present be said
that the existing observations on the development of the noto-
chord afford a strong presumption against its h3"poblastic origin.

In the second place, the remarkable investigations of
Hensen^, on the development of the notochord in Mammalia,
render it very probable that, in this group, the notochord is
developed from the hypoblast.

Hensen finds that in Mammalia, as in Elasmobranchs, the
mesoblast forms two independent lateral masses, one on each
side of the medullary canal.

After the commencing formation of the proto vertebra the
hypoblast becomes considerably thickened beneath the medul-
lary groove; and, though he has not followed out all the steps of
the process by which this thickening is converted into the noto-
chord, yet his observations go very far towards proving that
it does become the notochord.

1 Zeitschrift f. Anat. u. Entwicklungsgeschichte, Vol. i. p. 366.
" ^'Sitz. der Gesell. zu Marhurq, Jan. 1876.

96 THE NOTOCHORD.

Against the observations of Hensen, there ought, however,
to be mentioned those of Lieberklihn^ He believes that the
two lateral masses of mesoblast, described by Hensen (in an
earlier paper than the one quoted), are in reality united by a
delicate layer of cells, and that the notochord is formed from a
thickening of these.

Lieberklihn gives no further statements or figures, and it is
clear that, even if there is present the delicate layer of meso-
blast, which he fancies he has detected, yet this cannot in any
w^ay invalidate such a section as that represented on PI. x.
fig. 40, of Hensen's paper.

In this figure of Hensen's, the hypoblast cells become dis-
tinctly more columnar, and the whole layer much thicker im-
mediately below the medullary canal than elsewhere, and this
independently of any possible layer of mesoblast.

It appears to me reasonable to conclude that Lieberkuhn's
statements do not seriously weaken the certainty of Hensen's
results.

In addition to the observations of Hensen's on Mammalia,
those of Kowalevsky and Kuppfer on Ascidians may fairly be
pointed to as favouring the hypoblastic origin of the notochord.

It is not too much to say that at the present moment the
balance of evidence is in favour of regarding the notochord as
a hypoblastic organ.

This conclusion is, no doubt, rather startling, and difficult
to understand. The only feature of the notochord in its favour
is the fact of its being unsegmented\

Should it eventually turn out that the notochord is de-
veloped in most vertebrates from the mesoblast, and only ex-
ceptionally from the hypoblast, the further question will have
to be settled as to whether it is primitively a hypoblastic or a
mesoblastic organ ; but, from whatever layer it has its source, an
excellent example will be afforded of an organ changing from
the layer in which it was originally developed into another
distinct layer.

^ In my earlier paper I suggested that the endostyle of Ascidians afforded an
instance of a supporting organ heiiig derived from the hypoWast. This parallel
does not. hold since the endor;tyle has been shewn to possess a secretory func-
tion. I'ncver intended (as has been imagined by Profes;;or Todaro) to regard
the endost^'le as the homologue of tlie notochord.

CHAPTER VI.
Development of the Trunk during stages G to K.

By the stage when the external gills have become conspicuous
objects, the rudiments of the greater number of the important
organs of the body are definitely established.

Owing to this fact the first appearance of the external gills
forms a very convenient break in the Elasmobranch develop-
ment; and in the present chapter the history is carried on
to the period of this occurrence.

While the last chapter dealt for the most part with the
formation of the main organic systems from the three embry-
onic layers, the present one has for its subject the gradual
differentiation of these systems into individual organs. In treat-
ing of the development of the separate organs a divergence
from the plan of the last chapter becomes necessary, and the
following arrangement has been substituted for it. First of
all an account is given of the development of the external
epiblast, which is followed by a description of the organs
derived from the mesoblast and of the notochord.

External Epiblast

During stages G to I the epiblast' is formed of a single
layer of flattened cells ; and in this, as in the earlier stages, it
deserves to be especially noticed that the epiblast is never more
than one cell deep, and is therefore incapable of presenting any
differentiation into nervous and epidermic layers. (PI. x. ficr.
1-5).

1 Unless the contrary is stated, the facts recorded in this chapter apply
only to the genera Scyllium and Pristiurus.

98 THE EPIBLAST.

The cells which compose it are flattened and polygonal in
outline, but more or less spindle-shaped in section. They present
a strong contrast to the remaining embryonic cells of the body
in possessing a considerable quantity of clear protoplasm, which
in most other cells is almost entirely absent. Their granular
nucleus is rounded or oval, and typically contains a single
nucleolus. Frequently, however, two nucleoli are present, and
when this is the case an area free from granules is to be seen
around each nucleolus, and a dark line, which could probably
be resolved into granules by the use of a sufficiently high
magnifying power, divides the nucleus into two halves. These
appearances probably indicate that nuclei, in which two nucleoli
are present, are about to divide.

The epiblast cells vary in diameter from "022 to '026 Mm.
and their nuclei from "014 to '018 Mm. They present a fairly
uniform character over the g;reater part of the body. In
Torpedo they present nearly the same characters as in Pristiurus
and Scyllium, but are somewhat more columnar. (PI. X. fig. 7.)

Along the summit of the back from the end of the tail to
the level of the anus, or slightly beyond this, epiblast cells
form a fold — the rudiment of the embryonically imdivided
dorsal fin — and the cells forming this, unlike the general epi-
blast cells, are markedly columnar ; they nevertheless, here as
elsewhere, form but a single layer. (PL x. fig. 3 and 5 df.)
Although at this stage the dorsal fin is not continued as a fold
anteriorly to the level of the anus, yet a columnar thickening
or ridge of epiblast, extending along the median dorsal line
nearly to the level of the heart, forms, a true morphological
prolongation of the fin.

On the ventral side of the tail is present a rudiment of the
ventral unpaired fin, which stops short of the level of the anus,
but, though less prominent, is otherwise quite similar to the
dorsal fin and continuous with it round the end of the tail.
At this stage the mesoblast has no share in forming either fin.

In many sections of the tail there may be seen on each
side two folds of skin, which are very regular, and strongly
simulate the rudimentary fins just described. The cells com-
posing them are, however, not columnar, and the folds them-
selves are merely artificial products due to shrinking.

DEVELOPMENT OF ELASMOBRANCH FISHES. 99

At a stage slightly younger than K an important change
takes place in the epiblast.

From being composed of a single layer of cells it becomes
two cells deep. The two layers appear first of all anteriorly,
and subsequently in the remaining parts of the body. At first,
both layers are formed of flattened cells (PL x. fig. 8, and xi. fig,
9) ; but at a stage slightly subsequent to that dealt with in the
present chapter, the cells of the inner of the two layers become
columnar, and thus are established the two strata always
present in the epidei*mis of adult vertebrates, viz. an outer layer
of flattened cells and an inner one of columnar cells\

The history of the epiblast in Elasmobranchs is interesting,
from the light which it throws upon the meaning of the nervous
and epidermic layers into which the epiblast of Amphibians
and some other Vertebrates is divided. The Amphibians and
Elasmobranchs present the strongest contrast in the develop-
ment of their epiblast, and it is worth while shortly to review
and compare the history of the layer in the two groups.

In Amphibians the epiblast is from the first divided into an
outer stratum formed of a single row of flattened cells, and an
inner stratum composed of several rows of more rounded cells.
These two strata were called by Strieker the nervous and
epidermic layers, and these names have been very generally
adopted.

Both strata have a share in forming the general epiblast, and
though eventually they partially fuse together, there can be
bat little doubt that the horny layer of the adult epiblast,
where such can be distinguished''^, is derived from the epidermic
layer of the embryo, and the mucous layer of the epiblast from
the embryonic nervous layer. Both layers of the epiblast assist
in the formation of the cerebro-spinal nervous system, and there
also at first fuse together^, though the epidermic layer probably
separates itself again, as the central epithelium of the spinal
canal. The lens and auditory sac are derived exclusively from

^ The layers are known as epidermic (liorny) and mncous layers by English
writers, and as Hornschicht and Schleimschicht by the Germans. For their
existence in all Vertebrates, vide Leydig Ueber allgemrhie Bedpchungen der
Ampldhien, p. 20, Bonn, 1876.

■^ Vide Leydig loc. cit.

3 Vide Gotte Entwickhivgagpschichte der Hike.

100 THE EPIBLAST.

the nervous layer of the epidermis, while this layer also has
the greater share in forming the olfactory sac.

In Elasmobranchs the epihlast is at first uniformly composed
of a single row of cells. The part of the layer which will form
the central nervous system next becomes two or three cells
deep, but presents no distinction into two layers ; the remain-
ing portions of the layer remain, as before, one cell deep.
Although the epiblast at first presents this simple structure, it
eventually, as we have seen, becomes divided throughout into
two layers, homologous with the two layers which arise so early
in Amphibians. The outer one of the two forms the homy
layer of the epidermis and the central epithelium of the neural
canal. The inner one, the mucous layer of the epidermis and
the nervous part of the brain and spinal cord. Both layers
apparently enter into the formation of the organs of sense.

While there is no great difficulty in determining the
equivalent parts of the epidermis in Elasmobranchs and Am-
phibians, it still remains an open question in which of these
groups the epiblast retains its primitive condition.

Though it is not easy to bring conclusive proofs on the one
side or the other, the balance of argument appears to me
to be decidedly in favour of regarding the condition of the
epiblast in Elasmobranchs, and most other Vertebrates, as the
primitive one, and its condition in Amphibians as a secondary
one, due to the throwing back of the differentiation of their
epiblast into two layers to a very early period in their develop-
ment.

In favour of this view are the following points ; (1) That
a primitive division of the epiblast into two layers is unknown
in the animal kingdom, except amongst Amphibians and
(?) Osseous Fish. (2) That it appears more likely for a particular
feature of development to be thrown back to an earlier period,
than for such an important feature as a distinction between
two primary layers to be absolutely lost during an early period
of development, and then to re-appear again in later stages.

The fact of the epiblast of the neural canal being divided,
like the remainder of the layer, into nervous and epidermic
parts, cannot, I think, be used as an argument in favour of the
opposite view to that here maintained.

DEVELOPMENT OF ELASMOBRANCH FISHES. 101

It seems probable that the central canal of the nervous
system arose as an involution from the exterior, and therefore
that the epidermis lining it is in reality merely a part of the
external epidermis, and as such is naturally separated from the
true nervous structures adjacent to it\

Leaving the general features of the external skin, I pass
to the special organs derived from it during the stage just
anterior to K.

The unpaired Fins. The unpaired fins have grown consider-
ably, and the epiblast composing them becomes, like the remainder
of the layer, divided into two strata, both however composed of
more or less columnar cells. The ventral fin has now become
more prominent than the dorsal fin; but the latter extends
forward as a fold quite to the anterior part of the body.

The pah^ed Fins. Along each side of the body there appears
during this stage a thickened line of epiblast, which from the
first exhibits two special developments : one of these just in front
of the anus, and a second and better marked one opposite the
front end of the segmental duct. These two special thickenings
are the rudiments of the paired fins, which thus arise as special
developments of a continuous ridge on each side, precisely like
the ridges of epiblast which form the rudiments of the un-
paired fins.

Similar thickenings to those in Elasmobranchs are found at
the ends of the limbs in the embryos of both Birds and Mammals,
in the form of caps of columnar epiblast ^

The ridge, of which the limbs are special developments, is
situated on a level slightly ventral to that of the dorsal aorta,
and extends from just behind the head to the level of the anus.
It is not noticeable in surface views, but appears in sections
as a portion of the epiblast w^here the cells are more columnar
than elsewhere ; precisely resembling in this respect the for-
ward continuation of the dorsal fin. At the present stage the
posterior thickenings of this ridge which forms the abdominal
fins are so slight as to be barely visible, and their real nature
can only be detected by a careful comparison between sections

^ Vide Self, Development of Spinal Nerves in Elasmobranchs. Phil. Transact.
1876.

3 For Birds, vide Elements of Embryology, Foster and Balfoar, pp. 144—145,
and for Mammals, Kolliker Entwicklungsgeschichte, p. 283.

102 THE NATURE OF THE PAIRED FINS.

of this and the succeeclmg stages. The rudiments of the anterior
pair of limbs are more visible than those of the posterior,
though the passage between them and the remainder of the
ridofes is most grradual. Thus at first the rudiments of both the
limbs are nothing more than slight thickenings of the epiblast,
where its cells are more columnar than elsewhere. During
stage K the rudiments of both pairs of limbs, but especially of
the anterior pair, grow considerably, while at the same time
the thickened ridge of epiblast which connects them together
rapidly disappears. The thoracic limbs develope into an elong-
ated projecting fold of epiblast, in every way like the folds
forming the unpaired fins ; while at the same time the cells of
the subjacent mesoblast become closely packed, and form a
slight projection, at the summit of which the fold of the epiblast
is situated (PI. xi. fig. 9). The maximum projection of the thoracic
fin is slightly in advance of the front end of the segmental duct.
The abdominal fins do not, during stage K, develope quite so
fast as the thoracic, and at its close are merely elongated areas
where the epiblast is much thickened, and below which the
mesoblast is slightly condensed. In the succeeding stages
they develope into projecting folds of skin, precisely as do the
thoracic fins.

The features of the development of the limbs just described,
are especially well shewn in Torpedo ; in the embryos of which
the passage from the general linear thickening of epiblast into
the but slightly better marked thickening of the thoracic fin
is very gradual, and the fact of the limb being nothing else than
a special development of the linear lateral thickening is proved
in a most conclusive manner.

If the account just given of the development of the limbs
is an accurate record of what really takes place, it is not possible
to deny that some light is thrown by it upon the first origin of
the vertebrate limbs. The facts can only bear one interpretation,
viz. : that the limbs are the remnants of continuous lateral fins.

The unpaired dorsal fin develops as a continuous thickening,
which then grows up into a projecting fold of columnar cells. The
greater part of this eventually atrophies, but three separate
lobes are left which form the two dorsal fins and the upper lobe
of the caudal fin.

DEVELOPMENT OF ELASMOBRANCH FISHES. 1(3

The development of the limbs is almost identically similar
to that of the dorsal fins. There appears a lateral linear thick-
ening of epiblast, which however does not, like the similar
thickening of the fins, grow into a distinct fold. Its develop-
ment becomes confined to two special points, at each of which
is formed a continuous elongated fold of columnar cells precisely
like the fold of skin forming the dorsal fins. These two folds
form the paired pins. If it be taken into consideration that the
continuous lateral fin, of which the rudiment appears inElasmo-
branchs, does not exist in any adult Vertebrate, and also that a
continuous dorsal fin exists in many Fishes, the small differences
in development between the paired fin and the dorsal fins will
be seen to be exactly those which might have been anticipated
beforehand. Whereas the continuous dorsal fin, which often
persists in adult fishes, attains a considerable development before
vanishing, the originally continuous lateral one has only a
very ephemeral existence.

While the facts of development strongly favour a view which
would regard the limbs as remnants of a primitively continuous
lateral fin, there is nothing in the structure of the limbs of
adult Fishes which is opposed to this view. Externally they
closely resemble the unpaired fins, and both their position and
nervous supply appear clearly to indicate that they do not
belong to one special segment of the body. They appear
rather to be connected with a varying number of segments ; a
fact which would receive a simple explanation on the hypo-
thesis here adopted \

My researches throw no light on the nature of the skeletal
parts of the limb, but the suggestion which has been made by
Giinther^ with reference to the limb of Ceratodus (the most
primitive known), that it is a modification of a series of parallel
rays, would very well suit the view here proposed.

1 For the nervous supply in fishes, vide Stannius Periplier. Kerv. System d.
Fische. In Osseous Fishes he states that the thoracic fin is supplied by branches
from the first three though sometimes from the first four spinal nerves. In
Accipenser there are branches from the first six nerves. In Spinax the limb is
supplied by the rami anteriores of the fomih and succeeding ten spinal nerves.
In the Ptays not only do the sixteen anterior spinal nerves unite to supply the
fin, but in all there are rami anteriores from thkty spiual nerves which pass to
the thoracic limb.

2 Philosophical Trajisactions, 1871.

104 THE MESOBLAST.

Dr Dohrn^ in speaking of the limbs, points out the diffi-
culties in the way of supposing that they can have originated
de novo, and not by the modification of some preexisting organ,
and suggests that the limbs are modified gill-arches; a view
similar to which has been hinted at by Professor Gegenbaur ^

Dr Dohrn has not as yet given the grounds for his determi-
nation, so that any judgment on his views is premature.

None of my observations on Elasmobranchs lends any sup-
port to these views ; but perhaps, while regarding the limbs as
the remains of a continuous fin, it might be permissible to
suppose that the pelvic and thoracic girdles are altered remnants
of the skeletal parts of some of the gill-arches which have
vanished in existing Vertebrates.

The absence of limbs in the Marsipobranchii and Am-
phioxus, for reasons already insisted upon by Dr Dohrn ^,
cannot be used as an argument against limbs having existed in
still more primitive Vertebrates.

Though it does not seem probable that a dorsal and ventral
fin can have existed contemporaneously with lateral fins (at
least not as continuous fins), yet, judging from such forms as
the Rays, there is no reason why small balancing dorsal and
caudal fins should not have coexisted with fully developed
lateral fins.

Mesoblast. G—IC

The mesoblast in Stage F forms two independent lateral
plates, each with a splanchnic and somatic layer, and divided,
as before explained, into a vertebral portion and a parietal
portion. At their peripheral edge these plates are continuous
with the general mesoblastic tissue of the non-embryonic part
of the blastoderm; except in the free parts of the embryo,
where they are necessarily separated from the mesoblast of the
yolk-sac, and form completely independent lateral masses of
cells.

During the stages G and H, the two layers of which the
mesoblast is composed cease to be in contact, and leave be-

1 Ursprung d. WirbeUhiere und Finictionswechsels.
^ Grundrias d. Vergleichenden Anat. p. 494.
■^ Loc. cit.

DEVELOPMENT OF ELASMOBRANCH FISHES. 105

tween them a space which constitutes the commencement of the
body-cavity (PL x. fig. 1). From the very first this cavity is more
or less clearly divided into two distinct parts; one of them in the
vertebral portion of the plates of mesoblast, the other in the
parietal. The cavity in the parietal part of the plates alone
becomes the true body-cavity. It extends uninterruptedly
through the anterior parts of the embryo, but does not appear
in the caudal region, being there indicated only by the presence
of two layers in the mesoblast plates. Though fairly wide
below, it narrows dorsally before becoming continuous with the
cavity in the vertebral plates. The line of junction of the
vertebral and parietal plates is a, little ventral to the dorsal
summit of the alimentary canal (PI. X. fig. 5). Owing to the
fact that the vertebral plates are split up into a series of seg-
ments (protovertebrse), the section of the body-cavity they
enclose is necessarily also divided into a series of segments, one
for each protovertebra.

Thus the whole body- cavity consists of a continuous parietal
space which communicates by a series of apertures with a
number of separate cavities enclosed in the protovertebrse. The
cavity in each of the protovertebrse is formed of a narrowed
dorsal and a dilated ventral segment, the latter on the level of
the dorsal aorta (PL x. fig. 5). Cavities are present in all the
vertebral plates with the exception of a few far back in the tail ;
and exist in part of the caudal region posterior to that in
which a cavity in the parietal plate is present.

ProtovertebrcB. Each protovertebra^ or vertebral segment
of the mesoblast plate forms a flattened rectangular body,
ventrally continuous with the parietal plate of mesoblast. During
stage G the dorsal edge of the protovertebrse is throughout
on about a level with the ventral third of the spinal cord. Each
vertebral plate is composed of two layers, a somatic and a
splanchnic, and encloses the already-mentioned section of the
body cavity. The cells of both layers of the plate are columnar,
and each consists of a very large nucleus, invested by a delicate
layer of protoplasm.

1 No attempt has been made to describe iu detail the dilTerent appearances
presented by the protovertebra in the various parts of the body, but in each
bfcage a protovertebra from the dorsal region is taken as typical.

106 THE PROTOVEKTEBR.E.

Before the end of stage H the inner or splanchnic wall of
the protovertebra loses its simple constitution, owing to the
middle part of it, opposite the dorsal two-thirds of the notochord,
undergoing peculiar changes. These changes are indicated in
transverse sections (PL x. fig. 5 and G mp), by the cells in the
part we are speaking of acquiring a peculiar angular appearance,
and becoming one or two dee]3 ; and the meaning of the changes
is at once shewn by longitudinal horizontal sections. These prove
(PL XI. fig. 10) that the cells in this situation have become elong-
ated in a longitudinal direction, and, in fact, form typical spindle-
shaped embryonic muscle-cells, each with a large nucleus.
Every muscle-cell extends for the whole length of a protover-
tebra, and in the present stage, or at any rate in stage I,
acquires a peculiar granulation, which clearly foreshadows
transverse striation (PL xi. fig. 11 — 18).

Thus by stage H a small portion of the splanchnopleure
which forms the inner layer of each protovertebra, becomes
differentiated into a distinct band of longitudinal striated
muscles ; these almost at once become functional, and produce
the peculiar serpentine movements of the embryo, spoken of in
a previous chapter, p. 76.

It may be well to say at once that these muscles form but a
very small part of the muscles which eventually appear ; which
latter are developed at a very much later period from the
remaining cells of the protovertebrse. The band developed at
this stage appears to be a special formation, which has arisen
through the action of natural selection, to enable the embryo
to meet its respiratory requirements, by continually moving
about, and so subjecting its body to fresh oxydizing influences ;
and as such affords an interesting example of an important
structure acquired during and for embryonic life.

Though the cavities in the protovertebrse are at first per-
fectly continuous with the general body-cavity, of which indeed
they merely form a specialized part, yet by the close of stage H
they begin to be constricted off from the general body-cavity,
and this process is continued rapidly, and completed shortly
after stage I, and considerably before the commencement of stage
K. (PL X. fig. 6 and 8). While this is taking place, part of the
splanchnic layer of each protovertebra, immediately below the

DEVELOPMENT OF ELASMOBRANCH FISHES. 107

muscle-band just described, begins to proliferate, and produce
a number of cells, which at once grow in between the muscles
and the notochord. These cells are very easily seen both in
transverse and lonoitudinal sections, and form the commencinsf
vertebral bodies (PL x. fig. 6, and PL XL fig. 10 and 11 Vr).

At first the vertebral bodies have the same segmentatioa as
the protovertebrae from which they sprang ; that is to say, they
form masses of embryonic cells separated from each other by
narrow slits, continuous with the slits separating the proto-
vertebrse. They have therefore at their first appearance a
segmentation completely different from that which they event-
ually acquire (PL XL fig. 11).

After the separation of the vertebral bodies from the proto-
vertebrse, the remaining parts of the protovertebrae may be
called muscle-plates ; since they become directly converted into
the whole voluntary muscular system of the trunk. At the time
when the cavity of the muscle-plates has become completely
separate from the body-cavity, the muscle-plates themselves
are oblong structures, with two walls enclosing the cavity just
mentioned, in which the original ventral dilatation is still visible.
The outer or somatic wall of the plates retains its previous
simple constitution. The splanchnic wall has however a some-
what complicated structure. It is composed dorsally and ven-
trally of a columnar epithelium, but in its middle portion of
the muscle-cells previously spoken of. Between these and the
central cavity of the plates the epithelium forming the re-
mainder of the layer commences to insert itself; so that be-
tween the first-formed muscle and the cavity of the muscle-
plate there appears a thin layer of cells, not however continuous
throughout.

At the end of the period K the muscle-plates have extended
dorsall}^ two-thirds of the way up the sides of the spinal cord,
and ventrally to the level of the segmental duct. Their edges are
not straight, but are bent into an angular form, with the apex
pointing forwards. Vide PL XL fig. 17 mp.

Before the end of the period a number of connective-tissue
cells make their appearance, and extend upwards from the
dorsal summit of the muscle-plates around the top of the
spinal cord. These cells are at first rounded, but become

108 VERTEBRAL BODIES.

typical branclied connective-tissue cells before tlie close of
the period (PI. x. fig. 7 and 8).

Between stages I and K the bodies of the vertebrse rapidly
increase in size and send prolongations downwards and inwards
to meet below the notochord.

These soon become indistinguishably fused with other cells
which appear in the area between the alimentary cavity and
the notochord, but probably serve alone to form the vertebral
bodies, while the cells adjoining them form the basis for the
connective tissue of the kidneys, &c.

The vertebral bodies also send prolongations dorsalwards
between the sides of the spinal cord and the muscle-plates.
These grow round till they meet above the spinal and enclose
the dorsal nerve-roots. They soon however become fused with
the dorsal prolongations from the muscle-plates, at least so far
as my methods of investigation enable me to determine ; but it
appears to me probable that they in reality remain distinct,
and become converted into the neural arches, while the con-
nective-tissue cells from the muscle-plates form the adjoining
subcutaneous and inter-muscular connective tissue.

All the cells of vertebral rudiments become stellate and
form typical embryonic connective tissue. The rudiments
however still retain their primitive segmentation, corresponding
with that of the muscle-plates, and do not during this period
acquire their secondary segmentation. Their segmentation is
however less clear than it was at an earlier period, and in
the dorsal part of the vertebral rudiments is mainly indicated
by the dorsal nerve-roots, which always pass out in the interval
between two vertebral rudiments. Vide PI. xi. fig. 12 pr.

Intermediate Cell-Mass. At about the period when the
muscle-plates become completely free, a fusion takes place be-
tween the somatopleure and splanchnopleure immediately above
the dorsal extremity of the true body-cavity (PL X. fig. 6).
The cells in the immediate neighbourhood of this fusion form
a special mass, which we may call the intermediate cell-mass —
a name originally used by Waldeyer for the homologous cells
in the Chick. Out of it are developed the urino-genital organs
and the adjoining tissues. At first it forms little more than a
columnar epithelium, but by the close of the period is divided

DEVELOPMENT OF ELASMOBRANCH FISHES. 109

into (1) An epitlieliura on the free surface ; from this are
derived the glandular parts of the kidneys and functional parts
of the genital glands; and (2) a subjacent stroma which forms
the basis for the connective tissue and vascular parts of these
organs.

To the histor}^ of these parts a special section is devoted ;
and I now pass to the description of the mesoblast which lines
the body-cavity and forms the connective tissue of the body-
wall, and the muscular and connective tissue of the wall of the
alimentary canal.

Body- Cavity and parietal plates. By the close of stage H, as
has beeft already mentioned, a cavity is formed between the
somatopleure and splanchnopleure in the anterior part of the
trunk, which rapidly widens during the succeeding stages.
Anteriorly, it invests the heart, which arises during stage G, as
a simple space between the ventral wall of the throat and the
splanchnopleure (PI. x. fig. 4). Posteriorly it ends blindly.

This cavity forms in the region of the heart the rudiment
of the pericardial cavity. The remainder of the cavity forms
the true body-cavity.

Immediately behind the heart the alimentary canal is still
open to the yolk-sac, and here naturally the two lateral halves
of the body-cavity are separated from each other. In the tail
of the embryo no body-cavity has appeared by stage I, although
the parietal plates of mesoblast are distinctly divided into
somatic and splanchnic layers. In the caudal region the lateral
plates of mesoblast of the two sides do not unite ventrally, but
are, on the contrary, quite disconnected. Their ventral edge is
moreover much swollen (PL X. fig. 1). At the caudal swelling
the mesoblast plates cease to be distinctly divided into soaiato-
pleure and splanchnopleure, and more or less fuse with the
hypoblast of the caudal vesicle (PI. x. fig. 2).

Between stages I and K the body-cavity extends backwards
behind the point where the anus is about to appear, though
it never reaches quite to the extreme end of the tail. The
backward extension of the body-cavity, as is primitively
the case everywhere, is formed of two independent lateral
halves (PL xi. fig. 9 a). Anteriorly, opposite the hind end of
the small intestine, these two lateral halves unite ventrallv to

110 THE BODY-CAVITY.

form a single cavity in which hangs the small intestine (PL x.
fig. 8) suspended by a very short mesentery.

The most important change which takes place in the body-
cavity during this period is the formation of a septum which
separates off a pericardial cavity from the true body-cavity.

Immediately in front of the liver the splanchnic and'
somatic w^alls of the body come into very close contact, and I
believe unite over the greater part of their extent. The septum
so formed divides the original body-cavity into an anterior
section or pericardial cavity, and a posterior section or true body-
cavity. There is left, however, on each side dorsalJy a rather
narrow passage which serves to unite the pericardial cavity in
front with the true body-cavity behind.

In PL X. fig. 8 a, there is seen on one side a section through
this passage, while on the other side the passage is seen to be
connected with the pericardial cavity.

It is not possible from transverse sections to determine for
certain whether the septum spoken of is complete. An exami-
nation of longitudinal horizontal sections from an embryo
belonfcino^ to the close of the sta^e K has however satisfied
me that this septum, by that stage at any rate, is fully formed.

The two lateral passages spoken of above probably unite
in the adult to form the passage connecting the pericardial
with the peritoneal cavity, which, though provided with but a
single orifice into the pericardial cavity, divides into two limbs
before opening into the peritoneal cavity.

The body-cavity undergoes no further changes of importance
till the close of the perio'i

Bomatopleure and Splanclinopleure. Both the somatic and
splanchnic walls of the body-cavity during stage I exhibit a
simple uniform character throughout their whole extent. They
are formed of columnar cells where they line the dorsal part
of the body-cavity, but ventrally of more rounded and irregular
cells (PI. X. fig. 5).

In them may occasionally be seen aggregations of very
peculiar and large cells with numerous highly refracting
spherules ; the cells forming these are not unlike the primitive
ova to be described subsequently, but are probably large cells
derived from the yolk.

DEVELOPMENT OF ELASMOBEAXCH FISHES. Ill

It is during the stage intermediate between I and K that
the first changes become visible which indicate a distinction
between an epithelium (endothelium) lining the body-cavity
and the connective tissue adjoining this.

There are at first but very few connective-tissue cells
between the epithelium of the somatic layer of the mesoblast
and the epiblast, but a connection between them is established
by peculiar protoplasmic processes which pass from the one to
the other (PI. x. fig. 8). Towards the end of stage K, however,
there appears between the two a network of mesoblastic cells
connecting them together. In the rudimentary outgrow^th to
form the limbs the mesoblast cells of the somatic layer are
crowded in an especially dense manner.

From the first the connective-tissue cells around the hypo-
blastic epithelium of the alimentary tract are fairly numerous
(PI. X. fig. 8), and by the close of this period become concentrically
arranged round the intestinal epithelium, though not divided
into distinct layers. A special aggregation of them is present
in the hollow of the rudimentary spiral valve.

Behind the anal region the two layers of the mesoblast
(somatic and splanchnic) completely fuse during stage K, and
form a mass of stellate cells in which no distinction into two
layers can be detected (PL xi. fig. 9 c, 9 d).

The alimentary canal, which at first lies close below the
aorta, becomes during this period gradually carried further and
further from this, remaining however attached to the roof of
the body-cavity by a thin layer of the mesoblast of the splanch-
nopleure formed of an epithelium on each side, and a few
interposed connective-tissue cells. This is the mesentery which
by the close of stage K is of considerable length in the region
of the stomach, though shorter elsewhere.

The above account of the protovertebrae and body-cavity ap-
plies solely to the genera Pristiurus and Scyllium. The changes
of these parts in Torpedo only differ from those of Pristiurus
in unimportant though fairly noticeable details. AVithout
entering into any full description of these it may be pointed
out that both the true body-cavity and its continuations into
the protovertebrae appear later in Torpedo than in Pristiurus

112 EESUME.

and Scy Ilium. In some cases even the muscle-plates become
definitely separated and independent before the true body-
cavity has appeared. As a result of this the primitive con-
tinuity of the body-cavity and cavity of the muscle-plates
becomes to a certain extent masked, though its presence may
easily be detected by the obvious continuity which at first
exists between the somatic and splanchnic layers of mesoblast
and the two layers of the muscle-plate. In the muscle-plate
itself the chief point to be noticed is the fact that the earlier
formed bands of muscles [mp) arise very much later, and are
less conspicuous, in Torpedo than in the genera first described.
They are however present and functional.

The anatomical relations of the body-cavity itself are pre-
cisely the same in Torpedo as in Pristiurus and Scyllium, and
the pericardial cavity becomes separated from the peritoneal
in same way in all the genera; the two lateral canals connect-
ing the two cavities being also present in all the three genera.
The two independent parietal plates of mesoblast of the posterior
parts of the body have ventrally a swollen edge, as in Pristiurus,
and in this a cavity appears which forms a posterior continua-
tion of the true body-cavity.

Resume, The primitive independent mesoblast plates of the
two sides of the body become divided into two layers, a somatic
and a splanchnic (Hautfaserblatt and Darmfaserblatt). At the
same time in the dorsal part of the mesoblast plate a series of
transverse splits appear which mark out the limits of the proto-
vertebra3 and serve to distinguish a dorsal or vertebral part of
the plate from a ventral or parietal jDart.

Between the somatic and splanchnic layers of the mesoblast
plate a cavity arises which is continued quite to the summit
of the vertebral part of the plate. This is the primitive body-
cavity ; and at first the cavity is divided into two lateral and
independent halves.

The next change which takes place is the complete separa-
tion of the vertebral portion of the plate from the parietal ;
thereby the upper segmented part of the body-cavity becomes
isolated and separated from the lower and unsegmented part.
In connection with this change in the constitution of the body-
cavity there are formed a series of rectangular plates, each com-

DEVELOPMENT OF ELASMOBRANCH FISHES. 113

posed of two layers, a somatic and a splanchnic, between which
is the cavity originally continuous with the body-cavity. The
splanchnic layer of the plates buds off cells to form the rudi-
ments of the vertebral bodies which are originally segmented
in the same planes as the proto vertebrae. The plates them-
selves remain as the muscle-plates and develop a special layer
of muscle {m p) in their splanchnic layer.

In the meantime the parietal plates of the two sides unite
ventrally throughout the intestinal and cardiac regions of the
body, and the two primitively isolated cavities contained in them
coalesce. Posteriorly however the plates do not unite ventrally,
and their contained cavities remain distinct.

At first the pericardial cavity is quite continuous with the
body-cavity ; but by the close of the period included in the
present chapter it becomes separated from the body-cavity by a
septum in front of the liver, which is however pierced dorsally
by two narrow channels.

The parts derived from the two layers of the mesoblast (not
including special organs or the vascular system) are as follow : —

From the somatic layer are formed

(1) A considerable part of the voluntary muscular system

of the body.

(2) The dermis.

(3) A large part of the intermuscular connective tissue.

(4) Part of the peritoneal epithelium.

From the splanchnic layer are formed

(1) A great part of the voluntary muscular system.

(2) Part of the intermuscular connective tissue (?).

(3) The axial skeleton.

(4) The muscular and connective-tissue wall of the

alimentary tract.

(5) A great part of the peritoneal epithelium.

General Considerations. In the history which has just been
given of the development of the mesoblast, there are several
points which appear to me to throw light upon the primitive
origin of that layer. Before entering into these it is however
necessary to institute a comparison between the history of the
B. 8

114 GENERAL CONSIDERATIONS.

mesoblast in Elasmobranchs and in other Vertebrates, in order
to distinguish as far as possible the primitive and the secondary-
characters present in the various groups.

Though the Mammals are to be looked on as the most
differentiated group amongst the Vertebrates, yet in their
embryonic history they retain many very primitive features,
and, as has been recently shewn by Hensen^ present numerous,
remarkable approximations to the Elasmobranchs. We find ac-
cordingly^ that the primitive lateral plates of mesoblast undergo,
nearly the same changes in these two groups. In Mammals
there is at first a continuous cavity extending through both
the parietal and vertebral portions of each plate, and dividing ^
the plates into a somatic and a splanchnic layer : this cavity is
the primitive body-cavity. The vertebral portion of each plate
with its contained cavity then becomes divided off from the
parietal. The later development of these parts is not accurately
known, but it seems that the outer portion of each vertebral
plate, composed of two layers (somatic and splanchnic) en-
closing between them a remnant of the primitive body-cavity,
becomes separated off as a muscle-plate. The remainder
forms a vertebral rudiment, &c. Thus the extension of the
body-cavity into the vertebral portion of the mesoblast, and the
constriction of the vertebral portion of the cavity from the re-
mainder, are as distinctive features of Mammals as they are of
the Elasmobranchs.

In Birds ^ the horizontal splitting of the mesoblast into
somatic and splanchnic layers appears, as in Mammals, to extend
at first to the summit of the protovertebrse, but these bodies
become so early separated from the parietal plates that this
fact has usually been overlooked or denied ; but even on the
second day of incubation the outer layer of the protovertebrse is
continuous with the somatic layer of the lateral plates, and
the inner layer and kernel of the protovertebrai with the
splanchnic layer of the lateral plates \ After the isolation

1 Zeitschrift f. Anat. EntwicJclungsgeschichte, Vol. i.

2 Hensen loc. cit.

3 For the history of protovertebrae and muscle-plates in Birds, vide Ele-
ments of Embryology, Foster and Balfour, The statement there made that the
horizontal splitting of the mesoblast does not extend to the summit of the
vertebral plate, must however be regarded as doubtful.

* Vide ElemeiUs of Embryology, p. 56.

DEVELOPMENT OF ELASMOBRAXCH FISHES. 115

of the protovertebra3 the primitive position of the split which
separated their somatic and splanchnic layers becomes obscured,
but when on the third day the muscle-plates are formed they
are found to be constituted of tiuo layers, an inner and an outer,
which enclose between them a central cavity. This remarkable
fact, which has not received much attention, though noticeable
in most figures, receives a simple explanation as a surviving
rudiment on Darwinian principles. The central cavity of the
muscle-plate is, in fact, a remnant of vertebral extension of the
body-cavity, and is the same cavity as that found in the muscle-
plates of Elasmobranchs. The two layers of the muscle-plate
also correspond with the two layers present in Elasmobranchs,
the one belonging to the somatic, the other to the splanchnic
layer of mesoblast. The remainder of the protovertebrss in-
ternal to the muscle-plates is very large in Birds, and is the
equivalent of that portion of the protovertebrse which in Elas-
mobranchs is split off to form the vertebral bodies^ (PL x. fig. G, 7,
8, Vr). Thus, though the history of the development of the
mesoblast is not precisely the same for Birds as for Elasmo-
branchs, yet the differences betw^een the two groups are of such
a character as to prove in a striking manner that the Avian
development is a derivation from a more primary form, like
that of the Elasmobranchs.

According to the statements of Bambeke and Gotte, the
Amphibians present rather remarkable peculiarities in the
development of their muscular system. Each side-plate of
mesoblast is divided into a somatic and a splanchnic layer,
continuous throughout the vertebral and parietal portions of
the plate. The vertebral portions (protovertebrse) of the plates
soon become separated from the parietal, and form an inde-
pendent mass of cells constituted of two layers, which w^ere
originally continuous with the somatic and splanchnic layers
of the parietal plates. The outer or somatic layer of the

^ Dr Gotte, Entwickhmgsgeschichte der Unke, p. 534, gives a difl'erent
account of the development of the protovertebrffi from that in the text. He
states that the muscle-plates do not give rise to the main dorso-lateral muscles,
but only to some superficial ventral muscles, while the dorso-lateral muscles are
according to him formed from part of the kernel of the proto vertebrae internal
to the muscle-plates. The account given in the text is the result of my own
investigations, and accords precisely with the recent statements of Professor
Kolliker, Enhokhhmgsge>schichte, 1876,

8—2

116 , THE VERTEBRAL PLATES.

vertebral plates is formed of a single row of cells, but the
inner or splanchnic layer is made up of a central kernel of
cells and an inner single layer. This central kernel is the
first portion of the vertebral body to undergo any change,
and it becomes converted into the main dorso-lateral muscles
of the body, which apparently correspond with the muscles
derived from the wliole muscle-plate of the Elasmobranchs.
From the inner layer of the splanchnic division there are next
formed the main internal ventral muscles, rectus abdominis, &c.,
as well as the chief connective- tissue elements of the parts
surrounding the spinal cord. The outer layer of the vertebral
plates forms the dermis and sub-cutaneous connective tissue, as
well as some of the superficial muscles of the trunk and the
muscles of the limbs.

Dr Gotte appears to think that the vertebral plates in Am-
phibians present a perfectly normal development very similar
to that of other Vertebrates. The divergences between Am-
phibians and other Vertebrates appear, however, to myself,
to be very great, and although the very careful account given
by Dr Gotte is probably to be relied on, yet some further
explanation than he has offered of the development of these
parts amongst the Amphibians would seem to be required.

A primary stage in which the two layers of the vertebral
plates are continuous with the somatic and splanchnic layers
of a body- wall is equally characteristic of Amphibians, Elas-
mobranchs and Mammals. In the subsequent development,
however, a great difference between the types becomes appar-
ent, for whereas in Elasmobranchs both layers of the vertebral
plates combine to form the muscle-plates, out of which the great
dorso-lateral muscles are formed, in Amphibians what appear
to be the equivalent muscles are derived from a few of the cells
(the kernel) of the inner layer of the vertebral plates only.
The cells which form the lateral muscles in Amphibians might
be thought to correspond in position with the cells which become,
in Elasmobranchs, converted into the special early formed band
of muscles {m. p'.), rather than, as their development seems
to indicate, with the whole Elasmobranch muscle-plates \

1 The type of development of the muscle-plates of Amphibians would become
identical with that of Elasmobranchs if their first-formed mass of muscle cor-

DEVELOPMENT OF ELASMOBRANCH FISHES. 117

Osseous Fishes are stated to agree with Amphibians in the
development of their proto vertebrae and muscular system \ but
further observations on this point are required.

Though the development of the general muscular system
and muscle-plates does not, according to existing statements,
take place on quite the same type throughout the Vertebrate
sub-kingdom, yet the comparison which has been instituted
between Elasmobranchs and other Vertebrates appears to prove
that there are one or two common features in their development,
which may be regarded as primitive, and as having been inherited
from the ancestors of Vertebrates. These features are (1) The
extension of the body-cavity into the vertebral plates, and
subsequent enclosure of this cavity between the two layers
of the muscle-plates ; (2) The primitive division of the vertebral
plate into a somatic and a splanchnic layer, and the formation
of a large part of the voluntary muscular system out of the
splanchnic layer.

The ultimate derivation of the mesoblast forms one of
the numerous burning questions of modern embryology, and
there are advocates to be found for almost every one of the
possible views the question admits of.

All who accept the doctrine of descent are agreed that
primitively only two embryonic layers were present — the
epiblast and the hypoblast — and that the mesoblast subse-
quently appeared as a distinct layer, after a certain com-
plexity of organization had been attained.

The general agreement stops, however, at this point, and
the greatest divergence of opinion exists wdth reference to all
further questions which bear on the development of the meso-
blast. There appear to be four possibilities as to the origin of
this layer.

It may be derived :

(1) entirely from the epiblast,

responded with the early-formed muscles of Elasmobranchs, and the remaining
cells of both layers of the protovertebrae became in the course of development
converted into muscle-cells indistinguishable from those formed at first. Is it
possible that, owing to the distinctness of the first-formed mass of muscle, Dr
Gotte can have overlooked the fact that its subsequent growth is carried on at
the expense of the adjacent cells of the somatic layer?

1 Ehrlich, Ueber den peripher. Theil d.Urwirbel. Archivf. Mic. Anat. Vol. xi.

118 DERIVATION OF THE MESOBLAST.

(2) partly from the epiblast, and partly from the hypoblast,

(3) entirely from the hypoblast,

(4) or may have no fixed origin.

The fourth of these possibilities may for the present be
dismissed, since it can be only maintained should it turn out
that all the other views are erroneous. The first possibility is
supported by the case of the Coelenterata, and we might almost
say by that of this group only\

Amongst the Coelenterata the mesoblast, when present, is
unquestionably a derivative of the epiblast, and when, as is fre-
(piently the case, a distinct mesoblast is not present, the muscle-
cells form a specialized part of the epidermic cells.

The condition of the mesoblast in these lowly organized
animals is exactly what might d priori have been anticipated,
but the absence throughout the group of a true body-cavity,
or specially developed muscular system of the alimentary tract,
prevents the possibility of generalizing for other groups, from
the condition of the mesoblast in this one.

In those animals in which a body-cavity and muscular
alimentary tract are present, it would certainly appear reasonable
to expect the mesoblast to be derived from both the primitive
layers : the voluntary muscular system from epiblast, and the
splanchnic system from the hypoblast. This view has been
taken and strongly advocated by so distinguished, an embry-
ologist as Professor Haeckel, and it must be admitted, that on
a iwiori grounds there is much to recommend it; there are,
however, so far as I am aware of, comparatively few observed
facts in its favour.

Professor Haeckel's own objective arguments in support of
his view are as follows :

(1) From the fact that some investigators derive the meso-

1 The most important other instances in addition to that of the Coelenterata
^Yhich can be adduced in favour of the epiblastic origin of the mesoblast are the
Bird and Mammal, in which according to the recent observations of Hensen for
the Mammal, and Kolliker for the Mammal and Bird, the mesoblast is split off
from the epiblast. If the views I have elsewhere put forward about the meaning
of the primitive gi'oove be accepted, the derivation of the mesoblast from the
epiblast in these instances would be apparent rather than real, and have no
deep morphological significance for the present question.

Other instances may be brought forward from various groups, but none
of these arc sufficiently well confirmed to be of any value in the determina,
iioii uf the present question.

DEVELOPMENT OF ELASMOBRA^X'H FISHES. 119

blast with absolute confidence from the hypoblast, while others
do so with equal confidence from the epiblast, he concludes that
it is really derived from both these layers.

(2) A second argument is founded on the supposed deriva-
tion of the mesoblast in Amphioxus from both epiblast and
hypoblast. Ko^valevsky's account (on w^hich apparently Prof.
Haeckel's^ statements are based) appears to me, however, too
vague, and his observations too imperfect, for much confidence
to be placed in his statements on this head. It does not indeed
appear to me that the formation of the layers in Amphioxus, till
better known, can be used as an argument for any special view
about this question.

(3) Professor Haeckel's own observations on the develop-
ment of Osseous fish form a third argument in support of his
views. These observations do not, however, accord with those of
the majority of investigators, and not having been made by
means of sections, require further confirmation before they can
be definitely accepted.

(4) A fourth argument rests on the fact that the various
embryonic layers fuse together to form the primitive streak or
axis-cord in higher vertebrates. This he thinks proves that the
mesoblast is derived from both the primitive layers. The primi-
tive streak has, however, according to my views, quite another
significance to that attributed to it by Professor HaeckeP; but
in any case Professor Kolliker's researches, and on this point my
own observations accord w^ith his, appear to me to prove that the
fusion which there takes place is only capable of being used as
an argument in favour of an epiblastic origin of the mesoblast,
and not of its derivation from both epiblast and hypoblast.

The objective arguments in favour of Professor Haeckel's
view^s are not very conclusive, and he himself does not deny
that the mesoblast as a rule apparently arises as a single and
undivided mass from one of the two primary layers, and only
subsequently becomes split into somatic and splanchnic strata.
This original fusion and subsequent splitting of the mesoblast

1 Vide Anthropogenie, p. 197.

2 Vide Self, Development of Elasmobranch Fishes, Journal of Anat. ar^d
Phys. Vol. X. note on p. 682, and also Eeview of Professor Kolliker's Entwick-
lungsgeschichte des Menschen u. d. hoheren Thiere, Journal of Anat. and Phys.
Vol. X.

120 DERIVATION OF THE MESOBLAST.

is explained by him^as a secondary condition, a possibility wliich
cannot by any means be thrown on one side. It seems therefore
worth while examining how far the history of the somatic and
splanchnic layers of the mesoblast in Elasmobranchs and other
Vertebrates accords with the supposition that they were primi-
tively split off from the epiblast and the hypoblast respectively.

It is well to consider first of all what parts of the mesoblast
of the body might be expected to be derived from the somatic
and splanchnic layers on this view of their origin'.

From the somatic layer of the mesoblast there would no
doubt be formed the whole of the voluntary muscular system of
the bodv, the dermis, the subcutaneous connective tissue, and
the connective tissue between the muscles. It is probable also,
thouo-h this point is less certain, that the skeleton would be
derived from the somatic layer. From the splanchnic layer
would be formed the connective tissue and muscular layers of
the alimentary tract, and possibly also the vascular system.

Turning to the actual development of these parts, the
discrepancy between theory and fact becomes very remark-
able. From the somatic layer of the mesoblast, part of
the voluntary muscular system and the dermis is no doubt
derived, but the splanchnic layer supplies the material,
not only for the muscular wall of the digestive canal and the
vascular system, but also for the whole of the axial skeleton
and a great part of the voluntary muscular system of the body,
including the first-formed muscles. Though remarkable, it is
nevertheless not inconceivable, that the skeleton might be
derived from the splanchoic mesoblast, but it is very difficult to
understand how there could be formed from it a part of the
voluntary muscular system of the body indistinguishably fused
with part of the muscular system derived from the somatopleure.

1 Professor Haeckel speaks of tlie splitting of the mesoblast in Vertebrates
into a somatic and splanchnic layer as a secondary process {Gastrula u. Eifur-
chiuu) d. Thiere), but does not make it clear whether he regards this secondary
Bplitting as taking place along the old lines. It appears to me to be fauiy
certain that even if the original unsplit condition of the mesoblast is to be
regarded as a secondary condition, yet that the splitting of this must take place
along the old lines, otherwise a change in the position of the body-cavity in the
alult would have to be supposed— an unlikely change producing unnecessary
complication. The succeeding argument is based on the assumption that the
unsplit condition is a secondary condition, but that the split which eventually
appears in this occurs along the old lines, separating the primitive splanchuo-
pleure from the primitive somatopleure.

DEVELOPMENT OF ELASMOBRANCH FISHES. 121

No fact in my investigations comes out more clearly than that
a great part of the voluntary muscular system is formed from
the splanchnic layer of the mesoblast, yet this fact presents
a most serious difficulty to the view that the somatic and
splanchnic layers of the mesoblast in Vertebrates are respectively
derived from the epiblast and hypoblast.

In spite, therefore, of general a 'priori considerations of
a very convincing kind which tell in favour of the double
origin of the mesoblast, this view is supported by so few
objective facts, and there exists so powerful an array of facts
against it, that at present, at least, it seems impossible to main-
tain it. The full strength of the facts against it will appear
more fully in a review of the present state of our knowledo-e as
to the development of the mesoblast in the different groups.

To this I now pass.

In a paper on the " Early stages of development in Verte-
brates^" a short resume was given of the development of the
mesoblast throughout the animal kingdom, which it may be
worth while repeating here with a few additions. So far as we
know at present, the mesoblast is derived from the hypoblast
in the following groups:

Echinoderms(Hensen, Agassiz, Metschnikofif, Selenka, Gotte),
Nematodes (Butschli), Sagitta (Kowalevsky, Butschli), Lum-
bricus and probably other Annelids (Kowalevsky), Brachiopoda
(Kowalevsky), Crustaceans (Bobretzky), Insects (Kowalevsky,
Ulianin, Dohrn), Myriapods (Metschnikoff), Tunicates (Kowa-
levsky, Kuppfer), Petromyzon (Owsjanikoff), Osseous fishes
(Oellacher, Gotte, Kowalevsky), Elasmobranchs (Self), Amphi-
bians (Remak, Strieker, Gotte).

The list includes members from the greater number of the
groups of the animal kingdom ; the most striking omis-
sions being the Coelenterates, Mollusks, and the Amniotic Ver-
tebrates. The absence of the Coelenterates has been already
explained, and my grounds for regarding the Amniotic Vertebrates
as apparent rather than real exceptions have also been pointed
out. The Mollusks, however, remain as a large group, in which
we as yet know very little as to the formation of the mesoblast.

^ Q,uaYt. JL of Micros. Science, July, 1875.

122 DERIVATION OF THE MESOBLAST.

Dr Rabl ^, who seems recently to have studied the develop-
ment of Lymnaeus by means of sections, gives some figures
shewing the origin of the mesoblast ; they are, however, too
diagrammatic to be of much service in settling the present
question, and the memoirs of Professor Lankester'^ and Dr
FoP are equally inconclusive for this purpose, for, though they
contain fio-ures of elonorated and branched mesoblast cells
passing from the epiblast to the hypoblast, no satisfactory
representations are given of the origin of these cells. I have
myself observed in embryos of Turbo or Trochus similar
elongated cells to those figured by Lankester and Fol, but was
unable clearly to determine whence the}^ arose. The most accu-
rate observations which we have on this question are those of
Professor Bobretzky ^ In Nassa he finds that the three embry-
onic layers are all established during segmentation. The outer-
most and smallest cells form the epiblast, somewhat larger cells
adjoining these the mesoblast, and the large yolk-cells the hypo-
blast. These observations do not, however, demonstrate from
which of the primary layers the mesoblast is derived.

The evidence at present existing is clearly in favour of the
mesoblast being, in almost all groups of animals, developed from
the hypoblast, but strong as this evidence is, it has not its full
weight unless the actual manner in which the mesoblast is in
many groups derived from the hypoblast, is taken into consider-
ation. The most important of these are the Echinoderms,
Brachiopods and Sagitta.

In the Echinoderms the mesoblast is in part formed by cells
budded off from the hypoblast, the remainder, however, arises as
one or more diverticula of the alimentary tract. From the separate
cells first budded off there are formed the cutis, part of the
connective tissue and the calcareous skeleton^. The diverticula
from the alimentary cavity form the water-vascular system and
the somatic and splanchnic layers of mesoblast. The cavity of

1 Jenaische Zeitschrift, Vol. ix.

2 Quart. Jl. of Micros. Science, Vol. xxv. 187-1, and Phil. Trans. 1875.

3 Archives de Zoologie, Vol. iv.

4 Archiv f. Micr. Anat. Vol. xiii.

^ The recent researches of Selenka, Zeitschrift f. Wiss. ZooJo(jie, Vol. xxvii.
1876, demonstrate that in Echinoderms the muscles are derived from the cells
fast split oif from the hypoblast, and that the diverticula only form the water-
vascular system and the epithelial lining of the body-cavity.

DEVELOPMENT OF ELASMOBRANCH FISHES. 123

the diverticula after the separation of the tuater-va^cidar' system,
forms the hody-cavity. The outer lining layer of the cavity forms
the somatic layer of mesoblast and the voluntary muscles ; the
inner lining layer the splanchnic mesoblast which unites luith the
epithelium of the alimentary tract. Though this fundamental
arrangfement would seem to be universal amons^st Echinoderms,
considerable variations of it are exhibited in different groups.

There is one outgrowth from the alimentary tract in Sy-
napta ; two in Echinoids, Asteroids and Ophiura ; three in
Comatula, and four (?) in Amphiura. The cavity of the out-
growth usually forms the body-cavity, but sometimes in Ophiura
and Amphiura (MetschnikofF) the outgrowths are from the first
or soon become solid, and only secondarily acquire a cavity,
which is however homologous with the body-cavity of the other
groups.

In Saoitta* the formation of the mesoblast and the ali-

o

mentary tract takes place in nearly the same fashion as in the
Echinoderms. The simple invaginate alimentary cavity becomes
divided into three lobes, a central and two lateral. The two
lateral lobes are gradually more and more constricted off from
the central one, and become eventually quite separated from it;
their cavities remain independent, and form in the adult the
hody-cavity, divided by a mesentery into two distinct lateral
sections. The inner layer of each of the two lateral lobes forms
the mesoblast of the splanchnopleure, the outer layer the meso-
blast of the somatopleure. The central division of the primitive
gastrsea cavity remains as the alimentary tract of the adult.

The remarkable observations of Kowalevsky^ on the devel-
opment of the Brachiopoda have brought to light the unex-
pected fact that in t\vo genera at least (Argiope and Tere-
bratula) the mesoblast and body-cavity develope as paired
constrictions from the alimentary tract in a manner almost
identically the same as in Sagitta.

It thus appears that, so far as can be determined from the
facts at our disposal, the mesoblast in almost all cases is derived

1 Kowalevsky, Wxirmer u. Artliropoden, Mem. Acad. Pctersbonrg, 1871.

2 Zur Entwickluugsgeschichte d. Brachiopoden Protokoll d. Ersten Session der
Versammlung Eussischer Naturforscher i. Kasan, 1873. Published in Kaiser-
liche Gesrllschaft Moskau, 187-4 (Russian). Abstracted in Hoffmann and
Scbwalbe, Jahreshericht f. 1873.

124 NATURE OF THE BODY-CAVITY.

from the hypoblast, and in three widely separated groups it
arises as a pair of diverticula from the alimentary tract, each
diverticulum containing a cavity which eventually becomes the
body-cavity. I have elsewhere suggested^ that the origin of
the mesoblast from alimentary diverticula is to be regarded as
primitive for all higher animals, and that the more general cases
in which the mesoblast becomes split off, as an undivided layer,
from the hypoblast, are in reality derivates from this. The
chief obstacle in the way of this view arises from the difficulty
of understanding how the whole voluntary muscular system can
have been derived at first from the alimentary tract. That part
of a voluntary system of muscles might be derived from the con-
tractile diverticula of the alimentary canal attached to the body-
wall is not difficult to understand, but it is not easy to believe
that the secondary system so formed could completely replace
the primitive muscular system, derived, as it must have been,
from the epiblast. In my paper above quoted will be found
various speculative suggestions for removing this difficulty,
which I do not repeat here. If it be granted, however, that
in Sagitta, Brachiopods, and Echinoderms we have genuine
examples of the formation of the whole mesoblast from ali-
mentary diverticula, it is easy to see how the formation of the
mesoblast in Vertebrates may be a second derivate from an
origin of this nature.

An attempt has been already made to shew that the meso-
blast in Elasmobranchs is formed in a very primitive fashion,
and for this reason the Elasmobranchs appear to be especially
adapted for determining whether any signs are exhibited of a
derivation of the mesoblast as paired diverticula of the ali-
mentary tract. There are, it appears to me, several such
features. In the first place, the mesoblast is split off from the
hypoblast not as a single mass but as a pair of distinct masses,
comparable with the paired diverticula already alluded to.
Secondly, the body-cavity when it appears in the mesoblast
plates, does not arise as a single cavity, but as a pair of cavities,
one for each plate of mesoblast, and these cavities remain
permanently distinct in some parts of the body, and nowhere
unite till a comparatively late period. Thirdly, the primitive
1 Comparison of Early Stages, Quart. Jl. Micros. Science, July, 1875.

DEVELOPMENT OF ELASMOBRANCH FISHES. 125

body-cavity of the embryo is not confined to tbe region in
which a body-cavity exists in the adult, hiot extends to the
summit of the muscle-plates, at first separating parts which
become completely fused in the adult to form the great lateral
muscles of the body. It is difficult to understand how the body-
cavity could have such an extension as this, on the supposition
that it represents a primitive split in the mesoblast between
the wall of the gut and the body-wall ; but its extension to this
part is quite intelligible, on the supposition that it represents
the cavities of two diverticula of the alimentary tract, from
whose muscular walls the voluntary muscular system has been
derived. Lastly, I would point out that the derivation of part
of the muscular system from what appears as the splanchno-
pleure is quite intelligible on the assumed hypothesis, but, as
far as I see, on no other.

Such are the main features presented by the mesoblast in
Elasmobranchs, which favour the view of its having originally
formed the walls of the alimentary diverticula. Against this
view of its nature are the facts (1) of the mesoblast plates
being at first solid, and (2), as a consequence of this, of the body-
cavity never communicating with the alimentary canal. These
points, in view of our knowledge of embryological modifica-
tions, cannot be regarded as great difficulties to my view. We
have many examples of organs, which, though in most cases
arising as involutions, yet appear in other cases as solid in-
growths. Such examples are afforded by the optic vesicle, audi-
tory vesicle, and probably also by the central nervous system, of
Osseous Fish. In most Vertebrates these organs are formed as
hollow involutions from the exterior ; in Osseous Fish, however,
as solid involutions, in which a cavity secondarily appears.

The segmental duct of Elasmobranchs or the Wolffian duct
(segmental duct) of Birds are cases of a similar kind, being
organs which must originally have been formed as hollow
involutions, but which now arise as solid bodies.

Only one more instance of this kind need be cited, taken
from the Echinoderms.

The body-cavity and the mesoblast investing it arise in
the case of most Echinoderms as hollow involutions of the
alimentary tract, but in some exceptional groups, Ophiura

126 SEGMENTATION OF THE VERTEBRAL BODIES,

and Ampliiura, are stated to be solid at first and only sub-
sequently to become hollow. Should the accuracy of Metsch-
nikoff 's account of this point be confirmed, an almost exact paral-
lel to what has been supposed by me to have occurred with the
mesoblast in Elasmobranchs, and other groups, will be supplied.

The tendency of our present knowledge appears to be in
favour of regarding the body-cavity in Vertebrates as having
been primitively the cavity of alimentary diverticula, and the
mesoblast as having formed the walls of the diverticula.

This view, to say the least of it, suits the facts which we
know far better than any other theory which has been pro-
posed, and though no doubt the a prioi^i difficulties in its way
are very great, yet it appears to me to be sufficiently strongly
supported to deserve the attention of investigators. In the
meantime, however, our knowledge of invertebrate embryology
is so new and imperfect that no certainty on a question like
that which has just been discussed can be obtained ; and any
generalizations made at present are not unlikely to be upset by
the discovery of fresh facts.

The only other point in connection with the mesoblast
which I would call attention to is the formation of the ver-
tebral bodies.

My observations confirm those of Remak and Gegenbaur,
shewing that there is a primary segmentation of the vertebral
bodies corresponding to that of the muscle-plates, followed by a
secondary segmentation in which the central lines of vertebral
bodies are opposite the partitions between the muscle-plates.

The explanation of these changes is not difficult to find.
The primary segmentation of the body is that of the muscle-
plates, which must have been present at a time when the
vertebral bodies had no existence. As soon however as the
notochordal sheath was required to be strong as well as flexible,
it necessarily became divided into a series of segments.

The conditions under which the lateral muscles can cause
the flexure of the vertebral column are clearly that each
muscle- segment shall be capable of acting on two vertebrae;
and this condition can only be fulfilled when the muscle-
segments are opposite the intervals between the vertebrae.
Owing to this necessity, when the vertebral segments became

DEVELOPMENT OF ELASMOBRANCH FISHES. 127

formed, their centres corresponded, not with the centres of
the muscle-plates, but with the inter-muscular septa.

These considerations fully explain the secondary segmen-
tation of the vertebrae by which they become opposite the
inter-muscular septa. On the other hand, the primary seg-
mentation is clearly a remnant of the time when no ver-
tebral bodies were present, and has no greater morphological
significance than the fact that the cells to form the unseg-
mented investment of the notochord were derived from the
segmented muscle-plates, and only secondarily became fused
into a continuous tube.

The Urino-genital System.

The first traces of the urinary system become visible at
about the time of the appearance of the third visceral cleft. At
about this period the somatopleure and splanchnopleure become
more or less fused together at the level of the dorsal aorta, and
thus, as has been already mentioned, each of the original plates
of mesoblast becomes divided into a vertebral plate and lateral
plate (PI. X. fig. 6). The mass of cells resulting from this fusion
corresponds with Waldeyer's intermediate cell-mass in the Fowl.

At about the level of fifth protovertebra the first trace of
the urinary system appears.

From the intermediate cell-mass a solid knob grows outwards
towards the epiblast (woodcut, fig. 4, pd). This knob consists at
first of 20 — 30 cells, which agree in character with the neigh-
bourinoj cells of the intermediate cell-mass, and are at this
period rounded. It is mainly, if not enthely, derived from the
somatic layer of the mesoblast.

From this knob there grows backwards a solid rod of cells
which keeps in very close contact with the epiblast, and
rapidly diminishes in size towards its posterior extremity. Its
hindermost part consists in section of at most one or two cells.
It keeps so close to the epiblast that it might be supposed to
be derived from that layer were it not for the sections shewing
its orio^in from the knob above mentioned. We have in this
rod the commencement of what I have elsewhere^ called the
segmental duct.

1 Urinogenital Organs of Vertebrates, Jonrn. of Anot. and Phys. Vol. x.

128

SEGMENTAL INVOLUTIONS.

Fig. 4. — Two sections of a Pristiurus Embryo with three visceral clefts.

spn

The sections are to shew the development of the segmental duct {{'pd) or primi-
tive duct of the kidneys. In A (the anterior of the two sections) this
appears as as a solid knob projecting towards the epiblast. In B is seen
a section of the column which has grown backwards from the knob in A.

spn. rudiment of a spinal nerve ; mc. medullary canal ; ch. notochord ;
X. string of cells below the notochord ; mp. muscle-plate ; mp'. specially
developed portion of muscle-plate ; ao. dorsal aorta ; pd. segmental
duct. so. somatopleura ; sp. splanchnopleura ; pp. pleuroperitoneal or
body-cavity ; ep. epiblast ; al. alimentary canal.

My observations shew that the segmental duct is developed
in the way just described in both Pristiurus and Torpedo. Its
origin in Pristiurus is shewn in the adjoining woodcut, and in
Torpedo in Plate X. fig. 7sd.

At a stage somewhat older than I, the condition of the
segmental duct has not very materially altered. It has in-
creased considerably in length, and the knob at its front end
is both absolutely smaller, and also consists of fewer cells than
before (PL x. fig. 7 sd). These cells have become more columnar,
and have begun to arrange themselves radially; thus indicating
the early appearance of the lumen of the duct. The cells
forming the front part of the rod, as well as those of the knob,
commence to exhibit a columnar character, but in the hinder
part of the rod the cells are still rounded. In no part of it
has a lumen appeared.

At this period also the knob, partly owing to the com-
mencing separation of the muscle-plate from the remainder of
the mesoblast, begins to pass inwards and approach the pleuro-
peritoneal cavity.

At the same stage the first not very distinct traces of the
remainder of the urinary system become developed. These

DEVELOPMENT OF ELASMOBRANXH FISHES. 129

appear in the form of solid outgrowths from the intermediate
cell-mass just at the most dorsal part of the body-cavity.

The outgrowths correspond in numbers with the vertebral
segments, and are at first quite disconnected with the seg-
mental duct. At this stage they are only distinctly visible
in the first few segments behind the front end of the segmental
duct. A full description of them will come more conveniently
in the next stage.

By a stage somewhat earlier than K important changes
have taken place in the urinary system.

The segmental duct has acquired a lumen in its anterior
portion, which opens at its front end into the body-cavity.
(PL XI. fig. 9 sd). The lumen is formed by the columnar cells
spoken of in the last stage, acquiring a radiating arrangement
round a central point, at which a small hole appears. After the
lumen has once become formed, it rapidly increases in size.

The duct has also grown considerably in length, but its
hind extremity is still as thin, and lies as close to the epiblast,
as at first. The segmental involutions which commenced to
be formed in the last stage, have now appeared for every
vertebral segment along the- whole length of the segmental
duct, and even for two or three segments behind this.

They are simple independent outgrowths arising from the
outer and uppermost angle of the body -cavity, and are at first
almost without a trace of a lumen, though their cells are arranged
as two layers. They grow in such a way as to encircle the
oviduct on its inner and upper side (PL X. fig. 8 and PL xi. fig.
14 b. st). When the hindermost ones are formed, a slight trace
of a lumen is perhaps visible in the front ones. At a stage
slightly subsequent to this, in Scyllium Canicula, I noticed 29
of them ; the first of them arising in the segment immediately
behind the front end of the oviduct (PL XI. fig. 17 5^' ^^'^ two
of them being formed in segments just posterior to the hinder
extremity of the oviduct.

PL XI. fig. 16 and 18 represent two longitudinal sections
shewing the segmental nature of the involutions and their
relation to the segmental duct.

Many of the points which have been mentioned can be
seen by referring to PL X. and XI. Anteriorly the segmental
duct opens into the pleuro-peritoneal cavity. In the sections

B. 9

130 SEGMENTAL DUCT.

behind this there maybe seen the segmental duct with a distinct
lumen, and also a pair of segmental involutions (PL xi. fig. 14 a).
In the still posterior sections the segmental duct would be quite
without a lumen, and would closely adjoin the epiblast.

It seems not out of place to point out that the modes of
the development of the segmental duct and of the segmental
involutions are strikingly similar. Both arise as solid involu-
tions, from homologous parts of the mesoblast. The seg-
mental duct arises in the vertebral segment immediately in
front of that in which the first segmental involution appears ;
so that the segmental duct appears to he equivalent to a single
segmental involution.

The next stage corresponds with the first appearance of the
external gills. The segmental duct now communicates by a
wide opening with the body-cavity (PI. XI. fig. 9 sd). It pos-
sesses a lumen along its whole length up to the extreme hind
end (PI. XI. fig. 9 a). It is, however, at this hinder extremity
that the most important change has taken place. This end has
grown downwards towards that part of the alimentary canal
which still lies behind the anus. This downgrowth is begin-
ning to shew distinct traces of a lumen, and will appear in the
next stage as one of the horns by which the segmental ducts
communicate with the cloaca (PI. xi. fig. 9 h). All the anterior
segmental involutions have now acquired a lumen. But this
is still absent in the posterior ones (PI. xi. fig. 9 a).

Owing to the disappearance of the body-cavity in the
region behind the anus, the primitive involutions there remain
as simple masses of cells still disconnected with the segmental
duct (PL XI. fig. 9 6, 9 c and 9 d).

Primitive Ova. The true generative products make their
first appearance as the primitive ova between stages I and K.

In the sections of one of my embryos of this stage they are
especially well shewn, and the following description is taken
from those displayed in that embryo.

They are confined to the region which extends posteriorly
nearly to the end of the small intestine and anteriorly to the
abdominal opening of the segmental duct.

Their situation in this region is peculiar. There is no trace
of a distinct genital ridge, but the ova mainly lie in the dorsal
portion of the mesentery, and therefore in a part of the mesoblast

DEVELOPMENT OF ELASMOBRANCH FISHES. 131

which distinctly belon^^s to the splanchnopleure (PI. XI. fig. 14a).
Some are situated external to the segmental involutions ; and
others again, though this is not common, in a part of the
mesoblast which distinctly belongs to the body-wall (PL xi.
fig. 14 6).

The portion of mesentery in which the primitive ova are
most densely aggregated, corresponds to the future position of
the genital ridge, but the other positions occupied by ova
are quite outside this. Some ova are in fact situated on the
outside of the segmental duct and segmented tubes, and must
therefore effect a considerable migration before reaching their
final positions in the genital ridge on the inner side of the
segmental duct (PL xi. fig. 14 6).

The condition of the tissue in which the ova appear may at
once be gathered from an examination of the figures given.
It consists of an irregular epithelium of cells partly belonging
to the somatopleure and partly to the splanchnopleure, but
passing uninterruptedly from one layer to the other. The
cells which compose it are irregular in shape, but frequently
columnar (PL xi. fig. 14 a and 14 6).

They are formed of a nucleus which stains deeply, invested
by a very delicate layer of protoplasm. At the junction of somato-
pleure and splanchnopleure they are more rounded than else-
where. Yery few loose connective-tissue cells are present. The
cells just described vary from "008 Mm. to "01 Mm. in diameter.

The primitive ova are situated amongst them and stand out
w^ith extraordinary clearness, to which justice is hardly done in
my figures.

The normal full-sized ova exhibit the following structure.
They consist of a mass of somewhat granular protoplasm of
irregular, but more or less rounded, form. Their size varies
from '016 — "OoG Mm. In their interior a nucleus is present,
which varies from "012 — '016 Mm., but its size as a rule bears no
relation to the size of the containing celL

This is illustrated by the subjoined list of measurements.

The numbers given refer to degrees on my micrometer scale.

Since it is the ratio alone which it is necessary to call
attention to, the numbers are not reduced to decimals of a
millimeter. Each degree of my scale is equal, however, with
the object glass employed, to '002 Mm.

132 PRIMITIVE OVA.

Size of Piimitive oTa in Size of nucleus of Primitive

degrees of micrometer scale ova iu degrees of micrometer

with F. ocul 2. scale with F. ocul 2.

10 8

13 8

13 8

14 7

15 7

13 7i

11 8

16 51

12 7

10 7

15 6

13 6

12 7

This series brings out the result I have just mentioned with
great clearness.

In one case we find a cell has three times the diameter of
the nucleus 16 : 5J; in another case 10 : 8, the nucleus has
only a slightl}^ smaller diameter than the cell. The irration-
ality of the ratio is fairly shewn in some of my figures, though
none of the largest cells with very small nuclei have been
represented.

The nuclei are granular, and stain fairly well with haema-
toxylin. They usually contain a single deeply stained nucle-
olus, but in many cases, especially where large (and this
independently of the size of the cell), they contain two nucleoli
(PL XT. fig. 14 c and 14* d), and are at times so lobed as to give
an apparent indication of commencing division.

A multi-nucleolar condition of the nuclei, like that figured
by Gotte\ does not appear till near the close of embryonic
life, and is then found equally in the large ova and in those
not larger than the ova which exist at this early date.

As regards the relation of the jDrimitive ova to each other
and the neighbouring cells, there are a few points which de-
serve attention. In the first place, the ova are, as a rule,
collected in masses at particular points, and not distributed uni-
formly (fig. 14 a.) The masses in some cases appear as if they had

^ EiiticicJdungsgeschichte der Unkc, PI. i. fig. 8.

DEVELOPMENT OF ELASMOBRANCH FISHES. 133

resulted from the division of one primitive ovum, but can hardly
be adduced as instances of a commencing coalescence ; since if
the ova thus aggregated were to coalesce, an ovum would be
produced of a very much greater size than any which is found
during the early stages. Though at this stage no indication
is present of such a coalescence of cells to form ova as is be-
lieved to take place by Gotte, still the origin of the primitive
ova is not quite clear. One would naturally expect to find a
great number of cells intermediate between primitive ova and
ordinary columnar cells. Cells which may be intermediate are
no doubt found, but not nearly so frequently as might have
been anticipated. One or two cells are shewn in PI. xi. fig.
14 a. X, which are perhaps of an intermediate character; but
in most sections it is not possible to satisfy oneself that any
such intermediate cells are present.

In one case what appeared to be an intermediate cell was
measured, and presented a diameter of *012 Mm. while its
nucleus was '008 Mm. Apart from certain features of the
nucleus, which at this stage are hardly veiy marked, the
easiest method of distinguishing a primitive ovum from an
adjacent cell is the presence of a large quantity of protoplasm
around the nucleus. The nucleus of one of the smallest primi-
tive ova is not larger than the nucleus of an ordinary cell (being
about "008 Mm. in both). It is perhaps the similarity in the size
of the nuclei which renders it difficult at first to distinguish
developing primitive ova from ordinary cells. Except with the
very thinnest sections a small extra quantity of protoplasm
around a nucleus might easily escape detection, and the de-
veloping cell might only become visible when it had attained
to the size of a small typical primitive ovum.

It deserves to be noticed that the nuclei even of some of
the largest primitive ova scarcely exceed the surrounding nuclei
in size. This appears to me to be an argument of some weight
in shewing that the great size of primitive ova is not due to
the fact of their having been formed by a coalescence of dif-
ferent cells (in which case the nucleus would have increased in
the same proportion as the cell) ; but to an increase by a normal
method of growth in the protoplasm around the nucleus.

It appears to me to be a point of great importance certainly
to determine whether the primitive ova arise by a meta-

134 PRIMITIVE OVA.

morphosis of adjoining cells, or may not be introduced from else-
where. In some of the lower animals, e. g. Hydrozoa, there is no
question that the ova are derived from the epiblast ; we might
therefore expect to find that they had the same origin in Verte-
brates. Further than this, ova are frequently capable in a
young state of executing amoeboid movements, and accordingly
of migrating from one layer to another. In the Elasmobranchs
the primitive ova exhibit in a hardened state an irregular form
which might appear to indicate that they possess a power of
altering their shape, a view which is further supported by some
of them being at the present stage situated in a position very
different from that which they eventually occupy, and which
they can only reach by migration. If it could be shewn that
there were no intermediate stages between the primitive ova
and the adjoining cells (their migratory powers being admitted)
a strong presumption would be offered in favour of their having
migrated from elsewhere to their present position. In view
of this possibility I have made some special investigations,
which have however led to no very satisfactory results. There
are to be seen in the stages immediately preceding the present
one, numerous cells in a corresponding position to that of the
primitive ova, v/hich might very well be intermediate between
the primitive ova and ordinary cells, but which offer no suffi-
ciently well marked features for a certain determination of
their true nature.

In the particular embryo whose primitive ova have been
described these bodies were more conspicuous than in the
majority of cases, but at the same time they presented no
special or peculiar characters. •

In a somewhat older embryo of Scyllium the cells amongst
which the primitive ova lay had become very distinctly dif-
ferentiated as an epithelium (the germinal epithelium of
Waldeyer) well separated by what might almost be called a
basement membrane from the adjoining connective-tissue cells.
Hardly any indication of a germinal ridge had appeared, but
the ova were more definitely confined than in previous embryos
to the restricted area which eventually forms this. The ova
on the average were somewhat smaller than in the previous
cases.

In several embryos intermediate in age between the embryo

DEVELOPMENT OF ELASMOBRANCH FISHES. 135

wliose primitive ova were described at tlie commencement of
this section and the embr)^o last described, the primitive ova
presented some peculiarities, about the meaning of which I
am not quite clear, but which may perhaps throw some light
on the origin of these bodies.

Instead of the protoplasm around the nucleus being clear or
slightly granular, as in the cases just described, it was filled in
the most typical instances with numerous highly refracting
bodies resembling yolk-spherules. In osmic acid specimens (PL
XI. fig. 15) these stain very darkly, and it is then as a rule very
difficult to see the nucleus ; in specimens hardened in picric
acid and stained with hsematoxylin these bodies are stained of
a deep purple colour, but the nucleus can in most cases be dis-
tinctly seen. In addition to the instances in which the proto-
plasm of the ova is quite filled with these bodies, there are
others in which they only occupy a small area adjoining the
nucleus (PI. XI. fig. 15 a), and finally some in which only one or
two of these bodies are present. The protoplasm of the
primitive ova appears in fact to present a series of gradations
between a state in which it is completely filled with highly
refracting spherules and one in which these are completely
absent.

This state of things naturally leads to the view that the
primitive ova, when they are first formed, are filled with these
spherules, which are probably yolk- spherules, but that they
gradually lose them in the course of development. Against
this interpretation is the fact that the primitive ova in the
younger embryo first described are completely without these
bodies; this embryo however unquestionably presented an
abnormally early development of the ova : and I am satisfied
that embryos present considerable variations in this respect.

If the primitive ova are in reality in the first instance
filled with yolk-spherules, the question arises as to whether,
considering that they are the only mesoblast cells filled at this
period with yolk-spherules, we must not suppose that they
have migrated from some peripheral part of the blastoderm
into their present position. To this question I can give no
satisfactory answer. Against a view which would regard the
spherules in the protoplasm as bodies which appear subsequently
to the first formation of the ova, is the fact that hitherto

136 ' NOTOCHORD.

no instances in which these spherules were present have been
met with in the late stages of development ; and they seem
therefore to be confined to the first stages.

NotocJiord,

The changes undergone by the notochord during this period
present considerable differences according to the genus ex-
amined. One iy^Q of development is characteristic of Scyllium
and Pristiurus ; a second type, of Torpedo.

My observations being far more complete for Scyllium and
Pristiurus than for Torpedo, it is to the two former genera
only that the following account applies, unless the contrary
is expressly stated. Only the development of the parts of
the notochord in the trunk are here dealt with ; the cephalic
section of the notochord is treated of in a subsequent section.

During stage G the notochord is composed of flattened
cells arranged vertically, rendering the histological characters
of the notochord difficult to determine in transverse sections.
In longitudinal sections, however, the form and arrangement
of the cells can be recognised with great ease. At the beginning
of stage G each cell is composed of a nucleus invested by
granular protoplasm frequently vacuolated and containing in
suspension numerous yolk-spherules. It is difficult to deter-
mine whether there is only one vacuole for each cell, or whether
in some cases there may not be more than one.

Round the exterior of the notochord there is present a
distinct though delicate cuticular sheath.

The vacuoles are at first small, but during stage G rapidly
increase in size, while at the same time the yolk-spherules
completely vanish from the notochord.

As a result of the rapid growth of the vacuoles, the nuclei,
surrounded in each case by a small amount of protoplasm,
become pushed to the centre of the notochord, the remainder
of the protoplasm being carried to the edge. Tlie notochord
thus becomes composed during stages H and I (PL x. fig. 4 — 6)
of a central area mainly formed of nuclei with a small quantity
of protoplasm around them, and of a thin peripheral layer of
protoplasm without nuclei, the widish space between the two
being filled with clear fluid. The exterior of the cells is

DEVELOPMENT OF ELASMOBRANCH FISHES. 187

indurated, so that they may be said to be invested by a mem-
brane^; the cells themselves have a flattened form, and each ex-
tends from the edge to the centre of the notochord, the long axis
of each being rather greater than half the diameter of the cord.

The nuclei of the notochord are elliptical vesicles, consisting
of a membrane filled with granular contents, amongst which is
situated a distinct nucleolus. They stain deeply with h^ema-
toxylin. Their long diameter in Scyllium is about 0-02 Mm.

The diameter of the whole notochord in Pristiurus durino-

o

stage I is about 01 Mm. in the region of the back, and about
0'08 Mm. near the posterior end of the body.

Owing to the form of its constituent cells, the notochord
presents in transverse sections a dark central area surrounded
by a lighter peripheral one, but its true structure cannot be
unravelled without the assistance of longitudinal sections. In
these (PL xi. fig. 10) the nuclei form an irregular double row in
the centre of the cord. Their outlines are very clear, but those
of the individual cells cannot for certain be made out. It is,
however, easy to see that the cells have a flattened and wedge-
shaped form, with the narrow ends overlapping and inter-
locking at the centre of the notochord.

By the close of stage I the cuticular sheath of the notochord
has greatly increased in thickness.

During the period intermediate between stages I and K
the notochord undergoes considerable transformations. Its
cells cease to be flattened, and become irregularly polygonal,
and appear but slightly more compressed in longitudinal sec-
tions than in transverse ones. The vacuolation of the cells pro-
ceeds rapidly, and there is left in each cell only a very thin
layer of protoplasm around the nucleus. Each cell, as in the
earlier stages, is bounded by a mernbrane-like wall.

Accompanying these general changes special alterations
take place in the distribution of the nuclei and the protoplasm.
The nuclei, accompanied by protoplasm, gradually leave the
centre and migrate towards the periphery of the notochord.
At the same time the protoplasm of the cells forms a special
layer in contact with the investing sheath.

1 This membrane is better looked upon, as is done by Gegeubaur and Gotte,
as intercellular matter.

^' 10

138 NOTOCHORD.

The changes by which this takes place can easily be followed
in longitudinal sections. In PL XL fig. 11 the migration of the
nuclei has commenced. They are still, however, more or less
aggregated at the centre, and very little protoplasm is present
at the edges of the notochord. The cells, though more or less
irregularly polygonal, are still somewhat flattened. In PL xi.
fio-. 12 the notochord has made a further progress. The nuclei
now mainly lie at the side of the notochord, where they exist in
a somewhat shrivelled state, though still invested by a layer of
protoplasm.

A large portion of the protoplasm of the cord forms an
almost continuous layer in close contact with the sheath,
which is more distinctly visible in some cases than in others.

While the changes above described are taking place the
notochord increases in size. At the age of fig. 11 it is in the
anterior part of the body of Pristiurus about O'll Mm. At the
age of fig. 12 it is in the same species 0*12 Mm., while in Scyl-
lium Stellare it reaches about 0*17 Mm.

During stage K (PL x. fig. 8) the vacuolation of the cells of
the notochord becomes even more complete than during the
earlier stages, and in the central cells hardly any protoplasm
is present, though a starved nucleus surrounded by a little
protoplasm may be found in an occasional corner.

The whole notochord becomes very delicate, and can with
great difficulty be conserved whole in transverse sections.

The layer of protoplasm which appeared during the last
stage on the inner side of the cuticular membrane of the
notochord becomes during the present stage a far thicker and
more definite structure. It forms a continuous layer with
irregular prominences on its inner surface ; and contains nume-
rous nuclei. The layer sometimes presents in transverse sec-
tions hardly any indication of a division into a number of
separate cells, but in longitudinal sections this is generally very
obvious. The cells are directed very obliquely forwards, and
consist of an oblong nucleus invested by protoplasm. The
layer formed by them is very delicate and very easily destroyed.
In one example its thickness varied from '004 to '006 Mm., in
another it reached '012 Mm. The thickness of the cuticular
membrane is about '002 Mm. or rather less.

DEVELOPMENT OF ELASMOBRA^X'H FISHES. 139

The diameter of a notochord in the anterior part of the
body of a Pristiurus embryo of this stage is about 021 Mm.
Round the exterior of the notochord the mesoblast cells are
commencing to arrange themselves as a special sheath.

In Torpedo the notochord at first presents the same
structure as in Pristiurus, i. e. it forms a cylindrical rod of
flattened cells.

The vacuolation of these cells does not however commence
till a relatively very much later period than in Pristiurus, and
also presents a very different character (PI. x. fig. 7).

The vacuoles are smaller, more numerous, and more rounded
than in the other genera, and there can be no question that in
many cases there is more than one vacuole in a cell. The
most striking point in which the notochord of Torpedo differs
from that of Pristiurus consists in the fact that in Torpedo
there is never any aggregation of the nuclei at the centre of the
cord, but the nuclei are always distributed uniformly through
it. As the vacuolation proceeds the differences between Tor-
pedo and the other genera become less and less marked. The
vacuoles become angular in form, and the cells of the cord
cease to be flattened, and become polygonal.

At my final stage for Torpedo (slightly younger than K)
the only important feature distinguishing the notochord from
that of Pristiurus, is the absence of any signs of nuclei
or protoplasm passing to the periphery. Around the exterior
of the cord there is early found in Torpedo a special invest-
ment of mesoblastic cells.

10—2

CHAPTER VII.

Genekal Development of the Trunk from Stage H
TO THE Close of Embryonic Life.

External Epihlast.

The change already alluded to in the previous chapter (p. 9D)
by which the external epiblast or epidermis becomes divided
into two layers, is completed before the close of stage L.

In the tail region at this stage three distinct strata may be
recognized in the epidermis. (1) An outer stratum of flattened
horny cells, which fuse together to form an almost continuous
membrane. (2) A middle stratum of irregular partly rounded
and partly flattened cells. (3) An internal stratum of columnar
cells, bounded towards the mesoblast by a distinct basement
membrane (PL XIL fig. 8), unquestionably pertaining to the
epiblast. This layer is especially thickened in the terminal parts
of the paired fins (PI. xii. fig. 1). The two former of these
strata together constitute the epidermic layer of the skin, and
the latter the mucous layer.

In the anterior parts of the body during stage L the skin
only presents two distinct strata, viz. an inner somewhat
irregular layer of rounded cells, the mucous layer, and an outer
layer of flattened cells (PI. XIL fig. 8).

The remaining history of the external epiblast, consisting as
it does of a record of the gradual increase in thickness of the
epidermic strata, and a topographical description of its varia-
tions in structure and thickness in different parts, is of no
special interest and need not detain us here.

In the late embryonic periods subsequent to stage Q the
layers of the skin cease to be so distinct as at an earlier period,
partly owing to the innermost layer becoming less columnar,
and partly to the presence of a large number of mucous cells,
which have by that stage made their appearance.

I have followed with some care the development of the
placoid scales, but my observations so completely accord with

DEVELOPMENT OF ELASMOBRAXCH FISHES. 141

those of Dr O. Hertwig\ that it is not necessary to record
them. The so-called enamel layer is a simple product of the
thickening and calcification of the basement membrane, and
since this membrane is derived from the mucous layer of epi-
dermis, the enamel is clearly to be viewed as an epidermic
product. There is no indication of a gradual conversion of the
bases of the columnar cells forming the mucous layer of the
epidermis into enamel prisms, as is frequently stated to occur
in the formation of the enamel of the teeth in higher Verte-
brates.

Lateral line.

The lateral line and the nervous structures appended to it
have been recently studied from an embryological point of
view by Gotte^ in Amphibians and by Semper^ in Elasmo-
branchs.

The most important morphological result which these two
distinguished investigators believe themselves to have arrived
at is the direct derivation of the lateral nerve from the ecto-
derm. On this point there is a complete accord between them,
and Semper especially explains that it is extremely easy to
establish the fact.

As w^ill appear from the sequel, I have not been so fortunate
as Semper in elucidating the origin of the lateral nerve, and
my observations bear an interpretation not in the least in
accordance with the \iews of my predecessors, though not
perhaps quite conclusive against them.

It must be premised that two distinct structures have to
be dealt with, viz. the lateral line formed of modified epidermis,
and the lateral nerve whose origin is in question.

The lateral line is the first of the two to make its appear-
ance, at a stage slightly subsequent to K, in the form of a linear
thickening of the inner row of cells of the external epiblast, on
each side, at the level of the notochord.

This thickening, in my youngest embryo in which it is
found, has but a very small longitudinal extension, being

1 Jenaische Zeitschrift, Vol, viii.

2 Entw'cklungsgeschichte d. Unke.

^ Urogenital-system d. Selachier. Semper's Arbeiten, Bd. ii.

14)2 THE LATERAL NERVE,

present through about 10 thin sections in the last part of the
head and first part of the trunk. The thickening, though short,
is very broad, measuring about 0*28 Mm. in transverse section,
and presents no signs of a commencing differentiation of
nervous structures. The large intestinal branch of the vagus
can be seen in all the anterior sections in close proximity to
this line, and appears to me to give off to it posteriorly a small
special branch which can be traced through a few sections, vide
PI. XIL fig. 2 n.l. But this branch is not sufficiently well
marked to enable me to be certain of its real character. In
any case the posterior part of the lateral line is absolutely
without any adjoining nervous structures or traces of such.

The rudiment of the epidermic pa.rt of the lateral line
is formed of specially elongated cells of the mucous layer of the
epiblast, but around the bases of these certain rounder cells of
a somewhat curious appearance are intercalated.

There is between this and my next youngest embryo an
unfortunately large gap with reference to the lateral line,
although in almost every other respect the two embryos might
be regarded as belonging to the same stage. The lateral line
in the older embryo extends from the hind part of the head to
a point well behind the anus, and is accompanied by a nerve for
at least two-thirds of its length.

In the foremost section in which it appears the intestinal
branch of the vagus is situated not far from it, and may he
seen at intervals giving off branches to it. There is no sign
that these are otherwise than perfectly normal branches of
the vagus. Near the level of the last visceral cleft the in-
testinal branch of the vagus gives off a fair-sized branch, which
from the first occupies a position close to the lateral line though
well within the mesoblast (PI. XIL fig. 3a, n.l). This branch
is the lateral nerve, and though somewhat larger, is otherwise
much like the nerve I fancied I could see originating from the
intestinal branch of the vagus during the previous stage.

It rapidly thins out posteriorly and also apioroaches closer
and closer to the lateral line. At the front end of the trunk
it is quite in contact with it, and a short way behind this region
the cells of the lateral line arrange themselves in a gable-like
form, in the angle of which the nerve is situated (PL xii.

DEVELOPMENT OF ELASMOBRANCH FISHES. 143

fig. Sh, and 8c). In this position the nerve though small
is still very distinct in all good sections, and is formed of a rod
of protoplasm, with scattered nuclei, in which I could not detect
a distinct indication of cell-areas. The hinder part of the nerve
becomes continually smaller and smaller, without however pre-
senting any indication of becoming fused with the epiblast, and
eventually ceases to be visible some considerable distance in
front of the posterior end of the lateral line.

The lateral line itself presents some points of not incon-
siderable interest. In the first place, it is very narrow an-
teriorly and throughout the greater part of its length, but
widens out at its hinder end, and is widest of all at its ter-
mination, which is perfectly abrupt. The following measure-
ments of it were taken from an embryo belonging to stage L,
which though not quite my second youngest embryo is only
slightly older. At its hinder end it was 0*17 Mm. broad. At
a point not far from this it was 0-09 Mm. broad, and anteriorly
it was 0*05 Mm. broad. These measurements clearly show that
the lateral line is broadest at what may be called its growing-
point, a fact which explains its extraordinary breadth in the
anterior part of the body at my first stage, viz. 0'28 Mm., a
breadth which strangely contrasts with the breadth, viz. 05 Mm.,
which it has in the same part of the body at the present stage.

It still continues to form a linear area of modified epidermis,
and has no segmental characters. Anteriorly it is formed by
the cells of mucous layer becoming more columnar (PL xil.
fig. 3a). In its middle region the cells of the mucous layer
in it are still simply elongated, but, as has been said above,
have a gable-like arrangement, so as partially to enclose the
nerve (PI. xil. fig. 35). Nearer the hind end of the trunk a
space appears in it between its columnar cells and the flattened
cells of the outermost layer of the skin (PL xii. fig. 3c), and
this space becomes posteriorly invested by a very definite layer
of cells. The space (PL xii. fig. Sd) or lumen has a slit-Hke
section, and is not formed by the closing in of an originally
open groove, but by the formation of a cavity in the midst
of the cells of the lateral line. Its walls are formed by a layer
of columnar cells on the inner side, and flattened cells on the
outer side, both layers however appearing to be derived from

144 THE LATERAL LINE.

the mucous layer of the epidermis. The outer layer of cells
attains its greatest thickness dorsally.

During stages M, N, O, the lateral nerve gradually passes
inwards into the connective tissue between the dorso-lateral
and the ventro-lateral muscles, and becomes even before the
close of stage N completely isolated from the lateral line.

The growth of the lateral line itself remains for some time
almost stationary; anteriorly the cells retain the gable-like
arrangement which characterised them at an earlier period,
but cease to enclose the nerve ; posteriorly the line retains its
original more complicated constitution as a closed canal. In
stage the cells of the anterior part of the line, as well as
those of the posterior, commence to assume a tubular arrange-
ment, and the lateral line takes the form of a canal. The tubular
form is due to a hollowing out of the lateral line itself and a
rearrangement of its cells. As the lateral line becomes con-
verted into a canal it recedes from the surface.

In stage P the first indication of segmental apertures to the
exterior make their appearance, vide PL Xll. fig. 4. The lateral
line forms a canal situated completely below the skin, but
at intervals (corresponding with segments) sends upwards and
outwards prolongations towards the exterior. These prolonga-
tions do not during stage P acquire external openings. As
is shown in my figure, a special area of the inner border of the
canal of the lateral line becomes distinguished by its structure
from the remainder.

No account of the lateral line would be complete without
some allusion to the similar sensory structures which have such
a wide distribution on the heads of Elasmobranchs ; and this is
especially important in the present instance, owing to the light
thrown by a study of their development on the origin of the
nerves which supply the sense-organs of this class. The so-
called mucous canals of the head originate in the same way as
does tlie lateral line ; they are products of the mucous layer of
the epidermis. They eventually form either canals with nume-
rous openings to the exterior, or isolated tubes with terminal
ampulliform dilatations.

I have not definitely determined whether the canal-system
of the head arises in connection with the lateral line, or only

DEVELOPMENT OF ELASMOBRANCH FISHES. 145

eventually becomes so connected. The important point to be
noticed is, that at first no nervous structures are to be seen in
connection with it. In stage nerves for the mucous canals
make their appearance as delicate branches of the main stems.
These nerve-stems are very much ramified, and their branches
have, in a large number of instances, an obvious tendency
towards a particular sense-organ (PL xii. figs. 5 and 6).

I have not during stage O been able to detect a case of
direct continuity between the two. This is, however, esta-
blished in the succeeding stage P, in the case of the canals, and
the facility with which it may be observed would probably
render the embryo Elasmobranch a very favourable object for
studying the connection between nerves and terminal sense-
organs. The nerve (PI. xii. fig. 7) dilates somewhat before
uniting with the sense-organ, and the protoplasm of the nerve
and the sense-organ become completely fused. The basement
membrane of the skin is not continuous across their point of
junction, and appears to unite with a delicate membrane-like
structure, which invests the termination of the nerve. The
ampullae Avould seem to receive their nervous supply somewhat
later than the canals, and the terminal swellings of the nerves
supplying them are larger than in the case of the canals, and
the connection between the ampullae and the nerves not so
clear. In the case of the head, there can for Elasmobranchs be
hardly a question that the nerves which supply the mucous
canals grow centrifugally from the original cranial nerve-stems,
and do not originate in a peripheral manner from the integument.

This is an important point to make certain of in settKng
any doubtful features in the nervous supply of the lateral
line. Professor Semper^ with whom as dealing with Elasmo-
branchs we are more directly concerned, makes the following
statement : "At the time when at the front end the lateral nerve
has already completely separated itself from the ectoderm, and
is situated amongst the muscles, it still lies in the middle of
the body close to the ectoderm, and at the hind end of the
body is not yet completely segmented off (abgegliedert) from
the ectoderm." Although the last sentence of this quotation
ma} seem to be opposed to my statements, yet it appears to mo
1 Loc. cit. p. 398.

146 DERIVATION OF THE LATERAL NERVE.

probable that Professor Semper has merely seen the lateral
nerve partially enclosed in the ectoderm. This position of the
nerve no doubt affords a presumption, hut only a presumj^tion,
in favour of a direct origin of the lateral nerve from the ecto-
derm ; but against this interpretation of it are the following facts :

(1) That the front part of the lateral line is undoubtedly
supplied by branches which arise in the ordinary way from the
intestinal branch of the vagus ; and we should not expect to
find part of the lateral line supplied by nerves which originate
in one way, and the remainder supplied by a nerve having
a completely difierent and abnormal mode of origin.

(2) The growth of the lateral line is quite independent of
that of the lateral nerve : the latter arises subsequently to the
lateral line, and, so far as is shown by the inconclusive observa-
tion of my earliest stage, as an offshoot from the intestinal
branch of the vagus; and though it grows along at first in close
contact with the lateral nerve, yet it never presents, so far as I
have seen, any indubitable indication of becoming split off from
this, or of fusing with it.

(3) The fact that the cranial representatives of the lateral
line are supplied with nerves which originate in the normal
way\ affords a strong argument in favour of the lateral line
receiving an ordinary nerve-supply.

Considering all these facts, I am led to the conclusion that
the lateral nerve in Elasmohranchs arises as a branch of the
vagus, and not as a direct p)roduct of the external epiblasf.

An interesting feature about the lateral line and the similar
cephalic structures, is the fact of these being the only sense
organs in Elasmohranchs which originate entirely from the
mucous layer of the epiblast. This, coupled with the well-
known facts about the Amj^hibian epiblast, and the fact that the
mucous canals are the only sense-organs which originate subse-
quently to the distinct differentiation of the epiblast into mu-
cous and horny layers, goes far to prove ^ that the mucous layer

1 Gotte extends his statements about the lateral nerve to the nerves supplying
the mucous canals in the head ; but my observations appear to me, as far as
Elasmohranchs are concerned, nearly conclusive against such a derivation of
the nerves in the head.

2 I believe that Gotte, amongst his very numerous valuable remarks in the
Eutwichhingsgcschichte der Unke, has put forward a view similar to this, though
I cannot put my band on the reference.

DEVELOPMENT OF ELASMOBRANCH FISHES. 147

is to be regarded as the active layer of the epiblast, and that
after this has become differentiated, an organ formed from the
epiblast is always a product of it.

Muscle- Plates.

The muscle-plates at the close of stage K were flattened
angular- bodies with the apex directed forwards, their ventral
edge being opposite the segmental duct, and their dorsal edge
on a level with the middle of the spinal cord. They were com-
posed of two layers, formed for the most part of columnar cells,
but a small part of their splanchnic layer opposite the noto-
chord had already become differentiated into longitudinal mus-
cles.

During stage L the growth of these plates is very rapid, and
their upper ends extend to the summit of the neural canal, and
their lower ones nearly meet in the median ventral line. The
original band of muscles (PI. X. fig. 8 7?i. ^V), whose growth was so
slow^ during stages I and K, now increases wdth great rapidity,
and forms the nucleus of the whole voluntary muscular system.
It extends upwards and downwards by the continuous conver-
sion of fresh cells of the splanchnic layer into muscle-cells. At
the same time it grows rapidly in thickness, but it requires some
little patience and care to unravel the details of this growth;
and it will be necessary to enter on a slight digression as to
the relations of the muscle-plates to the suiTounding connective
tissue.

As the muscle-plates grow dorsalwards and ventralw^ards
their ends dive into the general connective tissue, whose origin
has already been described (PL xii. fig. 1). At the same time
the connective-tissue cells, which by this process become situ-
ated between the ends of the muscle-plates and the skin, grow
upwards and dowmwards, and gradually form a complete layer
separating the muscle-plates from the skin. The cells forming
the ends of the muscle-plates retain unaltered their prmiitive
undifferentiated character, and the separation between them
and the surrounding connective-tissue cells is very marked.
This however ceases to be the case in the r^rts of the muscle-

148 THE MUSCLE-PLATES.

plates on a level with the notochord and lower part of the
medullary canal; the thinnest sections and most careful exami-
nation are needed to elucidate the changes taking place in this
region. The cells which form the somatic layer of the muscle-
plates then begin to elongate and become converted into muscle-
cells, at the same time that they are increasing in number to
meet the rapid demands upon them. One result of these changes
is the loss of the original clearness in the external boundary be-
tween the muscle-plates and the adjoining connective-tissue
cells, which is only in exceptional cases to be seen so distinctly as
it may be in PL xii. fig. 1 and 8. Longitudinal horizontal sections
are the most instructive for studying the growth of the muscles,
but transverse sections are also needed. The interpretation
of the transverse ones is however rendered difficult, both by
rapid alterations in the thickness of the connective-tissue layer
between the skin and the muscle-plates (shown in PL xii. fig. 8),
and by the angular shape of the muscle-plates themselves.

A careful study of both longitudinal and transverse sections
has enabled me to satisfy myself of the fact that the cells
of the somatic layer of the protovertebrse, equally with the
cells of the splanchnic layer, are converted into muscle-cells, and
some of these are represented in the act of undergoing this
conversion in PL XIL fig. 8 ; but the difficulty of distinguishing
the outline of the somatic layer of the muscle-plates, at the
time its cells become converted into muscle-cells, renders it
very difficult to determine whether any cells of this layer
join the surrounding connective tissue. General considerations
certainly lead me to think that they do not ; but my observa-
tions do not definitely settle the point.

From these facts it is clear, as was briefly stated in the
last chapter, that both layers of the muscle-jdate are concerned
in forming the great lateral muscle, though the splanchnic layer
is converted into muscles very much sooner than the somatic^.

^ The difference between Dr Gotte's account of the development of the
muscles and my own consists mainly in my attributing to the somatic layer of
the muscle-plates a share in the formation of the great lateral muscles, which he
denies to it. In an earlier section of this Monograph, pp. 115, 116, too much
stress was unintentionally laid on the divergence of our views; a divergence
which appears to have, in part at least, arisen, not from our observations
being opposed, but from Dr Gotte's having taken the highly differentiated
Eombinator as his type instead of the less differentiated Elasmobranch.

DEVELOPMENT OF ELASMOBRAXCH FISHES. 149

The remainder of the history of the muscle-plates presents
no points of special interest.

Till the close of stage L, the muscle-plates are not distinctly
divided into dorsal and ventral segments, but this division, which
is so characteristic of the adult, commences to manifest itself
during stage M, and is quite completed in the succeeding stage.
It is effected by the appearance, nearly opposite the lateral line,
of a layer of connective tissue which divides the muscles on each
side into a dorso-lateral and ventro-lateral section. Even during
stage O the ends of the muscle-plate are formed of undiffer-
entiated columnar cells. The peculiar outlines of the inter-
muscular septa gradually appear during the later stages of
development, causing the well-known appearances of the mus-
cles in transverse sections, but require no special notice here.

With reference to the histological features of the develop-
ment of the muscle-fibres, I have not pushed my investigations
very far. The primitive cells present the ordinary division,
well known since Remak, into a striated portion and a non-
striated portion, and in the latter a nucleus is to be seen which
soon undergoes division and gives rise to several nuclei in the
non-striated part, while the striated part of each cell be-
comes divided up into a number of fibrillae. I have not
however determined what exact relation the original cells hold
to the eventual primitive bundles, or anything with reference
to the development of the sarcolemma.

The Muscles of the Limbs. — These are formed during stage
coincidently with the cartilaginous skeleton, in the form
of two bands of longitudinal fibres on the dorsal and ventral
surfaces of the limbs. Dr Kleinenberg first called my attention
to the fact that he had proved the limb-muscles in Lacerta to
be derived from the muscle-plates. This I at first believed did
not hold good for Elasmobranchs, but have since determined
that it does so. Between stages K and L the muscle-plates
grow downwards as far as the limbs and then turn outwards and
grow into them (PL XYii. fig. 1). Small portions of several
muscle-plates come in this way to be situated in the limbs,
and are very soon segmented off from the remainder of the
muscle-plates. The portions of muscle-plates thus introduced
into the limbs soon lose their original distinctness, and can no

150 THE VERTEBRAL COLUMN.

longer be recognised in stage L. There can however be but
little doubt that they supply the tissue for the muscles of the
limbs. The muscle-plates themselves after giving off these
buds to the limbs grow downwards, and by stage L cease to
show any trace of what has occurred (PI. xii. fig. 1). This
fact, coupled with the late development of the muscles of the
limbs (stage 0), caused me to fall into my original error.

The Vertebral Column and Notocliord,

In the previous chapter (p. 107) an account was given of
the origin of the tissue destined to form the vertebral bodies;
it merely remains to describe the changes undergone by this
in becoming converted into the permanent vertebrae.

This subject has already been dealt with by a considerable
number of anatomists, and my investigations coincide in the
main with the results of my predecessors. Especially the re-
searches of Gegenbaur* may be singled out as containing the
pith of the whole subject, and my results, while agreeing
in all but minor points with his, do not supplement them
to any very great extent. I cannot do more than confirm
Gotte's^ account of the development of the haemal arches, and
may add that Cartier^ has given a good account of the later
development of the centra. Under the circumstances it has not
appeared to me to be worth while recording with great detail
my investigations ; but I hope to be able to give a somewhat
more complete history of the whole subject than has appeared
in any single previous memoir.

At their first appearance the cells destined to form the
permanent vertebrae present the same segmentation as the
muscle-plates. This segmentation soon disappears, and between
stages K and L the tissue of the vertebral column forms a
continuous investment of the notochord which cannot be distin-
guished from the adjoining connective tissue. Immediately
surrounding the notochord a layer formed of a single row of cells
may be observed, which is not however very distinctly marked''.

1 Das Kopfskelet d. SclacTiier, p. 123.

2 Entwicklungsgeschichte d. Unke^ p. 433-4.

3 Zeitschrift f. Wiss. Anat. Bd. xxv., Supplement.

4 Vide p. 138.

DEVELOPMENT OF ELASMOBRANCH FISHES. lol

During the stage L there appear four special concentrations
of mesoblastic tissue adjoining the notochord, two of them
dorsal and two of them ventral. They are not segmented, and
form four ridges seated on the sides of the notochord. They
are united with each other by a delicate layer of tissue, and
constitute the rudiments of the neural and hssmal arches. In
longitudinal sections of stage L special concentrated wedge-
shaped masses of tissue are to be seen between the muscle-
plates, which must not be confused with these rudiments.
Immediately around the notochord the delicate investment of
cells previously mentioned, is still present.

The rudiments of the arches increase in size and distinct-
ness in the succeeding stages, and by stage N have unques-
tionably assumed the constitution of embryonic cartilage. In
the meantime there has appeared surrounding the sheath of
the notochord a well-marked layer of tissue which stains deeply
with haematoxylin, and with the highest power may be observed
to contain flattened nuclei. It is barely thicker than the
adjoining sheath, but is nevertheless the rudiment of vertebral
bodies. PI. xii. fig. 9, vh. Whence does this layer arise ? To
this question I cannot give a quite satisfactory answer. It is
natural to conclude that it is derived from the previously
existing mesoblastic investment of the notochord, but in the
case of the vertebral column I have not been able to prove this.
Observations on the base of the brain afford fairly conclusive
evidence that the homologous tissue present there has this
origin. Gegenbaur apparently answers the question of the origin
of this layer in the way suggested above, and gives a figure in
support of his conclusion (PI. xxii. fig. 3) \

The layer of tissue which forms the vertebral bodies rapidly
increases in thickness, and very soon, at a somewhat earlier
period than represented in Gegenbaur's Plate xxil. fig. 4,
a distinct membrane (Kolliker's Membrana Elastica Externa)
may easily be recognised surrounding it and separating it

1 None of my specimens resembles this figure, and the layer when first
formed is in my embryos much thinner than represented by Gegenbaur, and
the histological structure of the embryonic cartilage is very different from that
of the cartilage in the figiu*es alluded to. Gotte's very valuable researches
with reference to the origin of this layer in Amphibians" tend to confirm the
view advocated in the text.

152 THE NEURAL AND H.EMAL ARCHES.

from the adjoining tissue of the arches. Gegenbaur's figure
gives an excellent representation of the appearance of this
layer at the period under consideration. It is formed of a
homogeneous basis containing elongated concentrically arranged
nuclei, and constitutes a uniform unsegmented investment for
the notochord (vide PL Xil. fig. 10).

The neural and hsemal arches now either cease altogether
to be united with each other by a layer of embryonic cartilage,
or else the layer uniting them is so delicate that it cannot be
recognised as true cartilage. They have moreover by stage P
undergone a series of important changes. The tissue of the
neural arches does not any longer form a continuous sheet, but
is divided into (1) a series of arches encircling the spinal cord,
and (2) a basal portion resting on the cartilaginous sheath of
the notochord. There are two arches to each muscle-plate, one
continuous with the basal portion of the arch-tissue and forming
the true arch, which springs opposite the centre of a vertebral
body, and the second not so continuous, which forms what is
usually known as the intercalated piece. Between every pair
of true arches the two roots of a single spinal nerve pass out.
The anterior root passes out in front of an intercalated piece
and the posterior behind it \

The basal portion of the arch-tissue likewise undergoes
differentiation into a vertebral part continuous with the true arch
and formed of hyaline cartilage, and an intervertebral segment
formed of a more fibrous tissue.

The hsemal arches, like the neural arches, become divided
into a layer of tissue adjoining the cartilaginous sheath of
the notochord, and processes springing out from this opposite
the centres of the vertebrae. These processes throughout
the region of the trunk in front of the anus pass into the
space between the dorsal and ventral muscles, and are to be
regarded as rudiments of ribs. The tissue with which they are
continuous, which is exactly equivalent to the tissue from
which the neural arches originate, is not truly a part of the
rib. In the tail, behind the anus and kidneys, the cardinal

1 In the adult Scyllium it is well known that the posterior root pierces the
intercalated cartilage and the anterior root the true neural arch. This however
does not seem to be the case in the embryo at stage P.

DEVELOPMENT OF ELASMOBRAXCH FISHES. 153

veins fuse to form an impaired caudal vein below the aorta,
and in this part a fresh series of processes originates on each
side from the haemal tissue adjoining the cartilaginous sheath
of the notochord, and eventually, by the junction of the pro-
cesses of the two sides, a canal which contains the aorta and
caudal vein is formed below the notochord. These processes for
a few segments coexist with small ribs (vide PL xii. fig. 10),
a fact which shows (1) that they cannot be regarded as modified
ribs, and (2) that the tissue from which they spring is to be
viewed as a kind of general basis for all the haemal processes
which may arise, and is not specially connected with any one
set of processes.

While these changes (all of which are effected during stage
P) are taking place in the arches, the tissue of the vertebral
bodies or cartilaginous investment of the notochord, though
much thicker than before, still remains as a continuous tube
whose wall exhibits no segmental differentiations.

It is in stage Q that these differentiations first appear in the
vertebral regions opposite the origin of the neural arches. The
outermost part of the cartilage at these points becomes hyaline
and almost undistinguishable in structure from the tissue of the
arches\ These patches of hyaline cartilage grow larger and cause
the vertebral parts of the column to constrict the notochord,
whilst the intervertebral parts remain more passive, but become
composed of cells with very little intercellular substance.
Coincidently also with these changes, part of the layer internal
to the hyaline cartilage becomes modified to form a somewhat
peculiar tissue, the intercellular substance of which does not
stain, and in which calcification eventually arises (PL xii. fig. 11).
The innermost layer adjoining the notochord retains its primi-
tive fibrous character, and is distinguishable as a separate layer
through both the vertebral and the intervertebral regions. As a
result of these changes a transverse section through the centre
of the vertebral reo^ions now exhibits three successive rings
(vide PL XII. fig. 11), an external ring of hyaline cartilage in-
vested by ' the membrana elastica externa' {m.el), followed by a

1 A good representation of a longitudinal section at this stage is given by
Cartier {Zeitschrift f. Wiss. Zoologie, Bd. xxv., Supplement PI. iv, fig. 1), who
also gives a fair description of the succeeding changes of the vertebral column.

B. 11

154 THE NOTOCHORD.

ring of calcifying cartilage, and internal to this a ring of fibrous
cartilage, which adjoins the now slightly constricted notochord.
A transverse section of an intervertebral region shows only a
thick outer and thin inner ring of fibrous cartilage, the latter in
contact with the sheath of the unconstricted notochord.

The constriction of the notochord proceeds till in the centre
of the vertebrae it merely forms a fibrous band. The tissue
internal to the calcifying cartilage then becomes hyaline, so that
there is formed in the centre of each vertebral body a ring
of hyaline cartilage immediately surrounding the fibrous band
which connects the two unconstricted segments of the noto-
chord. The intervertebral tissue becomes more and more
fibrous. In Cartier's paper before quoted there is a figure
(fig. 3) which represents the appearance presented by a longi-
tudinal section of the vertebral column at this stage.

The relation of the vertebral bodies to the arches requires a
short notice. The vertebral hyaline cartilage becomes almost
precisely similar to the tissue of the arches, and the result is,
that were it not for the 'membrana elastica externa' it would
be hardly possible to distinguish the limits of the two tissues.
This membrane how^ever persists till the hyaline cartilage has
become a very thick layer (PI. Xll. fig. 11), but I have failed
to detect it in the adult, so that I cannot there clearly dis-
tinguish the arches from the body of the vertebrae. From a
comparison however of the adult with the embryo, it is clear
that the arches at most form but a small part of what is usually
spoken of as the body of the vertebrae.

The chano^es in the notochord itself durinof the stao^es sub-
sequent to K are not of great importance. The central part
retains for some time its previous structure, being formed of
large vacuolated cells with an occasional triangular patch of
protoplasm containing the starved nucleus and invested by
indurated layers of protoplasm. These indurated layers are
all fused, and are probably rightly regarded by Gegenbaur
and Gotte as representing a sparse intercellular matter. The
external protoplasmic layer of the notochord ceases shortly
after stage K to exhibit any traces of a division into separate
cells, but forms a continuous layer with irregular prominences
and numerous nuclei (PI. xii. fig. 9). In the stages subsequent

DEYELOPMEXT OF ELASMOBRANX'H FISHES. 155

to P further changes take place in the notochord : the remains
of the cells become more scanty and the intercellular tissue
assumes a radiating arrangement, giving to sections of the noto-
chord the appearance of a number of lines radiating from the
centre to the periphery (PL xii. fig. 11).

The sheath of the notochord at first grows in thickness, and
during stage L there is no difficulty in seeing in it the fine radial
markings already noticed by Mliller^ and Gegenbaur^ and re-
garded by them as indicating pores. Closely investing the sheath
of the notochord there is to be seen a distinct membrane, which,
though as a rule closely adherent to the sheath, in some
examples separates itself from it. It is perhaps the membrane
identified by W. Miiller^ (though not by Gegenbaur) as Kol-
liker's ' membrana elastica interna.' After the formation of
the cartilaginous investment of the notochord, this membrane
becomes more difficult to see than in the earlier stage, though
I still fancy that I have been able to detect it. The sheath
of notochord also appears to me to become thinner, and its
radial striation is certainly less easy to detect ^

1 Jenaische Zeitschrift, Vol. vi. ^ ^ioc. cit. ^ Loc. cit.

^ Gegenbaur makes the reserve statement with reference to the sheath of
the notochord. For my own sections the statement in the text^ certainly holds
good. Fortunately the point is one of no importance.

11—2

CHAPTER VIIL

Development of the Spinal Nerves and of the
Sympathetic Nervous System.

The spinal nerves.
The development of the spinal nerves has been already
treated by me at considerable length in a paper read before
the Royal Society in December, 1875 ^ and I have but little
fresh matter to add to the facts narrated in that paper. The
succeeding account, though fairly complete, is much less full
than the previous one in the Philosophical Transactions, but a
number of morphological considerations bearing on this sub-
ject are discussed.

The rudiments of the posterior roots make their appear-
ance considerably before those of the anterior roots. They
arise during stage I, as outgrowths from the spinal cord, at
a time when the muscle-plates do not extend beyond a third of
the way up the sides of the spinal cord, and in a part where no
scattered mesoblast-cells are present. They are formed first in
the anterior part of the body and successively in the posterior
parts, in the following way. At a point where a spinal nerve
is about to arise, the cells of the dorsal part of the cord begin
to proliferate, and the uniform outline of the cord becomes
broken (PL xiii. fig. 3). There is formed in this way a small
prominence of cells springing from the sum.mit of the spinal
cord, and constituting a rudiment of a pair of posterior roots.
In sections anterior to the point where a nerve is about to
appear, the nerve-rudiments are always very distinctly formed.
Such a section is shown in PI. xiii. fig. 2, and the rudiments
may there be seen as two club-shaped masses of cells, which
have grown outwards and downwards from the extreme dorsal
summit of the neural canal and in contact with its walls. The
rudiments of the two sides meet at their point of origin at the
dorsal median line, and are dorsally perfectly continuous with
the walls of the canal.

» Phil Trans. Vol. 166, p. 175.

DEVELOPMENT OF ELASMOBRANCH FISHES. 157

It is a remarkable fact that rudiments of posterior roots
are to be seen in every section. This may be interpreted as
meaning that the rudiments are in very close contact with each
other, but more probably means, as I hope to show in the
sequel,, that there arises from the spinal cord a continuous
outgrowth from which discontinuous processes (the rudiments
of posterior roots) grow out.

After their first formation these rudiments grow rapidly ven-
tralwards in close contact with the spinal cord (vide PI. xiii. fig.
1, and PL X. figs. 6 and 7), but soon meet with and become
partially enclosed in the mesoblastic tissue (PI. x. fi^. 7). The
similarity of the mesoblast and nerve-tissue in Scyllium and
Pristiurus embryos hardened in picric or chromic acid, render
the nerves in these genera, at the stage when they first become
enveloped in mesoblast, difficult objects to observe ; but no
similar difficulty is encountered in the case of Torpedo embryos.

While the rudiments of the posterior roots are still quite
short, those of the anterior roots make their first appearance.
Each of these (PI. xiii. fig. 4 a.r.) arises as a very small but dis-
tinct conical outgrowth from a ventral corner of the spinal
cord. From the very first the rudiments of the anterior roots
have an indistinct form of peripheral termination and some-
what fibrous appearance, while the protoplasm of which they
are composed becomes attenuated towards its end. The
points of origin of the anterior roots from the spinal cord
are separated by considerable intervals. In this fact, and also
in the fact of the nerves of the two sides never being united
with each other in the median line, the anterior roots exhibit
a marked contrast to the posterior. There are thus constituted,
before the close of stage I, the rudiments of both the anterior
and posterior roots of the spinal nerves. The rudiments of both
of these take their origin from the involuted epiblast of the
neural canal, and the two roots of each spinal nerve are at
first quite unconnected with each other. It is scarcely neces-
sary to state that the pairs of roots correspond in number with
the muscle-plates.

It is not my intention to enter with any detail into the
subsequent changes of the rudiments whose origin has been
described, but a few points especially connected with their

158 FIRST FORMATION OF THE SPINAL NERVES.

early development are sufficiently important to call for atten-
tion.

One feature of the posterior roots at tlieir first formation
is the fact that they appear as processes of a continuous out-
growth of the spinal cord. This state of affairs is not of long
continuance, and before the close of stage I each posterior root
has a separate junction with the- spinal cord. What then be-
comes of the originally continuous outgrowth ? It has not
been possible for me to trace the fate of this step by step;
but the discovery that at a slightly later period (stage K) there
is present a continuous commissure independent of the spinal
cord connecting the dorsal and central extremities of all
the spinal nerves, renders it very probable that the original
continuous outgrowth becomes converted into this commissure.
Like all the other nervous structures, this commissure is far
more easily seen in embryos hardened in a mixture of osmic and
chromic acids or osmic acid, than in those hardened in picric
acid. Its existence must be regarded as one of the most re-
markable results of my researches upon the Elasmobranch
nervous system. At stage K it is fairly tliick, though it becomes
much thinner at a slightly later period. Its condition during
stage K is shown in Plate xi. fig. 18, com. What it has been
possible for me to make out of its eventual fate is mentioned
subsequently \

A second feature of the eaiiiest condition of the posterior
roots is their attachment to the extreme dorsal summit of the
spinal cord — a point of attachment very different from that
which they eventually acquire. Before the commencement of
stage K this state of things has become altered ; and the pos^
terior roots spring from the spinal cord in the position normal
for Vertebrates.

This apparent migration caused me at first great perplexity,
and I do not feel quite satisfied that I have yet got completely
to the bottom of its meaning. The explanation which appears-
to me most probable has suggested itself in the course of some
observations on the development of the thin roof of the fourth

1 It is not by any means always possible to detect tliis commissure in trans-
verse sections. As I hnve suggested, in connection with a similar commissure
counecting the vagus branches, it perhaps easily falls out of the section, and is
always so small that the hole left would certainly be invisible.

DEVELOPMENT OF ELASMOBRANX'H FISHES. 159

ventricle. A growth of cells appears to take place in the median
dorsal line of the roof of the spinal cord. This growth tends to
divaricate the two lateral parts of the cord, which are originall}'-
contiguous in the dorsal line, and causes therefore the posterior
roots, which at first spring from the dorsal summit, to assume
an apparent attachn^ent to the side of the cord at some little
distance from the summit. If this is the true explanation of
the change of position wdiich takes place, it must be regarded
as due rather to peculiar growths in the spinal cord, than
to any alteration in the absolute attachment of the nerves.

By stage K the rudiment of the posterior root has become
greatly elongated, and exhibits a division into three distinct
portions (PL XIII. fig. 6) :

(1) A proximal portion, in which is situated the pedicle of
attachment to the wall of the neural canal.

(2) an enlarged portion, which may conveniently from its
future fate be called the spinal ganglion.

(3) a distal portion beyond this.

The proximal portion presents a fairly uniform diameter, and
ends dorsally in a rounded expansion ; it is attached, remarkably
enough, not hy its extremity/, but hy its side, to the spinal cord.
The dorsal extremities of the posterior roots are therefore free.
It seems almost certain that the free dorsal extremities of these
roots serve as the starting points for the dorsal commissure
before mentioned, which connects the roots together. The
attachment of the posterior nerve-root to the spinal cord is,
on account of its small size, very difficult to observe. In
favourable specimens there may however be seen a distinct
cellular prominence froni the spinal cord, which becomes con-
tinuous with a small prominence on the lateral border of the
nerve-root near its distal extremity. The proximal extremity
of the rudiment is composed of cells, which, by their small size
and circular form, are easily distinguished from those which
form the succeeding or ganglionic portion of the nerve. This
succeeding part has a swollen configuration, and is composed
o.f large elongated cells with oval nuclei. The remainder of the
rudiment forms the commencement of the true nerve.

The anterior root, which, at the close of stage I, formed a
small and inconspicuous pruminencc from tlie spinal cord.

160 THE COMMISSURE OF THE POSTERIOR ROOTS.

grows rapidly duriDg the succeeding stages, and soon forms
an elongated cellular structure with a wide attachment to the

o

spinal cord (PL xiii. fig. 5). At first it passes obliquely and
nearly horizontally outwards, but, before reaching the muscle-
plate of its side, takes a bend downwards (PL xiii. fig. 7).

I have not definitely made out when the anterior and pos-
terior roots unite, but this may easily be seen to take place
before the close of stage K (PL xi. fig. 18).

One feature of some interest with reference to the anterior
roots, is the fact that they arise not vertically below, but alter-
nately with the dorsal roots, a condition which persists in the
adult.

Although I have made some efforts to determine the even-
tual fate of the commissure uniting the dorsal roots, these have
not hitherto been crowned with success. It grows thinner and
thinner, becoming at the same time composed of fibrous pro-
toplasm with imbedded nuclei (PL xiii. fig. 8 and 9). By stage
M it is so small as to be quite indistinguishable in transverse
sections ; and I have failed in stage P to recognize it at all. I can
only conclude that it gradually atrophies, and finally vanishes
without leaving a trace. Both its appearance and history are
very remarkable, and deserve the careful attention of future
investigators.

There can be little doubt that it is some sort of remnant of
an ancestral structure in the nervous system ; and it would
appear to indicate that the central nervous system must origi-
nally have been formed of a median and two lateral strands.
At the same time I very much doubt whether it can be
brought into relation with the three rows of ganglion-cells
(a median and two lateral) which are so frequently present
on the ventral side of annelidan nerve-cords.

My results may he summarised as follows: — Along the
extreme dorsal summit of the spinal cord there arises on each
side a continuous outgrowth. From each outgrowth processes
corresponding in number to the muscle-plates grow downwards.
These are the rudiments of the posterior nerve-roots. The
outgrowths, though at first attached to the spinal cord through-
out their whole length, soon cease to be so, and remain in
connection with it at certain points only, which form the

DEVELOPMENT OF ELASMOBRA^X'H FISHES. IGl

primitive juDctions of the posterior roots witli the spinal cord.
The original outgrowth on each side remains as a bridge,
uniting together the dorsal extremities of all the posterior roots.
The posterior roots, though primitively attached to the dorsal
summit of the spinal cord, eventually come to arise from its
sides. The original homogeneous rudiments before the close of
stage K become differentiated into a root, a ganglion, and a
nerve.

The anterior roots, like the posterior, are outgi'owths from
the spinal cord, but are united independently with it, and the
points from which they spring originally, remain as those by
which they are permanently attached. The anterior roots arise,
not vertically below, but in the intervals between the posterior
roots. They are at first quite separate from the posterior roots ;
but before the close of stage K a junction is effected between
each posterior root and the corresponding anterior root. The
anterior root joins the posterior at some little distance below
its ganglion.

The results here arrived at are nearly in direct opposition to
those of the majority of investigators, though in accordance, at
least so far as the posterior roots are concerned, with the beau-
tiful observations of Hensen ' on the Development of Mammalia \'

Mr MarshalP has more recently published a paper on the
development of the nerves in Birds, in which he shows in a
most striking manner that the observations recorded here for
Elasmobranchs hold good for the posterior roots of Birds. The
similarity between his figures and my own is very noticeable.
A further discussion of the literature would be quite unprofit-
able, and I proceed at once to certain considerations suggested
by the above observations.

General considerations. — One point of general anatomy upon
which my observations throw considerable light, is the primitive
origin of nerves. So long as it was admitted that the spinal
and cerebral nerves developed in the embryo independently
of the central nervous system, their mode of origin always
presented to my mind considerable difficulties. It never ap-

1 Zeit.f. Anat. w. Entivicl(hing>tgeschichtc, Vol. i.

' Journal of Anatomy and Physiology, Vol. xi. April, 1877.

1G2 ORIGIN OF NERVES.

peared clear how it was possible for a state of things to
have arisen in which the central nervous system as well
as the peripheral terminations of nerves, whether motor or
sensory, were formed independently of each other ; while be-
tween them a third structure was developed, which, growing
out either towards the centre or towards the periphery, ulti-
mately brought the two into connection. That such a con-
dition could be a primitive one seemed scarcely possible.

Still more remarkable did it appear, on the supposition that
the primitive mode of formation of these parts was represented
in the developmental history of Vertebrates, that we should find
similar structural elements in the central and in the peripheral
nervous systems. The central nervous system arises from the
epiblast, and yet contains precisely similar nerve-cells and nerve-
fibres to the peripheral nervous system, which, when derived
from the mesoblast, was necessarily supposed to have an origin
completely different from that of the central nervous system.
Both of these difficulties are to a great extent removed by the
facts of the development of these parts in Elasmobranchs.

It is possible to suppose that in their primitive differentia-
tion contractile and sensory systems may, as in Hydra\ have
been developed from the protoplasm of even the same cell.
As the sensory and motor systems became more complicated, the
sensory portion of a cell would become separated by an in-
creasing interval from the muscular part of a cell, and the two
parts of a cell would only be connected by a long protoplasmic
process. When such a condition as that was reached, the
sensory portion of the cell would be called a ganglion-cell or
terminal sensory organ, the connecting process a nerve, and the
contractile portion of the cell a muscle-cell. When these organs
were in this condition, it might not impossibly happen for
the general developmental growth which tended to separate the
ganglion-cell and the muscle-cell to be so rapid as to render it
impossible for the growth of the connecting nerve to keep pace
with it, and that thus the process connecting the ganglion- cell
and the muscle-cell might become ruptured. Nevertheless the
tendency of the process to grow from the ganglion cell to the

^ KlcincnbciG Hydra.

DEVELOPMENT OF ELASMOBRANCH FISHES. 163

muscle-cell, would remain, and when the rapid developmental
growth had ceased, the two would become united again by the
growth of the process which had previously been ruptured. It
will be seen that this hypothesis, which I have considered only
with reference ta a single nerve and muscle-cell, might be
extended so as to apply to a complicated central nervous system
and peripheral nerves and muscles, and also could apply equally
as well to the sensary as to the nK>tor terminations of a nerve.
In the case of the sensory termination, we should only have to
suppose that the centre nervous cell became more and more
separated by the general growth from the recipient terminal
sensory cell, and that during the general growth the connection
between the twa was mechanically ruptured but restored again
on the termination of the more rapid gTOwth.

As the descendants of the animal in which the rupture
occurred became progressively more complicated, the two ter-
minal cells must have become widely separated at a continu-
ally earlier period, till finally they may have been separated
at a period of development when they were indistinguishable
from the surrounding embryonic cells ; and since the rupture
would also occur at this period, the primitive junction between
the nerve-centre and termination would escape detection. The
object of this hypothesis is to explain the facts, so far as they
are known, of the development of the nervous system in Verte-
brates.

In Vertebrates we certainly appear to have an outgrowth
from the nervous system, which eventually becomes united
with the muscle or sensory terminal organs. The ingenious
hypothetical scheme of development of the nerves given by
Hensen^ would be far preferable to the one suggested if it
could be brought into conformity with the facts. There
is, however, at present no evidence for Hensen's view, as he
himself admits, but considering how little we know of the
finer details of the development of nerves, it seems not im-
possible that such evidence may be eventually forthcoming.
The evidence from my own observation is, so far as it goes,
against it. At a time anterior to the outgrowth of the spinal

1 Virchow's Archiv, Vol. xxxi. 1861.

16-t OKIGIN OF NERVES.

nerves, I have shewn* that the spinal cord is campletely in-
vested by a delicate hyaline membrane. It is difficult to
believe that this is pierced by a number of fine processes, which
completely escape detection, but which must, nevertheless, be
present on the hypothesis of Hensen.

The facts of the development of nerves in Vertebrates are
unquestionably still involved in considerable doubt. It may,
I think, be considered as certain, that in Elasmobranchs the
roots of the spinal and cranial nerves are outgrowths of the
central nervous system. How the final terminations of the
nerves are formed is, however, far from being settled. Gotte*^,
whose account of the development of the spinal ganglia is com-
pletely in accordance with the ordinary views, yet states^ that
the growth of the nerve-fibres themselves is a centrifugal one
from the ganglia. My own investigations prove that the ganglia
have a centrifugal development, and also appear to demonstrate
that the nerves themselves near the ganglion have a similar
manner of growth. Moreover, the account given in the pre-
ceding chapter of the manner in which the nerves become con-
nected with the mucous canals of the head, goes far to prove
that the whole growth of the nerves is a centrifugal one.
The combination of all these converging observations tells
strongly in favour of this view.

On the other hand, Calberla'^ believes that in the tails of
larval Amphibians he has seen connective-tissue cells unite with
nerve-processes, and become converted into nerves, but he ad-
mits that he cannot definitely prove that the axis-cylinder has
not a centrifugal growth, while the connective-tissue cells
merely become converted into the sheath of the nerve. If
Calberla's view be adopted, that the nerves are developed
directly out of a chain of originally indifferent cells, each cell
of the chain being converted in turn into a section of the nerve,
an altogether different origin of nerves from that I have just
suggested would seem to be indicated.

The obvious difficulty, already alluded to, of understanding
how it is, according to the generally accepted mode of develop-
ment of the spinal nerves, that precisely similar nerve-cells and

1 PhU. Travf!., 1876. '^ Enttcicklunpsgeschichte der Unke.

^ Loc. cit. p. 51G. •* Archiv filr Micros. Anat. Vol. xi. 1875.

DEVELOPMENT OF ELASMOBKAXCH FISHES. 1=65

nerves should arise in structures which have such different
origins as the central nervous system and the spinal nerves, is
completely removed if my statements on the development of
the nerves in Elasmobranch represent the truth.

One point brought out in my investigations appears to me
to have bearings upon the origin of the central canal of the
vertebrate nervous system, and in consequence upon the origin
of the V'ertebrate nervous system itself. This point is, that the
posterior nerve-rudiments make their first appearance at the
extreme dorsal summit of the spinal cord. The transverse
section of the ventral nervous cord of an ordinary segmented
Annelid consists of two symmetrical halves placed side by
side. If by a mechanical folding the two lateral halves of
the nervous cord became bent towards each other, while into
the groove between the tw^o the external skin became pushed,
we should have an approximation to the vertebrate nervous
system. Such a folding as this might take place to give extra
rigidity to the body in the absence of a vertebral column.

If this foldiug were then completed in such a way that
the groove, lined by external skin and situated between the two
lateral columns of the nervous system, became converted into
a canal, above and below which the two columns of the nervous
system united, we should have in the transformed nervous cord
an organ strongly resembling the spinal cord of Vertebrates.

It is well known that the nerve-cells are always situated on
the ventral side of the abdominal nerve-cord of Annelids, either
as a continuous layer, or in the form of two, or more usually,
three bands. The dorsal side of the cord is composed of nerve-
fibres or white matter. If the folding I have supposed were to
take place in the Annelid nervous-cord, the grey and white
matters would have very nearly the same relative situations as
they have in the Vertebrate spinal cord. The grey matter
would be situated in the interior and line the central canal, and
the white matter would nearly surround the grey. The nerves
would then arise, not from the sides of the nervous cord as in
existing Annelids, but from its extreme ventral summit. One
of the most striking features which I have brought to light with
reference to the development of the posterior roots, is the fact
of their growing out from the extreme dorsal summit of the

1G6 VERTEBRATE AND ANNELID AX NERVOUS SYSTEMS.

neural canal, a position analogous to the ventral summit of the'
Annelidan nervous cord. Thus the posterior roots of the
nerves in Elasmobranchs^ arise, in the exact manner which
might have been anticipated, were the spinal canal due to
such a folding as I have suggested.

The argument from the position of the outgrowth of nerves
becomes the more striking from its great peculiarity, and
forms a feature which would be most perplexing without
some such explanation as I have proposed. The central epi-
thelium of the neural canal, according to this view, represents
the external skin, and its ciliation in certain cases may, per-
haps, be explained as a remnant of the ciliation of the external
skin still found amongst many of the lower Annelids.

I have employed the comparison of the Vertebrate and
Annelidan nervous cords, not so much to prove a genetic rela-
tion between the two, as to show the a priori possibility of the
formation of a spinal cord, and the a posteriori evidence we
have of the vertebrate canal having been formed in the way
indicated. I have not made use of what is really my strongest
argument, viz. that the embryological mode of formation of the
spinal canal by a folding in of the external epiblast is the very
method by which I supposed the spinal canal to have been
formed in the ancestors of Vertebrates. My object has been to
suggest a meaning for the peculiar primitive position of the
posterior roots, rather than to attempt to explain in full the
origin of the spinal canal.

Although the homologies between the Vertebrate and the
Annelidan nervous systems are not necessarily involved in the
questions which arise with reference to the formation of the
spinal canal, they have nevertheless considerable bearings on it.

Two views have recently been put forward on this subject.
Professor Gegenbaur^ looks upon the central nervous system
of Vertebrates as equivalent to the superior oesophageal ganglia

1 There are strong reasons for regarding the posterior roots as the primitive
ones. These are spoken of later, but I may state that they depend:

(1) On the fact that only iwsterior roots exist in the brain.

(2) That only posterior roots exist in Amphioxus.

(3) That the posterior roots devclope at an earlier period than the
anterior.

2 Gnnidriss d. Yerpleiclienden Anat. p. 264.

DEVELOPMENT OF ELASMOBRANCH FISHES. 1G7

of Annelids and Arthropods only, while Professors Leydig^ and
Semper^ and Dr Dohrn^ compare it with the whole Annelidan
nervous system.

The first of these two views is only possible on the suppo-
sition that Vertebrates are descended from unsegmented ances-
tors, and even then presents considerable difficulties. If the
ancestors of Vertebrates were segmented animals, and several of
the recent researches tend to shew that they were, they must
almost certainly have possessed a nervous cord like that of exist-
ing Annelids. If such were the case, it is almost inconceivable
that the greater portion of the nervous system which forms the
ventral cord can have become lost, and the system reduced to
the superior oesophageal ganglia. Dr Dohrn*, who has specu-
lated very profoundly on this matter, has attempted to explain
and remove some of the difficulties which arise in comparing
the nervous systems of Vertebrates and Annelids. He supposes
that the segmented Annelids, from which Vertebrates are
descended, were swimming animals. He further supposes that
their alimentary canal was pierced by a number of gill-slits,
and that the anterior amongst these served for the intro-
duction of nutriment into the alimentary canal, in fact as
supplementary mouths as well as for respiration. Eventually
the old mouth and throat atroj)hied, and one pair of coa-
lesced gill-slits came to serve as the sole mouth. Thus it came
about that on the disappearance of that portion of the alimen-
tary canal, which penetrated the oesophageal nervous ring, the
latter structure ceased to be visible as such, and no part of the
alimentary canal was any longer enclosed by a commissure of the
central nervous system. With the change of mouth Dr Dohrn
also supposes that there took place a change, which would for a
swimming animal be one of no great difficulty, of the ventral for

1 Bau des tJiierischen Kdrpers.

2 Stammesvericandscliaftd.Wirhelthiere u. Wirbellosen and Die Verwandschafts-
heziehungen d. gegliederten Thiere. This latter work, for a copy of which I
return my best thanks to the author, came into my hands after what follows
was written, and I much regret only to have been able to make one or two passing
allusions to it. The work is a most important contribution to the questions
about to be discussed, and contains a great deal that is veiy suggestive ; some of
the conclusions with reference to the Nervous System appear to me however to
be directly opposed to the observations on Spinal Nerves above recorded.

^ Ur.^prung d. Wirbelthiere u. Frincip des Functionswechsels.
^ Loc. cit.

168 DR DOIIIIN S HYrOTHESIS.

the dorsal surface. This general explanation of Dr Dohrn's,
apart from the considerable difficulty of the fresh mouth, appears
to me to be fairly satisfactory. Dr Dohrn has not however in
my opinion satisfactorily dealt with the questions of detail
which arise in connection with this comparison. One of
the most important points for his theory is to settle the
position where the nervous system was formerly pierced by
the oesophagus. This position he fixes in the fourth ventricle,
and supports his hypothesis by the thinness of the roof of the
spinal canal in this place, and the absence (?) of nervous struc-
tures in it.

It appears to me that this thinness cannot be used as an
argument. In the first place, if the hypothesis I have suggested
as to the formation of the spinal canal be accepted, the forma-
tion of the canal must be supposed to have occurred in point of
time either after or before the loss of the primitive mouth.
If, on the one hand, the spinal canal made its appearance before
the atrophy of the primitive mouth, the folding to form it
must necessarily have ceased behind the mouth; and, on the
supposition of the oesophageal ring having been situated in the
region of the fourth ventricle, a continuation of the spinal canal
could not be present in front of this part. If, on the other hand,
the cerebro-spinal canal appeared after the disappearance of the
primitive mouth, its roof must necessarily also be a formation
subsequent to the atrophy of the mouth, and varieties of struc-
ture in it can have no bearing upon the previous position of
the mouth.

But apart from speculations upon the origin of the spinal
cord, there are strong arguments against Dr Dohrn's view about
the fourth ventricle. In the first place, were the fourth ven-
tricle to be the part of the nervous system which previously
formed the oesophageal commissures, we should expect to find
the opening in the nervous system at this point to be visible
at an early period of development, and at a later period to
cease to be so. The reverse is however the case. In early
embryonic life the roof of the fourth ventricle is indistinguish-
able from other parts of the nervous system, and only thins
out at a later period. Further than this, any explanation
of the thin roof of the fourth ventricle ouojht also to elucidate

DEVELOPMENT OF ELASMOBRAXCH FISHES. 1G9

the nearly similar striictiire in the sinus rhomboidalis, and
cannot be considered satisfactory unless it does so.

The peculiarities of the cerebro -spinal canal in the region
of the brain appear to me to present considerable difficulties in
the way of comparing the central nervous system of Vertebrates
and segmented Annelids. The manner in which the cerebro-
spinal canal is prolonged into the optic vesicles, the cerebral and
the optic lobes is certainly opposed both to an intelligible expla-
nation of the spinal canal itself, and also to a comparison of the
two nervous systems under consideration.

Its continuation into the cerebral hemispheres and into the
optic lobes (mid-brain) may perhaps be looked upon as due to
peculiar secondary growths of those two ganglia, but it is
very difficult to understand its continuation into the optic
vesicles.

If it be granted that the spinal canal has arisen from a
folding in of the external skin, then the present inner surface of
the optic vesicle must also have been its original outer surface,
and it follows as a necessary consequence that the present
position of the rods and cones behind and not in front of the
nervous structures of the retina w^as not the primitive one. The
rods and cones arise, as is well known, from the inner surface of
the outer portion of the optic vesicle, and must, according to
the above view, be supposed originally to have heen situated
on the external surface, and have only come to occupy their
present position duriug the folding in, w^hich resulted in the
spinal canal. On d ])riori grounds we should certainly expect
the rods and cones to have resulted from the differentiation of
a layer of cells external to the conducting nervous structures.
The position of the rods and cones posterior to these suggests
therefore that some peculiar infolding has occurred, and may be
used as an argument to prove that the medullary groove is no
mere embryonic structure, but the embryonic repetition of an
ancestral change. The supposition of such a change of position
in the rods and cones necessarily implies that the folding in
to form the spinal canal must have been a very slow one. It
must have given time to the refracting media of the eye
gradually to travel round, so as still to maintain their primitive
position, wdiile in successive generations a rudimentary spinal
B. 12

170 HOMOLOGIES OF THE VERTEBRATE NERVOUS SYSTEM.

furrow carrying with it the retina became gradually converted
into a canaP.

If Dr Dohrn's comparison of the vertebrate nervous system
with that of segmented Annelids be accepted, the following
two points must in my opinion be admitted : —

(1) That the formation of the cerebro- spinal canal was sub-
sequent to the loss of the old mouth.

(2) That the position of the old mouth is still unknown.
The well-known view of looking at the pituitary and pineal

growths as the remnants of the primitive oesophagus, has no
doubt some features to recommend it. Nearly conclusive
against it is the fact that the pituitary involution is not, as
used to be supposed, a growth towards the infundibulum of the
hypoblast of the oesophagus, but of the epiblast of the mouth.
It is almost inconceivable that an involution from the present
mouth can have assisted in forming part of the old oesophagus.

There is a view not involving the difficulty of the oeso-
phageal ring, fresh mouth ^, and of the change of the ventral to
the dorsal surface, which, though so far unsupported by any

1 Professor Huxley informs me that lie has for many years entertained some-
what similar views to those in the text ahout the position of the rods and
cones, and has been accustomed to teach them in his lectures.

2 Professor Semper i(Z)te Verwandtschaftsheziehungen d. gegliederten Thiere,
Arbeiten aus d. Zool.-zoot. Jnstitut,Wuvzhm-g, 1876) has some interesting specu-
lations on the difficult question of the vertebrate mouth, which have unfortu-
nately come to my knowledge too late to be either fully discussed or incorporated
in the text. These speculations are founded on a comparison of the condition
of the mouth in Tm-bellarians and Nemertines, He comes to the conclusion
that there was a primitive mouth on the cardiac side of the supra-oesophageal
ganglion, which is the existing mouth of Turbellarians and Vertebrates and the
opening of the proboscis of Nemertines, but which has been replaced by a
fresh mouth on the neural side in Annelids and Nem-ertines. In Nemertines
however the two mouths co exist — the vertebrate mouth as the opening of the
proboscis, and the Annelid mouth as the opening for the alimentary tract.
This ingenious hypothesis is supported by certain anatomical facts, which do
not appear to me of great weight, but for which the reader must refer to the
original paper. It no doubt avoids the difficulty of the present position of the
vertebrate mouth, but unfortunately at the same time substitutes an equal diffi-
culty in the origin of the Annelidan mouth. This Professor Semper attempts to
get over by an hypothesis wliicli to my mind is not very satisfactory (p. 878),
which, however, and this Professor Semper does not appear to have noticed,
could equally well be employed to explain the origin of a Vertebrate mouth as a
secondary formation subsequent to the Annelidan mouth. Under these circum-
stances this fresh hypothesis does not bring us very much nearer to a solution
of the vertebrate-annelid mouth question, but merely substitutes one difficulty
for another; and does not appear to me so satisfactory as the hypothesis sug-
gested in the text.

At the same time Professor Semjoer's h}i3othesis suggests an explanation
of that curious organ the Nemertine proboscis. If the order of changes

DEVELOPMENT OF ELASMOBRANCH FISHES. 171

firm basis of observed facts, nevertheless apjoears to me worth
suggesting. It assumes that Vertebrates are descended not
through the present line of segmented Vermes, but through
some other line which has now, so far as is known, completely
vanished. This line must be supposed to have originated from
the same unsegmented Vermes as the present segmented Anne-
lids. They therefore acquired fundamentally similar segmental
and other Annelidan organs.

The difference between the two branches of the Vermes
lay in the nervous system. The unsegmented ancestors of the
'present Annelids seem to have had a pair of super-oesophageal
ganglia, from which two main nervous stems extended back-
wards, one on each side of the body. Such a nervous system in
fact as is possessed by existing Nemertines or Turbellarians^.
As the Vermes became segmented and formed the Annelids,
these side nerves seem to have developed ganglia, corresponding
in number wdth the segments, and finally, approximating on
the ventral surface, to have formed the ventral cord^

The other branch of Vermes which I suppose to have been
the ancestors of Vertebrates started from the same stock as
existing Annelids, but I conceive the lateral nerve-cords, instead
of approximating ventrally, to have done so dorsally, and thus a
dorsal cord to have become formed analogous to the ventral
cord of living Annelids, only without an oesophageal nerve-ring ^

It appears to me, (if the difficulties of comparing the
Annelidan ventral cord with the spinal cord of Vertebrates are
found to be insurmountable), that this hypothesis would involve
far fewer improbabilities than one which supposes the whole
central nervous system of Vertebrates to be homologous with
the super-oesophageal ganglia. The mode of formation of a

suggested by him were altered it might be possible to suppose that there
never was more than one mouth for all Vermes, but that the proboscis in
Nemertines gradually spht itself off from the oesophagus to which it originally
belonged, and became quite free and provided with a separate opening and per-
haps carried with it the so-called vagus of Professors Semper and Leydig.

1 It is not of course to be supposed that the primitive nervous system was
pierced by a proboscis like that of the Nemertines.

2 This is Gegenbaur's view of the develoj^ment of the ventral cord, and I
regard it in the meantime as the most probable view which has been suggested.

2 A dorsal instead of a ventral approximation of the lateral nerve-cords
would be possible in the descendants of such living segmented Vermes as
Saccocirrus and Polygordius.

12—2

172 SYMPATHETIC NERYOUS SYSTEM.

nervous system presupposed in my hypothesis, well accords with
what w^e know of the formation of the ventral cord in exist-
ing Annelids.

The supposition of the existence of another branch of seg-
mented Vermes is not a very great difficulty. Even at the
present day we have possibly more than one branch of Vermes
which have independently acquired segmentation, viz. : the
Choetopodous Annelids and the Hirudinea. If the latter is an
isolated branch, it is especially interesting from having inde-
pendently developed a series of segmental organs like those of
Choetopodous Annelids, which we must suppose the ancestors of
Vertebrates also to have done if they too form an independent
branch.

In addition to the difficulty of imagining a fresh line of
segmented Vermes, there is another difficulty to my view, viz. :
the fact that in almost all Vermes, the blood flows forwards
in the dorsal vessel, and backwards in the ventral vessel.
This condition of the circulation very well suits the view of a
change of the dorsal for the ventral surfaces, but is opposed to
these surfaces being the same for Vertebrates and Vermes.
I cannot however regard this point as a very serious difficulty to
my view, considering how undefined is the circulation in the
unsegmented grouj)s of the Vermes.

Sympathetic nervous system.

Between stages K and L there may be seen short branches
from the spinal nerves, which take a course towards the median
line of the body, and terminate in small irregular cellular
masses immediately dorsal to the cardinal veins (PL xvii.
fig. 1, sy. g.). These form the first traces that have come under
my notice of the sympathetic nervous system. In the youngest
of my embryos in which I have detected these it has not been
possible for me either definitely to determine the antero-
posterior limits of the system, or to make certain whether the
terminal masses of cells which form the ganglia are connected
by a longitudinal commissure. In a stage slightly younger than
L the ganglia are much more definite, the anterior one is situ-
ated in the cardiac region close to the end of the intestinal
branch of the vagus, and the last of them quite at the posterior

DEVELOPMENT OF ELASMOBRANCH FISHES. 173

end of the abdominal cavity. The anterior ganglia are the
largest ; the commissural cord, if developed, is still very indistinct.
In stage L the commissural cord becomes definite, though not
very easy to see even in longitudinal sections, and the ganglia
become so considerable as not to be easily overlooked. They
are represented in PI. XIT. fig. 1, sy. g. and in PI. xvii. fig. 2
in the normal j)osition immediately above the cardinal veins.
The branches connecting them with the trunks of the spinal
nerves may still be seen without difficulty. In later stages these
branches cannot so easily be made out in sections, but the
ganglia themselves continue as fairly conspicuous objects. The
segmental arrangement of the ganglia is shewn in PI. xvii.
fig. 3, a longitudinal and vertical section of an embryo between
stages L and M with the junctions of the sympathetic ganglia
and spinal nerves. The gang-lia occupy the intervals bet^Yeen
the successive segments of the kiilneys.

The sympathetic system anly came under my notice at a
comparatively late period in my investigations, and the above
facts do not in all points clear up its development \ My obser-
vations seem to point to the sympathetic s^^stem arising as an
off-shoot from the cerebrospinal system. Intestinal branches
would seem to be developed on the main nerve stems of this in
the thoracic and abdominal regions, each of these then developes
a ganglion, and the ganglia become connected by a longitudinal
commissure. On this view a typical spinal nerve has the follow-
ing parts : two roots, a dorsal and ventral, the dorsal one
ganglionated, and three main branches, (1) a ramus dorsalis,
(2) a ramus ventralis, and (3) a ramus intestinalis. This scheme
may be advantageously compared with that of a typical cranial
nerve according to Gegenbaur. It may be noted that it brings
the sympathetic nervous system into accord with the other
parts of the nervous system as a product of the epiblast, and
derived from outgrowths from the neural axis. It is clear, how-
ever, that my investigations, though they may naturally be
interpreted in this wa}'", do not definitely exclude a completely
different method of development for the sympathetic system.

1 The formatiou out of tlie sympathetic gangha of the so-called pau-ecl supra-
renal bodies is dealt with iu couuection with the vascular system. The original
views of Leydig on these bodies are fully borne out by the facts of their develop-
ment.

CHAPTER IX.

The Development of the Organs in the Head.

Tlie Development of the Brain,

General History. In stage Q the braia presents a very
simple constitution (PI. VI. fig. G), and is in fact little more than
a dilated termination to the cerebro-spinal axis. Its length is
nearly one-third that of the whole body, being proportionately
very much greater than in the adult.

It is divided by very slight constrictions into three lobes,
the posterior of which is considerably the largest. These are
known as the fore- brain, the mid-brain, and the hind-brain.
The anterior part of the brain is bent slightly downwards about
an axis passing through the mid-brain. The walls of the brain,
composed of several rows of elongated columnar cells, have a
fairly uniform thickness, and even the roof of the hind-brain
is as thick as any other part. Towards the end of stage G
the section of the hind-brain becomes somewhat triangular
with the apex of the triangle directed downwards.

In Pristiurus during stage H no very important changes
take place in the constitution of the brain. In Scyllium,
however, indications appear in the hind- brain of its future
division into a cerebellum and medulla oblongata. The cavity
of the anterior part dilates and becomes rounded, while that
of the posterior part assumes in section an hour-glass shape,
owing to an increase in the thickness of the lateral parts of
the walls. At the same time the place of the original thick
roof is taken by a very thin layer, wliich is formed not so much
through a change in the character and arrangements of the
cells composing the roof, as by a divarication of the two
sides of the hind-brain, and the simultaneous introduction of a
fresh structure in the form of a thin sheet of cells connecting
dorsally the diverging lateral halves of this part of the brain.
By stage I, the hind-brain in Pristiurus also acquires an

DEVELOPMENT OF ELASMOBEANCH FISHES. 175

hour-glass shaped section, but the roof has hardly begun to
thin out (PL xiv. fig. 4a and 46).

Durinor stac^es I and K the cranial flexure becomes more and
more pronounced, and causes the mid-brain definitely to form
the termination of the long axis of the embryo (Ph XIV. fig. 1,
2, etc.), and before the close of stage K a thin coating of white
matter has appeared on the exterior of the whole brain, but
no other histoloo-ical chancres of interest have occurred.

During stage L an apparent rectification of the cranial flexure
commences, and is completed by &tage Q. The changes involved
in this process may be advantageously studied by comparing
the longitudinal sections of the brain during stages L, P, and Q,
represented in PL XV. fig. la, o and 7a.

It will be seen, first of all,, that so far from the flexure of the
brain itself being diminished, it is increased, and in P (fig. 5)
the angle in the floor of the mid-brain becomes very acute
indeed ; in other words, the anterior part of the brain has been
bent upon the posterior thro-ugh nearly two right angles, and the
infundibulum, or primitive front end of the brain, now points
nearly directly backwards. At the same time the cerebral hemi-
spheres have grown directly forwards, and if figures la and
5 in Plate XV. be compared it will be seen that in the older
brain of the two the cerebral hemispheres have assumed a
position which might be looked on as the result of their having
been pushed dorsalwards and forwards against the mid-brain,
and having in the process pressed in and nearly obliterated the
original thalamencephalon. The thalamencephalon in fig. la,
belonging to stage L, is relatively large, but in fig. 5, belonging
to stage P, it only occupies a very small space between the front
w^all of the mid-brain and the hind wall of the cerebral hemi-
spheres. It is therefore in part by the change in position of the
cerebral hemispheres that the angle between the trabeculse and
parachordals becomes increased, i.e. their flexure diminished,
while at the same time the flexure of the brain itself is in-
creased. More important perhaps in the apparent rectification
of the cranial flexure than any of the previously mentioned
points, is the appearance of a bend in the hind-brain which
tends to correct the original cranial flexure. The gradual growth
of this fresh flexure can be studied in the longitudinal sections

17G THE FORE-BRAIN.

which have been represented. It is at its maximum in stage Q.
This short preliminary sketch of the development of the brain
as a whole will serve as an introduction to the history of the
individual divisions of the brain.

Fore-hrain. In its earliest condition the fore-brain forms
a single vesicle without a trace of separate divisions, but buds
off very early the optic vesicles, whose history is described with
that of the eye (PL xiv. fig. 3, op. v). Between stages I and K
the posterior part of the fore-brain sends outwards a papiiliform
process towards the exterior, which forms the rudiment of the
pineal gland (PL XIV. fig. 1, _2^?i). Immediately in front of the
rudiment a constriction appears, causing a division of the fore-
brain into a large anterior and a small posterior portion. This
constriction is shallow at first, but towards the close of stage K
becomes much deeper (PL xiv. fig. 2 and fig. 16a), leaving
however the two cavities of the two divisions of the fore-brain
united ventrally by a somewhat wide canal.

The posterior of the two divisions of the fore-brain forms
the thalamencephalon. Its anterior wall adjoining the cerebral
rudiment becomes excessively thin (PI. xiv. fig. 11); and its base
till the close of stage K is in close contact with the mouth
involution, and presents but a very inconspicuous prominence
which marks the eventual position of the infundibulum (PL XIV.
fig. 9a, 12, 16, in). The anterior and larger division of the
fore-brain forms the rudiment of the cerebral hemispheres and
olfactory lobes. Up to stage K this rudiment remains per-
fectly simple, and exhibits no signs, either externally or
internally, of a longitudinal constriction into two lobes. From
the canal unitino' the two divisions of the fore-brain (which
eventually forms part of the thalamencephalon) there spring
the hollow optic nerves. A slight ventral constriction separating
the cerebral rudiment from that part of the brain where these
are attached appears even before the cJose of stage K (PL xiv.
fig. 11, op.n).

During stage L the infundibulum becomes much produced,
and forms a wide sack in contact with the pituitary body, and
its cavity communicates with that of the third ventricle by an
elongated slit-like aperture. This may be seen by comparing
PL XV. fig. la and Ic. In fig. Ic taken along the middle line,

DEVELOPMENT OF ELASMOBRANCH FISHES. 177

there is present a long opening into the infundibiihim (in), which
is shewn to be very narrov/ by being no longer present in fig. la
representing a section slightly to one side of the middle line.
During the same stage the pineal gland grows into a sack -like
body, springing from the roof of the thalamencephalon, fig. Ihypn.
This latter (the thalamencephalon) is now dorsally separated
from the cerebral rudiment by a deep constrictioD, an J also
ventrally by a less well marked constriction. At its side also a
deep constriction is being formed in it, immediately behind
the pineal gland. The cerebral rudiment is still quite un-
paired and exhibits no sign of becoming constricted into two
lobes.

During the next two stages the changes in the fore-brain
are of no great importance, and I pas& at once to stage 0.
The infundibulum is now nearly in the same condition as
during stage L, though (as is well shewn in the figure of a
longitudinal section of the next stage) it points more directly
backwards than before. The remaining parts of the thalamen-
cephalo-n have however undergone considerable changes. The
more important of these are illustrated by a section of stage O,
PI. XV. fig. 3, trans ver&e to the long axis of the embryo, and
therefore, owing to the cranial flexure, cutting the thalamen-
cephalon longitudinally and horizontally ; and for stage P in a
longitudinal and vertical section through the brain (PI. xv.
fig. 5). In the first place the roof of the thalamencephalon has
become very much shortened by the approximation of the cere-
bral rudiment to the mid-brain. The pineal sack has also
become greatly elongated, and its somewhat dilated extremity
is situated between the cerebral rudiment and the external
skin. It opens into the hind end of the third ventricle, and its
posterior w^all is continuous with the front wall of the mid-brain.
The sides of the thalamencephalon have become much thick-
ened, and form distinct optic thalami {op.) united by a very well
marked posterior commissure {p c). The anterior wall of the
thalamencephalon as well as its roof are very thin. The optic
nerves have become by stage O quite solid except at their
roots, into which the ventricles of the fore-brain are for a short
distance prolonged. This solidification is arrived at, so far as I
have determined, without the intervention of a fold. The

178 THE CEREBRAL HEMISPHERES.

nerves are fibrous, and a commencement of the chiasma is
certainly present. From the chiasma there appears to pass
out on each side a band of fibres, which runs near the outer
surface of the brain to the base of the optic lobes (mid-brain),
and here the fibres of the two sides again cross.

By stage important changes are perceptible in' the cere-
bral rudiment. In the first place there has appeared a slight
fold at its anterior extremity (PL XV. fig. 3, x), destined to form
a vertical septum dividing it into two hemispheres, and secondly,
lateral outgrowths (vide PI. xv. fig. 2, oil), to form^ the ol-
factory lobes. Its thin posterior wall presents on each side a
fold which projects into the central cavity. From the peri-
pheral end of each olfactory lobe a nerve similar in its histo-
logical constitution to any other cranial nerve makes its appear-
ance (PL XV. fig. 2) ; this divides into a number of branches,
one of which passes into the connective tissue between the two
layers of epithelium in each Schneiderian fold. On the root of
this nerve there is a large development of ganglionic cells. I
have not definitely observed its origin, but have no reason to
doubt that it is a direct outgrowth from the olfactory lobe,
exactly similar in its mode of development to any other nerve of
the body.

The cerebral rudiment undergoes great changes during
stage P. In addition to a great increase in the thickness of its
walls, the fold which appeared in tlie last stage has grown back-
wards, and now divides it in front into two lobes, the rudiments
of the cerebral hemispheres. The greater and posterior section
is still however quite undivided, and the cavities of the lobes
(lateral ventricles) though separated in front are still quite
continuous behind. At i\iQ same time, the olfactory lobes, each
containing a prolongation of the ventricle, have become much
more pronounced (vide PL xv. fig. 4a and 4c, oil). The root of
the olfactory nerve is now very thick, and the ganglion cells it
contains are directly prolonged into the ganglionic portion of
the olfactory bulb ; in consequence of which it becomes rather
difficult to fix on the exact line of demarcation between the
bulb and the nerve.

Stage Q is the latest period in which I have investigated
the development of the brain. Its structure is represented

DEVELOPMENT OF ELASMOBRANCH FISHES. 179

for this stage in general view in PI. XV. fig. 6a, Qh, Qc, in
longitudinal section in PI. XV. fig. 7a, 7b, and in transverse
section PL XV. fig. 8a — d. The transverse sections are taken
from a somewhat older embryo than the longitudinal. In the
thalamencephalon there is no fresh point of great importance
to be noticed. The pineal gland remains as before, and has
become, if anything, longer than it was, and extends further
forwards over the summit of the cerebrum. It is situated,
as might be expected, in the connective tissue within the
cranial cavity (fig. Sa,pn), and does not extend outside the skull,
as it appears to do, according to Gotte's investigations, in
Amphibians. Gotte^ compares the pineal gland with the lono-
persisting pore which leads into the cavity of the brain in the
embryo of Amphioxus, and we might add the Ascidians, and
calls it " ein Umbildungsprodukt einer letzten Yerbindung des
Hirns mit der Oberhaut." This suggestion appears to me a
very good one, though no facts have come under my notice
which confirm it. The sacci vasculosi are perhaps indicated at
this stage in the two lateral divisions of the trilobed ventricle
of the infundibulum (fig. 8c).

The lateral ventricles (fig. 8a) are now quite separated by a
median partition, and a slight external constriction marks the
lobes of the two hemispheres ; these, however, are still united
by nervous structures for the greater part of their extent. The
olfactory lobes are formed of a distinct bulb and stalk (fig. 8a,
oil), and contain, as before, prolongations of the lateral ven-
tricles. The so-called optic chiasma is very distinct (fig. 8b,
op.n)y but the fibres from the optic nerves appear to me simply
to cross and not to intermingle.

The mid-hrain. The mid-brain is at first fairly marked off
from both the fore and hind brains, but less conspicuously
from the latter than from the former. Its roof becomes pro-
gressively thinner and its sides thicker up to stage P, its cavity
remaining quite simple. The thinness of the roof gives it, in
isolated brains of stage P, a bilobed appearance, (vide PL XV.
fig. 4Z>, mh, in which the distinctness of this character is by no
means exaggerated). During stage Q it becomes really bilobed
through the formation in its roof of a shallow median fun'ow,
1 Ent. d. Unke, p. 304.

ISO THE HIND-Br.AIN.

(PL XV. fig. Sh). Its cavity exhibits at tlie same time the indi-
cation of a division into a central and two lateral parts.

TJie hind-brain. The hind-brain has at first a ftiirly uniform
structure, but by the close of stage I, the anterior part becomes
distinguished from the remainder by the fact, that its roof does
not become thin as does that of the posterior part. This anterior,
and at first very insignificant poi^tion, forms the rudiment of
the cerebellum. Its cavity is quite simple and is continued
uninterruptedly into that of the remainder of the hind-brain.
The cerebellum assumes in the course of development a
greater and greater prominence, and eventually at the close
of stage Q overlaps both the optic lobes in front and the me-
dulla behind (PI. XV. fig. 7a). It exhibits in surface-views of
the liardened brain of stages P and Q the appearance of a
median constriction, and the portion of the ventricle contained
in it is prolonged into t\fO lateral outgrowths (PI. XV. fig. 8c
and 8d, ch).

The posterior section of the hind-brain which forms the me-
dulla undergoes changes of a somewhat complicated character.
In the first place its roof becomes in front very much extended
and thinned out. At the raphe, where the two lateral halves
of the brain originally united, a separation, as it were, takes
place, and the two sides of the brain become pushed apart, re-
maining united by only a very thin layer of nervous matter
(PL XIV. fig. 6, iv. v.). As a result of this peculiar growth in
the brain, the roots of the nerves of the tv\ro sides which were
originally in contact at the dorsal summit of the brain become
carried away from one another, and appear to arise at the sides
of the brain (PL xiv. fig. 6 and 7). Other changes also take
place in the wails of the brain. Each lateral wall presents two
projections towards the interior (PL xiv. fig. 5a). The ventral
of these vanish, and the dorsal approximate so as nearly to
divide the cavity of the hind-brain, or fourth ventricle, into a
large dorsal and a small ventral channel (PL xiv. fig. 6), and
this latter becomes con:ipletely obliterated in the later stages.
The dorsal pair, while approximating, also become more promi-
nent, aud stretch into the dorsal moiety of the fourth ventricle
(PL XIV. fig. G). They are still very prominent at stage Q (PL
fig. XV. Sd,ft}, and correspond in position with the fasciculi

DEVELOPMENT OF ELASMOBRANCH FISHES. 181

teretes of human anatomy. Part of tlie root of the seventh
nerve originates from them. They project freely in front into
the cavity of the fourth ventricle (PI. XV. fig. 7 ft).

By stage Q restiform tracts are indistinctly marked off from
the remainder of the brain, and are anteriorly continued into
the cerebellum, of which they form the peduncles. Near their
junction with the cerebellum they form prominent bodies (PI.
XV. fig. la, r t), which are regarded by Miklucho-Maclay^ as re-
presenting the true cerebellum.

By stage the medulla presents posteriorly, projecting into
its cavity, a series of lobes which correspond with the main
roots (not the branches) of the vagus and glosso-pharyngeal
nerves (PL xvi. fig. 5). There appear to me to be present
seven or eight projections : their number cannot however be
quite certainly determined. The first of them belongs to the
root of the glosso-pharyngeal, the next one is interposed between
the glosso-pharyngeal and the first root of the vagus, and is
without any corresponding nerve-root. The next five corre-
spond to the five main roots of the vagus. For each projec-
tion to which a nerve pertains there is a special nucleus of
nervous matter, from which the root springs. These nuclei do
not stain like the remainder of the walls of the medulla, and
stand out accordingly very conspicuously in stained sections.

The coating of white matter which appeared at the end of
stage K, on the exterior of each lateral half of the hind-brain,
extends from a point just dorsal to the attachment of the nerve-
roots to the ventral edge of the medulla, and is specially con-
nected with the tissue of the upper of the two already described
projections into the fourth ventricle.

A rudiment of the tela vasculosa makes its appearance during
stage Q, and is represented by the folds in the wall of the
fourth ventricle in my figure of that stage (PL XV. fig. 7a, t v).

The development of the brain in Elasmobranchs has already
been worked out by Professor Huxley, and a brief but in
many respects very complete account of it is given in his
recent paper on Ceratodus^ He says, pp. 30 and 31, ''The
development of the cerebral hemispheres in Plagiostome Fishes

1 Das Geliirn d. Selacliier, Leipzig, 1870.

■- Proceedings of tlie Zoological Society, 1876, Pt. I. p. 30 and 31.

182 THE VIEWS OF MIKLUCHO-MACLAY.

differs from the process by which they arise in the higher
Vertebrata. In a very early stage, when the first and second
visceral clefts of the embryo Scyllium are provided with only
a few short branchial filaments, the anterior cerebral vesicle is
already distinctly divided into the thalamencephalon (from
which the large infundibulum proceeds below, and the small
tubular peduncle of the pineal gland above, while the optic
nerve leaves its sides) and a large single oval vesicle of the
hemispheres. On the ventral face of the integument covering
these are two oval depressions, the rudimentary olfactory sacs.

"As development proceeds the vesicle of the hemispheres
becomes divided by the ingrowth of a median longitudinal
septum, and the olfactory lobes grow out from the posterior
lateral regions of each ventricle thus formed, and eventually
rise on to the dorsal faces of the hemispheres, instead of, as in
most Vertebrata, remaining on their ventral sides. I may
remark, that I cannot accept the views of Miklucho-Maclay,
whose proposal to alter the nomenclature of the parts of the
Elasmobranch's brain, appears to me to be based upon a misin-
terpretation of the facts of development."

The last sentence of the paragraph brings me to the one
part on which it is necessary to say a few words, viz. the views of
Miklucho-Maclay. His views have not received any general
acceptance, but the facts narrated in the preceding pages
shew, beyond a doubt, that he has ' misinterpreted ' the facts of
development, and that the ordinary view of the homology of
the parts is the correct one. A comparison of the figures I
have given of the embryo brain with similar figures of the
brain of higher Vertebrates shews this point conclusively.
Miklucho-Maclay has been misled by the large size of the
cerebellum, but, as we have seen, this body does not begin
to be conspicuous till late in embryonic life. Amongst the
features of the embryonic brain of Elasmobranchs, the long
persisting unpaired condition of the cerebral hemisphere, upon
which so much stress has already been laid by Professor Huxley,
appears to me to be one of great importance, and may not im-
probably be regarded as a real ancestral feature. Some obser-
vations have recently been published by Professor B. G. Wilder*

1 Anterior brain-mass with Sharks and Skates, American Journal of Science
and Arts, Vol. xii. 1870.

DEVELOPMENT OF ELASMOBRANCH FISHES. 183

upon this point, and upon the homologies and development of
the olfactory lobes. Fairly good figures are given to illustrate
the development of the cerebral hemispheres, but the con-
clusions arrived at are in part opposed to my own results.
Professor Wilder says : " The true hemispheres are the lateral
masses, more or less completely fused in the middle line, and
sometimes developing at the plane of union a buodle of longi-
tudinal commissural fibres. The hemispheres retain their
typical condition as anterior protrusions of the anterior vesicle ;
but they lie mesiad of the olfactory lobes, and in Mustelus at
least seem to he formed after them'' The italics are my own.
From what has been said above, it is clear that the statement
italicised, for Scyllium at least, completely reverses the order of
development. Still more divergent from my conclusions are
Professor Wilder's statements on the olfactory lobes. He says :
"The true olfactory lobe, or rhinencephalon, seems, therefore, to
embrace only the hollow base of the crus, more or less thickened,
and more or less distinguishable from the main mass as a hollow
process. The olfactory bulb, with the more or less elongated crus
of many Plagiostomes, seems to be developed independently, or
in connection with the olfactory sack, as are the general nerves ;"
and again, " But the young and adult brains since examined
shew that the ventricle {i.e. the ventricle of the olfactory lobe)
ends as a rounded cul-de-sac before reaching the 'lobe'."

The majority of the statements contained in the above
quotations are not borne out by my observations. Even the few
preparations of which I have given figures, appear to me to
prove that (1) the olfactory lobes (crura and bulbs) are direct
outgrowths from the cerebral rudiment, and develope quite
independently of the olfactory sack ; (2) that the ventricle of
the cerebral rudiment does not stop short at the base of the
crus ; (3) that from the bulb a nerve grows out which has a
centrifugal growth like other nerves of the body, and places
the central olfactory lobe in communication with the peripheral
olfactory sack. In some other Vertebrates this nerve seems
hardly to be developed, but it is easily intelligible, that if
in the ordinary course of growth the olfactory sack became
approximated to the olfactory lobe, the nerve which grew out
from the latter to the sack might become so short as to escape
detection.

184 THE OLFACTORY OEGAN,

Organs of Sense.

The olfactory organ. The olfactory pit is the latest formed
of the three organs of special sense. It appears during a stage
intermediate between / and K, as a pair of slight thickenings of
the external epiblast, in the normal vertebrate position on the
under side of the fore-brain immediately in front of the mouth
(PL XIY. fig. 1 and 2, ol).

The epiblast cells which form this thickening are very co-
lumnar, but present no special peculiarities. Each thickened
patch of skin soon becomes involuted as a shallow pit, which
remains in this condition till the close of the stage K. The
epithelium very early becomes raised into a series of folds
(Schneiderian folds). These are bilaterally symmetrical, and
diverge like the barbs of a feather from a median line
(PL XIV. fig. 14). The nasal pits at the close of stage K are
still sejDarated by a considerable interval from the walls of the
brain, and no rudiment of an olfactory lobe arises till a later
period ; but a description of the development of this as an in-
tegral part of the brain has already been given, p. 178.

Eye. The eye does not present in its early development any
very special features of interest. The optic vesicles arise as
hollow outgrowths from the base of the fore-brain (PL xiv. fig.
3, op.v), from which they soon become partially constricted, and
form vesicles united to the base of the brain by comparatively
narrow hollow stalks, the rudiments of the optic nerves. The con-
striction to which the stalk or optic nerve is due takes place
from above and backwards, so that the optic nerves open into
the base of the front part of the thalamencephalon (PL XIV.
fig. 13a, oj:) n). After the establishment of the optic nerves, there
take place the formation of the lens and the pushing iu of the
anterior wall of the optic vesicle towards the posterior.

The lens arises in the usual vertebrate fashion. The epiblast
in front of the optic vesicle becomes very much thickened, and
then involuted as a shallow pit, which eventually deepens and
narrows. The walls of the pit are soon constricted off as a
nearly spherical mass of cells enclosing a very small central
cavity, in some cases indeed so small as to be barely recog-
nisable (PL XIV. fig. 7, 1). The pushing in of the anterior wall

DEVELOPMENT OF ELASMOBRANCH FISHES. 185

of the optic vesicle towards the posterior takes place in quite
the normal manner; but, as has been already noticed by Gotte^
and others, is not a simple mechanical result of the formation
of the lens, as is shewn by the fact that the vesicle assumes a
flattened form even before the appearance of the lens. The
whole exterior of the optic cup becomes invested by mesoblast,
but no mesoblastic cells grow in between the lens and the ad-
joining wall of the oj^tic cup.

Round the exterior of the lens, and around the exterior and
interior of the optic cup, there appear membrane-like structures,
similar to those already described round the spinal cord and
other organs. These membrane-like structures appear with a
varying distinctness, but at the close of stage K stand out with
such remarkable clearness as to leave no doubt that they are
not artificial products (PL XIV. fig. 13a).^ They form the rudi-
ments of the hyaloid membrane and lens capsule. Similar,
though less well marked membranes, may often be seen lining
the central cavity of the lens and the space between the two
walls of the optic cup. The optic cup is at first very shallow,
but owing to the rapid growth of the free edge of its w^alls soon
becomes fairly deep. . The growth extends to the whole circum-
ference of the walls except the point of entrance of the optic
nerve (PL xiv. fig. 13a), where no grow^th takes place ; here
accordingly a gap is left in the walls, which forms the well
knowm choroid slit. While this double walled cup is increasing
in size, the wall lining the cavity of the cup becomes thick, and
the outer wall very thin (fig. 13a). No further differentiations
arise before the close of stage K.

The lens is carried outwards with the growth of the optic
cup, leaving the cavity of the cup quite empty. It also
grows in size, and its central cavity becomes larger. Still later
its anterior wall becomes very thin, and its posterior wall
thick, and doubly convex (fig. loa). Its changes, however,
so exactly correspond to those already known in other Verte-
brates, that a detailed description of them would be superfluous.

JS^o mesoblast passes into the optic cup round its edge, but a
process of mesoblast, accompanied by a blood-vessel, passes into

1 Entii'ickelungRgeschlclite d. Unke.

' The engraver has not been very successful in rendering these membranes.

B. 13

186 THE PROCESSUS FALCIFORMIS.

the space between the lens and the wall of the optic cup through
the choroid slit (fig. 13«, ch). This process of tissue is very easily-
seen, and swells out on entering the optic cup into a mushroom-
like expansion. It forms the processus falciformis, and from it
is derived the vitreous humour.

About the development of the parts of the eye, subsequently
to stage K, I shall not say much. The iris appears during
stage 0, as an ingrowing fold of both layers of the optic cup
with a layer of mesoblast on its outer surface, which tends to
close over the front of the lens. Both the epiblast layers com-
prising the iris are somewhat atrophied, and the outer one is
strongly pigmented.' At stage the mesoblast first also grows
in betAveen the external skin and the lens to form the rudiment
of the mesoblastic structures of the eye in front of the lens. The
layer, when first formed, is of a great tenuity.

The points in my observations, to which I attach the
greatest importance, are the formation of the lens capsule and
the hyaloid membrane ; with the development of these may be
treated also that of the vitreous humour and rudimentary pro-
cessus falciformis. The development of these parts in Elasmo-
branchs has recently been dealt with by Dr Bergmeister', and
his observations with reference to the vitreous humour and
processus falciformis, the discovery of which in embryo Elas-
mobranchs is due to him, are very complete. I cannot, however,
accept his view that the hyaloid membrane is a mesoblastic pro-
duct. Through the choroid slit there grows, as has been said,
a process of mesoblast, the processus falciformis, which on
entering the optic cup dilates, and therefore appears mush-
room-shaped in section. At the earliest stage (K) a blood-
vessel appeared in connection with it, but no vascular structure
came under my notice in the later stages. The structure of
this process during stage P is shewn in PI. xvi. fig. 6,p.fal.;
it is there seen to be composed of mesoblast-cells with fibrous
prolongations. The cells, as has been noticed by Bergmeister,
form a special border round its dilated extremity. This pro-
cess is formed much earlier than the vitreous humour, which is
first seen in stage O. In hardened specimens this latter
appears either as a gelatinous mass with a meshwork of fibres

1 Embryologie d. Colohoms, Sitz. d. I: Akad. Wien, Bd. lxxi. 1875.

DEVELOPMENT OF ELASMOBRANCH FISHES. 187

or (as shewn in PI. xvi. fig. 6) with elongated fibres proceeding
from the end of the processus falciformis. These fibres are
probably a product of the hardening reagent, but perhaps re-
present some preformed structure in the vitreous humour. I
have failed to detect in it any cellular elements. It is more or
less firmly attached to the hyaloid membrane.

On each side of the processus falciformis in stage P a slight
fold of the optic cup is to be seen, but folds so large as those
represented by Bergmeister have never come under my notice,
though this may be due to my not having cut sections of such
late embryos as he has. The hyaloid membrane appears long
before the vitreous humour as a delicate basement membrane
round the inner surface of the optic cup (PL XIV. fig. 18a), which
is perfectly continuous with a similar membrane round the outer
surface. In the course of development the hyaloid membrane
becomes thicker than the membrane outside the optic cup, with
which however it remains continuous. This is very clear in my
sections of stage M. By stage the membrane outside the cup
has ceased to be distinguishable, but the hyaloid membrane
may nevertheless be traced to the very edge of the cup round
the developing iris; but does not unite with the lens capsule.
It can also be traced quite to the junction of the two
layers of the optic cup at the side of the choroid slit
(PI. XVI. fig. 6, hy. m). When the vitreous humour becomes
artificially separated from the retina, the hyaloid meml)rane
sometimes remains attached to the former, but at other
times retains in preference its attachment to the retina. My
observations do not throw any light upon the junction of the
hyaloid membrane and lens capsule to form the suspensory
ligament, nor have I ever seen (as described by Bergmeister)
the hyaloid membrane extending across the free end of the
processus falciformis and separating the latter from the vitreous
humour. This however probably appears at a period subsequent
to the latest one investigated by me. The lens capsule arises at
about the same period as the hyaloid membrane, and is a pro-
duct of the cells of the lens. It can be very distinctly seen in
all the stages subsequent to its first formation. The proof of
its being a product of the epiblastic lens, and not of the
mesoblast, lies mainly in the fact of there being no mesoblast

13—2

188 THE HYALOID MEMBRANE.

at hand to give rise to it at the time of its formation,
vide PI. XIV, fig. 13 a. If the above observations are cor-
rect, it is clear that the hyaloid membrane and lens capsule
are respectively products of the retina and lens ; so that
it becomes necessary to go back to the older views of Kol-
liker and others in preference to the more modern ones of
Lieberkiihn and Arnold. It would take me too far from
my subject to discuss the arguments used by the later in-
vestigators to maintain their view that the hyaloid membrane
and lens capsule are mesoblastic products; but it will suffice
to say that the continuity of the hyaloid membrane over the
pec ten in birds is no conclusive argument against its retinal
origin, considering the great amount of apparently independent
growth which membranes, when once formed, are capable of
exhibiting.

Bergmeister's and my own observations on the vitreous
humour clearly prove that this is derived from an ingrowth
through the choroid-slit. On the other hand, the researches
of Lieberkiihn and Arnold on the Mammalian Eye appear to
demonstrate that a layer of mesoblast becomes in Mammalia
involuted with the lens, and from this the vitreous humour
(including the memhrana caj^sulo-pupillaiHs) is said to be in
part formed. Lieberkiihn states that in Birds the vitreous
humour is formed in a similar fashion. I cannot, however,
accept his results on this point. It appears, therefore, that,
so far as is known, all groups of Vertebrata, with the excep-
tion of Mammalia, conform to the Elasmobranch type. The
differences between the types of Mammalia and remaining
Vertebrata are, however, not so great as might at first sight
appear. They are merely dependent on slight differences in
the manner in which the mesoblast enters the optic cup. In
the one case it grows in round one specialized part of the edge
of the cup, i.e. the choroid-slit ; in the other, round the whole
edge, including the choroid-slit. Perhaps the mode of forma-
tion of the vitreous humour in Mammalia may be correlated
with the early closing of the choroid-slit.

Auditory Organ. With reference to the development of the
organ of hearing I have very little to say. Opposite the inter-
val between the seventh and the glosso-pharyngeal nerves the

DEVELOPMENT OF ELASMOBRANCH FISHES. 189

external epiblast becomes thickened, and eventually involuted
as a vesicle which remains however in communication with
the exterior by a naiTow duct. Towards the close of stage K
the auditory sack presents three protuberances — one pointing
forwards, a second backwards, and a third outwards. These
are respectively the rudiments of the anterior and posterior
vertical and external horizontal semicircular canals. These
rudiments are easily visible from the exterior (PI. xiv. fig. 2).

As has been already pointed out, the epiblast of Elasmo-
branchs during the early periods of development exhibits no
division into an epidermic and a nervous layer, and in accord-
ance with its primitive undifferentiated condition, those portions
of the organs of sense which are at this time directly derived from
the external integument are formed indiscriminately from the
whole, and not from an inner or so-called nervous part of it only.
In the Amphibians the auditory sack and lens are derived
from the nervous division of the epiblast only, while the same
division of the layer plays the major part in forming the olfac-
tory organ. It is also stated that in Birds and Mammals the
part of the epiblast corresponding to the nervous layer is alone
concerned in the formation of the lens, though this does not
appear to be the case with the olfactory or auditory organs
in these groups of Vertebrates,

Mouth involution and Pituitary body.

The development of the mouth involution and the pituitary
body is closely related to that of the brain, and may conve-
niently be dealt with here. The epiblast in the angle formed
by the cranial flexure becomes involuted as a hollow process situ-
ated in close proximity to the base of the brain. This hollow
process is the mouth involution, and it is bordered on its pos-
terior surface by the front wall of the alimentary tract, and on
its anterior by the base of the fore-brain.

The uppermost end of this does not till near the close of stage
K become markedly constricted off from the remainder, but is
nevertheless the rudiment of the pituitary body. PI. xiv.
figs. 9 a and 12 m shew in a most conclusive manner the cor-

190 THE PITUITARY BODY.

rectness of the above account, and demonstrate that it is from
the mouth invokition, and not, as has usually been stated, from
the alimentary canal, that the pituitary body is derived.

This fact was mentioned in my preliminary account of
Elasmobranch development^; and has also been shewn to be
the case in Amphibians by Gotte^ and in Birds by Mihal-
kowics^ The fact is of considerable importance with reference
to speculations as to the meaning of this body.

Plate XIV. fig. 7 represents a transverse section through the
head during a stage between I and K ; but, owing to the cranial
flexure, it -cuts the fore part of the head longitudinally and
horizontally, and passes through both the fore-brain {fh) and
the hind-brain {iv. v.). Close to the base of the fore-brain are
seen the mouth (m), and the pituitary involution from this
(pt). In contact with the pituitary involution is the blind an-
terior termination of the throat, which a little way back opens
to the exterior by the first visceral cleft (i. v.c). This figure
alone suffices to demonstrate the correctness of the above ac-
count of the pituitary body ; but the truth of this is still further
confirmed by other figures on the same plate (fig. 9 a and
12m); in which the mouth involution is in contact with, but
still separated from, the front end of the alimentary tract. By
the close of stage K, the septum between the mouth and
throat becomes pierced, and the two are placed in communica-
tion. This condition is shewn in PL xiv. fig. 16 a, and PI.
XV. fig. la, Ic, pt. In these figures the pituitary involution
has become very partially constricted off from the mouth in-
volution, though still in direct communication with it. In
later stages the pituitary involution becomes longer and dilated
terminally, while the passage connecting it with the mouth
becomes narrower and narrower, and is finally reduced to a
solid cord, which in its turn disappears. The remaining vesicle
then becomes divided into lobes, and connects itself closely with
the infundibulum (PL xv» figs. 5 and 6 pt). The later stages
for Elasmobranchs are fully described by W. Miiller in his im-

1 Quarterly Journal of Microscopic Science, Oct. 1874.

2 Entivicklungsgeschichte der Unke. Gtitte was the first to draw attention to
this fact. His observations were then shewn to hold true for Elasmobranchs
by myself, and subsequently for Birds by Mihalkowics.

3 Arch. f. viicr. Anat. Vol. xi;

DEVELOPMENT OF ELASMOBRANCH FISHES. 191

portant memoir on the Comparative Anatomy and development
of this orofan \

Development of the Cranial Nerves.

The present section deals with the whole development (so
far as I have succeeded in elucidating it) of the cranial
nerv^es (excluding the optic and olfactory nerves and the nerves
of the eye-muscles) from their first appearance to their attain-
ment of the adult condition. My description commences with
the first development of the nerves, to this succeeds a short
description of the nerves in the adult Scyllium, and the section
is completed by an account of the gradual steps by which the
adult condition is attained.

Early Development of the Cranial Nerves. — Before the close
of stage H the more important of the cranial nerves make
their appearance. The fifth and the seventh are the first
to be formed. The fifth arises by stage G (PL xiv. fig. 3 v),
near the anterior end of the hind-brain, as an outgrowth from
the extreme dorsal summit of the brain, in identically the
same luay as the dor^sal root of a spinal nerve.

The roots of the two sides sprout out from the summit of
the brain, in contact with each other, and grow ventralwards,
one on each side of the brain, in close contact with its walls.
I have failed to detect more than one root for the two embry-
onic branches of the fifth (ophthalmic and mandibular), and no
trace of anterior or ventral root has been met with in any of my
sections.

The seventh nerve is formed nearly simultaneously with or
shortly after the fifth, and some little distance behind and
independently of it, opposite the anterior end of the thickening
of the epiblast to form the auditory involution. It arises pre-
cisely like the fifth, from the extreme dorsal summit of the
neural axis (PI. XIV. fig. 4(X, vii). So far as I have been able
to determine, the auditory nerve and the seventh proper possess
only a single root common to the two. There is no anterior
root for the seventh any more than for the fifth.

1 W. Miiller, Ueber Entwicklung und Bau d. Hj'pophysis u. d. Processus
infundibuli cerebri, Jenaische Zeitschrift, Bd. vi.

192 FIRST FORMATION OF CRANIAL NERVES.

Behind the auditory involution, at a stage subsequent to
tliat in which the fifth and seventh nerves appear, there arise a
series of roots from the dorsal summit of the hind-brain, which
form the rudiments of the glosso-pharyngeal and vagus nerves.
These roots are formed towards the close of stage H, but are
still quite short at the beginning of stage I. Their manner of
development resembles that of the previously described cranial
nerves. The central ends of the roots of the opposite sides are
at first in contact with each other, and there is nothing to
distinguish the roots of the glosso-pharyngeal and of the vagus
nerves from the dorsal roots of spinal nerves. Like the dorsal
roots of the spinal nerves, they appear as a series of ventral
prolongations of a continuous outgrowth from the brain, which
outgrowth is moreover continuous with that for the spinal
nerves \ The outgrowth of the vagus and glosso-pharyngeal
nerves is not continuous with that of the seventh nerve. This
is shewn by PL xiv. figs. 4a and 45. The outgrowth of the
seventh nerve though present in 4a is completely absent in
45 which represents a section just behind 4a.

Thus, by the end of stage I, there have appeared the
rudiments of the 5th, 7th, 8th, 9th and 10th cranial nerves, all
of which spring from the hind-brain. These nerves all develope
precisely as do the posterior roots of the spinal nerves, and it
is a remarkable fact that hitherto I have failed to find a trace
in the hrain of a root of any cranial nerve arising from the
ventral corner of the hrain as do the anterior roots of the spinal
nerves'^.

1 In the presence of tins continuous outgrowth of the bram from which
spring the separate nerve stems of the vagus, may perhaps be found a reconcili-
ation of the apparently conflicting statements of Uotte and myself with reference
to the vagus nerve, Gotte regards the vagus as a single nerve, from its originating
as an undivided rudiment ; but it is clear from my researches that, for Elasmo-
branchs at least, this method of arguing will not hold good, since it would lead
to the conclusion that all the spinal nerves were branches of one single nerve,
si ace they too spring as processes from a continuous outgrowth from the brain !

2 The conclusion here arrived at with reference to the anterior roots, is
opposed to the observations of both Gegonbaur on Hexanchus, Jenaische Zeit-
scJirift, Vol. VI, and of Jackson and Clarke on Echinorhinus, Journal of Anatomy
and Physioloriy, Vol. x. These morphologists identify certain roots sj^ringing
from the medulla below and behind the main roots of the vagus as true anterior
roots of this nerve. The existence of these roots is not open to question, but
without asserting that it is impossible for me to have failed to detect such roots
had they been present in the embryo, I think I may maintain if these anterior
roots are not present in the embryo, tlieir idontitication as vagus roots must
be abandoned ; and they must be regarded as belonging to spinal nerves. This
point is more fully spoken of at p. 205.

DEVELOPMENT OF ELASMOBRANCH FISHES. 193

It is admittedly difficult to prove a negative, and it may
still turn out that there are anterior roots of the brain similar
to those of the spinal cord ; in the mean time, however, the
balance of evidence is in favour of there being none such. This
at first sight appears a somewhat startling conclusio-n, but a little
consideration shews that it is not seriously opposed to the facts
which we know. In the first place it has been shewn by
myself* that in Amphioxus (whose vertebrate nature I cannot
doubt) only dorsal nerve-roots are present. Yet the nerves of
Amphioxus are clearly mixed motor and sensory nerves, and it
appears to me far more probable that Amphioxus represents a
phase of development in which the nerves had not acquired
two roots, rather than one in which the anterior root has been
lost. In other words, the condition of the nerves in Amphioxus
appears to me to point to the conclusion that lyrimitively the
craniospinal nerves of vertebrates were nerves of mixed func-
tion with one root only, and that root a dorsal one; and that
the present anterior or ventral root is a secondary acquisition.
This conclusion is further supported by the fact that the posterior
roots develope in point o-f time befo-re the anterior roots. If it
be admitted that the vertebrate nerves primitively had only a
single root, then the retention of that condition in the brain
implies that this became differentiated from the remainder of
the nervous system at a very early period before the acquire-
ment of anterior nerve-roots, and that these eventually become
developed only in the case of spinal nerves, and not in the case
of the already highly modified cranial nerves.

Subsequent Changes of the Nerves. — To simplify my descrip-
tion of the subsequent growth of the cranial nerves, I have
inserted a short description of their distribution in the adult.
This is taken from a dissection of Scy Ilium Stellare, which
like other species has some individualities of its own not found
in the other Elasmobranchs. For points not touched on in this
description I must refer the reader to the more detailed ac-
counts of my predecessors, amongst whom may specially be
mentioned Stannius^ for Carcharias, Spinax, Raja, Chimsera,

1 Journal of Anatomy and Physiology, Vol. x.

2 Ncrvensystcm d. Fische, Eostock, 18i9.

194 CRANIAL NERVES IN THE ADULT.

&c. ; Gegenbaur* for Hexanchus ; Jackson and Clarke'' for
Echinorliinus.

The ordinary nomenclature has been employed for the
branches of the fifth and seventh nerves, though embryological
data to be adduced in the sequel throw serious doubts upon
it. Since I am without observations on the origin of the nerves
to the muscles of the eyes, all account of these is omitted.

The fifth nerve arises from the brain by three roots ^: (1) an ante-
rior more or less ventral root ; (2) a root slightly behind, but close
to the former"*, formed by the coalescence of two distinct strands, one
arising from a dorsal part of the medulla, and a second and larger
from the ventral; (3) a dorsal and posterior root, in its origin quite
distinct and well separated from the other two, and situated slightly
behind the dorsal strand of the second root. This root a little way
from its attachment becomes enclosed for a short distance in the same
sheath as the dorsal part of the second root, and a slight mixture of
fibres seems to occur, but the majority of its fibres have no connection
with those of the second root. The first and second roots of the fifth
appear to me partially to unite, but before their junction the ramus
ophthalmicus profundus is given off from the first of them.

The fifth nerve, according to the usual nomenclature, has three
main divisions. The first of these is the ophthalmic. It is formed
by the coalescence of two entirely independent branches of the
fifth, which unite on leaving the orbit. The dorsalmost of these,
or ramus ophthalmicus superficialis, originates from the third and
posterior of the roots of the fifth, nearly the whole of which appears
to enter into its formation. This root is situated on the dorsal part
of the "lobi trigemini," at a point 2?osterior to that of the other roots
of the fifth or even of the seventh nerve. The branch itself enters the
orbit by a separate foramen, and, keeping on the dorsal side of it,
reenters the cartilage at its anterior wall, and is there jomed by the
ramus ophthalmicus profundus. This latter nerve arises from the
anterior root of the fifth, separately pierces the wall of the orbit,
and takes a course slightly ventral to the superior ophthalmic nerve,
but does not (as is usual with Elasmobranchs) run below the superior
rectus and superior oblique muscles of the eye. The nerve formed
by the coalescence of the superficial and deep ophthalmic branches
courses a short way below the surface, and supplies the mucous
canals of the front of the snout. It is a purely sensory nerve.
Strong grounds will be adduced in the sequel for regarding the ramus
ophthalmicus superficialis, though not the op)hthalmicus profundus, as
in reality a branch of the seventh, and not of the fifth nerve.

1 Jenaische Zeitschrift, Vol. vi.

2 Journal of Anatomy and Physiology, Vol. x.

3 My results with reference to these roots accord exactly, so far as they go,
with the more carefully worked out conclusions of Stannius, loc. cit. p. 29 and 30.

4 The root of the seventh nerve cannot properly be distinguished from this
root.

DEVELOPMENT OF ELASMOBRANCH FISHES. .195

The second division of the fifth nerve is the superior maxillary,
which appears to me to arise from both the first and second roots of
the fifth, though mainly from the first. It divides once into two
main branches. The first of these — the buccal nerve of Stannius —
after passing forwards along the base of the orbit takes its course
obliquely across the palatine arch and behind and below the nasal
sack, supplying by the way numerous mucous canals, and dividing at
last into two branches, one of these passing directly forwards on the
ventral surface of the snout, and the second keeping along the front
border of the mouth. The second division of the superior maxillary
nerve (superior maxillary of Stannius), after giving ofi" a small
branch, which passes backwards in company with a branch from the
inferior maxillary nerve to the levator maxillae superioris, itself
keeps close to the buccal nerve, and eventually divides into numerous
fine twigs to the mucous canals of the skin at the posterior region of
the upper jaw. It anastomoses with the buccal nerve. The inferior
maxillary nerve arises mainly from the second root of the fifth. After
sending a small branch to the levator maxillae superioris, it passes
outwards along the line separating the musculus adductor man-
dibulse from the musculus levator labii superioris, and after giving
branches to these muscles takes a course forward along the border of
the lower jaw. It appears to be a mixed motor and sensoiy nerve.

The seventh or facial nerve arises by a root close to, but behind
and below the second root of the fifth, and is intimately fused with this.
It divides almost at once into a small anterior branch and large
posterior.

The anterior branch is the palatine nerve. It gives ofi" at first
one or two very small twigs, which pursue a course towards the
spiracle, and probably represent the spiracular nerves of other Elas-
mobranchs. Immediately after giving off these branches it divides
into two stems, a posterior smaller and an anterior larger one. The
former eventually takes a course which tends towards the angle of
the jaw, and is distributed to the mucous membrane of the roof
of the mouth, while the larger one bends forwards and supplies the
mucous membrane at the edge of the upper jaw. The main stem of
the seventh, after giving off a branch to the dorsal section of the
musculus constrictor superficialis, passes outwards to the junction of
the upper and lower jaws, where it divides into two branches, an
anterior superficial branch, which runs immediately below the skin
on the surface of the lower jaw, and a second branch, which takes a
deep course along the posterior border of the lower jaw, between it
and the hyoid, and sends a series of branches backwards to the ven*
tral section of the musculus constrictor superficialis. The main stem
of the facial is mixed motor and sensory. I have not noticed a
dorsal branch, similar to that described by Jackson and Clarke.

The auditory nerve arises immediately behind the seventh, but
requii'es no special notice here. A short way behind the auditory is
situated the root of the glossopharyngeal nerve. This nerve takes an
oblique course backwards through the skull, and gives off in its pas-

196 DEVELOPMENT OF THE FIFTH NEKVE.

sage a veiy small dorsal branch, which passes upwards and back-
wards through the cartilage towards the roof of the skull. At the
point where the main stem leaves the cartilage it divides into two
branches, an anterior smaller branch to the hinder border of the hyoid
arch, and a posterior and larger one to anterior border of the first
branchial arch. It forks, in fact, over the first visceral cleft.

The vagiis arises by a great number of distinct strands from the
sides of the medulla. In the example dissected there were twelve in
all. The anterior three of these were the largest ; the middle one
having the most ventral origin. The next four were very small and
in pairs, and were sejDarated by a considerable interval from the next
four, also very small, and these again by a marked interval from the
hindermost strand.

The common stem formed by the junction of these gives off im-
mediately on leaving the skull a branch which forks on the second
branchial cleft : a second for the third cleft is next given off ; the main
stem then divides into a dorsal branch — the lateral nerve — and a
ventral one — the branchio-intestinal nerve — which, after giving off
the branches for the two last branchial clefts, supplies the heart and
intestinal tract. The lateral nerve passes back towards the posterior
end of the body, internal to the lateral line, and between the dorso-
lateral and ventro-lateral muscles. It gives off at its origin a fine nerve,
which has a course nearly parallel to its own. The main stem of the
vagus, at a short distance from its central end, receives a nerve which
springs from the ventral side of the medulla, on about a level with
the most posterior of the true roots of the vagus. This small nerve
corresponds with the ventral or anterior roots of the vagus described
by Gegenbaur, Jackson, and Clarke (though in the species investigated
by the latter authors these roots did not join the vagus, but the
anterior spinal nerves). Similar roots are also mentioned by Stan-
nius, who found two of them in the Elasmobranchs dissected by him ;
it is possible that a second may be present in Scyllium, but have
been overlooked by me, or perhaps may have been exceptionally
absent in the example dissected.

The Fifth Kerve. The thinning of the roof of the brain,
in the manner already described, produces a great change
in the apparent position of the roots of all the nerves. The
central ends of the rudiments of the two sides are, as has
been mentioned, at first in contact dorsally; but, when by
the growth of the roof of the brain its two lateral halves
become pushed apart, the nerves also shift their position and
become widely separated. The roots of the fifth nerve are
so influenced by these changes that they spring from the brain
about half way up its sides, and a little ventral to the border
of its thin roof. While this change has been taking place in

DEVELOPMENT OF ELASMOBRANCH FISHES. 197

the point of attachment of the fifth nerv^e, it has not remained
in other respects in a stationary condition.

During stage H it already exhibits two distinct branches
known as the mandibular and ophthalmic. These branches
first lie outside a section of the body cavity "which exists in the
front part of the head. The ophthalmic branch of the fifth
being situated near the anterior end of this, and the mandi-
bular near the posterior end.

In stage I the body cavity in this part becomes divided
into two parts one behind the other, the posterior being situ-
ated in the mandibular arch. The bifurcation of the nerve
then takes place over the summit of the posterior of the two
divisions of the body cavity, PL xiv. fig. 9 6 V. and 10 V, &c.,
and at first both branches keep close to the sides of this.

The anterior or ophthalmic branch of the fifth soon leaves
the walls of the cavity just spoken of and tends towards the
eye, and there comes in close contact with the most anterior
section of the body cavity which exists in the head. These
relations it retains unchanged till the close of stage K. Be-
tween stages I and K it may easily be seen from the surface ;
but, before the close of stage K, the increased density of the
tissues renders it invisible in the living embryo.

The posterior branch of the fifth extends downwards into
the mandibular arch in close contact with the posterior and
outer wall of the body space already alluded to. At first no
branches from it can be seen, but I have detected by the
close of stage K, by an examination of the living embryo, a
branch springing from it a short way from its central ex-
tremity, and passing forwards, PL XIV. fig. 2 v. This branch
I take to be the rudiment of the superior maxillary division of
the fifth nerve. It is shewn in section, PL xiv. fig. 15 a V.

In the stages after K the anatomy of the nerves becomes
increasingly difficult to follow, and accordingly I must plead
indulgence for the imperfections in my observations on all the
nerves subsequently to this date. In the fifth I find up to
stage a single ophthalmic branch (PL XVI. fig. 41) Y op. th.),
which passes forwards slightly dorsal to the eye and parallel and
ventral to a branch of the seventh, which will be described when
I come to that nerve. I have been unable to observe that this

198 SEVENTH AND AUDITORY NERVES.

branch divides into a ramus superficialis and ramus profundus,
and subsequently to stage O I have no observations on it.

By stage the fifth may be observed to have two very
distinct roots, and a large ganglionic mass is developed close
to their junction (Gasserian ganglion), PL XVI. fig. 4 a. But in
addition to this ganglionic enlargement, all of the branches
have special ganglia of their own, PI. xvi. fig. 4 h.

Summary. The fifth nerve has almost from the beginning
two branches, the ophthalmic (probably the inferior ophthalmic
of the adult) and the inferior maxillary. The superior maxillary
nerve arises later than the other two as a branch from the in-
ferior, originating comparatively far from its root. There is at
first but a single root for the whole nerve, which subsequently
becomes divided into two. Ganglionic swellings are developed
on the common stem and main branches of the nerve.

A general view of the nerve is shewn in the diagram in
PL XVI. fig. 1.

Seventh and Auditory JS'erves. — There appears in my earliest
sections a single large rudiment in the position of the seventh
and auditory nerves ; but in longitudinal sections of an embryo
somewhat older than stage I, in which the auditory organ forms
a fairly deep pit, still widely open to the exterior, there are to
be seen immediately in front of the ear the rudiments of two
nerves, which come into contact where they join the brain and
have their roots still closely connected at the end of stage K
(PL XIV. fig. 10 and 15 (X and 15 h). The anterior of these
pursues a straight course to the hyoid arch (PL xiv. fig. 10,
VII.), the second of the two (PL xiv. fig. 10, au. n), which is
clearly the rudiment of the auditory nerve, developes a gan-
glionic enlargement and, turning backward, closely hugs the
ventral wall of the auditory involution.

The observation just recorded appears to lead to the fol-
lowing conclusions with reference to the development of the
auditory nerve. A single rudiment arises from the brain for
the auditory and seventh nerves. This rudiment subsequently
becomes split into two parts, an anterior to form the seventh
nerve, and a posterior to form the auditory nerve. The gan-
glionic part of the auditory nerve is derived from the primitive

DEVELOPMENT OF ELASMOBRANCH FISHES. 199

outgrowtlis from the brain, and not from the auditory involu-
tion. I do not feel perfectly confident that an independent
origin of the auditory nerve might not have escaped my notice;
but, admitting the correctness of the view which attributes to
the seventh and auditory a common origin, it follows that the
auditory nerve primitively arose in connection with the seventh,
of which it may either, as Gegenbaur believes, be a distinct
part — the ramus dorsalis — or else may possibly have formed
part of a commissure, homologous with that uniting the dorsal
roots of the spinal nerves, connecting the seventh with the
glossopharyngeal nerve. In either case it must be supposed
secondarily to have become separate and independent in con-
sequence of the development of the organ of hearing.

My sections of embryos of stage K and the subsequent
stages do not bring to light many new facts with reference to
the auditory nerve : they demonstrate however that its gan-
glionic part increases greatly in size, and in stage O there is a
distinct root for the auditory nerve in contact with that for the
seventh.

The history of the seventh nerve in its later stages presents
points of great interest. Near the close of stage K there
may be observed, in the living embryos and in sections, two
branches of the seventh in addition to the original ti-unk to
the hyoid arch, both arising from its anterior side; one passes
straight forwards close to the external skin, but is at first only
traceable a short way in front of the fifth, and a second passes
downwards into the mandibular arch in such a fashion, that
the seventh nerve forks over the hyomandibular cleft (vide
PI. XIV. fig. 2, VII.; 15 a, vii.). My sections shew both these
branches with great clearness. A third branch has also come
under my notice, whose course leads me to suppose that it
supplies the roof of the palate.

In the later stages my attention has been specially directed
to the very remarkable anterior branch of the seventh. This
may, in stages L to 0, be traced passing on a level with the
root of the fifth nerve above the eye, and apparently termi-
nating in branches to the skin in front of the eye (PL xvi.
fig. 3, VII. ; 4 a, vii. a). It courses close beneath the skin (though
this does not appear in the sections represented on account of

200 RAMUS OPHTHALMICUS SUPERFICIALIS.

their obliqueness), and runs parallel and dorsal to the ophthalmic
branch of the fifth nerve, and may easily be seen in this posi-
tion in longitudinal sections belonging to stage O ; but its
changes after this stage have hitherto baffled me, and its final
fate is therefore, to a certain extent, a matter of speculation.

The two other branches of the seventh, viz., the hyoid or
main branch and mandibular branch, retain their primitive
arrangement till the clos€ of stage O.

The fate of the remarkable anterior branch of the seventh
nerve is one of the most interesting points v^hich has started
up in the course of my investigations on the development of
the cranial nerves, and it is a matter of very great regret to
me that I have not been able to clear up for certain its later
history.

Its primitive distribution leads to the supposition that it
becomes the nerve known in the adult as the ramus ophthahnicus
superficialis of the fifth nerve, and this is the view which I
admit myself to be inclined to adopt. There are several
points in the anatomy of this nerve in the adult which tell in
favour of accepting this view with reference to it. In the first
place, the ramus ophthalmicus superficialis rises from the brain
(vide description above, p. 194), quite independently of the
ramus ophthalmicus profundus, and not in very close con-
nection with the other branches of the fifth, and also con-
siderably behind these, quite as far back indeed as the ventral
root of the seventh. There is therefore nothing in the position
of its root opposed to its being regarded as a branch of the
seventh nerve. Secondly, its distribution, which might at first
sight be regarded as peculiar, presents no very strange features
if it is looked on as a ramus dorsalis of the seventh, whose
apparent anterior instead of dorsal course is due to the cranial
flexure. If, however, the distribution of the ramus ophthal-
micus superficialis is used as an argument against my view,
a satisfactory reply is to be found in the fact that a branch
of the seventh nerve certainly has the distribution in question
i/n the embryo, and that there is no reason why it should
not retain it in the adult

Finally, the junction of the two rami ophthalmici, most
remarkable if they are branches of a single nerve, would present

DEVELOPMENT OF ELASMOBRAXCH FISHES. 201

nothing astonishing when they are regarded as branches of
two separate nerves.

If this view be adopted, certain modifications of the more
generally accepted view^s of the morphology of the cranial
nerves will be necessitated ; but this subject is treated of at the
end of this section.

Some doubt hangs over the fate of the other branches of
the seventh nerve, but their destination is not so obscure as
that of the anterior branch. The branch to the roof of the
mouth can be at once identified as the ' palatine nerve ', and
it only remains to speak of the mandibular branch

It may be noticed first of all wdth reference to this branch,
that the seventh behaves precisely like the less modified
succeeding cranial nerves. It forks in fact over a visceral
cleft (the hyomandibular) the two sides of which it supplies;
the branch at the anterior side of the cleft is the later
developed and smaller of the tw^o. There cannot be much
doubt that the mandibular branch must be identified with
the spiracular nerve (prse-spiracular branch Jackson and Clarke)
of the adult, and if the chorda tympani of Mammals is cor-
rectly regarded as the mandibular branch of the seventh
nerve, then the spiracular nerve must represent it. Jackson
and Clarke^ take a different view of the homology of the
chorda tympani, and regard it as equivalent to the ramus
mandibularis internus (one of the two branches into which
the seventh eventually divides), because this nerve takes
its course over the ligament connecting the mandible wdth
the hyoid. This view I cannot accept so long as it is ad-
mitted that the chorda tympani is the branch of a cranial
nerve supplying the anterior side of a cleft. The ramus man-
dibularis internus, instead of forming with the main branch
of the seventh a fork over the spiracle, passes to its destination
completely behind and below the spiracle, and therefore fails
to fulfil the conditions requisite for regarding it as a branch
to the anterior wall of a visceral cleft. It is indeed clear
that the ramus mandibularis internus cannot be identified
with the embryonic mandibular branch of the seventh (which
passes above the spiracle or hyomandibular cleft) when there is

1 Loc, cit.
B. 14

202 THE GLOSSOPHARYNGEAL AND VAGUS NERVES.

present in the adult another nerve (the spiracular nerve), which
exactly corresponds in distribution with the embryonic nerve
in question. My view accords precisely with that already
expressed by Gegenbaur in his masterly paper on the nerves
of Hexanchus, in wliich he distinctly states that he looks
upon the spiracular nerve as the homologue of an anterior
branchial branch of a division of the vagus. In the adult
the spiracular nerve is sometimes represented by one or two
branches of the palatine, e.g. Scy Ilium, but at other times
arises independently from the main stem of the seventh \
The only difficulty in my identification of the embryonic
mandibular branch with the adult spiracular nerve, is the
extremely small size of the latter in the adult, compared with
the size of mandibular in the embryo; but it is hardly sur-
prising to find an atrophy of the spiracular nerve accompanying
an atrophy of the spiracle itself. The palatine appears to me
to have been rightly regarded by Jackson and Clarke as the
great superficial petrosal of Mammals.

On the common root of the branches of the seventh nerve,
as well as on its hyoid branch, ganglionic enlarg-ements are
present at an early period of development.

The Glossopharyngeal and Vagus Nerves. Behind the ear
there are formed a series of five nerves which pass down
to respectively the first, second, third, fourth and fifth visceral
arches.

For each arch there is thus one nerve, whose course lies
close to the posterior margin of the preceding cleft, a second
anterior branch being developed later. These nerves are con-
nected with the brain (as I have determined by transverse
sections) by roots at first attached to the dorsal summit, but
eventually situated about half-way down the sides (PL xiv.
fig. 6), nearly opposite the level of the process which
divides the ventricle of the hind-brain into a dorsal and a
ventral moiety. The foremost of these nerves is the glosso-
pharyngeal. The next four are, as has been shewn by Gegen-
baur ^, equivalent to four independent nerves, but form, together
with the glossopharyngeal, a compound nerve, which we may
briefly call the vagus.

1 Hexanchns, Gegenbaur, Jenaische Zeitsclirift, Vol. vi. ^ Lqc. cit.

DEVELOPMENT OF ELASMOBRAXCH FISHES. 203

This compound nerve by stage K attains a very com-
plicated structure, and presents several remarkable and unex-
pected features. Since it has not been possible for me
completely to elucidate the origin of all its various parts, it
will conduce to clearness if I give an account of its structure
during stage K or L, and then return to what facts I can
mention with reference to its development. Its structure
during these stages is represented on the diagram, PI. xvi.
fig. 1. There are present five branches, viz. the glosso-
pharyngeal and four branches of the vagus, arising probably by
a considerably greater number of strands from the brain'.
All the strands from the brain are united together by a thin
commissure, Vg. com., continuous with the commissure of the
posterior roots of the spinal nerves, and from this commissure
the five branches are continued obliquely ventralwards and back-
wards, and each of them dilates into a ganglionic swelling.
They all become again united together by a second thick com-
missure, which is continued backwards as the intestinal branch
of the vagus nerve Vg. in. The nerves, however, are continued
ventralwards each to its respective arch. From the hinder
part of the intestinal nerve springs the lateral nerve n.L, at a
point whose relations to the branches of the vagus I have not
certainly determined.

The whole nerve-complex formed by the glossopharyngeal
and the vagus nerves cannot of course be shewn in any single
section. The various roots are shewn in PL XVL fig. 5. The
dorsal commissure is represented in longitudinal section in
PI. XIV. fig. 15 h, com., and in transverse section in PI. xvi,
%• 2 Vg, com. The lower commissure continued as the in-
testinal nerve is shewn in PL xiv, fig. 15 a, Vg., and as seen
in the living embryo in PL xiv. figs. 1 and 2. The ganglia
are seen in PL xiv. fig. 6, Vg. The junction of the vagus and
glossopharyngeal nerves is shewn in PL xiv. fig. 10. My obser-
vations have not taught me much with reference to the origin of
the two commissures, viz. the dorsal one and the one which forms
the intestinal branch of the vagus. Very possibly they originate
as a single commissure which becomes longitudinally seg-

1 In the diagram tliere are only five strands represented. This is due to
the fact that I have not certainly made out their true number.

14—2

204 THE BRANCHES OF THE VAGUS NERVE.

merited. It deserves to be noticed tliat the dorsal commissure
has a long stretch, from the last branch of the vagus to the
first spinal nerve, during which it is not c(>nnected with the
root of any nerve; vide fig. 15 6, com. This space probably
contained originally the now lost branches of the vagus. In
many transverse sections where the dorsal commissure might
certainly be expected to be present it cannot be seen, but this
is perhaps due to its easily falling out of the sections. I have
not been able to prove that the commissure is continued for-
wards into the auditory nerve.

The relation of the branches of the vagus and glossopharyn-
geal to the branchial clefts requires no special remark. It is
fundamentally the same in the embryo as in the adult. The
branches at the posterior side of the clefts are the first to
appear, those at the anterior side of the clefts being formed
subsequently to stage K.

One of the most interesting points wdth reference to the
vagus is the number of separate strands from the brain which
unite to form it. The questions connected with these have been
worked out in a masterly manner, both from an anatomical and
a theoretical standpoint, by Professor Gegenbaur\ It has not
been possible for me to determine the exact number of these in
my embryos, nor have I been able to shew whether they are as
numerous at the earliest appearance of the vagus as at a later
embryonic period. The strands are connected (PI. xvi. fig. 5)
with separate ganglionic centres in the brain, though in several
instances more than one strand is connected wdth a single
centre. In an embryo between stage O and P more than a
dozen strands are present. In an adult Scyllium I counted
twelve separate strands, but their number has been shewn by
Gegenbaur to be very variable. It is possible that they are
remnants of the roots of the numerous primary branches of the
vagus which have now vanished ; and this perhaps is the ex-
planation of their variability, since in the case of all organs
which are on the way to disappear variability is a precursor of
disappearance.

A second interesting point is the presence of the two
connecting commissures spoken of above. It was not till com-

' Loc. cit.

DEVELOPMENT OF ELASMOBRANCH FISHES. 205

paratively late in my investigations that I detected tlie dorsal
one. This has clearly the same characters as the dorsal com-
missure already described as connecting the roots of all the
spinal nerves, and is indeed a direct prolongation of this. It
becomes gradually thinner and thinner, and finally ceases to be
observable by about the close of stage L. It is of importance
as shewing the similarity of the branches of the vagus to
the dorsal roots of the spinal nerves. The ventral of the two
commissures persists in the adult as the common stem from
which all the branches of the vagus successively originate,
and is itself continued backwards as the intestinal branch of the
vagus. The glossopharyngeal nerve alone becomes eventually
separated from the succeeding branches. Stannius and Gegen-
baur have, as was mentioned above, detected in adult Elasmo-
branchs roots which join the vagus, and which resemble the
anterior or ventral roots of spinal nerves; and I have myself
described one such root in the adult Scyllium. I have searched
for these in my embryos, but without obtaining conclusive results.
In the earliest stages I can find no trace of them, but I have
detected in stage L one anterior root on debatable border-land,
which may conceivably be the root in question, but which I
should naturally have put down for the root of a spinal nerve.
Are the roots in question to be regarded as proper roots of the
vagus, or as ventral roots of spinal nerves whose dorsal roots
have been lost? The latter view appears to me the most pro-
bable one, partly from the embryological evidence furnished by
my researches, which is clearly opposed to the existence of ante-
rior roots in the brain, and partly from the condition of these
roots in Echinorhinus, in which they join the succeeding spinal
nerves and not the vagus \ The similar relations of the ap-
parently homologous branch or branches in many Osseous Fish
may also be used as an argument for my view.

If, as seems probable, the roots in question become the
hypoglossal nerve, this nerve must be regarded as formed
from the anterior roots of one or more spinal nerves. Without
embryological evidence it does not however seem possible to
decide whether the hypoglossal nerve contains elements only of
anterior roots or of botli anterior and posterior roots.

1 Vide Jackson and Clarke loc. cit. The authors take a different view to
that here advocated, and regard the ventral roots described by them as having
originally belonged to the vagus.

206 MYOTOMES OF THE HEAD.

Mesohlast of the Head.

Body Cavity and Myotomes of the Head. — During stage F
the appearance of a cavity on each side in the mesohlast of
the head was described. (Vide PL ix. fig. 3 b and Qpjj.) These
cavities end in front opposite the bhnd anterior extremity of the
alimentary canal ; behind they are continuous with the general
body-cavity. I propose calling them the head-cavities. The
cavities of the two sides have no communication with each other.

Coincidently with the formation of an outgrowth from the
throat to form the first visceral cleft, the head-cavity on each
side becomes divided into a section in front of the cleft and a
section behind the cleft (vide PI. xiv. fig. 4 a and 4 h pp.) ; and
durinof stagfe H it becomes, owino^ to the formation of a second
cleft, divided into three sections : (1) a section in front of the
first or hyomandibular cleft ; (2) a section in the hyoid arch
between the hyomandibular cleft and the hyobranchial or first
branchial cleft ; (3) a section behind the first branchial cleft.

The section in front of the hyomandibular cleft stands in a
peculiar relation to the two branches of the fifth nerve. The
ophthalmic branch of the fifth lies close to the outer side of its
anterior part, the mandibular branch close to the outer side of its
posterior part. During stage I this front section of the head-
cavity grows forward, and becomes divided, without the inter-
vention of a visceral cleft, into an anterior and posterior division.
The anterior lies close to the eye, and in front of the com-
mencing mouth involution, and is connected with the ophthalmic
branch of the fifth nerve. The posterior part lies completely
within the mandibular arch, and is closely connected with the
mandibular division of the fifth nerve.

As the rudiments of the successive visceral clefts are formed,
the posterior part of the head-cavity becomes divided into suc-
cessive sections, there being one section for each arch. Thus
the whole head-cavity becomes on each side divided into (1) a
premandibular section ; (2) a mandibular section ; (3) a hyoid
section ; (4) sections in the branchial arches.

The first of these divisions forms a space of a considerable
size, with epithelial walls of somewhat short columnar cells.
It is situated close to the eye, and presents a rounded or some-

DEVELOPMENT OF ELASMOBRANCH FISHES. 207

times triangular figure in sections (PL xiv. fig. 7, 9 6 and IG 6,
I. 2^P-)' The ophthalmic branch of the fifth nerve passes close
to its superior and outer wall.

Between stages I and K the anterior cavities of the two
sides are prolonged ventralwards and meet below the base of
the fore-brain (PI. xiv. fig. 8, i.pj).). The connection between
the two cavities appears to last for a considerable time, and still
persists at the close of stage L. The anterior or premandibular
pair of cavities are the only parts of the body-cavity within
the head which unite ventrally. In the trunk, however, the
primitively independent lateral halves of the body-cavity always
unite in this way. The section of the head-cavity just described
is so similar to the remaining posterior sections that it must be
considered as equivalent to them.

The next division of the head-cavity, which from its position
may be called the mandibular cavity, presents during the stages
I and K a spatulate shape. It forms a flattened cavity, dilated
dorsally, and produced ventrally into a long thin process parallel
to the hyomandibular gill-cleft, PI. XIV. fig. 1 j)}). and fig. 7,
9 6 and 15 a, 2pp. Like the previous space it is lined by a
short columnar epithelium.

The fifth nerve, as has already been mentioned, bifurcates
over its dorsal summit, and the mandibular branch of that
nerve passes down on its posterior and outer side. The man-
dibular aortic arch is situated close to its inner side, PL xiv.
fig. 7. Towards the close of this period the upper part of the
cavity atrophies. Its lower part also becomes much narrowed,
but its walls of columnar cells persist and lie close to one
another. The outer or somatic wall becomes very thin indeed,
the splanchnic wall, on the other hand, thickens and forms a
layer of several rows of elongated cells. This thicker wall is
on its inner side separated from the surrounding tissue by a
small space lined by a membrane-like structure. In each of
the remaining arches there is a segment of the original body-
cavity fundamentally similar to that in the mandibular arch.
A dorsal dilated portion appears, however, to be present in the
third or hyoid section alone, and even there disappears by the
close of stage K. The cavities in the posterior parts of the head
become much reduced like those in its anterior part, though at

208 MYOTOMES OF THE HEAD.

rather a later period. Their walls however persist, and become
more columnar. In PL xiv. fig. 13 6, pp., is represented the
cavity in the last arch but one, at a period when the cavity in
the mandibular arch has become greatly reduced. It occupies
the same position on the outer side of the aortic trunk of its
arch as does the cavity in the mandibular arch (PL Xiv.
fig. 7, 2pp), In Torpedo embryos the head-cavity is much
smaller, and atrophies earlier than in the embryos of Pristiurus
and Scy Ilium.

It has been shewn that, with the exception of the most
anterior, the divisions of the body-cavity in the head become
atrophied, not so however their tvalls. The cells forming these
become elongated, and by stage N become distinctly developed
into muscles. Their exact history I have not followed in its
details, but they almost unquestionably become the musculus
contrictor superficial is and musculus interbranchialis^ ; and pro-
bably also musculus levator mandibuli and other muscles of
the front part of the head.

The most anterior cavity close to the eye remains unaltered
much longer than the remaining cavities, and its two halves
are still in communication at the close of stage L. I have not
yet succeeded in tracing the subsequent fate of its walls, hut
think it probable that they develojje into the muscles of the eye.
The morphological importance of the sections of the body-
cavity in the head cannot be over-estimated, and the fact that
the walls become developed into the muscular system of the
head renders it almost certain that we must regard them as
equivalent to the muscle-plates of the body, which originally con-
tain, equally with those of the head, sections of the body-cavity.
If this determination is correct, there can be no doubt that they
ought to serve as valuable guides to the number of segments
which have coalesced to form the head. This point is, how-
ever, discussed in a subsequent section.

General mesoblast of the head. — In stage G no mesoblast is
present in the head, except that which forms the walls of the
head-cavity.

During stage H a few cells of undifferentiated connective

1 Vide Vetter, Die Kicmen wid Kiefermusculatur d. Fische. Jenaische Zcit-
schrift, Vol. vii.

DEVELOPMENT OF ELASMOBEANCH FISHES. 209

tissue appear around the stalk of tlie optic vesicle, and in the
space between the front end of the alimentary tract and the
base of the brain in the angle of the cranial flexure. They are
probably budded off from the walls of the head-cavities. Their
number rapidly increases, and they soon form an investment
surrounding all the organs of the head, and arrange themselves
as a layer, between the walls of the roof of the fore and mid-
brain and the external skin. At the close of stage K they are
still undifferentiated and embryonic, each consisting of a large
nucleus surrounded by a very delicate layer of protoplasm pro-
duced into numerous thread-like processes. They form a regular
meshwork, the spaces of which are filled up by an intercellular
fluid.

I have not worked out the development of the cranial and
visceral skeleton ; but this has been made the subject of an in-
vestigation by Mr Parker, who is more competent to deal
with it than any other living anatomist. His results were in
part made known in his lectures before the Royal College of
Surgeons \ and will be published in full in the Transactions of
the Zoological Society.

All my efforts have hitherto failed to demonstrate any
segmentation in the mesoblast of the head, other than that indi-
cated by the sections of the body-cavity before mentioned; but
since these, as above stated, must be regarded as equivalent to
muscle-plates, any further segmentation of mesoblast could not
be anticipated. To this statement the posterior part of the
head forms an apparent exception. Not far behind the audi-
tory involution there are visible at the end of period K a few
longitudinal muscles, forming about three or four muscle-plates,
the ventral part of which is wanting. I have not the means
of deciding whether they properly belong to the head, or may
not really be a part of the trunk system of muscles which
has, to a certain extent, overlapped the back part of the head,
but am inclined to accept the latter view. These cranial muscle-
plates are shewn in PI. xiv. fig. 15 6, and in PI. xvi. fig. 2.

Notochord in the Head.
The notochord during stage G is situated for its whole length
1 A report of the lectures appeared in Nature.

210 THE HYPOBLASTIC LINING GILL-SLITS.

close under the brain, and terminates opposite the base of the
mid-brain. As the cranial flexure becomes greater and meso-
blast is collected in the angle formed by this, the termination of
the notochord recedes from the base of the brain, but remains
in close contact with the front end of the alimentary canal. At
the same time its terminal part becomes very much thinner
than the remainder, ends in a point, and exhibits signs of a
retrogressive metamorphosis. It also becomes bent upon itself
in a ventral direction through an angle of 180*^ ; vide PL XIV.
fior. 9 a and 16 a. In some cases this curvature is even more

o

marked than is represented in these figures.

The bending of the end of the notochord is not directly
caused by the cranial flexure, as is proved by the fact that
the end of the notochord becomes bent through a far greater
angle than does the brain. During the stages subsequent to K
the ventral flexure of the notochord disappears, and its term-
inal part acquires by stage a distinct dorsal curvature.

Hypoblast of the head.

The only feature of the alimentary tract in the head which
presents any special interest is the formation of the gill-slits and
of the thyroid body. In the present section the development of
the former alone is dealt with : the latter body will be treated
in the section devoted to the general development of the ali-
mentary tract.

The gill-slits arise as outgrowths of the lining of the
throat towards the external skin. In the gill-slits of Torpedo
I have observed a very slight ingrowth of the external skin
tow^ards the hypoblastic outgrowth in one single case. In all
other cases observed by me, the outgrowth from the throat
meets the passive external skin, coalesces with it, and then, by
the dissolution of the wall separating the lumen of the throat
from the exterior, a free communication from the throat out-
wards is effected ; vide PI. xiv. fig. 5 a and h, and 13 6. Thus
it happens that the walls lining the clefts are entirely formed
of hypoblast. The clefts are formed successively \ the anterior
appearing first, and it is not till after the rudiments of three
have appeared, that any of them become open to the exterior,
i Vide riates vi. and vii.

DEVELOPMENT OF ELASMOBRANCH FISHES. 211

In stage K, four if not five are open to the exterior, and the
rudiments of six, the full number, have appeared \ Towards the
close of stage K there arise, from the walls of the 2nd, 3rd,
and 4th clefts, very small knob-like processes, the rudiments of
the external gills. These outgrowths are formed both by the
lining of the gill-cleft and by the adjoining mesoblast'^

From the mode of development of the gill-clefts, it appears
that their walls are lined externally by hypoblast, and therefore
that the external gills are processes of the walls of the alimen-
tary tract, i.e. are covered by an hypoblastic, and not an epiblastic
layer. It should be remembered, however, that after the gill-
slits become open, the point where the hypoblast joins the
epiblast ceases to be determinable, so that some doubt hangs
over the above statement.

The identification of the layer to which the gills belong is not
without interest. If the external gills have an epiblastic origin,
they may be reasonably regarded^ as homologous with the ex-
ternal gills of Annelids ; but, if derived from the hypoblast, this
view becomes, to say the least, very much less probable.

Segmentation of the Head.

The nature of the vertebrate head and its relation to the
trunk forms some of the oldest questions of Philosophical
Morphology.

The answers of the older anatomists to these questions
are of a contradictory character, but within the last few
years it has been more or less generally accepted that
the head is, in part at least, merely a modified portion
of the trunk, and composed, like that, of a series of homo-
dynamous segments*. While the researches of Huxley, Parker,
Gegenbaur, Gotte, and other anatomists, have demonstrated
in an approximately conclusive manner that the head is com-
posed of a series of segments, great divergence of opinion
still exists both as to the number of these segments, and

1 The description of stage K and L, pp. 77 and 78, is a little inaccurate with
reference to the number of the visceral clefts, though the number visible in the
hardened embryos is correctly described.

2 Vide on the development of the gills, Schenk, Sitz. d. k. Akad. Wien,
Vol. Lxxi., 1875.

^ Vide Dohrn, Ursprung d. Wirhelthiere.

* Semper, in his most recent work, maintains, if I understand him rightly,
that the head is in no sense a modified part of the trunk, but admits that it is
segmented in a similar fashion to the trunk.

212 SEGMENTATION OF THE HEAD.

as to the modifications which they have undergone, especially
in the anterior part of the head. The questions involved
are amongst the most difficult in the whole range of morpho-
logy, and the investigations recorded in the preceding pages do
not, I am very well aware, go far towards definitely solving
them. At the same time my observations on the nerves and on
the head-cavities appear to me to throw a somewhat new light
upon these questions, and it has therefore appeared to me worth
while shortly to state the results to which a consideration of
these organs points. There are three sets of organs, whose de-
velopment has been worked out, each of which pi-esents more or
less markedly a segmental arrangement: — (1) The cranial nerves ;
(2) the visceral clefts ; (3) the divisions of the head-cavity.

The first and second of these have often been employed in
the solution of the present problem, while the third, so far as is
known, exists only in the embryos of Elasmobranchs.

The development of the cranial nerves has recently been
studied with great care by Dr Gotte, and his investigations have
led him to adopt very definite views on the segments of head.
The arrangement of the cranial nerves in the adult has fre-
quently been used in morphological investigations about the skull,
but there are to my mind strong grounds against regarding it as
a-ffording a safe basis for speculation. The most important of
these depends on the fact that nerves are liable to the greatest
modification on any changes taking place in the organs they
suppl}^ On this account it is a matter of great difficulty,
amounting in many cases to actual impossibility, to determine
the morphological significance of the different nerve-branches,
or the nature of the fusions and separations which have taken
place at the roots of the nerves. It is, in fact, only in those
parts of the head which have, relatively speaking, undergone
but slight modifications, and which require no s]3ecial elucidation
from the nerves, that these sufficiently retain in the adult their
primitive form to serve as trustworthy morphological guides.

I propose to examine separately the light thrown on the
segmentation of the head by the development of (1) the nerves,
(2) the visceral clefts, (3) the head-cavities ; and then to com-
pare the three sets of results so obtained.

The post-auditory nerves present no difficulties ; they are all
organized in the same fashion, and, as was first pointed out by

DEVELOPMENT OF ELASMOBRANCH FISHES. 213

Gegenbaur, form five separate nerves, each indicating a seg-
ment. A comparison of the post-auditory nerves of Scj^Uium
and other typical Elasmobranchs with those of Hexanchus and
Heptanchus proves, however, that other segments w^ere origin-
ally present behind those now found in the more typical forms.
And the presence in Scyllium of numerous (twelve) strands
from the brain to form the vagus, as well as the fact that a
large section of the commissure connecting the vagus roots wdth
the posterior roots of the spinal nerves is not connected with
the brain, appear to me to shew that all traces of the lost nerves
have not yet vanished.

Passing forwards from the post-auditory nerves, we come to
the seventh and auditory nerves. Th« embryological evidence
brought forward in this paper is against regarding these nerves
as representing two segments. Although it must be granted
that my evidence is not conclusive against an independent
formation of these two nerves, yet it certainly tells in favour of

their orio^inatinoj from a common rudiment, and Marshall's re-
ts o '

suits on the origin of the two nerves in Birds (published in
the Journal of Anatomy and Physiology ^ Vol. XT. Part 3) sup-
port, I have reason to believe, the same conclusion. Even
were it eventually to be proved that the auditory nerve origin-
ated independently of the seventh, the general relations of
this nerve, embryological and otherwise, are such that, pro-
visionally at least, it could not be regarded as belonging to
the same category as the facial or glossopharyngeal nerves, and
it has therefore no place in a discussion on the segmentation
of the head.

The seventh norve of the embryo (PL xvi. fig. 1, vii.) is
formed by the junction of three conspicuous branches, (1) an
anterior dorsal branch which takes a more or less horizontal
course above the eye (vii. a) ; (2) a main branch to the hyoid
arch (vii. hy); (3) a smaller branch to the posterior edge of the
mandibular arch (vii. mn). The first of these branches can
clearly be nothing else but the typical "ramus dorsalis," of
which however the auditory may perhaps be a specialized
part. The fact that this branch pursues an anterior and not
a directly dorsal course is probably to be explained as a con-
sequence of the cranial flexure. The two other branches of

214 SEGMENTATION OF THE HEAD.

tlie seventh nerve are the same as those present in all the
posterior nerves, viz. the branches to the two sides of a branchial
cleft, in the present instance the spiracle ; the seventh nerve
being clearly the nerve of the hyoid arch.

The fifth nerve presents in the arrangement of its branches
a similarity to the seventh nerve so striking that it cannot be
overlooked. This similarity is at once obvious from an inspec-
tion of the diagram of the nerves on PL xvi. fig. 1, v., or from an
examination of the sections representing these nerves (PI. xvi.
figs. 3 and 4). It divides like the seventh nerve into three
main branches : (1) an anterior and dorsal branch {r. ophthal-
micus profundus), whose course lies parallel to but ventral to
that of the dorsal branch of the seventh nerve; (2) a main
branch to the mandibular arch (r. maxillsG inferioris); and (3) an
anterior branch to the palatine arcade (?'. maxillae superioris).
I was at first inclined to regard the anterior branch of the fifth
(ophthalmic) as representing a separate nerve, and was supported
in this view by its relation to the most anterior of the head-
cavities ; but the unexpected discovery of an exactly similar
branch in the seventh nerve has induced me to modify this
view, and I am now constrained to view the fifth as a single
nerve, whose branches exactly correspond with those of the
seventh. The anterior branch of the fifth is, like the corre-
sponding branch of the seventh, the ramus doi^salis, and the two
other branches are the equivalent of the branches of the
seventh, which fork over the spiracle, though in the case of
the fifth nerve no distinct cleft is present unless we regard the
mouth as such. Embryology thus appears to teach us that
the fifth nerve is a single nerve supplying the mandibular arch,
and not, as has been usually thought, a complex nerve result-
ing from the coalescence of two or three distinct nerves. My
observations do not embrace the origin or history of the third,
fourth, and sixth nerves, but it is hardly possible to help sus-
pecting that in these we have the nerve of one or more segments
in front of that supplied by the fifth nerve; a view which well
accords with the most recent morphological