THE LIBRARY

OF

THE UNIVERSITY
OF CALIFORNIA

PRESENTED BY

PROF. CHARLES A. KOFOID AND
MRS. PRUDENCE W. KOFOID

THE

ELEMENTS OF EMBKYOLOGY.

THE

ELEMENTS OF EMBBYOLOGY

M. FOSTER, M.A., M.D., LL.D., F.R.S.,

FELLOW OF AND PB^LECTOB IN PHYSIOLOGY IN TRINITY COLLEGE, CAMBRIDGE,

AND THE LATE

FRANCIS M. BALFOUR, M.A., LL.D., F.R.S.,

FELLOW OF TBINITY COLLEGE, CAMBBIDGE,
AND PBOFESSOB OF ANIMAL MOBPHOLOGY IN THE UNIVERSITY.

SECOND EDITION REVISED.

EDITED BY
ADAM SEDGWICK, M.A.,

FELLOW AND ASSISTANT LECTUBER OF TBINITY COLLEGE, CAMBBIDGE,

WALTER HEAPE,

DEMONSTBATOR IN THE MOBPHOLOGICAL LABOBATOBY OF THE UNIVEESITY
OF CAMBBIDGE.

Honiron :
MACMILLAN AND CO.

1883

[The Right of Translation is reserved.]

PRINTED BY C. J. CLAY, M.A. & SON,
AT THE UNIVERSITY PRESS.

V

PREFACE TO THE SECOND EDITION.

WHEN this little work first appeared, it was put for-
ward as a Part I, to be followed by other Parts. That
plan was however soon abandoned. Nevertheless the
volume seemed to have a place of its own ; and my dear
lost friend undertook to prepare a second edition, in-
tending to add some account of the development of
the Mammal with a view of making the work an
elementary introduction to vertebrate embryology more
particularly suited for medical students. He was occu-
pied with the task at the time of his sad death; and
indeed a melancholy interest is attached to some of the
sheets, by the fact that he had taken them to Switzer-
land with him, on that fatal journey.

All the first part up to p. 160 he had passed for
press ; and he had further revised up to about p. 202.
The whole of the rest of the volume has been under-

M3698S5

VI PREFACE.

taken by Mr Adam Sedgwick and Mr Walter Heape.
They have attempted to carry out as far as possible
what we believe to have been Balfour's views, and
trust that the public will judge leniently of their
efforts to perform a difficult task. I have myself been
able to do no more than offer general advice from time
to time; and though it has not been thought advisable
to change the title, the merits as well as the responsi-
bilities of the latter part of the work must rest with
them.

M. FOSTER.

TRINITY COLLEGE,
CAMBRIDGE,
March, 1883.

TABLE OF CONTENTS.

PART I. THE HISTORY OF THE CHICK.

CHAPTER I.

THE STRUCTURE OF THE HEN'S EGG, AND THE CHANGES WHICH TAKE

PLACE UP TO THE BEGINNING OF INCUBATION . pp. I — 24.

The shell and shell-membrane, i — 3. The albumen, 3. The
vitelline membrane, 4. The yolk, 5—7. Area opaca, 7. Area
pellucida, 8. The structure of the blastoderm, 7 — 10. Eecapitu-
lation, 10. The ovarian ovum, n — 15. The descent of the ovum
along the oviduct, 15 — 17. Impregnation, 17. Segmentation,
18—24.

CHAPTER II.

BRIEF SUMMARY OF THE WHOLE HISTORY OF INCUBATION,

PP- 25—47-

The embryo is formed in the area pellucida, 25. The germinal
layers, 25, 26. The extension of the blastoderm over the yolk, 26.
The vascular area, 27. The head-fold, 27 — 36. The tail-fold, 37.
The lateral folds, 37. The yolk-sac, 37. The alimentary canal, 39.
The neural tube, 39, 40. The body-cavity, 41. The somatopleure, 41.
The splanchnopleure, 42. The stalk of the yolk-sac, 42, 43. The
amnion, 43 — 46. The allantois, 46, 47.

viii TABLE OF CONTENTS.

CHAPTER III.

THE CHANGES WHICH TAKE PLACE DURING THE FIRST DAY OF INCU-
BATION ... pp. 48 — 76.

Variations in the progress of development, 48, 49. The embryonic
shield, 49. Formation of hypoblast, 51. The germinal wall, 52.
The primitive streak, 52 — 54. Formation of primitive streak meso-
blast, 54, 55. Hypoblastic mesoblast, 55. Primitive groove, 56, 57.
The notochord, 59 — 62. The medullary groove, 62, 63. Amnion, 63.
The changes taking place in the three layers, 63 — 66. The germinal
wall, 65, 66. The increase of the head-fold, 66. The closure of the
medullary canal, 66, 67. The cleavage of the mesoblast ; formation
of spanchnopleure and somatopleure, 68. The vertebral and lateral
plates, 69. The mesoblastic somites, 70. The sinus rhomboidalis, 71.
The neurenteric passage, 71 — 74. Formation of the vascular area,
74, 75. Recapitulation, 75, 76.

CHAPTER IV.

THE CHANGES WHICH TAKE PLACE DURING THE FIRST HALF OF THE
SECOND DAY ... pp. 77 — 95.

Increasing distinctness and prominence of embryo, 77. The first
cerebral vesicle, 78, 79. The auditory pits, 81. Increase in number
of mesoblastic somites, 81. The fore-gut, 82. The heart, 82 — 89.
The vascular system, 89 — 94. Formation of blood-vessels, 92—94.
The rudiment of the Wolffian duct, 94. Summary, 94, 95.

CHAPTER V.

THE CHANGES WHICH TAKE PLACE DURING THE SECOND HALF OF THE
SECOND DAY ... pp. 96—108.

Increasing prominence of the embryo; the tail-fold and lateral
folds, 96. Continued closure of medullary canal, 96—98. The
brain, 98 — 101. The optic vesicles, 98. The second and third cerebral
vesicles, 100. The cerebral hemispheres, 100. First appearance of
cranial nerves, 100, 101. The notochord, 101. The cranial flexure,
TOT. The auditory vesicle, 101. Increase of curvature of heart, 101,

TABLE OF CONTENTS. IX

102. Auricular appendages, 102. Vascular System, 102 — 106.
Commencement of circulation, 102. The primitive aortae and first
pair of aortic arches, 102, 103. The vitelline vessels and sinus ter-
minalis, 103, 104. The course of the circulation, 105. The second
and third pairs of aortic arches, 105, 106. The Wolffian duct and
first appearance of Wolffian body, 106. The growth of the amnion,

107. The first appearance of the allantois, 107. Summary, 107,

1 08.

CHAPTER VI.

THE CHANGES WHICH TAKE PLACE DURING THE THIRD DAY,

pp. 109—194.

The diminution of the albumen, 109. The spreading of the opaque
and vascular areas, 109, no. The vascular area, no — 113. The
continued folding-in of the embryo, 113. The increase of the amnion,
113. The change in position of the embryo, 1 1 3 — 1 16. The curvature
of the body, 116. The cranial flexure, 116, 117. The brain, 117 — 123.
Growth of the vesicle of the cerebral hemispheres, 117. The lateral
ventricles, 117. The vesicle of the 3rd ventricle or thalamencephalon,
117. The rudiment of the pineal gland, 117, 118. The infundibulum,
119. The stomodaeum, 119. The pituitary body, 119 — 121. Changes
in the mid-brain, the corpora bigemina, crura cerebri and iter, 121.
Changes in the hind-brain, the medulla, cerebellum; 4th ventricle,
121, 122. Changes in the neural canal, 122, 123. The cranial
and spinal nerves, 123 — 132. The neural band, 123 — 126. The fifth,
seventh, ninth and tenth cranial nerves, 126, 127. Later develop-
ment of cranial nerves, 127 — 129. The spinal nerves, 129. The
shifting of point of attachment of nerves, 131. Anterior roots, 131.
The eye, 137 — 156. The first changes in the optic vesicles, 132, 133.
The secondary optic vesicle and development of the lens, 134 — 137.
The choroidal fissure, 137 — 140. The choroid, sclerotic and cornea,
140, 141. The further development of the optic vesicle, 141. The
ora serrata, 142. The iris, 142. Pigment epithelium of choroid, 142.
The ciliary processes, uvea, ciliary muscle and ligamentum pectinatum,
144. The histological changes in the retina, 144 — 146. Optic nerve,
146, 147. The choroid fissure, 147. The pecten, 148. The histo-
logical changes in ths lens, 149, 150. The vitreous humour, 150.
The cornea, 1 50 — 153. The aqueous humour, 153. Summary of the
development of the eye, 154, 155. The eyelids, 155. The lacrymal
glands and duct, 155, 156. The organ of hearing, 156 — 161. Closure

X TABLE OF CONTENTS.

of the auditory involution, 157. The otic vesicle, 157. The mem-
branous labyrinth, 158, 159. The osseous labyrinth, 159, 160. Com-
parison of ear with eye, 160, 161. The organ of smell, 161, 162.
The olfactory lobes and nerves, 162. The visceral arches and visceral
clefts, 162 — 167. Superior maxillary, and fronto-nasal processes,
164, 165. Fate of first visceral cleft, 165, 166. The meatus audi-
torius externus, 166. The tympanic membrane, 166. The Eustachian
tube and tympanic cavity, 165, 166. The fenestra ovalis and rotunda,
166. The columella, 166, 167. The vascular system, i6j — 170. The
aortic arches, 167. Changes in the heart, 167, 168. The venous
system, 169, 170. The meatus venosus, cardinal veins and ductus
Cuvieri, 169, 170. The alimentary canal, 171 — 185. .Folding in of
the splanchnopleure, tail-fold, 171, 172. The mesentery, 172, 173.
(Esophagus and stomach, 173. The intestine, 174. The postanal
gut, neurenteric canal and proctodaaum, 174 — 176. The lungs,
176 — 178. The liver, 178 — 181. The pancreas, 181. The thyroid
body, 1 8 1, 182. The spleen, 182. The growth and blood-vessels of the
allantois, 182 — 184. The mesollast, 185 — 193. The muscle-plates,
186 — 189. The intermediate cell-mass and Wolffian body, 189 — 193.
A typical Wolffian tubule, 193. Change of position of Wolffian duct,
193. Summary, 193, 194.

CHAPTER VII.

THE CHANGES WHICH TAKE PLACE ON THE EOUBTH DAY, pp. 195 — 231.

Appearance on opening the egg, 195. Growth of amnion, 195, 196.
The vitelline duct, 196. Increase of cranial flexure and tail-fold, 196 —
198. The first appearance of the limbs, 198. The growth of the
brain, 200. The face, 202. Changes in the nasal pits, 202. The sto-
modaeum and mouth, 202, 203. The cranial nerves, 203. Changes in
the mesoblastic somites, 204 — 212. The membranous vertebral
column, 205. The secondary segmentation of the vertebral column
and formation of the permanent vertebras, 205 — 207. Recapitulation,
207, 208. The changes in the notochord, 208—211. Ossification of
vertebrae, 209, 210. The changes in the muscle plates, 211, 212.
Wolffian body and duct, 212—214. The Miillerian duct, 214 — 218.
The kidney and ureter, 218 — 220. The ovaries and testes, 220 — 223,
Fate of the embryonic urinogenital organs, 223, 224. Changes in the
arterial system, 224 — 226. Changes in the venous system ; veins of
the liver, 226 — 229. Changes in the heart ; the ventricular septum,
229, 230. Summary, 230, 231.

TABLE OF CONTENTS. XI

CHAPTER VIII.

THE CHANGES WHICH TAKE PLACE ON THE FIFTH DAY, pp. 232 — 274.

Appearance on opening the egg, 232. The changes in the limbs,
233, 234. The pectoral and pelvic girdles ; the ribs and sternum, 234,
235. The development of the skull, 235—246. The cranium, 235.
The parachordals and notochord, 237, 238. The trabeculse, 239 — 241.
The sense capsules, 241, 242. Membrane and cartilage bones, 242.
Skeleton of visceral arches, 242 — 245. Table of bones, 246. The
changes in the face, 246 — 251. The posterior nares, 251. Changes in
the spinal cord; its histological differentiation, 251 — 254. The central
canal; and the posterior and anterior fissures, 254 — 256. Changes
in the heart, 256 — 264. Septum in the bulbus and semilunar valves,
257 — 259. The cardiac valves, 262. The foramen ovale and Eustachian
valve, 262 — 264. The pericardial and pleural cavities, 264 — 269.
Histological differentiation and the fate of the three primary layers,
269—273. Summary, 273, 274.

CHAPTER IX.

FROM THE SIXTH DAY TO THE END OF INCUBATION, pp. 275 — 303.

The appearance of distinct avian characters, 275. The foetal
appendages during the 6th and 7th days, 276 — 278. During the
8th, gth and roth days, 278. From the i ith to the i6th days, 278, 279.
From the i6th day onwards, 279, 280. Changes in the general form
of the embryo during the 6th and 7th days, 280 — 282. During the
8th — loth days, 282. From the i ith day onwards, 282. Feathers, 282.
Nails, 283. Ossification, 283. Changes in the venous system before
and after the commencement of pulmonary respiration, 283 — 289.
Changes in the arterial system, the modification of the aortic arches,
289 — 297. Summary of the chief phases of the circulation, 297—303.
Hatching, 303.

xii TABLE OF CONTENTS.

PART II. THE HISTORY OF THE MAMMA-
LIAN EMBRYO.

INTRODUCTION, pp. 307, 308.

CHAPTER X.

GENERAL DEVELOPMENT OF THE EMBRYO . . pp. 309 — 341.

The ovarian ovum, 309, 310. The egg- membranes, 310. Ma-
turation and impregnation, 310 — 312. Segmentation, 312 — 314. The
blastodermic vesicle, 314 — 316. The formation of the layers, 316 —
320. The primitive streak and groove, 319, 320. The medullary
groove, 320, 321. The mesoblast, 321 — 325. The notochord, 325, 326.
The rudiment of the neurenteric canal, 326. Recapitulation, 326.
The vascular area, 326. General growth of the embryo, 327 — 334.
The human embryo, 335 — 341. Embryos of guinea-pig, etc. with so-
called inversion of the layers, 341.

CHAPTER XI.

EMBRYONIC MEMBRANES AND YOLK-SAC ... pp. 342 — 364.

The typical development of the embryonic membranes, 342 — 352.
Vascular area of rabbit, 343 — 346. The yolk-sac or umbilical vesicle;
amnion, 343. The subzonal membrane, 346. Attachment of blasto-
dermic vesicle to uterine walls, 347. The formation of the chorion,
348. Mesoblast and blood-supply of the allantois, 348, 349. The
placenta, 349, 350. The fate of the embryonic membranes, 350 — 352.
Deciduate and non-deciduate type of placenta, 352. Comparative
history of the mammalian fatal membranes, 352 — 359. Foetal mem-
branes of Monotremata and Marsupialia, 352. The discoidal pla-
centa, 353, 354. The metadiscoidal placenta, 354 — 358. The zonary
placenta, 358, 359. The diffuse and polycotyledonary placenta, 359.
Comparative histology of the placenta, 359 — 363. Evolution of the
placenta, 364.

TABLE OF CONTENTS. Xlll

CHAPTER XII.

THE DEVELOPMENT OF THE ORGANS IN MAMMALIA, pp. 365 — 422.

The organs derived from the epiblast, 365 — 400. Hairs, 365.
Glands, 366. The hind-brain, 367—370. The mid-brain, 370, 371.
General development of fore-brain, 371. Thalamencephalon, 371
— 376. Pituitary body, 372, 373. Pineal gland, 373 — 376. Cerebral
hemispheres, 376 — 385. The olfactory lobes, 385. Histogenesis of
brain, 385 — 387. The eyes, 387 — 390. Choroid slit, membrana
capsulo-pupillaris and arteria centralis retinae, 389. The auditory
organ, 390 — 397. Accessory auditory structures, 397 — 399. The nasal
organ, and organ of Jacobson, 399. Cranial and spinal nerves ; sym-
pathetic system, 400.

Organs derived from the mesoblast, 400 — 417. The vertebral
column, 400, 401. The skull, 401. The visceral arches, 402. Man-
dibular and hyoid arches ; malleus, incus, and stapes, 403 — 405.
Eibs ; sternum; pectoral and pelvic girdles, 405. Skeleton of the
limbs, 406. Body-cavity ; pericardial, pleural cavities and dia-
phragm, 406.

The vascular system, 406 — 413. The heart, 406, 407. The ar-
terial system, 407 — 409. The venous system, 409 — 413. The supra-
renal bodies, 413, 414. The urinogenital organs, 414 — 417.
Wolffian duct and body ; kidney ; ovary and testis, 414, 415. Genital
cord, 415. Urinogenital sinus and external generative organs, 415
—417.

Alimentary canal and its appendages, 477 — 422. Splanchnic
mesoblast and mesentery, 419, 420. Stomodaeum, 420, 421. Hard
and soft palate, 420, 421. Teeth, 421. Proctodaeum, 422.

APPENDIX . . pp. 423—471.

Incubators, 423 — 425. Hardening reagents, 425 — 428. Staining
reagents, 428 — 432. Imbedding, 432 — 434. Cutting sections, 434,
435. Mounting sections, 436. Preservation of embryos as a whole,
43<5, 437-

xiv TABLE OF CONTENTS.

Practical directions for obtaining and studying chick embryos, 437
— 460. Examination of a 36 — 48 hours embryo, 437 — 444. Of an
embryo of about 48—50 hours, 444—447. Of an embryo at the end
of the 3rd day, 447—451. Of an embryo of the 4th day, 451—453-
Of a blastoderm of 20 hours, 453 — 456. Of an unincubated blasto-
derm, 457. Of the process of segmentation, 458. Of the later changes
of the embryo, 459. Of the development of blood-vessels, 459, 460.

Practical directions for obtaining and studying Mammalian em-
bryos, 460 — 471. Animals and breeding, 460, 461. Examination and
treatment of segmenting ova, 461 — 464. Of the blastodermic vesicle,
j2 — 90 hours, 465. Of the blastodermic vesicle of 7 days, 465, 466.
Of an 8 days embryo, 466 — 468. Of an embryo of 8 days 12 hours,
468, 469. Of the foatal membranes of an embryo of 14 days, 469,
470.

NOTE A. Automatic microtome, 471.

NOTE B. New method of mounting sections, 47 1 .

PART I.
THE HISTORY OF THE CHICK.

CHAPTER I.

THE STRUCTURE OF THE HEN'S EGG, AND THE CHANGES
WHICH TAKE PLACE UP TO THE BEGINNING OF IN-
CUBATION.

IN a lien's egg quite newly laid we meet with the
following structures. Most external is the shell (Fig.
1, s.\ composed of an organic basis, impregnated with
calcic salts. It is sufficiently porous to allow of the
interchange of gases between its interior and the exter-
nal air, and thus the chemical processes of respiration,
feeble at first, but gradually increasing in intensity, are
carried on during the whole period of incubation.

It is formed of two layers, both of which may contain
pigment. The inner layer is by far the thickest, and is
perforated by vertical canals which open freely on its
inner aspect. Superficially these canals appear to be
closed by the extremely thin outer layer. They are
probably of some importance in facilitating the pene-
tration of air through the shell.

Lining the shell, is the shell-membrane, which is
double, being made up of two layers : an outer thicker
F. & B. 1

THE HEN'S EGG.

[CHAP.

(Fig. 1, s. m.), and an inner thinner one (i. s. m.). Both
of these layers consist of several laminae of felted fibres
of various sizes, intermediate in nature between connec-
tive and elastic fibres.

FIG. I.

ch.l.

DIAGRAMMATIC SECTION OF AN UNINCUBATED FOWL'S EGG
(modified from Allen Thomson).

bl. blastoderm, w. y. white yolk. This consists of a central
flask-shaped mass and a number of layers arranged con-
centrically around this. y. y. yellow yolk. v. t. vitelline
membrane, x. layer of more fluid albumen immediately
surrounding the yolk. w. albumen consisting of alternate
denser and more fluid layers, ch. I. chalaza. a. ch. air-
chamber at the broad end of the egg. This chamber is
merely a space left between the two layers of the shell-mem-
brane, i. s. m. internal layer of shell-membrane, s. m.
external layer of shell-membrane, s. shell.

I.] THE WHITE OF THE EGG. 3

Over the greater part of the egg the two layers of
the shell-membrane remain permanently in close appo-
sition ; but at the broad end they tend to separate, and
thus to develope between them a space into which air
finds its way. This air-chamber, as it is called, is not
to be found in perfectly fresh eggs, but makes its
appearance in eggs which have been kept for some
time, whether incubated or not, and gradually increases
in size, as the white of the egg shrinks in bulk from
evaporation.

Immediately beneath the shell-membrane is the
white of the egg or albumen (Fig. 1, w.), which is, chemi-
cally speaking, a mixture of various forms of proteid
material, with fatty, extractive, and saline bodies. The
outer part of the white, especially in eggs which are not
perfectly fresh, is more fluid than that nearer the yolk.

Its average composition may be taken as
12*0 p. c. proteid matter,
1*5 p. c. fat and extractives,
5 p. c. saline matter, chiefly sodic and potassic chlorides,

with phosphates and sulphates,
86*0 p. c. water.

The white of the egg when boiled shews in section alter-
nate concentric layers of a transparent and of a finely granular
opaque material. In the natural condition, the layers corre-
sponding to these opaque layers are composed of more fluid
albumen, while those corresponding to the transparent layers
are less fluid, and consist of networks of fibres, containing fluid
in their meshes. The innermost layer, however, immediately
surrounding the yolk (Fig. 1, #.), is of the more fluid finely
granular kind.

In eggs which have been hardened a spiral arrange-
ment of the white may be observed, and it is possible to

1—2

4 THE HEN'S EGG. [CHAP.

tear off laminae in a spiral direction from left to right,
from the broad to the narrow end of the egg.

Two twisted cords called the chalazce (Fig. 1, ch. £.),
composed of coiled membranous layers of denser albu-
men, run from the two extremities of the egg to the
opposite portions of the yolk. Their inner extremities
expand and merge into a layer of denser albumen sur-
rounding the fluid layer next the yolk. Their outer
extremities are free, and do not quite reach the outer
layer of the white. Thus they cannot serve to suspend
the yolk, although they may help to keep it in position,
by acting as elastic pads. The interior of each chalaza
presents the appearance of a succession of opaque white
knots ; hence the name chalazae (hailstones).

The yolk is enclosed in the vitelline membrane (Fig.
1, v. £.), a transparent somewhat elastic membrane easily
thrown into creases and wrinkles. It might almost be
called structureless, but under a high power a fine
fibrillation is visible, and a transverse section has a
dotted or punctuated appearance ; it is probably there-
fore composed of fibrils. Its affinities are with elastic
connective tissue.

The whole space within the vitelline membrane is
occupied by the yolk. To the naked eye this appears
tolerably uniform throughout, except at one particular
point of its surface, at which may be seen, lying imme-
diately under the vitelline membrane, a small white
disc, about 4 mm. in diameter. This is the blastoderm,
or cicatricula.

A tolerably typical cicatricula in a fecundated egg
will shew an outer white rim of some little breadth, and
within that a circular transparent area, in the centre of

I.] THE WHITE YOLK. 5

which, again, there is an opacity, varying in appearance,
sometimes homogeneous, and sometimes dotted.

The disc is always found to be uppermost whatever
be the position of the egg, provided there is no restraint
to the rotation of the yolk. The explanation of this is
to be sought for in the lighter specific gravity of that
portion of the yolk which is in the neighbourhood of the
disc, and the phenomenon is not in any way due to the
action of the chalazae.

A section of the yolk of a hard-boiled egg will shew
that it is not perfectly uniform throughout, but that
there is a portion of it having the form of a flask, with
a funnel-shaped neck, which, when the egg is boiled,
does not become so solid as the rest of the yolk, but
remains more or less fluid.

The expanded neck of this flask-shaped space is
situated immediately underneath the disc, while its
bulbous enlargement is about in the middle of the yolk.
We shall return to it directly.

The great mass of the yolk is composed of what is
known as the yellow yolk (Fig. 1, y. y.). This consists
of spheres (Fig. 2, A.) of from 25/4 to lOOyu,1 in diameter
filled with numerous minute highly refractive granules ;
these spheres are very delicate and easily destroyed by
crushing. When boiled or otherwise hardened in situ,
they assume a polyhedral form, from mutual pressure.
The granules they contain seem to be of an albuminous
nature, as they are insoluble in ether or alcohol.

Chemically speaking the yolk is characterized by the presence
in large quantities of a proteid matter, having many affinities
with globulin, and called vitellin. This exists in peculiar associa-

6 THE HEN'S EGG. [CHAP.

tion with the remarkable body Lecithin. (Compare Hoppe-
Seyler, Hdb. Phys. Chem. Anal.) Other fatty bodies, colouring
matters, extractives (and, according to Dareste, starch in small
quantities), &c. are also present. Miescher (Hoppe-Seyler,
Chem. Untersuch. p. 502) states that a considerable quantity of
nuclein may be obtained from the yolk, probably from the
spherules of the white yolk.

FIG. 2.

A. Yellow yolk-sphere filled with fine granules. The outline of
the sphere has been rendered too bold.

B. White yolk-spheres and spherules of various sizes and pre-
senting different appearances. (It is very difficult in a
woodcut to give a satisfactory representation of these pe-
cub'ar structures.)

The yellow yolk, thus forming the great mass of the
entire yolk, is clothed externally by a thin layer of a
different material, known as the white yolk, which at
the edge of the blastoderm passes underneath the disc,
and becoming thicker at this spot forms, as it were, a
bed on which the blastoderm rests. Immediately under
the middle of the blastoderm this bed of white yolk is
connected, by a narrow neck, with a central mass of
similar material, lying in the middle of the yolk (Fig. 1,
w. y.). When boiled, or otherwise hardened, the white
yolk does not become so solid as the yellow yolk ; hence
the appearances to be seen in sections of the hardened
yolk. The upper expanded extremity of this neck of

I.] THE YELLOW YOLK. 7

white yolk is generally known as the "nucleus of
Pander."

Concentric to the outer enveloping layer of white
yolk there are within the yolk other inner layers of the
same substance, which cause sections of the hardened
yolk to appear to be composed of alternate concentric
thicker laminae of darker (yellow) yolk, and thinner
laminae of lighter (white) yolk (Fig. 1, wy ?/.).

The microscopical characters of the white yolk
elements are very different from those of the yellow
yolk. It is composed of vesicles (Fig. 2, J?.) for the most
part smaller than those of the yellow yolk (4yi6 — 75^),
with a highly refractive body, often as small as 1/-&, in
the interior of each ; and also of larger spheres, each of
which contains a number of spherules, similar to the
smaller spheres.

Another feature of the white yolk, according to His,
is that in the region of the blastoderm it contains
numerous large vacuoles filled with fluid; they are
sufficiently large to be seen with the naked eye, but do
not seem to be present in the ripe ovarian ovum.

It is now necessary to return to the blastoderm. In
this, as we have already said, the naked eye can distin-
guish an opaque white rim surrounding a more trans-
parent central area, in the middle of which again is a
white spot of variable appearance. In an unfecundated
cicatricula the white disc is simply marked with a
number of irregular clear spaces, there being no proper
division into a transparent centre and an opaque rim.

The opaque rim is the commencement of what we
shall henceforward speak of as the area opaca; the
central transparent portion is in the same way the

8 THE HEN'S EGG. [CHAP.

beginning of the area pellucida. In the part corre-
sponding to the area opaca the blastoderm rests imme-
diately on the white yolk ; underneath the area pellu-
cida is a shallow space containing a nearly clear fluid,
to the presence of which the central transparency seems
to be due. The white spot in the middle of the area
pellucida appears to be the nucleus of Pander shining
through.

Vertical sections of the blastoderm shew that it is
formed of two layers. The upper of these two layers
is composed, see Fig. 3, ep, of a single row of cells,
with their long axes arranged vertically, adhering
together so as to form a distinct membrane, the edge of
which rests upon the white yolk. After staining with
silver nitrate, this membrane viewed from above shews
a mosaic of uniform polygonal cells.

Each cell is composed of granular protoplasm filled
with highly refractive globules ; and in each an oval nu-
cleus may be distinguished. They are of a nearly uniform
size (about 9 yu,) over the opaque and the pellucid areas.

The under layer (Fig. 3, I), is composed of cells
which vary considerably in diameter; but even the
smaller cells of this layer are larger than the cells of the
upper layer. They are spherical, and so filled with
granules and highly refractive globules, that a nucleus
can rarely be seen in them : in the larger cells these
globules are identical with the smaller white yolk
spheres.

The cells of this layer do not form a distinct mem-
brane like the cells of the upper layer, but lie as a
somewhat irregular network of cells between the upper
layer and the bed of white yolk on which the blastoderm

I.] THE BLASTODERM.

rests. The lowest are generally the
largest. The layer is thicker at the peri-
phery than at the centre : and rests on
a bed of white yolk, from which it is in
parts separated by a more or less de-
veloped cavity, containing probably fluid
yolk matter about to be absorbed. In
the bed of white yolk nuclei are present,
which are destined to become the nuclei
of cells about to join the lower layer of
the blastoderm. These nuclei are gene-
rally more numerous in the neighbour-
hood of the thickened periphery of the
blastoderm than elsewhere. Amongst
the lower layer cells are to be found

FIG. 3.
SECTION OF A BLASTODERM OF A FOWL'S EGG

AT THE COMMENCEMENT OF INCUBATION.

The thiii but complete upper layer ep com-
posed of columnar cells rests on the in-
complete lower layer I, composed of larger
and more granular cells. The lower layer
is thicker in some places than in others,
and is especially thick at the periphery.
The line below the under layer marks the
upper surface of the white yolk. The larger
so-called formative cells are seen at 6,
lying on the white yolk. The figure does
not take in quite the whole breadth of the
blastoderm ; but the reader must under-
stand that both to the right hand and the
left ep is continued farther than I, so that
at the extreme edge it rests directly on
the white yolk.

10 THE HEN'S EGG* [CHAP.

peculiar large spherical bodies, which superficially re-
semble the larger cells around them, and have been
called formative cells. Their real nature is still very
doubtful, and though some are no doubt true cells,
others are perhaps only nutritive masses of yolk.

The opacity of the peripheral part of the blastoderm
is in a large measure due to the collection of the lower
layer cells in this region, and the thickening, so caused,
appears to be more pronounced for a small arc which
subsequently constitutes the hinder border of the area
pellucida.

Over nearly the whole of the blastoderm the upper
layer rests on the under layer. At the circumference
however the upper layer stretches for a short distance
beyond the under layer, and here consequently rests
directly on the white yolk.

To recapitulate : — In the normal unincubated hen's
egg we recognize the blastoderm, consisting of a com-
plete upper layer of smaller nucleated granular cells
and a more or less incomplete under layer of larger
cells, filled with larger granules; in these lower cells
nuclei are rarely visible. The thin flat disc so formed
rests, at the uppermost part of the entire yolk, on a
bed of white yolk, and a peripheral thickening of the
lower layer causes the appearance in the blastodermic
disc of an area opaca and an area pellucida. The great
mass of the entire yolk consists of the so-called
yellow yolk composed of granular spheres. The
white yolk is composed of smaller spheres of pecu-
liar structure, and exists, in small part, as a thin
coating around, and as thin concentric laminae in
the substance of the yellow yolk, but chiefly in the

10 THE HEN'S EGG* [CHAP.

peculiar large spherical bodies, which superficially re-
semble the larger cells around them, and have been
called formative cells. Their real nature is still very
doubtful, and though some are no doubt true cells,
others are perhaps only nutritive masses of yolk.

The opacity of the peripheral part of the blastoderm
is in a large measure due to the collection of the lower
layer cells in this region, and the thickening, so caused,
appears to be more pronounced for a small arc which
subsequently constitutes the hinder border of the area
pellucida.

Over nearly the whole of the blastoderm the upper
layer rests on the under layer. At the circumference
however the upper layer stretches for a short distance
beyond the under layer, and here consequently rests
directly on the white yolk.

To recapitulate : — In the normal unincubated hen's
egg we recognize the blastoderm, consisting of a com-
plete upper layer of smaller nucleated granular cells
and a more or less incomplete under layer of larger
cells, filled with larger granules; in these lower cells
nuclei are rarely visible. The thin flat disc so formed
rests, at the uppermost part of the entire yolk, on a
bed of white yolk, and a peripheral thickening of the
lower layer causes the appearance in the blastodermic
disc of an area opaca and an area pellucida. The great
mass of the entire yolk consists of the so-called
yellow yolk composed of granular spheres. The
white yolk is composed of smaller spheres of pecu-
liar structure, and exists, in small part, as a thin
coating around, and as thin concentric laminae in
the substance of the yellow yolk, but chiefly in the

I.J THE OVARIAN OVUM. 11

form of a flask-shaped mass in the interior of the yolk,
the upper somewhat expanded top of the neck of
which forms the bed on which the blastoderm rests.
The whole yolk is invested with the vitelline mem-
brane, this again with the white ; and the whole is
covered with two shell-membranes and a shell.

Such an egg has however undergone most important
changes while still within the body of the hen; and
in order to understand the nature of the structures
which have just been described, it will be necessary to
trace briefly the history of the egg from the stage when
it exists as a so-called ovarian ovum in the ovary of a
hen up to the time when it is laid.

In birds the left ovary alone is found in the adult ;
and is attached by the mesovarium to the dorsal wall
of the abdominal cavity, on the left side of the vertebral
column. It consists of a mass of vascular stroma in
which the ova are imbedded, is covered superficially
by a layer of epithelium, continuous with the epithelial
lining of the peritoneal cavity. The appearance of the
ovary varies greatly according to the age of the indi-
vidual. In the mature and sexually active females
it is almost wholly formed of pedunculated and highly
vascular capsules of various sizes, each containing a more
or less developed ovum ; in the young animal however
it is much more compact, owing to the absence of
advanced ova.

If one of the largest capsules of the ovary of a hen
which is laying regularly be opened, it will be found to
contain a nearly spherical (or more correctly, ellipsoidal
with but slightly unequal axes) yellow body enclosed in
a delicate membrane. This is the ovarian ovum or egg.

12 THE HEN'S EGG. [CHAP.

Examined with care the ovum, which is tolerably uni-
form in appearance, will be found to be marked at one
spot (generally facing the stalk of the capsule and form-
ing the pole of the shorter axis of the ovum) by a small
disc differing in appearance from the rest of the ovum.
This disc which is known as the germinal disc or discu-s

SECTION THROUGH THE GERMINAL Disc OF THE RIPE OVARIAN
OVUM OF A FOWL WHILE YET ENCLOSED IN ITS CAPSULE.

a. Connective-tissue capsule of the ovum. 6. follicular epithe-
lium, at the surface of which nearest the ovum lies the
vitelline membrane, c. granular material of the germinal
disc, which becomes converted into the blastoderm. (This
is not very well represented in the woodcut. In sections
which have been hardened in chromic acid it consists of fine
granules.) w. y, white yolk, which passes insensibly into
the fine granular material of the disc. #, germinal vesicle
enclosed in a distinct membrane, but shrivelled up by the
action of the chromic acid, y, space originally completely
filled up by the germinal vesicle, before the latter was
shrivelled up by the action of the chromic acid.

proligerus, consists of a lenticular mass of protoplasm
(Fig. 4, c), imbedded in which is a globular or ellipsoidal
body (Fig. 4, #), about SlO/,6 in diameter, called the
germinal vesicle. This has a delicate wall, and its con-
tents are clear and fluid in the fresh state, but become
granular upon the addition of reagents.

I.]

THE OVARIAN OVUM.

13

The rest of the ovum is known as the yolk. This
consists of two elements, the white yolk- and the yellow
yolk-spheres, which are distributed respectively very
much in the same way as in the laid egg, the yellow
yolk forming the main mass of the ovum, and the white
yolk being gathered underneath and around the disc
(Fig. 4, w. y\ and also forming a flask-shaped mass in
the interior. The delicate membrane surrounding the
whole is the vitelline membrane.

The youngest ova in the ovary of a fowl, in common
with those of all other animals, present the characters
of a simple cell. Such a cell is dia grammatically repre-
sented in Fig. 5.

It is seen to consist of a naked protoplasmic body
containing in its interior a nucleus — the germinal vesi-
cle— which in its turn envelopes
a nucleolus — constituting what is
known as the germinal spot.
Such young ova are enclosed in
a capsule of epithelium, named
the follicle or follicular mem-
brane, and are irregularly scat-
tered in the stroma of the ovary.
The difference between such
an immature ovum and the ripe
ovum just described is very great,
but throughout its growth the
ovum retains the characters of a
cell, so that the mature ova-
rian ovum, equally with the
youngest ovum in the ovary, is a single cell.
The most striking changes which takes place in the

FIG. 5.

DIAGRAM OF THE
OVUM. (From Gegen-
baur.)

a. Granular proto-
plasm, b. Nucleus (ger-
minal vesicle), c. Nu-
cleolus (germinal spot).

14 THE HEN'S EGG. [CHAP.

course of the maturation of the ovum concern the body
of the cell rather than the germinal vesicle. As the
body grows in size a number of granules make their
appearance in its interior. These granules are formed
by the inherent activity of the protoplasm, which is
itself nourished, in a large measure at any rate, by the
cells of the follicle. The outermost layer of the proto-
plasm remains free from these granules. As the ovum
grows older the granules become larger, first of all in
the centre, and subsequently at the periphery, and take
the form of white yolk-spherules. The greater part of
them become at a later stage converted into yellow
yolk-spheres, while a portion of them, situated in the
position of the white yolk of the ripe ovum, retain their
original characters.

The germinal vesicle, which in the youngest ova is
situated centrally or subcentrally, travels in the course
of the growth of the ovum towards the periphery, and
the protoplasm immediately surrounding it remains
relatively free from yolk granules, and so constitutes
the germinal disc. In the younger ova there is but a
single germinal spot in the germinal vesicle, but as the
ova enlarge several accessory germinal spots make their
appearance, while in the ripe ovum it seems doubtful
whether there is any longer a trace of a germinal
spot.

The cells of the follicular epithelium are at first
arranged in a single row, but at a later stage become
two or more rows deep : they undergo however a
nearly complete atrophy in the ripe ovum. Around
the follicular epithelium, there is present a membrana
propria, and in the later stages of the growth of the

I.] THE OVARIAN OVUM. 15

ovum this is in its turn embraced by a highly vascular
connective-tissue capsule.

The youngest ova are, as has already been stated,
quite naked. In ova of about 1*5 mm. the superficial
layer of the ovum becomes converted into a radiately
striated membrane called the zona radiata. At a later
period a second membrane, placed between the zona
radiata and the cells of the follicle, makes its appearance,
but its mode of origin is still unknown. As the ovum
approaches maturity the zona radiata disappears, and in
the ripe ovum the second membrane, which has already
been spoken of as the vitelline membrane, alone
remains.

From what has just been stated it follows that in
an egg which has been laid the yolk alone constitutes
the true ovum. The white and the shell are in fact
accessory structures formed during the passage of the
ovum down the oviduct.

When the ovarian ovum is ripe and about to be
discharged from the ovary, its capsule is clasped by
the open infundibulum of the oviduct. The capsule
then bursts, and the ovum escapes into the oviduct, its
longer axis corresponding with the long axis of the
oviduct, the germinal disc therefore being to one
side.

In describing the changes which take place in the
oviduct, it will be convenient, following the order pre-
viously adopted, to treat first of all of the formation
of the accessory parts of the egg. These are secreted
by the glandular walls of the oviduct. This organ
therefore requires some description. It may be said to
consist of four parts : — 1st. The dilated infundibulum

16 THE HEN'S EGG. [CHAP.

with an abdominal opening. 2nd. A long tubular
portion — the oviduct proper — opening by a narrow neck
or isthmus into the 3rd portion, which is much dilated,
and has been called the uterus ; the 4th part is some-
what narrow, and leads from the uterus into the cloaca.
The whole of the mucous membrane lining the oviduct
is largely ciliated.

The accessory parts of the egg are entirely formed
in the 2nd and 3rd portions. The layer of albumen
which immediately surrounds the yolk is first de-
posited ; the chalazse are next formed. Their spiral
character and the less distinctly marked spiral arrange-
ment of the whole albumen is brought about by the
motion of the egg along the spiral ridges into which
the interior of the second or tubular portion of the
oviduct is thrown. The spirals of the two chalazae are
in different directions. This is probably produced by
their peripheral ends remaining fixed while the yolk to
which their central ends are attached is caused to
rotate by the contractions of the oviduct. During the
formation of the chalazse the rest of the albumen is
also deposited ; and finally the shell-membrane is formed
in the narrow neck of the 2nd portion, by the fibrilla-
tion of the most external layer of albumen. The e^sr

tf OO

passes through the 2nd portion in little more than
3 hours. In the 3rd portion the shell is formed. The
mucous membrane of this part is raised into nume-
rous flattened folds, like large villi, containing follicu-
lar glands. From these a thick white fluid is poured
out, wnich soon forms a kind of covering to the egg, in
which the inorganic particles are deposited. In this
portion of the oviduct the egg remains from 12 to 18

I.] IMPREGNATION. 17

hours, during which time the shell acquires its normal
consistency. At the time of laying it is expelled from
the uterus by violent muscular contractions, and passes
with its narrow end downwards along the remainder of
the oviduct, to reach the exterior.

Impregnation. This process occurs in the upper
portion of the oviduct; the spermatozoa being found
actively moving in a fluid which is there contained.

We have as yet, as far as the fowl is concerned, no
direct observations concerning the changes preceding
and following upon impregnation ; nor indeed concern-
ing the actual nature of the act of impregnation.

In other types however these processes have been
followed with considerable care, and the result has been
to shew that prior to impregnation a division of the
ovum takes place into two very unequal parts. The
smaller of these parts is known as the polar body, and
plays no further part in the development. In the
course of the division of the ovum into these two parts
the germinal vesicle also divides, and one part of it
enters the polar body, while a portion remains in the
larger segment which continues to be called the ovum,
and is there known as the female pronudeus. Im-
pregnation has been found to consist essentially in
the entrance of a single spermatozoon into the ovum,
followed by the fusion of the two. The spermatozoon
itself is to be regarded as a cell, the head of which
corresponds to the nucleus. When the spermatozoon
enters the ovum the substance forming its tail becomes
mingled with the protoplasm of the latter, but the head
enlarges and constitutes a distinct body called the male
pronudeus, which travels towards and finally fuses with
F. & B. 2

18 THE HEN'S EGG. [CHAP.

the female pronucleus to constitute the nucleus of the
impregnated ovum.

Segmentation.- There follows upon the impregna-
tion a remarkable process known as the segmentation.
The process consists essentially in the division of the
impregnated ovum by a series of successive segmenta-
tions into a number of cells, of which the whole of the
cells of the future animal are the direct descendants.
In the majority of instances this process results in the
division of the whole ovum into cells ; but in cases of
ova where there is a large amount of food yolk, only
that part of the ovum in which the protoplasm is but
slightly loaded with food material, and which we have
already described as the germinal disc, becomes so
divided. The remainder of the ovum constitutes a
food reservoir for the use of the developing embryo
and is known as the food yolk. The segmentation in
such ova, of which that of the fowl is one of the
best known examples, is described as being partial or
meroblastic1.

In order to understand the process of segmentation
in the fowl's ovum it must be borne in mind that the
germinal disc is not sharply separated from the re-
mainder of the ovum, but that the two graduate insen-
sibly into each other.

The segmentation commences in the lower part of
the oviduct, shortly before the shell has begun to be
formed.

Viewed from above, a furrow is seen to make its

1 For a fuller account of the relation between holoblastic and
meroblastic segmentation the reader is referred to the treatise on
Comparative Embryology by Balfour, Vol. i. chapter iii.

SEGMENTATION.
FIG. 6.

19

A

SURFACE VIEWS OF THE EARLY STAGES OF THE SEGMENTATION
IN A FOWL'S EGG. (A and G after Coste.)

A represents the earliest stage. The first furrow (b) has
begun to make its appearance in the centre of the germinal disc,
whose periphery is marked by the line a. In J3} the first furrow
is completed nearly across the disc, and a second similar furrow
at right angles to the first has appeared. The disc thus
becomes divided somewhat irregularly into quadrants by four
(half) furrows. In a later stage ((7) the meridian furrows b have
increased in number, from four, as in B, to nine, and cross
furrows have also made their appearance. The disc is thus cut
up into small central (c) and larger peripheral (d) segments.
Several new cross furrows are seen just beginning, as ex. gr. close
to the end of the line of reference d.

appearance, running across the germinal disc, though
not for the whole breadth, and dividing it into two
halves (Fig. 6, A). This primary furrow is succeeded
by a second at right angles to itself. The surface thus
becomes divided into four segments or quadrants (Fig.

2—2

20 THE HEN'S EGG. [CHAP.

The second furrow cuts the first somewhat excen-
trically.

The first four furrows do not extend through the
whole thickness of the germinal disc, and the four seg-
ments marked out by them are not separated from the
disc on their lower aspect.

Each of these is again bisected by radiating furrows,
and thus the number of segments is increased from four
to eight (it may be seven or nine). The central portion
of each segment is then, by a cross furrow, cut off from
the peripheral portion, giving rise to the appearance of
a number of central smaller segments, surrounded by
more external elongated segments (Fig. 6, (7).

The excentricity in the arrangement of the segments
is moreover still preserved, the smaller segments being
situated nearer one side of the germinal disc. The
excentricity of the segmentation gives to the segmenting
germinal disc a bilateral symmetry, but the relation
between the axis of symmetry of the segmenting germinal
disc and the long axis of the embryo is not known.

Division of the segments now proceeds rapidly by
means of furrows running in various directions. And
it is important to note that the central segments
divide more rapidly than the peripheral, and con-
sequently become at once smaller and more numerous
(Fig. 7).

Meanwhile sections of the hardened blastoderm
teach us that segmentation is not confined to the sur-
face, but extends through the mass of the blastoderm ;
they shew us moreover that division takes place by
means of not only vertical, but also horizontal furrows,
i. e. furrows parallel to the surface of the disc (Fig. 8).

I.]

SEGMENTATION.
FIG. 7.

21

SURFACE VIEW OF THE GERMINAL Disc OF A HEN'S EGG

DURING THE LATER STAGES OF SEGMENTATION.

(Chromic Acid Preparation.)

At c in the centre of the disc the segmentation masses are
very small and numerous. At b, nearer the edge, they are
larger and fewer ; while those at the extreme margin a are largest
and fewest of all. It will be noticed that the radiating furrows
marking off the segments a do not reach to the extreme margin
e of the disc.

The drawing is completed in one quadrant only ; it will of
course be understood that the whole circle ought to be filled up
in a precisely similar manner.

In this way, by repeated division or segmentation,
the original germinal disc is cut up into a large number
of small rounded masses of protoplasm, which are small-
est in the centre, and increase in size towards the peri-
phery. The segments lying uppermost are moreover
smaller than those beneath, and thus the establishment
of the two layers of the blastoderm is foreshadowed.

22

["CHAP.

SECTION OF THE GERMINAL Disc OF A FOWL DURING THE

LATER STAGES OF SEGMENTATION.

The section, which represents rather more than half the
breadth of the blastoderm (the middle line being shewn at c),
shews that the upper and central parts of the disc segment
faster than those below and towards the periphery. At the
periphery the segments are still very large. One of the larger
segments is shewn at a. In the majority of segments a nucleus
can be seen ; and it seems probable that a nucleus is present in
all. Most of the segments are filled with highly refracting
spherules, but these are more numerous in some cells (especially
the larger cells near the yolk) than in others. In the central
part of the blastoderm the upper cells have commenced to form
a distinct layer.

a. large peripheral cell. b. larger cells of the lower parts of the
blastoderm, c. middle line of blastoderm, e. edge of the
blastoderm adjoining the white yolk. w. white yolk.

In the later stages of segmentation not only do the
first-formed segments become further divided, but seg
mentation also extends into the remainder of the germi-
nal disc.

The behaviour of the nucleus during the segmenta-
tion has not been satisfactorily followed, but there is,

I.] SEGMENTATION. 23

from the analogy of other forms, no doubt that in the
formation of the first two segments the original nucleus,
formed by the fusion of the male and female pronuclei,
becomes divided, and that a fresh division of the nucleus
takes place with the formation of each fresh segment.
Nuclei make their appearance moreover in the part of
the' ovum immediately below that in which the segmen-
tation has already taken place ; these are in all proba-
bility also derived from the primitive nucleus. The
substance round some of these nuclei rises up in the
form of papillae, which are subsequently constricted off
and set free as supplementary segmentation masses;
while some of the nuclei remain and form the nuclei
already spoken of as existing in the bed of white yolk
below the blastoderm in the unincubated egg.

Between the segmented germinal disc, which we
may now call the blastoderm, and the bed of white yolk
on which it rests, a space containing fluid makes its
appearance.

As development proceeds, segmentation reaches its
limits in the centre, but continues at the periphery, and
thus eventually the masses at the periphery become of
the same size as those in the centre.

The distinction however between an upper and a
lower layer becomes more and more obvious.

The masses of the upper layer arrange themselves,
side by side, with their long axes vertical ; their nuclei
become very distinct. In fact they form a membrane
of columnar nucleated cells.

The masses of the lower layer, remaining larger than
those of the upper layer, continue markedly granular
and round, and form rather a close irregular network

24t THE HEN'S EGG. [CHAP. i.

than a distinct membrane. Their nuclei are not readily
visible.

At the time when the segmentation-spheres in the
centre are smaller than those at the periphery, and
those above are also smaller than those below, a few
large spherical masses, probably containing each one of
the nuclei already spoken of, arise by a process of seg-
mentation from the bed of white yolk, and rest directly
on the white yolk at the bottom of the shallow cavity
below the mass of segmentation- spheres. They contain
either numerous small spherules, or fine granules;
the spherules precisely resembling the smaller spheres
of white yolk. These loose spherical masses form the
majority of the formative cells already spoken of.

Thus the original germinal disc of the ovarian ovum
becomes, by the process of segmentation, converted into
the blastoderm of the laid egg. with its upper layer of
columnar nucleated cells, and its lower layer of irregu-
larly disposed cells, accompanied by a few stray "forma-
tive " cells lying loose in the cavity below.

CHAPTER II.

A BRIEF SUMMARY OF THE WHOLE HISTORY OF
INCUBATION.

STEP by step the simple two-layered blastoderm
described in the previous chapter is converted into the
complex organism of the chick. The details of the
many changes through which this end is reached will
perhaps be rendered more intelligible if we prefix to the
special history of them a brief summary of the general
course of events from the beginning to the end of incu-
bation.

In the first place, it is to be borne in mind that the
embryo itself is formed in the area pellucida, and in the
area pellucida alone. The area opaca in no part enters
directly into the body of the chick; the structures to
which it gives rise are to be regarded as appendages,
which sooner or later disappear.

Germinal layers. The blastoderm at starting con-
sists of two layers. Very soon a third layer makes its
appearance between the other two. These three layers,
known as the germinal layers, the establishment of which
is a fact of fundamental importance in the history of the
embryo, are called respectively the upper, middle and
lower layers, or epiblast, mesoblast and Tiypoblast. Of

26 PKELIMINARY ACCOUNT. [CHAP.

these the epiblast and hypoblast constitute the primary
layers.

Three similar germinal layers are found in the
embryos of all vertebrate and most invertebrate forms,
and their history is one of the most important parts of
comparative embryology.

The epiblast gives rise to the epidermis, the central
and peripheral parts of the nervous system, and to the
most important parts of the organs of special sense.
The hypoblast is essentially the secretory layer, and
furnishes the whole epithelial lining of the alimentary
tract and its glands, with the exception of part of the
mouth and anus which are lined by the epiblast and
are spoken of by embryologists as the stomodceum and
proctodceum. Finally the mesoblast is a source from
which the whole of the vascular system, the muscular
and skeletal system, and the connective tissue of all
parts of the body, are developed. It gives in fact origin
to the connective-tissue basis both of the skin and of
the mucous membrane of the alimentary tract, and to
all the structures lying between these two with the
exceptions already indicated. It is more especially to
be noted that it gives rise to the excretory organs and
generative glands.

Formation of the embryo. The blastoderm which
at first, as we have seen, lies like a watch-glass over the
cavity below, its margin resting on the circular germinal
wall of white yolk, spreads, as a thin circular sheet, over
the yolk, immediately under the vitelline membrane.
Increasing uniformly at all points of its circumference,
the blastodermic expansion covers more and more of the
yolk, and at last, reaching the opposite pole, completely
envelopes it. Thus the whole yolk, instead of being

II. J THE HEAD-FOLD. 27

enclosed as formerly by the vitelline membrane alone,
comes to be also enclosed in a bag formed by the blasto-
derm.

It is not however until quite a late period that the
complete closing in at the opposite pole takes place ; in
fact the extension of the blastoderm must be thought
of as going on during the first seven days of incubation.

Both the area opaca and the area pellucida share in
this enlargement, but the area opaca increases much
more rapidly than the area pellucida, and plays the
principal part in encompassing the yolk.

The mesoblast, in that part of the area opaca which
is nearest to the area pellucida, becomes the seat of
peculiar changes, which result in the formation of blood-
vessels. Hence this part of the area opaca is called the
vascular area.

The embryo itself may be said to be formed by a
folding off the central portion of the area pellucida from
the rest of the blastoderm. At first the area pellucida
is quite flat, or, inasmuch as it forms part of the circum-
ference of the yolk, slightly but uniformly curved. Very
soon, however, there appears at a certain spot a semi-
lunar groove, at first small, but gradually increasing in
depth and extent; this groove, which is represented in
section in the diagram (Fig. 9, A\ breaks the uni-
formity of the level of the area pellucida. It may be
spoken of as a tucking in of a small portion of the
blastoderm in the form of a crescent. When viewed
from above, it presents itself as a curved line (the hinder
of the two concentric curved lines in front of A in Fig.
22), which marks the hind margin of the groove, the
depression itself being hidden.

28

PRELIMINARY ACCOUNT.
FIG. 9.

Fig. 9, A to N forms a series of purely diagrammatic repre-
sentations introduced to facilitate the comprehension of the
manner in which the body of the embryo is formed, and of the
various relations of the yolk-sac, amnion and allantois.

In all vt is the vitelline membrane, placed, for convenience
sake, at some distance from its contents, and represented as per-
sisting in the later stages ; in the actual egg it is in direct contact
with the blastoderm (or yolk), and early ceases to have a separate
existence. In all e indicates the embryo, pp the general pleuro-
peritoneal space, of the folds of the amnion proper ; ae or ac the
cavity holding the liquor amnii ; al the allantois ; a the ali-
mentary canal ; y or ys the yolk or yolk-sac.

A, which maybe considered as a vertical section taken longi-
tudinally along the axis of the embryo, represents the relations of
the parts of the egg at the time of the first appearance of the
head-fold, seen on the right-hand side of the blastoderm e. The

II.] THE EMBRYONIC APPENDAGES. 29

blastoderm is spreading both behind (to the left hand in the
figure), and in front (to right hand) of the head-fold, its limits
being indicated by the shading and thickening for a certain dis-
tance of the margin of the yolk y. As yet there is no fold on the
left side of e corresponding to the head-fold on the right.

B is a vertical transverse section of the same period drawn
for convenience sake on a larger scale (it should have been made
flatter and less curved). It shews that the blastoderm (vertically
shaded) is extending laterally as well as fore and aft, in fact in
all directions ; but there are no lateral folds, and therefore no
lateral limits to the body of the embryo as distinguished from
the blastoderm.

Incidentally it shews the formation of the medullary groove
by the rising up of the laminae dorsales. Beneath the section of
the groove is seen the rudiment of the notochord. On either side
a line indicates the cleavage of the mesoblast just commencing.

In C, which represents a vertical longitudinal section of later
date, both head-fold (on the right) and tail-fold (on the left) have
advanced considerably. The alimentary canal is therefore closed
in, both in front and behind, but is in the middle still widely
open to the yolk y below. Though the axial parts of the embryo
have become thickened by growth, the body- walls are still thin ;
in them however is seen the cleavage of the mesoblast, and the
divergence of the somatopleure and splanchnopleure. The
splanchnopleure both at the head and at the tail is folded in to
a greater extent than the somatopleure, and forms the still wide
splanchnic stalk. At the end of the stalk, which is as yet short,
it bends outwards again and spreads over the surface of the yolk.
The somatopleure, folded in less than the splanchnopleure to
form the wider somatic stalk, sooner bends round and runs out-
wards again. At a little distance from both the head and the
tail it is raised up into a fold, a/, a/, that in front of the head
being the highest. These are the amniotic folds. Descending from
either fold, it speedily joins the splanchnopleure again, and the
two, once more united into an uncleft membrane, extend some
way downwards over the yolk, the limit or outer margin of the
opaque area not being shewn. All the space between the soma-
topleure and the splanchnopleure, pp} is shaded with dots. Close

30

PRELIMINARY ACCOUNT,

[CHAP.

to the body this space may be called the pleuroperitoneal cavity ;
but outside the body it runs up into either amniotic fold, and
also extends some little way over the yolk.

D represents the tail end at about the same stage on a more
enlarged scale, in order to illustrate the position of the allantois
al (which was for the sake of simplicity omitted in (7), shewn as a
bud from the splanchnopleure, stretching downwards into the pleu-
roperitoneal cavity^. The dotted area representing as before the

II.] THE EMBRYONIC APPENDAGES. 31

whole space between the splanchnopleure and the somatopleure,
it is evident that a way is open for the allantois to extend from
its present position into the space between the two limbs of the
amniotic fold af.

Ej also a longitudinal section, represents a stage still farther
advanced. Both splanchnic and somatic stalks are much nar-
rowed, especially the former, the cavity of the alimentary canal
being now connected with the cavity of the yolk-sack by a mere
canal. The folds of the amnion are spreading over the top of
the embryo and nearly meet. Each fold consists of two walls
or limbs, the space between which (dotted) is as before merely
a part of the space between the somatopleure and splanchno-
pleure. Between these arched amniotic folds and the body of
the embryo is a space not as yet entirely closed in.

F represents on a different scale a transverse section of E
taken through the middle of the splanchnic stalk. The dark ring
in the body of the embryo shews the position of the neural canal,
below which is a black spot, marking the notochord. On either
side of the notochord the divergence of somatopleure and splanch-
nopleure is obvious. The splanchnopleure, more or less thick-
ened, is somewhat bent in towards the middle line, but the two
sides do not unite, the alimentary canal being as yet open below
at this spot ; after converging somewhat they diverge again and
run outwards over the yolk. The somatopleure, folded in to
some extent to form the body- walls, soon bends outwards again,
and is almost immediately raised up into the lateral folds of the
amnion af. The continuity of the pleuroperitoneal cavity within
the body with the interior of the amniotic fold outside the body
is evident ; both cavities are dotted.

(7, which corresponds to D at a later stage, is introduced to
shew the manner in which the allantois, now a distinctly hollow
body, whose cavity is continuous with that of the alimentary
canal, becomes directed towards the amniotic fold.

In H a longitudinal, and / a transverse section of later date,
great changes have taken place. The several folds of the amnion
have met and coalesced above the body of the embryo. The inner
limbs of the several folds have united into a single membrane (a),
which encloses a space (ae or ac) round the embryo. This mem-

32

PRELIMINARY ACCOUNT.

[CHAP.

brane(a)is-the amnion proper, and the cavity within it, i.e. between
it and the embryo, is the cavity of the amnion containing the
liquor amnii. The allantois is omitted for the sake of sim-
plicity.

It will be seen that the amnion a now forms in every direc-
tion the termination of the somatopleure ; the peripheral portions
of the somatopleure, the united outer or descending limbs of the
folds af in (7, D, F, G having been cut adrift, and now forming
an independent continuous membrane, the serous membrane,
immediately underneath the vitelline membrane.

In / the splanchnopleure is seen converging to complete the
closure of the alimentary canal a' even at the stalk (elsewhere
the canal has of course long been closed in), and then spreading
outwards as before over the yolk. The point at which it unites
with the somatopleure, marking the extreme limit of the cleavage
of the mesoblast, is now much nearer the lower pole of the
diminished yolk.

II.] THE EMBRYONIC APPENDAGES. 33

As a result of these several changes, a great increase in the
dotted space has taken place. It is now possible to pass from
the actual peritoneal cavity within the body, on the one hand
round a great portion of the circumference of the yolk, and on the
other hand above the amnion a, in the space between it and the
serous envelope.

Into this space the allantois is seen spreading in K at al.

In L the splanchnopleure has completely invested the yolk-
sac, but at the lower pole of the yolk is still continuous with
that peripheral remnant of the somatopleure now called the
serous membrane. In other words, the cleavage of the mesoblast
has been carried all round the yolk (ys) except just at the lower
pole.

In M the cleavage has been carried through the pole itself ;
the peripheral portion of the splanchnopleure forms a complete
investment of the yolk, quite unconnected with the peripheral
portion of the somatopleure, which now exists as a continuous
membrane lining the interior of the shell. The yolk-sac (ys) is
therefore quite loose in the pleuroperitoneal cavity, being con-
nected only with the alimentary canal (a') by a solid pedicle.

Lastly, in ^V the yolk-sac (ys} is shewn being withdrawn into
the cavity of the body of the embryo. The allantois is as before,
for the sake of simplicity, omitted ; its pedicle would of course lie
by the side of ys in the somatic stalk marked by the usual dotted
shading.

It may be repeated that the above are diagrams, the various
spaces being shewn distended, whereas in many of them in the
actual egg the walls have collapsed, and are in near juxta-
position.

In a vertical longitudinal section carried through the
middle line, we may recognize the following parts (Fig.
9, A, or on a larger scale Fig. 10, which also shews details
which need not be considered now). Beginning at what
will become the posterior extremity of the embryo (the
left-hand side of the figure in each case), and following the
surface of the blastoderm forwards (to the right in the
F. & B. 3

PRELIMINARY ACCOUNT.

FIG. 10.

N.C.

[CHAP.

F.So.

DIAGRAMMATIC LONGITUDINAL SECTION THROUGH THE Axis OF
AN EMBRYO.

The section is supposed to be made at a time when the head-
fold has commenced but the tail-fold has not yet appeared.
F. So. fold of the somatopleure.
F. Sp. fold of the splanchnopleure.

The line of reference F. So. is placed in the lower bay, outside
the embryo. The line of D is placed in the upper bay inside
the embryo ; this will remain as the alimentary canal. Both
folds (F. So., F. Sp.) are parts of the head- fold, and are to be
thought of as continually travelling onwards (to the left) as de-
velopment proceeds.
pp. space between somatopleure and splanchnopleure : pleuro-

peritoneal cavity.

Am. commencing (head) fold of the amnion.
A fuller explanation is given under Fig. 29.

figures), the level is maintained for some distance, and
then there is a sudden descent, the blastoderm bending
round and pursuing a precisely opposite direction to its
previous one, running backwards instead of forwards, for
some distance. It soon, however, turns round again, and
once more running forward, with a gentle ascent, regains
the original level. As seen in section, then, the blasto-
derm at this spot may be said to be folded up in the

II.] THE HEAD-FOLD. 35

form of the letter Q. This fold we shall always speak of
as the head-fold. In it we may recognize two limbs:
an upper limb in which the curve is directed forwards,
and its bay, opening backwards, is underneath the blas-
toderm, i.e. as we shall see, inside the embryo (Fig. 10.
If) ; and an under limb in which the curve is directed
backwards, and its bay, opening forwards, is above the
blastoderm, i.e. outside the embryo. If an 8 like the above,
made of some elastic material, were stretched laterally,
the effect would be to make both limbs longer and
proportionally narrower, and their bays, instead of being
shallow cups, would become more tubular. Such a
result is in part arrived at by the growth of the blasto-
derm; the upper limb of the gj is continually growing
forward (but, unlike the stretched elastic model, in-
creases in all its dimensions at the same time), and the
lower limb is as continually lengthening backwards;
and thus both upper and lower bays become longer and
longer. This we shall hereafter speak of as the travel-
ling backwards of the head-fold.

The two bays do not however both become tubular.
The section we have been speaking of is supposed to be
taken vertically along a line, which will afterwards be-
come the axis of the embryo; and the lower bay of the
8 is a section of the crescentic groove mentioned above,
in its middle or deepest part. On either side of the
middle line the groove gradually becomes shallower.
Hence in sections taken on either side of the middle
line or axis of the embryo (above or below the plane
of the figures), the groove would appear the less marked
the farther the section from the middle line, and at a
certain distance would disappear altogether. It must be

3—2

36 PRELIMINARY ACCOUNT. [CHAP.

remembered that the groove is at first crescent-shaped,
with the concavity of the crescent turned towards what
will be the hind end of the embryo (Fig. 22). As the whole
head-fold is carried farther and farther back, the horns
of the crescent are more and more drawn in towards the
middle line, the groove becoming first semicircular, then
horse-shoe-shaped. In other words, the head-fold,
instead of being a simple fold running straight back-
wards, becomes a curved fold with a central portion in
front running backwards, and two side portions running
in towards the middle line. The effect of this is that
the upper bay of the 8 (that within the embryo) gets
closed in at the sides as well as in the front, and thus
speedily becomes tubular. The under bay of the <3
(that outside the embryo) remains of course open at the
sides as in front, and forms a sort of horse-shoe-shaped
ditch surrounding the front end of the embryo.

We have dwelt thus at length on the formation of
the head-fold, because, unless its characters are fairly
grasped, much difficulty may be found in understanding
many events in the history of the chick. The reader
will perhaps find the matter easier to comprehend if he
makes for himself a rough model, which he easily can
do by spreading a cloth out flat to represent the blasto-
derm, placing one hand underneath it, to mark the axis
of the embryo, and then tucking in the cloth from above
under the tips of his fingers. The fingers, covered with
the cloth and slightly projecting from the leve] of the
rest of the cloth, will represent the head, in front of
which will be the semicircular or horse-shoe-shaped
groove of the head-fold.

At its first appearance the whole 8 may be spoken

II.] THE TAIL-FOLD. 37

of as the head-fold, but later on it will be found con-
venient to restrict the name chiefly to the lower limb
of the 3.

Some time after the appearance of the head-fold, an
altogether similar but at first less conspicuous fold
makes its appearance, at a point which will become the
posterior end of the embryo. This fold, which travels
forwards just as the head-fold travels backwards, is the
tail-fold (Fig. 9, (7).

In addition, between the head- and the tail-fold two
lateral folds appear, one on either side. These are
simpler in character than either head-fold or tail-fold,
inasmuch as they are nearly straight folds directed
inwards towards the axis of the body (Fig. 8, F), and not
complicated by being crescentic in form. Otherwise they
are exactly similar, and in fact are formed by the con-
tinuations of the head- and tail-folds respectively.

As these several folds become more and more de-
veloped, the head-fold travelling backwards, the tail-
fold forwards, and the lateral folds inwards, they tend to
unite in the middle point ; and thus give rise more and
more distinctly to the appearance of a small tubular
sac seated upon, and connected, by a continually-nar-
rowing hollow stalk, with that larger sac which is formed
by the extension of the rest of the blastoderm over the
whole yolk.

The smaller sac we may call the " embryonic sac,"
the larger one " the yolk-sac." As incubation proceeds,
the smaller sac (Fig. 9) gets larger and larger at the
expense of the yolk-sac (the contents of the latter being
gradually assimilated by nutritive processes into the
tissues forming the growing walls of the former, not

38 PRELIMINARY ACCOUNT. [CHAP.

directly transferred from one cavity into the other).
Within a day or two of the hatching of the chick, at a
time when the yolk-sac is still of some considerable size,
or at least has not yet dwindled away altogether, and
the development of the embryonic sac is nearly com-
plete, the yolk-sac (Fig. 9, N) is slipped into the body
of the embryo, so that ultimately the embryonic sac
alone remains.

The embryo, then, is formed by a folding-off of a
portion of the blastoderm from the yolk-sac. The
general outline of the embryo is due to the direction
and shape of the several folds which share in its forma-
tion ; these, while preserving a nearly perfect bilateral
symmetry, present marked differences at the two ends
of the embryo. Hence from the very first there is no
difficulty in distinguishing the end which will be the
head from that which will be the tail.

In addition to this, the tubular sac of the embryo,
while everywhere gradually acquiring thicker and
thicker walls, undergoes at various points, through local
activities of growth in the form of thickenings, ridges,
buds or other processes, many modifications of the
outline conferred upon it by the constituent folds. Thus
bud-like processes start out from the trunk to form the
rudiments of the limbs, and similar thickenings and
ridges give rise to the jaws and other parts of the face.
By the unequal development of these outgrowths the
body of the chick is gradually moulded into its proper
outward shape.

Were the changes which take place of this class
only, the result would be a tubular sac of somewhat com-
plicated outline, but still a simple tubular sac. Such

IT.] THE MEDULLARY CANAL. 39

a simple sac might perhaps be roughly taken to repre-
sent the body of many an invertebrate animal ; but the
typical structure of a bird or other vertebrate animal is
widely different. It may very briefly be described as
follows.

First there is, above, a canal running lengthways
along the body, in which are lodged the brain and
spinal cord. Below this neural tube is an axis repre-
sented by the bodies of the vertebrae and their con-
tinuation forwards in the structures which form the base
of the skull. Underneath this, again, is another tube
closed in above by the axis, and on the sides and below
by the body-walls. Enclosed in this second tube, and
suspended from the axis, is a third tube, consisting of
the alimentary canal with its appendages (liver, pan-
creas, lungs, &c., which are fundamentally mere diver-
ticula from one simple canal). The cavity of the outer
tube, which also contains the heart and other parts of
the vascular system, is the general body cavity ; it con-
sists of a thoracic or pleural, and an abdominal or peri-
toneal section ; these two parts are, however, from their
mode of origin, portions of one and the same tube.
Thus a transverse section of a vertebrate animal always
shews the same fundamental structure : above a single
tube, below a double tube, the latter consisting of one
tube enclosed within another, the inner being the ali-
mentary canal, the outer the general cavity of the body.
Into such a triple tube the simple tubular embryonic
sac of the chick is converted by a series of changes of a
remarkable character.

The upper or neural tube is formed in the following
way. At a very early period the upper layer of the

40 PKELIMINARY ACCOUNT. [CHAP.

blastoderm or epiblast in the region which will become
the embryo, is raised up into two ridges or folds, which
run parallel to each other at a short distance on either
side of what will be the long axis of the embryo, and
thus leave between them a shallow longitudinal groove
(Fig. 9, B, also Figs. 21, m.c). As these ridges, which
bear the name of medullary folds, increase in height
they arch over towards each other, and eventually meet
and coalesce in the middle line, thus converting the
groove into a canal, which at the same time becomes
closed at either end (Fig. 8, F. /, also Fig. 34. Me.}.
The cavity so formed is the cavity of the neural tube,
and eventually becomes the cerebro-spinal canal. Its
walls are wholly formed of epiblast.

The lower double tube, that of the alimentary canal,
and of the general cavity of the body, is formed in an
entirely different way. It is, broadly speaking, the
result of the junction and coalescence of the funda-
mental embryonic folds, the head-fold, tail-fold, and
lateral folds ; in a certain sense the cavity of the body
is the cavity of the tubular sac described in the last
paragraph.

But it is obvious that a tubular sac formed by the
folding-in of a single sheet of tissue, such as we have
hitherto considered the blastoderm to be, must be a
simple tubular sac possessing a single cavity only. The
blastoderm however does not long remain a single
sheet, but speedily becomes a double sheet of such a
kind that, when folded in, it gives rise to a double
tube.

Very early the blastoderm becomes thickened in the
region of the embryo, the thickening being chiefly due

II.] THE BODY CAVITY. 41

to an increase in the middle layer or mesoblast, while
at the same time it becomes split or cleft horizontally
over the greater part of its extent into two leaves, an
upper leaf and a lower leaf. In the neighbourhood of
the axis of the body, beneath the neural tube, this
cleavage is absent (Fig. 9, B ; also Figs. 24, 34), in fact,
it begins at some little distance on either side of the
axis and spreads thence into the periphery in all direc-
tions. It is along the mesoblast that the cleavage
takes place, the upper part of the mesoblast uniting
with epiblast to form the upper leaf, and the lower
part with the hypoblast to form the lower leaf.

In the fundamental folds both leaves are involved,
both leaves are folded downwards and inwards, both
leaves tend to meet in the middle below ; but the
lower leaf is folded in more rapidly, and thus diverges
from the upper leaf, a space being gradually developed
between them (Fig. 9). In course of time the several
folds of the lower leaf meet and unite to form an inner
tube quite independently of the upper leaf, whose own
folds in turn meet and unite to form an outer tube
separated from the inner one by an intervening space.
The inner tube which from its mode of formation is
clearly lined by hypoblast is the alimentary canal which
is subsequently perforated at both ends to form the
mouth and anus ; the walls of the outer tube are the
walls of the body ; and the space between the two tubes
is the general body or pleuroperitoneal cavity.

Hence the upper (or outer) leaf of the blastoderm,
from its giving rise to the body-walls, is called the
somatopleure 1 ; the lower (or inner) leaf, from its form-

1 Soma, body, pleuron, side.

42 PRELIMINARY ACCOUNT. [CHAP.

ing the alimentary canal and its tributary viscera, the
splanchnopleure *.

This horizontal splitting of the blastoderm into a
somatopleure and a splanchnopleure, which we shall
hereafter speak of as the cleavage of the mesoblast, is
not confined to the region of the embryo, but gradually
extends over the whole of the yolk-sac. Hence in the
later days of incubation the yolk-sac comes to have
two distinct coats, an inner splanchnopleuric and an
outer somatopleuric, separable from each other all
over the sac. We have seen that, owing to the
manner of its formation, the ' embryonic sac ' is con-
nected with the ' yolk-sac ' by a continually narrowing
hollow stalk ; but this stalk must, like the embryonic
sac itself, be a double stalk, and consist of a smaller
inner stalk within a larger outer one, Fig. 9, E} H.
The folds of the splanchnopleure, as they tend to
meet and unite in the middle line below, give
rise to a continually narrowing hollow stalk of their
own, a splanchnic stalk, by means of which the walls of
the alimentary canal are continuous with the splanch-
nopleuric investment of the yolk-sac, and the interior
of that canal is continuous with the cavity inside the
yolk-sac. In the same way the folds of the somato-
pleure form a similar stalk of their own, a somatic
stalk, by means of which the body- walls of the chick
are continuous (for some time ; the continuity, as we
shall see, being eventually broken by the development
of the amnion) with the somatopleuric investment of
the yolk-sac ; and the pleuroperitoneal cavity of the

1 Splanchnon, viscus, pleuron, side.

II.] THE AMNION. 43

body of the chick is continuous with the narrow space
between the two investments of the yolk-sac.

At a comparatively early period the canal of the
splanchnic stalk becomes obliterated, so that the
material of the yolk can no longer pass directly into
the alimentary cavity, but has to find its way into
the body of the chick by absorption through the blood-
vessels. The somatic stalk, on the other hand, remains
widely open for a much longer time ; but the somatic
shell of the yolk-sac never undergoes that thickening
which takes place in the somatic walls of the embryo
itself; on the contrary, it remains thin and insignificant.
When, accordingly, in the last days of incubation the
greatly diminished yolk-sac with its splanchnic invest-
ment is withdrawn into the rapidly enlarging abdominal
cavity of the embryo, the walls of the abdomen close
in and unite, without any regard to the shrivelled,
emptied somatopleuric investment of the yolk-sac,
which is cast off as no longer of any use. (Fig. 9. Com-
pare the series.)

The Amnion. Very closely connected with the
cleavage of the mesoblast and the division into soma-
topleure and splanchnopleure, is the formation of the
amnion, all mention of which was, for the sake of
simplicity, purposely omitted in the description just
given.

The amnion is a peculiar membrane enveloping the
embryo, which takes its origin from certain folds of
the somatopleure, and of the somatopleure only, in the
following way.

At a time when the cleavage of the mesoblast has
somewhat advanced, there appears, a little way in front

44 PRELIMINARY ACCOUNT. [CHAP.

of the semilunar head-fold, a second fold (Fig. 22, also
Fig. 9, (7.), running more or less parallel or rather con-
centric with the first, and not unlike it in general
appearance, though differing widely from it in nature.
In the head-fold the whole thickness of the blastoderm
is involved ; in it both somatopleure and splanchno-
pleure (where they exist, i. e. where the mesoblast is
cleft) take part. This second fold, on the contrary, is
limited entirely to the somatopleure. Compare Figs.
9 and 10. In front of the head-fold, and therefore alto-
gether in front of the body of the embryo, the somato-
pleure is a very thin membrane, consisting only of
epiblast and a very thin layer of mesoblast ; and the
fold we are speaking of is, in consequence, itself thin
and delicate. Rising up as a semilunar fold with its
concavity directed towards the embryo (Fig. 9, C, q/!),
as it increases in height it is gradually drawn back-
wards over the developing head of the embryo. The
fold thus covering the head is in due time accompanied
by similar folds of the somatopleure starting at some
little distance behind the tail, and at some little dis-
tance from the sides (Fig. 9, G, D, E, F, and Fig. 11 am.).
In this way the embryo becomes surrounded by a
series of folds of thin somatopleure, which form a con-
tinuous wall all round it. All are drawn gradually
over the body of the embryo, and at last meet and
completely coalesce (Fig. 9, H, 7), all traces of their
junction being removed. Beneath these united folds
there is therefore a cavity, within which the embryo
lies (Fig. 9, H, ae). This cavity is the cavity of the
amnion. The folds which we have been describing are
those which form the amnion.

II.]

THE AMNION.
FIG. 11.

45

DIAGRAMMATIC LONGITUDINAL SECTION THROUGH THE POS-
TERIOR END OF AN EMBRYO BIRD, AT THE TIME OF THE
FORMATION OF THE ALLANTOIS.

ep. epiblast ; Sp.c. spinal canal ; ch. notochord ; n.e. neurenteric
canal ; liy. hypoblast ; p.a.g. postanal gut ; pr. remains of
primitive streak folded in on the ventral side ; al. allantois ;
me. mesoblast; an. point where anus will be formed; p.c.
perivisceral cavity ; am. amnion ; so. somatopleure ; sp.
splanchnopleure.

Each fold, of course, necessarily consists of two
limbs, both limbs consisting of epiblast and a very thin
layer of mesoblast ; but in one limb the epiblast looks
towards the embryo, while in the other it looks away
from it. The space between the two limbs of the fold,
as can easily be seen in Figs. 9 and 11, is really part
of the space between the somatopleure and splanch-
nopleure ; it is therefore continuous with the general
space, part of which afterwards becomes the pleuro-
peritoneal cavity of the body, shaded with dots in
figure 9 and marked (p p). It is thus possible 'to
pass from the cavity between the two limbs of each

46 PRELIMINARY ACCOUNT. [CHAP.

fold of the amnion into the cavity which surrounds
the alimentary canal. When the several folds meet
and coalesce together above the embryo, they unite
in such a way that all their inner limbs go to form a
continuous inner membrane or sac, and all their outer
limbs a similarly continuous outer membrane or sac.
The inner membrane thus built up forms a completely
closed sac round the body of the embryo, and is called
the amniotic sac, or amnion proper (Fig. 9, H, J, &c. a.),
and the fluid which it afterwards contains is called
the amniotic fluid, or liquor amnii. The space between
the inner and outer sac, being formed by the united
cavities of the several folds, is, from the mode of its
formation, simply a part of the general cavity found
everywhere between somatopleure and splanchnopleure.
The outer sac over the embryo lies close under the
vitelline membrane, while its periphery is gradually
extended over the yolk as the somatopleuric invest-
ment of the yolk-sac described in the preceding para-
graph. It constitutes the false amnion while the mem-
brane of which it forms a part is frequently known as
the serous membrane.

The Allantois. If the mode of origin of these two
sacs (the inner or true amnion, and the outer or false
amnion, as Baer called it) and their relations to the
embryo be borne in mind, the reader will have no diffi-
culty in understanding the course taken in its growth
by an important organ, the allantois, of which we shall
hereafter have to speak more in detail.

The allantois is essentially a diverticulum of the
alimentary tract, into which it opens immediately in
front of the anus. It at first (Fig. 11, al) forms a

II.] THE ALLANTOIS. 47

flattened sac projecting into the pleuroperitoneal cavity,
the walls of the sac being formed of a layer of splanchnic
mesoblast lined by hypoblast.

It grows forwards in the peritoneal cavity until it
reaches the stalk connecting the embryo with the yolk-
sac, and thence very rapidly pushes its way into the space
between the true and false amniotic sacs (Fig. 9, G, K}.
Curving over the embryo, it comes to lie above the
embryo and the amnion proper, separated from the
shell (and vitelline membrane) by nothing more than
the thin false amnion. In this position it becomes
highly vascular, and performs the functions of a respi-
ratory organ. It is evident that though now placed
quite outside the embryo, the space in which it lies is a
continuation of that peritoneal cavity in which it took
its origin.

It is only necessary to add, that the serous mem-
brane, including the false amnion, either coalesces with
the vitelline membrane, in contact with which it lies,
or else replaces it ; and in the later days of incubation
was called by the older embryologists the chorion — a
name however which we shall not adopt.

CHAPTER III.

THE CHANGES WHICH TAKE PLACE DURING THE FIRST
DAY OF INCUBATION.

DURING the descent of the egg along the oviduct,
where it is exposed to a temperature of about 40° C., the
germinal disc, as we have seen, undergoes important
changes. When the egg is laid and becomes cold these
changes all but entirely cease, and the blastoderm
remains inactive until, under the influence of the higher
temperature of natural or artificial incubation, the vital
activities of the germ are brought back into play, the
arrested changes go on again, and usher in the series of
events which we have now to describe in detail.

The condition of the blastoderm at the time when
the egg is laid is not exactly the same in all eggs ; in
some the changes being farther advanced than in others,
though the differences of course are slight. In some
eggs, especially in warm weather, changes of the same
kind as those caused by actual incubation may take
place, to a certain extent, in the interval between
laying and incubation ; lastly, in all eggs, both under
natural and especially under artificial incubation, the

CHAP. III.] THE EMBRYONIC SHIELD. 49

dates of the several changes are, within the limits of
some hours, very uncertain, particularly in the first few
days ; one egg being found, for example, at 36 hours in
the same stage as another at 24 or 30 hours, or a third
at 40 or 48 hours. When we speak therefore of any
event as taking place at any given hour or part of any
given day, we are to be understood as meaning that
such an event will generally be found to have taken
place at about that time. We introduce exact dates
for the convenience of description.

The changes which take place during the first day
will be most easily considered under several periods.

Prom the 1st to about the 8th hour. — During this
period the blastoderm, when viewed from above, is
found to have increased in size. The pellucid area,
which at the best is but obscurely marked in the unin-
cubated egg, becomes very distinct (the central opacity
having disappeared), and contrasts strongly with the
opaque area, which has even still more increased both
in distinctness and size.

For the first few hours both the pellucid and opaque
areas remain approximately circular, and the most im-
portant change, besides increase in size and greater
distinctness which can be observed in them, is a slight
ill-defined opacity or loss of transparency, which makes
its appearance in the hinder half of the pellucid area.
This is known as the embryonic shield.

Slight as are the changes which can at this stage be1
seen from surface views, sections taken from hardened
specimens bring to light many most important changes
in the nature and arrangement of the constituent
cells.

F. & B. 4

50

THE FIRST DAY.

[CHAP.

FIG. 12.

i

SECTION OF A BLASTODERM OF A FOWL'S EGG
AT THE COMMENCEMENT OF INCUBATION.

The thin but complete upper layer ep
composed of columnar cells rests on the in-
complete lower layer I, composed of larger
and more granular cells. The lower layer is
thicker in some places than in others, and is
especially thick at the periphery. The line
below the under layer marks the upper sur-
face of the white yolk. The larger so-called
formative cells are seen at 6, lying on the
white yolk. The figure does not take in quite
the whole breadth of the blastoderm ; but the
reader must understand that both to the right
hand and the left ep is continued farther than
I, so that at the extreme edge it rests directly
on the white yolk.

It will be remembered that the
blastoderm in the unincubated egg is
composed of two layers, an upper (Fig.
12, ep) and an under layer; that the
upper is a coherent membrane of colum-
nar nucleated cells, but that the lower
one (Fig. 12, 1) is formed of an irregular
network of larger cells in which the
nuclei are with difficulty visible; and
that in addition to this there are certain
still larger cells, called 'formative cells'
(Fig. 12,6), lying at the bottom of the
segmentation-cavity.

Under the influence of incubation
changes take place very rapidly, which

THE HYPOBLAST.

51

III.]

result in the formation of the three layers of the blasto-
derm.

The upper layer, which is the epiblast already
spoken of (Fig. 13), takes at first but little share in
these changes.

In the lower layer, however, certain of the cells
begin to get flattened horizontally, their granules become
less numerous, and the nucleus becomes distinct; the
cells so altered cohere together and form a membrane.
The membrane thus formed, which is first completed in

TRANSVERSE SECTION THROUGH THE BLASTODERM OF A CHICK
BEFORE THE APPEARANCE OF THE PRIMITIVE STREAK.

The epiblast is represented somewhat diagrammatically. The
hyphens shew the points of junction of the two halves of the
section. The hypoblast is already constituted as a membrane of
flattened cells, and a number of scattered cells are seen between
it and the epiblast.

the centre of the pellucid area, constitutes the hypoblast.
Between the hypoblastic membrane and the epiblast
there remain a number of scattered cells (Fig. 13) which
cannot however be said to form a definite layer altogether
distinct from the hypoblast. They are almost entirely
confined to the posterior part of the area pellucida, and

4-2

52 THE FIRST DAY. [CHAP.

give rise to the opacity of that part, which we have
spoken of as the embryonic shield.

At the edge of the area pellucida the hypoblast
becomes continuous with a thickened rim of material,
underlying the epiblast, and derived from the original
thickened edge of the blastoderm and the subjacent
yolk. It is mainly formed of yolk granules, with a
varying number of cells and nuclei imbedded in it. It
is known as the germinal wall, and is spoken of more in
detail on pp. 65 and 66.

The epiblast is the Hornblatt (corneal layer), and the hypo-
blast the Darmdrusenblatt (epithelial glandular layer) of the
Germans, while those parts of the mesoblast which take part in
the formation of the somatopleure and splanchnopleure cor-
respond respectively to the Haut-musJcel-platte and Darm-faser-
platte.

All blood-vessels arise in the mesoblast. Hence the vascular
layer of the older writers falls entirely within the mesoblast.

The serous layer of the old authors includes the whole of
the epiblast, but also comprises a certain portion of mesoblast ;
for they speak of all the organs of animal life (skin, bones,
muscle, &c.) as being formed out of the serous layer, whereas the
epiblast proper gives rise only to the epidermis and to certain
parts of the nervous system. In the same way their mucous layer
corresponds to the hypoblast with so much of the mesoblast as
takes part in the formation of the organs of organic life. Their
vascular layer therefore answers to a part only of the mesoblast
viz. that part in which blood-vessels are especially developed.

From the 8th to the 12th hour. The changes
which next take place result in the complete differen-
tiation of the embryonic layers, a process which is inti-
mately connected with the formation of a structure known
as the primitive streak. The full meaning of the

III.] THE PRIMITIVE STREAK. 53

latter structure, and its relation to the embryo, can how-
ever only be understood by comparison with the develop-
ment of the lower forms of vertebrate life.

It will be remembered that in surface views of the
unincubated blastoderm a small arc, at what we stated
to be the posterior end, close to the junction between
the area opaca and the area pellucida is distinguished
by its more opaque appearance. In the surface view
the primitive streak appears as a linear opacity, which
gradually grows forwards from the middle of this arc
till it reaches about one-third of the diameter of the

FIG. 14.

pr s

AREA PELLUCIDA OF A VERY YOUNG BLASTODERM OF A CHICK,
SHEWING THE PRIMITIVE STREAK SHORTLY AFTER ITS
FIRST APPEARANCE.
pr.s. primitive streak ; ap. area pellucida ; a.op. area opaca.

area pellucida. During the formation of the primitive
streak the embryonic shield grows fainter and finally
vanishes. When definitely established the primitive
streak has the appearance diagrammatically represented
in Fig. 14.

54 THE FIRST DAY. [CHAP.

Sections at this stage throw a very important light
on the nature and mode of origin of the primitive
streak. In the region in front of it the blastoderm is
still formed of two layers only, but in the region of the
streak itself the structure of the blastoderm is greatly
altered. The most important features in it are repre-
sented in Fig. 15. This figure shews that the median

FIG. 15.

TRANSVERSE SECTION THROUGH A BLASTODERM OF ABOUT THE
AGE REPRESENTED IN FlG. 14, SHEWING THE FlRST DIF-
FERENTIATION OF THE PRIMITIVE STREAK.

The section passes through about the middle of the primitive
streak.

pvs. primitive streak ; ep. epiblast ; Jiy. hypoblast ; yk. yolk of
the germinal wall.

portion of the blastoderm has become very much thick-
ened (thus producing the opacity of the primitive streak),
and that this thickening is caused by a proliferation of
rounded cells from the epiblast. In the very young
primitive streak, of which Fig. 15 is a section, the rounded
cells are still continuous throughout with the epiblast, but
they form nevertheless the rudiment of the greater part
of a sheet of mesoblast, which will soon arise in this
region.

III.] THE PRIMITIVE STREAK. 55

In addition to the cells clearly derived from the
epiblast, there are certain other cells (Fig. 15), closely
adjoining the hypoblast ; these are derivatives of the
cells, interposed between the epiblast and hypoblast,
which gave rise to the appearance of the embryonic
shield during the previous stage. In our, opinion these
cells also have a share in forming the future meso-
blast.

It thus appears that the primitive streak is essen-
tially a linear proliferation of epiblast cells; the cells
produced being destined to give rise to the mesoblast.
This proliferation first commences at the hinder end of
the area pellucida, and thence proceeds forwards.

While the primitive streak is being established, the
epiblast becomes two or more rows of cells deep in the
region of the area pellucida.

Soon after this, the hitherto circular pellucid area
becomes oval (the opaque area remaining circular). The
oval is, with remarkable regularity, so placed that its
long axis forms a right angle, or very nearly a right
angle, with the long axis of the egg itself. Its narrow
end corresponds with the future hind end of the embryo.
If an egg be placed with its broad end to the right hand
of the observer, the head of the embryo will in nearly
all cases be found pointing away from him.

The 12th to the 16th hour. The primitive streak
at its first appearance is shadowy and ill-defined; gradu-
ally however it becomes more distinct; and during the
same period the pellucid area rapidly increases in size,
and from being oval becomes pear-shaped (Fig. 16). The
primitive streak grows even more rapidly than the
pellucid area; so that by the 16th hour it is not only

56 THE FIRST DAY. [CHAP.

absolutely, but also relatively to the pellucid area,
longer than it was at the 12th hour.

It finally occupies about two-thirds of the length of
the area pellucida; but its hinder end in many instances
appears to stop short of the posterior border of the
area pellucida (Fig. 16). The median line of the

Fm. 16.

SURFACE VIEW OF THE AREA PELLUCIDA OF A CHICK'S
BLASTODERM SHORTLY AFTER THE FORMATION OF THE
PRIMITIVE GROOVE.

pr. primitive streak with primitive groove ;
of. amniotic fold.

The darker shading round the primitive streak shews the
extension of the mesoblast.

primitive streak becomes marked by a shallow furrow
running along its axis. In fresh specimens, viewed with
transmitted light, this furrow appears as a linear trans-
parency, but in hardened specimens seen under reflected
light may be distinctly recognized as a narrow groove,

III.] THE PRIMITIVE GROOVE. 57

the bottom of which, being thinner than the sides,
appears more transparent when viewed with transmitted
light. It is known as the primitive groove. Its depth
and the extent of its development are subject to great
variations.

During these changes in external appearance there
grow from the edges of the cord of cells constituting the
primitive streak two lateral wings of mesoblast cells,
whiclj gradually extend till they reach the sides of the
area pellucida (Fig. 17). The two wings of mesoblast
meet along the line of the primitive streak, where they
still remain attached to the epiblast. During this period
many sections through the primitive streak give an
impression of the mesoblast being involuted along the lips
of a groove. The hypoblast below the primitive streak
is always quite independent of the mesoblast above,
though much more closely attached to it in the median
line than at the sides. The part of the mesoblast, which
we believe to be derived from the primitive lower layer
cells, can generally be distinctly traced. In many cases,
especially at the front end of the primitive streak, it
forms, as in Fig. 17, a distinct layer of stellate cells, quite
unlike the rounded cells of the mesoblastic involution
of the primitive streak.

In the region in front of the primitive streak, where
the first trace of the embryo will shortly appear, the
layers at first undergo no important changes, except
that the hypoblast becomes somewhat thicker. Soon,
however, as shewn in longitudinal section in Fig. 18, the
hypoblast along the axial line becomes continuous be-
hind with the front end of the primitive streak. Thus
at this point, which is the future hind end of the

58

THE FIRST DAY.

[CHAP.

Fig. 17.

Fig. 18.

III.] FORMATION OF THE EMBRYO. 59

FIG. 17.

TRANSVERSE SECTION THROUGH THE FRONT END OP THE PRI-
MITIVE STREAK OF A BLASTODERM OF THE SAME AGE AS
FIG. 16.

pv. primitive groove ; m. mesoblast ; ep. epiblast ; hy. hypo-
blast ; yh. yolk of germinal wall.

FIG. 18.

LONGITUDINAL SECTION THROUGH THE AXIAL LINE OF THE
PRIMITIVE STREAK, AND THE PART OF THE BLASTODERM
IN FRONT OF IT, OF THE BLASTODERM OF A CHICK SOME-
WHAT YOUNGER THAN FlG. 19.

pr.s. primitive streak ; ep. epiblast ; hy. hypoblast of region in
front of primitive streak ; n. nuclei ; yk. yolk of germinal
wall.

embryo, the mesoblast, the epiblast, and the hypoblast
all unite together.

From the 16th to the 20th hours. At about the
16th hour, in blastoderms of the stage represented in
Fig. 16, an important change takes place in the constitution
of the primitive hypoblast in front of the primitive streak.
The rounded cells, of which it is at first composed (Fig.
18), break up into (1) a layer formed of a single row of
more or less flattened elements below — the hypoblast
proper — and (2) into a layer formed of several rows of
stellate elements, between the hypoblast and the epiblast
— the mesoblast (Fig. 19 m). A separation between these
two layers is at first hardly apparent, and before it has
become at all well marked, especially in the median line,
an axial opaque line makes its appearance in surface
views, continued forwards from the front end of the
primitive streak, but stopping short at a semicircular

60

THE FIRST DAY.

FIG. 19.

[CHAP.

TRANSVERSE SECTION THROUGH THE EMBRYONIC KEGION OF
THE BLASTODERM OF A CHICK SHORTLY PRIOR TO THE
FORMATION OF THE MEDULLARY GROOVE AND NOTO-
CHORD.

m. median line of the section ; ep. epiblast ; 11. lower layer cells
(primitive hypoblast) not yet completely differentiated into
mesoblast and hypoblast ; n. nuclei.

fold — the future head-fold — near the front end of the
area pellucida. In section (Fig. 20) this opaque line is
seen to be due to a special concentration of cells in the
form of a cord. This cord is the commencement of an
extremely important structure found in all vertebrate
embryos, which is known as the notochord (ch). In most
instances the commencing notochord remains attached
to the hypoblast, after the mesoblast has at the sides
become quite detached (vide Fig. 20), but in other cases
the notochord appears to become differentiated in the
already separated layer of mesoblast. In all cases the
notochord and the hypoblast below it unite with the front
end of the primitive streak; with which also the two
lateral plates of mesoblast become continuous.

From what has just been said it is clear that in the
region of the embryo the mesoblast originates as two
lateral plates split off from the primitive hypoblast, and

III.]

THE NOTOCHORD.
FIG. 20.

61

TRANSVERSE SECTION THROUGH THE EMBRYONIC REGION OF THE
BLASTODERM OF A CHICK AT THE TIME OF THE FORMATION
OF THE NOTOCHORD, BUT BEFORE THE APPEARANCE OF
THE MEDULLARY GROOVE.

ep. epiblast ; hy. hypoblast ; ch. notochord ; me. mesoblast ;
ylc. yolk of germinal wall.

FIG. 21.

TRANSVERSE SECTION OF A BLASTODERM INCUBATED FOR
18 HOURS.

The section passes through the medullary groove we., at some
distance behind its front end.

A. Epiblast. B. Mesoblast. C. Hypoblast.

m.c. medullary groove ; m.f. medullary fold ; ch. notochord.

62 THE FIRST DAY. [CHAP.

that the notochord originates simultaneously with the
mesoblast,with which it is at first continuous, as a median
plate similarly of hypoblastic origin.

Kolliker 1 holds that the mesoblast of the region of the em-
bryo is derived from a forward growth from the primitive streak.
There is no theoretical objection to this view, and we think it would
be impossible to shew for certain by sections whether or no
there is a growth such as he describes ; but such sections as that
represented in Fig. 19 (and we have series of such sections from
several embryos) appear to us to be conclusive in favour of the
view that the mesoblast of the region of the embryo is to a large
extent derived from a differentiation of the primitive hypoblast.
The mesoblast of the primitive streak forms in part the vascular
structures found in the area pellucida, and probably also in part
the mesoblast of the allantois.

The differentiation of the embryo may be said to
commence with the formation of the notochord and the
lateral plates of mesoblast. Very shortly after the for-
mation of these parts, the axial part of the epiblast
above the notochord and in front of the primitive streak,
being here somewhat thicker than in the lateral parts,
becomes differentiated into a distinct medullary plate, the
sides of which form two folds known as the medullary
folds, enclosing between them a groove known as the
medullary groove. The medullary plate itself consti-
tutes that portion of the epiblast which gives rise to the
central nervous system.

Between the 18th to the 20th hour the medullary
groove, with its medullary folds or laminae dorsales, is
fully established. It then presents the appearance, to-
wards the hinder extremity of the embryo, of a shallow

1 Entwick. d. Menschen u. hoheren Thiere. Leipzig, 1879.

III.] THE NOTOCHORD. 63

groove with sloping diverging walls, which embrace be-
tween them the front end of the primitive streak.
Passing forwards towards what will become the head
of the embryo the groove becomes narrower and deeper
with steeper walls. On reaching the head-fold (Fig. 22),
which continually becomes more and more prominent,
the medullary folds curve round and meet each other in
the middle line, so as to form a somewhat rounded end
to the groove. In front therefore the canal does not
become lost by the gradual flattening and divergence of
its walls, as is the case behind, but has a definite termi-
nation, the limit being marked by the head-fold.

In front of the head-fold, quite out of the region of
the medullary folds, there is usually another small fold
formed earlier than the head-fold, which is the begin-
ning of the amnion (Fig. 22).

The appearance of the embryo and its relation to
the surrounding parts are somewhat diagrammatically
represented in Fig. 22. The primitive streak now ends
with an anterior swelling (not represented in the figure),
and is usually somewhat unsymmetrical. In most cases
its axis is more nearly continuous with the left, or
rarely the right, medullary fold than with the medullary
groove. In sections its front end appears as a ridge on
one side or rarely in the middle of the floor of the wide
medullary groove.

The general structure of the developing embryo at
the present stage is best understood from such a section
as that represented in Fig. 21. The medullary groove
(m. c.) lined by thickened epiblast is seen in the median
line of the section. Below it is placed the notochord (cA),
which at this stage is a mere rod of cells, and on each

THE FIRST DAY.

Fm. 22.

[CHAP.

^IT-

VIEW OF THE PELLUCID AREA OF A BLASTODERM OF
18 HOURS.

None of the opaque area is shewn, the pear-shaped outline
indicating the limits of the pellucid area.

At the hinder part of the area isseen the primitive groove
pr., with its nearly parallel walls, fading away behind, but curv-
ing round and meeting in front so as to form a distinct anterior
termination to the groove, about half way up the pellucid area.

Above the primitive groove is seen the medullary groove m.c.,
with the medullary folds A. These diverging behind, slope away
on either side of the primitive groove, while in front they curve
round and meet each other close upon a curved line which repre-
sents the head-fold.

The second curved line in front of and concentric with the
first is the commencing fold of the amnion.

III.] THE GERMINAL WALL. 65

side are situated the mesoblastic plates (B). The hypo-
blast forms a continuous and nearly flat layer below.

While the changes just described have been occur-
ring in the area pellucida, the growth of the area opaca
has also progressed actively. The epiblast has greatly
extended itself, and important changes have taken place
in the constitution of the germinal wall already spoken
of.

The mesoblast and hypoblast of the area opaca do
not arise by simple extension of the corresponding layers
of the area pellucida ; but the whole of the hypoblast
of the area opaca, and a large portion of the meso-
blast, and possibly even some of the epiblast, take their
origin from the peculiar material which forms the
germinal wall and which is continuous with the hypo-
blast at the edge of the area opaca (vide figs. 15, 17,
18, 19, 20).

The exact nature of this material has been the subject of
many controversies. Into these controversies it is not our purpose
to enter, but subjoined are the results of our own examination.
The germinal wall first consists, as already mentioned, of the
lower cells of the thickened edge of the blastoderm, and of the
subjacent yolk material with nuclei. During the period before
the formation of the primitive streak the epiblast appears to
extend itself over the yolk, partly at the expense of the cells of
the germinal wall, and possibly even of cells formed around the
nuclei in this part. The cells of the germinal wall, which are at
first well separated from the yolk below, become gradually ab-
sorbed in the growth of the hypoblast, and the remaining cells
and yolk then become mingled together, and constitute a com-
pound structure, continuous at its inner border with the hypo-
blast. This structure is the germinal wall usually so described.
It is mainly formed of yolk granules with numerous nuclei, and
a somewhat variable number of rather large cells imbedded
F. & B. 5

66 THE FIRST DAY. [CHAP.

amongst them. The nuclei, some of which are probably enclosed
by a definite cell body, typically form a special layer immedi-
ately below the epiblast. A special mass of nuclei (vid-e Tigs. 18
and 20, n) is usually present at the junction of the hypoblast
with the germinal wall.

The germinal wall retains the characters just enumerated till
near the close of the first day of incubation. One function of its
cells appears to be the absorption of yolk material for the growth
of the embryo.

The chief events then of the second period of the
first day are the appearance of the medullary folds
and groove, the formation of the notochord and lateral
plates of mesoblast, the beginning of the head-fold and
amnion, and the histological changes taking place in the
several layers.

Prom the 20th to the 24th hour. A view of
the embryo during this period is given in Fig. 23.
The head-fold enlarges rapidly, the crescentic groove
becoming deeper, while at the same time the over-
hanging margin of the groove (the upper limb of the
g), rises up above the level of the blastoderm ; in fact,
the formation of the head of the embryo may now be
said to have definitely begun.

The medullary folds, increasing in size in every
dimension, but especially in height, lean over from
either side towards the middle line, and thus tend
more and more to roof in the medullary canal, espe-
cially near the head. About the end of the first day
they come into direct contact in the region which
will afterwards become the brain, though they do not
as yet coalesce. In this way a tubular canal is formed.
This is the medullary or neural canal (Fig. 23, Fig. 24,

III.] THE MEDULLARY CANAL. 67

FIG. 23.

DORSAL VIEW OF THE HARDENED AREA PELLUCIDA OF A CHICK
WITH FIVE MESOBLASTIG SOMITES. THE MEDULLARY
FOLDS HAVE MET FOR PART OF THEIR EXTENT, BUT HAVE
NOT UNITED.

a.pr. anterior part of the primitive streak ; p.pr. posterior part
of the primitive streak.

Me.). It is not completely closed in till a period con-
siderably later than the one we are considering.

Meanwhile important changes are taking place in
the axial portions of the mesoblast, which lie on each
side of the notochord beneath the medullary folds.

In an embryo of the middle period of this day,
examined with transmitted light, the notochord is
seen at the bottom of the medullary groove between
the medullary folds, as a transparent line shining
through the floor of the groove when the embryo is
viewed from above. On either side of the notochord
the body of the embryo appears somewhat opaque,

5—2

68 THE FIKST DAY. [CHAP.

owing to the thickness of the medullary folds; as
these folds slope away outwards on either side, so
the opacity gradually fades away in the pellucid area.
There is present at the sides no sharp line of demarca-
tion between the body of the embryo and the rest of
the area; nor will there be any till the lateral folds
make their appearance ; and transverse vertical sections
shew (Fig. 21) that there is no break in the mesoblast,
from the notochord to the margin of the pellucid area,
but only a gradual thinning.

During the latter period of the day, however, the
plates of mesoblast on either side of the notochord
begin to be split horizontally into two layers, the one
of which attaching itself to the epiblast, forms with
it the somatopleure (shewn for a somewhat later stage
in Fig. 24), while the other, attaching itself to the
hypoblast, forms with it the splanchnopleure. By
the separation of these two layers from each other,
a cavity (Pp), containing fluid only, and more con-
spicuous in certain parts of the embryo than in others,
is developed. This cavity is the beginning of that
great serous cavity of the body which afterwards becomes
divided into separate cavities. We shall speak of it as
the pleuro-peritoneal cavity.

This cleavage into somatopleure and splanchno-
pleure extends close up to the walls of the medullary
canal, but close to the medullary canal a central or
axial portion of each plate becomes marked off by
a slight constriction from the peripheral (Fig. 24), and
receives the name of vertebral plate, the more external
mesoblast being called the lateral plate. The cavity
between the two layers of the lateral plate rapidly

III.]

VERTEBRAL PLATE.

69

enlarges, while that in the vertebral plate remains in
the condition of a mere split.

FIG. 24.

TRANSVERSE SECTION THROUGH THE DORSAL REGION OP AN
EMBRYO OF THE SECOND DAY (copied from His), intro-
duced here to illustrate the formation of the mesoblastic
somiti^, and the cleavage of the mesoblast.

M. medullary canal ; Pv. mesoblastic somite ; w. rudiment of
Wolffian duct; A. epiblast; C. hypoblast ; Ch. notochord ;
Ao. aorta ; BO. splanchnopleure.

At first each vertebral plate is not only unbroken
along its length, but also continuous at its outer edge
with the upper and lower layers of the lateral plate
of the same side. Very soon, however, clear trans-
verse lines are seen, in surface views (Fig. 23), stretch-
ing inwards across each vertebral plate from the edge
of the lateral plate towards the notochord ; while a
transparent longitudinal line makes its appearance on
either side of the notochord along the line of junction
of the lateral with the vertebral plate.

The transverse lines are caused by the formation
of vertical clefts, that is to say, narrow spaces containing
nothing but clear fluid ; and sections shew that they

70 THE FIRST DAY. [CHAP.

are due to breaches of continuity in the mesoblast
only, the epiblast and hypoblast having no share in the
matter.

Thus each vertebral plate appears in surface views
to be cut up into a series of square plots, bounded by
transparent lines (Fig. 23). Each square plot is the
surface of a corresponding cubical mass (Fig. 24, Pv.).
The two such cubical masses first formed, lying one on
each side of the notochord, beneath and a little to
the outside of the medullary folds, are the first pair
of mesoblastic somites1.

The mesoblastic somites form the basis out of which
the voluntary muscles of the trunk and the bodies of
the vertebrae are formed.

The first somite rises close to the anterior ex-
tremity of the primitive streak, but the next is stated
to arise in front of this, so that the first-formed so-
mite corresponds to the second permanent vertebra.
The region of the embryo in front of the second formed
somite — at first the largest part of the whole embryo — is
the cephalic region (Fig. 23). The somites following the
second are formed in regular succession from before
backwards, out of the unsegmented mesoblast of the
posterior end of the embryo, which rapidly grows in
length to supply the necessary material. With the
growth of the embryo the primitive streak is con-
tinually carried back, the lengthening of the embryo
always taking place between the front end of the
primitive streak and the last somite ; and during this

1 These bodies are frequently called protovertebra, but we shall
employ for them the term mesoblastic somites.

III.] THE NEURENTERIC PASSAGE. 71

process the primitive streak undergoes important
changes both in itself and in its relation to the embryo.
Its anterior thicker part, which is embraced by the
diverging medullary folds, soon becomes distinguished
in structure from the posterior part, and is placed
symmetrically in relation to the axis of the embryo,
(Fig. 23 a.pr)-, at the same time the medullary folds,
which at first simply diverge on each side of the
primitive streak, bend in again and meet behind so
as completely to enclose this front part of the primi-
tive streak. The region, where the medullary folds
diverge, is known as the sinus rhomboidalis of the
embryo bird, though it has no connection with the
similarly named structure in the adult.

This is a convenient place to notice remarkable appearances
which present themselves close to the junction of the neural plate
and the primitive streak. These are temporary passages leading
from the hinder end of the neural groove or tube into the alimen-
tary canal. They vary somewhat in different species of birds, and
it is possible that in some species there may be several openings
of the kind, which appear one after the other and then close again.
They were first discovered by Gasser, and are spoken of as the
neur "enteric passages or canals1. In all cases, with some doubtful
exceptions, they lead round the posterior end of the notochord, or
through the point where the notochord falls into the primitive
streak.

The largest of these passages is present in the embryo duck
with twenty-six mesoblastic somites, and is represented in the
series of sections (Fig. 25). The passage leads obliquely back-
wards and ventralwards from the hind end of the neural tube

1 "Die Primitivstreifen bei Vogelembryonen." Schrift. d. Gesell.
z. Beford d. Gesammten Naturwiss. zu Marburg. Vol. u. Supple-
ment i. 1879.

72

THE FIRST DAY.
FIG. 25.

[CHAP.

FOUR TRANSVERSE SECTIONS THROUGH THE NEURENTERIC
PASSAGE AND ADJOINING PARTS IN A DUCK EMBRYO
WITH TWENTY-SIX MESOBLASTC SOMITES.

A. Section in front of the neurenteric canal, shewing a lumen
in the notochord.

B. Section through the passage from the medullary canal
into the notochord.

C. Section shewing the hypoblastic opening of the neuren-
teric canal, and the groove on the surface of the primitive streak,
which opens in front into the medullary canal.

D. Primitive streak immediately behind the opening of the
neurenteric passage.

me. medullary canal ; ep. epi blast ; hy. hypoblast ; ch. noto-
chord ; pr. primitive streak.

in.]

THE NEUKENTERIC PASSAGE.

73

into the notochord, where the latter joins the primitive streak
(B). A narrow diverticulum from this passage is continued for-
wards for a short distance along the axis of the notochord (A,
cfi). After traversing the notochord, the passage is continued
into a hypoblastic diverticulum, which opens ventrally into the
future lumen of the alimentary tract (C). Shortly behind the
point where the neurenteric passage communicates with the
neural tube the latter structure opens dorsally, and a groove on
the surface of the primitive streak is continued backwards from
it for a short distance (C). The first part of this passage to
appear is the hypoblastic diverticulum above mentioned.

FIG. 20.

DIAGRAMMATIC LONGITUDINAL SECTION THROUGH THE POS-
TERIOR END OF AN EMBRYO BIRD AT THE TIME OF THE
FORMATION OF THE ALLANTOIS.

ep. epiblast ; Sp.c. spinal canal ; ch. notochord ; n.e. neurenteric
canal ; hy. hypoblast ; p.a.g. post-anal gut ; pr. remains of
primitive streak folded in on the ventral side ; al. allantois ;
me. mesoblast ; an. point where anus will be formed ; p.c.
perivisceral cavity ; am. amnion ; so. somatopleure ; sp.
splanchnopleure.

In the chick we have found in some cases an incomplete pas-
prior to the formation of the first somite. At a later stage

74 THE FIRST DAY. [CHAP.

there is a perforation on the floor of the neural canal, which is
not so marked as those in the goose or duck, and never results
in a complete continuity between the neural and alimentary
tracts ; but simply leads from the floor of the neural canal into
the tissues of the tail-swelling, and thence into a cavity in the
posterior part of the notochord. The hinder diverticulum of the
neural canal along the line of the primitive groove is, moreover,
very considerable in the chick, and is not so soon obliterated as
in the goose. The incomplete passage in the chick arises at a
period when about twelve somites are present. The third
passage is formed in the chick during the third day of incuba-
tion.

The anterior part of the primitive streak becomes con-
verted into the tail-swelling ; the groove of the posterior part
gradually shallows and finally disappears. The hinder part itself
atrophies from behind forwards, and in the course of the folding
off of the embryo from the yolk the part of the blastoderm where
it was placed becomes folded in, so as to form part of the ventral
wall of the embryo. The apparent hinder part of the primitive
streak is therefore in reality ventral and anterior in relation to
the embryo.

Since the commencement of incubation the area
opaca has been spreading outwards over the surface of the
yolk, and by the end of the first day has reached about
the diameter of a sixpence. It appears more or less
mottled over the greater part of its extent, but this is
more particularly the case with the portion lying next
to the pellucid area ; so much so, that around the pel-
lucid area an inner ring of the opaque area may be
distinguished from the rest by the difference of its
aspect.

The mottled appearance of this inner ring is due to
changes taking place in the mesoblast above the germi-
nal wall — changes which eventually result in the forma-

III.] SUMMARY. 75

tion of what is called the vascular area, the outer border
of which marks the extreme limit to which the meso-
blast extends.

The changes then which occur during the first day
may thus be briefly summarized :

(1) The hypoblast is formed as a continuous layer
of plate-like cells from the lower layer of the segmenta-
tion spheres.

(2) The primitive streak is formed in the hinder
part of the area pellucida as a linear proliferation of
epiblast cells. These cells spread out as a layer on
each side of the primitive streak, and form part of the
mesoblast.

(3) The primitive groove is formed along the axis
of the primitive streak.

(4) The pellucid area becomes pear-shaped, the
broad end corresponding with the future head of the
embryo. Its long axis lies at right angles to the long
axis of the egg.

(5) The medullary plate with the medullary groove
makes its appearance in front of the primitive groove.

(6) The primitive hypoblast in the region of the
medullary plate gives rise to an axial rod of cells forming
the notochord, and to two lateral plates of mesoblast.
The innermost stratum of the primitive layer forms the
permanent hypoblast.

(7) The development of the head-fold gives rise
to the first definite appearance of the head.

(8) The medullary folds rise up and meet first in
the region of the mid-brain to form the neural tube.

(9) By the cleavage of the mesoblast, the somato-
pleure separates from the splanchnopleure.

76 THE FIRST DAY. [CHAP. III.

(10) One or more pairs of mesoblastic somites make
their appearance in the vertebral portion of the meso-
blastic plates.

(11) The first trace of the amnion appears in front
of the head-fold.

(12) The vascular area begins to be distinguished
from the rest of the opaque area.

CHAPTER IV.

THE CHANGES WHICH TAKE PLACE DURING THE FIRST
HALF OF THE SECOND DAY.

General development. In attempting to remove
the blastoderm from an egg which has undergone
from 30 to 36 hours' incubation, the observer can-
not fail to notice a marked change in the consist-
ency of the blastodermic structures. The excessive
delicacy and softness of texture which rendered the
extraction of an 18 or 20 hours' blastoderm so difficult,
has given place to a considerable amount of firmness;
the outlines of the embryo and its appendages are much
bolder and more distinct; and the whole blastoderm
can be removed from the egg with much greater ease.

In the embryo itself viewed from above one of the
features which first attracts attention is the progress
in the head-fold (Fig. 27). The upper limb or- head
has become much more prominent, while the lower
groove is not only proportionately deeper, but is also
being carried back beneath the body of the embryo.

The medullary folds are closing rapidly. In the
region of the head they have quite coalesced, a slight
notch in the middle line at the extreme front marking

78

THE SECOND DAY.

[CHAP.

for some little time their line of junction (Fig. 23).
The open medullary groove of the first day has thus
become converted into a tube, the neural canal, closed
in front, but as yet open behind. Even before the

p.r

EMBRYO OF THE CHICK BETWEEN THIRTY AND THIRTY-SIX HOURS,

VIEWED FROM ABOVE AS AN OPAQUE OBJECT.

(Chromic acid preparation.)

f.b. front-brain : mb. mid-brain ; h. b. hind-brain ; op.v. optic vesi-
cle ; au.p. auditory pit ; o.f. vitelline vein ; p.v. mesoblastic
somite; m.f. line of junction of the medullary folds above the

IV.] THE BRAIN. 79

medullary canal ; s.r. sinus rhomboidalis ; t. tail-fold ; p.r.
remains of primitive groove (not satisfactorily represented) ;
a.p. area pellucida.

The line to the side between p.v. and m.f. represents the true
length of the embryo.

The fiddle-shaped outline indicates the margin of the pellucid
area. The head, which reaches as far back as o.f., is dis-
tinctly marked off; but neither the somatopleuric nor
splanchnopleuric folds are shewn in the figure ; the latter
diverge at the level of o./., the former considerably nearer
the front, somewhere between the lines m.b. and h. b. The
optic vesicles op.v. are seen bulging out beneath the superfi-
cial epiblast. The heart lying underneath the opaque body
cannot be seen. The tail-fold t. is just indicated ; no dis-
tinct lateral folds are as yet visible in the region midway
between head and tail. At m.f. the line of junction between
the medullary folds is K still visible, being lost forwards over
the cerebral vesicles, while behind may be seen the remains
of the sinus rhomboidalis, s.r.

medullary folds coalesce completely in the cephalic
region, the front end of the neural canal dilates into
a small bulb, whose cavity remains continuous with
the rest of the canal, and whose walls are similarly
formed of epiblast. This bulb is known as the first
cerebral vesicle, Fig. 27, /A, and makes its appearance
in the early hours of the second day. From its sides
two lateral processes almost at once grow out : they are
known as the optic vesicles (Fig. 27, op. v.), and their
history will be dealt with at length somewhat later.
Behind the first cerebral vesicle a second and a third
soon make their appearance ; they are successively
formed very shortly after the first vesicle ; but the
consideration of them may be conveniently reserved to-
a later period. At the level of the hind end of the

80

THE SECOND DAY.

[CHAP.

AN EMBRYO CHICK OF ABOUT THIRTY-SIX HOURS, VIEWED
FROM BELOW AS A TRANSPARENT OBJECT.

FB. the fore-brain or first cerebral vesicle, projecting from the
sides of which are seen the optic vesicles, op. A definite
head is now constituted, the backward limit of the somato-
pleure fold being indicated by the faint line S.O. Around
the head are seen the two limbs of the amniotic head-fold :
one, the true amnion a, closely enveloping the head, the
other, the false amnion a', at some distance from it. The
head is seen to project beyond the anterior limit of the
pellucid area.

The splanchnopleure folds extend as far back as sp. Along its
diverging limbs are seen the conspicuous venous roots of

IV.] THE MESOBLASTIC SOMITES. 81

the vitelline veins, uniting to form the heart h, already
established by the coalescence of two lateral halves which,
continuing forward as the bulbus arteriosus b. a., is lost in
the substance of the head just in front of the somatopleure
fold.

HE. hind-brain ; ME. mid-brain ; p.v. and v.pl. mesoblastic
somites ; ch. front end of notochord ; me. posterior part of
notochord ; e. parietal mesoblast ; pi. outline of area pellu-
cida ; pv. primitive streak.

head two shallow pits are visible. They constitute the
first rudiments of the organ of hearing, and are known as
the auditory pits (Fig. 27, au.p.*). fc <fa InAj^&vnJi

The number of mftsubhBtio pomi-tea increases rapidly
by a continued segmentation of the vertebral plates of
mesoblast. The four or five pairs formed during the
first day have by the middle of the second increased to
as many as fifteen. The addition takes place from
before backwards ; and the hindermost one is for some
time placed nearly on a level with the boundary be-
tween the hind end of the trunk of the embryo, and
the front end of the primitive /screak. For some time
the already formed goalies oo^ilSt increase in size,
so that at first the embryo clearly elongates by addi-
tions to its hinder end.

Immediately behind the level of the last uajeso*'
Elastic- s~6Tnhe there is placed an enlargement of the
unclosed portion of the medullary canal. This enlarge-
ment is the sinus rhomboid alis already spoken off It
is shewn in Fig. 23. On its floor is placed the' front
end of the primitive streak. It is a purely embryonic
structure which disappears during the second day.

In a former chapter it was pointed out (p. 27)
that the embryo is virtually formed by a folding

"B-

82 THE SECOND DAY. [CHAP.

or tucking in of a limited portion of the blastoderm,
first at the anterior extremity, and afterwards at the
posterior extremity and at the sides. One of the results
of this doubling up of the blastoderm to form the head
is the appearance, below the anterior extremity of the
medullary tube, of a short canal, ending blindly in
front, but open widely behind (Fig. 29, Z>), a cul de
sac, in fact, lined with hypoblast and reaching from the
extreme front of the embryo to the point where the
splanchnopleuric leaf of the head-fold (Fig. 29, F. Sp)
turns back on itself. This cul de sac, which of course be-
comes longer and longer the farther back the head-fold is
carried, is the rudiment of the front end of the alimen-
tary canal, the fore-gut, as it might be called. In trans-
verse section it appears to be flattened horizontally,
and also bent, so as to have its convex surface looking
downwards (Fig. 30, al). At first the anterior end is
quite blind, there being no mouth as yet; the formation
of this at a subsequent date will be described later on.

At the end of the first half of the second day the
head-fold has not proceeded very far backwards, and
its limits can easily be seen in the fresh embryo both
from above and from below (Fig. 28).

The heart. It is in the head-fold that the forma-
tion of the heart takes place, its mode of origin being
connected with that cleavage of the mesoblast and con-
sequent formation of splanchnopleure and somatopleure
of which we have already spoken.

At the extreme end of the embryo (Fig. 29), where
the blastoderm begins to be folded back, the mesobtast
is never cleft, and here consequently there is neither
somatopleure nor splanchnopleure ; but at a point a

IV.]

THE HEART.
FIG. 29.

83

N.C.

F.So.

DIAGRAMMATIC LONGITUDINAL SECTION THROUGH THE Axis OF
AN EMBRYO.

The section is supposed to be made at a time when the head-
fold has commenced but the tail-fold has not yet appeared.
N.C. neural canal, closed in front but as yet open behind. Ch.
notochord. The section being taken in the middle line,
the protovertebrse are of course not shewn. In front of
the notochord is seen a mass of uncleft mesoblast, which
will eventually form part of the skull. D. the commencing
foregut or front part of the alimentary canal. F. So.
Somatopleure, raised up in its peripheral portion into the
amniotic fold Am. Sp. Splanchnopleure. At Sp. it forms
the under wall of the foregut ; at F. Sp. it is turning round
and about to run forward. Just at its turning point the
cavity of the heart Ht. is being developed in its mesoblast.
pp. pleuroperitoneal cavity. A epiblast, B mesoblast, C
hypoblast, indicated in the rest of the figure by differences in
the shading. At the part where these three lines of reference
end the mesoblast is as yet uncleft.

very little further back, close under the blind end of
the foregut, the cleavage (at the stage of which we are
speaking) begins, and the somatopleure, F. So, and
Splanchnopleure, F. Sp. diverge from each other. They

6—2

84 THE SECOND DAY. [CHAP.

thus enclose between them a cavity, pp, which rapidly
increases behind by reason of the fact that the fold of
the splanchnopleure is carried on towards the hinder
extremity of the embryo considerably in advance of
that of the somatopleure. Both folds, after running a
certain distance towards the hind end of the embryo,
are turned round again, and then course once more for-
wards over the yolk-sac. As they thus return (the
somatopleure having meanwhile given off the fold of
the amnion, Am.), they are united again to form the
uncleft blastodermic investment of the yolk-sac. In
this way the cavity arising from their separation is
closed below.

It is in this cavity, which from its mode of forma-
tion the reader will recognise as a part (and indeed at
this epoch it constitutes the greater part) of the general
pleuroperitoneal cavity, that the heart is formed.

This makes its appearance at the under surface and
hind end of the foregut, just where the splanchnopleure
folds turn round to pursue a forward course (Fig. 29,
Ht.) ; and by the end of the first half of the second day
(Fig. 28, h) has acquired somewhat the form of a flask
with a slight bend to the right. At its anterior end a
slight swelling marks the future bulbus arteriosus ; and
a bulging behind indicates the position of the auricles.
It is hollow, and its cavity opens behind into two
vessels called the vitelline veins (Figs. 27, o.f. and 28 sp.),
which pass outwards in the folds of the splanchno-
pleure at nearly right angles to the axis of the embryo.
The anterior extremity of the heart is connected with
the two aortae.

The heart, including both its muscular wall and its

IV.] THE HEART. 85

epitheloid lining, is developed out of the splanchnic
mesoblast on the ventral side of the throat. But
since the first commencements of the heart make
their appearance prior to the formation of the throat,
the development of this organ is somewhat complicated;
and in order to gain a clear conception of the manner
in which it takes place the topography of the region
where it is formed needs to be very distinctly under-
stood.

In the region where the heart is about to appear,
the splanchnopleure is continually being folded in on
either side, and these lateral folds are progressively
meeting and uniting in the middle line to form the under
or ventral wall of the foregut. At any given moment
these folds will be found to have completely united in
the middle line along a certain distance measured from
the point in front where the cleavage of the mesoblast
(i.e. the separation into somatopleure and splanch-
nopleure) begins, to a particular point farther back.
They will here be found to be diverging from the point
where they were united, and not only diverging late-
rally each from the middle line, but also both turning
so as to run in a forward direction to regain the surface
of the yolk and rejoin the somatopleure, Fig. 29. In a
transverse section taken behind this extreme point of
union, or point of divergence, as we may call it, the
splanchnopleure on either side when traced downwards
from the axis of the embryo may be seen to bend in
towards the middle so as to approach its fellow, and then
to run rapidly outwards, Fig. 31, B. A longitudinal
section shews that it runs forwards also at the same
time, Fig. 29. A section through the very point of

86 THE SECOND DAY. [CHAP.

divergence shews the two folds meeting in the middle
line and then separating again, so as to form something
like the letter x, with the upper limbs converging, and
the lower limbs diverging. In a section taken in
front of the point of divergence, the lower diverging
limbs of the x have disappeared altogether; nothing
is left but the upper limbs, which, completely united
in the middle line, form the under-wall of the fore-
gut.

As development proceeds, what we have called the
point of divergence is continually being carried farther
and farther back, so that the distance between it and
the point where the somatopleure and splanchnopleure
separate from each other in front, i. e. the length of the
foregut, is continually increasing.

In the chick, as we have already stated, the heart
commences to be formed in a region where the folds of
the splanchnopleure have not yet united to form the
ventral wall of the throat, and appears in the form of
two thickenings of the mesoblast of the splanchno-
pleure, along the diverging folds, i.e. along the lower
limbs of the x, just behind the point of divergence.
These thickenings are continued into each other by a
similar thickening of the mesoblast extending through
the point of divergence itself.

The heart has thus at first the form of an inverted
V, and consists of two independent cords of splanchnic
mesoblast which meet in front, without however uniting.
As the foldirig-in of the splanchnopleure is continued
backwards the two diverging halves of the heart are
gradually brought together. Thus very soon the develop-
ing heart has the form of an inverted Y, consisting of an

IV.]

THE HEART.

87

unpaired portion in front and two diverging limbs be-
hind. The unpaired portion is the true heart, while the
diverging limbs are the vitelline veins already spoken
of (Fig. 28, sp). While the changes just spoken of
have been taking place in the external form of the
heart, its internal parts have also become differentiated.
A cavity is formed in each of the halves of the
heart before even they have coalesced. Each of these
cavities has at first the form of an irregular space

FIG. 30.

TRANSVERSE SECTION THROUGH THE POSTERIOR PART OF THE
HEAD OF AN EMBRYO CHICK OF THIRTY HOURS.

hb. hind-brain ; vg. vagus nerve ; ep. epiblast ; ch. notochord ;
x. thickening of hypoblast (possibly a rudiment of the sub-
notochordal rod) ; al. throat ; lit. heart ; pp. body cavity ;
so. somatic mesoblast ; sf. splanchnic mesoblast ; hy. hypo-
blast.

88 THE SECOND DAY. [CHAP.

between the splanchnic mesoblast and the wall of the
throat (Fig. 30, fit.). During their formation (Fig. 30),
a thin layer of mesoblast remains in contact with the
hypoblast, but connected with the main mass of the
mesoblast of the heart by protoplasmic processes. A
second layer next becomes split from the main mass of
mesoblast, being still connected with the first layer by the
above-mentioned protoplasmic processes. These two
layers unite to form a tube which constitutes the epithe-
lioid lining of the heart ; the lumen of this tube is the
cavity of the heart, and soon loses the protoplasmic
trabeculse which at first traverse it. The cavity of the
heart may thus be described as being formed by a
hollowing out of the splanchnic mesoblast. Some of the
central cells of the original thickenings probably become
blood-corpuscles.

The thick outer part of the cords of splanchnic meso-
blast which form the heart become the muscular walls
and peritoneal covering of this organ. The muscular
wall of each division of the heart has at first the form
of a half tube widely open on its dorsal aspect, that
is towards the hypoblast of the gut (Fig. 30 and 32).
After the two halves of the heart have coalesced in the
manner already explained, the muscular walls grow in
towards the middle line on the dorsal side until they
meet each other and coalesce, thus forming a complete
tube as shewn diagrammatically in Fig. 31, A. They
remain, however, at first continuous with the splanchnic
mesoblast surrounding the throat, thus forming a pro-
visional mesentery — the mesocardium — attaching the
heart to the ventral wall of the throat. The epithelioid
tubes formed in the two halves of the heart remain for

IV.] THE VASCULAR SYSTEM. 89

some time separate, and cause the cavity of the heart to
be divided into two tubes even after its two halves have
to all appearance completely coalesced1.

Soon after its formation the heart begins to beat ;
its at first slow and rare pulsations beginning at the
venous and passing on to the arterial end. It is of some
interest to note that its functional activity commences
long before the cells of which it is composed shew any
distinct differentiation into muscular or nervous ele-
ments.

Vascular system. To provide channels for the
fluid thus pressed by the contractions of the heart, a
system of tubes has made its appearance in the meso-
blast both of the embryo itself and of the vascular and
pellucid areas. In front the single tube of the bulbus
arteriosus bifurcates into two primitive aortce, each
of which bending round the front end of the foregut,
passes from its under to its upper side, the two forming
together a sort of incomplete arterial collar imbedded
in the mesoblast of the gut. Arrived at the upper side
of the gut, they turn sharply round, and run separate
but parallel to each other backwards towards the tail, in
the mesoblast on each side of the notochord immediately
under the mesoblastic somites (Figs. 32, Ao, 34, ao).
About half way to the hinder extremity each gives off
at right angles to the axis of the embryo a large branch,
the vitelline artery (Fig. 36, Of, A.}, which, passing
outwards, is distributed over the pellucid and vascular
areas, the main trunk of each aorta passing on with
greatly diminished calibre towards the tail, in which it
becomes lost.

1 This is not shewn in the diagram, Fig. 31, A.

90

THE SECOND DAY.

[CHAP.

B

lib.

E

Two DIAGRAMMATIC SECTIONS OF A THIRTY-SIX HOURS' EMBRYO

ILLUSTRATING THE STRUCTURE OF THE HEART SHORTLY
AFTER ITS FORMATION. A IS THE ANTERIOR SECTION.

hb. hind brain ; nc. notochord ; E. epiblast ; so. somatopleure ;
sp. splanchnopleure ; d. alimentary canal ; hy. hypoblast ;
hz. (in A) heart ; of. vitelline vein.

In A the two halves of the heart have coalesced to form an
unpaired tube suspended from the ventral wall of the throat.

IV.] THE VASCULAR SYSTEM. 91

In B are seen in the diverging folds of the splanchnopleure
the two vitelline veins (of) which will shortly unite to form
the ductus venosus.

"IT

TRANSVERSE SECTION OF AN EMBRYO AT THE END OF THE
SECOND DAY PASSING THROUGH THE REGION OF THE BULBUS
ARTERIOSUS. (Copied from His.)

M. medullary canal in the region of the hind brain ; V. anterior
cardinal vein ; Ao. Aorta ; Ch. Notochord ; al. alimentary
canal ; H. Heart (bulbus arteriosus) ; Pp. Pleuroperitoneal
cavity; am. amnion.

In the vascular and pellucid areas, the formation of
vascular channels with a subsequent differentiation
into arteries, capillaries and veins, is proceeding rapidly.
Blood-corpuscles too are being formed in considerable
numbers. The mottled yellow vascular area becomes
covered with red patches consisting of aggregations of
blood-corpuscles, often spoken of as blood-islands.

Round the extreme margin of the vascular area and
nearly completely encircling it, is seen a thin red line,
the sinus or vena terminalis (Fig. 36, Sv.). This will soon
increase in size and importance.

From the vascular and pellucid area several large
channels are seen to unite and form two large trunks,

92 THE SECOND DAY. [CHAP.

one on either side, which running along the splanch-
nopleure folds at nearly right angles to the axis of the
embryo, unite at the " point of divergence " to join the
venous end of the heart. These are the vitelline veins
spoken of above.

Both vessels and corpuscles are formed entirely
from the cells of the mesoblast; arid in the regions
where the mesoblast is cleft, are at first observed ex-
clusively in the splanchnopleure. Ultimately of course
they are found in the mesoblast everywhere.

In the pellucid area, where the formation of the blood-vessels
may be most easily observed, a number of mesoblastic cells are
seen to send out processes (Fig. 33). These processes unite, and
by their union a protoplasmic network is formed containing
nuclei at the points from which the processes started. The
nuclei, which as a rule are much elongated and contain large oval
nucleoli, increase very rapidly by division, and thus form groups
of nuclei at the, so to speak, nodal points of the network.
Several nuclei may also be seen here and there in the processes
themselves. The network being completed, these groups, by
continued division of the nuclei, increase rapidly in size ; the
protoplasm around them acquires a red colour, and the whole
mass breaks up into blood-corpuscles (Fig. 33, b.c.) The proto-
plasm on the outside of each group, as well as that of the uniting
processes, remains granular, and together with the nuclei in it
forms the walls of the blood-vessels. A plasma is secreted by
the walls, and in this the blood-corpuscles float freely.

Each nodal point is thus transformed into a more or less
rounded mass of blood-corpuscles floating in plasma but en-
veloped by a layer of nucleated protoplasm, the several groups
being united by strands of nucleated protoplasm. These uniting
strands rapidly increase in thickness; new processes are also
continually being formed ; and thus the network is kept close
and thickset while the area is increasing in size.

By changes similar to those which took place in the nodal

IV.]

THE VASCULAR SYSTEM.

93

points, blood-corpuscles make their appearance in the pro-
cesses also, the central portions of which become at the same
time liquefied.

By the continued widening of the connecting processes and
solution of their central portions, accompanied by a corresponding
increase in the enveloping nucleated cells, the original proto-

FIG. 33.

SURFACE VIEW FROM BELOW OF A SMALL PORTION OF THE
POSTERIOR END OF THE PELLUCID AREA OF A THIRTY-SIX
HOURS' CHICK. To illustrate the formation of the blood-
capillaries and blood-corpuscles, magnified 400 diameters.

b.c. Blood-corpuscles at a nodal point, already beginning to
acquire a red colour. They are enclosed in a layer of proto-
plasm, in the outermost part of which are found nuclei, a.
These nuclei subsequently become the nuclei of the cells
forming the walls of the vessels. The nodal groups are
united by protoplasmic processes (p*pr\ also containing
nuclei with large nucleoli (ri).

94 THE SECOND DAY. [CHAP.

plasmic network is converted into a system of communicating
tubes, the canals of which contain blood-corpuscles and plasma,
and the walls of which are formed of flattened nucleated cells.

The blood-corpuscles pass freely from the nodal points into
the hollow processes, and thus the network of protoplasm be-
comes a network of blood-vessels, the nuclei of the corpuscles and
of the walls of which have been, by separate paths of development,
derived from the nuclei of the original protoplasm.

The formation of the corpuscles does not proceed equally
rapidly or to the same extent in all parts of the blastoderm. By
far the greater part are formed in the vascular area, but some
arise in the pellucid area, especially in the hinder part. In the
front of the pellucid area the processes are longer and the network
accordingly more open ; the corpuscles also are both later in
appearing and less numerous when formed.

Assuming the truth of the above account, it is evident that
the blood-vessels of the yolk-sack of the chick do not arise as
spaces or channels between adjacent cells of the mesoblast, but
are hollowed out in the communicating protoplasmic substance
of the cells themselves. The larger vessels of the trunk are
however probably formed as spaces between the cells, much as is
the case with the heart.

Wolffian duct. About this period there may be
seen in transverse sections, taken through the embryo
in the region of the seventh to the eleventh somite a
small group of cells (Fig. 34, W. d) projecting on either
side from the mass of uncleft mesoblast on the outside
of the mesoblastic somites, into the somewhat triangular
space bounded by the epiblast above, the upper and
outer angle of the mesoblastic somite on the inside,
and the somatic mesoblast on the outside.

This group of cells is the section of a longitudinal
ridge, the rudiment of the Wolffian duct or primitive
duct of the excretory system ; while the mass of cells

IV.] SUMMARY. 95

from which it springs is known as the intermediate
cell mass. We shall return to them immediately.

Summary. The most important changes then which
take place during the first half of the second day are,
the closure of the medullary folds, especially in the
anterior part, and the dilatation of the canal so formed
into the first cerebral vesicle ; the establishment of a
certain number of mesoblastic somites ; the elevation of
the head from the plane of the blastoderm ; the forma-
tion of the tubular heart and of the great blood-vessels ;
and the appearance of the rudiment of the Wolffian
duct.

It is important to remember that the embryo of which
we are now speaking is simply a part of the whole
germinal membrane, which is gradually spreading over
the surface of the yolk. It is important also to bear in
mind that all that part of the embryo which is in front
of the foremost somite corresponds to the future head,
and the rest to the neck, body and tail. During this
period the head occupies about a third of the whole
length of the embryo.

CHAPTER V.

THE CHANGES WHICH TAKE PLACE DURING THE
SECOND HALF OF THE SECOND DAY.

ONE important feature of this stage is the rapid
increase in the process of the folding-off of the embryo
from the plane of the germ, and its consequent con-
version into a distinct tubular cavity. At the begin-
ning of the second day, the head alone projected from
the rest of the germ, the remainder of the embryo
being simply a part of a flat blastoderm, nearly com-
pletely level from the front mesoblastic somite to the hind
edge of the pellucid area. At this epoch, however, a
tail-fold makes its appearance, elevating the tail above
the level of the blastoderm in the same way that the
head was elevated. Lateral folds also, one on either
side, soon begin to be very obvious. By the progress
of these, together with the rapid backward extension
of the head-fold and the slower forward extension of
the tail- fold, the body of the embryo becomes more and
more distinctly raised up and marked off from the rest
of the blastoderm.

The medullary canal closes up rapidly. The wide
sinus rhomboidalis becomes a narrow fusiform space,

CHAP. V.] THE BRAIN. 97

and at the end of this period is entirely roofed over.
The conversion of the original medullary groove into
a closed tube is thus completed.

The brain. In the region of the head most im-
portant changes now take place. We saw that at the
beginning of this day the front end of the medullary
canal was dilated into a bulb, the first cerebral vesicle,
which by budding off two lateral vesicles became con-
verted into three vesicles : a median one connected
by short hollow stalks with a lateral one on either side.
The lateral vesicles known as the optic vesicles (Fig.
27, op. v, Fig. 35, a), become converted into parts of the
eyes ; the median one still retains the name of the first
cerebral vesicle.

The original vesicle being primarily an involution
of the epiblast, the walls of all three vesicles are formed
of epiblast ; all three vesicles are in addition covered
over with the common epiblastic investment which will
eventually become the epidermis of the skin of the
head. Between this superficial epiblast and the invo-
luted epiblast of the vesicles, there exists a certain
quantity of mesoblast to serve as the material out of
which will be formed the dermis of the scalp, the skull,
and other parts of the head. At this epoch, however,
the mesoblast is found chiefly underneath the several
vesicles (Fig. 30). A small quantity may in section be
seen at the sides ; but at the top the epidermic epiblast
is either in close contact with the involuted epiblast of
the cerebral and optic vesicles or separated from it by
fluid alone, there being as yet in this region between
the two no cellular elements representing the mesoblast.

The constrictions marking off the optic vesicles also
F. & B. 7

98

THE SECOND DAY.
FIG. 34.

[CHAP.

TRANSVERSE SECTION THROUGH THE DORSAL REGION OF AN
EMBRYO OF 45 HOURS.

V.] THE BRAIJS. 99

A. epiblast. B. mesoblast. C. hypoblast consisting of a single
row of flattened cells. M. c. medullary canal. P. v. meso-
blastic somite. W. d. Wolffian duct. S. o. Somatopleure.
S.p. Splanchnopleure. p.p. pleuroperitoneal cavity, c. h.
notochord. a. o. dorsal aorta, v. blood-vessels of the yolk-
sac, o.p. line of junction between opaque and pellucid
areas ; w. palisade-like yolk spheres which constitute the ger-
minal wall.

Only one-half of the section is represented in the figure — if
completed it would be bilaterally symmetrical about the line of
the medullary canal.

take place of course beneath the common epiblastic
investment, which is not involved in them. As a con-
sequence, though easily seen in the transparent fresh

HEAD OF A CHICK AT THE END OF THE SECOND DAY VIEWED

FROM BELOW AS A TRANSPARENT OBJECT.

(Copied from Huxley).

7. first cerebral vesicle, a. optic vesicle, d. infundibulum.

The specimen shews the formation of the optic vesicles (a),
as outgrowths from the 1st cerebral vesicle or vesicle of the 3rd
ventricle, so that the optic vesicles and vesicle of the 3rd ven-
tricle at first freely communicated with each other, and also the
growth of the lower wall of the vesicle of the 3rd ventricle into a
process which becomes the infundibulum (d).

7—2

100 THE SECOND DAY. [CHAP,

embryo (Fig. 28), they are but slightly indicated in
hardened specimens (Fig. 27).

When an embryo of the early part of the second
day is examined as a transparent object, that portion of
the medullary canal which lies immediately behind the
first cerebral vesicle is seen to be conical in shape, with
its walls thrown into a number of wrinkles. These
wrinkles may vary a good deal in appearance, and shift
from time to time, but eventually, before the close of
the second day, after the formation of the optical
vesicles, settle down into two constrictions, one separat-
ing the first cerebral vesicle from that part of the
medullary canal which is immediately behind it, and
the other separating this second portion from a third.
So that instead of there being one cerebral vesicle onlyr
as at the commencement of the second day, there is now,
in addition to the optic vesicles, a series of three, one
behind the other : a second and third cerebral vesicle
have been added to the first (Fig. 27, mb, hb). They
may be also called the " fore brain," the " mid brain,""
and the "hind brain," for into these parts will they
eventually be developed.

The optic vesicles, lying underneath the epiblast,,
towards the end of the day are turned back and pressed
somewhat backwards and downwards against the sides
of the first cerebral vesicle or fore brain, an elongation
of their stalks permitting this movement to take place.
The whole head becomes in consequence somewhat
thicker and rounder.

Before the end of the day the fore brain elongates
anteriorly. The part so established is not at first sepa-
rate from that behind, but it is . nevertheless the first

V.] THE CRANIAL FLEXURE. 101

unpaired commencement of two vesicles which develop
into the cerebral hemispheres ; but up to the end of the
day it is still very small and inconspicuous.

Early on the second day the commencements of
several of the cranial nerves make their appearance
as outgrowths of the (Fig. 30, vg) roof of the mid and
hind brains, but their development, together with that
of the spinal nerves, will be dealt with in the next
chapter.

The notochord. The notochord, whose origin
was described in the account of the first day, is during
the whole of the second day a very conspicuous object.
It is seen as a transparent rod, somewhat elliptical in
section (Fig. 34, ch), lying immediately underneath
the medullary canal for the greater part of its length,
and reaching forward in front as far as below the
hind border of the first cerebral vesicle.

Cranial flexure. Round the anterior termination
of the notochord, the medullary canal, which up to the
present time has remained perfectly straight, towards
the end of the day begins to curve. The front portion
of the canal, i.e. the fore-brain with its optic and cere-
bral vesicles, becomes slightly bent downwards, so as to
form a rounded obtuse angle with the rest of the
embryo. This is the commencement of the so-called
cranial flexure and is, mechanically speaking, a con-
sequence of the more rapid growth of the dorsal wall of
the anterior part of the brain as compared with that of
the ventral.

Auditory vesicle. Lastly, as far as the head is
concerned, the epiblastic plates forming the rudiments of
the auditory vesicles become converted into deep pits

102 THE SECOND DAY. [CHAP.

opening one on each side of the hind-brain (Fig. 27,
au. p).

Heart. We left the heart as a fusiform body
slightly bent to the right, attached to the under wall
of the foregut by the mesocardium. The curvature
now increases so much that the heart becomes almost
GQ -shaped, the venous portion being drawn up towards
the head so as to lie somewhat above (dorsal to) and
behind the arterial portion. (It would perhaps be more
correct to say that the free intermediate portion is by
its own growth bent downwards, backwards, and some-
what to the right, while the venous root of the heart is-
at the same time continually being lengthened by the
carrying back of that "point of divergence" of the
splanchnopleure folds which marks the union of the
vitelline veins into a single venous trunk.) The heart
then has at this time two bends, the one, the venous
bend, the right-hand curve of the uz ; the other, the
arterial bend, the left-hand curve of the cc. The
venous bend which, as we have said, is placed above
and somewhat behind the arterial bend, becomes marked
by two bulgings, one on either side. These are the
rudiments of the auricles, or rather of the auricular
appendages. The ascending limb of the arterial bend
soon becomes conspicuous as the bulbus arteriosus>
while the rounded point of the bend itself will here-
after grow into the ventricles.

Vascular system. The blood-vessels, whose origin
during the first half of this day has been already
described, become during the latter part of the day so
connected as to form a complete system, through which
a definite circulation of the blood is now for the first

V.'J THE VASCULAR SYSTEM. 103

time (consequently some little while after the com-
mencement of the heart's pulsation) carried on.

The two primitive aortce have already been de-
scribed as encircling the foregut, and then passing
along the body of the embryo immediately beneath
the mesoblastic somites on each side of the notochord.
They are shewn in Figs. 32 A.o. and 34 a.o in section as
two large rounded spaces lined with flattened cells. At
first they run as two distinct canals along the whole
length of the embryo ; but, after a short time, unite at
some little distance behind the head into a single trunk,
which lies in the middle line of the body immediately
below the notochord (Fig. 57). Lower down, nearer the
tail, this single primitive trunk again divides into two
aorte, which, getting smaller and smaller, are finally
lost in the small blood-vessels of the tail. At this
epoch, therefore, there are two aortic arches springing
from the bulbus arteriosus, and uniting above the ali-
mentary canal in the back of the embryo to form the
single dorsal aorta, which travelling backwards in the
median line divides near the tail into two main
branches. From each of the two primitive aortae, or
from each of the two branches into which the single
aorta divides, there is given off on either side a large
branch. These have been already spoken of as the
vitelline arteries. At this stage they are so large that
by far the greater part of the blood passing down the
aorta finds its way into them, and a small remnant only
pursues a straight course into the continuations of the
aorta towards the tail.

Each vitelline artery leaving the aorta at nearly
right angles (at a point some little way behind the

104 THE SECOND DAY. [CHAP.

backward limit of the splanchnopleure fold which is
forming the alimentary canal), runs outwards beneath
the mesoblastic somites in the lower range of the meso-
blast, close to the hypoblast. Consequently, when in its
course outwards it reaches the point where the meso-
blast is cleft to form the somatopleure and splanchno-
pleure, it attaches itself to the latter. Travelling along
this, and dividing rapidly into branches, it reaches the
vascular area in whose network of small vessels (and
also to a certain extent in the similar small vessels of
the pellucid area) it finally loses itself.

The terminations of the vitelline arteries in the
vascular and pellucid areas are further connected with
the heart in two different ways. From the network of
capillaries, as we may call them, a number of veins take
their origin, and finally unite into two main trunks, the
vitelline veins. These have already been described as
running along the folds of the splanchnopleure to form
the venous roots of the heart. Their course is conse-
quently more or less parallel to that of the vitelline
arteries, but at some little distance nearer the head,
inasmuch as the arteries run in that part of the splanch-
nopleure which has not yet been folded in to form the ali-
mentary canal. Besides forming the direct roots of the
vitelline veins, the terminations of the vitelline arteries
in the vascular area are also connected with the sinus
terminates spoken of above as running almost completely
round, and forming the outer margin of the vascular
area. This (Fig. 36, ST.), may be best described as
composed of two semicircular canals, which nearly meet
at points opposite the head and opposite the tail, thus all
but encircling the vascular area between them. At the

V.] THE VASCULAR SYSTEM. 105

point opposite the head the end of each semicircle is
connected with vessels (Fig. 36), which run straight in
towards the heart along the fold of the splanchnopleure,
and join the right and left vitelline veins. At the
point opposite the tail there is at this stage no such
definite connection. At the two sides, midway between
their head and tail ends, the two semicircles are espe-
cially connected with the vitelline arteries.

The circulation of the blood then during the latter
half of the second day may be described as follows. The
blood brought by the vitelline veins falls into the
twisted cavity of the heart, and is driven thence through
the bulbus arteriosus and aortic arches into the aorta.
From the aorta, by far the greater part of the blood
flows into the vitelline arteries, only a small remnant
passing on into the caudal terminations. From the
capillary net-work of the vascular and pellucid areas
into which the vitelline arteries discharge their
contents, part of the blood is gathered up at once
into the lateral or direct trunks of the vitelline
veins. Part however goes into the middle region
of each lateral half of the sinus terminalis, and there
divides on each side into two streams. One stream,
and that the larger one, flows in a forward direction
until it reaches the point opposite the head, thence it
returns by the veins spoken of above, straight to the
vitelline trunks. The other stream flows backward,
and becomes lost at the point opposite to the tail.
This is the condition of things during the second day;
it becomes considerably changed on the succeeding day.

At the time that the heart first begins to beat the
capillary system of the vascular and pellucid areas is

106 THE SECOND DAY. [CHAP,

not yet completed ; and the fluid which is at first driven
by the heart contains, according to most observers, very
few corpuscles.

At the close of the second day the single pair of
aortic arches into which the bulbus arteriosus divides
is found to be accompanied by a second pair, formed
in the same way as the first, and occupying a position a
little behind it. Sometimes even a third pair is added.
Of these aortic arches we shall have to speak more fully
later on.

Wolffian duct. During the latter half of the second
day the Wolfrlan duct to which we have already alluded
becomes fully established, while the first traces of the
embryonic excretory organs or kidneys, known as the
Wolman bodies, make their appearance. The develop-
ment of the latter will be dealt with in the history of
the third day, but the history of the duct itself may
conveniently be completed here.

The first trace of it is visible in an embryo Chick
with eight somites, as a ridge projecting from the inter-
mediate cell mass towards the epiblast in the region of
the seventh somite. In the course of further develop-
ment it continues to constitute such a ridge as far as
the eleventh somite (Fig. 34 TFd), but from this point it
grows backwards by the division of its cells, as a free
column in the space between the epiblast and rnesoblast.
In an embryo with fourteen somites of about the
stage represented in fig. 28 a small lumen has appeared
in its middle part, and in front it is connected with
rudimentary Wolffian tubules, which develop in con-
tinuity with it. In the succeeding stages the lumen of
the duct gradually extends backwards and forwards.

V.] THE AMNION. 107

and the duct itself also passes inwards relatively to the
epiblast (fig. 43 wd). Its hind end elongates till it
comes into connection with, and opens on the fourth
day into the cloacal section of the hind-gut.

The amnion and allantois. The amnion, especially
the anterior or head fold, advances in growth very
rapidly during the second day, and at the close of the
day completely covers the head and neck of the embryo ;
so much so that it is necessary to tear or remove it when
the head has to be examined in hardened opaque speci-
mens. The tail and lateral folds of the amnion, though
still progressing, lag considerably behind the head-fold.

The side -folds eventually meet in the median dorsal
line, and their coalescence proceeds backwards from the
head-fold in a linear direction, till there is only a small
opening left over the tail of the embryo. This finally
becomes closed early on the third day.

In Figs. 32 and 43 am. the folds of the amnion are
shewn before they have coalesced. After the coalescence
of the folds of the amnion above the embryo the two
limbs of which each is formed become, as already ex-
plained in chapter IL, separate from each other: the
inner, forming a special investment of the embryo, and
constituting the amnion proper (Fig. 65), the outer at-
taching itself to the vitelline membrane and becoming
the serous envelope.

The development of the allantois commences during
the second day, but since it is mainly completed during
the third day we need not dwell upon it further in this
place.

Summary. The chief events, then, which occur
during the second half of the second day are as follow: —

108 THE SECOND DAY. [CHAP. V.

1. The second and third cerebral vesicles make
their appearance behind the first.

2. The optic vesicles spring as hollow buds from
the lateral, and the unpaired commencement of the cere-
bral hemispheres from the front, portions of the first
cerebral vesicle.

3. The auditory plate becomes converted into a
pit, opening at the side of the hind-brain or third cere-
bral vesicle.

4. The first indications of the cranial flexure be-
come visible.

5. The head-fold, and especially the splanchno-
pleure moiety, advances rapidly backwards ; the head of
the embryo is in consequence more definitely formed.
The tail-fold also becomes distinct.

6. The curvature of the heart increases; the first
rudiments of the auricles appear.

7. The circulation of the yolk-sac is established.

8. The amnion grows rapidly, and the allantois
commences to be formed.

CHAPTER VI.

THE CHANGES WHICH TAKE PLACE DURING THE THIRD

DAY.

OF all days in the history of the chick within
the egg this perhaps is the most eventful; the rudi-
ments of so many important organs now first make their
appearance.

In many instances we shall trace the history of these
organs beyond the third day of incubation, in order ta
give the reader a complete view of their development.

On opening an egg on the third day the first thing
which attracts notice is the diminution of the white of
the egg. This seems to be one of the consequences of
the functional activity of the newly-established vascular
area whose blood-vessel are engaged either in directly
absorbing the white or, as is more probable, in absorbing
the yolk, which is in turn replenished at the expense of
the white. The absorption, once begun, goes on so
actively that, by the end of the day, the decrease of the
white is very striking.

The blastoderm has now spread over about half
the yolk, the extreme margin of the opaque area reach-

110 THE THIRD DAY. [CHAP.

ing about half-way towards the pole of the yolk opposite
to the embryo.

The vascular area, though still increasing, is much
smaller than the total opaque area, being in average-
sized eggs about as large as a florin. Still smaller than
the vascular area is the pellucid area in the centre of
which lies the rapidly growing embryo.

During the third day the vascular area is not
only a means for providing the embryo with nourish-
ment from the yolk, but also, inasmuch as by the dimi-
nution of the white it is brought close under the shell
and therefore fully exposed to the influence of the
atmosphere, serves as the chief organ of respiration.

This in fact is the period at which the vascular area
may be said to be in the stage of its most complete de-
velopment; for though it will afterwards become larger,
it will at the same time become less definite and rela-
tively less important. We may therefore, before we
proceed, add a few words to the description of it given
in the last chapter.

The blood leaving the body of the embryo by the
vitelline arteries (Fig. 36, R. Of. A., L. Of. A.} is
carried to the small vessels and capillaries of the vascu-
lar area, a small portion only being appropriated by the
pellucid area.

From the vascular area part of the blood returns
directly to the heart by the main lateral trunks of the
vitelline veins, R. Of., L. Of. During the second day
these venous trunks joined the body of the embryo
considerably in front of, that is, nearer the head than,
the corresponding arterial ones. Towards the end of
the third day, owing to the continued lengthening of

THE VASCULAR AREA

FIG. 36.
AA

III

. \

SXr-

sa:

L.0f

DIAGRAM OF THE CIRCULATION OF THE YOLK-SACK AT THE END
OF THE THIRD DAY OF INCUBATION.

H. heart. A A. the second, third and fourth aortic arches ; the
first has become obliterated in its median portion, but is
continued at its proximal end as the external carotid, and at
its distal end as the internal carotid. AO. dorsal aorta.
L. Of. A. left vitelline artery. R. Of. A. right vitelline
artery. S. T. sinus terminalis. L. Of. left vitelline vein.
R. Of. right vitelline vein. S. V. sinus venosus. D. C.
ductus Cuvieri. S. Ca. V. superior cardinal or jugular vein.
V. Ca. inferior cardinal vein. The veins are marked in

112 THE THIRD DAY. [CHAP,

outline and the arteries are made black. The whole blasto-
derm has been removed from the egg and is supposed to be
viewed from below. Hence the left is seen on the right, and

vice '

the heart, the veins and arteries run not only parallel
to each other, but almost in the same line, the points at
which they respectively join and leave the body being
nearly at the same distance from the head.

The rest of the blood brought by the vitelline
arteries finds its way into the lateral portions of the
sinus terminalis, S.T., and there divides on each side
into two streams. Of these, the two which, one on
each side, flow backward, meet at a point about oppo-
site to the tail of the embryo, and are conveyed along a
distinct vein which, running straight forward parallel to
the axis of the embryo, empties itself into the left vitel-
line vein. The two forward streams reaching the gap
in the front part of the sinus terminalis fall into either
one, or in some cases two veins, which run straight
backward parallel to the axis of the embryo, and so
reach the roots of the heart. When one such vein only
is present, it joins the left vitelline trunk; where there
are two they join the left and right vitelline trunks
respectively. The left vein is always considerably
larger than the right; and the latter when present
rapidly gets smaller and speedily disappears.

The chief differences, then, between the peripheral
circulation of the second and of the third day are due
to the greater prominence of the sinus terminalis and
the more complete arrangements for returning the blood
from it to the heart. After this day, although the vas-
cular area will go on increasing in size until it finally

VI.] CHANGE OF POSITION OF THE EMBRYO. 113

all but encompasses the yolk, the prominence of the
sinus terminalis will become less and less in proportion
as the respiratory work of the vascular area is shifted
on to the allantois, and its activities confined to absorb-
ing nutritive matter from the yolk.

The folding-in of the embryo makes great pro-
gress during this day. Both head and tail have become
most distinct, and the side folds which are to constitute
the lateral walls have advanced so rapidly that the
embryo is now a bond fide tubular sac, connected with
the rest of the yolk by a broad stalk. This stalk,
as was explained in Chap. II, is double, and consists of
an inner splanchnic stalk continuous with the alimen-
tary canal, which is now a tube closed at both ends and
open to the stalk along its middle third only, and an
outer somatic stalk continuous with the body-walls of
the embryo, which have not closed nearly to the same
extent as the walls of the alimentary canal. (Compare
Fig. 9, A and J5, which may be taken as diagrammatic
representations of longitudinal and transverse sections
of an embryo of this period.)

The embryo is almost completely covered by the
amnion. Early in this day the several amniotic folds
will have met and completely coalesced along a line
over the back of the embryo in the manner already
explained in the last chapter.

During this day a most remarkable change takes
place in the position of the embryo. Up to this
time it has been lying symmetrically upon the yolk
with the part which will be its mouth directed straight
downwards. It now turns round so as to lie on its left
side.

F. & B. 8

114

THE THIRD DAY.
FIG. 37.

[CHAP.

CHICK OF THE THIRD DAY (FIFTY-FOUR HOURS) VIEWED FROM

UNDERNEATH AS A TRANSPARENT OBJECT.

of. the outer amniotic fold or false amnion. This is very con-
spicuous around the head, but may also be seen at the tail.

a. the true amnion, very closely enveloping the head, and here
seen only between the projections of the several cerebral
vesicles. It may also be traced at the tail.
In the embryo of which this is a drawing, the head-fold of the

amnion reached a little farther backward than the reference u,

VI.] GENERAL VIEW OF EMBRYO. 115

but its limit could not be distinctly seen through the body of the
embryo. The prominence of the false arnnion at the head is apt
to puzzle the student ; but if he bears in mind the fact, which
could not well be shewn in Fig. 9, that the whole amniotic fold,
both the true and the false limb, is tucked in underneath the
head, the matter will on reflection become intelligible.

C. H. cerebral hemisphere. F. B. thalamencephalon or vesicle of
the third ventricle. M. B. mid-brain. H. B. hind-brain. Op.
optic vesicle. Ot. otic vesicle. Of V. vitelline veins forming
the venous roots of the heart. The trunk on the right hand
(left trunk when the embryo is viewed in its natural position
from above) receives a large branch, shewn by dotted lines,
coming from the anterior portion of the sinus terminalis.
Ht. the heart, now completely twisted on itself. Ao. the
bulbus arteriosus, the three aortic arches being dimly seen
stretching from it across the throat, and uniting into the
aorta, still more dimly seen as a curved dark line running
along the body. The other curved dark line by its side,
ending near the reference y, is the notochord ch.

About opposite the line of reference x the aorta divides into two
trunks, which, running in the line of the somewhat opaque
mesoblastic somites on either side, are not clearly seen.
Their branches however, Ofa, the vitelline arteries, are
conspicuous and are seen to curve round the commencing
side folds.

Pv. mesoblastic somites. Below the level of the vitelline arteries
the vertebral plates are but imperfectly cut up into meso-
blastic somites, and lower down still, not at all.

x is placed at the "point of divergence" of the splanchnopleure
folds. The blind foregut begins here and extends about up
to y. x therefore marks the present hind limit of the
splanchnopleure folds. The limit of the more transparent
somatopleure folds is not shewn.

It will be of course understood that all the body of the embryo
above the level of the reference #, is seen through the portion of
the yolk-sac (vascular and pellucid area), which has been removed

8—2

116 THE THIRD DAY. [CHAP.

with the embryo from the egg, as well as through the double
amniotic fold.

We may repeat that, the view being from below, whatever is
described in the natural position as being to the right here
appears to be left, and vice versa.

This important change of position at first affects
only the head (Fig. 37), but subsequently extends also to
the trunk. It is not usually completed till the fourth
day. At the same time the left vitelline vein, the one on
the side on which the embryo comes to lie, grows very
much larger than the right, which henceforward gradu-
ally dwindles and finally disappears.

Coincidently with the change of position the whole
embryo begins to be curved on itself in a slightly
spiral manner.' This curvature of the body becomes
still more marked on the fourth day, Fig. 67.

In the head very important changes take place.
One of these is the cranial flexure, Figs. 37, 38. This
(which must not be confounded with the curvature of
the body just referred to) we have already seen was
commenced in the course of the second day, by the
bending downwards of the head round a point which
may be considered as the extreme end either of the
notochord or of the alimentary canal.

The flexure progresses rapidly, the front-brain being
more and more folded down till, at the end of the third
day, it is no longer the first vesicle or fore-brain, but
the second cerebral vesicle or mid-brain, which occupies
the extreme front of the long axis of the embryo. In
fact a straight line through the long axis of the embryo
would now pass through the mid-brain instead of, as at
the beginning of the second day, through the fore-brain,

VI.] THE BRAIN. 117

so completely has the front end of the neural canal
been folded over the end of the notochord. The com-
mencement of this cranial flexure gives the body of an
embryo of the third day somewhat the appearance of a
retort, the head of the embryo corresponding to the
bulb. On the fourth day the flexure is still greater
than on the third, but on the fifth and succeeding days
it becomes less obvious, owing to the filling up of the
parts of the skull.

The brain. The vesicle of the cerebral hemispheres,
which on the second day began to grow out from the
front of the fore -brain, increases rapidly in size during
the third day, growing out laterally, so as to form two
vesicles, so much so that by the end of the day it (Fig.
37, CH, Fig. 38) is as large or larger than the original
vesicle from which it sprang, and forms the most con-
spicuous part of the brain. In its growth it pushes
aside the optic vesicles, and thus contributes largely to
the roundness which the head is now acquiring. Each
lateral vesicle possesses a cavity, which afterwards
becomes one of the lateral ventricles. These cavities are
continuous behind with the cavity of the fore-brain.

Owing to the development of the cerebral vesicle the
original fore -brain no longer occupies the front position
(Fig. 37, FB, Fig. 38, /&), and ceases to be the con-
spicuous object that it was. Inasmuch as its walls will
hereafter be developed into the parts surrounding the
so-called third ventricle of the brain, we shall hence-
forward speak of it as the vesicle of the third ventricle,
or thalamencephalon.

On the summit of the thalamencephalon there may
now be seen a small conical projection, the rudiment of

118 THE THIRD DAY. [CHAP.

FIG. 38.

HEAD OF A CHICK OF THE THIRD DAY VIEWED SIDEWAYS AS A
TRANSPARENT OBJECT. (From Huxley.)

I a. the vesicle of the cerebral hemisphere. 1 6. the vesicle of
the third ventricle (the original fore-brain) ; at its summit
is seen the projection of the pineal gland e.

Below this portion of the brain is seen, in optical section, the
optic vesicle a already involuted with its thick inner and thinner
outer wall (the letter a is placed on the junction of the two, the
primary cavity being almost obliterated). In the centre of the
vesicle lies the lens, the shaded portion being the expression of
its cavity. Below the lens between the two limbs of the horse-
shoe is the choroidal fissure.

II. the mid-brain. III. the hind-brain. V. the rudiments of
the fifth cranial nerve, VII. of the seventh. Below the seventh
nerve is seen the auditory vesicle b. The head having been
subjected to pressure, the vesicle appears somewhat distorted as
if squeezed out of place. The orifice is not yet quite closed up.

I, the inferior maxillary process of the first visceral or man-
dibular fold. Below, and to the right of this, is seen the first
visceral cleft, below that again the second visceral fold (2), and
lower down the third (3) and fourth (4) visceral folds. In front
of the folds (i.e. to the left) is seen the arterial end of the heartr
the aortic arches being buried in their respective visceral folds.

f. represents the mesoblast of the base of the brain and spinal
cord.

VI.] THE PITUITAKY BODY. 119

the pineal gland (Fig. 38, e), while the centre of the

floor is produced into a funnel-shaped process, the infun-

dibulum (Fig. 39, In), which, stretching towards the

FIG. 39.

LONGITUDINAL SECTION THROUGH THE BRAIN OF A YOUNG
PRISTIURUS EMBRYO.

cer. commencement of cerebral hemisphere ; pn. pineal gland ;
In. infundibulum ; pt. ingrowth of mouth to form the
pituitary body ; mb. mid-brain ; cb. cerebellum ; ch. noto-
chord ; al. alimentary tract ; laa. artery of mandibular arch.

extreme end of the oral invagination or stomodceum,
joins a diverticulum of this which becomes the pituitary
body.

The development of the pituitary body or hypophysis cerebri
has been the subject of considerable controversy amongst embryo-
logists, and it is only within the last few years that its origin
from the oral epithelium has been satisfactorily established.

In the course of cranial flexure the epiblast on the under side
of the head becomes tucked in between the blind end of the
throat and the base of the brain. The part so tucked in constitutes
a kind of bay, and forms the stomodaeum or primitive buccal
cavity already spoken of. The blind end of this bay becomes
produced as a papilliform diverticulum which may be called the
pituitary diverticulum. It is represented as it appears in a

120 THE THIRD DAY. [CHAP.

lower vertebrate embryo (Elasmobranch) in Fig. 39, but is in all
important respects exactly similar in the chick. Very shortly after
the pituitary diverticulum becomes first established the boundary
wall between the stomodseum and the throat becomes perforated,
and the limits of the stomodaeum obliterated, so that the pituitary
diverticulum looks as if it had arisen from the hypoblast. During
the third day of incubation the front part of the notochord
becomes bent downward, and, ending in a somewhat enlarged
extremity, comes in contact with the termination of the pituitary
diverticulum. The mesoblast around increases and grows up, in
front of the notochord and behind the vesicle of the third
ventricle, to form the posterior clinoid process. The base of the
vesicle of the third ventricle at the same time grows downwards
towards the pituitary diverticulum, and forms what is known as the
infundibulum. On the fourth day the mesoblastic tissue around
the notochord increases in quantity, and the end of the notochord,
though still bent downwards, recedes a little from the termination
of the pituitary diverticulum, which is still a triangular space with
a wide opening into the alimentary canal.

On the fifth day, the opening of the pituitary diverticulum
into the alimentary canal has become narrowed, and around the
whole diverticulum an investment of mesoblast-cells has appeared.
Behind it the clinoid process has become cartilaginous, while to
the sides and in front it is enclosed by the trabeculse. At this
stage, in fact, we have a diverticulum from the alimentary canal
passing through the base of skull to the infundibulum.

On the seventh day the communication between the cavity
of the diverticttlum and that of the throat has become still
narrower. The diverticulum is all but converted into a vesicle,
and its epiblastic walls have commenced to send out into the
mesoblastic investment solid processes. The infundibulum now
appears as a narrow process from the base of the vesicle of the
third ventricle, which approaches, but does not unite with, the
pituitary vesicle.

By the tenth day the opening of the pituitary vesicle into
the throat becomes almost obliterated, and the lumen of the
vesicle itself very much diminished. The body consists of
anastomosing cords of epiblast-cells, the mesoblast between

VI.] THE PITUITARY BODY. 121

which has already commenced to become vascular. The cords
or masses of epiblast cells are surrounded by a delicate mem-
brana propria, and a few of them possess a small lumen. The
infundibulum has increased in length. The relative positions of
the pituitary body and infundibulum are shewn in the figure of
the brain in Chapter vni.

On the twelfth day the communication between the pituitary
vesicle and the throat is entirely obliterated, but a solid cord of
cells still connects the two. The vessels of the pia mater of the
vesicle of the third ventricle have become connected with the
pituitary body, and the infundibulum has grown down along its
posterior border.

In the later stages all connection is lost between the pituitary
body and the throat, and the former becomes attached to the
elongated processus infundibuli.

The real nature of the pituitary body is still extremely obscure,
but it is not improbably the remnant of a glandular structure
which may have opened into the mouth in primitive vertebrate
forms, but which has ceased to have a function in existing
vertebrates1.

Beyond an increase in size, which it shares with
nearly all parts of the embryo, and the change of
position to which we have already referred, the mid-
brain undergoes no great alteration during the third
day. Its roof will ultimately become developed into
the corpora Ugemina or optic lobes, its floor will form
the crura cerebri, and its cavity will be reduced to the
narrow canal known as the iter a tertio ad quartum
ventriculum.

In the hind-brain, or third cerebral vesicle, that
part which lies nearest to the mid-brain, is during

1 Wilhelm M tiller Ueber die Entwicklung und Ban der Hypophysis
und des Processus Infundibuli Cerebri. Jenaische Zeitschrift, Bd. vi.
1871, and V. von Mihalkovics, Wirbelsaite u. Hirnanhang , Archiv f.
mikr. Anat. Vol. xi. 1875.

122 THE THIRD DAY. [CHAP.

the third day marked off from the rest by a slight
constriction. This distinction, which becomes much
more evident later on by a thickening of the walls and
roof of the front portion, separates the hind-brain into
the cerebellum in front, and the medulla oblongata
behind (Figs. 38 and 39). While the walls of the
cerebellar portion of the hind-brain become very much
thickened as well at the roof as at the floor and sides,
the roof of the posterior or medulla oblongata portion
thins out into a mere membrane, forming a delicate
covering to the cavity of the vesicle (Fig. 40, iv), which
here becoming broad and shallow with greatly thick-
ened floor and sides, is known as the fourth ventricle,
subsequently overhung by the largely developed pos-
terior portion of the cerebellum.

The third day, therefore, marks the differentiation
of the brain into five distinct parts : the cerebral
hemispheres, the central masses round the third
ventricle, the corpora bigemina or optic lobes, the
cerebellum and the medulla oblongata ; the original
cavity of the neural canal at the same time passing
from its temporary division of three single cavities into
the permanent arrangement of a series of connected
ventricles, viz. the lateral ventricles, the third ventricle,
the iter (with a prolongation into the optic lobe on
each side), and the fourth ventricle.

At the same time that the outward external shape
of the brain is thus being moulded, internal changes
are taking place in the whole neural canal. These are
best seen in sections.

At its first formation, the section of the cavity of
the neural canal is round, or nearly so.

VI.] THE CRANIAL AND SPINAL NERVES. 123-

About this time, however, the lining of involuted
epiblast along the length of the whole spinal cord
becomes very much thickened at each side, while
increasing but little at the mid-points above and below.
The result of this is that the cavity as seen in section
(Figs. 64 and 65), instead of being circular, has become
a narrow vertical slit, almost completely filled in on
each side.

In the region of the brain the thickening of the
lining epiblast follows a somewhat different course.
While almost everywhere the sides and floor of the
canal are greatly thickened, the roof in the region of
the various ventricles, especially of the third and fourth,
becomes excessively thin, so as to form a membrane
reduced to almost a single layer of cells. (Fig. 40, IV.)

Cranial and spinal nerves. A most important
event which takes place during the second and third
days, is the formation of the cranial and spinal nerves.
Till within a comparatively recent period embryologists
were nearly unanimous in believing that the peripheral
nerves originated from the mesoblast at the sides of
the brain and spinal cord. This view has now however
been definitely disproved, and it has been established
that both the cranial and spinal nerves take their origin
as outgrowths of the central nervous system.

The cranial nerves are the first to be developed and
arise before the complete closure of the neural groove.
They are formed as paired outgrowths of a continuous
band known as the neural band, composed of two
laminae, which connects the dorsal edges of the incom-
pletely closed neural canal with the external epiblast.
This mode of development will best be understood by

124

THE THIRD DAY.

[CHAP.

CC

AQA

SECTION THROUGH THE HIND-BRAIN OF A CHICK AT THE END
OF THE THIRD DAT OF INCUBATION.

IV. Fourth ventricle. The section shews the very thin roof and
thicker sides of the ventricle.

Ch. Notochord — (diagrammatic shading).

CV. Anterior cardinal or jugular vein.

CC. Involuted auditory vesicle. CC points to the end which
will form the cochlear canal. RL. Kecessus labyrinthi. hy.
hypoblast lining the alimentary canal, hy is itself placed in
the cavity of the alimentary canal, in that part of the canal
which will become the throat. The ventral (anterior) wall of
the canal is not shewn in the section, but on each side are
seen portions of a pair of visceral arches. In each arch
is seen the section of the aortic arch AOA belonging to the
visceral arch. The vessel thus cut through is running
upwards towards the head, being about to join the dorsal
aorta AO. Had the section been nearer the head, and
carried through the plane at which the aortic arch curves

VI.]

THE CRANIAL AND SPINAL NERVES.

125

round the alimentary canal to reach, the mesoblast above it,
AOA and AO would have formed one continuous curved
space. In sections lower down in the back the two aortse,
AO, one on each side, would be found fused into one median
canal.

an examination of Fig. 41, where the two roots of the
vagus nerve (vg) are shewn growing out from the neural
band. Shortly after this stage the neural band becomes
separated from the external epiblast, and constitutes

FIG. 41.

TRANSVERSE SECTION THROUGH THE POSTERIOR PART OF THE
HEAD OF AN EMBRYO CHICK OF THIRTY HOURS.

hb. hind-brain ; vg. vagus nerve ; ep. epiblast ; ch. notochord ;
x. thickening of hypoblast (possibly a rudiment of the sub-
notochordal rod) ; al. throat ; ht. heart ; pp. body cavity ;
so. somatic mesoblast ; sf. splanchnic mesoblast ; hy. hypo-
blast.

126 THE THIRD DAY. [CHAP.

a crest attached to the roof of the brain, while its two
laminae become fused.

Anteriorly, the neural crest extends as far as the
roof of the mid-brain. The pairs of nerves which
undoubtedly grow out from it are the fifth pair, the
seventh and auditory (as a single root), the glosso-
pharyngeal and the various elements of the vagus (as a
single root).

After the roots of these nerves have become estab-
lished, the crest connecting them becomes partially
obliterated. The roots themselves grow centrifugally,
and eventually give rise to the whole of each of the
cranial nerves. Each complete root develops a gan-
glionic enlargement near its base, and (with the ex-
ception of the third nerve) is distributed to one of the
visceral arches, of which we shall say more hereafter.
The primitive attachment of the nerves is to the roof
of the brain, but in most instances this attachment is
replaced by a secondary attachment to the sides or
floor.

The rudiments of four cranial nerves, of which two
lie in front of and two behind the auditory vesicle,
are easily seen during the third day at the sides of the
hind-brain. They form a series of four small opaque
masses, somewhat pearshaped, with the stalk directed
away from the middle line.

The most anterior of these is the rudiment of the
fifth nerve (Figs. 42 and 67, V). Its narrowed outer
portion or stalk divides into two bands or nerves. Of
these one passing towards the eye terminates at present
in the immediate neighbourhood of that organ. The
other branch (the rudiment of the inferior maxillary

VI.]

THE CRANIAL NERVES.

FIG. 42.

127

HEAD OF AN EMBRYO CHICK OF THE THIRD DAY (SEVENTY-
FIVE HOURS) VIEWED SIDEWAYS AS A TRANSPARENT OBJECT.

(From Huxley.)

la. cerebral hemispheres. Ib. vesicle of the third ventricle. II.
mid-brain. III. hind-brain, g. nasal pit. a. optic vesicle.
6. otic vesicle, d. infundibulum. e. pineal body. h. noto-
chord. V. fifth nerve. VII. seventh nerve. VIII. united
glossopharyngeal and pneumogastric nerves. I, 2, 3, 4, 5
the five visceral folds.

branch of the fifth nerve) is distributed to the first
visceral arch.

The second mass (Figs. 42 and 67, VII) is the rudi-
ment of the seventh, or facial nerve, and of the audi-
tory nerve. It is the nerve of the second visceral arch.

The two masses behind the auditory vesicle repre-
sent the glossopharyngeal and pneumogastric nerves
(Fig. 42, VIII, Fig. 67, G. Ph. and Pg). At first
united, they subsequently become separate. The glosso-
pharyngeal supplies the third arch, and the pneumo-
gastric the fourth and succeeding arches.

The later development of the cranial nerves has only been
partially worked out, and we will confine ourselves here to a very

128 THE THIRD DAY. [CHAP.

brief statement of some of the main results arrived at. The
outgrowth for the vagus nerve supplies in the embryo the fourth
and succeeding visceral arches, and from what we know of it
in the lower vertebrate types, we may conclude that it is a
compound nerve, composed of as many primitively distinct
nerves as there are branches to the visceral arches.

The glossopharyngeal nerve is the nerve supplying the third
visceral arch, the homologue of the first branchial arch of Fishes.
The development of the hypoglossal nerve is not known, but it is
perhaps the anterior root of a spinal nerve. The spinal accessory
nerve has still smaller claims than the hypoglossal to be regarded
as a true cranial nerve. The primitively single root of the
seventh auditory nerves divides almost at once into two branches.
The anterior of these pursues a straight course to the hyoid arch
and forms the rudiment of the facial nerve, Fig. 67, vn ; the second
of the two, which is the rudiment of the auditory nerve, develops
a ganglionic enlargement, and, turning backwards, closely hugs
the ventral wall of the auditory involution. The sixth nerve
appears to arise later than the seventh nerve from the ventral
part of the hind-brain, and has no ganglion near its root.

Shortly after its development the root of the fifth nerve shifts
so as to be attached about half-way down the side of the brain.
A large ganglion is developed close to the root, which becomes
the Gasserian ganglion. The main branch of the nerve grows
into the-mandibular arch (Fig. 67), maintaining towards it similar
relations to those of the nerves behind it to their respective
arches.

An important branch becomes early developed which is
directed straight towards the eye (Fig. 67), near which it meets
and unites with the third nerve, where the ciliary ganglion
is developed. This branch is usually called the ophthalmic
branch of the fifth nerve, and may perhaps represent an inde-
pendent nerve.

Later than these two branches there is developed a third
branch, passing the upper process of the first visceral arch.
It forms the superior maxillary branch of the adult.

Nothing is known with reference to the development of the
fourth nerve.

VI.] THE SPINAL NERVES. 129

The history of the third nerve is still imperfectly known.
There is developed early on the second day from the neural
crest, on the roof of the mid-brain, an outgrowth on each side,
very similar to the rudiment of the posterior nerves. This out-
growth is believed by Marshall to be the third nerve, but it must
be borne in mind that there is no direct evidence on the point,
the fate of the outgrowth in question not having been satisfac-
torily followed.

At a very considerably later period a nerve may be found
springing from the floor of the mid-brain, which is undoubtedly
the third nerve. If identical with the outgrowth just spoken of,
it must have shifted its attachment from the roof to the floor of
the brain.

The nerve when it springs from the floor of the brain runs
directly backwards till it terminates in the ciliary ganglion,
from which two branches to the eye-muscles are given off.

[A. Marshall. " The development of the cranial nerves in the
Chick." Quart. Journal of Microscop. Science, Vol. xvin.]

In the case of the spinal nerves the posterior roots
originate as outgrowths of a series of median processes
of cells, which make their appearance on the dorsal side
of the spinal cord. The outgrowths, symmetrically
placed on each side, soon take a pyriform aspect, and
apply themselves to the walls of the spinal cord. They
are represented as they appear in birds in Fig. 43, sp. g.}
and as they appear in a lower vertebrate form in Fig. 44.

The original attachment of the nerve -rudiment to
the medullary wall is not permanent. It becomes, in
fact, very soon either extremely delicate or absolutely
interrupted.

The nerve-rudiment now becomes divided into three
parts, (1) a proximal rounded portion; (2) an enlarged
middle portion, forming the rudiment of a ganglion ; (3)
a distal portion, forming the commencement of the nerve.
The proximal portion may very soon be observed to be
F. & B. 9

130

TRANSVERSE SECTION THROUGH THE TRUNK OF A DUCK EMBRYO
WITH ABOUT TWENTY-FOUR MESOBLASTIC SOMITES.

am. amnion ; so. somatopleure ; sp. splanchnopleure ; wd. Wolffian
duct ; st. segment al tube ; ca.v. cardinal vein ; ms. muscle-
plate ; sp.g. spinal ganglion; sp.c. spinal cord; ch. notochord;
ao. aorta ; hy. hypoblast.

united with the side of the spinal cord at a very con-
siderable distance from its original point of origin. It is
moreover attached, not by its extremity, but by its side.
The above points, which are much more easily
studied in some of the lower vertebrate forms than in
Birds, are illustrated by the subjoined section of an
Elasmobranch embryo, Fig. 45.

VI ] THE SPINAL NERVES. ] 31

FIG. 44.

pr

trip

TRANSVERSE SECTION THROUGH THE TRUNK OF A YOUNG EMBRYO

OF A DOG-FlSH.

nc. neural canal ; pr. posterior root of spinal nerve ; x. sub-
notochordal rod ; ao. aorta ; sc. somatic mesoblast ; sp.
splanchnic mesoblast ; mp. muscle-plate ; mp'. portion of
muscle-plate converted into muscle ; Vv. portion of the
vertebral plate which will give rise to the vertebral bodies ;
al. alimentary tract.

It is extremely difficult to decide whether the per-
manent attachment of the posterior nerve-roots to the
spinal cord is entirely a new formation, or merely due
to the shifting of the original point of attachment.
We are inclined to adopt the former view.

The origin of the anterior roots of the spinal nerves
has not as yet been satisfactorily made out in Birds ;
but it appears probable that they grow from the ventral
corner of the spinal cord, considerably later than the
posterior roots, as a number of strands for each nerve,

9—2

132

THE THIRD DAY.
FIG. 45.

[CHAP.

cfa

SECTION THROUGH THE DORSAL REGION OF AN EMBRYO DOG-FISH.

pr. posterior root ; sp.g. spinal ganglion ; n. nerve ; x. attach-
ment of ganglion to spinal cord ; nc. neural canal ; mp.
muscle-plate ; ck. notochord ; i. investment of spinal cord.

which subsequently join the posterior roots below the
ganglia. The shape of the root of a completely formed
spinal nerve, as it appears in an embryo of the fourth
day, is represented in Fig. 68.

The Eye. In the preceding chapter we saw how
the first cerebral vesicle, by means of lateral outgrowths
followed by constrictions, gave rise to the optic vesicles.
These and the parts surrounding them undergo on the
third day changes which result in the formation of the
eyeball.

At their first appearance the optic vesicles stand
out at nearly right angles to the long axis of the
embryo (Fig. 27), and the stalks which connect them

VL]

THE EYE.

133

with the fore-brain are short and wide. The con-
strictions which give rise to the stalks take place chiefly
from above downwards, and also somewhat inwards and
backwards. Thus from the first the vesicles appear to
spring from the under part of the fore-brain.

These stalks soon become comparatively narrow,
and constitute the rudiments of the optic nerves (Fig.
46 b). The constriction to which the stalk or optic

FIG. 46.

SECTION THROUGH THE HEAD OF AN EMBRYO TELEOSTEAN, TO
SHEW THE FORMATION OF THE OPTIC VESICLES, ETC. (From
Gegenbaur ; after Schenk.)

c. fore-brain ; a. optic vesicle ; b. stalk of optic vesicle ; d.
epidermis.

nerve is due takes place obliquely downwards and
backwards, so that the optic nerves open into the base
of the front part of the thalamencephalon (Fig. 46 b).

While these changes have been going on in the
optic stalks, development has also proceeded in the
region of the vesicles themselves, and given rise to the
rudiments of the retina, lens, vitreous humour, and
other parts of the eye.

134 THE THIRD DAY. [CHAP.

Towards the end of the second day the external
or superficial epiblast which covers, and is in all but
immediate contact with, the most projecting portion of
the optic vesicle, becomes thickened. This thickened
portion is then driven inwards in the form of a shallow
open pit with thick walls (Fig. 47 A, o), carrying before
it the front wall (r) of the optic vesicle. To such an
extent does this involution of the superficial epiblast
take place, that the front wall of the optic vesicle is
pushed close up to the hind wall, and the cavity of the
vesicle becomes almost obliterated (Fig. 47, #).

The bulb of the optic vesicle is thus converted into
a cup with double walls, containing in its cavity the
portion of involuted epiblast. This cup, in order to
distinguish its cavity from that of the original optic
vesicle, is generally called the secondary optic vesicle.
We may, for the sake of brevity, speak of it as the
optic cup; in reality it never is a vesicle, since it
always remains widely open in front. Of its double
walls the inner or anterior (Fig. 47 B, r) is formed
from the front portion, the outer or posterior (Fig. 47
5, u) from the hind portion of the wall of the primary
optic vesicle. The inner or anterior (r), which very
speedily becomes thicker than the other, is converted
into the retina; in the outer or posterior (u), which
remains thin, pigment is eventually deposited, and it
ultimately becomes the tesselated pigment-layer of the
choroid.

By the closure of its mouth the pit of involuted
epiblast becomes a completely closed sac with thick
walls and a small central cavity (Fig. 47 B, I}. At
the same time it breaks away from the external epi-

VI.]

THE EYE.
FIG. 47.

135

DIAGRAMMATIC SECTIONS ILLUSTRATING THE FORMATION OF
THE EYE. (After Kemak.)

In A, the thin superficial epiblast h is seen to be thickened at #,
in front of the optic vesicle, and involuted so as to form
a pit o, the mouth of which has already begun to close in.
Owing to this involution, which forms the rudiment of the
lens, the optic vesicle is doubled in, its front portion r being
pushed against the back portion u, and the original cavity
of the vesicle thus reduced in size. The stalk of the vesicle
is shewn as still broad.

In J?, the optic vesicle is still further doubled in so as to form a
cup with a posterior wall u and an anterior wall r. In the
hollow of this cup lies the lens £, now completely detached
from the superficial epiblast x. Its cavity is still shewn.
The cavity of the stalk of the optic vesicle is already much
narrowed.

blast, which forms a continuous layer in front of it,
all traces of the original opening being lost. There is
thus left lying in the cup of the secondary optic vesicle,
an isolated elliptical mass of epiblast. This is the
rudiment of the lens. The small cavity within it
speedily becomes still less by the thickening of the
walls, especially of the hinder one.

At its first appearance the lens is in immediate
contact with the anterior wall of the secondary optic
vesicle (Fig. 47 B}. In a short time, however, the lens

136 THE THIRD DAY. [CHAP.

is seen to lie in the mouth of the cup (Fig. 50 A), a
space (vh) (which is occupied by the vitreous humour)
making its appearance between the lens and anterior
wall of the vesicle.

In order to understand how this space is developed,
the position of the optic vesicle and the relations of
its stalk must be borne in mind.

The vesicle lies at the side of the head, and its
stalk is directed downwards, inwards and backwards.
The stalk in fact slants away from the vesicle. Hence
when the involution of the lens takes place, the direc-
tion in which the front wall of the vesicle is pushed in
is not in a line with the axis of the stalk, as for
simplicity's sake has been represented in the diagram
Fig. 47, but forms an obtuse angle with that axis, after
the manner of Fig. 48, where s represents the cavity

FIG. 48.

DIAGRAMMATIC SECTION OF THE EYE AND THE OPTIC NERVE
AT AN EARLY STAGE (from Lieberklihn),

bo shew the lens I occupying the whole hollow of the optic cup,
the inclination of the stalk s to the optic cup, and the
continuity of the cavity of the stalk s with that of the
primary vesicle c ; r} anterior, u posterior wall of the optic
cup,

VI.] THE EYE, 137

of the stalk leading away from the almost obliterated
cavity of the primary vesicle.

Fig. 48 represents the early stage at which the
lens fills the whole cup of the secondary vesicle. The
subsequent state of affairs is brought about through
the growth of the walls of the cup taking place more
rapidly than that of the lens. But this growth or this
dilatation does not take place equally in all parts of
the cup. The walls of the cup rise up all round except
that part of the circumference of the cup which
adjoins the stalk. While elsewhere the walls increase
rapidly in height, carrying so to speak the lens with
them, at this spot, which in the natural position of the
eye is on its under surface, there is no growth: the
wall is here imperfect, and a gap is left. Through this
gap, which afterwards receives the name of the cho-
roidal fissure, a way is open from the mesoblastic tissue
surrounding the optic vesicle and stalk into the interior
of the cavity of the cup.

From the manner of its formation the gap or fissure
is evidently in a line with the axis of the optic stalk,
and in order to be seen must be looked for on the
under surface of the optic vesicle. In this position it
is readily recognized in the transparent embryo of the
third day, Figs. 37 and 48.

Bearing in mind these relations of the gap to the
optic stalk, the reader will understand how sections of
the optic vesicle at this stage present very different
appearances according to the plane in which the
sections are taken.

When the head of the chick is viewed from under-
neath as a transparent object the eye presents very

138

THE THIRD DAY.

[CHAP.

in the diagram

much the appearance represented
Fig. 49.

A section of such an eye taken along the line y,
perpendicular to the plane of the paper, would give a
figure corresponding to that of Fig. 50 A. The lens,
the cavity and double walls of the secondary vesicle, and
the remains of the primary cavity, would all be repre-

DlAGRAMMATIC REPRESENTATION OF THE EYE OF THE CHICK
OF ABOUT THE THIRD DAY AS SEEN WHEN THE HEAD IS
VIEWED FROM UNDERNEATH AS A TRANSPARENT OBJECT.

I the lens, I' the cavity of the lens, lying in the hollow of the

optic cup.
r the anterior, u the posterior wall of the optic cup, c the cavity

of the primary optic vesicle, now nearly obliterated. By

inadvertence u has been drawn thicker than r, it should

have been thinner throughout.
s the stalk of the optic cup with s its cavity, at a lower level

than the cup itself and therefore out of focus ; the dotted

line indicates the continuity of the cavity of the stalk with

that of the primary vesicle.
The line 0, 0, through which the section shewn in Fig. 50 C is

supposed to be taken, passes through the choroidal fissure.

VI.]

139

B.

C.

Diagrammatic section taken perpendicular to the plane of
the paper, along the line y, y, Fig. 49. The stalk is not
seen, the* section falling quite out of its region, vh, hollow
of optic cup filled with vitreous humour ; other letters as in
Fig. 47 B.

Section taken parallel to the plane of paper through Fig. 49,
so far behind the front surface of the eye as to shave off a
small portion of the posterior surface of the lens I, but so
far in front as not to be carried at all through the stalk.
Letters as before ; /, the choroidal fissure.
Section along the line z, z, perpendicular to the plane of the
paper, to shew the choroidal fissure /, and the continuity of
the cavity of the optic stalk with that of the primary optic
vesicle. Had this section been taken a little to either side of
the line z, 2, the wall of the optic cup would have extended
up to the lens below as well as above. Letters as above.

sented (the superficial epiblast of the head would also
be shewn) ; but there would be nothing seen of either
the stalk or the fissure. If on the other hand the
section were taken in a plane parallel to the plane of
the paper, at some distance above the level of the
stalk, some such figure would be gained as that shewn
in Fig. 50 B. Here the fissure / is obvious, and the
communication of the cavity vh of the secondary vesicle
with the outside of the eye evident; the section of
course would not go through the superficial epiblast,

140 THE THIRD DAY. [CHAP.

Lastly, a section, taken perpendicular to the plane of
the paper along the line z, i.e. through the fissure
itself, would present the appearances of Fig. 50 C,
where the wall of the vesicle is entirely wanting in the
region of the fissure marked by the position of the
letter /. The external epiblast has been omitted in
the figure.

The fissure such as we have described it exists for
a short time only. Its lips come into contact, and
unite (in the neighbourhood of the lens, directly, but in
the neighbourhood of the stalk, by the intervention of
a structure which we shall describe presently), and thus
the cup-like cavity of the secondary optic vesicle is
furnished with a complete wall all round. The interior
of the cavity is filled by the vitreous humour, a clear
fluid in which are a few scattered cells.

With reference to the above description, two points require
to be noticed. Firstly it is extremely doubtful whether the
invagination of the secondary optic vesicle is to be viewed as an
actual mechanical result of the ingrowth of the lens. Secondly
it seems probable that the choroid fissure is not simply due to a
deficiency in the growth of part of the walls of the secondary
optic cup, but is partly due to a more complicated inequality of
growth resulting in a doubling up of the primary vesicle from
the side along the line of the fissure, at the same time that the
lens is being thrust in in front. In Mammalia, the doubling up
involves the optic stalk, which becomes flattened (whereby its
original cavity is obliterated) and then folded in on itself, so as
to embrace a new central cavity continuous with the cavity of
the vitreous humour.

During the changes in the optic vesicle just de-
scribed, the surrounding mesoblast takes on the cha-
racters of a distinct investment, whereby the outline of

VI.] THE EYE. 141

the eyeball is definitely formed. The internal portions
of this investment, nearest to the retina, become the
choroid (i.e. the chorio-capillaris, and the lamina
fusca, the pigment epithelium, as we have seen, being
derived from the epiblastic optic cup), and pigment is
subsequently deposited in it. The remaining external
portion of the investment forms the sclerotic.

The complete differentiation of these two coats
of the eye does not however take place till a late
period.

In front of the optic cup the mesoblastic invest-
ment grows forwards, between the lens and the super-
ficial epiblast, and so gives rise to the substance of
the cornea; the epiblast supplying only the anterior
epithelium.

We may now proceed to give some further details
with reference to the histological differentiation of the
parts, whose general development has been dealt with
in the preceding pages.

The histological condition of the eye in its earliest
stages is very simple. Both the epiblast forming the
walls of the optic vesicle, and the superficial layer
which is thickened to become the lens, are composed of
simple columnar cells. The surrounding mesoblast is
made up of cells whose protoplasm is more or less
branched and irregular. These simple elements are
gradually modified into the complicated tissues of the
adult eye, the changes undergone being most marked
in the cases of the retina, the optic nerve, and the
lens with its appendages.

The optic vesicle. We left the original cavity of
the primary optic vesicle as a nearly obliterated space

142 THE THIRD DAY. [CHAP.

between the two walls of the optic cup. By the end
of the third day the obliteration is complete, and the
two walls are in immediate contact.

The inner or anterior wall is, from the first, thicker
than the outer or posterior ; and over the greater part
of the cup this contrast increases with the growth of
the eye, the anterior wall becoming markedly thicker
and undergoing changes of which we shall have to
speak directly (Fig. 51).

In the front portion however, along, so to speak, the
lip of the cup, anterior to a line which afterwards be-
comes the ora serrata, both layers not only cease to
take part in the increased thickening, accompanied by
peculiar histological changes, which the rest of the cup
is undergoing, but also completely coalesce together.
Thus a hind portion or true retina is marked off from a
front portion.

The front portion, accompanied by the choroid
which immediately overlays it, is, behind the lens,
thrown into folds, the ciliary ridges ; while further for-
ward it bends in between the lens and the cornea to
form the iris. The original wide opening of the optic
cup is thus narrowed to a smaller orifice, the pupil ;
and the lens, which before lay in the open mouth, is
now inclosed in the cavity of the cup. While in the
hind portion of the cup, or retina proper, no deposit of
black pigment takes place in the layer formed out of
the inner or anterior wall of the vesicle, in the front
portion we are speaking of, pigment is largely deposited
throughout both layers, so that eventually this portion
seems to become nothing more than a forward pro-
longation of the pigment-epithelium of the choroid.

VI.J

THE OPTIC VESICLE.
FIG. 51.

143

Kt.

p.Ch

SECTION OF THE EYE OF CHICK AT THE FOURTH DAY.

ep. superficial epiblast of the side of the head.

R. true retina : anterior wall of the optic cup. p. Gh. pigment-
epithelium of the choroid : posterior wall of the optic cup.
6 is placed at the extreme lip of the optic cup at what will
become the margin of the iris.

I. the lens. The hind wall, the nuclei of whose elongated cells
are shewn at ril, now forms nearly the whole mass of the lens,
the front wall being reduced to a layer of flattened cells el.

m. the mesoblast surrounding the optic cup and about to form
the choroid and sclerotic. It is seen to pass forward between
the lip of the optic cup and the superficial epiblast.

144 THE THIRD DAY. [CHAP.

Filling up a large part of the hollow of the optic cup is seen
a hyaline mass forming the hyaloid membrane and the coagulum
of the vitreous humour. In the neighbourhood of the lens it
seems to be continuous as at d with the tissue a, which in turn
is continuous with the mesoblast m, and appears to be the
rudiment of the capsule of the lens and suspensory ligament.

Thus while the hind moiety of the optic cup be-
comes the retina proper, including the choroid-pigment
in which the rods and cones are imbedded, the front
moiety is converted into the ciliary portion of the
retina, covering the ciliary processes, and into the uvea
of the iris ; the bodies of the ciliary processes and the
substance of the iris, their vessels, muscles, connective
tissue and ramified pigment, being derived from the
mesoblastic choroid. The margin of the pupil marks
the extreme lip of the optic vesicle, where the outer or
posterior wall turns round to join the inner or anterior.

The ciliary muscle and the ligamentum pectinatum
are both derived from the mesoblast between the
cornea and the iris.

The retina. At first, as we have said, the two walls
of the optic cup do not greatly differ in thickness. On
the third day the outer or posterior becomes much
thinner than the inner or anterior, and by the middle
of the fourth day is reduced to a single layer -of flat-
tened cells (Fig. 51, p. Gh.). At about the 80th hour
its cells commence to receive a deposit of pigment, and
eventually form the so-called pigmentary epithelium of
the choroid ; from them no part of the true retina (or
no other part of the retina, if the pigment-layer in
question be supposed to belong more truly to the retina
than to the choroid) is derived.

VI.] THE RETINA. 145

On the fourth day, the inner (anterior) wall of the
optic cup (Fig. 51, R) is perfectly uniform in structure,
being composed of elongated somewhat spindle-shaped
cells, with distinct nuclei. On its external (posterior)
surface a distinct cuticular membrane, the membrana
limitans externa, early appears.

As the wall increases in thickness, its cells multiply
rapidly, so that it soon appears to be several cells thick :
each cell being however probably continued through
the whole thickness of the layer. The wall at this
stage corresponds closely in its structure with the brain,
of which it may properly be looked upon as part. Ac-
cording to the usual view, which is not however fully
supported by recent observations, the retina becomes
divided in its subsequent growth into (1) an outer
part, corresponding morphologically to the epithelial
lining of the cerebro-spinal canal, composed of what
may be called the visual tells of the eye, i. e. the cells
forming the outer granular (nuclear) layer and the rods
and cones attached to them ; and (2) an inner portion
consisting of the inner granular (nuclear) layer, the
inner molecular layer, the ganglionic layer and the
layer of nerve-fibres corresponding morphologically to
the substance of the brain and spinal cord.

The actual development of the retina is not thoroughly
understood. According to the usual statements (Kolliker1) the
layer of ganglion cells and the inner molecular layer are first
differentiated, while the remaining cells give rise to the rest
of the retina proper, and are bounded externally by the membrana
limitans externa. On the inner side of the ganglionic layer the
stratum of nerve-fibres is also very early established. The rods

1 Entwick. d. Menschen, etc.<} 1879.
F, & B. 10

146 THE THIRD DAY. [CHAP.

and cones are formed as prolongations or cuticularizations of the
cells which eventually form the outer granular layer. The layer
of cells external to the molecular layer is not divided till
comparatively late into the inner and outer granular (nuclear)
layers, and the interposed outer molecular layer.

Lowe1 has recently written an elaborate paper on this subject
in which he arrives at very different results from Kolliker
and other observers.

According to him only the outer limbs of the rods and
cones, which he holds to be metamorphosed cells, correspond to
the epithelial layer of the brain.

The changes described above are confined to that
portion of the retina which, lies behind the ora serrata.
In front of this both walls of the cup coalesce as we
have said into a cellular layer in which a deposit of
pigment takes place.

At a very early period a membrane appears on the side of
the retina adjoining the vitreous humour. This membrane is
the hyaloid membrane. It is formed at a time when there is no
trace of mesoblastic structures in the cavity of the vitreous
humour, and must therefore be regarded as a cuticular deposit
of the cells of the optic cup.

The optic nerve. The optic nerves are derived,
as we have said, from the at first hollow stalks of the
optic vesicles. Their cavities gradually become oblite-
rated by a thickening of the walls, the obliteration
proceeding from the retinal end inwards towards the
brain. While the proximal ends of the optic stalks
are still hollow, the rudiments of the optic chiasma
are formed at the roots of the stalks, the fibres of
the one stalk growing over into the attachment of the
other. The decussation of the fibres would appear

1 Archiv fur mikr. Anat. Vol. xv.

VI.] THE CHOROID FISSURE. 147

to be complete. The fibres arise in the remainder of
the nerves somewhat later. At first the optic nerve
is equally continuous with both walls of the optic cup ;
as must of necessity be the case, since the interval
which primarily exists between the two walls is con-
tinuous with the cavity of the stalk. When the cavity
within the optic nerve vanishes, and the fibres of the
optic nerve appear, all connection between the outer
wall of the optic cup and the optic nerve disappears,
and the optic nerve simply perforates the outer wall,
remaining continuous with the inner one.

The choroid fissure. During the third day of incu-
bation there passes in through the choroid slit a vas-
cular loop, which no doubt supplies the transuded
material for the growth of the vitreous humour. Up to
the fifth day this vascular loop is the only structure
passing through the choroid slit. On this day however
a new structure appears, which remains permanently
through life, and is known as the pecten. It consists
of a lamellar process of the mesoblast cells round the
eye, passing through the choroid slit near the optic
nerve, and enveloping part of the afferent branch of
the vascular loop above mentioned. The proximal part
of the free edge of the pecten is somewhat swollen, and
sections through this part have a club-shaped form.
On the sixth day the choroid slit becomes rapidly
closed, so that at the end of the sixth day it is reduced
to a mere seam. There are however two parts of this
seam where the edges of the optic cup have not
coalesced. The proximal of these adjoins the optic
nerve, and permits the passage of the pecten, and at a
later period of the optic nerve ; and the second or distal

10—2

148 THE THIRD DAY. [CHAP.

one is placed near the ciliary edge of the slit, and is
traversed by the efferent branch of the above-men-
tioned vascular loop. This vessel soon atrophies, and
with it the distal opening in the choroid slit completely
vanishes. In some varieties of domestic Fowl (Lieber-
kiihn) the opening however persists. The seam which
marks the original site of the choroid slit is at first con-
spicuous by the absence of pigment, and at a later
period by the deep colour of its pigment. Finally, a
little after the ninth day, no trace of it is to be
seen.

Up to the eighth day the pecten remains as a simple
lamina; by the tenth or twelfth day it begins to be
folded or rather puckered, and by the seventeenth or
eighteenth day it is richly pigmented, and the pucker-
ings have become nearly as numerous as in the adult,
there being in all seventeen or eighteen. The pecten
is now almost entirely composed of vascular coils, which
are supported by a sparse pigmented connective tissue ;
and in the adult the pecten is still extremely vascular.
The original artery which became enveloped at the
formation of the pecten continues, when the latter be-
comes vascular, to supply it with blood. The vein is
practically a fresh development after the atrophy of
the distal portion of the primitive vascular loop of the
vitreous humour.

There are no true retinal blood-vessels.

The permanent opening in the choroid fissure for
the pecten is intimately related to the entrance of the
optic nerve into the eyeball; the fibres of the optic
nerve passing in at the inner border of the pecten,
coursing along its sides to its outer border, and radi-

VI.] THE LENS. 149

ating from it as from a centre to all parts of the
retina.

The lens. This when first formed is somewhat
elliptical in section with a small central cavity of a
similar shape, the front and hind walls being of nearly
equal thickness, each consisting of a single layer of
elongated columnar cells.

In the subsequent growth of the lens, the develop-
ment of the hind wall is of a precisely opposite cha-
racter to that of the front wall. The hind wall becomes
much thicker, and tends to obliterate the central cavity
by becoming convex on its front surface. At the same
time its cells, still remaining as a single layer, become
elongated and fibre-like. The front wall on the con-
trary becomes thinner and thinner and its cells more
and more flattened and pavement-like.

These modes of growth continue until at the end of
the fourth day, as shewn in Fig. 51, the convex hind
wall I comes into absolute contact with the front wall
el and the cavity is thus entirely obliterated. The cells
of the hind wall have by this time become veritable
fibres, which, when seen in section, appear to be arranged
nearly parallel to the optic axis, their nuclei nl being
seen in a row along their middle. The front wall, some-
what thickened at either side where it becomes continu-
ous with the hind wall, is now a single layer of flattened
cells separating the -hind wall of the lens, or as we may
now say the lens itself, from the front limb of the
lens-capsule ; of this it becomes the epithelium.

The subsequent changes undergone consist chiefly in
the continued elongation and multiplication of the lens-
fibres, with the partial disappearance of their nuclei.

150 THE THIRD DAY. [CHAP.

During their multiplication they become arranged
in the manner characteristic of the adult lens.

The lens capsule is probably formed as a cuticular
membrane deposited by the epithelial cells of the lens.
But it should be stated that many embryologists regard
it as a product of the mesoblast.

The vitreous humour. The vitreous humour is a
mesoblastic product, entering the cavity of the optic
cup by the choroid slit just spoken of. It is nourished by
the vascular ingrowths through the choroid slit. Its
exact nature has been much disputed. It arises as a
kind of transudation, but frequently however contains
blood-corpuscles and embryonic mesoblastic cells. It
is therefore intermediate in its character between or-
dinary intercellular substance, and the fluids contained
in serous cavities.

The integral parts of the eye in front of the lens are
the cornea, the aqueous humour, and the iris. The
development of the latter has already been sufficiently
described in connection with the retina, and there re-
main to be dealt with the cornea, and the cavity con-
taining the aqueous humour.

The cornea. The cornea is formed by the coales-
cence of two structures, viz. the epithelium of the
cornea and the cornea proper. The former is directly
derived from the external epiblast, which covers the
eye after the invagination of the lens. The latter is
formed in a somewhat remarkable manner, first clearly
made out by Kessler.

When the lens is completely separated from the epi-
dermis the central part of its outer wall remains directly

VI.] THE CORNEA. 151

in contact with the epidermis (future corneal epithelium).
At its edge there is a small ring-shaped space bounded
by the outer skin, the lens and the edge of the optic cup.
There appears, at about the time when the cavity of
the lens is completely obliterated, a structureless layer
external to the above ring-like space and immediately
adjoining the inner face of the epidermis. This layer,
which forms the commencement of the cornea proper,
at first only forms a ring at the border of the lens,
thickest at its outer edge, and gradually thinning
away towards the centre. It soon however becomes
broader, and finally forms a continuous stratum of con-
siderable thickness, interposed between the external
skin and the lens. As soon as this stratum has
reached a certain thickness, a layer of flattened cells
grows in along its inner side from the mesoblast sur-
rounding the optic cup (Fig. 52, dm). This layer is
the epithelioid layer of the membrane of Descemet1.
After it has become completely established, the meso-
blast around the edge of the cornea becomes divided
into two strata ; an inner one (Fig. 52 cb) destined to
form the mesoblastic tissue of the iris already described,
and an outer one (Fig. 52 cc) adjoining the epidermis.
The outer stratum gives rise to the corneal corpuscles,
which are the only constituents of the cornea not yet
developed. The corneal corpuscles make their way

1 It appears possible that Lieberkiihn may be right in stating
that the epithelium of Descemet 's membrane grows in between the
lens and the epiblast before the formation of the cornea proper, and
that Kessler's account, given above, may on this point require correc-
tion. From the structure of the eye in some of the lower forms it
seems probable that Descemet's membrane is continuous with the
choroid.

THE THIRD DAY, [CHAP.

FIG. 52.

SECTION THROUGH THE EYE OF A FOWL ON THE EIGHTH DAT
OF DEVELOPMENT, TO SHEW THE IRIS AND CORNEA IN THE
PROCESS OF FORMATION. (After Kessler.)

ep. epiblastic epithelium of cornea ; cc. corneal corpuscles growing
into the structureless matrix of the cornea ; dm. Descemet's
membrane ; ir. iris ; cb. mesoblast of the iris (this reference
letter points a little too high).

The space between the layers dm. and ep. is filled with the
structureless matrix of the cornea.

through the structureless corneal layer, and divide it
into two strata, one adjoining the epiblast, and the
other adjoining the inner epithelium. The two strata
become gradually thinner as the corpuscles invade a
larger and larger portion of their substance, and finally
the outermost portion of each alone remains to form
above and below the membrana elastica anterior and
posterior (Descemet's membrane) of the cornea. The
corneal corpuscles, which have grown in from the sides,
thus form a layer which becomes continually thicker,
and gives rise to the main substance of the cornea.

Whether the increase in the thickness of the layer is
due to the immigration of fresh corpuscles, or to the
division of those already there, is not clear. After the

VI.] THE AQUEOUS HUMOUR. 153

cellular elements have made their way into the cornea,
the latter becomes continuous at its edge with the meso-
blast which forms the sclerotic.

The derivation of the original structureless layer of the cornea
is still uncertain. Kessler derives it from the epiblast, but it
appears more probable that Kolliker1 is right in regarding it
as derived from the mesoblast. The grounds for this view are,
(1) the fact of its growth inwards from the border of the meso-
blast round the edge of the eye, (2) the peculiar relations between
it and the corneal corpuscles at a later period. This view would
receive still further support if a layer of mesoblast between the
lens and the epiblast were really present as believed by Lieber-
kiihn. It must however be admitted that the objections to
Kessler's view of its epiblastic nature are rather a priori than
founded on definite observation.

The observations of Kessler, which have been mainly followed
in the above account, are strongly opposed by Lieberkiihn and
other observers, and are not entirely accepted by Kolliker. It
is however especially on the development of these parts in Mam-
malia (to be spoken of in the sequel) that the above authors
found their objections.

The aqueous humour. The cavity for the aqueous
humour has its origin in the ring-shaped space round
the front of the lens, which, as already mentioned, is
bounded by the external skin, the edge of the optic cup,
and the lens. By the formation of the cornea this
space is shut off from the external skin, and on the
appearance of the epithelioid layer of Descemet's
membrane a continuous cavity is developed between
the cornea and the lens. This cavity enlarges and

1 L. Kessler, Zur Entwick. d. Auges d. Wirbelthiere. Leipzig, 1874.
N. Lieberkiihn, " Beitrage z. Anat. d. embryonalen Auges," Archiv
f. Anat. u. Phys., 1879. Kolliker, Entwick. d. Henschen, etc. Leipzig,

1879.

154 THE THIRD DAY. [CHAP.

receives its final form upon the full development of the
iris.

Summary. We may briefly recapitulate the main
facts in the development of the eye as follows.

The eye commences as a lateral outgrowth of the
fore-brain, in the form of a stalked vesicle.

The stalk, becoming narrowed and subsequently
solid, is converted into the optic nerve.

An involution of the superficial epiblast over the
front of the optic vesicle, in the form first of a pit, then
of a closed sac with thick walls, and lastly, of a solid
rounded mass (the small central cavity being entirely
obliterated by the thickening of the hind wall), gives
rise to the lens. Coincidently with this involution of
the lens, the optic vesicle is doubled up on itself, and
its cavity obliterated ; thus a secondary optic vesicle
or optic cup with a thick anterior and a thin posterior
wall is produced. As a result of the manner in which
the doubling up takes place, or of the mode of growth
afterwards, the cup of the secondary optic vesicle is at
first imperfect along its under surface, where a gap, the
choroidal fissure, exists for some little time, but subse-
quently closes up.

The mesoblast in which the eye is imbedded gathers
itself together around the optic cup into a distinct in-
vestment, of which the internal layers become the
choroid, the external the sclerotic. An ingrowth of
this investment between the front surface of the lens
and the superficial epiblast furnishes the body of the
cornea, the epiblast itself remaining as the anterior
corneal epithelium.

The mesoblast entering on the under side through

VI.] THE LACRYMAL DUCT. 155

the choroidal fissure gives rise to the vitreous humour,
while at a later stage a definite process of this meso-
blast becomes the pecten.

Of the walls of the optic cup, the thinner outer
(posterior) wall becomes, behind the line of the ora
serrata, the pigment-epithelium of the choroid, while
the thicker inner (anterior) wall supplies all the ele-
ments of the retina, including the rods and cones which
grow out from it into the pigment-epithelium.

In front of the line of the ora serrata, both walls of
the optic cup, quite thin and wholly fused together, give
rise to the pigment- epithelium of the ciliary processes
and iris, the bodies of both these organs being formed
from the mesoblastic investment.

Accessory Organs connected with the Eye.

Eyelids. The most important accessory structures connected
with the eye are the eyelids. They are developed as simple folds
of the integument with a mesoblastic prolongation between their
two laminae. They are three in number, viz. an upper and lower,
and a lateral one — the nictitating membrane — springing from
the inner or anterior border of the eye. Their inner face is lined
by a prolongation of conjunctiva, which is the modified epiblast
covering the cornea and part of the sclerotic.

The Lacrymal glands and Lacrymal duct.

The lacrymal glands are formed as solid ingrowths of the
conjunctival epithelium. They appear on the eighth day of
incubation.

The lacrymal duct begins as a solid ridge of the epidermis,
projecting inwards along the line of the so-called lacrymal groove,
from the eye to the nasal pit.

At the end of the sixth day this ridge begins to be separated
from the epidermis, remaining however united with it on the
inner side of the lower eyelid.

156 THE THIRD DAY. [CHAP.

After it has become free, it forms a solid cord, the lower end
of which unites with the wall of the nasal cavity. The cord
so formed gives rise directly to the whole of the duct proper and
to the lower branch of the collecting tube. The upper branch of
the collecting tube is formed as an outgrowth from it. A lumen
begins to be formed in it on the twelfth day of incubation, and first
appears at the nasal end. It arises as a space amongst the cells
of the cord, but is not due to an absorption of the central cells1.

Organ of hearing. During the second day the ear
first made its appearance on either side of the hind-
brain as an involution of the external epiblast, thrust
down into the mass of mesoblast rapidly developing
between the epiblast of the skin and that of the neural

FIG. 53.

£P

SECTION THROUGH THE HEAD OF AN ELASMOBRANCH EMBRYO,
AT THE LEVEL OF THE AUDITORY INVOLUTION.

aup. auditory pit ; aun. ganglion of auditory nerve ; iv.v. roof
of fourth ventricle ; a.c.v. anterior cardinal vein ; aa. aorta ;

1 G. Born: "Die Nasenhohlen u. Thranennasengang d. amnioten
Wirbelthiere, i. Lacertilia n. Aves." Morphologisches Jahrbuch, Vol.
v., 1879.

VI-]

THE EAR.

157

Laa. aortic trunk of mandibular arch ; pp. head cavity of
mandibular arch ; Ivc. alimentary pouch which will form the
first visceral cleft ; Th. rudiment of thyroid body.

canal (Fig. 27, au. £>.). It then had the form of a
shallow pit with a widely open mouth, similar in form
to that shewn for an embryo dog-fish in Fig. 53, au. p.
Before the end of the third day, its mouth closes up and
all signs of the opening are obliterated. The pit thus
becomes converted into a closed vesicle, lined with
epiblast, and surrounded by mesoblast. This vesicle is
the otic vesicle, whose cavity rapidly enlarges while its
walls become thickened (Fig. 54, (7(7).

cc

AOA

SECTION THROUGH THE HlND-BRAIN OF A CHICK AT THE END
OF THE THIRD DAY OF INCUBATION.

IV. Fourth ventricle. The section shews the very thin roof and
thicker sides of the ventricle.

158 THE TH1KD DAY. [CHAP.

Ch. Notochord — (diagrammatic shading).

CV. Anterior cardinal or jugular vein.

CO. Involuted auditory vesicle. CO points to the end which
will form the cochlear canal. RL. Recessus labyrinthi. hy.
hypoblast lining the alimentary canal, hy is itself placed in
the cavity of the alimentary canal, in that part of the canal
which will become the throat. The lower (anterior) wall of
the canal is not shewn in the section, but on each side are
seen portions of a pair of visceral arches. In each arch
is seen the section of the aortic arch AOA belonging to the
visceral arch. The vessel thus cut through is running
upwards towards the head, being about to join the dorsal
aorta AO. Had the section been nearer the head, and
carried through the plane at which the aortic arch curves
round the alimentary canal to reach the mesoblast above it,
AOA and AO would have formed one continuous curved
space. In sections lower down in the back the two aorta,
AO, one on either side, would be found fused into one median
canal.

The changes by which this simple otic vesicle is
converted into the complicated system of parts known
as the internal ear, have been much more completely
worked out for Mammals than for Birds. We shall
therefore reserve a full account of them for a later
portion of this work. Meanwhile a brief statement of
the essential nature of the changes may be useful ; and
will be most conveniently introduced here.

The internal ear consists essentially of an inner
membranous labyrinth lying loosely in and only partially
attached to an outer osseous labyrinth.

The membranous labyrinth (Fig. 55) consists of two
parts : (1) the vestibule, with which are connected three
pairs of semicircular canals, pag', fr, hor ', and a long
narrow hollow process, the aqueductus or recessus vesti-

VI.]

A.

THE EAR,
FIG. 55.

159

13.

T mr

TWO VIEWS OF THE MEMBRANOUS LABYRINTH OF COLUMBA

DOMESTICA (copied from Hasse).
A, from the exterior, B, from the interior.

hor'. horizontal semicircular canal, hor. ampulla of ditto, pag'. pos-
terior vertical semicircular canal, pag. ampulla of ditto,
//. anterior vertical semicircular canal, fr. ampulla of ditto,
u. utriculus, ru. recessus utriculi, v. the connecting tube
between the ampulla of the anterior vertical semicircular
canal and the utriculus, de. ductus endolymphaticus (recessus
vestibuli), s. sacculus hemisphericus, cr. canalis reuniens, lag.
lagena, mr. membrane of Reissner, pb. Basilar membrane.

buli, and (2) the ductus cochlearis, which in birds is a
flask-shaped cavity slightly bent on itself, the dilated
end of which is called the lagena. The several parts of
each of these cavities freely communicate, and the two
are joined together by a narrow canal, the canalis re-
uniens, cr.

The osseous labyrinth has a corresponding form,
and may be similarly divided into parts : into a bony
vestibule, with its bony semicircular canals and recessus

160 THE THIRD DAY. [CHAP.

vestibuli, and into a bony cochlea; but the junction
between the cochlea and the bony vestibule is much
wider than the membranous canalis reuniens.

The cavity of the osseous cochlea is partially divided
lengthways by the ductus cochlearis into a scala tym-
pani and a scala vestibuli, which do not however extend
to the lagena.

The auditory nerve, piercing the osseous labyrinth
in various points, is distributed in the walls of the mem-
branous labyrinth.

All these complicated structures are derived from
the simple primary otic vesicle and the surrounding
mesoblast by changes in its form and differentiation of
its walls. . All the epiblast of the vesicle goes to form
the epithelium of the membranous labyrinth, whose
cavity, filled with endolymph, represents the original
cavity which was first open to the surface but subse-
quently covered in. It gradually attains its curiously
twisted form by a series of peculiar processes of unequal
growth in the, at first, simple walls of the vesicle. The
corium of the membranous labyrinth, and all the tissues
of the osseous labyrinth, are developed out of the meso-
blastic investment of the vesicle. The space between
the osseous and membranous labyrinths, including the
scala vestibuli and scala tympani, may be regarded as
essentially a series of lymphatic cavities hollowed out
in the mesoblast.

It will be seen then that the ear, while resembling
the eye in so far as the peculiar structures in which the
sensory nerve in each case terminates are formed of
involuted epiblast, differs from it inasmuch as it arises
by an independent involution of the superficial epiblast,

VI.] THE OLFACTORY ORGAN. 161

whereas the eye is a constricted portion of the general
involution which gives rise to the central nervous
system.

The origin of the auditory nerve has already been
described. It is shewn in close contact with the walls
of the auditory pit in Fig. 53.

Organ of Smell. The organ of smell makes its ap-
pearance during the third day, as two depressions or
pits, on the under surface of the head, a little in front
of the eye (Fig. 56, N).

FIG. 56.

ot

HEAD OP AN EMBRYO CHICK OF THE THIRD DAY VIEWED

SIDEWAYS AS AN OPAQUE OBJECT.

(Chromic acid preparation.)

C.H. Cerebral hemispheres. F.B. Vesicle of third ventricle.
M.B. Mid-brain. Cb. Cerebellum. H.B. Medulla ob-
longata.

N. Nasal pit. ot. otic vesicle in the stage of a pit with the open-
ing not yet closed up. op. Optic vesicle, with I. lens and
ch.f. choroidal fissure. The superficial epiblast moulds
itself to the form of the optic vesicle and the lens ; hence
the choroidal fissure, though formed entirely underneath the
superficial epiblast, is distinctly visible from the outside.

1 F. The first visceral fold; above it is seen a slight indication of
the superior maxillary process.

2, 3, 4 F. Second, third and fourth visceral folds, with the vis-
ceral clefts between them.
F.&B. 11

162 THE THIRD DAY. [CHAP.

Like the lens and the labyrinth of the ear, they are
formed from the external epiblast; unlike them they
are never closed up.

The olfactory nerves arise as outgrowths of the front
end of the cerebral hemispheres, before any trace of a
special division of the brain, forming an olfactory lobe,
has become established. Their peripheral extremities
unite with the walls of the olfactory pits during the
third day. The olfactory lobes arise as outgrowths of
the cerebral hemispheres on the seventh day of incuba-
tion.

Visceral Arches and Visceral Clefts. It must be
borne in mind that, especially in the early stages of
development, owing to the very unequal growth of
different parts, the relative position of the various
structures is continually shifting. This is very well
seen in the instance of the heart. At its first appear-
ance, the heart is lodged immediately beneath the
extreme front of the alimentary canal, so far forwards
as to underlie that portion of the medullary canal which
will form the brain. It is, in fact, at that epoch a part
of the head. From that early position it gradually
recedes farther and farther backward, until, at the end
of the third day, a considerable interval is observed
between it and the actual head. In other words, a
distinct neck has been formed, in which most important
changes take place.

The neck is distinguished from the trunk in which
the heart now lies by the important feature that in it
there is no cleavage of the mesoblast into soinatopleure
and splanchnopleure, and consequently no pleuroperito-
neal cavity. In passing from the exterior into the ali-

VI.] THE VISCERAL CLEFTS. 163

mentary canal, the three layers of the blastoderm are
successively traversed, without any breach of continuity,
save such as is caused by the cavities of the blood-
vessels. In this neck, so constituted, there appear on
the third day certain fissures or clefts, the visceral or
branchial clefts. These are real clefts or slits passing
right through the walls of the throat, and are placed in
series on either side across the axis of the alimentary
canal, lying not quite at right angles to that axis and
parallel to each other, but converging somewhat to the
middle of the throat in front (Fig. 56). Viewed from
the outside in either fresh or preserved embryos they
are not very distinctly seen to be clefts ; but when they
are seen from within, after laying open the throat, their
characters as elongated oval slits can easily be recog-
nised.

Four in number on either side, the most anterior is
the first to be formed, the other three following in suc-
cession. Their formation takes place from within out-
wards. The hypoblast is pushed outwards as a pouch,
which grows till it meets the epiblast, which is then
broken through, while the hypoblast forms a junction
with the epiblast at the outside of the throat.

No sooner has a cleft been formed than its anterior
border (i.e. the border nearer the head) becomes raised
into a thick lip or fold, the visceral or branchial fold.
Each cleft has its own fold on its anterior border, and in
addition the posterior border of the fourth or last visceral
cleft is raised into a similar fold. There are thus five
visceral folds to four visceral clefts (Fig. 56). The last
two folds however, and especially the last, are not nearly
so thick and prominent as the other three, the second

11—2

164 THE THIRD DAY. [CHAP.

being the broadest and most conspicuous of all. The
first fold meets, or nearly meets, its fellow in the middle
line in front, but the second falls short of reaching the
middle line, and the third, fourth and fifth do so in an
increasing degree. Thus in front views of the neck a
triangular space with its apex directed towards the
head is observed between the ends of the several folds.

Into this space the pleuroperitoneal cavity extends,
the somatopleure separating from the splanchnopleure
along the ends of the folds ; and it is here that the
aorta plunges into the mesobkst of the body.

The visceral clefts and arches to a large extent dis-
appear in the adult, and constitute examples of an inte-
resting class of embryonic organs, whose presence is
only to be explained by the fact that, in the ancestors of
the types in which they are now developed in the
embryo, they performed an important function in the
adult. The visceral arches and clefts are in fact the
homologues of the branchial arches and branchial clefts
of Fishes, which continue to be formed in the embryos
of the higher vertebrate types, although they have
ceased to serve as organs of respiration. The skeletal
structures developed in the visceral arches persist as
the jaw-bones and hyoid bone, but the clefts, with the
exception of the first, become obliterated.

Of the history of the skeletal elements we shall
speak in detail hereafter; meanwhile we may briefly
deal with the general history of these parts.

The first fold on either side, increasing rapidly in
size and prominence, does not, like the others, remain
single, but sends off in the course of the third day a
branch or bud-like process from its anterior edge. This

VI.] THE VISCERAL ARCHES. 165

branch, starting from near the dorsal beginning of the
fold, runs ventralwards and forwards, tending to meet the
corresponding branch from the fold on the other side, at
a point in the middle line nearer the front of the head
than the junction of the main folds. The two branches
do not quite meet, being separated by a median process,
which at the same time grows down from the extreme
front of the head, and against which they abut. Between
the main folds, which are directed somewhat backwards
and the branches which slant forwards, a somewhat
lozenge-shaped space is developed which, as the folds
become more and more prominent, grows deeper and
deeper. In the main folds are developed the man-
dibles, and in the branches the superior maxillce : the
lozenge-shaped cavity between them is the cavity of the
mouth, and the descending process which helps to
complete the upper margin of this cavity is called, from
the parts which will be formed out of it, the fronto-
nasal process.

Part of the mesoblast of the two succeeding pairs of
visceral folds is transformed into the hyoid bone, which
will be best considered in connection with the develop-
ment of the skull. The two last arches disappear with-
out giving rise to any permanent structures.

With the exception of the first the visceral clefts
become obliterated at an early stage of embryonic life ;
but the first persists, although it loses all trace of its
original branchial function and becomes intimately con-
nected with the organ of hearing, of which in fact it
forms a most essential part, becoming converted into
the Eustachian tube and tympanic cavity. The outer
opening and the outer part also of the cleft become

166 THE THIRD DAY. [CHAR

obliterated at an early date, but from the inner part of
the cleft a diverticulum is given off towards the ex-
terior, which becomes the tympanic cavity. The inner
part of the cleft itself forms the Eustachian tube, while
its mouth forms the oral aperture of this tube.

The meatus auditorius externus first appears as a
shallow depression at the region where the closure of
the first visceral cleft takes place. It is in part formed
by the tissue surrounding this depression growing up in
the form of a wall, but the blind end of the meatus also
becomes actually pushed in towards the tympanic
cavity.

The tympanic membrane is derived from the tissue
which separates the meatus auditorius externus from
the tympanic cavity. This tissue is obviously consti-
tuted of an hypoblastic epithelium on its inner aspect,
an epiblastic epithelium on its outer aspect, and a layer
of mesoblast between them, and these three layers give
rise to the three layers of which this membrane is
formed in the adult. During the greater part of foetal
life it is relatively very thick, and presents a structure
bearing but little resemblance to that in the adult.

The tympanic cavity is bounded on its inner aspect
by the osseous investment of the internal ear, but at
two points, known as the fenestra ovalis and fenestra
rotunda, the bone is deficient and its place is taken by
a membrane.

These two fenestrse appear early, and are probably
formed by the nonchrondrification of a -small area of
the embryonic cartilage. The upper of the two, or
fenestra ovalis, contains the base of a bone, known as
the columella. The main part of the columella is

VJ,] THE AORTIC ARCHES. 167

formed of a stalk which is held by Parker to be derived
from part of the skeleton of the visceral arches, while
the base, forming the stapes, appears to be an inde-
pendent formation.

The stalk of the columella extends to the tympanic
membrane; its outer end becoming imbedded in this
membrane, and serving to transmit the vibrations of
the membrane to the fluid in the internal ear.

Vascular system. By the end of the second day
three pairs of aortic arches had been established in
connection with the heart. When the visceral folds
and clefts are formed, a definite arrangement between
them and the aortic arches is always observed. The
first visceral cleft runs between the first and second
aortic arches. Consequently the first aortic arch runs
in the first visceral fold, and the second in the second.
In the same way, the second visceral cleft lies between
the second and third aortic arches, the third aortic arch
running in the third visceral fold. Each aortic arch runs
in the thickened mesoblast of the corresponding fold.

Arrived at the dorsal surface of the alimentary canal,
these arches unite at acute angles to form a common
trunk, the dorsal aorta (Fig. 57, A.0\ which runs along
the back immediately under the notochord. The length
of this common single trunk is not great, as it soon
divides into two main branches, each of which, after
giving off the large vitelline artery, Of. A., pursues its
course with diminished calibre to the tail, where it is
finally lost in the capillaries of that part.

The heart is now completely doubled up on itself.
Its mode of curvature is apparently somewhat compli-
cated. Starting from the point of junction of the vitel-

168 THE THIRD DAY. [CHAP.

Fia 57.
IMA. E.CA.

DIAGRAM OF THE ARTERIAL CIRCULATION ON THE
THIRD DAY.

1, 2, 3. The first three pairs of aortic arches. A. The vessel
formed by the junction of the three pairs of arches. A.O.
Dorsal aorta formed by the junction of the two branches A
and A ; it quickly divides again into two branches which
pass down one on each side of the notochord. From each of
these is given off a large branch Of. A., the vitelline artery.
E.CA, LCA, external and internal carotid arteries.

line veins (Fig. 37, Ht), there is first a slight curvature
towards the left; this is followed by a turn to the right,
and then the heart is completely bent on itself, so that
afterwards it pursues a course directed from behind
quite straight forwards (except perhaps for a little incli-
nation to the left) to the point where the aortic arches
branch off. In this way, as shewn in section in Fig. 59, A,
the end of the bulbus arteriosus (v) comes to lie just
underneath (or in front of according to the position of

VI.] THE HEART. 169

the embryo) that part which has already been marked
off by the lateral bulgings as the auricular portion (au).

That part of the heart which is turned to the right,
including the point of doubling up, is the ventricular
portion, and is even at this stage separated from the
auricular portion by a slight neck. This external con-
striction corresponds to an internal narrowing of the
lumen of the heart, and marks the position of the future
canalis auric u laris.

The ventricular portion is, on the other hand, like-
wise separated by a fainter constriction from the ante-
rior continuation of the heart which forms the bulbus
arteriosus. The projecting part where the doubling
takes place is at this stage still quite round ; we shall
see that later on it becomes pointed and forms the apex
of the heart.

The whole venous portion of the heart (if we may so
speak of it, though of course at this stage blood of the
same quality passes right along the whole cardiac canal)
lies in a plane which is more dorsal than the arterial por-
tion. The point at which the venous roots of the heart,
i.e. the two vitelline trunks, unite into a single canal, is
on this day carried farther and farther away from the
heart itself. By the end of the day there is a consider-
able distance between the auricular portion of the actual
heart and the point where the venous roots separate,
each to pursue its course along the splanchnopleure-fold
of its own side. This distance is traversed by a single
venous trunk, of which the portion close to the auricles
is called the sinus venosus, and the more distant the
ductus venosus. We shall give to the whole trunk the
name used by the older observers, the meatus venosus.

170 THE THIRD DAY.

Small arteries to various parts of the body are now
being given off by the aorta and its branches. The
capillaries in which these end are gathered into veins
which unite to form two main trunks on either side, the
cardinal veins, anterior and posterior (Fig. 36, Fig. 58

FIG. 58.

DIAGRAM OF THE VENOUS CIRCULATION ON THE

THIRD DAY.

H. Heart. J. Jugular or anterior cardinal vein. C. Inferior
or posterior cardinal vein. Of. Vitelline vein. dc. Ductus
Cuvieri.

J and (7), which run parallel to the long axis of the body
in the upper part of the mesoblast, a little external to
the mesoblastic somites. These veins, which do not
attain to any great importance till well on in the third
day, unite opposite to the heart, on each side, into a
short common trunk at right angles to themselves.
The two short trunks thus formed, which bear the
name of ductus Cuvieri (Fig. 36, Fig. 58, dc), running
ventralwards and then transversely straight inwards
towards the middle line fall into the sinus venosus.

The two ductus Cuvieri pass from the heart to the
body walls in a special horizontal mesentery, whose for-
mation and function we shall return to in speaking of
the formation of the pericardial cavity. The position of
one of them is shewn in section in Fig. 59 B, dc.

VI.]

THE TAIL-FOLD.
FIG. 59.

171

B.

TRANSVERSE SECTIONS THROUGH A CHICK EMBRYO WITH
TWENTY-ONE MESOBLASTIC SOMITES TO SHEW THE FOR-
MATION OF THE PERICARDIAL CAVITY, A. BEING THE

ANTERIOR SECTION.

pp. body cavity ; pc. pericardial cavity ; al. alimentary cavity ;
au. auricle ; v. ventricle ; sv. sinus venosus ; dc. ductu&
Cuvieri ; ao. aorta ; mp. muscle-plate ; me. medullary cord.

The alimentary canal. As we stated above, the
folding in of the splanchnopleure to form the alimentary
canal is proceeding with great rapidity, the tail-fold as
well as the head-fold contributing largely to this result.

The formation of the tail-fold is very similar to that
of the head-fold. The tail is a solid, somewhat curved,,
blunt cone of mesoblast, immediately coated with the

172 THE THIED DAY. [CHAP.

superficial epiblast except at the upper surface (corre-
sponding to the back of the embryo), where lies the
pointed termination of the neural tube.

So rapid is the closure of the splanchnopleure both
in front and behind, that two of the three parts into
which the digestive tract may be divided, are brought,
on this day, to the condition of complete tubes.

The first division, including the region from the
mouth to the duodenum, is completely folded in by the
end of the day; so likewise is the third division com-
prising the large intestine and the cloaca. The middle
division, corresponding to the future small intestine,
still remains quite open to the yolk-sac below.

The attachment of the newly formed alimentary
canal to the body above is at first very broad, and only
a thin stratum of mesoblast separates the hypoblast of
the canal from the notochord and mesoblastic somites;
even that maybe absent under the notochord. During the
third day, however, along such portions of the canal as
have become regularly enclosed, i.e. the hinder division
and the posterior moiety of the anterior division, the
mesoblastic attachment becomes narrower and (in a ver-
tical direction) longer, the canal appearing to be drawn
more ventralwards (or according to the position of the
embryo forwards), away from the vertebral column.

In what may be regarded as the pleural division of
the general pleuroperitoneal space, along that part of
the alimentary canal which will form the oesophagus,
this withdrawal is very slight (Fig. 59), but it is very
marked in the peritoneal space. Here such parts of the
digestive canal as are formed come to be suspended from
the body above by a narrow flattened band of mesoblas-

VI.] THE (ESOPHAGUS. 173

tic tissue which reaches from the neighbourhood of the
notochord, and becomes continuous with the mesoblas-
tic coating which wraps round the hypoblast of the
canal. This flattened band is the mesentery, shewn
commencing in Fig. 65, and much more advanced in
Fig. 68, M. It is covered on either side by a layer of
flat cells forming the epithelioid lining of the peritoneal
membrane, while its interior is composed of indifferent
tissue.

The front division of the digestive tract consists of
three parts. The most anterior part, the oesophagus,
still ending blindly in front reaches back as far as the
level of the hind end of the heart ; and is divided into
two regions, viz. an anterior region, characterized by the
presence of the visceral clefts, whose development has
already been dealt with, and a posterior region without
such clefts.

Its transverse section, which up to the end of the
second day was somewhat crescent-shaped, with the
convexity downwards, becomes on this day more nearly
circular. Close to its hinder limit, the lungs (Fig. 60,
Ig), of whose formation we shall speak directly, take
their origin.

The portion of the digestive canal which succeeds
the oesophagus, becomes towards the close of the third
day somewhat dilated (Fig. 60, St) ; the region of the
stomach is thus indicated.

The hinder or pyloric end of the stomach is separated
by a very small interval from the point where the com-
plete closing in of the alimentary canal ceases, and where
the splanchnopleure-folds spread out over the yolk,
This short tract is nevertheless clearly marked out as

174

THE THIRD DAY.

FIG. 60.

[CHAP.

DIAGRAM

OF A PORTION OF THE DIGESTIVE TRACT OF A
CHICK UPON THE FOURTH DAY.
(Copied from Gotte.)

The black inner line represents the hypoblast, the outer shading
the mesoblast. Ig. lung-diverticulum with expanded termi-
nation, forming the primary lung-vesicle. St. stomach.
I. two hepatic diverticula with their terminations united by
cords of hypoblast cells, p. diverticulum of the pancreas
with the vesicular diverticula coming from it.

the duodenum by the fact that from it, as we shall
presently point out, the rudiments of the ducts of the
liver and pancreas are beginning to be formed.

The posterior division of the digestive tract, cor-
responding to the great intestine and cloaca, is from
its very first formation nearly circular in section and
of a larger bore than the oesophagus.

During part of the third day the hinder end of this
section of the gut is in communication with the neural
tube by the neur enteric canal already spoken of (Fig.
61, ne). The communication between the two tubes

VI.]

THE PKOCTOD^EUM
FIG. 61.

175

DIAGRAMMATIC LONGITUDINAL SECTION THROUGH THE POS-
TERIOR END OF AN EMBRYO BIRD, AT THE TIME OF THE
FORMATION ON THE ALLANTOIS.

ep. epiblast; Sp.c. spinal canal ; ch. notochord ; n.e. neurenteric
canal ; hy. hypoblast ; p.a.g. postanal gut ; pr. remains of
primitive streak folded in on the ventral side ; al. allantois ;
me. mesoblast ; an. point where anus will be formed ; p.c.
perivisceral cavity ; am. amnion ; so. somatopleure ; sp.
splanchnopleure.

does not last long, but even after its rupture there re-
mains a portion of the canal continuous with the gut ;
this, however, constitutes a purely embryonic and tran-
sient section of the alimentary canal, and is known
as the postanal gut. Immediately in front of it is a
deep infolding of the epiblast, which nearly meets the
hypoblast (Fig. 61, an) and forms the rudiment of the
anus and of the outer section of the cloaca into which
the bursa Fabricii opens in the adult. It is known to
embryologists as the proctodceum, but does not open
into the alimentary tract till considerably later. The

176 THE THIKD DAY. [CHAP.

section of the alimentary tract immediately in front of
the postanal gut is somewhat enlarged, and becomes the
inner section of the adult cloaca receiving the urinary
and genital ducts. The allantois, to whose develop-
ment we shall return directly, opens into it ventrally.

It is to be noted that the two sections of the cloaca
of adult birds have a different origin. The inner section
being part of the primitive alimentary tract and lined by
hypoblast ; the outer being part of an involution of the
outer skin and lined by epiblast.

The lungs are in their origin essentially buds or
processes from the primitive oesophagus.

At a point immediately behind the region of the
visceral clefts the cavity of the alimentary canal be-
comes compressed laterally, and at the same time con-
stricted in the middle so that its transverse section (Fig.
62, 1) is somewhat hourglass-shaped, and shews an upper
or dorsal chamber d, joining on to a lower or ventral
chamber I by a short narrow neck.

The hinder end of the lower tube enlarges (Fig. 62,
2), and then becomes partially divided into two lobes
(Fig. 62, 3). All these parts at first freely communicate,
but the two lobes behind, partly by their own growth,
and partly by a process of constriction, soon become
isolated posteriorly (Fig. 60, lg)\ while in front they
open into the lower chamber of the oesophagus.

By a continuation forwards of the process of con-
striction the lower chamber of the oesophagus, carrying
with it the two lobes above mentioned, becomes gradu-
ally transformed into an independent tube, opening in
front by a narrow slit-like aperture into the oesophagus.
The single tube in front is the rudiment of the trachea

VI.] THE LUNGS. 177

and larynx, while the two diverticula behind (Fig. 60,
Ig) become the bronchial tubes and lungs.

While the above changes are taking place in the
hypoblastic walls of the alimentary tract, the splanchnic

FOUR DIAGRAMS ILLUSTRATING THE FORMATION OF THE
LUNGS. (After Gotte.)

a. mesoblast ; b. hypoblast ; d. cavity of digestive canal ; I.
cavity of the pulmonary diverticulum.

In (1) the digestive canal has commenced to be constricted
into a dorsal and ventral canal ; the former the true alimentary
canal, the latter the pulmonary tube ; the two tubes communi-
cate with each other in the centre.

In (2) the ventral (pulmonary) tube has become expanded.

In (3) the expanded portion of the tube has become con-
stricted into two tubes, still communicating with each other and
with the digestive canal.

In (4) these are completely separated from each other and
from the digestive canal, and the mesoblast has also begun to
exhibit externally changes corresponding to the internal changes
which have been going on.

F. &B. 12

178 THE THIRD DAY. [CHAP.

mesoblast surrounding these structures becomes very
much thickened; but otherwise bears no marks of the
internal changes which are going on, so that the above
formation of the lungs and trachea cannot be seen from
the surface. As the paired diverticula of the lungs grow
backwards, the mesoblast around them takes however
the form of two lobes, into which they gradually bore
their way.

The further development of the lungs is, at first,
essentially similar to that of a racemose gland. From
each primitive diverticulum numerous branches are
given off. These branches, which are mainly confined
to the dorsal and lateral parts, penetrate into the sur-
rounding mesoblast and continue to give rise to second-
ary and tertiary branches. At right angles to the
finest of these the arborescent branches so charac-
teristic of the avian lung are given off. In the meso-
blast around them numerous capillaries make their
appearance.

The air sacs, which form such important adjuncts
of the avian lungs, are the dilated extremities of
the primary pulmonary diverticula and of their main
branches.

The whole pulmonary structure is therefore the
result of the growth by budding of a system of branched
hypoblastic tubes in the midst of a mass of mesoblastic
tissue, the hypoblastic elements giving rise to the epi-
thelium of the tubes and the mesoblast providing the
elastic, muscular, cartilaginous, connective and other
tissues of the tracheal and bronchial walls.

The liver is the first formed chylopoietic appendage
of the digestive canal, and arises between the 55th and

VI.] THE LIVER. 179

60th hour as a couple of diverticula one from either
side of the duodenum immediately behind the stomach
(Fig. 60, I). These diverticula are of course lined by
hypoblast. The right one is, in all cases, from the first
longer, but of smaller diameter than the left. Situated
a little behind the heart, they embrace between them
the two vitelline veins forming the roots of the meatus
venosus.

The diverticula soon give rise to numerous hollow
branches or processes, which extend into the surround-
ing mesoblast.

Towards the end of the third day there may further
be observed in the greatly thickened mesoblastic invest-
ment of either diverticulum a number of cylindrical
solid cords of hypoblast which are apparently out-
growths from the hypoblast of the branches of the di-
verticula. These cylinders rapidly increase in number,
apparently by a process of sprouting, and their some-
what swollen peripheral extremities come into contact
and unite. And thus, about the ninetieth hour, a sort
of network of solid thick strings of hypoblastic cells is
formed, the mesoblast in the meshes of the network
becoming at the same time largely converted into
blood-vessels. Each diverticulum becomes in this way
surrounded by a thick mass composed partly of solid
cylinders, and to a less extent of hollow processes, con-
tinuous with the cylinders on the one hand, and the
main diverticulum on the other, all knit together with
commencing blood-vessels and unchanged mesoblastic
tissue. Between the two masses runs the now fused
roots of the meatus venosus with which the blood-
vessels in each mass are connected.

12—2

180 THE THIRD DAY. [CHAP.

Early on the fourth day each mass sends out ventral
to the meatus venosus a solid projection of hypoblas-
tic cylinders towards its fellow, that from the left side
being much the longest. The two projections unite
and form a long solid wedge, which passes obliquely
down from the right (or from the embryo lying on its
left side, the upper) mass to the left (or lower) one. In
this new wedge may be seen the same arrangement of a
network of hypoblastic cylinders filled in with vascular
mesoblast as in the rest of the liver. The two original
diverticula with their investing masses represent respec-
tively the right and left lobes of the liver, and the wedge-
like bridge connecting them is the middle lobe.

During the fourth and fifth days the growth of the
solid, lobed liver thus formed is very considerable; the
hypoblastic cylinders multiply rapidly, and the network
formed by them becomes very close, the meshes contain-
ing little more than blood-vessels. The hollow processes
of the diverticula also ramify widely, each branch being
composed of a lining of hypoblast enveloped in a coating
of spindle-shaped mesoblastic cells. The blood-vessels
are in direct connection with the meatus venosus — have
become, in fact, branches of it. It may soon be observed,
that in those vessels which are connected with the pos-
terior part of the liver (Fig. 74), the stream of blood is
directed from the meatus venosus into the network of
the liver. In those connected with the anterior part the
reverse is the case ; here the blood flows from the liver
into the meatus venosus. The thick network of solid
cylinders represents the hepatic parenchyma of the adult
liver, while the hollow processes of the diverticula are
the rudiments of the biliary ducts; and we may suppose

VI.] THE PANCREAS. 181

each solid cylinder to represent a duct with its lumen
almost, but perhaps not quite, completely obliterated.

During the fifth day, a special sac or pouch is deve-
loped from the right primary diverticulum. This pouch,
consisting of an inner coat of hypoblast, and an outer of
mesoblast, is the rudiment of the gall-bladder.

The Pancreas arises nearly at the same time as the
liver in the form of an almost solid outgrowth from the
dorsal side of the intestine nearly opposite but slightly
behind the hepatic outgrowths (Fig. 60, p). Its blind
end becomes somewhat enlarged and from it numerous
diverticula grow out into the passive splanchnic meso-
blast.

As the ductules grow longer and become branched,
vascular processes grow in between them, and the whole
forms a compact glandular body in the mesentery on
the dorsal side of the alimentary tract. The primitive
outgrowth elongates and assumes the character of a duct.

On the sixth day a new similar outgrowth from
the duodenum takes place between the primary diver-
ticulum and the stomach. This, which ultimately
coalesces with its predecessor, gives rise to the second
duct, and forms a considerable part of the adult pan-
creas. A third duct is formed at a much later period.

The Thyroid body. The thyroid body arises at the end of
the second or beginning of the third day as an outgrowth from
the hypoblast of the ventral wall of the throat opposite the
point of origin of the anterior aortic arch. It has at first the
form of a groove extending forwards up to the future mouth, and
in its front part extending ventrally to the epiblast. It has not
been made out whether the whole groove becomes converted into
the permanent thyroid. By the fourth day it becomes a
solid mass of cells, and by the fifth ceases to be connecter!

182 THE THIED DAY. [CHAP,

with the epithelium of the throat, becoming at the same time
bilobed. By the seventh day it has travelled somewhat back-
wards, and the two lobes have completely separated from each
other. By the ninth day the whole is invested by a capsule of
connective tissue, which sends in septa dividing it into a number
of lobes or solid masses of cells, and by the sixteenth day its two
lobes are composed of a number of follicles, each with a 'mein-
brana propria,' and separated from each other by septa of con-
nective tissue, much as in the adult x.

The spleen. Although the spleen cannot be reckoned
amongst the glands of the alimentary tract its development may
conveniently be dealt with here. It is formed shortly after the
first appearance of the pancreas, as a thickening of the me-
sentery of the stomach (mesogastrium) and is therefore entirely
a mesoblastic structure. The mass of mesoblast which forms
the spleen becomes early separated by a groove on the one side
from the pancreas and on the other from the mesentery. Some
of its cells become elongated, and send out processes which,
uniting with like processes from other cells, form the trabecular
system. From the remainder of the tissue are derived the cells
of the spleen pulp, which frequently contain more than one
nucleus. Especial accumulations of these take place at a later
period to form the so-called Malpighian corpuscles of the spleen.

The Allantois. We have already had occasion to
point out that the allantois is essentially a diverticulum
of the alimentary tract into which it opens immediately
in front of the anus. Its walls are formed of vascular
splanchnic mesoblast, within which is a lining of hypo-
blast. It becomes a conspicuous object on the third
day of incubation, but its first development takes place
at an earlier period, and is intimately connected with
the formation of the posterior section of the gut.

At the time of the folding in of the hinder end of

1 Miiller Ueber die Entwiclcelung der Schilddruse. Jenaischa
Zeitschrift, 1871.

VI.] THE ALLANTOIS. 183

the gut the splitting of the mesoblast into somatopleure
and splanchnopleure has extended up to the border of
the hinder division of the primitive streak. The ventral
wall of what we have already termed the postanal
section of the alimentary tract is formed by the primi-
tive streak. Immediately in front of this is the involu-
tion which forms the proctodseum; while the wall of
the hindgut in front of the proctodseum owes its origin
to a folding in of the splanchnopleure.

The allantois first appears as a narrow diverticulum
formed by a special fold of the splanchnopleure just in
front of the proctodaeum. This protuberance arises, how-
ever, before the splanchnopleure has begun to be tucked
in so as to form the ventral wall of the hindgut ; and it
then forms a diverticulum (Fig. 63 A, All) the open
end of which is directed forward, while its blind end
points somewhat dorsalwards and towards the peritoneal
space behind the embryo.

As the hindgut becomes folded in the allantois shifts
its position, and forms (Figs. 63 B and 61) a rather wide
vesicle lying immediately ventral to the hind end of the
digestive canal, with which it communicates freely by a
still considerable opening; its blind end projects into
the pleuroperitoneal cavity below.

Still later the allantois grows forward, and becomes
a large spherical vesicle, still however remaining con-
nected with the cloaca by a narrow canal which forms
its neck or stalk (Fig. 9 G, al). From the first the
allantois lies in the pleuroperitoneal cavity. In this
cavity it grows forwards till it reaches the front limit of
the hindgut, where the splanchnopleure turns back to
enclose the yolk-sac. It does not during the third

184

A.

THE THIRD DAY.
Fm. 63.

[CHAP.

B.

Two LONGITUDINAL SECTIONS OF THE TAIL-END OF AN EM-
BRYO CHICK TO SHEW THE ORIGIN OF THE ALLANTOIS.
A AT THE BEGINNING OF THE THIRD DAY; B AT THE
MIDDLE OF THE THIRD DAY. (After Dobrynin.)

t. the tail ; m. the mesoblast ; ccf. the epiblast ; x". the neural
, canal ; Dd. the dorsal wall of the hindgut ; SO. somato-
pleure ; Spl. splanchnopleure ; u. the mesoblast of the
splanchnopleure carrying the vessels of the yolk-sac ; pp.
pleuroperitoneal cavity ; Df. the epithelium lining the
pleuroperitoneal cavity; All. the commencing allantois ;
w. projection formed by anterior and posterior divisions of
the primitive streak; y. hypoblast which will form the
ventral wall of the hindgut ; v. anal invagination (procto-
dseum) ; 6f. cloaca.

day project beyond this point ; but on the fourth day
begins to pass out beyond the body of the chick, along
the as yet wide space between the splanchnic and soma-
tic stalks of the embryo, on its way to the space between
the external and internal folds of the amnion, which it
will be remembered, is directly continuous with the
pleuroperitoneal cavity (Fig. 9 K). In this space it

VI.] THE MESOBLASTIC SOMITES. 185

eventually spreads out over the whole body of the
chick. On the first half of the fourth day the vesicle is
still very small, and its growth is not very rapid. Its
mesoblast wall still remains very thick. In the latter
half of the day its growth becomes very rapid, and it
forms a very conspicuous object in a chick of that date
(Fig. 67, Al}. At the same time its blood-vessels be-
come important. It receives its supply of blood from
two branches of the aorta known as the allantoic arte-
ries, and the blood is brought back from it by two allan-
toic veins which run along in the body walls, and after
uniting into a single trunk fall into the vitelline vein
close behind the liver.

Mesoblast of the trunk. Coincidently with the
appearance of these several rudiments of important
organs in the more or less modified splanchnopleure-
folds, the solid trunk of the embryo is undergoing
marked changes.

When we compare a transverse section taken through
say the middle of the trunk at the end of the third day
(Fig. 65), with a similar one of the second day (Fig. 34),
or even the commencement of the third day (Fig. 64),
we are struck with the great increase of depth (from
dorsal to ventral surface) in proportion to breadth. This
is partly due to the slope of the side walls of the body
having become much steeper> as a direct result of the
rapidly progressing folding off of the embryo from the
yolk-sac. But it is also brought about by the great
changes both of shape and structure which are taking
place in the mesoblastic somites, as well as by the
development of a mass of tissue between the notochord
and the hypoblast of the alimentary canal.

186

THE THIRD DAY.

[CHAP.

It will be remembered that the horizontal splitting

of the mesoblast into somatic and splanchnic layers

extends at first to the dorsal summit of the vertebral

plates, but after the formation of the somites the split

FIG. 64.

am.

so.

TRANSVERSE SECTION THROUGH THE TRUNK OF A DUCK
EMBRYO WITH ABOUT TWENTY-FOUR MESOBLASTIC SO-
MITES.

am. amniou ; so. somatopleure ; sp. splanchnopleure ; wd. Wolf-
nan duct ; st. segmental tube ; ca.v. cardinal vein ; ms.
muscle -plate ; sp.g. spinal ganglion ; sp.c. spinal cord ; ch,
notochord ; ao. aorta ; Jiy. hypoblast.

between the somatic and splanchnic layers becomes to
a large extent obliterated, though in the anterior somites

VI.] THE MUSCLE-PLATES. 187

it appears in part to persist. The somites on the second
day, as seen in a transverse section (Fig. 34, P.v), are
somewhat quadrilateral in form but broader than they
are deep.

Each at that time consists of a somewhat thick
cortex of radiating rather granular columnar cells,
enclosing a small kernel of spherical cells. They are
not, as may be seen in the above figure, completely
separated from the ventral (or rather at this period
lateral) parts of the mesoblastic plate, and the dorsal
and outer layer of the cortex of the somites is continuous
with the somatic layer of mesoblast, the remainder of
the cortex, with the central kernel, being continuous
with the splanchnic layer. Towards the end of the
second and beginning of the third day the dorsal and
outer layer of the cortex, together probably with some
of the central cells of the kernel, becomes separated
off as a special plate. From this plate, which is
shewn in the act of being formed in Fig. 64, ms, the
greater part of the voluntary muscular system of the
trunk is developed. When once formed the muscle-
plates have in surface views a somewhat oblong form,
and consist of two layers, an inner and an outer, which
enclose between them an almost obliterated central
cavity (Fig. 65, mp). No sooner is the muscle-plate
formed than the middle portion of the inner layer be-
comes converted into longitudinal muscles. The central
space in the muscle -plates is clearly a remnant of the
vertebral portion of the body cavity, which, though it
wholly or partially disappears in a previous stage, re-
appears again on the formation of the muscle-plate.

It is especially interesting to note that the first

188

THE THIRD DAY.
FIG. 65.

[CHAP.

SO

SECTION THROUGH THE DORSAL REGION OF AN EMBRYO CHICK
AT THE END OF THE THIRD DAY.

Am. amnion. m. p. muscle-plate. C. V. cardinal vein. Ao. dorsal
aorta. The section passes through the point where the
dorsal aorta is just commencing to divide into two branches.
Ch. notochord. W. d. Wolman duct. W. b. commencing
differentiation of the mesoblast cells to form the Wolman
body. ep. epiblast. SO. somatopleure. Sp. splaiichno-
pleure. hy. hypoblast. The section passes through the
point where the digestive canal communicates with the yolk-
sac, and is consequently still open below.

This section should be compared with the section through
the dorsal region of an embryo at the commencement of the third

VI.] THE INTERMEDIATE CELL-MASS. 189

day (Fig. 64). The chief differences between them arise from
the great increase in the space (now filled with mesoblast-cells)
between the notochord and the hypoblast. In addition to this
we have in the later section the completely formed amnion, the
separation of the muscle-plate from the mesoblastic somites, the
formation of the Wolman body, etc.

The mesoblast including the Wolman body and the muscle-
plate (m.p.) is represented in a purely diagrammatic manner.
The amnion, of which only the inner limb or true amnion is
represented in the figure, is seen to be composed of epiblast and a
layer of mesoblast ; though in contact with the body above the
top of the medullary canal, it does not in any way coalesce with
it, as might be concluded from the figure.

formed muscles in embryo birds have an arrangement
like that which is permanent in fishes; being longi-
tudinal in direction, and divided into segments.

The remainder of the somites, after the formation of
the muscle-plates, is of very considerable bulk ; the cells
of the cortex belonging to them lose their distinctive
characters, and their major part becomes converted, in a
manner which will be more particularly described in a
future chapter, into the bodies of the permanent ver-
tebrae.

We may merely add here that each of these bodies
sends a process inwards ventral to the medullary cord,
and that the processes from each pair of these bodies
envelope between them the notochord.

The intermediate cell-mass and the Wolffian body.

In a transverse section of a 45 hours' embryo a consider-
able mass of cells may be seen collected between the meso-
blastic somites and the point where the divergence into
somatopleure and splanchnopleure begins (Fig. 34, just
below W.d). This mass of cells, which we have already

190 THE THIRD DAY. [CHAP.

spoken of as the intermediate cell-mass, is at first indis-
tinguishable from the cells lining the inner end of the
body cavity ; but on the third day, a special peritoneal
lining of epithelioid cells is developed which is more or
less sharply marked off from the adjoining part of the
intermediate cell-mass. This latter now also passes
without any very sharp line of demarcation into the
mesoblastic somite itself; and as the folding in of the
side wall progresses, the mass of cells in this position
increases in size and grows in between the notochord
and the hypoblast, but does not accumulate to a suffi-
cient extent to separate them widely until the end of
the third or beginning of the fourth day.

The fusion between the intermediate cell-mass and the inner
portions of the somites becomes so complete on the third day
that it is almost impossible to say which of the cells in the
neighbourhood of the notochord are derived from the somites
and which form the intermediate cell-mass. It seems almost
certain however that the cells which form the immediate invest-
ment of the notochord really belong to the somites.

The intermediate cell-mass is of special importance
to the embryologist, in that the excretory and generative
systems are developed from it.

We have already described (p. 106) the development
of the Wolffian duct, and we have now to deal with the
Wolffian body which is, as the reader has no doubt
gathered, the embryonic excretory organ.

The structure of the fully developed Wolffian body
is fundamentally similar to that of the permanent kid-
neys, and consists essentially of convoluted tubules,
commencing in Malpighian bodies with vascular glome-
ruli, and opening into the duct.

VI.] THE WOLFFIAN BODY. 191

The tubules of the Wolffian body are developed
independently of the Wolffian duct, and are derived
from the intermediate cell-mass, shewn in Fig. 34,
between the upper end of the body-cavity and the meso-
blastic somite. In the chick the mode of development
of this mass into the segmental tubules is different in
the regions in front of and behind about the sixteenth
segment. In front of about the sixteenth segment
special parts of the intermediate cell-mass remain
attached to the peritoneal epithelium, on this layer
becoming differentiated ; there being several such parts
to each segment The parts of the intermediate cell-
mass attached to the peritoneal epithelium become
converted into S-shaped cords (Fig. 64 sf) which soon
unite with the Wolffian duct (wd), and constitute the
primitive Wolffian tubules. Into the commencement
of each of these cords the lumen of the body-cavity is
for a short distance prolonged, so that this part con-
stitutes a rudimentary peritoneal funnel leading from
the body-cavity into the lumen of the Wolffian tubule.

In the foremost Wolffian tubules, which never reach
very complete development, the peritoneal funnels
riden considerably. The section of the tube adjoining
tie wide peritoneal funnel becomes partially invaginated

the formation of a vascular ingrowth known as a
glomerulus, and this glomerulus soon grows to such an
extent as to project through the peritoneal funnel, the
neck of which it completely fills, into the body-cavity
(Fig. 66, gl). There is thus formed a series of glomeruli
belonging to the anterior Wolffian tubuli projecting
freely into the body-cavity. These glomeruli with
their tubuli become however early aborted.

192 THE THIRD DAY. [CHAP.

Fia. 66.

SECTION THROUGH THE EXTERNAL GLOMERULUS OF ONE OF
THE ANTERIOR SEGMENTAL TUBES OF AN EMBRYO CHICK
OF ABOUT 100 HOURS.
gl. glomerulus ; ge. peritoneal epithelium ; Wd. Wolffian duct ;

ao. aorta ; me. mesentery.

The Wolffian tubule, and the connection between the external
and internal parts of the glomerulus are not shewn in this figure.

In the case of the remaining tubules developed from
the S-shaped cords, the attachment to the peritoneal
epithelium is very soon lost. The cords acquire a
lumen, and open into the Wolffian duct. Their blind
extremities constitute the commencements of Malpi-
ghian bodies.

In the posterior part of the Wolffian body of the
chick the intermediate cell-mass becomes very early
detached from the peritoneal epithelium, and at a con-
siderably later period breaks up into oval vesicles, which
elongate into Wolffian tubules. In addition to the
primary tubules, whose development has just been
described, secondary and tertiary tubules are formed
on the dorsal side of the primary tubules. They are

VI.] THE WOLFFIAN BODY. 193

differentiated out of the mesoblast of the intermediate
cell-mass and open independently into the Wolffian
duct.

A tubule of the Wolffian body typically consists of the follow -
ing parts, (1) a section carrying the peritoneal opening, and
known as the peritoneal funnel, (2) a dilated vesicle into which
this opens, (3) a coiled tubulus proceeding from (2), and termi-
nating in (4) a wider portion opening into the Wolffian duct.

In the chick, the peritoneal funnel is only found in the most
anterior tubules and soon atrophies; it is not developed in the
tubules of the posterior part of the Wolffian body. Region No.
4 also is not clearly marked off from region No. 3. One part of
the wall of the dilated vesicle (2) is invaginated by a bunch of
capillaries and gives rise to the Malpighian body.

In consequence of the continual folding in of the
somatopleure and especially of the splanchnopleure, as
well as owing to the changes taking place in the meso-
blastic somites, the Wolffian duct undergoes on the
third day a remarkable change of position. Instead of
lying, as on the second day, immediately under the
epiblast (Fig. 34, W.d), it is soon found to have appa-
rently descended into the middle of the intermediate
cell-mass (Fig. 64, w.d) and at the end of the third day
occupies a still lower position and even projects some-
what towards the pleuroperitoneal cavity. (Fig. 65,
W.d.)

The chief events then which take place on the third
day are as follows :

1. The turning over of the embryo so that it now
lies on its left side.

2. The cranial flexure round the anterior extremity
of the notochord.

F. &B. 13

194 THE THIRD DAY. [CHAP. VI.

3. The completion of the circulation of the yolk-
sac; the increased curvature of the heart, and the
demarcation of its several parts; the appearance of new
aortic arches, and of the cardinal veins.

4. The formation of four visceral clefts and five
visceral arches.

5. The involution to form the lens, and the forma-
tion of the secondary optic vesicle.

6. The closing in of the otic vesicle.

7. The formation of the nasal pits.

8. The appearance of the vesicles of the cerebral
hemispheres ; the separation of the hind-brain into cere-
bellum and medulla oblongata.

9. The definite establishment of the cranial and
spinal nerves as outgrowths of the central nervous
system.

10. The completion of the fore-gut and of the
hind-gut; the division of the former into oasophagus,
stomach and duodenum, of the latter into large intestine
and cloaca.

11. The formation of the lungs from a diverticulum
of the alimentary canal immediately in front of the
stomach.

12. The formation of the liver and pancreas: the
former as two diverticula from the duodenum, which
subsequently become united by nearly solid outgrowths ;
the latter as a single diverticulum also from the duo-
denum.

13. The changes in the mesoblastic somites and
the appearance of the muscle-plates.

14. The definite formation of the Wolffian bodies
and the change in position of the Wolffian duct.

CHAPTER VII.

THE CHANGES WHICH TAKE PLACE DURING THE
FOURTH DAY.

ON opening an egg in the middle or towards the end
of the fourth day, a number of points in which progress
has been made since the third day are at once apparent.
In the first place, the general growth of the embryo has
been very rapid, so that its size is very much greater
than on the previous day. In the second place, the
white of the egg has still further diminished, the em-
bryo lying almost in immediate contact with the shell
membrane.

The germinal membrane embraces more than half
the yolk, and the vascular area is about as large as a
halfpenny.

Corresponding to the increased size of the embryo,
there is a great increase in the quantity of blood circu-
lating in the vascular area as a whole, though the sinus
terminalis is already less distinct than it was.

The amnion becomes increasingly conspicuous. It
is now seen as a distinct covering obscuring to a certain
extent the view of the body of the chick beneath, and

13—2

196 THE FOUETH DAY. [CHAP.

all traces of the junction of its folds are by this time
lost. As yet there is very little fluid in the amniotic
sac proper, so that the true amnion lies close upon the
embryo.

The folding off of the embryo from the yolk sac has
made great progress. The splanchnic stalk, which on
the third day was still tolerably wide, inasmuch as about
one third of the total length of the alimentary canal
was as yet quite open to the yolk sac below, now be-
comes so much constricted by the progressive closing in
of the splanchnopleure folds, that the alimentary canal
may be said to be connected with the yolk sac by a very
narrow neck only. This remnant of the splanchnic
stalk we may now call the vitelline duct; though narrow,
it is as yet quite open, affording still free communica-
tion between the inside of the yolk sac and the interior
of the alimentary canal.

The somatic stalk, though narrowing somewhat, is
much wider than the splanchnic stalk, so that a con-
siderable ring-shaped space exists between the two.

Another very prominent feature is the increase in
the cranial flexure. During the third day, the axis of
the front part of the head was about at right angles to
the long axis of the body; the whole embryo being still
somewhat retort-shaped. On this day, however, the
flexure has so much increased that the angle between
the long axis of the body and that of the front segment
of the head is an acute one.

The tail-fold, which commenced to be noticeable
during the third day, has during this day increased very
much, and the somewhat curved tail (Fig. 67) forms
quite a conspicuous feature of the embryo. The general

EMBRYO AT THE END OF THE FOURTH DAY SEEN AS
A TRANSPARENT OBJECT.

The amnion has been completely removed, the cut end of the
somatic stalk is shewn at S.S. with the allantois (Al.} protruding
from it.

C.ff. cerebral hemisphere. F.B. fore-brain or vesicle of the third
ventricle (thalamencephalon) with the pineal gland (Pn.)
projecting from its summit. M.B. mid-brain. Cb. cerebellum.
1 V. V. fourth ventricle. L. lens. ch.s. choroid slit. Owing to
the growth of the optic cup the two layers of which it is com-
posed cannot any longer be seen from the surface ; the pos-
terior surface of the choroid layer alone is visible. Cen. V.
auditory vesicle, s.m. superior maxillary process. IF, %F, etc.
first, second, third and fourth visceral folds. V. fifth nerve
sending one branch to the eye, the ophthalmic branch, and

198 THE FOURTH DAY. [CHAP.

another to the first visceral arch. VII. seventh nerve passing
to the second visceral arch. G. Ph. glossopharyngeal nerve
passing towards the third visceral arch. Pg. pneumogastric
nerve passing towards the fourth visceral arch. iv. investing
mass (basilar plate). No attempt has been made in the figure
to indicate the position of the dorsal wall of the throat, which
cannot be easily made out in the living embryo, ch. noto-
chord. The front end of this cannot be seen in the living
embryo. It does not end however as shewn in the figure,
but takes a sudden bend downwards and then terminates in
a point. Ht. heart seen through the walls of the chest. M.P.
muscle-plates. W. wing. H.L. hind limb. Beneath the
hind limb is seen the curved tail.

curvature of the body has also gone on increasing, and
as the result of these various flexures, the embryo has
become somewhat spirally curled up on itself (Fig. 67).

The distinct appearance of the limbs must be
reckoned as one of the most important events of the
fourth day.

Owing to the continued greater increase of depth
than of breadth, the body of the embryo appears in
section (Fig. 68) higher and relatively narrower than
even on the third day, and the muscle-plates, instead of
simply slanting downwards, come to be nearly vertical
in position. Not far from the line which marks their
lower ends, the somatopleure, almost immediately after
it diverges from the splanchnopleure, is raised up (Fig.
68, TF..R.) into a low rounded ridge which runs along
nearly the whole length of the embryo from the neck
to the tail.

It is on this ridge, which is known as the Wolffian
ridge, that the limbs first appear as flattened conical
buds projecting outwards. They seem to be local de-

VII.]

THE LIMBS.
FIG. 68.

199

WM

SECTION THROUGH THE LUMBAR KEGION OF AN EMBRYO AT
THE END OF THE FOURTH DAY.

n. c. neural canal, p.r. posterior root of spinal nerve with gan-
glion, a.r. anterior root of spinal nerve. A.G.C. anterior
grey column of spinal cord. A. W.C. anterior white column
of spinal cord just commencing to be formed, and not very
distinctly marked in the figure, m.p. muscle-plate, ch.
notochord. W.R. Wolfnan ridge. A.O. dorsal aorta. V.c.a.
posterior cardinal vein. W.d. Wolffian duct. W.b. Wolffian
body, consisting of tubules and Malpighian corpuscles. One
of the latter is represented on each side. g.e. germinal

200 THE FOUKTH DAY. [CHAP.

epithelium, d. alimentary canal. M. commencing me-
sentery. S.O. somatopleure. 8.P. splanchnopleure. V.
blood-vessels, pp. pleuroperitoneal cavity.

velopments of the ridge, the rest of which becomes less
and less prominent as they increase in size. Each bud,
roughly triangular in section, consists of somewhat
dense mesoblast covered by epiblast which on the sum-
mit is thickened into a sort of cap. The front limbs or
wings (Fig. 67) arise just behind the level of the heart,
and the hind limbs in the immediate vicinity of the
tail. The first traces of them can be seen towards the
end of the third, but they do not become conspicuous
till the fourth day, by the end of which the two pairs
may be already distinguished by their different shapes.
The front limbs are the narrowest and longest, the hind
limbs being comparatively short and broad. Both are
flattened from above downwards and become more so as
their growth continues.

In the head, the vesicles of the cerebral hemispheres
are rapidly increasing in size, their growth being enor-
mous as compared with that of the thalamencephalon or
vesicle of the third ventricle. The mid-brain is now, as
compared to the other parts of the brain, larger than at
any other epoch, and an indistinct median furrow on its
upper surface indicates its division into two lateral
halves. The great increase of the mesoblastic contents
of the secondary optic vesicle or involuted retinal cup
causes the two eyeballs to project largely from the sides
of the head (Fig. 69, Op). The mass of mesoblast which
invests all the various parts of the brain, is not only
growing rapidly below and at the sides, but is also
undergoing developments which result in the formation

VII.] THE HEAD. 201

FIG. 69.

A. HEAD OF AN EMBRYO CHICK OF THE FOURTH DAY

VIEWED FROM BELOW AS AN OPAQUE OBJECT. (Chromic

acid preparation.)

C.H. cerebral hemispheres. F.B. vesicle of the third ventricle
or thalamencephalon. Op. eyeball, nf. naso-frontal process.
M. cavity of mouth. S.M. superior maxillary process of F. 1,
the first visceral fold (mandibular arch). F. 2, F. 3 second
and third visceral arches. N. nasal pit.

In order to gain the view here given the neck was cut across
between the third and fourth visceral folds. In the section e
thus made are seen the alimentary canal al, the neural canal n.c.,
the notochord ch., the dorsal aorta AO., and the jugular veins V.
Ao. bulbus arteriosus.

In the drawing the nasal groove has been rather exaggerated
in its upper part. On the other hand the lower and shallower
part of the groove where it runs between the superior maxillary
process S.M. and the broad naso-frontal process has not been
satisfactorily rendered. Hence the end of the superior maxillary
process seems to join the inner and not, as described in the text,
the outer margin of the nasal groove. A few hours later the
separation of the two would have been very visible.

B. The same seen sideways, to shew the visceral folds, ot. otic
vesicle. Remaining letters as before.

202 THE FOURTH DAY. [CHAP.

of the primitive skull. All these events, added to the
cranial flexure spoken of above, give to the anterior
extremity of the embryo a shape which it becomes more
and more easy to recognize as that of a head.

Meanwhile the face is also being changed. The two
nasal pits were on the third day shallow depressions com-
plete all round. As the pits deepen on the fourth day
by the growth upwards of a rim round them, a deficiency
or break in the ridge may be observed on that side of it
turned towards the mouth; which constitutes a kind of
shallow groove (Fig. 69 N) directed obliquely downwards
towards the cavity of the mouth. The fronto-nasal
process or median ridge (Fig. 69, nf), which on the third
day rose up between the superficial projections caused by
the bulging anterior extremities of the vesicles of the
cerebral hemispheres, and on the fourth day becomes
increasingly prominent, separates the two grooves from
each other, and helps to form the inner wall of each of
them, while the depth of the groove also becomes in-
creased by the prolongation along its inner side of the
rim surrounding the nasal pit. Abutting on the outer
side of each groove near the mouth and so helping to
form the outer wall of each, lie the ends of the superior
maxillary processes of the first visceral arch (Fig. 69 B,
SM), which like the fronto-nasal process are increasing
in size. By their continued growth, the groove is more
and more deepened, and leading as it does from the
nasal pit to the cavity of the mouth, may already be
recognized as the rudiment of the passage of the pos-
terior nares.

During the latter half of the fourth day there ap-
pears at the bottom of the deep lozenge-shaped cavity

VII.] THE CKANIAL NERVES. 203

of the stomodseum or primitive buccal cavity, in the now
thin wall dividing it from the alimentary canal, a longi-
tudinal, or according to Gotte a vertical slit which, soon
becoming a wide opening, places the two cavities in
complete communication.

The cavity of the mouth, being, it will be remember-
ed, formed partly by depression, partly -by the growth
of the surrounding folds, is lined entirely with epiblast,
from which the epithelium of its surface and of its
various glands is derived. In this respect, as Remak
pointed out, it differs from all the rest of the alimentary
canal, whose whole epithelium is formed out of hypoblast.

By the side of the hind-brain the cerebellum, through
the increased thickening of its upper walls, is becoming
more and more distinct from the medulla oblongata;
while both in front and behind the auditory vesicle,
in which the rudiments of the cochlea and recessus ves-
tibuli are already visible, the cranial ganglia and nerves
are acquiring increased distinctness and size. They may
be very plainly seen when the head of the fresh embryo
is subjected to pressure.

The foremost is the fifth cranial nerve (Fig. 67, F.)
with its Gasserian ganglion; it lies a little way behind
the anterior extremity of the notochord immediately
below the cerebellum. Next to this comes the seventh
nerve (Fig. 67, VII.), starting just in front of the ear-
vesicle, and extending far downwards towards the second
visceral arch. The two nerves which lie behind the ear-
vesicle are now obviously separate from each other; the
front one is the glossopharyngeal (Fig. 67, G.Ph.), and
the hinder one already shews itself to be the pneumo-
gastric (Fig. 67, Pg.).

204 THE FOUKTH DAY. [CHAP.

The mesoblastic somites, which by the continued
differentiation of the axial mesoblast at the tail end of
the embryo have increased in number from thirty to
forty, undergo during this day changes of great import-
ance. Since these changes are intimately connected
with the subsequent development of the vertebral
column, it will perhaps be more convenient to describe
briefly here the whole series of events through which
the somites become converted into the permanent
structures to which they give rise, though many of the
changes do not take place till a much later date than
the fourth day.

The separation of the muscle-plates (p. 187) left the
remainder of each somite as a somewhat triangular
mass lying between the neural canal and notochord on
the inside, and the muscle-plate and intermediate cell-
mass on the outside (Fig. 64). Already on the third day
(Fig. 65) the upper angle of this triangle grows upwards,
between its muscle-plate and the neural canal, and
meeting its fellow in the middle line above, forms a
roof of mesoblast over the neural canal, between it and
the superficial epiblast. At about the same time, the
inner and lower angle of the triangle grows inwards to-
wards the notochord, and passing both below it (between
it and the aorta) and above it (between it and the
neural canal), meets a similar growth from its fellow
somite of the other side, and thus completely invests
the notochord with a coat of mesoblast, which, as seen in
Fig. 68, is at first much thicker on the under than on
the upper side.

Both neural canal and notochord are thus furnished
from neck to tail with a complete investment of meso-

VII.] THE PERMANENT VERTEBRAE. 205

blast, still marked, however, by the transparent lines
indicating the fore and aft limits of the several somites.
This mesoblastic investment is sometimes spoken of as
the "membranous" vertebral column.

The portions of the somites thus forming the primary
vertebrae or membranous vertebral column are converted
into the permanent vertebrae; but their conversion is
complicated by a remarkable new or secondary segmen-
tation of the whole vertebral column.

On the fourth day, the transparent lines marking
the fore and aft limits of the somites are still distinctly
visible. On the fifth day, however, they disappear, so
that the whole vertebral column becomes fused into a
homogeneous mass whose division into vertebrae is only
indicated by the series of ganglia. This fusion, which
does not extend to the muscle-plates in which the
primary lines of division still remain visible, is quickly
followed by a fresh segmentation, the resulting segments
being the rudiments of the permanent vertebrae. The
new segmentation, however, does not follow the lines of
the segmentation of the muscle-plates, but is so effected
that the centres of the vertebral bodies are opposite the
septa between the muscle-plates.

The explanation of this character in the segmentation is not
difficult to find. The primary segmentation of the body is that
of the muscle-plates, which were present in the primitive forms
in which vertebrae had not appeared. 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 condition under which the lateral muscles can best cause
the flexure of the vertebral column is clearly that each muscle-
plate shall be capable of acting on two vertebrae ; and this con-
dition can only be fulfilled when the muscle-segments are oppo-

206 THE FOURTH DAY. [CHAP.

site the intervals between the vertebrae. For this reason, when
the vertebras became formed, their centres were opposite not the
middle of the muscle- plates but the inter-muscular septa.

These considerations fully explain the characters of the
secondary segmentation of the vertebral column. On the other
hand the primary segmentation of the vertebral rudiments is
clearly a remnant of a condition when no vertebral bodies were
present ; and has no greater morphological significance than the
fact that the cells of the vertebrae were derived from the seg-
mented muscle-plates, and then became fused into a continuous
sheath around the notochord and nervous axis ; till finally they
became in still higher forms differentiated into vertebrae and
their arches.

By these changes this remarkable result is brought
about, that each permanent vertebra is formed out of
portions of two consecutive mesoblastic somites. Thus,
for instance, the tenth permanent vertebra is formed
out of the hind portion of the tenth somite, and the
front portion of the eleventh somite.

The new segmentation is associated with or rather is
caused by histological changes. At the time when
the fusion takes place, the mesoblast, which in the form
of processes from the somites surrounds and invests
the notochord, has not only increased in mass but also
has become cartilaginous, so that, as Gegenbaur 1 points
out, there is present for a short period on the fifth day
a continuous and unsegmented cartilaginous investment
of the notochord.

This cartilaginous tube does not however long re-
main uniform. At a series of points corresponding in
number to the original somites it becomes connected

1 Untersuchung zur vergleichenden Anatomic der Wirbelsaule lei
Amphibien und Reptilien, Leipzig, 1862.

VII.] THE PERMANENT VERTEBRA. 207

with a number of cartilaginous arches which appear in
the mesoblastic investment of the neural canal. These
arches, which thus roof in the neural canal, are the
cartilaginous precursors of the osseous vertebral arches.
We further find that the portions of the cartilaginous
tube from which the arches spring come to differ histo-
logically from the portions between them not connected
with arches : they are clearer and their cells are less
closely packed. There is however at this period no
distinct segmentation of the cartilaginous tube, but
merely a want of uniformity in its composition.

The clearer portions, from which the arches spring,
form the bodies of the vertebrae, the segments between
them the intervertebral regions of the column.

On the fifth day a division takes place of each of the
intervertebral segments into two parts, which respec-
tively attach themselves to the contiguous vertebral
regions. A part of each intervertebral region, immedi-
ately adjoining the notochord, does not however undergo
this division, and afterwards gives rise to the ligamen-
tum suspensorium.

This fresh segmentation is not well marked, if in-
deed it takes place at all in the sacral region.

To recapitulate: — the original somites lying side by
side along the notochord, after giving off the muscle -
plates, grow around, and by fusing together completely
invest, with mesoblast, both neural canal and notochord.

This investment, of which by reason of its greater
growth the original bodies of the somites now seem to be
only an outlying part, becomes cartilaginous in such a
way that while the notochord becomes surrounded with
a thick tube of cartilage bearing no signs of segmenta-

208 THE FOURTH DAY. [CHAP.

tion, but having the ganglia lodged on its exterior at
intervals, the neural canal is covered in with a series of
cartilaginous arches, connected to each other by ordinary
mesoblastic tissue only, but passing at their bases di-
rectly into the cartilaginous tube around the notochord.

By a process of histological differentiation the carti-
laginous tube is divided into vertebral and interverte-
bral portions, the vertebral portions corresponding to
the arches over the neural canal. Fresh lines of seg-
mentation then appear in the intervertebral portions,
dividing each of them into two parts, of which one at-
taches itself to the vertebra in front and the other to
the vertebra behind.

The notochord. Meanwhile from the fourth to the
sixth day important changes take place in the notochord
itself.

On its first appearance the notochord was, as we
have seen, composed of somewhat radiately arranged
but otherwise perfectly typical mesoblast-cells.

On the third day some of the central cells become
vacuolated, while the peripheral cells are still normal.
The vacuolated cells exhibit around the vacuole a peri-
pheral layer of granular protoplasm in which the nucleus
lies embedded, whilst the vacuoles themselves are filled
with a perfectly clear and transparent material, which
in an unaltered condition is probably fluid. Towards
the end of the day the notochord acquires a delicate
structureless sheath which is no doubt a product of its
peripheral cells.

On the fourth day all the cells become vacuolated
with the exception of a single layer of flattened cells at
the periphery. The vacuoles go on enlarging unti]

VII.] THE NOTOCHORD. 209

on the sixth day the vacuoles in each cell have so much
increased at the expense of the protoplasm that only a
very thin layer of the latter is left at the circumference
of the cell, at one part of which, where there is gene-
rally more protoplasm than elsewhere, the starved re-
mains of a nucleus may generally be detected. Thus
the whole notochord becomes transformed into a spongy
reticulum, the meshes of which correspond to the vacu-
oles of the cells and the septa to the remains of their
cell-walls.

The notochord is on the sixth day at the maximum
of its development, the change which it henceforward
undergoes being of a retrograde character.

From the seventh day onward, it is at various points
encroached upon by its investment. Constrictions are
thus produced which first make their appearance in the
intervertebral portions of the sacral region. In the cer-
vical region, according to Gegenbaur, the intervertebral
portions are not constricted till the ninth day, though in
the vertebral portions of the lower cervical vertebrae con-
strictions are visible as early as the seventh day. By
the ninth and tenth days, however, all the interverte-
bral portions have become distinctly constricted, and at
the same time in each vertebral portion there have also
appeared two constrictions giving rise to a central and
to two terminal enlargements. In the space therefore
corresponding to each vertebra and its appropriate in-
tervertebral portion, there are in all three constrictions
and three enlargements.

On the twelfth day the ossification of the bodies
of the vertebrae commences. The first vertebra to ossify
is the second or third cervical, and the ossification gradu-
F. & B. 14

210 THE FOURTH DAY. [CHAP.

ally extends backwards. It does not commence in the
arches till somewhat later. For each arch there are
two centres of ossification, one on each side.

The notochord persists for the greater part of foetal
life and even into post-foetal life. The larger vertebral
portions are often the first completely to vanish. They
would seem in many cases at any rate (Gegenbaur) to
be converted into cartilage and so form an integral part
of the permanent vertebrae. Rudiments of the inter-
vertebral portions of the notochord may long be detected
in the ligamenta suspensoria.

"We may remind the reader that in the adult bird we find
between each of the vertebrae of a neck and back a cartilaginous
disc — the meniscus — which is pierced in the centre. These discs
are thick at the circumference but thin off to a fine edge round
the central hole. Owing to the shape of these discs there are left
between each pair of vertebrae two cavities, which only commu-
nicate through the central aperture of the meniscus. Through
this central aperture there passes a band called the 'ligamen-
tum suspensorium,' connecting the two vertebrae.

In the tail the menisci are replaced by bodies known as the
'annuli fibrosi/ which precisely resemble the similarly named
bodies in mammals. They differ from the menisci in being
attached over their whole surface to the ends of the vertebral
bodies, so that the cavities between the menisci and the vertebrae
do not exist. They are pierced however by a body corre-
sponding with the ligamentum suspensorium and known as the
1 nucleus pulposus.'

In the intervertebral regions the chorda, soon after the com-
mencement of ossification, entirely disappears. The cartilage
around it however becomes converted (in the region of the neck)
into the ligamentum suspensorium, which unites the two ver-
tebrae between which it is placed.

In the tail the cartilage becomes the nucleus pulposus, which
corresponds exactly to the l ligamentum suspensorium ' of the
neck and back.

VII.] THE MUSCLE-PLATES. 211

Shortly after the formation of the ligamentum suspensorium
the remaining cartilage of the intervertebral segments is con-
verted into the meniscus between each two vertebrae, and in the
tail into the annulus fibrosus. Both are absent in the sacrum.

Muscle-plates. We shall conclude our account of
the mesoblastic somites by describing the changes which
take place in the muscle-plates.

In the chick these are somewhat complicated, and
have not been fully worked out.

On the third day the muscle-plates end opposite the
point where the mesoblast becomes split into somato-
pleure and splanchnopleure. On the fourth day how-
ever (Fig. 68 mp.) they extend a certain distance into
the side walls of the body beyond the point of the
division into somatopleure and splanchnopleure.

Into what muscles of the trunk they become con-
verted has been somewhat disputed. Some embryolo-
gists have stated that they only form the muscles of
the back. We have, however, little doubt that all the
episkeletal muscles, to use Professor Huxley's term
(Vertebrates, p. 46), are their products; a view also
adopted by Professors Huxley and Kolliker.

The development of the subvertebral system of muscles
(hyposkeletal of Huxley) has not been worked out, but on the
whole there is reason to believe that it is derived from the
muscle-plates. Kolliker, Huxley and other embryologists believe
however that these muscles are independent of the muscle-plates
in their origin.

Whether the muscle of the diaphragm is to be placed in the
same category as the hyposkeletal muscles has not been made out.

It is probable that the cutaneous muscles of the trunk are
derived from the cells given off from the muscle-plates. Kolliker
however believes that they have an independent origin.

14—2

212 THE FOURTH DAY. [CHAP.

The limb-muscles, both extrinsic and intrinsic, are in certain
fishes (Elasmobranchii), derived from the muscle-plates, and a
similar origin has been observed in Lacertilia and Amphibia.

In the Chick and other higher Vertebrata on the other hand
the entrance of the muscle-plates into the limbs has not been
made out (Kolliker). It seems therefore probable that by an
embryological modification, of which instances are so frequent>
the cells which give rise to the muscles of the limbs in the higher
Vertebrata can no longer be traced into a direct connection with
the muscle-plates.

At first, as is clear from their mode of origin, the
muscle-plates correspond in number with the protover-
tebrae, and this condition is permanent in the lower
vertebrates, such as fishes, where we find that the
lateral muscle is divided by septa into a series of
segments corresponding in number with the vertebrae.

Wolffian body or mesonephros. Of all the events
of the fourth day, none perhaps are more important than
those by which the rudiments of the complex urinary
and generative systems are added to the simple Wolffian
duct and body, which up to that time are the sole repre-
sentatives of both systems.

We saw that the duct arose on the second day (pp.
94, 106) as a solid ridge which subsequently became a
tube, lying immediately underneath the epiblast above
the intermediate cell-mass, close against the upper and
outer angles of the somites, and reaching from about
opposite to the seventh somite away to the hinder end
of the embryo.

At first the duct occupies a position immediately
underneath the superficial epiblast, but very soon after
its formation the growth of the somites and the changes
which take place in the intermediate cell-mass, together

VII.] THE WOLFFIAN BODY. 213

with the general folding in of the body, cause it to
appear to change its place and travel downwards (p.
193). While the shifting is going on, the cells lining
the upper end of the pleuroperitoneal cavity (the kind
of bay which, as seen in sections, is formed by the diver-
gence of the somatopleure and splanchnopleure) become
columnar, and constitute a distinct epithelium. This
epithelium, which is clearly shewn in Fig. 64, and is
also indicated in Fig. 65, is often called the germinal
epithelium, because some of its cells subsequently take
part in the formation of the ovary. Soon after the ap-
pearance of the germinal epithelium, the intermediate
cell-mass increases in size and begins to grow outwards
into the pleuroperitoneal cavity, as a rounded projection
which lies with its dorsal surface towards the somato-
pleure, and its ventral surface towards the splanchno-
pleure, but is in either case separated from these layers
by a narrow chink. The Wolffian duct (Figs. 65, 68,
Wd) travels down, and finally before the end of the third
day is found in the upper part of this projection, near
that face of it which is turned towards the somatopleure.

The tubules of the anterior part of the Wolffian
body have by the end of the fourth day almost entirely
disappeared; but the tubules of that part of the Wolf-
fian body which is found behind the 16th segment
undergo a further development.

Each increases in size especially that portion which
proceeds from the Malpighian body and is known as the
coiled tubulus (region No. 3, see p. 193). This becomes
much elongated and twisted. As a consequence of this
increase in size the intermediate cell-mass comes to
project more and more into the pleuroperitoneal cavity.

214 THE FOURTH DAY. [CHAP.

The large size of the hinder part of the Wolffian body as
compared with that of the anterior part is due to the presence of
the dorsally placed secondary tubules, whose development was
mentioned on p. 192. These are more numerous in the posterior
than in the anterior part of the Wolffian body. At the hind end
of the Wolffian body there are as many as four to each primary
tubule.

The tubules, which from their contorted course are
in sections (Figs. 68, 71) seen cut at various angles,
possess an epithelium which is thicker than that of the
Wolffian duct. From this difference it is generally easy
to distinguish the sections of the tubules from those of
the duct. The glomeruli of the Malpighian bodies are
in sections of hardened embryos usually filled with
blood-corpuscles.

Towards the hind end of the embryo, the projection
of the intermediate cell-mass spoken of above becomes
smaller and smaller, and the Wolffian duct is thus
brought nearer to the splanchnopleure, and in the
region of the hind-gut comes to lie close to the walls of
the alimentary canal. On the fourth day, the two ducts
meet and open into two horns, into which the side- walls
of the recently formed cloaca are at that time produced,
one on either side.

As we shall afterwards see, the ducts of the perma-
nent kidneys and Miiller's duct also fall into these two
horns of the cloaca.

The Wolffian bodies thus constituted perform the
offices of kidneys for the greater part of embryonic life.
In the chick they disappear before birth; but in most
of the Ichthyopsida they remain for life as the perma-
nent kidneys.

Miillerian duct. After the establishment of the

VII.] THE MULLERIAN DUCT. 215

Wolffian body there is formed in both sexes a duct,
which in the female becomes the oviduct, but which in
the male is functionless and usually disappears. This
duct, in spite of certain peculiarities in its development,
is without doubt homologous with the Miillerian duct
of the Ichthyopsida.

The first rudiment of the Miillerian duct appears at
the end of the fourth day, as three successive open involu-
tions of the peritoneal epithelium, connected together
by more or less well-defined ridge-like thickenings of
the epithelium. It takes its origin from the layer of
thickened peritoneal epithelium situated near the dorsal
angle of the body-cavity, close to the Wolffian duct, and
some considerable distance behind the front end of the
Wolffian duct.

In a slightly later stage the ridges connecting the
grooves become partially constricted off from the peri-
toneal epithelium, and develop a lumen. The condition
of the structure at this stage is illustrated by Fig. 70,
representing three transverse sections through two
grooves, and through the ridge connecting them.

The Miillerian duct may in fact now be described as
a short but slightly convoluted duct, opening into the
body-cavity by three groove-like apertures, and con-
tinued for a short distance behind the last of these.

In an embryo not very much older than the one last
described the two posterior apertures vanish and the
anterior opening alone remains as the permanent open-
ing of the Miillerian duct.

The position of these openings in relation to the
Wolffian body is shewn in Fig. 71, which probably passes
through a region between two of the peritoneal openings.

SECTIONS SHEWING TWO OF THE PERITONEAL INVAGINATIONS
WHICH GIVE RISE TO THE ANTERIOR PART OF THE MlTL-
LERIAN DUCT (PRONEPHROS).

A is the llth section of the series.

B „ 15th „ „

C „ 18th „

gr 2 second groove, gr 3 third groove, r 2 second ridge, wd.
Wolffian duct.

As long as the openings persist, the Mullerian duct
consists merely of a very small rudiment, continuous
with the hindermost of them, and its solid extremity
appears to unite with the walls of the Wolffian duct.

After the closure of the two hinder openings the
Miillerian duct commences to grow rapidly backwards,
and for the first part of its subsequent course it
appears to be split off as a solid rod from the outer or
ventral wall of the Wolffian duct (Fig. 72). Into this
rod the lumen, present in its front part, subsequently
extends. Its mode of development in front is thus pre-
cisely similar to that of the Miillerian duct in Elasmo-
branchii and Amphibia.

This mode of development only occurs however in
the anterior part of the duct. In the posterior part of

VII.]

THE MULLERIAN DUCT.

FIG. 71.

217

SECTION OF THE INTERMEDIATE CELL-MASS ON THE FOURTH
DAY. (From Waldeyer.) Magnified 160 times.

m. mesentery. L. somatopleure. a', portion of the germinal
epithelium from which the involution to form the duct of
Muller (z) takes place, a. thickened portion of the germinal
epithelium in which the primitive ova O and o are lying.
E. modified mesoblast which will form the stroma of the
ovary. WK. Wolman body. y. Wolman duct.

its course its growing point lies in a bay formed by the
outer wall of the Wolffian duct, but does not become
definitely attached to that duct. It seems however
possible that, although not actually split off from the

218 THE FOURTH DAY. [CHAP.

FIG. 72.

Two SECTIONS SHEWING THE JUNCTION OF THE TERMINAL
SOLID PORTION OF THE MULLERIAN DUCT WITH THE
WOLFFIAN DUCT.

In A the terminal portion of the duct is quite distinct ; in B
it has united with the walls of the Wolffian duct.

md. Miillerian duct. Wd. Wolffian duct.

walls of the Wolffian duct, it may grow backwards from
cells derived from that duct.

The Miillerian duct finally reaches the cloaca though
it does not in the female for a long time open into it,
and in the male never does so.

The anterior part of the commencing Miillerian duct with its
three openings into the body-cavity is probably homologous with
the head kidney or pronephros of the Ichthyopsida.

Permanent kidney or metanephros. Between the
80th and 100th hour of incubation, the permanent kid-
neys begin to make their appearance, and as is the case
with the Wolffian bodies, the first portion of them to
appear is their duct. Near its posterior extremity the
Wolffian duct becomes expanded, and from the expand-
ed portion a diverticulum is constricted off which in a

VII. J THE PERMANENT KIDNEY. 219

transverse section lies dorsal to the original duct, and
the blind end of which points forwards, that is, towards
the head of the chick. This is the duct of the perma-
nent kidney or ureter. At first the ureter and the
Wolffian duct open by a common trunk into the cloaca,
but this state of things lasts for a short time only, and
by the sixth day the two ducts have independent open-
ings.

The ureter thus beginning as an outgrowth from
the Wolffian duct grows forwards, and extends along
the outer side of a mass of mesoblastic tissue which
lies mainly behind, but somewhat overlaps the dorsal
aspect of, the Wolffian body.

This mass of mesoblastic cells may be called the
metanephric blastema. It is derived from the interme-
diate cell-mass of the region reaching from about the
thirty-first to the thirty-fourth somite. It is at first
continuous with, and indistinguishable in structure
from, the portion of the intermediate cell-mass of the
region immediately in front of it, which breaks up into
Wolffian tubules. The metanephric blastema remains
however quite passive during the formation of the
Wolffian tubules in the adjoining blastema; and on the
formation of the ureter breaks off from the Wolffian
body in front, and, growing forwards and dorsalwards,
becomes connected with the inner side of the ureter
in the position just described.

In the subsequent development of the kidney col-
lecting tubes grow out from the ureter, and become
continuous with masses of cells of the metanephric
blastema, which then differentiate themselves into the
kidney tubules.

220 THE FOURTH DAY. [CHAP.

The formation of the kidneys takes place before the
end of the seventh day, but they do not become of func-
tional importance till considerably later.

From their mode of development it clearly follows
that the permanent kidneys are merely parts of the
same system as the Wolffian bodies, and that their se-
paration from these is an occurrence of a purely second-
ary importance.

The generative ridge. Before describing the sub-
sequent fate of the Wolffian and Miillerian ducts, it will
be necessary to give an account of the formation of the
true sexual glands, the ovaries and testes.

At the first appearance of the projection from the in-
termediate cell-mass, which we may now call the genital
ridge, a columnar character is not only visible in the
layer of cells covering the nascent ridge itself along its
whole length, but may also be traced for some little dis-
tance outwards on either side of the ridge in the cells
lining the most median portions of both somatopleure and
splanchnopleure. Passing outwards along these layers,
the columnar cells gradually give place to a flat tesse-
lated epithelium. As the ridge continues to increase
and project, the columnar character becomes more and
more restricted to cells covering the ridge itself, over
which at the same time it becomes more distinct. On
the outer side of the ridge, that is on the side which
looks towards the somatopleure, the epithelium under-
goes, as we have seen, an involution to form the com-
mencement of the duct of Muller, and for some little
time retains in the immediate neighbourhood of that
duct its columnar character (Fig. 71, a'), though even-
tually losing it.

VII.] THE SEXLUAL EMINENCE. 221

The median portion of the ridge is occupied by the
projection of the Wolffian body, and here the epithelium
rapidly becomes flattened.

On the inside of the ridge however, that is on the side
looking towards the splanchnopleure, the epithelium not
only retains its columnar character, but grows several
cells deep (Fig. 71, a), while at the same time the meso-
blast (E) underlying it becomes thickened. In this
way, owing partly to the increasing thickness of the
epithelium, and partly to the accumulation of mesoblast
beneath it, a slight eminence is formed, which when
viewed from below, after opening the abdominal cavity,
appears in direct light as a fusiform white patch or
streak, in its early stages extending along the whole
length of the Wolffian body and genital ridge, but sub-
sequently restricted to its anterior portion. Its appear-
ance under these circumstances has been well described
by Von Baer.

This 'sexual eminence' is present in the early stages
of both sexes. In both the epithelium consists of several
layers of short cylindrical cells, a few of which are con-
spicuous on account of their size and their possessing a
highly refractive oval nucleus of considerable bulk; in
both, the underlying thickened mesoblast consists — as
indeed at this epoch it does generally in all parts of the
body — of spindle-shaped cells.

The larger conspicuous cells of the epithelium
which appear to have quite a common origin with their
fellow cells and to arise from them by direct differen-
tiation, and which are seen at the first in male as
well as female embryos, are the primordial ova or pri-
mitive germinal cells (Fig. 71, o). Thus in quite early

222 THE FOURTH DAY. [CHAP.

stages it is impossible to detect the one sex from the
other.

The ovary. In the female the primordial ova en-
large and become more numerous, the whole epithelium
growing thicker and more prominent, and the spindle-
shaped cells of the underlying mesoblast also increase
rapidly and thus form the stroma of the ovary. The
primordial ova after undergoing some further changes,
into which it is not within the scope of this work to
enter, become surrounded by a number of the ordinary
epithelium cells. These form a distinct layer, the folli-
cular epithelium, round the ovum. After a time there
appear numerous vascular ingrowths from the stroma,
which penetrate through all parts of the germinal epi-
thelium and break it up into a sponge-like structure
formed of trabeculse of germinal epithelium interpene-
trated by vascular strands of stroma. The trabeculae
of the germinal epithelium form the egg-tubes of
Pfliiger.

In this way each ovum becomes invested by a cap-
sule of vascular connective tissue lined internally by
a layer of epithelium; the whole constituting a Graafian
follicle.

The large nucleus of the primordial ovum becomes
the germinal vesicle, while the ovum itself remains as
the true ovum ; this subsequently becomes enlarged by
the addition of a quantity of yolk derived from a differ-
entiation of its protoplasm.

The testis. The first traces of the testes are found
in the dorsal and inner side of the intermediate cell-
mass, and appear about the sixth day. From the first
they differ from the rudimentary ovaries, by coming into

VII.] THE TESTIS. 223

somewhat close connection with the Wolffian bodies;
but occupy about the same limits from before back-
wards. The mesoblast in the position we have men-
tioned begins to become somewhat modified, and by
the eighth day the testis is divided by septa of connec-
tive tissue into a number of groups of cells; which are
the commencing tubuli seminiferi. By the sixteenth
day the cells of the tubuli have become larger and
acquired a distinctly epithelial character.

The history of the primordial cells in the male has
not been so thoroughly worked out as in the female.
The spermatozoa appear to arise by the division of
the primitive ova (present, as we have stated, in the
early stages of both sexes), which probably migrate
into the adjacent stroma, accompanied by some of the
indifferent epithelial cells. Here the primitive germi-
nal cells increase in number and give rise to the cells
lining the secretory tubules of the testes.

Outgrowths from the Malpighian bodies of the
Wolffian body appear to be developed, which extend
into the testis and come into connection with the true
seminiferous stroma.

It is evident from the above account that the male
and female generative products are homodynamous,
but the consideration of the development of the pro-
ducts in the two sexes shows that a single spermatozoon
is not equivalent to an ovum, but rather that the whole
of the spermatozoa derived from a primordial ovum are
together equivalent to one ovum.

We have now described the origin of all the parts
which form the urinary and sexual systems, both of the
embryo and adult. It merely remains to speak briefly

224 THE FOURTH DAY. [CHAP.

of the changes, which on the attainment of the adult
condition take place in the parts described.

The Wolffian body, according to Waldeyer, may be
said to consist of a sexual and urinary part, which can,
he states, be easily distinguished in the just-hatched
chick. The sexual part becomes in the cock the after-
testes or coni vasculosi, and consists of tubules which
lose themselves in the seminiferous tubules. In the
hen it forms part of the epoophoron1 of Waldeyer, and
is composed of well-developed tubes without pigment.
The urinary part forms in both sexes a small rudiment,
consisting of blindly ending tubes with yellow pigment ;
it is most conspicuous in the hen. This rudiment
has been called by Waldeyer parepididymis in the male
and paroophoron in the female.

The Wolffian duct remains as the vas deferens in
the male. In the female it becomes atrophied and
nearly disappears.

The duct of Miiller on the right side (that on the
left side with the corresponding ovary generally dis-
appearing) remains in the female as the oviduct. In
the male it is almost entirely obliterated on both
sides.

Vascular system. We may return to the changes
which are taking place in the circulation.

On the fourth day, the point at which the dorsal
aorta divides into two branches is carried much further
back towards the tail.

A short way beyond the point of bifurcation, each
vessel gives off a branch to the newly-formed allantois.

1 This is also called parovarium (His), and Bosenmiiller's organ.

VII.] THE ARTERIAL ARCHES. 225

It is not, however, till the second half of the fourth day,
when the allantois grows rapidly, that these allantoicy
or, as they are sometimes called, umbilical, arteries
acquire any importance, if indeed they are present
before.

The vitelline arteries are before the end of the day
given off from the undivided aortic trunk as a single
but quickly bifurcating vessel, the left of the two
branches into which it divides being much larger than
the right.

During this day, the arterial arch running in the
first visceral fold becomes obliterated, the obliteration
being accompanied by the appearance of a new (fourth)
arch running in the fourth visceral fold on either
side.

The second pair of arterial arches also becomes
nearly (if not entirely) obliterated; but a new pair of
arches is developed in the last (fifth) visceral fold,
behind the last visceral cleft; so that there are still
three pairs of arterial arches, which however now run
in the third, fourth and fifth visceral folds, the last of
these being as yet small. If we reckon in the slight
remains of the second pair of arches we may consider
that there are in all four pairs of arches. When the
first and second arches are obliterated, it is only the
central portion of each arch on either side which abso-
lutely disappears. The ventral portion connected with
the bulbus arteriosus, and the dorsal portion which
joins the dorsal aorta, both remain, and are both carried
straight forward towards the head. The ventral por-
tions of both first and second arches unite on each side
to form a branch, the external carotid (Fig. 73, E.C.A.),
F. & B. 15

226 THE FOURTH DAY. [CHAP.

which runs straight up from the bulbus arteriosus to
the head.

FIG. 73.

STATE OF ARTERIAL CIRCULATION ON THE FIFTH OR SIXTH

DAY.

E.G. A. external carotid. I.C.A. internal carotid. D. A. dorsal
aorta. Of. A. vitelline artery. U.A. allantoic arteries.

In the same way the dorsal portions form a branch,
the internal carotid, which takes its origin from the
dorsal or far end of the third arch.

In the venous system important changes also occur.

As the liver in the course of its formation wraps
round the common trunk of the vitelline veins, or
meatus venosus, it may be said to divide that vessel
into two parts : into a part nearer the heart which is
called the sinus venosus (Fig. 74, S. V.), and into a part
surrounded by the liver which is called the ductus
venosus. Beyond, i.e. behind the liver, the ductus veno-
sus is directly continuous with the vitelline veins, or, as
we may now say, vein, for the right trunk has become
so small as to appear a mere branch of the left (Fig.
74, 0/0.

\
VII.] THE VEINS OF THE LIVER. 227

DIAGRAM OF THE VENOUS CIRCULATION AT THE COMMENCE-
MENT OF THE FIFTH DAY.

H. heart, d.c. ductus CuvierL Into the ductus Cuvieri of each
side fall J. the jugular vein or superior cardinal vein,

W. the vein from the wing, and c. the inferior cardinal vein.
&. V. sinus venosus. Of. vitelline vein. U. allantoic vein,
which at this stage gives off branches to the body-walls.

V.C.I, vena cava inferior. L liver.

The hepatic circulation, which was commenced on
the third day, becomes completely established. Those
branches which come off from the ductus venosus soon
after its entrance between the liver lobes carry blood
into the substance of the liver and are called vence
advehentes, while those which join the ductus venosus
shortly before it leaves the liver (i. e. nearer the heart)
carry blood away from the hepatic substance into the
ductus and are called vence revehentes. As a result of
this arrangement there is a choice of paths for the
blood in passing from the vitelline vein to the sinus
venosus; it may pass through the capillary net- work
of the liver, going in by the venae advehentes and

15—2

228 THE FOURTH DAY. [CHAP.

coming back again by the vense revehentes, or it may
go straight through the ductus venosus without passing
at all into the substance of the liver.

As the alimentary canal by its continued closing in
becomes on the fourth day more and more distinct from
the yolk-sac, it gradually acquires veins of its own, the
mesenteric veins, which first appear as small branches
of the vitelline vein, though eventually, owing to the
change in the relative size and importance of the yolk-
sac and intestine, the latter seems to be a branch of
one of the former.

Corresponding to the increase in the size of the
head, the superior cardinal veins (Fig. 74, J.) become
larger and more important and are joined by the wing
veins (Tf.). As before, they form the ductus Cuvieri
(d.c.) by joining with the inferior cardinal veins (c.).

The latter are now largely developed, they seem to
take origin from the Wolffian bodies, and their size and
importance is in direct proportion to the prominence of
these bodies. They might be called the veins of the
Wolffian bodies.

As the kidneys begin to be formed a new single
median vein makes its appearance, running from them
forwards, beneath the vertebral column, to fall into the
sinus venosus (Fig. 74, V.C.I.). This is the vena cava
inferior.

As the lungs are being formed the pulmonary veins
also make their appearance and become connected with
the left side of the auricular division of the heart.

The blood carried to the allantois by the allantoic
arteries is brought back by two veins, which very soon
after their appearance unite, close to the allantois, into

VII.] THE HEART. 229

a single trunk, the allantoic vein, which, running along
the splanchnopleure, falls into the vitelline vein (Fig.
74, K).

Meanwhile the heart is undergoing considerable
changes. Though the whole organ still exhibits a
marked curvature to the right, the ventricular portion
becomes directed more distinctly ventralwards, forming
a blunted cone whose apex will eventually become the
apex of the adult heart.

The concave (or dorsal) walls of the ventricles be-
come much thicker, as did the convex or ventral walls
on the third day.

Well-marked constrictions now separate the ven-
tricles from the bulbus arteriosus on the one hand, and
from the auricles on the other. The latter constriction
is very distinct, and receives the name of canalis auri-
cularis (Fig. 75, C. A.) ; the former, sometimes called
the /return Halleri, is far less conspicuous.

FIG. 75.

C.A

HEART OF A CHICK ON THE FOURTH DAY OF INCUBATION
VIEWED FROM THE VENTRAL SURFACE.

La. left auricular appendage. C.A. canalis auricularis. v. ven-
tricle. 6. bulbus arteriosus.

230 THE FOURTH DAY. [CHAP.

The most important event is perhaps the formation
of the ventricular septum. This, which commenced on
the third day as a crescentric ridge or fold springing
from the convex or ventral side of the rounded ven-
tricular portion of the heart, now grows rapidly across
the ventricular cavity towards the concave or dorsal
side. It thus forms an incomplete longitudinal par-
tition, extending from the canalis auricularis to the
commencement of the bulbus arteriosus, and dividing
the twisted ventricular tube into two somewhat curved
canals, one more to the left and above, the other to the
right and below. These communicate freely with each
other, above the free edge of the partition, along its
whole length.

Externally the ventricular portion as yet shews no
sign of the division into two parts.

The bulbus arteriosus (Fig. 75, 6.) has increased in
size, and is now very conspicuous.

The venous end of the heart is placed still more
dorsal, and to the left of the arterial end; its walls are
beginning to become thicker.

The auricles are nearly if not quite as far forward
as the ventricles, and the auricular appendages (Fig.
75, La.), which were visible even on the third day, are
exceedingly prominent, giving a strongly marked ex-
ternal appearance of a division of the auricular portion
of the heart into two chambers; but Von Baer was
unable to detect at this date any internal auricular
septum.

The chief events then of the fourth day are : —
(1) The increase of the cranial and body flexure.

VII.] SUMMARY. 231

(2) The increase in the tail-fold.

(3) The formation of the limbs as local thickenings
of the Wolffian ridge.

(4) The formation of the olfactory grooves.

(5) The absorption of the partition between the
mouth and the throat.

(6) The vacuolation of the cells of the notochord.

(7) The formation of the ureter.

(8) The formation of the duct of Muller.

(9) The appearance of the primitive ova in the
germinal epithelium.

(10) The development of a fifth pair of arterial
arches, and the obliteration of the first, and partial
obliteration of the second pair.

(11) The development of the 'canalis auricularis,'
the growth of the septum of the ventricles and of the
auricular appendages.

CHAPTER VIII.

THE CHANGES WHICH TAKE PLACE ON THE FIFTH

DAY.

ON opening an egg about the middle of the fifth
day, the observer's attention is not arrested by any new
features ; but he notices that the progress of develop-
ment, which was so rapid during the later half of
the fourth day, is being continued with undiminished
vigour.

The allantois, which on the fourth day began to
project from the pleuroperitoneal cavity, has grown very
rapidly, and now stretches away from the somatic stalk
far over the right side of the embryo (which it will be
remembered is lying on its left side) in the cavity
between the two amniotic folds (Fig. 9, K.\ It is
very vascular, and already serves as the chief organ of
respiration.

The blastoderm has spread over the whole of the
yolk-sac, and the yolk is thus completely enclosed in
a bag whose walls however are excessively delicate and
easily torn. The vascular area extends over about two-
thirds of the yolk.

The splanchnic stalk or vitelline duct has now
reached its greatest narrowness ; it has become a solid

CHAP. VIII.] THE LIMBS. 233

cord, and will undergo no further change till near the
time of hatching. The space between it and the so-
matic stalk is still considerable, though the latter is
narrower than it was on the fourth day.

The embryo remains excessively curved, so much
so indeed that the head and the tail are nearly in
contact.

The limbs have increased, especially in length ; in
each a distinction is now apparent between the more
cylindrical stalk and the flattened terminal expansion ;
and the cartilaginous precursors of the several bones
have already become visible.

The fore and hind limbs are still exceedingly alike,
and in both the stalk is already beginning to be bent
about its middle to form the elbow and knee respec-
tively.

The angles of both knee and elbow are in the first
instance alike directed outwards and somewhat back-
wards. By the eighth day, however, the elbow has
come to look directly backwards and the knee forwards.
In consequence of this change, the digits of the fore
limb point directly forwards, those of the hind limb
directly backwards. This state of things is altered by
a subsequent rotation of the hand and foot on the arm
and leg, so that by the tenth day the toes are directed
straight forwards, and the digits of the wing backwards
and somewhat ventralwards, the elbow and knee almost
touching each other.

While these changes are taking place the differences
between wing and foot become more and more distinct.
The cartilages of the digits appear on the fifth day as
streaks in the broad flat terminal expansions, from the

234 THE FIFTH DAY. [CHAP.

even curved edge of which they do not project. On the
sixth or seventh day the three digits of the wing (the
median being the longest) and the four (or in some
fowls five) digits of the foot may be distinguished, and
on the eighth or ninth day these begin to project from
the edge of the expanded foot and wing, the substance
of which, thin and more or less transparent, remains for
some time as a kind of web between them. By the
tenth day the fore and hind extremities, save for the
absence of feathers and nails, are already veritable
wings and feet.

Within the mesoblast of the limbs a continuous
blastema becomes formed, which constitutes the first
trace of the skeleton of the limb. The corresponding
elements of the two limbs, viz. the humerus and femur,
radius and tibia, ulna and fibula, carpal and tarsal
bones, metacarpals and metatarsals, and phalanges, be-
come differentiated within this, by the conversion of defi-
nite regions into cartilage, which probably are at first
united. These cartilaginous elements subsequently ossify.

The pectoral girdle. The scapulo-coracoid elements of the
shoulder girdle are formed as a pair of cartilaginous plates,
one on each side of the body. The dorsal half of each plate
ossifies as the scapula, the ventral as the coracoid. The clavicles
are probably membrane bones.

The pelvic girdle is derived from a pair of cartilaginous
plates, one on each side. Each of them is developed in con-
tinuity with the femur of its side. The dorsal half of each plate
ossifies as the ilium ; the ventral half becomes prolonged into
two processes, the anterior of which ossifies as the pubis, the
posterior at the ischium.

Ribs and sternum. The ribs appear to arise as
cartilaginous bars in the connective tissue of the body

VIII.] THE CRANIUM. 235

walls. They are placed opposite the intervals between
the muscle-plates, and are developed independently of
the vertebrae, with the transverse processes of which
they subsequently become closely united by fibrous
tissue.

The sternum appears to be formed from the fusion
of the ventral extremities of a certain number of the
ribs. The extremities of the ribs unite with each other
from before backwards, and thus give rise to two car-
tilaginous bands. These bands become segmented off
from the ribs with which they are at first continuous,
and subsequently fuse in the median ventral line to
form the unpaired sternum.

The skull. Two distinct sets of elements enter into
the composition of the avian skull. These are (1) the
cranium proper, (2) the skeleton of the visceral arches.

The cranium. As we mentioned in the last chap-
ter, the formation of the primitive cranium commenced
upon the fourth day. This primitive cranium, in its
earliest stage, inasmuch as it is composed of condensed
but otherwise only slightly differentiated mesoblast, may
be spoken of as the membranous cranium. On the sixth
day true hyaline cartilage makes its appearance as a
differentiation within the membranous cranium. The
cartilaginous cranium is composed of the following parts.

(1) A pair of cartilaginous plates placed on each
side of the cephalic section of the notochord, and known
as the parachordals (Fig. 76, iv.). These plates, together
with the notochord (nc.) enclosed between them, form a
floor for the hind- and mid-brain. The continuous plate,
formed by them and the notochord, is known as the
basilar plate.

236

THE FIFTH DAY.
FIG. 76.

[CHAR

YlEW FROM ABOVE OF THE PARACHORDALS AND OF THE TRABE-

CUL.E ON THE FIFTH DAY OF INCUBATION. (From Parker.)

In order to shew this the whole of the upper portion of the
head has been sliced away. The cartilaginous portions of the
skull are marked with the dark horizontal shading.

c.v. 1. cerebral vesicles (sliced off), e. eye. nc. notochord.
iv. parachordal. 9. foramen for the exit of the ninth nerve.
d. cochlea, h.s.c. horizontal semi-circular canal, q. quad-
rate. 5. notch for the passage of the fifth nerve. Ig. ex-
panded anterior end of the paracbordals. pt.s. pituitary
space, tr. trabeculse. The reference line tr has accidentally
been made to end a little short of the cartilage.

(2) A pair of bars forming the floor for the fore-
brain, and known as the trabeculce (tr.). These bars are
continued forward from the parachordals, with which, in
the chick, they are from the first continuous. United

VIII.] THE PARACHORDALS. 237

behind where they embrace the front end of the noto-
chord, they diverge anteriorly for some little distance and
then bend in again in such a way as to enclose a space
— the pituitary space. In front of this space they again
unite and extend forwards into the nasal region.

(3) The cartilaginous capsules of the sense organs.
Of these the auditory and olfactory capsules unite more
or less intimately with the cranial walls, while the optic
capsules, forming the sclerotics, remain distinct.

The parachordals and notochord. The first of
these sets of elements, viz. the parachordals *and noto-
chord, forming together the basilar plate, is an unseg-
mented continuation of the axial tissue of the vertebral
column. It forms the floor for that section of the brain
which belongs to the primitive postoral part of the
head, and its extension is roughly that of the basiocci-
pital of the adult skull.

Laterally it encloses the auditory sacs (Fig. 76), the
tissue surrounding these (forming the so-called ' periotic
capsules') is in. the chick never separate from the basi-
lar plate. In front it becomes narrowed, and at the
same time excavated so as to form a notch on each side
(Fig. 76, 5) through which the fifth nerve passes ; and
in front of this it again becomes expanded.

In order to render our subsequent account more
intelligible, we may briefly anticipate the fate of the
basilar plate. Behind it grows upwards on both sides, and
the two outgrowths meet above so as completely to enclose
the medulla oblongata, and to circumscribe a hole known
as the ' occipital foramen? And it is at this point only
that the roof of the skull is at any period formed of
cartilage.

238 THE FIFTH DAY. [CHAP.

It will be convenient to say a few words here with reference
to the notochord in the head. It always extends along the floor
of the mid- and hind-brains, but ends immediately behind the
infundibulum. The front end of the notochord becomes more or
less ventrally flexed in correspondence with the cranial flexure ;
its anterior end being in some animals (Elasmobranchii) almost
bent backwards (Fig. 77).

FIG. 77.
ml)

LONGITUDINAL SECTION THROUGH THE HEAD OF A YOUNG
PRISTIURUS EMBRYO.

cer. commencement of the cerebral hemisphere, pn. pineal gland.
In. infundibulum. pt. ingrowth from mouth to form the
pituitary body. mb. mid-brain, cb. cerebellum, ch. noto-
chord. al. alimentary tract. laa. artery of mandibular
arch.

Kolliker has shewn that in the Rabbit, and a more or less
similar phenomenon may also be observed in Birds, the anterior
end of the notochord is united to the hypoblast of the throat in
immediate contiguity with the opening of the pituitary body ;
but it is not clear whether this is to be looked upon as the
remnant of a primitive attachment of the notochord to the hypo-
blast, or as a secondary attachment.

Within the basilar plate the notochord often exhibits two or
more dilatations, which have been regarded by Parker and
Kolliker as indicative of a segmentation of this plate ; but they
hardly appear to be capable of this interpretation.

VIII.] THE TRABECULSE. 239

The trabeculae. The trabeculse, so far as their
mere anatomical relations are concerned, play the same
part in forming the floor for the front cerebral vesicle
as the parachordals for the mid- and hind-brains. They
differ however from the parachordals in one important
feature, viz. that, except at their hinder end, they do
not embrace between them the notochord.

The notochord constitutes, as we have seen, the
primitive axial skeleton of the body, and its absence in
the greater part of the region of the trabeculse would
probably seem to indicate, as pointed out by Gegen-
baur, that these parts, in spite of their similarity to
the parachordals, have not the same morphological
significance.

While this distinction between the parachordals and the
trabeculse must be admitted, there seems to be no reason against
supposing that the trabeculse may be plates developed to support
the floor of the fore-brain, for the same physiological reasons
that the parachordals have become formed at the sides of the
notochord to support the floor of the hind-brain. By some
anatomists the trabeculse have been held to be a pair of branchial
bars ; but this view has now been generally given up. They
have also been regarded as equivalent to a complete pair of
neural arches enveloping the front end of the brain. The primi-
tive extension of the base of the fore-brain through the pituitary
space is an argument, not without force, which has been appealed
to in support of this view.

In the majority of the lower forms the trabeculse
arise quite independently of the parachordals, though
the two sets of elements soon unite ; while in Birds
(Fig. 76) and Mammals the parachordals and trabeculse
are formed as a continuous whole. The junction be-

240 THE FIFTH DAY. [CHAP.

tween the trabeculse and parachordals becomes marked
by a cartilaginous ridge known as the posterior clinoid.
The trabeculse are somewhat lyre-shaped, meeting in
front and behind, and leaving a large pituitary space
between their middle parts (Fig. 76). Into this space
there primitively projects the whole base of the fore-brain,
but the space itself gradually becomes narrowed, till it
usually contains only the pituitary body. The carotid
arteries pass through it in the embryo ; but it ceases to
be perforated in the adult. The trabeculso soon unite
together, both in front and behind, and form a complete
plate underneath the fore-brain, ending in two horns in
the interior of the fronto-nasal process. A special ver-
tical growth of this plate in the region of the orbit
forms the interorbital plate (Fig. 78, ps.), on the upper
surface of which the front part of the brain rests. The
trabecular floor of the brain does not long remain
simple. Its sides grow vertically upwards, forming a
lateral wall for the brain, in which two regions may
be distinguished, viz. an alisphenoidal region (Fig. 78,
as.) behind, growing out from what is known as the
basisphenoidal region of the primitive trabeculse, and
an orbitosphenoidal region in front growing out from
the presphenoidal region of the trabeculse. These
plates form at first on each side a continuous lateral
wall of the cranium. At the front end of the
brain they are continued inwards, and more or less
completely separate the true cranial cavity from the
nasal region in front. The region of the trabeculse in
front of the brain is the ethmoidal region ; it forms the
anterior boundary of the cranial cavity. The basal part
of this region forms an internasal plate, from which an

VIII.]

THE SENSE CAPSULES.

241

internasal septum, continuous behind with the inter-
orbital septum, grows up (Fig. 78); while the lateral

FIG. 78.

bb r

SlDE VIEW OF THE CARTILAGINOUS CRANIUM OF A FOWL ON THE

SEVENTH DAY OF INCUBATION. (After Parker.)

pn. preuasal cartilage, aln. alinasal cartilage, ale. aliethmoid ;
immediately below this is the aliseptal cartilage, eth. eth-
moid, pp. pars plana. ps. presphenoid or inter-orbital.
pa. palatine, pg. pterygoid. z. optic nerve, as. alisphenoid.
q. quadrate, st. stapes, fr. fenestra rotunda, hso. horizon-
tal semicircular canal, psc. posterior vertical semicircular
canal : both the anterior and the posterior semicircular
canals are seen shining through the cartilage, so. supra-
occipital, eo. exoccipital. oc. occipital condyle. nc. noto-
chord. mJc. Meckel s cartilage, ch. cerato-hyal. bh. basi-
hyal. cbr. and ebr. cerato-branchial. bbr. basibranchial.

part is known as the lateral ethmoid region, which
is always perforated for the passage of the olfactory
nerve.

The sense capsules. The most important of these

is the auditory capsule, which, as we have seen, fuses

intimately with the lateral walls of the skull. In front

there is usually a cleft separating it from the alisphe-

F. &B. 16

242 THE FIFTH DAY. [CHAP.

noid region of the skull, through which the third
division of the fifth nerve passes out. This cleft be-
comes narrowed to a small foramen. The sclerotic is
free, but profoundly modifies the region of the cranium
near which it is placed. The nasal investment is de-
veloped in continuity, and is closely united, with the
ethmoid region.

The cartilaginous cranium, the development of
which has been thus briefly traced, persists in the
adult without even the addition of membrane bones
in certain fishes, e.g. the Elasmobranchii. In the Sela-
chioid Ganoids it is also found in the adult, but is
covered over by membrane bones. In all other types
it is invariably present in the embryo, but becomes in
the adult more or less replaced by osseous tissue.

The bones in the adult skull may be divided
roughly into two categories according to their origin.

(1) Cartilage bones, i.e. ossifications in the primi-
tive cartilaginous cranium.

(2) Membrane bones, i.e. ossifications in membrane
without any cartilaginous precursors.

The names which have been given to the various
parts of the cartilaginous cranium in the above account
are derived from the names given to the bones appear-
ing in the respective regions in the more developed
skull.

The skeleton of the visceral arches. The visceral
arches were all originally branchial in function. They
supported the walls between successive branchial clefts.

The first arch (mandibular) has in all living forms
lost its branchial function, and its bar has become con-
verted into a supporting skeleton for the jaws.

VIII.] THE MANDIBULAR ARCH. 243

The second arch (hyoid), with its contained bar,
though retaining in some forms (Elasmobranchii) its
branchial function, has in most acquired additional
functions, and has undergone in consequence various
peculiar modifications.

The succeeding arches and their contained bars
retain their branchial function in Pisces and some
Amphibia, but are secondarily modified and largely
aborted in the abranchiate forms.

The ordinary visceral arches in the chick are, as we
have seen, sufficiently obvious, while as yet their meso-
blast is quite undifferentiated ; but in the three ante-
rior of them rods of cartilage are subsequently deve-
loped and begin to make their appearance about the
fifth day.

The first arch (mandibular), it will be remembered,
budded off a process called the superior maxillary pro-
cess. The whole arch, therefore, comes to consist of
two parts, viz. a superior and an inferior maxillary pro-
cess ; it is in the latter of these that the cartilaginous rod
on each side is developed. The membranous tissue in the
superior maxillary process is called, from its subsequent
fate, the ptery go-palatine bar, and is in the chick ossified
directly without the intervention of cartilage. In the
inferior maxillary process two developments of cartilage
take place, a proximal and a distal. The proximal
cartilage is situated (Figs. 76 and 79, q.) at the side
of the periotic capsule, but is not united with it. It is
known as the quadrate, and in the early stage is merely
a small knob of cartilage. The quadrate cartilage os-
sifies as the quadrate bone, and supplies the permanent
articulation for the lower jaw. The distal rod is called

16—2

244 THE FIFTH DAY. [CHAP.

MeckeUs cartilage (Fig. 79, mk.) ; it soon becomes
covered by investing (membrane) bones which form
the mandible ; and its proximal end ossifies as the
articulare.

FIG. 79.

VIEW FROM BELOW OF THE PAIRED APPENDAGES OF THE SKULL
OF A FOWL ON THE FlFTH DAT OF INCUBATION. (From

Parker.)

cv. 1. cerebral vesicles, e. eye. fn. fronto-nasal process, n. nasal
pit. tr. trabeculse. pts. pituitary space, mr. superior
maxillary process, pg. pterygoid. pa. palatine, q. quad-
rate, mk. Meckel's cartilage, ch. cerato-hyal. bh. basi-
hyal. cbr. ceratobranchial. ebr. proximal portion of the
cartilage in the third visceral arch. bbr. basibranchial.
1. first visceral cleft. 2. second visceral cleft. 3. third vis-
ceral arch.

In the next arch, usually called the second visceral
or hyoid arch, there is a very small development of
cartilage. This consists of a central azygos piece, the

VIII.] THE COLUMELLA. 245

'basi-hyal' (Fig. 79, bh.), and two rods, one on each
side, the ' cerato-hyals ' (Fig. 79, ch.).

In the third arch, which corresponds with the first
branchial arch of the Ichthyopsida, there is on each
side a large distal cartilaginous rod (Fig. 79, cbr.), the
' cerato-branchial/ and a smaller proximal piece (Fig.
79, ebr.), between the two arches lies an undefined
mass (Fig. 79, bbr), the ' basibranchial.' In the arches
behind this one there is in the bird no development of
cartilage.

The lower part of the hyoid arch, including the
basi-hyal, unites with the remnants of the arch behind
to form the hyoid bone of the adult.

The fenestra ovalis and fenestra rotunda appear
on the seventh day as spaces in the side walls of the
periotic cartilage. The former is filled up by a small
piece of cartilage, the stapes (Fig. 78, st.), which in the
adult forms part of the columella (see pp. 166, 167).

The columella is believed by Huxley and Parker to represent
the independently developed dorsal element of the hyoid, together
with the stapes with which it has become united.

For further details of the development of the skull
we must refer the student to Professor Parker's Memoir
upon the Development of the Skull of the Common
Fowl (Gallus domesticus), Phil. Trans., 1866, Vol. CLVL,
pt. 1, and to the chapter on the Bird's skull in the
Morphology of the Skull, by Professor Parker and
Mr Bettany.

We shall conclude this account by giving a table of
those bones which are preformed in cartilage, and of the
purely splint or membrane bones.

246 THE FIFTH DAY. [CHAP.

Parts of the bird's skull which are either preformed
in cartilage or remain cartilaginous.

Formed from the parachordal cartilages and their
upgrowths around the foramen magnum. — Supraocci-
pital. Exoccipital. Basioccipital.

Formed in the periotic cartilage. — Epiotic. Prootia
Opisthotic.

Formed from the trabeculse and their upgrowths,
— Alisphenoid. Basisphenoid. Orbitosphenoid. Pre-
sphenoid. Ethmoid. Septum nasi, turbinals, prenasal
and nasal cartilages.

Articulare and quadrate belonging to the first
visceral arch. Skeleton of the second and third visceral
arches and stapes.

Splint-bones not preformed in cartilage.

Parietals. Squamosals. Frontals. Lacrymals.
Nasals. Premaxillse. Maxillae. Maxillo-palatines,
Vomer. Jugals. Quadrato-jugals. Dentary and
bones of mandible. Basi-temporal and rostrum. Ptery-
goid and palatine (superior maxillary process).

The face. Closely connected with the development
of the skull is the formation of the parts of the face.

After the appearance of the nasal grooves on the
fourth day the mouth (Fig. 80, M.) appears as a deep
depression inclosed by five processes. Its lower border
is entirely formed by the two inferior maxillary pro-
cesses (Fig. 80, F. 1), at its sides lie the two superior
maxillary processes S. M., while above it is bounded by
the fronto-nasal process nf.

VIII.] THE FRONTO-NASAL PROCESS.

FIG. 80.

247

A. HEAD OF EMBRYO CHICK OF THE FOURTH DAY VIEWED FROM

BELOW AS AN OPAQUE OBJECT. (Chromic acid preparation.)

C.ff. cerebral hemispheres. FB. vesicle of the third ventricle.
Op. eyeball, nf. naso-frontal process. M. cavity of mouth.
S.M. superior maxillary process of F. 1, the first visceral
fold (mandibular arch). F. 2, F. 3, second and third
visceral folds. N. nasal pit.

In order to gain the view here given the neck was cut across
between the third and fourth visceral folds. In the section e
thus made, are seen the alimentary canal al with its collapsed
walls, the neural canal ra.c., the notochord ch., the dorsal aorta
AO.j and the jugular veins V.

B. The same seen sideways, to shew the visceral folds.
Letters as before.

After a while the outer angles of the fronto-nasal
process, enclosing the expanded termination of the
trabeculae, project somewhat outwards on each side,
giving the end of the process a rather bilobed appear-
ance. These projecting portions of the fronto-nasal pro-
cess form on each side the inner margins of the rapidly

248 THE FIFTH DAY. [CHAP.

deepening nasal grooves, and are sometimes spoken of
as the inner nasal processes. The outer margin of each
nasal groove is raised up into a projection frequently
spoken of as the outer nasal process, which runs down-
wards to join the superior maxillary process, from which
however it is separated by a shallow depression. This
depression, which runs nearly horizontally outwards
towards the eyeball, is known as the lacrymal groove
(see p. 155).

On the fifth day the inner nasal processes, or lower
and outer corners of the fronto-nasal process, arching
over, unite on each side with the superior maxillary
processes. (Compare Fig. 81, which, however, is a view
of the head of a chick of the sixth day.) In this way
each nasal groove is converted into a canal, which leads

FIG. 81.

HEAD OF A CHICK AT THE SIXTH DAY FROM BELOW. (From
Huxley.)

la. cerebral vesicles, a. eye, in which the remains of the choroid
slit can still be seen. g. nasal pits. k. fronto-nasal process.
1. superior maxillary process. 1. inferior maxillary process

VIII. ] THE MOUTH. 249

or first visceral arch. 2. second visceral arch. x. first vis-
ceral cleft between the first and second visceral arches.

The cavity of the mouth is seen enclosed by the fronto-nasal
process, the superior maxillary processes and the first pair of
visceral arches. At the back of it is seen the opening leading
into the throat. The nasal grooves leading from the nasal pits
to the mouth are already closed over and converted into canals.

from the nasal pit above, into the cavity of the mouth
below, and places the two in direct communication.
This canal, whose lining consists of epiblast, is the
rudiment of the nasal labyrinth.

By the seventh day (Fig. 82), not only is the union
of the superior maxillary and fronto-nasal processes
completed, and the upper boundary of the mouth thus
definitely constituted, but these parts begin to grow
rapidly forward, thus deepening the mouth and giving
rise to the appearance of a nose or beak (Fig. 82),
which, though yet blunt, is still distinct. The whole of
the lower boundary of the buccal cavity is formed by
the inferior maxillary processes.

As we have before mentioned (p. 240), cartilage suc-
ceeded by bone is developed in the fronto-nasal process ;
the pterygo-palatine osseous bar (membranous ossifica-
tion) in the superior maxillary process; Meckel's cartilage
the main part of which atrophies, the proximal end only
ossifying as the articulare, and the quadrate succeeded
by bone in the inferior maxillary process; the other
bones which form the boundaries of the mouth in the
adult are developed later after all external trace of these
parts as separate processes has "disappeared.

At first the mouth is a simple cavity into which the
nasal canals open directly. When however the various

250 THE FIFTH DAY. [CHAP.

FIG. 82.

HEAD OF A CHICK OF THE SEVENTH DAY FROM BELOW. (From
Huxley.)

la. cerebral vesicles, a. eye. g. nasal pits. L fronto-nasal
process. 1. superior maxillary process. 1. first visceral
arch. 2. second visceral arch. x. first visceral cleft.

The external opening of the mouth has become much con-
stricted, but it is still enclosed by the fronto-nasal process and
superior maxillary processes above, and by the inferior maxillary
processes (first pair of visceral arches) below.

The superior maxillary processes have united with the fronto-
nasal process, along the whole length of the latter, with the
exception of a small space in front, where a narrow angular
opening is left between the two.

processes unite together to form the upper boundary of
the mouth, each superior maxillary process sends in-
wards a lateral bud. These buds become flattened and
form horizontal plates which stretch more and more
inward towards the middle line. There they finally
meet, and by their union, which is effected first in front
and thence extends backwards, constitute a horizontal

VIII.] THE SPINAL CORD. 251

plate stretching right across the mouth and dividing
it into two cavities — an upper and a lower one.

In the front part of the mouth their union is quite
complete, so that here there is no communication between
the two cavities. Behind, however, the partition is not
a complete one, so that the two divisions of the buccal
cavity communicate at the back of the mouth. The
external opening of * the mouth passes into the lower of
these two cavities, which may therefore be called the
mouth proper. Into the upper chamber the nasal
ducts open ; it may be called the respiratory chamber,
and forms the commencement of the chamber of the
nose. In birds generally the upper nasal cavity be-
comes subsequently divided by a median partition into
two chambers, which communicate with the back of
the mouth by separate apertures, the posterior nares.
The original openings of the nasal pits remain as the
nostrils.

The spinal cord. — On this day important changes
take place in the spinal cord; and a brief history of
the development of this organ may fitly be introduced
here.

At the beginning of the third day the cavity of the
neural canal is still of considerable width, and when
examined in vertical section its sides may be seen to be
nearly parallel, though perhaps approximating to each
other more below than above.

The exact shape varies according to the region of
the body from which the section is taken.

The epiblast walls are at this time composed of
radiately arranged columnar cells. The cells are much
elongated, but somewhat irregular; and it is very

252 THE FIFTH DAY. [CHAP.

difficult in sections to make out their individual
boundaries. They contain granular oval nuclei in
which a nucleolus can almost always be seen. The
walls of the canal are both anteriorly and posteriorly
considerably thinner in the median plane than in the
middle.

Towards the end of the third day changes take
place in the shape of the cavity. In the lumbar region
its vertical section becomes more elongated, and at the
same time very narrow in the middle while expanded
at each end into a somewhat bulbous enlargement, pro-
ducing an hour-glass appearance (Fig. 65). Its walls
however still preserve the same histological characters
as before.

On the fourth day (Fig. 68) coincidently with the
appearance of the spinal nerves, important changes
may be observed in the hitherto undifFerentiated epi-
blastic walls, which result in its differentiation into (1)
the epithelium of the central canal, (2) the grey matter
of the cord, and (3) the external coating of white
matter.

The white matter is apparently the result of a
differentiation of the outermost parts of the superficial
cells of the cord into longitudinal nerve-fibres, which
remain for a long period without a medullary sheath.
These fibres appear in transverse sections as small dots.
The white matter forms a transparent investment of
the grey matter; it arises as four patches, viz. an anterior
and a posterior white column on each side, which lie on
a level with the origin of the anterior and posterior
nerve-roots. It is always, at first, a layer of extreme
tenuity, but rapidly increases in thickness in the sub-

VIII.]

THE GREY MATTER.

258

sequent stages, and extends so as gradually to cover the
whole cord (Fig. 83).

FIG. 83.

pen

lew

SECTION THROUGH THE SPINAL CORD OF A SEVEN DAYS'
CHICK.

pew. dorsal white column, lew. lateral white column, acw. ven-
tral white column, c. dorsal tissue filling up the part where
the dorsal fissure will be formed, pc. dorsal grey cornu.
ac. anterior grey cornu. ep. epithelial cells, age. anterior
commissure, pf. dorsal part of spinal canal, spc. ventral
part of spinal canal, af. anterior fissure.

The grey matter and the central epithelium are
formed by a differentiation of the main mass of the
walls of the medullary canal. The outer cells lose their

254 THE FIFTH DAY. [CHAP.

epithelial-like arrangement, and, becoming prolonged
into fibres, give rise to the grey matter, while the inner-
most cells retain their primitive arrangement, and con-
stitute the epithelium of the canal. The process of
formation of the grey matter would appear to proceed
from without inwards, so that some of the cells which
have, on the formation of the grey matter, an epithelial-
like arrangement, subsequently become converted into
true nerve-cells.

The central epithelium of the nervous system pro-
bably corresponds with the so-called epidermic layer of
the epiblast.

The grey matter soon becomes prolonged dorsally
and ventrally into the posterior and anterior horns. Its
fibres may especially be traced in two directions : — (1)
round the anterior end of the spinal canal, immediately
outside its epithelium and so to the grey matter on
the opposite side, forming in this way an anterior grey
commissure, through which a decussation of the fibres
from the opposite sides is effected : (2) dorsalwards
along the outside of the lateral walls of the canal.

There is at this period (fourth day) no trace of the
ventral or dorsal fissure, and the shape of the central
canal is not very different from what it was at an earlier
period. This condition of the spinal cord is especially
instructive as it is very nearly that which is permanent
in Amphioxus.

The next event of importance is the formation of
the ventral or anterior fissure. This begins on the fifth
day and owes its origin to a downgrowth of the an-
terior horns of the cord on each side of the middle line.
The two downgrowths enclose between them a some-

VIII.] THE POSTEKIOK FISSURE. 255

what linear space — the anterior fissure — which in-
creases in depth in the succeeding stages (Fig. 83, a/).

The dorsal or posterior fissure is formed at a later
period (about the seventh day) than the anterior, and
accompanies the atrophy of the dorsal section of the
embryonically large canal of the spinal cord. The exact
mode of its formation appears to be still involved in
some obscurity.

It seems probable, though further investigations on the point
are still required, that the dorsal fissure is a direct result of the
atrophy of the dorsal part of the central canal of the spinal
cord. The walls of this coalesce dorsally, and the coalescence
gradually • extends inwards, so as finally to reduce the central
canal to a minute tube, formed of the ventral part of the original
canal. The epithelial wall formed by the coalesced walls on the
dorsal side of the canal is gradually absorbed.

The epithelium of the central canal, at the period when its
atrophy commences, is not covered dorsally either by grey or
white matter, so that, with the gradual reduction of the dorsal
part of the canal and the absorption of the epithelial wall formed
by the fusion of its two sides, a fissure between the two halves of
the spinal cord becomes formed. This fissure is the posterior or
dorsal fissure. In the process of its formation the white matter
of the dorsal horns becomes prolonged so as to line its walls ; and
shortly after its formation the dorsal grey commissure makes its
appearance ; this is not improbably derived from part of the
epithelium of the original central canal.

Meanwhile an alteration is taking place in the ex-
ternal outline of the cord. From being, as on the
fourth and fifth days, oval in section, it becomes, chiefly
through the increase of the white matter, much more
nearly circular.

By the end of the seventh day the following im-

256 THE FIFTH DAY. [CHAP.

portant parts of the cord have been definitely es-
tablished :

(1) The anterior and posterior fissures.

(2) The anterior and posterior horns of grey
matter.

(3) The anterior, posterior and lateral columns
of white matter.

(4) The spinal canal.

As yet, however, the grey masses of the two sides of
the cord only communicate by the anterior grey com-
missure, and the white columns of opposite sides do
not communicate at all. The grey matter, moreover,
still far preponderates over the white matter in
quantity.

By the ninth day the posterior fissure is fully
formed, and the posterior grey commissure has also
appeared.

In the centre of the sacral enlargement this com-
missure is absent, and the posterior columns at a later
period separate widely and form the ' sinus rhomboi-
dalis/ which is not, as has been sometimes stated, the
remains of the primitive 'sinus rhomboidalis ' visible
during the second day.

The anterior white columns have much increased on
this day, and now form the sides of the already deep
anterior fissure. The anterior white commissure does
not however appear till somewhat later.

The heart. The fifth day may perhaps be taken
as marking a most important epoch in the history of
the heart. The changes which take place on that and
on the sixth day, added to those previously undergone,

VIII.] THE VENTRICULAR SEPTUM. 257

transform the simple tube of the early days of in-
cubation into an almost completely formed heart.

The venous end of the heart, though still lying
somewhat to the left and dorsal, is now placed as far
forwards as the arterial end, the whole organ appearing
to be drawn together. The ventricular septum is com-
plete.

The apex of the ventricles becomes more and more
pointed. In the auricular portion a small longitudinal
fold appears as the rudiment of the auricular septum,
while in the canalis auricula ris, which is now at its
greatest length, there is also to be seen a commencing
transverse partition tending to separate the cavity of
the auricles from those of the ventricles.

About the 106th hour, a septum begins to make its
appearance in the bulbus arteriosus in the form of a
longitudinal fold, which according to Tonge (Proc.
of Royal Soc. 1868) starts, not (as Von Baer thought)
at the end of the bulbus nearest to, but at that farthest
removed from, the heart. It takes origin from the wall
of the bulbus between the fifth and fourth pairs of
arches and grows backwards in such a manner as to
divide the bulbus into two channels, one of which leads
from the heart to the fourth and third pair of arches
and the other to the fifth pair. The free edge of the
septum is somewhat Y-shaped, so that its two legs as
it were project backwards towards the heart, further
than its central portion ; and this shape of the free
edge is maintained during the whole period of its
growth. Its course backwards is not straight but
spiral, and thus the two channels into which it divides
the bulbus arteriosus wind spirally the one over the
F. &B. 17

258 THE FIFTH DAY. [CHAP.

other. The existence of the septum can only be as-
certained at this stage by dissection or by sections,
there being as yet no external signs of the division.

At the time when the septum is first formed, the
opening of the bulbus arteriosus into the ventricles is
narrow or slit-like, apparently in order to prevent the
flow of the blood back into the heart. Soon after the
appearance of the septum, however, semilunar valves
(Tonge, loc. cit.) are developed from the wall of that
portion of the bulbus which lies between the free edge
of the septum and the cavity of the ventricles.

These arise as six solid outgrowths of the wall
arranged in pairs, a ventral, a dorsal, and an outer pair,
one valve of each pair belonging to the one and the
other to the other of the two main divisions of the
bulbus which are now being established.

The ventral and the dorsal pairs of valves are the
first to appear: the former as two small prominences
separated from each other by a narrow groove, the
latter as a single shallow ridge, in the centre of which
is a prominence indicating the point where the ridge
will subsequently become divided into two. The outer
pair of valves appear opposite each other, at a con-
siderably later period, between the ends of the other
pair of valves on each side.

As the septum grows backwards towards the heart,
it finally reaches the position of these valves. One of
its legs then passes between the two ventral valves,
and the other unites with the prominence on the dorsal
valve-ridge. At the same time the growth of all the
parts causes the valves to appear to approach the heart
and thus to be placed quite at the top of the ventricular

VIII.] THE BULBUS ARTERIOSUS. 259

cavities. The free edge of the septum of the bulbus
now fuses with the ventricular septum, and thus the
division of the bulbus into two separate channels, each
provided with three valves, and each communicating
with a separate side of the heart, is complete, the po-
sition of the valves not being very different from what
it is in the adult heart.

That division of the bulbus which opens into the
fifth pair of arches is the one which communicates with
the right ventricle, while that which opens into
the third and fourth pairs communicates with the left
ventricle (vide Fig. 93). The former becomes the pul-
monary artery, the latter the commencement of the
systemic aorta.

The external constriction actually dividing the bul-
bus into two vessels does not begin to appear till the
septum has extended some way back towards the heart.

The semilunar valves become pocketed at a period
considerably later than their first formation (from the
147th to the 165th hour) in the order of their ap-
pearance.

Towards the end of the fifth and in the course of the
sixth day further important changes take place in the
heart.

The venous end with its two very conspicuous au-
ricular appendages, comes to be situated more dorsal
to the arterial end, though it still turns rather towards
the left. The venous portion of the heart undergoes
on the sixth day, or even near to the end of the fifth,
such a development of the muscular fibres of its walls
that the canalis auricularis becomes almost entirely
concealed. The point of the heart is now directed

17—2

260

THE FIFTH DAY.

[CHAP,

nearly backwards (i. e. towards the tail), but also a little
ventralwards.

An alteration takes place during the sixth day in
the relative position of the parts of the ventricular
division of the heart. The right ventricle is now turned
towards the abdominal surface, and also winds to a
certain extent round the left ventricle. It will be
remembered that on the fourth day the right ventricle
was placed dorsal to the left.

The right ventricle is now also the smaller of the
two, and the constriction which divides it from the left
ventricle does not extend to the apex of the heart
(Fig. 84). It has, however, a very marked bulge to-
wards the right.

FIG. 84.
A B

Two VIEWS OF THE HEART OF A CHICK UPON THE FIFTH

DAY OF INCUBATION.
A. from the ventral, B. from the dorsal side.

l.a. left auricular appendage, r.a. right auricular appendage.
r.v. right ventricle, l.v. left ventricle, b. bulbus arteriosus.

At first the bulbus arteriosus appeared to come off
chiefly from the left ventricle ; during the fifth day, and
still more on the sixth, it appears to come from the

VIII.] THE BULBUS ARTERIOSUS. 261

right chamber. This is caused by the canal from the
right ventricle into the bulbus arteriosus passing to-
wards the left, and on the ventral side, so as entirely
to conceal the origin of the canal from the left chamber
of the heart. On the seventh day the bulbus arteriosus
appears to come less markedly from the right side of
the heart.

All these changes, however, of position of the bulbus
arteriosus only affect it externally; during the whole
time the two chambers of the heart open respectively
into the two divisions of the bulbus arteriosus. The
swelling of the bulbus is much less marked on the
seventh day than it was before.

At the end of the sixth day, and even on the fifth
day (Figs. 84, 85), the appearance of the heart itself,

FIG. 85.

Iv

HEART OF A CHICK UPON THE SIXTH DAY OF INCUBATION,
FROM THE VENTRAL SURFACE.

La. left auricular appendage, r.a. right auricular appendage.
r.v. right ventricle, l.v. left ventricle, b. bulbus arteriosus.

without reference to the vessels which come from it,
is not very dissimilar from that which it presents when
adult.

262 THE FIFTH DAY. [CHAP.

The original curvature to the right now forms the
apex of the ventricles, and the two auricular appendages
are placed at the anterior extremity of the heart.

The most noticeable difference (in the ventral view)
is the still externally undivided condition of the bulbus
arteriosus.

The subsequent changes which the heart undergoes
are concerned more with its internal structure than
with its external shape. Indeed, during the next three
days, viz. the eighth, ninth, and tenth, the external
form of the heart remains nearly unaltered.

In the auricular portion, however, the septum which
commenced on the fifth day becomes now more con-
spicuous. It is placed vertically, and arises from the
ventral wall; commencing at the canalis auricularis
and proceeding backwards, it does not as yet reach the
opening into the sinus venosus.

The blood from the sinus, or, as we may call it, the
inferior vena cava, enters the heart obliquely from the
right, so that it has a tendency to flow towards the left
auricle of the heart, which is at this time the larger of
the two.

The valves between the ventricles and auricles are
now well developed, and it is about this time that the
division of the bulbus arteriosus into the aorta and
pulmonary artery becomes visible on the exterior.

By the eleventh or thirteenth day the right auricle
has become as large as the left, and the auricular sep-
tum much more complete, though there is still a small
opening, the foramen ovale, by which the two cavities
communicate with each other. Through this foramen
the greater part of the blood of the vena cava inferior,

VIII.] THE EUSTACHIAN VALVE. 263

which is now joined just at its entrance into the heart
by the right vena cava superior, is directed into the left
auricle. The left vena cava superior enters the right
auricle independently; between it and the inferior vena
cava is a small valve which directs its blood entirely
into the right auricle.

On the sixteenth day the right vena cava superior,
when viewed from the exterior, still appears to join the
inferior vena cava before entering the heart ; from the
interior however the two can now be seen to be sepa-
rated by a valve. This valve, called the 'Eustachian
valve/ extends to the opening of the left vena cava
superior, and into it the valve which in the earlier
stage separated the left superior and inferior vense
cavse has apparently become merged. There is also on
the left side of the opening of the inferior cava a mem-
brane stretching over the foramen ovale, and serving as
a valve for that orifice. The blood from the inferior
cava still passes chiefly into the left auricle through
the foramen ovale, while the blood from the other
two vense cavse now falls into the right auricle, being
prevented from entering the left chamber by the
Eustachian valve.

Hence, since at this period also the blood from the
left ventricle passes to a great extent to the anterior
portion of the body, there is a species of double-circula-
tion going on. The greater part of the blood from the
allantois entering the left auricle from the inferior vena
cava passes into the left ventricle and is thence sent
chiefly to the head and anterior extremities through the
third and fourth arches ; from these it is brought back
through the right auricle to the right ventricle, from

264 THE FIFTH DAY. [CHAP.

whence through the fifth arch it is returned along the
aorta to the allantois.

From the seventeenth to the nineteenth day the
right auricle becomes larger than the left. The large
Eustachian valve still prevents the blood from the
superior cavae from entering the left auricle, while it
conducts the blood from the inferior vena cava into that
chamber through the foramen ovale. The entrance of
the inferior vena cava is however further removed than
it was from the foramen ovale, and the increased flow
of blood from the lungs prevents all the blood of the
inferior cava from entering into the left auricle. At
the same time the valve of the foramen ovale prevents
the blood in the left auricle from entering the right
auricle.

During the period from the seventh day onwards
the apex of the heart becomes more marked, the arte-
rial roots are more entirely separated and the various
septa completed, so that when the foramen ovale is
closed and the blood of the inferior vena cava thereby
entirely confined to the right auricle, the heart has
practically acquired its adult condition.

The pericardial and pleural cavities. The heart
at first lies in the general body cavity attached to the
ventral wall of the gut by a mesocardium (Fig. 86, A),
but the part of the body cavity containing it afterwards
becomes separated off as a distinct cavity known as the
pericardial cavity. It is formed in the following way.
When the two ductus Cuvieri leading transversely from
the sinus venosus to the cardinal veins become deve-
loped (p. 170), a horizontal septum is formed to support
them, stretching across from the splanchnic to the so-

VIII ]

THE PERICARDIAL CAVITY.

265

matic side of the body cavity, dividing the body cavity
for a short distance in this region into a dorsal section,
{formed of a right and a left division) constituting the
true body cavity (Fig. 86 B, p.p), and a ventral section
(Fig. 86, B, p.c.), the pericardial cavity. The two parts
of the body cavity thus formed are at first in free com-
munication both in front of and behind this septum. The

FIG. 86.

TRANSVERSE SECTIONS THROUGH A CHICK EMBRYO WITH
TWENTY-ONE MESOBLASTIC SOMITES TO SHEW THE FORMA-
TION OP THE PERICARDIAL CAVITY, A. BEING THE ANTE-
RIOR SECTION.

pp. body cavity, pc. pericardial cavity, al. alimentary cavity.
au. auricle, v. ventricle, sv. sinus venosus. dc. ductus
Cuvieri. ao. aorta, mp. muscle-plate, me. medullary cord

266

THE FIFTH DAY.

[CHAP.

septum however is soon continued forwards so as com-
pletely to separate the ventral pericardial and the
dorsal body cavity in front, the pericardial cavity ex-
tending considerably further forwards than the body
cavity.

FIG. 87.

SECTION THROUGH THE CARDIAC EEGION OF AN EMBRYO OF
LACERTA MURALIS OF 9 M.M. TO SHEW THE MODE OF
FORMATION OF THE PERICARDIAL CAVITY.

Atf. heart, pc. pericardial cavity, al. alimentary tract. Ig. lung.
1. liver, pp. body cavity, md. open end of Mullerian duct.
wd. Wolffian duct. vc. vena cava inferior, ao. aorta, ch.
notochord. me. medullary cord.

Since the horizontal septum, by its mode of origin,
is necessarily attached to the ventral side of the gut,
the dorsal part of the primitive body space is, as we
have already mentioned, divided into two halves by
a median vertical septum formed of the gut and its

VIII.] THE PERICARDIAL CAVITY. 267

mesentery (Fig. 86, B). Posteriorly the horizontal sep-
tum grows in a slightly ventral direction along the
under surface of the liver (Fig. 87), till it meets the
abdominal wall of the body at the insertion of the
falciform ligament, and thus completely shuts off the
pericardial cavity from the body cavity. The horizontal
septum forms, as is obvious from the above description
the dorsal wall of the pericardial cavity.

After the completion of this separation the right
and left sections of the body cavity, dorsal to the peri-
cardial cavity, rapidly become larger and receive the
lungs which soon sprout out from the throat.

The diverticula which form the lungs grow out into
splanchnic mesoblast, in front of the body cavity, but
as they grow they extend into the two anterior com-
partments of the body cavity, each attached by its
mesentery to the mesentery of the gut (Fig. 87, lg.).
They soon moreover extend beyond the posterior limit of
the pericardium into the undivided body cavity behind.

To understand the further changes in the peri-
cardial cavity it is necessary to bear in mind its rela-
tions to the adjoining parts. It lies at this period
completely ventral to the two anterior prolongations of
the body cavity containing the lungs. Its dorsal wall is
attached to the gut, and is continuous with the me-
sentery of the gut passing to the dorsal abdominal wall,
forming the posterior mediastinum of human anatomy.

The changes which next ensue consist essentially in
the enlargement of the sections of the body cavity
dorsal to the pericardial cavity. This enlargement
takes place partly by the elongation of the posterior
mediastinum, but still more by the two divisions of the-

268 THE FIFTH DAY. [CHAP.

body cavity which contain the lungs extending them-
selves ventrally round the outside of the pericardial
cavity. This process is illustrated by Fig. 88, taken

Fm. 88.

/». c.

SECTION THROUGH AN ADVANCED EMBRYO OF A RABBIT TO
SHEW HOW THE PERICARDIAL CAVITY BECOMES SUR-
ROUNDED BY THE PLEURAL CAVITIES.

ht. heart, pc. pericardial cavity, pl.p. pleural cavity. Ig. lung.
al. alimentary tract, ao. dorsal aorta, ch. notochord. rp.
rib. st. sternum, sp.c. spinal cord.

from an embryo rabbit. The two dorsal sections of the
body cavity (pl.p.) finally extend so as completely to
envelope the pericardial cavity (pc.), remaining how-
ever separated from each other below by a lamina ex-
tending from the ventral wall of the pericardial cavity

VIII.] HISTOLOG1CAL DIFFERENTIATION. 26D

to the body wall, which forms the anterior mediastinum
of human anatomy.

By these changes the pericardial cavity is converted
into a closed bag, completely surrounded at its sides by
the two lateral halves of the body cavity, which were
primitively placed dorsally to it. These two sections of
the body cavity, which in the chick remain in free
communication with the undivided peritoneal cavity
behind, may, from the fact of their containing the
lungs, be called the pleural cavities.

Histological differentiation. The fifth day may also
be taken as marking the epoch at which histological
differentiation first becomes distinctly established and
begins to make great progress.

It is of course true that loi)g before this date, even
from the earliest hours, the cells in each of the three
fundamental layers have ceased to be everywhere alike.
Nevertheless the changes undergone by the several cells
have been few and slight. The cells of epiblastic origin,
both those going to form the epidermis and those in-
cluded in the neural involution, are up to this time
simple more or less columnar cells ; they may be seen
here elongated, there oval, and in another spot spheroi-
dal ; here closely packed, with scanty protoplasm, there
scattered, with each nucleus well surrounded by cell-
substance ; but wherever they are found they may still
be recognized as cells of a distinctly epithelial character.
So also with the cells of hypoblastic origin, whether
simply lining the alimentary canal or taking part in the
formation of the compound glands. Even in the meso-
blast, which undergoes far more changes than either of
the other layers, not only increasing more rapidly in

270 THE FIFTH DAY. [CHAP.

bulk but also serving as the mother tissue for a far
greater number of organs, the alterations in the indi-
vidual cells1 are, till near upon the fifth day, insignifi-
cant. Up to this time the mesoblast may be spoken of
as consisting for the most part of little more than in-
different tissue : — of nuclei imbedded in a protoplasmic
cell-substance. In one spot the nuclei are closely
packed together, and the cell-substance scanty and
compact; at another the nuclei are scattered about
with spindle-shaped masses of protoplasm attached to
each, and there is a large development either of inter-
cellular spaces or of intracellular vacuoles filled with
clear fluid. The protoplasm differs in various places,
chiefly in being more or less granular, and less or more
transparent, having as yet undergone but slight chemi-
cal transformation. Up to this epoch (with the excep-
tion of the early differentiated blood and muscles of the
muscle plates) there are no distinct tissues, and the
rudiments of the various organs are simply marked out
by greater or less condensation of the simple meso-
blastic substance.

From the fifth day onwards, however, histological
differentiation takes place rapidly, and it soon becomes
possible to speak of this or that part as being composed
of muscular, or cartilaginous, or connective, &c. tissue.
It is not within the scope of the present work to treat
in detail of these histogenetic changes, for information
concerning which we would refer the reader to histolo-
gical treatises. We have already had occasion to refer

1 With the exception of the cells of the middle part of the inner
layer of the muscle-plates, which we have seen become converted into
longitudinal muscles on the third day (p. 187).

VIII.] THE EPIBLAST. 271

incidentally to many of the earliest histological events,
and shall content ourselves by giving a brief summary
of the derivation of the tissues of the adult animal from
the three primary layers of the blastoderm.

The epiblast or upper layer of many embryologists
forms primarily two very important parts of the body,
viz. the central nervous system and the epidermis.

It is from the involuted epiblast of the neural tube
that the whole of the grey and white matter of the
brain and spinal cord appears to be developed, the
simple columnar cells of the epiblast being apparently
directly transformed into the characteristic multipolar
nerve-cells. The whole of the sympathetic1 nervous
system and the peripheral nervous elements of the
body, including both the spinal and cranial nerves and
ganglia, are epiblastic in origin.

The epithelium (ciliated in the young animal) lining
the canalis centralis of the spinal cord, together with
that lining the ventricles of the brain, all which cavities
and canals are, as we have seen, derivatives of the
primary neural canal, is the undifferentiated remnant of
the primitive epiblast.

The epiblast, as we have said, also forms the epider-
mis, not however the dermis, which is of mesoblastic
origin. The line of junction between the epiblast and
the mesoblast coincides with that between the epidermis

1 The details of the development of the sympathetic system have
only been imperfectly worked out in the chick. We propose deferring
our account of what is known on this head to the second part of this
work dealing with the Mammalia. We may here state, however, that the
whole of the chain of the sympathetic ganglia is developed in con-
tinuity with the outgrowths from the wall of the neural tube which
give rise to the spinal nerves.

272 THE FIFTH DAY. [CHAP.

and the dermis. From the epiblast are formed all such
tegumentary organs or parts of organs as are epidermic
in nature.

In addition to these, the epiblast plays an important
part in the formation of the organs of special sense.

According to their mode of formation these organs
may be arranged into two divisions. In the first come
the cases where the sensory expansion of the organ of
special sense is derived from the involuted epiblast of
the medullary canal. To this class belongs the retina,
including the epithelial pigment of the choroid, which
is formed from the original optic vesicle budded out
from the fore-brain.

To the second class belong the epithelial expansions
of the membranous labyrinth of the ear and the cavity
of the nose, which are formed by involution from the
superficial epiblast covering the external surface of the
embryo. These accordingly have no primary connection
with the brain. We may also fairly suppose that the
'taste bulbs' and the nervous cells, which have lately
been described as present in the epidermis, are also
structures formed from the epiblast.

In addition to these we have the crystalline lens
formed of involuted epiblast, and the cavity of the
mouth and anus lined by it. The pituitary body is
also epiblastic in origin. These are the most important
parts which are derived from the epiblast.

From the hypoblast are derived the epithelium of
the digestive canal, the epithelium of the trachea,
bronchial tubes and air cells, the cylindrical epithelium
of the ducts of the liver, pancreas and other glands of
the alimentary canal, as well as the hepatic cells con-

VIII.] THE HYPOBLAST AND MESOBLAST. 273

stituting the parenchyma of the liver, developed, as we
have seen, from the hypoblast cylinders given off around
the primary hepatic diverticula.

Homologous, probably with the hepatic cells, and
equally of hypoblastic origin, are the more spheroidal
c secreting cells ' of the pancreas and other glands. The
epithelium of the salivary glands, though these so exactly
resemble the pancreas, is of epiblastic origin, inasmuch
as the cavity of the mouth (p. 119) is entirely lined by
epiblast.

The hypoblast lines the allantois, and the notochord
also is an hypoblastic product.

From the mesoblast are formed all the remaining
parts of the body. The muscles, the bones, the connec-
tive tissue and the vessels, both arteries, veins, capillaries
and lymphatics, with their appropriate epithelium, are
entirely formed from the mesoblast.

The generative and urinary organs are also de-
rived from the mesoblast. It is worthy of notice that
their epithelium, though resembling the hypoblastic
epithelium of the alimentary canal, is undoubtedly
mesoblastic.

From the mesoblast lastly are derived all the mus-
cular, connective and vascular elements, as well of the
alimentary canal and its appendages as of the skin and
the tegumentary organs. Just as it is only the epider-
mic moiety of the latter which is derived from the
epiblast, so it is only the epithelium of the former
which comes from the hypoblast.

The important events then which characterize the
fifth day are : —

1. The growth of the allantois.
F. & B. 18

274 THE FIFTH DAY. [CHAP. VIII.

2. The appearance of the knee and elbow, and of
the cartilages which precede the bones of the digits and
limbs.

3. The formation of the primitive cartilaginous
cranium, more especially of the investing mass and the
trabeculse, and the appearance of rods of cartilage in
the visceral arches.

4. The developments of the parts of the face : the
closing in of the nasal passages by the nasal processes.

5. A large development of grey matter in the
spinal cord as the anterior and posterior cornua; con-
siderable growth both of the anterior and posterior
white columns, and the commencement of the anterior
and posterior fissures.

6. The appearance of the auricular septum, of a
septum in the bulbus arteriosus, and of the semilunar
valves.

7. The establishment of the several tissues.

CHAPTER IX.

FROM THE SIXTH DAY TO THE END OF INCUBATION.

THE sixth day marks a new epoch in the develop-
ment of the chick, for distinctly avian characters then
first make their appearance.

Striking and numerous as are the features, which
render the class Aves one of the most easily recognizable
in the whole animal kingdom, the embryo of a bird does
not materially differ in its early phases from that of a
reptile or a mammal, even in the points of structure
which are most distinctively avian. It may, it is true,
be possible to infer, even at a comparatively early stage,
from some subsidiary tokens, whether any given em-
bryo belongs to this class or that (and indeed the same
inference may be drawn from the ovum itself) ; but up
to a certain date it is impossible to point out, in the
embryo of the fowl, the presence of features which may
be taken as broadly characteristic of an avian organiza-
tion. This absence of any distinctive avian differen-
tiation lasts in the chick roughly speaking till the com-
mencement of the sixth day.

18—2

276 THE SIXTH DAY. [CHAP.

We do not mean that on the sixth day all the organs
suddenly commence to exhibit peculiarities which mark
them as avian. There are no strongly marked breaks
in the history of development; its course is perfectly
gradual, and one stage passes continuously into the
next. The sixth and seventh days do however mark
the commencement of the period in which the spe-
cialization of the bird begins to be apparent. Then for
the first time there become visible the main features
of the characteristic manus and pes ; the crop and the
intestinal caeca make their appearance; the stomach
takes on the form of a gizzard ; the nose begins to de-
velope into a beak ; and the commencing bones of the
skull arrange themselves after an avian type. Into
these details we do not propose to enter, and shall
therefore treat the history of the remaining days with
great brevity.

We will first speak of the FCETAL APPENDAGES.

On the sixth and seventh days these exhibit
changes which are hardly less important than the
events of previous days.

The amnion at its complete closure on the fourth
day very closely invested the body of the chick; the
true cavity of the amnion was at that time therefore very
small. On the fifth day fluid begins to collect in the
cavity, and raises the membrane of the amnion to some
distance from the embryo. The cavity becomes still
larger by the sixth day, and on the seventh day is of
very considerable dimensions, the fluid increasing with
it. On the sixth day Von Baer observed movements of
the embryo, chiefly of the limbs; he attributes them
to the stimulation of the cold air on opening the egg.

IX.] THE YOLK. 277

By the seventh day very obvious movements begin to
appear in the amnion itself; slow vermicular con-
tractions creep rythmically over it. The amnion in
fact begins to pulsate slowly and rythmically, and by
its pulsation the embryo is rocked to and fro in the
egg. This pulsation is due probably to the contraction
of involuntary muscular fibres, which seem to be present
in the attenuated portion of the mesoblast, forming
part of the amniotic fold. (Of. Chap. II. p. 45.) Similar
movements are also seen in the allantois at a con-
siderably later period.

The growth of the allantois has been very rapid,
and it forms a flattened bag, covering the right side of
the embryo and rapidly spreading out in all directions,
between the primitive folds of the amnion, that is be-
tween the amnion proper and the false amnion (serous
membrane). It is filled with fluid, so that in spite of
its flattened form its opposite walls are distinctly sepa-
rated from each other.

The vascular area has become still further extended
than on the previous day, but with a corresponding loss
in the definite character of its blood-vessels. The sinus
terminalis has indeed by the end of the seventh day
lost all its previous distinctness, and the vessels which
brought back the blood from it to the heart are no
longer to be seen.

Both the vitelline arteries and veins now pass to
and from the body of the chick as single trunks, as-
suming more and more the appearance of being merely
branches of the mesenteric vessels.

The yolk is still more fluid than on the previous
day, and its bulk has (according to Yon Baer) increased.

278 THE SIXTH DAY. [CHAP.

This can only be due to its absorbing the white of the
egg, which indeed is diminishing rapidly.

During the eighth, ninth, and tenth days the
aninion does not undergo any very important changes.
Its cavity is still filled with fluid, and on the eighth
day its pulsations are at their height, henceforward
diminishing in intensity.

The splitting of the mesoblast has now extended to
the outer limit of the vascular area, viz. over about
three quarters of the yolk-sac. The somatopleure at
this point is continuous (as can be easily seen by
reference to Fig. 9) with the original outer fold of
the amnion.

It thus comes about that the further splitting of the
mesoblast merely enlarges the cavity in which the
allantois lies. The growth of this organ keeps pace
with that of the cavity in which it is placed. Spread
out over the greater part of the yolk-sac as a flattened
bag filled with fluid, it now serves as the chief organ of
respiration.

Hence it is very vascular, the vessels on that side of
the bag which is turned to the serous membrane and
shell being especially large and numerous.

The yolk now begins to diminish rapidly in bulk.
The yolk-sac becomes flaccid, and on the eleventh day
is thrown into a series of internal folds, abundantly
supplied with blood-vessels. By this means the surface
of absorption is largely increased, and the yolk is more
and more rapidly taken up by the blood-vessels, and in
a partially assimilated condition transferred to the body
of the embryo.

By the eleventh day the abdominal parietes though

IX.] THE ALLANTOIS. 279

still much looser and less firm than the walls of the
chest may be said to be definitely established, and the
loops of intestine, which have hitherto been hanging
down into the somatic stalk, are henceforward confined
within the cavity of the abdomen. The body of the
embryo is therefore completed; but it still remains
connected with its various appendages by a narrow
somatic umbilicus, in which run the stalk of the allan-
tois and the solid cord suspending the yolk-sac.

The cleavage of the mesoblast still progressing, the
yolk is completely invested by the (splanchnopleuric)
yolk-sac except at the pole opposite to the embryo,
where for some little time a small portion remains
unenclosed ; at this spot the diminished white of the
egg adheres as a dense viscid plug.

The allantois meanwhile spreads out rapidly, and
lies over the embryo close under the shell, being sepa-
rated from the shell membrane by nothing more than
an attenuated membrane, the serous membrane, formed
out of the outer primitive fold of the amnion and the
remains of the vitelline membrane. With this serous
membrane the allantois partially coalesces, and in
opening an egg at the later stages of incubation, unless
care be taken the allantois is in danger of being torn
in the removal of the shell membrane. As the allantois
increases in size and importance, the allantoic vessels
are correspondingly developed. They are very con-
spicuous when the egg is opened, the pulsations of the
allantoic arteries at once attracting attention.

On about the sixteenth day, the white having
entirely disappeared, the cleavage of the mesoblast is
carried right over the pole of the yolk opposite the

280 THE SIXTH DAY. [CHAP.

embryo, and is thus completed (Fig. 9). The yolk-sac
now, like the allantois which closely wraps it all round,
lies loose in a space bounded outside the body by the
serous membrane, and continuous with the pleuro-
peritoneal cavity of the body of the embryo. Deposits
of urates now become abundant in the allantdic fluid.

The loose and flaccid walls of the abdomen enclose
a space which the empty intestines are far from filling,
and on the nineteenth day the yolk-sac, diminished
greatly in bulk but still of some considerable size, is
withdrawn through the somatic stalk into the ab-
dominal cavity, which it largely distends. Outside the
embryo there remains nothing now but the highly
vascular allantois and the practically bloodless serous
membrane and amnion. The amnion, whose fluid during
the later days of incubation rapidly diminishes, is con-
tinuous at the umbilicus with the body-walls of the
embryo. The serous membrane (or outer primitive
amniotic fold) is by the completion of the cleavage of
the mesoblast and the invagination of the yolk-sac,
entirely separated from the embryo. The cavity of the
allantois by means of its stalk passing through the um-
bilicus is of course continuous with the cloaca.

In the EMBRYO itself a few general points only de-
serve notice.

By the sixth or seventh day the flexure of the
body has become less marked, so that the head does
not lie so near to the tail as on the previous days ; at
the same time a more distinct neck makes its ap-
pearance.

Though the head is still disproportionately large, its
growth ceases to be greater than that of the body.

IX.] THE BRAIN. 281

Up to this period the walls of the somatic stalk
have remained thin and flaccid, almost membranous in
fact, the heart appearing to hang loosely out of the
body of the embryo. About this time however the
stalk, especially in front, rapidly narrows and its meso-
blast becomes thickened. In this way the heart and
the other thoracic viscera are enclosed by definite firm
chest walls, along the sides of which the ribs grow
forwards and in front of which the cartilaginous rudi-
ments of the sternum appear.

The abdominal walls are also being formed, but not
to the same extent, and the stalk of the allantois still
passes out from the peritoneal cavity between the
somatic and the splanchnic stalks.

In the brain one of the most marked features is the
growth of the cerebral hemispheres. The median division
between these has in front increased in depth, so that
the lateral ventricles are continued forwards as two
divergent horns, while backwards they are also con-
tinued as similar divergent horns separated from one
another by the vesicle of the third ventricle.

We propose to treat more fully of the development of the
brain in the second part of this work, the importance of the
mammalian brain rendering it undesirable to go too much into
the details of the brain of the bird.

All the visceral clefts are closed by the seventh day.
It will be remembered that the inner part of the first
cleft persists as the Eustachian tube (p. 166).

The structures which surround the mouth are be-
ginning to become avian in form, though the features
are as yet not very distinctly marked

282 THE SIXTH DAY. [CHAP.

The tongue has appeared on the floor of the mouth
as a bud of mesoblast covered by epiblast.

During the eighth, ninth, and tenth days the
embryo grows very rapidly, the head being still especially
large, and at the same time becoming more round, the
mid-brain not being so prominent.

Prom the eleventh day onwards the embryo suc-
cessively puts on characters which are not only
avian, but even distinctive of the genus, species and
variety.

So early as the ninth or tenth day the sacs con-
taining the feathers begin to protrude from the surface
of the skin as papillae, especially prominent at first along
the middle line of the back from the neck to the rump,
and over the thighs, the sacs of the tail feathers being
very conspicuous. On the thirteenth day these sacs,
generally distributed over the body, and acquiring the
length of a quarter of an inch or more, appear to the
naked eye as feathers, the thin walls of the sacs allow-
ing their contents, now coloured according to the variety
of the bird, to shine through. They are still however
closed sacs, and indeed remain such even on the nine-
teenth day, when many of them are an inch in length.

Feathers are epidermal structures. They arise from an in-
duration of the epidermis of papillae containing a vascular core

On the eighth day a chalky-looking patch is ob-
servable on the tip of the nose. This by the twelfth
day has become developed into a horny but still soft
beak.

On the thirteenth day, nails are visible at the ex-
tremities, and scales on the remaining portions of the

IX.] OSSIFICATION. 283

toes. These on the sixteenth day become harder and
more horny, as does also the beak.

Nails are developed on special regions of the epidermis,
known as the primitive nail beds. They are formed by the
cornification of a layer of cells which makes its appearance
between the horny and mucous layers of the epidermis. The
distal border of the nail soon becomes free, and the further
growth is effected by additions to the under side and attached
extremity of the nail.

By the thirteenth day the cartilaginous skeleton is
completed and the various muscles of the body can be
made out with tolerable clearness.

Ossification begins according to Von Baer on the
eighth or ninth day by small deposits in the tibia, in
the metacarpal bones of the hind-limb, and in the sca-
pula. On the eleventh or twelfth day a multitude of
points of ossification make their appearance in the
limbs, in the scapular and pelvic arches, in the ribs, in
the bodies of the cervical and dorsal vertebrae and in
the bones of the head, the centres of ossification of the
vertebral arches not being found till the thirteenth day.

The events which we have thus briefly narrated are
accompanied by important changes in the arterial
and venous systems.

The condition of the venous system at about the
end of the third day was fully described in Chap. VI.
p. 170, and the changes which have taken place between
that date and the latter days of incubation may be seen
by comparing the diagram Fig. 58 with the diagrams
Figs. 89 and 90.

On the third day, nearly the whole of the venous
blood from the body of the embryo was carried back to

284 THE SIXTH DAY. [CHAP.

the heart by two main venous trunks, the superior (Fig.
58, J) and inferior (Fig. 58, (7) cardinal veins, joining
on each side to form the short transverse ductus Cuvieri,
both which in turn united with the sinus venosus close
to the heart. As the head and neck continue to enlarge
and the wings become developed, the single superior
cardinal or jugular vein, as it is usually called (Figs. 89,
90, J"), of each side, is joined by two new veins : the

FIG. 89.

DIAGRAM OF THE VENOUS CIRCULATION AT THE COMMENCEMENT
OF THE FIFTH DAT.

H. heart, d.c. ductus Cuvieri. Into the ductus Cuvieri of each
side fall J. the jugular vein, W. the vein from the wing and

0. the inferior cardinal vein. & V. sinus venosus. Of. vitel-
line vein. U. allantoic vein, which at this stage gives off
branches to the body- walls. V.O.I, inferior vena cava.

1. liver.

vertebral vein (Su. J. F.), bringing back blood from the
head and neck, and the vein from the wing ( W).

The inferior cardinal veins have their roots in the
Wolffian bodies; they become developed, pari passu,

IX.] THE VENOUS SYSTEM. 285

with those organs, and may be called the veins of the
Wolffian bodies. On the third day they are the only
veins which bring the blood back from the hinder part
of the body of the embryo.

About the fourth or fifth day, however, a new single
venous trunk, the vena cava inferior (Fig. 89, V.G.I.\
makes its appearance in the middle line, in a plane more
dorsal than that of the cardinal veins. This, starting
from the sinus venosus not far from the heart, is on the
fifth day a short trunk running backward in the middle
line below the aorta, and speedily losing itself in the
tissues above the Wolffian bodies. When the kidneys
are formed it receives blood from them, and thencefor-
ward enlarging rapidly eventually becomes the channel
by which the greater part of the blood from the hind limbs
and the hinder part of the body finds its way to the heart.
In proportion as this vena cava inferior increases in size,
and the Wolffian bodies give place to the permanent
kidneys, the posterior cardinal veins diminish. The
blood originally coming to the posterior cardinals from
the posterior part of the spinal cord and trunk is trans-
ported into two posterior vertebral veins; which are
placed dorsal to the heads of the ribs and join the
anterior vertebral veins. With the appearance of these
veins the anterior part of the posterior cardinals dis-
appears.

At its first appearance the vena cava inferior may
be considered as a branch of the trunk which we have
called the sinus venosus, but as development proceeds,
and the vena cava becomes larger and larger, the sinus
venosus assumes more and more the appearance of being
merely the cardiac termination of the vena cava, and

286 THE SIXTH DAY. [CHAP.

the ductus venosus from the liver may now be said to join
the vena cava instead of being prolonged into the sinus.
While this growth of the vena cava is going on, the
points at which the ductus Cuvieri enter into the sinus
venosus are drawn in towards the heart itself, and finally
these trunks fall directly and separately into the auricular
cavities, and are henceforward known as the right and
left vena cava superior (Fig. 90, V.S.R., V.8.L.). There

DIAGRAM OF THE VENOUS CIRCULATION DURING THE LATER
DATS OF INCUBATION.

H. heart. V.S.R. right vena cava superior. V.S.L. left vena cava
superior. The two venae cavse superiores are the original
c ductus Cuvieri,' they still open into the sinus venosus and
not independently into the heart. J. jugular vein. $£7. V.
superior vertebral vein. In. V. inferior vertebral vein. W.
vein for the wing. V.C.I, vena cava inferior, which receives
most of the blood from the inferior extremities, etc. D. V.
ductus venosus. P. V. portal vein. M. a vein bringing
blood from the intestines into the portal vein. Of. vitelline
vein. U. allantoic vein. The three last mentioned veins
unite together to form the portal vein. I. liver.

The remnants of the inferior cardinal veins are not shewn.

IX.] THE VENOUS SYSTEM. 287

are therefore, when these changes have been effected,
three separate channels, with their respective orifices,
by which the blood of the body is brought back to the
heart, viz. the right and left superior and the inferior
venae cavae.

While the auricular septum is as yet unformed, the
blood from these veins falls into both auricles, perhaps
more into the left than into the right. As the septum
however grows up, the three vessels become connected
with the right auricle only while the left receives the
two pulmonary veins coming from the lungs. (Compare
Chap. VII. p. 228).

On the third day the course of the vessels from the
yolk-sac is very simple. The two vitelline veins, of
which the right is already the smaller, form the meatus
venosus from which, as it passes through the liver on its
way to the heart, are given off the two sets of venae
advehentes and venae revehentes.

With the appearance of the allantois on the fourth
day, a new feature is introduced. From the meatus
venosus, a short distance behind the liver, there is given
off a vein which quickly divides into two branches.
These, running along the ventral side of the body from
the walls of which they receive some amount of blood,
pass to the allantois. They are the allantoic or um-
bilical veins. The single vein which they unite to form
becomes, by reason of the rapid growth of the allantois,
very long ; and hence it is perhaps better to speak of it
as the allantoic vein (Fig. 90, U}. The right branch
soon diminishes in size and finally disappears. Mean-
while the left on reaching the allantois bifurcates ; and,
its two branches becoming large and conspicuous, there

288 THE SIXTH DAY. [CHAP.

still appear to be two main allantoic veins uniting at a
short distance from the allantois to form the single long
allantoic vein. At its first appearance the allantoic
vein seems to be but a small branch of the vitelline,
but as the allantois grows rapidly, and the yolk-sac
dwindles, this state of things is reversed, and the less
conspicuous vitelline appears as a branch of the larger
allantoic.

On the third day the blood returning from the walls
of the intestine is insignificant in amount. As however
the intestine becomes more and more developed, it
acquires a distinct venous system, and the blood sent to
it by branches of the aorta is returned by veins which
form a trunk, the mesenteric vein (Fig. 90, Jtf ), falling
into the vitelline vein at its junction with the allantoic
vein.

These three great veins in fact, viz. the vitelline,
the allantoic, and the mesenteric, form a large common
trunk which enters at once into the liver, and which we
may now call the portal vein (Fig. 90, P. F.). This, at
its entrance into the liver, partly breaks up into the
venae advehentes, and partly continues as the ductus
venosus straight through the liver, emerging from which
it joins the vena cava inferior. Before the establish-
ment of the vena cava inferior, the venae revehentes,
carrying back the blood which circulates through the
hepatic capillaries, joined the ductus venosus close to
its exit from the liver (Fig. 89). By the time however
that the vena cava has become a large and important
vessel it is found that the venae revehentes or as we
may now call them the hepatic veins have shifted their
embouchment and now fall directly into that vein, the

IX.] THE VENOUS SYSTEM. 289

ductus venosus making a separate junction rather higher
up (Fig. 90).

This state of things continues with but slight changes
till near the end of incubation, when the chick begins
to breathe the air in the air-chamber of the shell, and
respiration is no longer carried on by the allantois.
Blood then ceases to flow along the allantoic vessels;
they become obliterated. The vitelline vein, which as
the yolk becomes gradually absorbed proportionately
diminishes in size and importance, comes to appear as
a mere branch of the portal vein. The ductus venosus
becomes closed, remaining often as a mere ligament;
and hence the whole of the blood coming through the
portal vein flows into the substance of the liver, and
so by the two hepatic veins into the vena cava (Fig.
91, HP}.

Previous to these changes one of the veins passing
from the rectum into the vena cava has given off a
branch which effects a junction with one of the mesen-
teric veins. This now forms a somewhat conspicuous
connecting branch between the systems of the vena
cava and the portal vein (Fig. 91, Cy. M.).

All three venae cavae now fall exclusively into the
right auricle, and by the closure of the foramen ovale
the blood flowing through them is entirely shut off from
the left auricle, into which passes the blood from the
two pulmonary veins (Fig. 91, L. V.).

Such is the history of the veins in the chick. As
will be seen in the second part of this work, the course
of events in the mammal, though in the main similar,
differs in some unimportant respects.

It remains for us to speak of the changes which
F. & B. 19

290

THE SIXTH DAY.
FIG. 91.

[CHAP.

SuV.

DIAGRAM OF THE VENOUS CIRCULATION OF THE CHICK AFTER
THE COMMENCEMENT OF KESPIRATION BY MEANS OF THE
LUNGS.

W. wing vein. J. jugular vein. Su. V. superior vertebral vein.
In. V. inferior vertebral vein. These unite together on each
side to form the corresponding superior vena cava. L. V.
pulmonary veins. V.C.L vena cava inferior. H.P. hepatic
veins. P. V. portal vein. M. mesenteric veins. Gy.M. con-
necting vessel between the branches of the portal vein and
the system of the vena cava inferior. It is called the coccy-
geo-mesenteric vein, and unites the cross branch connecting
the two hypogastrics with the mesenteric vein. The ductus
venosus has become obliterated. The three vense cavse fall
independently into the right auricle and the pulmonary
veins into the left auricle. Cr. crural vein. Jc. kidney.
1. liver, pp. hypogastric veins. C. V. caudal vein.

have in the meantime been taking place in the arterial
system. The condition of things which exists on the
fifth or sixth day is shewn in the diagram (Fig. 92).

IX.]

THE ARTERIAL ARCHES.

FIG. 92.

291

STATE OF ARTERIAL CIRCULATION ON THE FIFTH OR SIXTH

DAY.

E.G. A. external carotid. I.C.A. internal carotid. D.A. dorsal
aorta. Of. A. vitelline artery, U. A. allantoic artery.

We have already seen (Chap. vn. p. 225) that of
the three aortic arches which make their appearance on
the third day, the first two disappear : the first on the
fourth, the second on the fifth day ; but that their dis-
appearance is accompanied by the formation behind
them of two new aortic arches, the fourth and the fifth.
Thus there are generally three, never more than three,
pairs of aortic arches present and functional at one time.

This statement needs some limitation ; for according to Von
Baer there are four arches present both on the fourth and
fifth days. In the case of the fourth day a slight remnant of the
first pair of arches still persists when the fourth pair is already
formed ; and on the fifth day the second pair has not entirely
disappeared when the fifth pair is formed. In both of these
cases however the first pair of arches of the four is only present
for a very short time, and then is so diminished in size as to be
of no importance.

19—2

292 THE SIXTH DAY. [CHAP.

The first pair of arches, before it entirely disappears,
sends off on each side two branches towards the head.
Of these, one forms the direct continuation of the bulbus
arteriosus in a straight line from the point where the
first aortic arch leaves it ; primarily distributed to the
tongue and inferior maxillary region, it becomes the
external carotid (Fig. 92, E.G. A.}. The other, starting
from the point where the aortic arch of each side joins
its fellow, dorsal to the alimentary canal, to form the
dorsal aorta, is primarily distributed to the brain, and
becomes the internal carotid (Fig. 92, I.G.A.).

When the first arch disappears, the external carotid
arteries still remain as the anterior continuations of the
bulbus arteriosus. And since the dorsal trunks uniting
the distal ends of the first and second arches do not
become obliterated at the time when the first pair of
arches disappears, the internal carotids remain as
branches springing from the distal ends of the second
pair of arches ; they are supplied with blood from that
pair, the stream in which flows chiefly towards the head
instead of backwards towards the dorsal aorta, as is the
case with the succeeding arches. When the second
pair of arches is obliterated, the connecting branch with
the next arch is again left, and thus the internal carotids
appear as branches from the distal ends of the third
pair of arches.

On the third day the dorsal aorta does not for any
distance remain single in its backward course along the
body, but soon divides into two trunks which run one
on either side of the middle line of the body. These
two trunks, as development proceeds, gradually unite
along their whole length, and there is thus formed a

IX.] THE PULMONARY ARTERIES. 293

single median aorta terminating behind in the caudal
artery (Figs. 92, 94). The arteries to the kidneys,
hind limbs, etc. are developed as branches of this aorta.

As the allantois grows rapidly and becomes an im-
portant respiratory organ, the allantoic or umbilical
arteries increase in size. As a general, though ap-
parently not invariable rule, the right allantoic artery
gets gradually smaller and soon disappears.

The vitelline artery (Of. -A.) now leaves the aorta
as a single but quickly bifurcating trunk, which at the
end of the fifth day is still very large.

By the fifth day the ventricular portion of the heart
(compare Chap. vn. p. 257) is completely divided into
two chambers. The bulbus arteriosus is also divided
by a septum into two channels, one of which com-
municates with the right ventricle of the heart and the
other with the left.

One result of this arrangement is that all the
blood which passes to the anterior extremity of the
body comes from the left ventricle of the heart.

At about the seventh day an entire separation
begins to take place between the arterial roots which
come respectively from the right and left chambers of
the heart. The root from the right chamber (Fig. 93)
remains connected with the fifth pair of arches. The
root from the left ventricle is connected with the third
and fourth pairs of arches.

The lower part of the body still receives blood from
both the right and left ventricles, since the blood which
enters the fifth arch still flows into the common dorsal
aorta. As the lungs however increase in size, a com-
munication is set up between them and the fifth pair of

294

THE SIXTH DAY.

[CHAP.

FIG. 93.

C.CA

R.S.C

DIAGRAM OF THE CONDITION OF THE ARCHES OF THE AORTA

TOWARDS THE CLOSE OF INCUBATION.

i, 2, 3, 4, 5. the several aortic arches. E.G. A. external carotid.
I.C. A. internal carotid. C.C.A. common carotid. V.a. ver-
tebral artery. R.sc. right subclavian. L.sc. left subclavian.
R.P., L.P. right and left pulmonary arteries. R.P.A. right
arterial root or division of the bulbus arteriosus, or pul-
monary artery ; the left root or division, constituting the
aorta, is seen by its side. The system of the fifth arch is
in lighter shading. The dotted lines shew the portions of
the arches which have been obliterated.

arches in the shape of two vessels which, springing one
from the arch of each side, grow downwards towards the
lungs. At first small and narrow, these pulmonary
arteries, for such they are, grow rapidly larger and
larger, so that more and more of the blood from the
right ventricle is carried to the lungs.

At the same time the connection between the third
and fourth pairs of arches on each side grows weaker ; so

IX.] THE CAROTID ARTERIES. 295

that less and less of tlie blood which flows along the
third pair of arches is able to pass backwards to the
hind end of the body.

The fourth arch of the right side now becomes the
most important of all the arches ; and nearly the whole
of the blood supplying the hinder parts of the body
passes through it. It is this arch which remains as
the permanent aortic arch of the adult ; and it is im-
portant to notice that the arch which forms the great
dorsal aorta in birds is the fourth on the right side, and
not as in mammals the fourth on the left side. The
fourth arch of the left side in birds, after giving off the
subclavian, is continued as an exceedingly small and
unimportant vessel to join the fourth right arch. It is
soon obliterated.

In consequence of these changes the condition of
the aortic arches during the latter days of incubation,
before respiration by the lungs has commenced, is as
follows (Fig. 93).

The first and second arches are completely ob-
literated. The third arch on each side is continued at
its dorsal end as the internal carotid, I.C.A, the con-
nection between it and the fourth arch having become
entirely obliterated. From its ventral end as the direct
continuation of the trunk which originally supplied the
first and second arches the external carotid, E. G.A., is
given off. Each pair of carotids arises therefore from a
common trunk — the common carotid (C.C.A.). Each
of these trunks gives off near its proximal end a branch,
the vertebral artery (F.a.).

The common carotid on the right side comes off
from the fourth arch of the right side (the arch of the

296 THE SIXTH DAY. [CHAP.

dorsal aorta), and is not as yet connected with the right
subclavian, R.sc. The common carotid of the left side
comes off from the fourth arch of the left side ; but since
this arch becomes the left subclavian, L.sc. (the connec-
tion between the fourth and fifth left arches being
obliterated), the portion of the trunk between the fourth
arch and the bulbus arteriostis (or as it must now be
called the common aortic root) is called the left
innominate artery.

The fourth arch of the right side forms the com-
mencement of the great dorsal aorta, and gives off the
right subclavian (R.sc.) just before it is joined by the
fifth arch.

The fifth arch of each side gives off branches (R.P.,
L.P.) to the lungs ; their distal continuations, by which
these arches are connected with the. systemic circulation,
though much reduced, are not obliterated.

The final changes undergone by the arterial system
after the commencement of the pulmonary respiration
consist chiefly in the complete separation of the pul-
monary and systemic circulations. As the branches to
the lungs become stronger and stronger, less and less
blood from the right ventricle enters into the dorsal
aorta; and the connecting vessels become smaller and
smaller.

Each of these fifth arches from the right ventricle
may therefore be considered at about the sixteenth or
eighteenth day as divided into two parts, an inner part
which connects the heart with the lung, and an outer
part which still connects the arch with the main dorsal
aorta. As these outer parts become smaller they re-
ceive the name of the ' ductus or canales Botalli' or

IX.] SUMMAKY. 297

'ductus arteriosi.' The one on the right side is short;
that on the left side is much longer and narrower.

When respiration commences the blood ceases to
pass through these canals, which either remain as mere
ligaments or else become absorbed altogether. By this
means, the foramen ovale becoming at the same time
closed, a complete double circulation is established. All
the blood from the right ventricle passes into the lungs,
and all that from the left ventricle into the body at
large.

Two other changes take place about the same time
in the aortic branches. That portion of the right fourth
or aortic arch which lies between the origin of the right
subclavian and the common carotid becomes shortened,
and is finally swallowed up in such a fashion that the
right subclavian (Fig. 94, R. sc.) comes off from the
right common carotid, a very short trunk being formed
by the union of the two to serve as the right innomi-
nate artery.

At the same time, corresponding to the increase in
the length of the neck, the common carotids are very
greatly lengthened. They lie close together in the
neck, and in many birds actually unite to form a com-
mon trunk.

It will of course be understood that with the dis-
appearance of the allantois and the absorption of the
yolk, the allantoic and vitelline arteries also disappear.

It may perhaps be of advantage to the reader if we
here briefly summarize the condition of the circulation
at its four most important epochs; viz. on the third
day, on the fifth day, during the later days of incu-
bation before respiration by the lungs has commenced,

298

[CHAP.

DIAGRAM OF THE ARTERIAL SYSTEM OF THE ADULT FOWL.

P.A. root of pulmonary artery. L.in. left innominate artery.
D.A. dorsal aorta. COB. cseliac arteries, mes. mesenteric
artery, ar.r. renal arteries, fern, femoral arteries. Is.
ischiatic arteries, hyp. hypogastric arteries, cau. caudal
artery. The other letters as in Fig. 93.

and after the chick has begun to breathe by the
lungs.

On the third day the circulation is of an exceed-
ingly simple character.

IX.] SUMMARY. 299

The heart is to all intents and purposes a simple
twisted tube marked off by constrictions into a series of
three consecutive chambers. The blood coming from
the venous radicles passes through the heart and then
through the three pairs of arterial arches.

From these it is collected into the great dorsal
aorta. Upon this dividing into two branches, the stream
of blood passes down on each side of the notochord
along the body, and thence out by the vitelline arteries,
which distribute it to the yolk-sac.

In the yolk-sac it partly passes into the sinus termi-
nalis and so into the fore and aft trunks, partly directly
into the lateral trunks, of the vitelline veins. In both
cases it is brought back to the two venous radicles and
so to the heart.

On this day the blood is aerated in the capillaries of
the yolk-sac.

On the fifth or sixth day the two auricles are
present though having a common cavity. The septum
of the ventricles is nearly complete, so that the blood
on entering the ventricles from the auricles is divided
into two streams. These two streams pass respectively
from the right and left chambers of the heart into the
two divisions of the bulbus arteriosus. The blood from
the right ventricle passes into the fifth pair of arches
and that from the left ventricle into the third and
fourth pairs of arches.

From the anterior parts the blood is brought back
by the anterior cardinal or jugular veins; from the
hinder parts of the body, chiefly by the cardinal veins,
but also in part by the now commencing vena cava
inferior.

300 THE SIXTH DAY. [CHAP.

The blood from the yolk-sac and allantois, together
with a small quantity from the intestine, is collected
into the portal vein, and by that vessel carried to the
liver. Here it becomes divided into two streams, part
flowing directly by the ductus venosus into the sinus
venosus, and the remainder passing through the capil-
laries of the liver, being brought back to the ductus
venosus by the hepatic veins.

During this period the blood is aerated both by the
allantois and yolk-sac, but as yet chiefly by the latter.

At a somewhat late period of incubation the
blood from the ventricles passes into two entirely dis-
tinct roots. The one of these, that from the right
chamber, sends the blood to the fifth pair of arches;
passing through which the greater part of the blood
flows into the dorsal aorta, a small portion only finding
its way into the lungs through the as yet unimportant
pulmonary arteries.

Through the other aortic root, viz. that from the
left ventricle, the blood flows into the third and fourth
pairs of arches. That part of the blood which flows
into the third pair, passes almost entirely to the head
and upper extremities by the external and internal
carotids ; that which flows into the right arch of
the fourth pair is chiefly brought to the dorsal aorta,
but some of it passes to the right wing ; that, on the
contrary, which goes into the left fourth arch is for the
most part sent to the left wing, a small part only reach-
ing the dorsal aorta. There is still a mixture of the
blood from the two chambers of the heart, so that the
blood in the dorsal aorta is composed partly of blood
from the left, and partly from the right chambers.

IX.] SUMMARY. 301

The blood of the upper (anterior) end of the body
comes entirely from the left ventricle.

The blood of the dorsal aorta passes to the yolk-
sac and allantois, and to all the hinder parts of the
body. It is brought back from the yolk-sac, from the
allantois, and to a certain extent from the intestines, by
the portal vein, part of the blood from which passes to
the inferior vena cava by the direct course (ductus
venosus), and part indirectly by the more circuitous
course of the capillaries of the liver and hepatic veins.

The blood from the generative and urinary organs,
and from the hinder extremities, is brought back to the
heart by the vena cava inferior ; that from the upper
extremities and head by the jugular, vertebral and
wing veins into the two venae cavae of the right and
left side, and so to the heart. Of these three venae
cavae, the right superior and the inferior join the
auricle by a common entrance, but the left superior
has an entrance of its own. All of these open into
the cavity of the right auricle, but the opening of
the inferior vena cava is so directed (vide Chap. vill.
p. 263) that the blood carried by this vessel flows
chiefly through the foramen ovale into the left auricle.
The blood from the two superior venae cavae enters the
right auricle only. Now the blood of the inferior
vena cava has been partly aerated by the allantois;
and, since it is this blood which passing through the
left auricle and ventricle is distributed to the third
aortic arch, unmixed by any blood from the right ven-
tricle (the mixture with the blood from the fifth arch
reaching only as far as the fourth arch), it happens
that the blood which flows to the anterior extremities

302 THE SIXTH DAY. [CHAP.

and head is more aerated than that in any other part
of the body.

From the anterior extremities the blood is to a
great extent returned by the left superior cava, and
goes into the right auricle, whence, by the right ven-
tricle, it is distributed through the fifth pair of arches
over the body, after joining the more aerated blood
passing through the fourth pair of arches.

The blood from the lungs is brought back by two
small veins into the left auricle.

The characteristics of the circulation at this time are
that the blood is aerated by the allantois, and that there
is a partial double circulation. (Vide Chap. vm. p. 263.)

As soon as respiration commences the canals
leading to the dorsal aorta from the fifth pair of arches,
which communicate only with the right ventricle, be-
come closed. The blood passing along the fifth arch
now flows only into the lungs, through the pulmonary
arteries. The blood from the left ventricle owing to
the cessation of the circulation of the yolk-sac and of the
allantois is distributed exclusively to the body of the
chick, from whence it is all brought back into the right
auricle by the three now independently opening vena3
cavse.

The portal veins henceforward receive blood from
the intestines only, and the ductus venosus is ob-
literated, so that all the blood of the portal vein passes
through the capillaries of the liver.

The partition between the auricles is rendered com-
plete by the closure of the foramen ovale; into the
right auricle the veins of the body enter, and into the
left the pulmonary veins.

IX.] HATCHING. 303

There is thus a completely double circulation formed,
in which all the blood of the left ventricle is arterial,
and all the blood of the right ventricle venous, and
there is at no part of the circulation a mixture of venous
and arterial blood.

As early as the sixth day movements, as we have
said, may be seen in the limbs of the embryo upon
opening the egg. We may conclude that after this
epoch spontaneous movements occur from time to time
in the unopened egg. They cannot however be of any
great extent until the fourteenth day, for up to this
time the embryo retains the position in which it was
first formed, viz. with its body at right angles to the
long axis of the egg.

On the fourteenth day a definite change of position
takes place ; the chick moves so as to lie lengthways in
the egg, with its beak touching the chorion and shell
membrane where they form the inner wall of the
rapidly increasing air-chamber at the broad end (Chap,
i. p. 3).

On the twentieth day or thereabouts the beak is
thrust through these membranes, and the bird begins
to breathe the air contained in the chamber. There-
upon the pulmonary circulation becomes functionally
active, and at the same time blood ceases to flow
through the umbilical arteries. The allantois shrivels
up, the umbilicus becomes completely closed, and the
chick piercing the shell at the broad end of the egg
with repeated blows of its beak, casts off the dried re-
mains of allantois, amnion and chorion, and steps out
into the world.

GENERAL LIBRARY
UNIVERSITY OF CALIFORNIA— BERKELEY

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