THE WORKS

OF

FRANCIS MAITLAND BALFOUR.

VOL.

dftritiom

(Cambrtoge :

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

S>

THE WORKS

OF

FRANCIS MAITLAND BALFOUR,

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

FELLOW OF TRINITY COLLEGE,

AND PROFESSOR OF ANIMAL MORPHOLOGY IN THE UNIVERSITY OF

CAMBRIDGE.

EDITED BY
M. FOSTER, F.R.S.,

PROFESSOR OF PHYSIOLOGY IN THE UNIVERSITY OF CAMBRIDGE;

AND

ADAM SEDGWICK, M.A.,

FELLOW AND LECTURER OF TRINITY COLLEGE, CAMBRIDGE.

VOL. III.

A TREATISE ON COMPARATIVE EMBRYOLOGY.
Vol. II. Vertebrata.

Hontron :

MACMILLAN AND CO.
1885

(\

s A v

[The Right of Translation is reserved.}

PREFACE TO VOLUME II.

THE present volume completes my treatise on Com-
parative Embryology. The first eleven chapters deal
with the developmental history of the Chordata. These
are followed by three comparative chapters completing
the section of the work devoted to Systematic Embry-
ology. The remainder of the treatise, from Chapter
XIV. onwards, is devoted to Organogeny. For the
reasons stated in the introduction to this part the or-
ganogeny of the Chordata has been treated with much
greater fulness than that of the other groups of Metazoa.

My own investigations have covered the ground of
the present volume much more completely than they did
that of the first volume ; a not inconsiderable proportion
of the facts recorded having been directly verified by
me.

The very great labour of completing this volume has
been much lightened by the assistance I have received

B. III.

vi PREFACE.

from my friends and pupils. Had it not been for their
co-operation a large number of the disputed points, which
I have been able to investigate during the preparation of
the work, must have been left untouched.

My special thanks are due to Mr Sedgwick, who has
not only devoted a very large amount of time and labour
to correcting the proofs, but has made for me an index of
this volume, and has assisted me in many other ways.

Dr Allen Thomson and Professor Kleinenberg of
Messina have undertaken the ungrateful task of looking
through my proof-sheets, and have made suggestions
which have proved most valuable. To Professors
Parker, Turner, and Bridge, I am also greatly indebted
for their suggestions with reference to special chapters of
the work.

CONTENTS OF VOLUME II.

CHAPTER I. CEPHALOCHORDA. Pp. t — 8.

Segmentation and formation of the layers, pp. I — 3. Central nervous system,
pp. 3, 4. Mesoblast, p. 5. General history of larva, pp. 6 — 8.

CHAPTER II. UROCHORDA. Pp. 9 — 39.

Solitaria, pp. 9 — 23. Development of embryo, pp. 9 — 15. Growth and
structure of free larva, pp. 15 — 19. Retrogressive metamorphosis, pp. 19 — 23.
Sedentaria, p. 23. Natantia, pp. 23 — 28. Doliolida:, pp. 28, 29. Salpida, pp.
29 — 34. Appendicularia, p. 34. Metagenesis, pp. 34 — 38.

CHAPTER III. ELASMOBRANCHII. Pp. 40 — 67.

Segmentation and formation of the layers, pp. 40 — 47. Epiblast, p. 47.
Mesoblast, pp. 47 — 51. Hypoblast and notochord, pp. 51 — 54. General
features of the embryo at successive stages, pp. 55 — 62. The yolk-sack, pp.
62-66.

CHAPTER IV. TELEOSTEI. Pp. 68 — 82.

Segmentation and formation of the layers, pp. 68—73. General history of
the layers, pp. 73 — 75. General development of the embryo, pp. 76 — 81.

CHAPTER V. CYCLOSTOMATA. Pp. 83 — 101.

Segmentation and formation of the layers, pp. 83 — 86. Mesoblast and noto-
chord, pp. 86, 87. General history of the development, pp. 87 — 97. Metamor-
phosis, pp. 97 — 100. Myxine, p. 100.

CHAPTER VI. GANOIDEI. Pp. 102 — 119.

Acipenser, pp. 102 — no. Segmentation and formation of the layers, pp. 102
— 104. General development of the embryo and larva, pp. 104 — 1 10. Lepidosleus,
pp. 111—119. Segmentation, pp. in, 112. General development of embryo and
larva, pp. 1 1 2 — 1 19. General observations on the embryology of Ganoids, p. 119.

CHAPTER VII. AMPHIBIA. Pp. 120 — 144.

Oviposition and impregnation, pp. 120, 121. Formation of the layers, pp.
I2I — 124. Epiblast, pp. 125—127. Mesoblast and notochord, pp. 128, 129.
Hypoblast, pp. 129—131. General groivth of the embryo, pp. 131 — 143. Anura,
pp. 131— 141. Urodela, pp. 141— 143. Gymnophiona, p. 143.

CONTENTS OF VOLUME II.

CHAPTER VIII. AVES. Pp. 145—201.

- Segmentation and formation of the layers, pp. 145—166. General history of
the germinal layers, pp. 166—169. General development of the embryo, pp. 169
— 180. Fa-tal membranes, pp. 185—199- Amnion, pp. 185—191. Allantois,
pp. 191 — 193. Yolk-sack, pp. 193— 199-

CHAPTER IX. REPTILIA. Pp. 202—213.

Lacertilia, pp. 202 --209. Segmentation and formation of the layers, pp. 202
—207. General development of the embryo, p. 208. Embryonic membranes
and yolk-sack, pp. 208—210. Ophidia, p. 210. Ckelonia, pp. 210—212.

CHAPTER X. MAMMALIA. Pp. 214 — 274.

Segmentation and formation of the layers, pp. 214 — 227. General growth of
the embryo, pp. 227 — 232. Embryonic membranes and yolk-sack, pp. 232 — 239.
Comparative history of the Mammalian foetal membranes, pp. 239—257. Com-
parative histology of the placenta, pp. 257—259. Evolution of the placenta,
ppt 25g — 26i. Development of the Guinea-pig, pp. 262 — 265. The human
embryo, pp. 265 — 270.

CHAPTER XI. COMPARISON OF THE FORMATION OF THE GERMINAL
LAYERS AND OF THE EARLY STAGES IN THE DE-
VELOPMENT OF VERTEBRATES. Pp. 275 — 310.

Formation of the gastrula, pp. 275—292. The formation of the mesoblast
and of the notochord, pp. 292—300. The epiblast, pp. 300—304. Formation of
the central nervous system, pp. 301—304. Formation of the organs of special
sense, p. 304. Summary of organs derived from the three germinal layers, pp.
304 — 306. Growth in length of the Vertebrate embryo, pp. 306 — 309. The
evolution of the allantois and amnion, pp. 309, 310.

CHAPTER XII. OBSERVATIONS ON THE ANCESTRAL FORM OF THE
CHORDATA. Pp. 311 — 330.

General considerations, pp. 311 — 316. The medullary canal, pp. 316, 317.
The origin and nature of the mouth, pp. 317 — 321. The cranial flexure, pp. 321,
322. The postanal gut and neurentcric canal, pp. 322 — 325. The body-cavity
and mesoblastic somites, p. 325. The notochord, pp. 325, 326. Gill clefts,
PP 326, 327- Phylogeny of the Chordata, pp. 327 — 329.

CHAPTER XIII. GENERAL CONCLUSIONS. Pp. 331 — 388.

I. Mode of origin and homologies of the germinal layers, pp- 33 E
— 360. Formation of the primary germinal layers, pp. 332, 333. Invagination,
pp. 333 — 335. Delamination, pp. 335 — 338. Phylogenetic significance of delami-
nation and invagination, pp. 338 — 345. Homologies of the germinal layers,
IT- .545' 346- The origin of the mesoblast, pp. 346 — 360.

EL Larval forms: their nature, origin, and affinities. Preliminary
considerations, pp. 360 — 362. Types of larva-, pp. 363 — 384. Phylogenetic
conclusions, pp. 384, 385. General conclusions and summary, pp. 385, 386.

CONTENTS OK VOLUME II.

IX

PART II. ORGANOGENY;

INTRODUCTION. Pp. 391, 392.

CHAPTER XIV. THE EPIDERMIS AND ITS DERIVATIVES. Pp. 393 — 399.
Protective epidermic structures, pp. 393 — 397. Dermal skeletal structures,
p. 397. Glands, pp. 397, 398.

CHAPTER XV. THE NERVOUS SYSTEM. Pp. 400—469.

The origin of the nervous system, pp. 400 — 405. Nervous system of the
Invertebrata, pp. 405 — 414. Central nervous system of the Vertebrata, pp. 415 —
447. Spinal chord, pp. 415 — 419. General development of the brain, pp. 419 —
423. Hind-brain, pp. 424 — 427. Mid-brain, pp. 427, 428. General develop-
ment of fore-brain, pp. 428 — 430. Thalamencephalon, pp. 430 — 435. Pituitary
body, pp. 435 — 437. Cerebral Hemispheres, pp. 437 — 444. Olfactory lobes,
pp. 444, 445. General conclusions as to the central nervous system of the Verte-
brata, pp. 445 — 447. Development of the cranial and spinal nerves, pp. 448 — 466.
Spinal nerves, pp. 448 — 455. Cranial nerves, pp. 455 — 466. Sympathetic nervous
system, pp. 466 — 468.

CHAPTER XVI. ORGANS OF VISION. Pp. 470 — 511.

Ccelenterata, pp. 471, 472. Mollusca, pp. 472 — 479. Chsetopoda, p. 479.
Chastognatha, p. 479. Arthropoda, pp. 479 — 483. Vertebrata general, pp. 483 —
490. Retina, pp. 490 — 492. Optic nerve, pp. 492, 493. Choroid fissure, p. 493.
Lens, pp. 494, 495. Vitreous humour, pp. 494, 495. Cornea, pp. 495 — 497.
Aqueous humour, p. 497. Comparative development of Vertebrate eye, pp. 497 — 506.
Ammoccete eye, pp. 498, 499. Optic vesicles, p. 499. Lens, p. 499. Cornea,
p. 500. Optic nerve' and choroid fissure, pp. 500 — 505. Iris and ciliary pro-
cesses, p. 506. Accessory organs connected with the eye, p. 506. Eyelids,
p. 506. Lacrymal glands, p. 506. Lacrymal duct, pp. 506, 507. Eye of the
Tunicata, pp. 507 — 509. Accessory eyes in the Vertebrata, pp. 509, 510.

CHAPTER XVII. AUDITORY ORGAN, OLFACTORY ORGAN, AND SENSE

ORGANS OF THE LATERAL LINE. Pp. 5 12 — 541.

Auditory organs, pp. 512 — 531. General structure of auditory organs,
PP- S1^, 513. Auditory organs of the Coelenterata, pp. 513 — 515. Auditory
organs of the Mollusca, pp. 515, 516. Auditory organs of the Crustacea, p. 516.
Auditory organs of the Verlebrata, pp. 516 — 530. Auditory vesicle, pp. 517 —
524. Organ of Corti, pp. 524 — 527. Accessory structures connected with the
organ of hearing of terrestrial vertebrata, pp. 527 — 530. Auditory organ of the
Tunicata, pp. 530, 531. Bibliography of Auditory organs, p. 531.

Olfactory organs, pp. 531 — 538. Bibliography of Olfactory organs, p. 538.

Sense organs of the lateral line, pp. 538—540. Bibliography of sense

organs of lateral line, pp. 540, 541.

CHAPTER XVIII. THE NOTOCHORD, THE VERTEBRAL COLUMN, THE
RIBS, AND THE STERNUM. Pp. 542 — 563.

Introductory remarks on the origin of the skeleton, pp. 542 — 544. Biblio-
graphy of the origin of the skeleton, pp. 544, 545. The notochord and its cartilagi-

CONTENTS OF VOLUME II.

nous sheath, pp. 545 — 549. The vertebral arches and the vertebral bodies, pp. 549
—559- Cyclostomata, p. 549. Elasmobranchii, pp. 549—553. Ganoidei, p. 553.
Teleostei, p. 553. Amphibia, pp. 553— 556. Reptilia, pp. 556, 557. Aves,
pp. 557, 558. Mammalia, pp. 558, 559. Bibliography of the notochord and
vertebral column, p. 560. Ribs, pp. 560—562. Sternum, pp. 562, 563.
Bibliography of the ribs and sternum, p. 563.

CHAPTER XIX. THE SKULL. Pp. 564—598.

Preliminary remarks, pp. 564, 565. The cartilaginous cranium, pp.

565—571. The parachordals and notochord, pp. 566, 567. The trabecula',
pp. 567—570. The sense capsules, pp. 570, 571. The branchial skeleton,
pp. 572 — 591. General structure of, pp. 572 — 575. Mandibular and hyoid arches,
pp. 575 — 591. Elasmobranchii, pp. 576—579. Teleostei, pp. 579—581. Am-
phibia, pp. 581—588. Sauropsida, pp. 588, 589. Mammalia, pp. 589—591.

Membrane bones and ossifications of the cranium, pp. 592 — 597.

Membrane bones, pp. 592 — 595. Ossifications of the cartilaginous cranium, pp.
595 — 597. Labial cartilages, p. 597. Bibliography of the skull, p. 598.

CHAPTER XX. PECTORAL AND PELVIC GIRDLES AND THE SKELETON

OF THE LIMBS. Pp. 599 — 622.
The Pectoral girdle, pp. 599 — 606. Pisces, pp. 599—601. Amphibia

and Amniota, pp. 601, 602. Lacertilia, p. 603. Chelonia, p. 603. Aves, pp.
603, 604. Mammalia, p. 604. Amphibia, p. 605. Bibliography of Pectoral
girdle, pp. 605, 606.

The Pelvic girdle, pp. 606 — 608. Pisces, pp. 606, 607. Amphibia and
Amniota, pp. 606, 607. Amphibia, p. 607. Lacertilia, p. 607. Mammalia,
p. 608. Bibliography of Pelvic girdle, p. 608. Comparison of pectoral and -pelvic
girdles, pp. 608, 609.

Limbs, pp. 609- -622. The piscine fin, pp. 609 — 618. The cheiroptery-
gium, pp. 618—622. Bibliography of limbs, p. 622.

CHAPTER XXI. THE BODY CAVITY, THE VASCULAR SYSTEM AND THE
VASCULAR GLANDS. Pp. 623 — 666.

The body cavity, pp. 623—632. General, pp. 623, 624. Chordat'a, pp.
624—632. Abdominal pores, pp. 626, 627. Pericardial cavities, pleural cavities
and diaphragm, pp. 627 — 632. Bibliography of body cavity, p. 632.

The Vascular System, pp. 632—663. General, pp. 632, 633. The heart,
pp. 633—643. Bibliography of the heart, p. 643. Arterial system, pp. 643—651.
Bibliography of the arterial system, p. 651. Venous system, pp. 651 — 663.
Bibliography of the venous system, p. 663. Lymphatic system and spleen,
p. 664. Bibliography of spleen, p. 664. Suprarenal bodies, pp. 664—666.
Bibliography of suprarenal bodies, p. 666.

CHAPTER XXII. THE MUSCULAR SYSTEM. Pp. 667 — 679.

Evolution of muscle-cells, pp. 667, 668. Voluntary muscular system of the Chor-
data, pp. 668 — 679. Muscular fibres, pp. 668, 669. Muscular system of the trunk and
limbs, pp. 673 — 6/6. The somites and muscular system of the head, pp. 676 —
671;. Bibliography of muscular system, p. 679.

CONTENTS OF VOLUME II. xi

CHAPTER XXIII. EXCRETORY ORGANS. Pp. 680 — 740.

Platyelminthes, pp. 680, 681. Mollusca, pp. 681, 682. Polyzoa, pp. 682, 683.
Branchiopoda, p. 683. Choctopoda, pp. 683 — 686. Gephyrea, pp. 686, 687.
Discophora, pp.687, 688. Arthropocla, pp.688, 689. Nematoda, p. 689. Excre-
tory organs and generative ducts of the Craniata, pp. 689—737.
General, pp. 689, 690. Elasmobranchii, pp. 690 — 699. Cyclostomata, pp. 700,
701. Teleostei, pp. 701 — 704. Ganoidei, pp. 704—707. Dipnoi, p. 707.
Amphibia, pp. 707 — 713. Amniota, pp. 713 — 727. General conclusions
and summary, pp. 728—737. Pronephros, pp. 728, 729. Mesonephros, pp.
729—732. Genital ducts, pp. 732—736. Metanephros, pp. 736, 737. Com-
parison of the excretory organs of the Chordata and Invertebrata, pp. 737, 738.
Bibliography of Excretory organs, pp. 738 — 740.

CHAPTER XXIV. GENERATIVE ORGANS AND GENITAL DUCTS. Pp.

741—753-

Generative organs, pp. 741—748. Porifera, p. 741. Ccelenterata, pp.
741 — 743. Chtetopoda and Gephyrea, p. 743. Chastognatha, pp. 743 — 745.
Polyzoa, p. 745. Nematoda, p. 745. Insecta, p. 745. Crustacea, pp. 745,
746. Chordata, pp. 746—748. Bibliography of generative organs, p. 748.

Genital ducts, pp. 748—753.

CHAPTER XXV. THE ALIMENTARY CANAL AND ITS APPENDAGES IN
THE CHORDATA. Pp. 754—780.

Mesenteron, pp. 754—774- Subnotochordal rod, pp. 7S4—756- Splanch-
nic mesoblast and mesentery, pp. 756—758. Respiratory division of the Mesen-
teron, pp. 758—766. Thyroid body, pp. 759—762. Thymus gland, pp. 762, 763.
Swimming bladder and lungs, pp. 763—766, The middle division of the Mesen-
teron, pp. 766—771. Cloaca, pp. 766, 767. Intestine, pp. 767, 768. Liver,
pp. 769, 770. Pancreas, pp. 770, 771. Posjtanal section of the Mesenteron, pp.
771—774.

The stomodseum, pp. 774 — 778. Comparative development of oral cavity,
PP- 774—776. Teeth, pp. 776—778.

The proctodseum, pp. 778 — 780. Bibliography of alimentary canal, p. 780.

EMBRYOLOGY.

CHAPTER I.
CEPHALOCHORDA.

THE developmental history of the Chordata has been studied
far more completely than that of any of the groups so far con-
sidered ; and the results which have been arrived at are of
striking interest and importance. Three main subdivisions of
this group can be recognized : (i) the Cephalochorda containing
the single genus Amphioxus ; (2) the Urochorda or Tunicata ;
and (3) the Vertebrata1. The members of the second and
probably of the first of these groups have undergone degenera-
tion, but at the same time the members of the first group
especially undergo a less modified development than that of
other Chordata.

CEPHALOCHORDA.

Our knowledge of the development of Amphioxus is mainly
due to Kowalevsky (Nos. 1 and 2). The ripe eggs appear to be
dehisced into the branchial or atrial cavity, and to be transported
thence through the branchial clefts into the pharynx, and so
through the mouth to the exterior. (Kowalevsky, No. 1, and
Marshall, No. 5.)

1 The term Vertebrata is often used to include the Cephalochorda. It is in many
ways convenient to restrict its use to the forms which have at any rate some indica-
tions of vertebrae ; a restriction which has the further convenience of restoring to the
term its original limitations. In the first volume of this work the term Craniata was
used for the forms which I now propose to call Vertebrata.

B. III. I

FORMATION OF THE LAYERS.

When laid the egg is about O'iO5 mm. in diameter. It is in-
vested by a delicate membrane, and is somewhat opaque owing
to the presence of yolk granules, which are however uniformly
distributed through it, and proportionately less numerous than
in the ova of most Chordata. Impregnation is external and the
segmentation is nearly regular (fig. i). A small segmentation

FIG. i. THE SEGMENTATION OF AMPHIOXUS.

A. Stage with two equal segments.

B. Stage with four equal segments.

(Copied from Kowalevsky. )

C. Stage after the four segments have become divided by an equatorial furrow
into eight equal segments.

D. Stage in which a single layer of cells encloses a central segmentation cavity.

E. Somewhat older stage in optical section.
sg. segmentation cavity.

cavity is visible at the stage with four segments, and increases
during the remainder of the segmentation ; till at the close (fig.
I E) the embryo consists of a blastosphere formed of a single
layer of cells enclosing a large segmentation cavity. One side
of the blastosphere next becomes invaginated, and during the
process the embryo becomes ciliated, and commences to rotate.
The cells forming the invaginated layer become gradually more
columnar than the remaining cells, and constitute the hypoblast;
and a structural distinction between the epiblast and hypoblast
is thus established. In the course of the invagination the seg-

CEPHALOCHORDA.

mentation cavity becomes gradually obliterated, and the embryo
first assumes a cup-shaped form with a wide blastopore, but soon
becomes elongated, while the communication of the archenteron,
or cavity of invagination, with the exterior is reduced to a small
blastopore (fig. 2 A), placed at the pole of the long axis which
the subsequent development shews to be the hinder end oj the

FIG. i. EMBRYOS OF AMPHIOXUS. (After Kowalevsky.)

The parts in black with white lines are epiblastic; the shaded parts are hypo-
blastic.

A. Gastrula stage in optical section.

B. Slightly later stage after the neural plate np has become differentiated, seen as
a transparent object from the dorsal side.

C. Lateral view of a slightly older larva in optical section.

D. Dorsal view of an older larva with the neural canal completely closed except
for a small pore (no) in front.

E. Older larva seen as a transparent object from the side.

bl. blastopore (which becomes in D the neurenteric canal) ; ne. neurenteric canal ;
np. neural or medullary plate ; no. anterior opening of neural canal ; ch. notochord ;
so1, so11, first and second mesoblastic somites.

embryo. The blastopore is often known in other Chordata as
the anus of Rusconi. Before the invagination is completed the
larva throws off the egg-membrane, and commences to lead a
free existence.

Up to this stage the larva, although it has acquired a
cylindrical elongated form, has only the structure of a simple
two-layered gastrula; but the changes which next take place

I — 2

MEDULLARY CANAL.

give rise on the one hand to the formation of the central nervous
system, and on the other to the formation of the notochord and
mesoblastic somites1. The former structure is developed from
the epiblast and the two latter from the hypoblast.

The formation of the central nervous system commences
with the flattening of the dorsal surface of the embryo. The
flattened area forms a plate (fig. 2 B and fig. 3 A, «/), extending
backwards to the blastopore, which has in the meantime passed
round to the dorsal surface. The sides of the plate become
raised as two folds, which are most prominent posteriorly, and
meet behind the blastopore, but shade off in front. The two
folds next unite dorsally, so as to convert the previous groove
into a canal2 — the neural or medullary canal. They unite first
of all over the blastopore, and their line of junction extends
from this point forwards (fig. 2 C, D, E). There is in this way
formed a tube on the floor of which the blastopore opens behind,
and which is itself open in front. Finally the medullary canal
is formed for the whole length of the embryo. The anterior
opening persists however for some time. The communication
between the neural and alimentary tracts becomes interrupted
when the caudal fin appears and the anus is formed. The
neural canal then extends round the end of the notochord to the
ventral side, but subsequently retreats to the dorsal side and
terminates in a slight dilatation.

In the formation of the medullary canal there are two points
deserving notice — viz. (i) the connection with the blastopore;
(2) the relation of the walls of the canal to the adjoining
epiblast. With reference to the first of these points it is clear
that the fact of the blastopore opening on the floor of the neural
canal causes a free communication to exist between the archen-
teron or gastrula cavity and the neural canal ; and that, so long
as the anterior pore of the neural canal remains open, the
archenteron communicates indirectly with the exterior (vide
fig. 2 E). It must not however be supposed (as has been done
by some embryologists) that the pore at the front end of the
neural canal represents the blastopore carried forwards. It is

1 The protovertebrae of most embryologists will be spoken of as mesoblastic
somites.

2 The details of this process are spoken of below.

CEPHALOCHORDA.

5

even probable that what Kowalevsky describes as the carrying
of the blastopore to the dorsal side is really the commencement
of the formation of the neural canal, the walls of which are con-
tinuous with the lips of the blastopore. This interpretation
receives support from the fact that at a later stage, when the
neural and alimentary canals become separated, the neural
canal extends round the posterior end of the notochord to the
ventral side. The embryonic communication between the neural
and alimentary canals is common to most Chordata ; and the
tube connecting them will be called the neurenteric canal.
It is always formed in fundamentally the same manner as in
Amphioxus. With reference to the second point it is to be
noted that Amphioxus is exceptional amongst the Chordata in
the fact that, before the closure of the neural groove, the layer
of cells which will form the neural tube becomes completely
separated from the adjoining epiblast (fig. 3 A), and forms a

FIG. 3. SECTIONS OF AN AMPHIOXUS EMBRYO AT THREE STAGES.
(After Kowalevsky.)

A. Section at gastrula stage.

B. Section of an embryo slightly younger than that represented in fig. 2 D.

C. Section through the anterior part of an embryo at the stage represented in
fig. 2 E.

np. neural plate ; nc. neural canal ; mes. archenteron in A and B, and mesenteron
in C ; ch. notochord ; so. mesoblastic somite.

structure which may be spoken of as the medullary plate ; and
that in the closure of the neural canal the lateral epiblast forms
a complete layer above this plate before the plate itself is folded
over into a closed canal. This peculiarity will be easily under-
stood from an examination of fig. 3 A, B and C.

The formation of the mesoblastic somites commences, at
about the same time as that of the neural canal, as a pair of
hollow outgrowths of the walls of the archenteron. These

MESOBLASTIC SOMITES.

outgrowths, which are shewn in surface view in fig. 2 B and D,
so, and in section in fig. 3 B and C, so, arise near the front end
of the body and gradually extend backwards as wing-like diver-
ticula of the archenteric cavity. As they grow backwards their
dorsal part becomes divided by transverse constrictions into
cubical bodies (fig. 2 D and E), which, with the exception of the
foremost, soon cease to open into what may now be called the
mesenteron, and form the mesoblastic somites. Each mesoblastic
somite, after its separation from the mesenteron, is constituted
of two layers, an inner one — the splanchnic — and an outer — the
somatic, and a cavity between the two which was originally con-
tinuous with the cavity of the mesenteron. Eventually the
dorsal parts of the outgrowths become separated from the
ventral, and form the muscle-plates, while their cavities
atrophy. The cavity of the ventral part, which is not divided
into separate sections by the above described constrictions,
remains as the true body cavity. The ventral part of the inner
layer of the mesoblastic outgrowths gives rise to the muscular
and connective tissue layers of the alimentary tract, and the
dorsal part to a section of the voluntary muscular system. The
ventral part of the outer layer gives rise to the somatic meso-
blast, and the dorsal to a section of the voluntary muscular
system. The anterior mesoblastic somite long retains its com-
munication with the mesenteron, and was described by Max
Schultze, and also at first by Kowalevsky, as a glandular organ.
While the mesoblastic somites are becoming formed the dorsal
wall of the mesenteron develops a median longitudinal fold
(fig. 3 B, c/i), which is gradually separated off from before back-
wards as a rod (fig. 3 C, c/i), underlying the central nervous system.
This rod is the notochord. After the separation of those
parts the remainder of the hypoblast forms the wall of the
mesenteron.

With the formation of the central nervous system, the meso-
blastic somites, the notochord, and the alimentary tract the
main systems of organs are established, and it merely remains
briefly to describe the general changes of form which accompany
the growth of the larva into the adult. By the time the larva
is but twenty-four hours old there are formed about seventeen
mesoblastic somites. The body, during the period in which

CEPIIALOCHORDA.

these are being formed, remains cylindrical, but shortly after-
wards it becomes pointed at both ends, and the caudal fin
appears. The fine cilia covering the larva also become replaced
by long cilia, one to each cell. The mesenteron is still completely
closed, but on the right side of the body, at the level of the front
end of the mesenteron, the hypoblast and epiblast now grow
together, and a perforation becomes formed through their point

br.c

FIG. 4. SECTIONS THROUGH TWO ADVANCED EMBRYOS OF AMPHIOXUS TO

SHEW THE FORMATION OF THE PERIBRANCHIAL CAVITY. (After Kowalevsky.)

In A are seen two folds of the body wall with a prolongation of the body cavity.
In B the two folds have coalesced ventrally, forming a cavity into which a branchial
cleft is seen to open.

tttes. mesenteron ; br.c. branchial cavity; //. body cavity.

of contact, which becomes the mouth. The anus is probably
formed about the same time if not somewhat earlier1.

Of the subsequent changes the two most important are (i)
the formation of the gill slits or clefts ; (2) the formation of the
peribranchial or atrial cavity.

The formation of the gill slits is, according to Kowalevsky's description,
so peculiar that one is almost tempted to suppose that his observations were
made on pathological specimens. The following is his account of the
process. Shortly after the formation of the mouth there appears on the
ventral line a coalescence between the epiblast and hypoblast. Here an
opening is formed, and a visceral cleft is thus established, which passes to
the left side, viz. the side opposite the mouth. A second and apparently a
third slit are formed in the same way. The stages immediately following
were not observed, but in the next stage twelve slits were present, no longer
however on the left side, but in the median ventral line. There now appears
on the side opposite the mouth, and the same therefore as that originally
occupied by the first three clefts, a series of fresh clefts, which in their

1 The lateral position of the mouth in the embryo Amphioxus has been regarded
as proving that the mouth represents a branchial cleft, but the general asymmetry
of the organs is such that no great stress can, I think, be laid on the position of the
mouth.

8 BRANCHIAL CAVITY.

growth push the original clefts over to the same side as the mouth. Each of
the fresh clefts becomes divided into two, which form the permanent clefts of
their side.

The gill slits at first open freely to the exterior, but during
their formation two lateral folds of the body wall, containing a
prolongation of the body cavity, make their appearance (fig. 4
A), and grow downwards over the gill clefts, and finally meet
and coalesce along the ventral line, leaving a widish cavity
between themselves and the body wall. Into this cavity, which
is lined by epiblast, the gill clefts open (fig. 4 B, br.c). This
cavity — which forms a true peribranchial cavity — is completely
closed in front, but owing to the folds not uniting completely
behind it remains in communication with the exterior by an
opening known as the atrial or abdominal pore.

The vascular system of Amphioxus appears at about the
same time as the first visceral clefts.

BIBLIOGRAPHY.

(1) A. Kowalevsky. " Entwicklungsgeschichte des Amphioxus lanceolatus."
AIJHI. Acad. Imper. des Sciences de St Petersbourg, Series vil. Tom. xi. 1867.

(2) A. Kowalevsky. " Weitere Studien iiber die Entwicklungsgeschichte des
Amphioxus lanceolatus." Archivf. mikr. Anat., Vol. XIII. 1877.

(3) Leuckart u. Pagenstecher. " Untersuchungen iiber niedere Seethiere."
Mailer's Arckiv, 1858.

(4) Max Schultze. " Beobachtung junger Exemplare von Amphioxus." Zeit.
f. wiss. Zool., Bd. in. 1851.

(5) A. M. Marshall. "On the mode of Oviposition of Amphioxus." your,
of Anat. and Phys., Vol. X. 1876.

CHAPTER II.

UROCHORDA1.

IN the Solitaria, except Cynthia, the eggs are generally laid,
and impregnation is effected sometimes before and sometimes
after the eggs have left the atrial cavity. In Cynthia and most
Caducichordata development takes place within the body of the
parent, and in the Salpidae a vascular connection is established
between the parent and the single foetus, forming a structure
physiologically comparable with the Mammalian placenta.

Solitaria. The development of the Solitary Ascidians has
been more fully studied than that of the other groups, and appears
moreover to be the least modified. It has been to a great
extent elucidated by the splendid researches of Kowalevsky
(Nos. 18 and 20), whose statements have been in the main
followed in the account below. Their truth seems to me to be
established, in spite of the scepticism they have met with in
some quarters, by the closeness of their correspondence with
the developmental phenomena in Amphioxus.

1 The following classification of the Urochorda is adopted in the present chapter.

I. Caducichordata.

( Solitaria ex. Ascidia.
A. SIMPLICIA \

( Sociaha ex. Clavelhna.

B. COMPOSITA Sedentaria «. Botryllus.

( Natantia ex. Pyrosoma.

j^*--
( Doliohdse.

Pyros
C. CONSERTA

II. Perennichordata.

Ex. Appendicularia.

10 MEDULLARY GROOVE.

The type most fully investigated by Kowalevsky is Ascidia
(Phallusia) mammillata ; and the following description must be
taken as more especially applying to this type.

The segmentation is complete and regular. A small seg-
mentation cavity appears fairly early, and is surrounded, ac-
cording to Kowalevsky, by a single layer of cells, though on
this point Kupffer (No. 27) and Giard (No. 11) are at variance
with him.

The segmentation is followed by an invagination of nearly
the same character as in Amphioxus. The blastosphere resulting
from the segmentation first becomes flattened on one side, and
the cells on the flatter side become more columnar (fig. 8 I.).
Very shortly a cup-shaped form is assumed, the concavity
of which is lined by the more columnar cells. The mouth of the
cup or blastopore next becomes narrowed ; while at the same
time the embryo becomes oval. The blastopore is situated not
quite at a pole of the oval but in a position which subsequent
development shews to be on the dorsal side close to the posterior
end of the embryo. The long axis
of the oval corresponds with the
long axis of the embryo. At this
stage the embryo consists of two
layers ; a columnar hypoblast
lining the central cavity or archen-
teron, and a thinner epiblastic
layer. The dorsal side of the
embryo next becomes flattened ^^teteaiaflP^X,/

(fig. 8 II.), and the epiblast cover-

.,.,,, r, , j FIG. 5. TRANSVERSE SECTION

mg it is shortly afterwards marked THROUGH THE FRONT KND OF AN EM-

by an axial groove continued for- BRYO OF PHALLUSIA MAMMILLATA.
, . (After Kowalevsky.)

wards from the blastopore to near

,, c i /• i i , //- The embryo is slightly younger

the front end of the body (fig. 5, than that represented in fig. 8 in.

This is the medullary mg, medullary groove; al. ali-

groove, and it soon becomes con- mentary tract.
verted into a closed canal — the medullary or neural canal—
below the external skin (fig. 6, n.c). The closure is effected by
the folds on each side of the furrow meeting and coalescing
dorsally. The original medullary folds fall into one another
behind the blastopore. so that the blastopore is situated on the

UROCHORDA. 1 1

floor of the groove, and, on the conversion of the groove into a
canal, the blastopore connects the canal with the archenteric
cavity, and forms a short neurenteric canal. The closure of the
medullary canal commences at the
blastopore and is thence continued
forwards, the anterior end of the
canal remaining open. The above me-

processes are represented in longitu-
dinal section in fig. 8 III, n. When
the neural canal is completed for its
whole length, it still communicates
by a terminal pore with the exterior. FIG. 6. TRANSVERSE OPTICAL

T ,. , , . c ., , „ SECTION OF THE TAIL OF AN EM-

In the relation of the medullary BRYO OF PHALLUSIA MAMMIL-
canal to the blastopore, as well as LATA- (After Kowalevsky.)

in the closure of the medullary The section is from an embryo

r i 1 • i r j . i_ of the same age as fig. 8 IV.

groove from behind forwards, the cA. notocbhord ; %.,. neurai

Solitary Ascidians agree closely with canal ; me- mesoblast ; al. hypo-

7 blast of tail.

Amphioxus.

The cells of the dorsal wall of the archenteron immediately
adjoining the front and sides of the blastopore have in the mean-
time assumed a somewhat different character from the remaining
cells of the archenteron, and give rise to a body which, when
viewed from the dorsal surface, has somewhat the form of a
horseshoe. This body was first observed by Metschnikoff. On
the elongation of the embryo and the narrowing of the blasto-
pore the cells forming this body arrange themselves as a broad
linear cord, two cells wide, underlying about the posterior half
of the neural canal (fig. 7, ch}. They form the rudiment of the
notochord, which, as in Amphioxus, is derived from the dorsal
wall of the archenteron. They are seen in longitudinal section
in fig. 8 II. and ill. ch.

With the formation of the notochord the body of the embryo
becomes divided into two distinct regions — a posterior region
where the notochord is present, and an anterior region into
which it is not prolonged. These two regions correspond with
the tail and the trunk of the embryo at a slightly later stage.
The section of the archenteric cavity in the trunk dilates and
constitutes the permanent mesenteron (figs. 7, al, and 8 III. and
IV. dd\ It soon becomes shut off from the slit-like posterior

12

NOTOCHORD.

ch-

part of the archenteron. The nervous ai

system in this part also dilates and
forms what may be called the ce-
phalic swelling (fig. 8 IV.), and the
pore at its anterior extremity
gradually narrows and finally dis-
appears. In the region of the tail
we have seen that the dorsal wall of
the archenteron becomes converted
into the notochord, which imme-
diately underlies the posterior part
of the medullary canal, and soon
becomes an elongated cord formed
of a single or double row of flattened
cells. The lateral walls of the archen-
teron (fig. 7, me) in the tail become
converted into elongated cells ar-
ranged longitudinally, which form
powerful lateral muscles (fig. 8 IV.
tri). After the formation of the noto-
chord and of the lateral muscles

there remains of the archenteron in the tail only the ventral wall,
which according to Kowalevsky forms a simple cord of cells
(fig. 6, at). It is however not always present, or else has escaped
the attention of other observers. It is stated by Kowalevsky to
be eventually transformed into blood corpuscles. The neuren-
teric canal leads at first into the narrow space between the above
structures, which is the remnant of the posterior part of the
lumen of the archenteron. Soon both the neurenteric canal and
the caudal remnant of the archenteron become obliterated.

During the above changes the tail becomes considerably
elongated and, owing to the larva being still in the egg-shell, is
bent over to the ventral side of the trunk.

The larva at this stage is represented in a side view in fig. 8
IV. The epidermis is formed throughout of a single layer of
cells. In the trunk the mesenteron is shewn at dd and the
dilated part of the nervous system, no longer communicating
with the exterior, at n. In the tail the notochord is shewn at
ch, the muscles at m, and the solid remnant of the ventral wall

FIG. 7. OPTICAL SECTION OF
AN EMBRYO OF PHALLUSIA MAM-
MI LLAT A. (After Kowalevsky.)

The embryo is of the same age
as fig. 8 ill, but is seen in longitu-
dinal horizontal section.

al. alimentary tract in anterior
part of body ; ch. notochord ; me.
mesoblast.

UROCHORDA.

FIG. 8. VARIOUS STAGES IN THE DEVELOPMENT OF PHALLUSIA MAMMILLATA.

(From Huxley; after Kowalevsky.)
The embryos are represented in longitudinal vertical section.

I. Commencing gastrula stage, fh. segmentation cavity.

II. Late gastrula stage with flattened dorsal surface, eo. blastopore; ch. noto-
chord ; dd. hypoblast.

III. A more advanced embryo with a partially-formed neural tube, ch, and dd.
as before; n. neural tube; c. epiblast.

IV. Older embryo in which the formation of the neural tube is completed, dd.
hypoblast enclosing persistent section of alimentary tract; dd' . hypoblast in the tail ;
m. muscles.

V. Larva just hatched. The end of the tail is not represented, a. eye; gb.
dilated extremity of neural tube with otolith projecting into it; Rg. anterior swelling
of the spinal division of the neural tube ; f. anterior pore of neural tube ; Rm. posterior
part of neural tube; o. mouth; Chs. notochord; kl. atrial invagination ; dd. branchial
region of alimentary tract ; d. commencement of oesophagus and stomach ; dd' . hypo-
blast in the tail ; m. muscles ; hp. papilla for attachment.

VI. Body and anterior part of the tail of a two days' larva, kirn, atrial aperture;
en. endostyle; ks. branchial sack; iks, iks. branchial slits; bb. branchial vessel
between them; ch. axial portion of notochord ; chs. peripheral layer of cells. Other
reference letters as before.

14 THE TEST.

of the archenteron at dd '. The delicate continuation of the
neural canal in the tail is seen above the notochord at n. An
optical section of the tail is shewn in fig. 6. It is worthy of
notice that the notochord and muscles are formed in the same
manner as in Amphioxus, except that the process is somewhat
simplified. The mode of disappearance of the archenteric cavity
in the tail, by the employment of the whole of its walls in the
formation of various organs, is so peculiar, that I feel some
hesitation in accepting Kowalevsky's statements on this head1.

The larva continues to grow in length, and the tail becomes
further curled round the ventral side of the body within the
egg-membrane. Before the tail has nearly reached its full length
the test becomes formed as a cuticular deposit of the epiblast
cells (O. Hertwig, No. 13, Semper, No. 37). It appears first in
the tail and gradually extends till it forms a complete invest-
ment round both tail and trunk, and is at first totally devoid of
cells. Shortly after the establishment of the test there grow out
from the anterior end of the body three peculiar papillae, deve-
loped as simple thickenings of the epidermis. At a later stage,
after the hatching of the larva, these papillae develop glands at
their extremities, secreting a kind of glutinous fluid2. After
these papillae have become formed cells first make their appear-
ance in the test ; and there is simultaneously formed a fresh
inner cuticular layer of the test, to which at first the cells are
confined, though subsequently they are found in the outer layer
also. On the appearance of cells in the test the latter must be
regarded as a form, though a very abnormal one, of connective
tissue. When the tail of the larva has reached a very con-
siderable length the egg-membrane bursts, and the larva becomes
free. The hatching takes place in Asc. canina about 48 — 60
hours after impregnation. The free larva (fig. 8 V.) has a
swollen trunk, and a very long tail, which soon becomes

1 It is more probable that this part of the alimentary tract is equivalent to the
post-anal gut of many Vertebrata, which is at first a complete tube, but disappears
later by the simple absorption of the walls.

z It is probable that these papillae are very primitive organs of the Chordata.
Structures, which are probably of the same nature, are formed behind the mouth in
the larva^ of Amphibia, and in front of the mouth in the larvce of Ganoids (Acipenser,
Lepidosteus), and are used by these larvre for attaching themselves.

UROCHORDA. 1 5

straightened out. It has a striking resemblance to a tadpole
(vide fig. 10).

In the free larval condition the Ascidians have in many
respects a higher organization than in the adult state. It is
accordingly convenient to divide the subsequent development
into two periods, the first embracing the stages from the con-
dition represented in fig. 8 V. up to the full development of the
free larva, and the second the period from the full development
of the larva to the attainment of the fixed adult condition.

Growth and Structure of the free larva.

The nervous system. The nervous system was left as a
closed tube consisting of a dilated anterior division, and a
narrow posterior one. The former may be spoken of as the
brain, and the latter as the spinal cord ; although the homologies
of these two parts are quite uncertain. The anterior part of the
spinal cord lying within the trunk dilates somewhat (fig. 8 V. and

FIG. 9. LARVA OF ASCIDIA MENTULA. (From Gegenbaur; after Kupffer.)
Only the anterior part of the tail is represented.

N'. anterior swelling of neural tube; N. anterior swelling of spinal portion of
neural tube; n. hinder part of neural tube; ch. notochord; K. branchial region of
alimentary tract ; d. cesophageal and gastric region of alimentary tract ; 0. eye ;
a. otolith ; o. mouth ; s. papilla for attachment.

VI. Rg) and there may thus be distinguished a trunk and a
caudal section of the spinal cord.

The original single vesicle of the brain becomes divided by
the time the larva is hatched into two sections (fig. 9) — (i) an
anterior vesicle with, for the most part, thin walls, in which

1 6 EYE.

unpaired auditory and optic organs make their appearance, and

(2) a posterior nearly solid cephalic ganglion, through which

there passes a narrow continuation of the central canal of the

nervous system. This ganglion consists of a dorsal section

formed of distinct cells, and a ventral section formed of a

punctated material with nuclei. The auditory organ1 consists

of a 'crista acustica' (fig. 9), in the form of a slight prominence

of columnar cells on the ventral side of the anterior cerebral

vesicle ; to the summit of which a spherical otolith is attached

by fine hairs. In the crista is a cavity containing clear fluid.

The dorsal half of the otolith is pigmented : the ventral half is

without pigment. The crista is developed in situ, but the otolith

is formed from a single cell on the dorsal side of the cerebral

vesicle, which forms a projection into the cavity of the vesicle,

and then travels (in a manner not clearly made out) round the

right side of the vesicle till it comes to the crista ; to which it is

at first attached by a narrow pedicle. The fully developed eye

(figs. 8 VI. and 9, O) consists of a cup-shaped retina, which forms

a prominence slightly on the right side of the posterior part of

the dorsal wall of the anterior cerebral vesicle, and of refractive

media. The retina is formed of columnar cells, the inner ends

of which are imbedded in pigment, The refractive media of the

eye are directed towards the cavity of the cerebral vesicle, and

consist of a biconvex lens and a meniscus. Half the lens is

imbedded in the cavity of the retina and surrounded by the

pigment, and the other half is turned toward a concavo-convex

meniscus which corresponds in position with the cornea. The

development of the meniscus and lens is unknown, but the

retina is formed (fig. 8 V. a] as an outgrowth of the wall of the

brain. At the inner ends of the cells of this outgrowth a deposit

of pigment appears.

The trunk section of the spinal cord (fig. 9, N) is separated
by a sharp constriction from the brain. It is formed of a super-
ficial layer of longitudinal nervous fibres, and a central core of
ganglion cells. The layer of fibres diminishes in thickness
towards the tail, and finally ceases to be visible. Kupffer
detected three pairs of nerves passing off from the spinal cord to

1 For a fuller account of the organs of sense vide the chapters on the eye and ear.

UROCHORDA.

the muscles of the tail. The foremost of these arises at the
boundary between the trunk and the tail, and the two others at
regular intervals behind this point.

The mesoblast and muscular system. It has already been
stated that the lateral walls of the archenteron in the tail give
rise to muscular cells. These cells lie about three abreast, and
appear not to increase in number ; so that with the growth
of the tail they grow enormously in length, and eventually
become imperfectly striated. The mesoblast cells at the hinder
end of the trunk, close to its junction with the tail, do not
become converted into muscle cells, but give rise to blood
corpuscles ; and the axial remnant of the archenteron undergoes
a similar fate. According to Kowalevsky the heart is formed
during larval life as an elongated closed sack on the right side of
the endostyle.

The notochord. The notochord was left as a rod formed of
a single row of cells, or in As. canina and some other forms of
two rows, extending from just within the border of the trunk to
the end of the tail.

According to Kowalevsky, Kupffer, Giard, etc. the notochord undergoes
a further development which finds its only complete parallel amongst
Chordata in the doubtful case of Amphioxus.

There appear between the cells peculiar, highly refractive discs (fig. 8 v.
Chs). These become larger and larger, and finally, after pushing the
remnants of the cells with their nuclei to the sides, coalesce together to form
a continuous axis of hyaline substance. The remnants of the cells with
their nuclei form a sheath round the hyaline axis (fig. 8 vi. ch.}. Whether
the axis is to be regarded as formed of an intercellular substance, or of a
differentiation of parts of the cells is still doubtful. Kupffer inclines to the
latter view : the analogy of the notochord of higher types appears to me to
tell in favour of the former one.

The alimentary tract. The anterior part of the primitive
archenteron alone retains a lumen, and from this part the whole
of the permanent alimentary tract (mesenteron) becomes deve-
loped. The anterior part of it grows upwards, and before
hatching an involution of the epiblast on the dorsal side, just in
front of the anterior extremity of the nervous system, meets and
opens into this upgrowth, and gives rise to the permanent mouth
(fig. 8 v. o\

B. III. 2

1 8 ALIMENTARY TRACT.

Kowalevsky states that a pore is formed at the front end of the nervous
tube leading into the mouth (fig. 8 v. and vi. /) which eventually gives rise
to the ciliated sack, which lies in the adult at the junction between the mouth
and the branchial sack. Kupffer however was unable to find this opening ;
but Kowalevsky's observations are confirmed by those of Salensky on
Salpa.

From the hinder end of the alimentary sack an outgrowth
directed dorsalwards makes its appearance (figs. 8 V. and 9, d),
from which the oesophagus, stomach and intestine become
developed. It at first ends blindly. The remainder of the
primitive alimentary sack gives rise to the branchial sack of the
adult. Just after the larva has become hatched, the outgrowth
to form the stomach and oesophagus, etc. bends ventralwards
and to the right, and then turns again in a dorsal and left
direction till it comes close to the dorsal surface, somewhat to
the left of and close to the hinder end of the trunk. The first
ventral loop of this part gives rise to the oesophagus, which
opens into the stomach ; from this again the dorsally directed
intestine passes off.

On the ventral wall of the branchial sack there is formed a
narrow fold with thickened walls, which forms the endostyle.
It ends anteriorly at the stomodaeum and posteriorly at the
point where the solid remnant of the archenteron in the tail was
primitively continuous with the branchial sack. The whole of the
alimentary wall is formed of a single layer of hypoblast cells.

A most important organ connected with the alimentary
system still remains to be dealt with, viz. the atrial or peri-
branchial cavity. The first rudiments of it appear at about the
time of hatching, in the form of a pair of dorsal epiblastic
involutions (fig. 8 V. £/), at the level of the junction between the
brain and the spinal cord. These involutions grow inwards, and
meet corresponding outgrowths of the branchial sack, with
which they fuse. At the junction between them is formed an
elongated ciliated slit, leading from the branchial sack into the
atrial cavity of each side. The slits so formed are the first pair
of branchial clefts. Behind the first pair of branchial clefts a
second pair is formed during larval life by a second outgrowth
of the branchial sack meeting the epiblastic atrial involutions
(fig. 8 vi. \ks and 2ks). The intestine at first ends blindly close

UROCHORDA.

to the left atrial involution, but the anus becomes eventually
formed by an opening being established between the left atrial
involution and the intestine.

During the above described processes the test remains quite
intact, and is not perforated at the oral or the atrial openings.

The retrogressive metamorphosis of the larva.

The development of the adult from the larva is, as has
already been stated, in the main a retrogressive metamorphosis.
The stages in this metamorphosis are diagrammatically shewn
in figs. 10 and n. It commences with the attachment of the
larva (fig. 10 A) which takes place by one of the three papillae.
Simultaneously with the attachment the larval tail undergoes a
complete atrophy (fig. 10
B), so that nothing is
left of it but a mass of
fatty cells situated close
to the point of the pre-
vious insertion of the
tail in the trunk.

The nervous system
also undergoes a very
rapid retrogressive meta-
morphosis ; and the only
part of it which persists
would seem to be the
dilated portion of the
spinal cord in the trunk
(KupfTer, No. 28).

The three papillae, in-
cluding that serving for
attachment, early disappear, and the larva becomes fixed by a
growth of the test to foreign objects.

An opening appears in the test some time after the larva is
fixed, leading into the mouth, which then becomes functional.
The branchial sack at the same time undergoes important
changes. In the larva it is provided with only two ciliated slits,
which open into the, at this stage, paired atrial cavity (fig. 10).

2 — 2

FIG. 10. DIAGRAM SHEWING THE MODE OF
ATTACHMENT AND SUBSEQUENT RETROGRESSIVE

METAMORPHOSIS OF A LARVAL ASCIDIAN. (From

Lankester.)

2O

METAMORPHOSIS.

BRAIN

The openings of the atrial cavity at first are shut off from
communication with the exterior by the test, but not long after
the larva becomes fixed, two perforations are formed in the test,
which lead into the openings of the two atrial cavities. At the
same time the atrial cavities dilate so as gradually to embrace the
whole branchial sack to which their inner walls attach themselves.
Shortly after this the branchial clefts rapidly increase in
number1.

The increase of the branchial clefts is somewhat complicated. Between
the two primitive clefts two new ones appear, and then a third appears
behind the last cleft. In the interval
between each branchial cleft is placed
a vascular branchial vessel (fig. 8 vi.
bb\ Soon a great number of clefts
become added in a row on each side
of the branchial sack. These clefts
are small ciliated openings placed
transversely with reference to the
long axis of the branchial sack, but
only occupying a small part of the
breadth of each side. The intervals
dorsal and ventral to them are soon
filled by series of fresh rows of slits,
separated from each other by longi-
tudinal bars. Each side of the
branchial sack becomes in this way
perforated by a number of small
openings arranged in rows, and
separated by transverse and longitu-
dinal bars. The whole structure forms the commencement of the branchial
basketwork of the adult ; the arrangement of which differs considerably in
structure and origin from the simple system of branchial clefts of normal
vertebrate types. At the junction of the transverse and longitudinal bars
papillas are formed projecting into the lumen of the branchial sack.

After the above changes are far advanced towards com-
pletion, the openings of the two atrial sacks gradually approxi-
mate in the dorsal line, and finally coalesce to form the single
atrial opening of the adult. The two atrial cavities at the same
time coalesce dorsally to form a single cavity, which is con-

TAIL-

FIG. IT. DIAGRAM OF A VERY YOUNG
ASCIDIAN. (From Lankester.)

1 The account of the multiplication of the branchial clefts is taken from Krohn's
paper on Phallusia mammillata (No. 24), but there is every reason to think that it
holds true in the main for simple Ascidians.

UROCHORDA. 21

tinuous round the branchial sack, except along the ventral line
where the endostyle is present. The atrial cavity, from its
mode of origin as a pair of epiblastic involutions1, is clearly a
structure of the same nature as the branchial or atrial cavity of
Amphioxus; and has nothing whatever to do with the true body
cavity.

It has already been stated that the anus opens into the
original left atrial cavity; when the two cavities coalesce the
anus opens into the atrial cavity in the median dorsal line.

Two of the most obscure points in the development are the
origin of the mesoblast in the trunk, and of the body cavity.
Of the former subject we know next to nothing, though it seems
that the cells resulting from the atrophy of the tail are em-
ployed in the nutrition of the mesoblastic structures of the
trunk.

The body cavity in the adult is well developed in the region
of the intestine, where it forms a wide cavity lined by an
epithelioid mesoblastic layer. In the region of the branchial
sack it is reduced to the vascular channels in the walls of the
sack.

Kowalevsky believes the body cavity to be the original seg-
mentation cavity, but this view can hardly be regarded as
admissible in the present state of our knowledge. In some
other Ascidian types a few more facts about the mesoblast will
be alluded to.

With the above changes the retrogressive metamorphosis
is completed ; and it only remains to notice the change in
position undergone in the attainment of the adult state. The
region by which the larva is attached grows into a long process
(fig. 10 B), and at the same time the part carrying the mouth is
bent upwards so as to be removed nearly as far as possible from
the point of attachment. By this means the condition in the

1 In the asexually produced buds of Ascidians the atrial cavity appears, with the
exception of the external opening, to be formed from the primitive branchial sack.
In the buds of Pyrosoma however it arises independently. These peculiarities in the
buds cannot weigh against the embryonic evidence that the atrial cavity arises from
involutions of the epiblast, and they may perhaps be partially explained by the fact
that in the formation of the visceral clefts outgrowths of the branchial sack meet the
atrial involutions.

22 MOLGULA.

adult (fig. u) is gradually brought about; the original dorsal
surface with the oral and atrial openings becoming the termina-
tion of the long axis of the body, and the nervous system being
placed between the two openings.

The genus Molgula presents a remarkable exception amongst the simple
Ascidians in that, in some if not all the species belonging to it, development
takes place (Lacaze Duthiers 29 and 30, Kupffer 28) quite directly and
without larval metamorphosis.

The ova are laid either singly or adhering together, and are very opaque.
The segmentation (Lacaze Duthiers) commences by the formation of four
equal spheres, after which a number of small clear spheres are formed
which envelope the large spheres. The latter give rise to a closed enteric
sack, and probably also to a mass of cells situated on the ventral side,
which appear to be mesoblastic. The epiblast is constituted of a single
layer of cells which completely envelopes the enteric sack and the
mesoblast.

While the ovum is still within the chorion five peculiar processes of
epiblast grow out ; four of which usually lie in the same sectional plane of
the embryo. They are contractile and contain prolongations of the body
cavity. Their relative size is very variable.

The nervous system is formed on the dorsal side of the embryo before
the above projections make their appearance, but, though it seems probable
that it originates in the same manner as in the more normal forms, its
development has not been worked out. As soon as it is formed it consists of
a nervous ganglion similar to that usually found in the adult. The history
of the mass of mesoblast cells has been inadequately followed, but it
continuously disappears as the heart, excretory organs, muscles, etc. become
formed. So far as can be determined from Kupffer's descriptions the body
cavity is primitively parenchymatous — an indication of an abbreviated
development — and does not arise as a definite split in the mesoblast.

The primitive enteric 'cavity becomes converted into the branchial sack,
and from its dorsal and posterior corner the oesophagus, stomach and
intestine grow out as in the normal forms. The mouth is formed by the
invagination of a disc-like thickening of the epidermis in front of the nervous
system on the dorsal side of the body ; and the atrial cavity arises behind
the nervous system by a similar process at a slightly later period. The gill
clefts opening into the atrial cavity are formed as in the type of simple
Ascidians described by Krohn.

The embryo becomes hatched not long after the formation of the oral and
atrial openings, and the five epiblastic processes undergo atrophy. They
are not employed in the attachment of the adult.

The larva when hatched agrees in most important points with the adult ;
and is without the characteristic provisional larval organs of ordinary
forms ; neither organs of special sense nor a tail becoming developed. It
has been suggested by Kupffer that the ventrally situated mesoblastic mass

UROCHORDA. 23

is the same structure as the mass of elements which results in ordinary types
from the degeneration of the tail. If this suggestion is true it is difficult to
believe that this mass has any other than a nutritive function.

The larva of Ascidia ampulloides described by P. van Beneden is
regarded by Kupffer as intermediate between the Molgula larva and the
normal type, in that the larval tail and notochord and a pigment spot are
first developed, while after the atrophy of these organs peculiar processes
like those of Molgula make their appearance.

Sedentaria. The development of the fixed composite Ascidians is, so
far as we know, in the main similar to that of the simple Ascidians. The
larvae of Botryllus sometimes attain, while still in the free state, a higher
stage of development with reference to the number of gill slits, etc. than
that reached by the simple Ascidians, and in some instances (Botryllus
auratus Metscknikoff} eight conical processes are found springing in a ring-
like fashion around the trunk. The presence of these processes has led to
somewhat remarkable views about the morphology of the group ; in that
they were regarded by Kolliker, Sars, etc. as separate individuals, and it was
supposed that the product of each ovum was not a single individual, but a
whole system of individuals with a common cloaca.

The researches of Metschnikoff (No. 32), Krohn (No. 25), and Giard
(No. 12), etc. demonstrate that this paradoxical view is untenable, and that
each ovum only gives rise to a single embryo, while the stellate systems are
subsequently formed by budding.

Natantia. Our knowledge of the development of Pyrosoma
is mainly due to Huxley (No. 16) and Kowalevsky (No. 22).
In each individual of a colony of Pyrosoma only a single egg
comes to maturity at one time. This egg is contained in a
capsule formed of a structureless wall lined by a flattened epi-
thelioid layer. From this capsule a duct passes to the atrial
cavity, which, though called the oviduct, functions as an afferent
duct for the spermatozoa.

The segmentation is meroblastic, and the germinal disc
adjoins the opening of the oviduct. The segmentation is very
similar to that which occurs in Teleostei, and at its close the
germinal disc has the form of a cap of cells, without a trace
of stratification or of a segmentation cavity, resting upon the
surface of the yolk, which forms the main mass of the ovum.

After segmentation the blastoderm, as we may call the layer
of cells derived from the germinal disc, rapidly spreads over the
surface of the yolk, and becomes divided into two layers, the
epiblast and the hypoblast. At the same time it exhibits a
distinction into a central clearer and a peripheral more opaque

PYROSOMA.

,at

region. At one end of the blastoderm, which for convenience
sake may be spoken of as the posterior end, a disc of epiblast
appears, which is the first rudiment of the nervous system, and
on each side of the middle of the blastoderm there arises an epi-
blastic involution. The epiblastic involutions give rise to the
atrial cavity.

These involutions rapidly grow in length, and soon form
longish tubes, opening at the surface by pores situated not far
from the posterior end of the blastoderm.

The blastoderm at this stage, as seen on the surface of the
yolk, is shewn in fig. 12 A. It is somewhat broader than long.
The nervous system
is shewn at n, and at
points to an atrial
tube. A transverse
section, through about
the middle of this
blastoderm, is repre-
sented in fig. 12 B.
The epiblast is seen
above. On each side
is the section of an
atrial tube (af). Below
is the hypoblast which
is separated from the
yolk especially in the
middle line; at each

side it is beginning to

, , A. SURFACE VIEW OF THE OVUM OF PYROSOMA

grow in below, on the NOT FAR ADVANCED IN DEVELOPMENT. The em-

SUrface of the volk bryonic structures are developed from a disc-like

The space below the
hypoblast is the ali-
mentary cavity, the
ventral wall of which
is formed by the cells growing in at the sides. Between the
epiblast and hypoblast are placed scattered mesoblast cells, the
origin of which has not been clearly made out.

In a later stage the openings of the two atrial tubes gradually
travel backwards, and at the same time approximate, till finally

FIG. 12.

blastoderm.

B. TRANSVERSE SECTION THROUGH THE M i DDI.K

PART OF THE SAME BLASTODERM.

at. atrial cavity ; hy. hypoblast ; n. nervous disc
in the region of the future Cyathozooid.

UROCHORDA.

they meet and coalesce at the posterior end of the blastoderm
behind the nervous disc (fig. 13, cl}. The tubes themselves at
the same time become slightly constricted not far from their
hinder extremities, and so divided into a posterior region nearly
coterminous with the nervous system (fig. 13), and an anterior
region. These two regions have very different histories in the
subsequent development.

The nervous disc has during these changes become marked
by a median furrow (fig. 13, ng}, which is soon converted into a
canal by the same process as in the simple Ascidians. The
closure of the groove commences
posteriorly and travels forwards.
These processes are clearly of
the same nature as those which
take place in Chordata generally
in the formation of the central
nervous system.

In the region of the germinal
disc which contains the anterior
part of the atrial tubes, the ali-
mentary cavity becomes, by the
growth of the layer of cells de-
scribed in the last stage, a com-
plete canal, on the outer wall of
which the endostyle is formed
as a median fold. The whole
anterior part of the blastoderm
becomes at the same time
gradually constricted off from
the yolk.

The fate of the anterior and

posterior parts of the blastoderm is very different. The anterior
part becomes segmented into four zooids or individuals, called
by Huxley Ascidiozooids, which give rise to a fresh colony of
Pyrosoma. The posterior part forms a rudimentary zooid,
called by Huxley Cyathozooid, which eventually atrophies.
These five zooids are formed by a process of embryonic fission.
This fission commences by the appearance of four transverse
constrictions in the anterior part of the blastoderm; by which

en

-at

FIG. 13. BLASTODERM OF PYRO-
SOMA SHORTLY BEFORE ITS DIVISION

INTO CYATHOZOOID AND ASCIDIO-
ZOOIDS. (After Kowalevsky.)

cl. cloacal (atrial) opening; en. en-
dostyle ; at. atrial cavity ; ng. nervous
groove.

The heart and pericardial cavity are
seen to the left.

26 I'VROSOMA.

the whole blastoderm becomes imperfectly divided into five
regions, fig. 14 A.

The hindermost constriction (uppermost in my figure) lies
just in front of the pericardial cavity; and separates the Cyatho-
zooid from the four ascidiozooids. The three other constrictions
mark off the four Ascidiozooids. The Cyathozooid remains for
its whole length attached to the blastoderm, which has now
nearly enveloped the yolk. It contains the whole of the nervous
system (ng), which is covered behind by the opening of the
atrial tubes (cl}. The alimentary tract in the Cyathozooid
forms a tube with very delicate walls. The pericardial cavity is
completely contained within the Cyathozooid, and the heart
itself (///) has become formed by an involution of the walls of the
cavity.

The Ascidiozooids are now completely separated from the
yolk. They have individually the same structure as the un-
divided rudiment from which they originated ; so that the
organs they possess are simply two atrial tubes, an alimentary
tract with an endostyle, and un differentiated mesoblast cells.

In the following stages the Ascidiozooids grow with great
rapidity. They soon cease to lie in a straight line, and eventu-
ally form a ring round the Cyathozooid and attached yolk
sack.

While these changes are being accomplished in the external
form of the colony, both the Cyathozooids and the Ascidiozooids
progress considerably in development. In the Cyathozooid the
atrial spaces gradually atrophy, with the exception of the ex-
ternal opening, which becomes larger and more conspicuous.
The heart at the same time comes into full activity and drives
the blood through the whole colony. The yolk becomes more
and more enveloped by the Cyathozooid, and is rapidly ab-
sorbed ; while the nutriment derived from it is transported to
the Ascidiozooids by means of the vascular connection. The
nervous system retains its previous condition ; and round the
Cyathozooid is formed the test into which cells migrate, and
arrange themselves in very conspicuous hexagonal areas. The
delicate alimentary tract of the Cyathozooid is still continuous
with that of the first Ascidiozooid. After the Cyathozooid has
reached the development just described it commences to atrophy.

UROCHORDA.

The changes in the Ascidiozooids are even more considerable
than those in the Cyathozooid. A nervous system appears as a
fresh formation close to the end of each Ascidiozooid turned
towards the Cyathozooid. It forms a tube of which the open
A -B

FlG. 14. TWO STAGES IN THE DEVELOPMENT OF PYROSOMA IN WHICH THE
CYATHOZOOID AND FOUR ASCIDIOZOOIDS ARE ALREADY DISTINCTLY FORMED.
(After Kowalevsky.)

cy. Cyathozooid ; as. ascidiozooid ; ng. nervous groove ; /it. heart of Cyathozooid ;
cl. cloacal opening.

front end eventually develops into the ciliated pit of the
mouth, and the remainder into the actual nervous ganglion.
Between the nervous system and the endostyle an involution
appears, which gives rise to the mouth. On each side of the
primitive alimentary cavity of each Ascidiozooid branchial slits
make their appearance, leading into the atrial tubes; so that the
primitive alimentary tract becomes converted into the branchial
sacks of the Ascidiozooids. The remainder of the alimentary
tract of each zooid is formed as a bud from the hind end of the
branchial sack in the usual way. The alimentary tracts of the
four Ascidiozooids are at first in free communication by tubes
opening from the hinder extremity of one zooid into the dorsal
side of the branchial sack of the next zooid. At the hinder end
of each Ascidiozooid is developed a mass of fatty cells known
as the elaeoblast, which probably represents a rudiment of the
larval tail of simple Ascidians. (Cf. pp. 30 — 32.)

The further changes consist in the gradual atrophy of the
Cyathozooid, which becomes more and more enclosed within the
four Ascidiozooids. These latter become completely enveloped

28 PYROSOMA.

in a common test, and form a ring round the remains of the
yolk and of the Cyathozooid, the heart of which continues how-
ever to beat vigorously. The cloacal opening of the Cyathozooid
persists through all these changes, and, after the Cyathozooid
itself has become completely enveloped in the Ascidiozooids and
finally absorbed, deepens to form the common cloacal cavity of
the Pyrosoma colony.

The main parts of the Ascidiozooids were already formed
during the last stage. The zooids long remain connected to-
gether, and united by a vascular tube with the Cyathozooid, and
these connections are not severed till the latter completely atro-
phies. Finally, after the absorption of the Cyathozooid, the
Ascidiozooids form a rudimentary colony of four individuals
enveloped in a common test. The two atrial tubes of each
zooid remain separate in front but unite posteriorly. An anus
is formed leading from the rectum into the common posterior
part of the atrial cavity; and an opening is established between
the posterior end of the atrial cavity of each Ascidiozooid and
the common axial cloacal cavity of the whole colony. The
atrial cavities in Pyrosoma are clearly lined by epiblast, just as
in simple Ascidians.

When the young colony is ready to become free, it escapes
from the atrial cavity of the parent, and increases in size by
budding.

Doliolidae. The sexually developed embryos of Doliolum have been
observed by Krohn (No. 23), Gegenbaur (No. 10), and Keferstein and Ehlers
(No. 17); but the details of the development have been very imperfectly
investigated.

The youngest embryo observed was enveloped in a large oval trans-
parent covering, the exact nature of which is not clear. It is perhaps a
larval rudiment of the test which would seem to be absent in the adult.
Within this covering is the larva, the main organs of which are already
developed ; and which primarily differs from the adult in the possession of a
larval tail similar to that of simple Ascidians.

In the body both oral and atrial openings are present, the latter on the
dorsal surface ; and the alimentary tract is fully established. The endostyle
is already formed on the ventral wall of the branchial sack, but the bran-
chial slits are not present. Nine muscular rings are already visible. The
tail, though not so developed as in the simple Ascidians, contains an axial
notochord of the usual structure, and lateral muscles. It is inserted on the
ventral side, and by its slow movements the larva progresses.

UROCHORDA. 29

In succeeding stages the tail gradually atrophies, and the gill slits, four
in number, develop ; at the same time a process or stolon, destined to give
rise by budding to a second non-sexual generation, makes its appearance on
the dorsal side in the seventh inter-muscular space. This stolon is
comparable with that which appears in the embryo of Salpa. When the
tail completely atrophies the larva leaves its transparent covering, and
becomes an asexual Doliolum with a dorsal stolon.

SalpidaB. As is well known the chains of Salpa alone are sexual, and
from each individual of the chain only a single embryo is produced. The
ovum from which this embryo takes its origin is visible long before the
separate Salps of the chain have become completely developed. It is
enveloped in a capsule continuous with a duct, which opens into the atrial
cavity, and is usually spoken of as the oviduct. The capsule with the ovum
is enveloped in a maternal blood sinus. Embryonic development com-
mences after the chain has become broken up, and the spermatozoa derived
from another individual would seem to be introduced to the ovum through
the oviduct.

At the commencement of embryonic development the oviduct and
ovicapsule undergo peculiar changes ; and in part at least give rise to a
structure subservient to the nutrition of the embryo, known as the placenta.
These changes commence with the shortening of the oviduct, and the
disappearance of a distinction between oviduct and ovicapsule. The cells
lining the innermost end of the capsule, i.e. that at the side of the ovum
turned away from the atrial cavity, become at the same time very columnar.
The part of the oviduct between the ovum and the atrial cavity dilates into
a sack, communicating on the one hand with the atrial cavity, and on the
other by a very narrow opening with the chamber in which the egg is
contained. This sack next becomes a prominence in the atrial cavity, and
eventually constitutes a brood-pouch. The prominence it forms is covered
by the lining of the atrial cavity, immediately within which is the true wall
of the sack. The external opening of the sack becomes gradually narrowed,
and finally disappears. In the meantime the chamber in which the embryo
is at first placed acquires a larger and larger opening into the sack ; till
finally the two chambers unite, and a single brood-pouch containing the
embryo is thus produced. The inner wall of the chamber is formed by the
columnar cells already spoken of. They form the rudiment of the placenta.
The double wall of the outer part of the brood-pouch becomes stretched by
the growth of the embryo ; the inner of its two layers then atrophies. The
outer layer subsequently gives way, and becomes rolled back so as to lie at
the inner end of the embryo, leaving the latter projecting freely into the
atrial cavity.

While these changes are taking place the placenta becomes fully
developed. The first rudiment of it consists, according to Salensky, of the
thickened cells of the ovicapsule only, though this view is dissented from by
Brooks, Todaro, etc. Its cells soon divide to form a largish mass, which
becomes attached to a part of the epiblast of the embryo.

30 SALPA.

On the formation of the body cavity of the embryo a central axial
portion of the placenta becomes separated from a peripheral layer ; and a
channel is left between them which leads from a maternal blood sinus into
the embryonic body cavity. The peripheral layer of the placenta is formed
of cells continuous with the epiblast of the embryo ; while the axial portion
is constituted of a disc of cells adjoining the embryo, with a column of
fibres attached to the maternal side. The fibres of this column are believed
by Salensky to be products of the original rudiment of the placenta. The
placenta now assumes a more spherical form, and its cavity becomes shut off
from the embryonic body cavity. The fibrous column breaks up into a
number of strands perforating the lumen of the organ, and the cells of the
wall become stalked bodies projecting into the lumen.

When the larva is nearly ready to become free the placenta atrophies.

The placenta functions in the nutrition of the embryo in the following
way. It projects from its first formation into a maternal blood sinus, and,
on the appearance of a cavity in it continuous with the body cavity of the
embryo, the blood of the mother fully intermingles with that of the embryo.
At a later period the communication with the body cavity of the embryo is
shut off, but the cavity of the placenta is supplied with a continuous stream
of maternal blood, which is only separated from the foetal blood by a thin
partition.

It is now necessary to turn to the embryonic development about which it
is unfortunately not as yet possible to give a completely satisfactory account.
The statements of the different investigators contradict each other on most
fundamental points. I have followed in the main Salensky (No. 34), but
have also called attention to some points where his observations diverge
most from those of other writers, or where they seem unsatisfactory.

The development commences at about the period when the brood-pouch
is becoming formed ; and the ovum passes entirely into the brood-pouch
before the segmentation is completed. The segmentation is regular, and
the existence of a segmentation cavity is denied by Salensky, though
affirmed by Kowalevsky and Todaro1.

At a certain stage in the segmentation the cells of the ovum become
divided into two layers, an epiblast investing the whole of the ovum with the
exception of a small area adjoining the placenta, where the inner layer or
hypoblast, which forms the main mass of the ovum, projects at the surface.
The epiblast soon covers the whole of the hypoblast, so that there would
seem (according to Salensky's observations) to be a kind of epibolic
invagination : a conclusion supported by Todaro's figures.

At a later stage, on one side of the free apex of the embryo, a
mesoblastic layer makes its appearance between the epiblast and hypoblast.
This layer is derived by Salensky, as it appears to me on insufficient
grounds, from the epiblast. Nearly at the same time there arises not far

1 From Todaro's latest paper (No. Hit) it would seem the segmentation cavity has
very peculiar relations.

UROCHORDA. 31

from the same point of the embryo, but on the opposite side, a solid thick-
ening of epiblast which forms the rudiment of the nervous system. The
nervous system is placed close to the front end of the body ; and nearly at
the opposite pole, and therefore at the hind end, there appears immediately
below the epiblast a mass of cells forming a provisional organ known as the
elaeoblast. Todaro regards this organ as mesoblastic in origin, and Salensky
as hypoblastic. The organ is situated in the position which would be
occupied by the larval tail were it developed. It may probably be regarded
(Salensky) as a disappearing rudiment of the tail, and be compared in this
respect with the more or less similar mass of cells described by Kupffer in
Molgula, and with the elaeoblast in Pyrosoma.

After the differentiation of these organs a cavity makes its appearance
between the epiblast and hypoblast, which is regarded by Salensky as the
body cavity. It appears to be equivalent to the segmentation cavity of
Todaro. According to Todaro's statements, it is replaced by a second
cavity, which appears between the splanchnic and somatic layers of
mesoblast, and constitutes the true body cavity. The embryo now begins to
elongate, and at the same time a cavity makes its appearance in the centre
of the hypoblast cells. This cavity is the rudiment of the branchial and
alimentary cavities : on its dorsal wall is a median projection, the rudiment
of the so-called gill of Salpa.

At two points this cavity comes into close contact with the external skin.
At one of these, situated immediately ventral to the nervous system, the
mouth becomes formed at a later period. At the other, placed on the dorsal
surface between the nervous system and the elaeoblast, is formed the cloacal
aperture.

By the stage under consideration the more important systems of organs
are established, and the remaining embryonic history may be very briefly
narrated.

The embryo at this stage is no longer covered by the walls of the brood-
pouch but projects freely into the atrial cavity, and is only attached to its
parent by means of the placenta. The epiblast cells soon give rise to a
deposit which forms the mantle. The deposit appears however to be formed
not only on the outer side of the epiblast but also on the inner side ; so that
the epiblast becomes cemented to the subjacent parts, branchial sack, etc.,
by an intercellular layer, which would seem to fill up the primitive body
cavity with the exception of the vascular channels (Salensky).

The nervous system, after its separation from the epiblast, acquires a
central cavity, and subsequently becomes divided into three lobes, each with
an internal protuberance. At its anterior extremity it opens into the
branchial sack ; and from this part is developed the ciliated pit of the
adult. The nervous ganglion at a later period becomes solid, and a median
eye is subsequently formed as an outgrowth from it.

According to Todaro there are further formed two small auditory
(? olfactory) sacks on the ventral surface of the brain, each of them placed in
communication with the branchial cavity by a narrow canal.

32 SALPA.

The mesoblast gives rise to the muscles of the branchial sack, to the
heart, and to the pericardium. The two latter are situated on the ventral
side of the posterior extremity of the branchial cavity.

Branchial sack and alimentary tract. The first development of the
enteric cavity has already been described. The true alimentary tract is
formed as a bud from the hinder end of the primitive cavity. The remainder
of the primitive cavity gives rise to the branchial sack. The so-called gill
has at first the form of a lamella attached dorsally to the walls of the
branchial sack ; but its attachment becomes severed except at the two ends,
and it then forms a band stretching obliquely across the branchial cavity,
which subsequently becomes hollow and filled with blood corpuscles The
whole structure is probably homologous with the peculiar fold, usually
prolonged into numerous processes, which normally projects from the
dorsal wall of the Ascidian branchial sack.

On the completion of the gill the branchial sack becomes divided into a
region dorsal to the gill, and a region ventral to it. Into the former the
single atrial invagination opens. No gill slits are formed comparable with
those in simple Ascidians, and the only representative of these structures is
the simple communication which becomes established between the dorsal
division of the branchial sack and the atrial opening. The whole branchial
sack of Salpa, including both the dorsal and ventral divisions, corresponds
with the branchial sack of simple Ascidians. On its ventral side the
endostyle is formed in the normal way. The mouth arises at the point
already indicated near the front end of the nervous system1.

1 Brooks takes a very different view of the nature of the parts in Salpa. He says,
No. 7, p. 322, "The atrium of Salpa, when first observed, was composed of two
"broad lateral atria within the body cavity, one on each side of the branchial sack,
"and a very small mid-atrium — The lateral atria do not however, as in most Tuni-
"cata, remain connected with the mid-atrium, and unite with the wall of the branchial
"sack to form the branchial slits, but soon become entirely separated, and the two
"walls of each unite so as to form a broad sheet of tissue, which soon splits up to
"form the muscular bands of the branchial sack." Again, p. 324, "During the
"changes which have been described as taking place in the lateral atria, the mid-

"atrium has increased in she The branchial and atrial tunics now unite upon

" each side, so that the sinus is converted into a tube which communicates, at its pos-
"terior end, with the heart and peri visceral sinus, and at the anterior end with the
"neural sinus. This tube is the gill. ...The centres of the two regions upon the sides
"of the gill, where these two tissues have become united, are now absorbed, so that a
"single long and narrow branchial slit is produced on each side of the gill. The
"branchial cavity is thus thrown into communication with the atrium, and the upper
"surface of the latter now unites with the outer tunic, and the external atrial opening
"is formed by absorption."

The above description would imply that the atrial cavity is a space lined by meso-
blast, a view which would upset the whole morphology of the Ascidians. Salensky's
account, which implies only an immense reduction in the size of the atrial cavity as
compared with other types, appears to me far more probable. The lateral atria of

UROCIIORDA. 33

Development of the chain of sexual Salps. My description
of the embryonic development of Salpa would not be complete without some
reference to the development of the stolon of the solitary generation of Salps
by the segmentation of which a chain of sexual Salps originates.

The asexual Salp, the embryonic development of which has just been
described, may be compared to the Cyathozooid of Pyrosoma, from which it
mainly differs in being fully developed. While still in an embryonic
condition it gives rise to a process or stolon, which becomes divided into a
number of zooids by transverse constrictions, in the same manner that part
of the germ of the ovum of Pyrosoma is divided by transverse constrictions
into four Ascidiozooids.

The stolon arises as a projection on the right side of the body of the
embryo close to the heart. It is formed (Salensky, No. 35) of an outgrowth
of the body wall, into which there grow the following structures :

(1) A central hollow process from the end of the respiratory sack.

(2) A right and left lateral prolongation of the pericardial cavity.

(3) A solid process of cells on the ventral side derived from the same
mass of the cells as the elasoblast.

(4) A ventral and a dorsal blood sinus.

Brooks appear to be simply parts of the body cavity, and have certainly no connection
with the lateral atria of simple Ascidians or Pyrosoma.

The observations of Todaro upon Salpa (No. 38) are very remarkable, and illustrated
by beautifully engraved plates. His interpretations do not however appear quite satis-
factory. The following is a brief statement of some of his results.

During segmentation there arises a layer of small superficial cells (epiblast) and
a central layer of larger cells, which becomes separated from the former by a segmenta-
tion cavity, except at the pole adjoining the free end of the brood-pouch. At this point
the epiblast cells become invaginated into the central cells and form the alimentary
tract, while the primitive central cells remain as the mesoblast. A fold arises from the
epiblast which Todaro compares to the vertebrate amnion, but the origin of it is un-
fortunately not satisfactorily described. The folds of the amnion project towards the
placenta, and enclose a cavity which, as the folds never completely meet, is perma-
nently open to the maternal blood sinus. This cavity corresponds with the cavity of
the true amnion of higher Vertebrates. It forms the cavity of the placenta already
described. Between the two folds of the amnion is a cavity corresponding with the
vertebrate false amnion. A structure regarded by Todaro as the notochord is formed
on the neck, connecting the involution of the alimentary tract with the exterior. It
has only a very transitory existence.

In the later stages the segmentation cavity disappears and a true body cavity is
formed by a split in the mesoblast.

Todaro's interpretations, and in part his descriptions also, both with reference to
the notochord and amnion, appear to me quite inadmissible. About some other parts
of his descriptions it is not possible to form a satisfactory judgment. He has recently
published a short paper on this subject (No. 39) preliminary to a larger memoir, which
is very difficult to understand in the absence of plates. He finds however in the
placenta various parts which he regards as homologous with the decidua vera and
reflexa of Mammalia.

B. III. 1

34 APPENDICULARIA.

Besides these parts there appears on the dorsal side a hollow tube, the
origin of which is unknown, which gives rise to the nervous system.

The hollow process of the respiratory sack is purely provisional, and
disappears without giving rise to any permanent structure. The right and
left prolongations of the pericardial cavity become solid and eventually give
origin to the mesoblast. The ventral process of cells is the most important
structure in the stolon in that it gives rise both to the alimentary and respir-
atory sacks, and to the generative organs of the sexual Salps. The stolon
containing the organs just enumerated becomes divided by transverse
constrictions into a number of rings. These rings do not long remain
complete, but become interrupted dorsally and ventrally. The imperfect
rings so formed soon overlap, and each of them eventually gives rise to a
sexual Salp. Although the stolon arises while the asexual Salp is still in an
embryonic condition, it does not become fully developed till long after the
asexual Salp has attained maturity.

Appendicularia. Our only knowledge of the development of
Appendicularia is derived from Fol's memoir on the group (No. 8). He
simply states that it develops, as far as he was able to follow, like other
Ascidians ; and that the extremely minute size of the egg prevented him
from pursuing the subject. He also states that the pair of pores leading
from the branchial cavity to the exterior is developed from epiblastic
involutions meeting outgrowths of the wall of the branchial sack.

Metagenesis.

One of the most remarkable phenomena in connection with
the life history of many Ascidians is the occurrence of an
alternation of sexual and gemmiparous generations. This alter-
nation appears to have originated from a complication of the
process of reproduction by budding, which is so common in this
group. The mode in which this very probably took place will
be best understood by tracing a series of transitional cases
between simple budding and complete alternations of gene-
rations.

In the simpler cases, which occur in some Composita
Sedentaria, the process of budding commences with an out-
growth of the body wall into the common test, containing a
prolongation of part of the alimentary tract1.

1 It is not within the scope of this work to enter into details with reference
to the process of budding. The reader is referred on this head more especially to the
papers of Huxley (No. 16) and Kowalevsky (No. 22) on Pyrosoma, of Salensky
(No. 35) on Salpa, and Kowalevsky (No. 21) on Ascidians generally. It is a question
of very great interest how budding first arose, and then became so prevalent in these

UROCHORDA. 35

Between the epiblastic and hypoblastic layers of the bud so
formed, a mesoblastic and sometimes a generative outgrowth of
the parent also appears.

The systems of organs of the bud are developed from the
corresponding layers to those in the embryo1. The bud even-
tually becomes detached, and in its turn gives rise to fresh
buds. Both the bud and its parent reproduce sexually as well
as by budding : the new colonies being derived from sexually
produced embryos.

The next stage of complication is that found in Botryllus
(Krohn, Nos. 25 and 26). The larva produced sexually gives
rise to a bud from the right side of the body close to the heart.
On the bud becoming detached the parent dies away without
developing sexual organs. The bud of the second generation
gives rise to two buds, a right one and a left one, and like the
larva dies without reaching sexual maturity. The buds of the
third generation each produce two buds and then suffer the
same fate as their parent.

The buds of the third generation arrange themselves with
their cloacal extremities in contact, and in the fourth generation
a common cloaca is formed, and so a true radial system of
zooids is established ; the zooids of which are not however
sexual.

The buds of the fourth generation in their turn produce two
or three buds and then die away.

Fresh systems become formed by a continuation of the
process of budding, but the zooids of the secondary systems so

degenerate types of Chordata. It is possible to suppose that budding may have com-
menced by the division of embryos at an early stage of development, and have gradu-
ally been carried onwards by the help of natural selection till late in life. There is
perhaps little in the form of budding of the Ascidians to support this view — the early
budding of Didemnum as described by Gegenbaur being the strongest evidence for it
— but it fits in very well with the division of the embryo in Lumbricus trapezoides
described by Kleinenberg, and with the not unfrequent occurrence of double monsters
in Vertebrata which may be regarded as a phenomenon of a similar nature (Rauber).
The embryonic budding of Pyrosoma, which might perhaps be viewed as supporting
the hypothesis, appears to me not really in favour of it; since the Cyathozooid of
Pyrosoma is without doubt an extremely modified form of zooid, which has obviously
been specially developed in connection with the peculiar reproduction of the Pyro-
somidse.

1 The atrial spaces form somewhat doubtful exceptions to the rule.

3—2

36 METAGENESIS.

formed are sexual. The ova come to maturity before the
spermatozoa, so that cross fertilization takes place.

In Botryllus we have clearly a rudimentary form of alter-
nations of generations, in that the sexually produced larva is
asexual, and, after a series of asexual generations, produced
gemmiparously, there appear sexual generations, which however
continue to reproduce themselves by budding.

The type of alternations of generations observable in
Botryllus becomes, as pointed out by Huxley, still more marked
in Pyrosoma.

The true product of the ovum is here (vide p. 25) a rudi-
mentary individual called by Huxley the Cyathozooid. This
gives rise, while still an embryo, by a process equivalent to
budding to four fully developed zooids (Ascidiozooids) similar
to the parent form, and itself dies away. The four Ascidio-
zooids form a fresh colony, and reproduce (i) sexually, whereby
fresh colonies are formed, and (2) by ordinary budding, whereby
the size of the colony is increased. All the individuals of the
colony are sexual.

The alternation of generations in Pyrosoma widely differs
from that in Botryllus in the fact of the Cyathozooid differing
so markedly in its anatomical characters from the ordinary
zooids.

In Salpa the process is slightly different1. The sexual forms
arc noiv incapable of budding, and, although at first a series of
sexual individuals are united together in the form of a chain, so
as to form a colony like Pyrosoma or Botryllus, yet they are so
loosely connected that they separate in the adult state. As in
Botryllus, the ova are ripe before the spermatozoa. Each
sexual individual gives rise to a single offspring, which, while
still in the embryonic condition, buds out a 'stolon' from its
right ventral side. This stolon is divided into a series of lateral
buds after the solitary asexual Salp has begun to lead an inde-
pendent existence. The solitary asexual Salp clearly corre-
sponds with the Cyathozooid of Pyrosoma, though it has not,
like the Cyathozooid, undergone a retrogressive metamorphosis.
By far the most complicated form of alternation of gene-

1 Vide p. 33.

UROCHORDA. 37

rations known amongst the Ascidians is that in Doliolum. The
discovery of this metamorphosis was made by Gegenbaur (No.
10). The sexual form of Doliolum is somewhat cask-shaped,
with ring-like muscular bands, and the oral and atrial apertures
placed at opposite ends of the cask. The number of gill slits
varies according to the species. The ovum gives rise, as already
described, to a tailed embryo which subsequently develops into
a cask-shaped asexual form. On attaining its full size it loses
its branchial sack and alimentary tract. While still in the
embryonic condition, a stolon grows out from its dorsal side in
the seventh inter-muscular space. The stolon, like that in Salpa,
contains a prolongation of the branchial sack1.

On this stolon there develop two entirely different types of
buds, (i) lateral buds, (2) dorsal median buds.

The lateral buds are developed in regular order on the two
sides of the stolon, and the most advanced buds are those
furthest removed from the base. They give rise to forms with a
very different organization to that of the parent. They are
compared by Gegenbaur to a spoon, the bowl of which is formed
by the branchial sack, and the handle by the stalk attaching the
bud to the stolon. The oral opening into the branchial sack is
directed upwards : an atrial opening is remarkably enough not
present. The branchial sack is perforated by numerous open-
ings. It leads into an alimentary tract which opens directly to
the exterior by an anus opposite the mouth.

The stalks attaching the more mature buds to the stolon are
provided with ventrally directed scales, which completely hide
the stolon in a view from the ventral surface.

These buds have, even after their detachment, no trace of
generative organs, and shew no signs of reproducing themselves
by budding. Their eventual fate is unknown.

The median dorsal buds have no such regular arrangement
as the lateral buds, but arise in irregular bunches, those furthest
removed from the base of the stolon being however the oldest.
These buds are almost exactly similar to the original sexual
form ; they do not acquire sexual organs, but are provided with

1 I draw this conclusion from Gegenbaur's fig. (No. 10), PI. XVI., fig. 15. The
body (x) in the figure appears to me without doubt the rudiment of the stolon, and
not, as believed by Gegenbaur, the larval tail.

38 BIBLIOGRAPHY.

a stolon attached on the ventral side, in the sixth inter-muscular
space.

This stolon is simply the stalk by which each median bud
was primitively attached to the stolon of the first asexual form.

From the stolon of the median buds of the second generation
buds are developed which grow into the sexual forms.

The generations of Doliolum may be tabulated in the follow-
ing way.

Sexual generation,

ist asexual form with dorsal stolon,
I

spoon-like forms developed as 2nd asexual forms developed as

lateral buds (eventual history median buds with ventral stolon,

unknown). |

sexual generation.

BIBLIOGRAPHY.

(6) P. J. van Beneden. " Recherches s. I'Embryogenie, 1'Anat. et la Physiol.
des Ascidies simples." Mem. Acad. Roy. de Belgiqiie, Tom. xx.

(7) W. K. Brooks. "On the development of Salpa." Bull, of the Museum of
Comp. Anat. at Harvard College, Cambridge, Mass.

(8) H. Fol. Eludes sur les Appendiculaires du detroit de Messine. Geneve et
Bale, 1872.

(!)) Ganin. "Neue Thatsachen a. d. Entwicklungsgeschichte d. Ascidien."
Zcit.f. wiss. Zool., Vol. xx. 1870.

(10) C. Gegenbaur. "Ueber den Entwicklungscyclus von Doliolum nebst
Bemerkungen liber die Larven dieser Thiere." Zeit.f. wiss. Zool., Bd. vu. 1856.

(11) A. Giard. "Etudes critiques des travaux d'embryogenie relatifs a la
parente des Vertebres et des Tuniciers." Archiv Zool. experiment., Vol. I. 1872.

(12) A. Giard. " Recherches sur les Synascidies. " Archiv Zool. exper., Vol. I.
1872.

(13) O. Hertwig. " Untersuchungen lib. d. Bau u. d. Entwicklung des Cellu-
lose-Mantels d. Tunicaten." yenaische Zeitschrift, Bd. vil. 1873.

(14) Th. II. Huxley. " Remarks upon Appendicularia and Doliolum. " Phil.
Trans., 1851.

(15) Th. II. Huxley. "Observations on the anatomy and physiology of Salpa
and Pyrosoma." Phil. Trans., 1851.

(16) Th. H. Huxley. "Anatomy and development of Pyrosoma." Linnean
Trans., 1860, Vol. XXIII.

(17) Keferstein u. Ehlers. Zoologische Beitrdge, 1861. Doliolum.

(18) A. Kowalevsky. "Entwicklungsgeschichte d. einfachen Ascidien." Mem.
Acad. Pclersbourg, vil. s<5rie, T. X. 1866.

(19) A. Kowalevsky. "Beitrag z. Entwick. d. Tunicaten." Nachrichten d.
konigl. Gesell.zu Goltingen. 1868.

(20) A. Kowalevsky. "Weitere Studien lib. d. Entwicklung d. einfachen As-
cidien." Archiv f. mikr. Anat., Vol. vil. 1871.

BIBLIOGRAPHY. 39

(21) A. Kowalevsky. " Ueber Knospung d. Ascidien. " Archiv f. mikr. Anat.,
Vol. X. 1874.

(22) A. Kowalevsky. "Ueber die Entwicklungsgeschichte d. Pyrosoma."
Archiv f. mikr. Anat., Vol. XI. 1875.

(23) A. Krohn. "Ueber die Gattung Doliolum u. ihre Arten." Archiv f.
Naturgeschichte, Bd. XVIII. 1852.

(24) A. Krohn. "Ueber die Entwicklung d. Ascidien." Mailer's Archiv,
1852.

(25) A. Krohn. "Ueber die Fortpflanzungsverhaltnisse d. Botrylliden. " Archiv
f. Naturgeschichte, Vol. xxxv. 1869.

(26) A. Krohn. "Ueber die fruheste Bildung d. Botryllenstocke." Archiv f.
Naturgeschichte, Vol. xxxv. 1869.

(27) C. Kupffer. "Die Stammverwandschaft zwischen Ascidien u. Wirbelthie-
ren." Archiv f. mikr. Anat., Vol. vi. 1870.

(28) C. Kupffer. "Zur Entwicklung d. einfachen Ascidien." Archiv f. mikr.
Anat., Vol. vin. 1872.

(29) H. Lacaze Duthiers. "Recherches sur 1'organisation et 1'Embryogenie
des Ascidies (Molgula tubulosa)." Comptes rendus, May 30, 1870, p. 1154.

(30) H. Lacaze Duthiers. "Les Ascidies simples des Cotes de France" (De-
velopment of Molgula). Archiv Zool. exper., Vol. III. 1874.

(31) R. Leuckart. "Salpa u. Verwandte." Zoologische Untersuchungen,
Heft u.

(32) E. Metschnikoff. " Observations sur le developpement de quelques ani-
maux (Botryllus and Simple Ascidians)." Bull. d. FAcad. Petersbourg, Vol. xin.
1869.

(33) H. Milne-Edwards. "Observations s. 1. Ascidies composees des cotes de
la Manche." Memoir es d. rinstitut, T. xvill. 1842.

(34) W. Salensky. " Ueber d.embryonale Entwicklungsgeschichte derSalpen."
Zeit.f. wiss. Zool., B. xxvii. 1877.

(35) W. Salensky. "Ueber die Knospung d. Salpen." Morphol. Jahrbuch,
Bd. in. 1877.

(36) W. Salensky. "Ueber die Entwicklung d. Hoden u. liber den Genera-
tionswechsel d. Salpen." Zeit. f. wiss. Zool., Bd. xxx. Suppl. 1878.

(37) C. Semper. " Ueber die Entstehung d. geschichteten Cellulose- Epidermis
d. Ascidien." Arbeit, a. d. zool.-zoot. Instit. Wiirzburg, Vol. II. 1875.

(38) Fr. Todaro. Sopra lo sviluppo e f anatomia delle Salpe. Roma, 1875.

(39) Fr. Todaro. "Sui primi fenomeni dello sviluppo delle Salpe." Reale
Accademia dei Lincei, Vol. iv. 1880.

CHAPTER III.

ELASMOBRANCHII.

THE impregnation of the ovum is effected in the oviduct.
In most forms the whole of the subsequent development, till the
time when the embryo is capable of leading a free existence,
takes place in the uterus ; but in other cases the egg becomes
enveloped, during its passage down the oviduct, first in a layer
of fluid albumen, and finally in a dense horny layer, which
usually takes the form of a quadrilateral capsule with characters
varying according to the species. After the formation of this
capsule the egg is laid, and the whole of the development,
with the exception of the very first stages, takes place
externally.

In many of the viviparous forms (Mustelus, Galeus, Car-
charias, Sphyrna) the egg is enclosed, during the early stages
of development at any rate, in a very delicate shell homologous
with that of the oviparous forms ; there is usually also a scanty
albuminous layer. Both of these are stated by Gerbe (No. 42)
to be absent in Squalus spinax.

The following are examples of viviparous genera : Hexanchus, Noti-
danus, Acanthias, Scymnus, Galeus, Squalus, Mustelus, Carcharias, Sphyrna,
Squatina, Torpedo ; and the following of oviparous genera : Scyllium, Pris-
tiurus, Cestracion, Raja1.

The ovum at the time of impregnation has the form of a
large spherical mass, similar to the yolk of a bird's egg, but
without a vitelline membrane2. The greater part of it is formed
of peculiar oval spherules of food-yolk, held together by a
protoplasmic network. The protoplasm is especially concen-
trated in a small lens-shaped area, known as the germinal disc,
which is not separated by a sharp line from the remainder of

1 For further details, vide Miiller (No. 48). - Vide Vol. II., p. 62.

ELASMOBRANCHII.

the ovum. Yolk spherules are present in this disc as elsewhere,
but are much smaller and of a different character. The segmen-
tation has the normal meroblastic character (fig. 15) and is
confined to the germinal disc. Before it commences the ger-
minal disc exhibits amoeboid movements. During the segmen-
tation nuclei make their appearance spontaneously (?) in the
yolk adjoining the germinal- disc (fig. 15, nx'}, and around them
portions of the yolk with its protoplasmic network become
segmented off. Cells are thus formed which are added to those
resulting from the segmentation proper. Even after the seg-
mentation numerous nuclei are present in the granular matter
below the blastoderm (fig. 16 A, n')\ and around these cells

FIG. 15.

SECTION THROUGH GERMINAL DISC OF A PRISTIURUS EMBRYO DURING
THE SEGMENTATION.

«. nucleus ; nx. nucleus modified prior to division ; nx'. modified nucleus in the
yolk ; f. furrow appearing in the yolk adjacent to the germinal disc.

are being continually formed, which enter the blastoderm, and
are more especially destined to give rise to the hypoblast. The
special destination of many of these cells is spoken of in detail
below.

At the close of segmentation the blastoderm forms a some-
what lens-shaped disc, thicker at one end than at the other ; the
thicker end being the embryonic end. It is divided into two
strata — an upper one, the epiblast — formed of a single row of
columnar cells ; and a lower one, the primitive hypoblast,
consisting of the remaining cells of the blastoderm, and forming
a mass several strata deep. These cells will be spoken of as the

SEGMENTATION.

lower layer cells, to distinguish them from the true hypoblast
which is one of their products.

A cavity very soon appears in the lower layer cells, near the
non-embryonic end of the blastoderm, but the cells afterwards

C—

FlG. 16. TWO LONGITUDINAL SECTIONS OF THE BLASTODERM OF A PfUSTIURUS
EMBRYO DURING STAGES PRIOR TO THE FORMATION OF THE MEDULLARY GROOVE.
ep. epiblast ; //. lower layer cells or primitive hypoblast ; m. mesoblast ; hy. hypo-
blast ; sc. segmentation cavity ; es. embryo swelling ; «'. nuclei of yolk ; er. embryonic
rim. c. lower layer cells at the non-embryonic end of the blastoderm.

disappear from the floor of this cavity, which then lies between
the yolk and the lower layer cells (fig. 16 A, sc}. This cavity is
the segmentation cavity equivalent to that present in Amphi-
oxus, Amphibia, etc. The chief peculiarity about it is the
relatively late period at which it makes its appearance, and the
fact that its roof is formed both by the epiblast and by the

FIG. 17. LONGITUDINAL SECTION THROUGH THE BLASTODERM OF A PRISTIURUS

EMBRYO OF THE SAME AGE AS FIG. 28 B.
ep. epiblast ; er. embryonic rim ; m. mesoblast ; al. mesenteron.

lower layer cells. Owing to the large size of the segmentation
cavity the blastoderm forms a thin layer above the cavity and a
thickened ridge round its edge.

The epiblast in the next stage is inflected for a small arc at
the embryonic end of the blastoderm, where it becomes con-
tinuous with the lower layer cells ; at the same time some of the
lower layer cells of the embryonic end of the blastoderm assume

ELASMOBRANCHII.

43

a columnar form, and constitute the true hypoblast. The
portion of the blastoderm, where epiblast and hypoblast are
continuous, forms a projecting structure which will be called the
embryonic rim (fig. 16 B, er).

This rim is a very important structure, since it represents the
dorsal portion of the lip of the blastopore of Amphioxus. The
space between it and the yolk represents the commencing
mesenteron, of which the hypoblast on the under side of the lip
is the dorsal wall. The ventral wall of the mesenteron is at
first formed solely of yolk held together by a protoplasmic net-
work with numerous nuclei. The cavity under the lip becomes
rapidly larger (fig. 17, al}, owing to the continuous conversion of
lower layer cells into columnar hypoblast along an axial line
passing from the middle of the embryonic rim towards the
centre of the blastoderm. The continuous differentiation of the
hypoblast towards the centre of the blastoderm corresponds with
the invagination in Amphioxus. During the formation of the
embryonic rim the blastoderm grows considerably larger, but,
with the exception of the formation of the embryonic rim, retains
its primitive constitution.

The segmentation cavity undergoes however important
changes. There is formed below it a floor of lower layer cells,
derived partly from an ingrowth from the two sides, but mainly
from the formation of cells around the nuclei of the yolk (fig.
1 6). Shortly after the floor of cells has appeared, the whole
segmentation cavity becomes obliterated (fig. 17).

The disappearance of the segmentation cavity corresponds
in point of time with the formation of the hypoblast by the
pseudo-invagination above described ; and is probably due to
this pseudo-invagination, in the same way that the disappear-
ance of the segmentation cavity in Amphioxus is due to the true
invagination of the hypoblast.

When the embryonic rim first appears there are no external
indications of the embryo as distinguished from the blastoderm,
but when it has attained to some importance the position of the
embryo becomes marked out by the appearance of a shield-like
area extending inwards from the edge of the embryonic rim,
and formed of two folds with a groove between them (fig. 28 B,
mg), which is deepest at the edge of the blastoderm, and

44

FORMATION OF MESOBLAST.

shallows out as it extends inwards. This groove is the me-
dullary groove ; and its termination at the edge of the blasto-
derm is placed at the hind end of the embryo.

At about the time of its appearance the mesoblast becomes
first definitely established.

At the edge of the embryonic rim the epiblast and lower
layer cells are continuous. Immediately underneath the me-
dullary groove, as is best seen in transverse section (fig. 18), the
whole of the lower layer cells become converted into hypoblast,
and along this line the columnar hypoblast is in contact with
the epiblast above. At the sides however this is not the case ;
but at the junction of the epiblast and lower layer cells the
latter remain undifferentia-
ted. A short way from the
edge the lower layer cells
become divided into two dis-
tinct layers, a lower one con-
tinuous with the hypoblast
in the middle line, and an
upper one between this and
the epiblast (fig. 18 B). The
upper layer is the commence-
ment of the mesoblast (m).
The mesoblast thus arises
as two independent lateral

, . - FlG. l8. TWO TRANSVERSE SECTIONS OF

plates, one on each side 01 AN EMBRYO OF THE SAME AGE AS FIG. 17.

it.at

A. Anterior section.

B. Posterior section.

mg. medullary groove ; ep. epiblast ; hy.
hypoblast ; n.al. cells formed round the
nuclei of the yolk which have entered the
hypoblast ; 111. mesoblast.

The sections shew the origin of the
mesoblast.

the medullary groove, which
are continuous behind with
the undifferentiated lower
layer cells at the edge of the
embryonic rim. The meso-
blast plates are at first very
short, and do not extend to the front end of the embryo. They
soon however grow forwards as two lateral ridges, attached to
the hypoblast, one on each side of the medullary groove (fig. 18
A, ;#). These ridges become separate from the hypoblast, and
form two plates, thinner in front than behind ; but still continu-
ous at the edge of the blastoderm with the undifferentiated cells
of the lip of the blastopore, and laterally with the lower layer

ELASMOBRANCHII.

45

cells of the non-embryonic part of the blastoderm. It results
from the above mode of development of the mesoblast, that it
may be described as arising in the form of a pair of solid out-
growths of the wall of the alimentary tract ; which differ from the
mesoblastic outgrowths of the wall of the archenteron in Amphi-
oxus in not containing a prolongation of the alimentary cavity.

A general idea of the structure of the blastoderm at this
stage may be gathered from the diagram representing a longi-

FIG. 19. DIAGRAMMATIC LONGITUDINAL SECTIONS OF AN ELASMOBRANCH

EMBRYO.

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

ep. epiblast ; m. mesoblast ; al. alimentary cavity ; sg. segmentation cavity ; nc.
neural canal ; ch. notochord ; x. point where epiblast and hypoblast become continu-
ous at the posterior end of the embryo ; ». nuclei of yolk.

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

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

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

46

FORMATION OF MESOBLAST.

tudinal section through the embryo (fig. 19 B). In this figure
the epiblast is represented in white and is seen to be continuous
at the lip of the blastopore (x) with the shaded hypoblast.
Between the epiblast and hypoblast is seen one of the lateral
plates of mesoblast, represented by black cells with clear out-
lines. The non-embryonic lower layer cells of the blastoderm
are represented in the same manner as the mesoblast of the
body. The alimentary cavity is shewn at al, and below it is
seen the yolk with nuclei (;/). The segmentation cavity is re-
presented as still persisting, though by this stage it would have
disappeared.

FIG. 20. THREE SECTIONS THROUGH A PRISTIURUS EMBRYO SOMEWHAT YOUNGER

THAN FIG. 28 C.

A. Section through the cephalic plate.

B. Section through the posterior part of the cephalic plate.

C. Section through the trunk.

ch. notochord ; mg. medullary groove ; al. alimentary tract ; lp. lateral plate of
mesoblast ; //. body cavity.

As to the growth of the blastoderm it may be noted that it
has greatly extended itself over the yolk. Its edge in the
meantime forms a marked ridge, which is due not so much
to a thickening as to an arching of the epiblast. This ridge
is continuous with the embryonic rim, which gradually con-
centrates itself into two prominences, one on each side of the tail
of the embryo, mainly formed of masses of undifferentiated lower
layer cells. These prominences will be called the caudal
swellings.

ELASMOBRANCHII. 47

By this stage the three layers of the body, the epiblast,
mesoblast, and hypoblast, have become definitely established.
The further history of these layers may now be briefly traced.

Epiblast. While the greater part of the epiblast becomes
converted into the external epidermis, from which involutions
give rise to the olfactory and auditory pits, the lens of the eye,
the mouth cavity, and anus, the part of it lining the medullary
groove becomes converted into the central nervous system and
optic cup. The medullary groove is at first continued to the
front end of the medullary plate ; but the anterior part of this
plate soon enlarges, and the whole plate assumes a spatula form
(fig. 28 C, h, and fig. 20 A and B). The enlarged part becomes
converted into the brain, and may be called the cephalic plate.

The posterior part of the canal deepens much more rapidly
than the rest (fig. 20 C), and the medullary folds unite dorsally
and convert the posterior end of the medullary groove into a
closed canal, while the groove is still widely open elsewhere.
The medullary canal does not end blindly behind, but simply
forms a tube not closed at either extremity. The importance of
this fact will appear later.

Shortly after the medullary folds have met behind the whole
canal becomes closed in. This occurs in the usual way by the
junction and coalescence of the medullary folds. In the course
of the closing of the medullary groove the edges of the cephalic
plate, which have at first a ventral curvature, become bent up in
the normal manner, and enclose the dilated cephalic portion
of the medullary canal. The closing of the medullary canal
takes place earlier in the head and neck than in the back.

An anterior pore at the front end of the canal, like that in
Amphioxus and the Ascidians, is not found. The further differ-
entiation of the central nervous system is described in a special
chapter: it may however here be stated that the walls of the
medullary canal give rise not only to the central nervous system
but to the peripheral also.

Mesoblast. The mesoblast was left as two lateral plates
continuous behind with the undifferentiated cells of the caudal
swellings.

The cells composing them become arranged in two layers
(fig. 20 C, lp\ a splanchnic layer adjoining the hypoblast, and a

48

THE MESOBLAST.

pr

somatic layer adjoining the epiblast.
Between these two layers there is
soon developed in the region of the
head a well-marked cavity (fig. 20 A,
//) which is subsequently continued
into the region of the trunk, and
forms the primitive body cavity, equi-
valent to the cavity originating as an
outgrowth of the archenteron in Am-
phioxus. The body cavities of the
two sides are at first quite inde-
pendent.

Coincidentally with the appear-
ance of differentiation into somatic
and splanchnic layers the mesoblast
plates become in the region of the
trunk partially split by a series of
transverse lines of division into meso-
blastic somites. Only the dorsal
parts of the plates become split in
this way, their ventral parts remain-
ing quite intact. As a result of this
each plate becomes divided into a dorsal portion adjoining the
medullary canal, which is di-
vided into somites, and may
be called the vertebral plate,
and a ventral portion not so
divided, which may be called
the lateral plate. These two
parts are at this stage quite
continuous with each other ;
and the body cavity origi-
nally extends uninterrupted-
ly to the summit of the ver-
tebral plates (fig. 21).

The next change results
in the complete separation
of the vertebral portion of
the plate from the lateral

Sf,

FIG. ii. TRANSVERSE SEC-
TION THROUGH THE TRUNK OF
AN EMBRYO SLIGHTLY OLDER
THAN FIG. 28 E.

nc. neural canal ; pr. posterior
root of spinal nerve ; x. subnoto-
chordal rod ; ao. aorta ; sc. soma-
tic mesoblast ; sp. splanchnic me-
soblast ; 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.

FIG. 22. HORIZONTAL SECTION THROUGH
THE TRUNK OF AN EMBRYO OF SCYLLIUM
CONSIDERABLY YOUNGER THAN 28 F.

The section is taken at the level of the
notochord, and shews the separation of the
cells to form the vertebral bodies from the
muscle-plates.

ch. notochord ; ep. epiblast ; Vr. rudiment
of vertebral body ; mp. muscle-plate ; mp' .
portion of muscle-plate already differentiated
into longitudinal muscles.

ELASMOBRANCHII. 49

portion ; thereby the upper segmented part of the body cavity
becomes isolated, and separated from the lower and unseg-
mented part. As a consequence of this change the vertebral
plate comes to consist of a series of rectangular bodies, the
mesoblastic somites, each composed of two layers, a somatic
and a splanchnic, between which is the cavity originally continu-
ous with the body cavity (fig. 23, mp}. The splanchnic layer of
the plates buds off cells to form the rudiments of the vertebral
bodies which are at first segmented in the same planes as the
mesoblastic somites (fig. 22, Vr\ The plates themselves re-
main as the muscle-plates (mp}, and give rise to the whole of the
voluntary muscular system of the body. Between the vertebral
and lateral plates there is left a connecting isthmus, with a
narrow prolongation of the body cavity (fig. 23 B, st], which
gives rise (as described in a special chapter) to the segmental
tubes and to other parts of the excretory system.

In the meantime the lateral plates of the two sides unite
ventrally throughout the intestinal and cardiac regions of the
body, and the two primitively isolated cavities contained in
them coalesce. In the tail however the plates do not unite
ventrally till somewhat later, and their contained cavities remain
distinct till eventually obliterated.

At first the pericardial cavity is quite continuous with the
body cavity ; but it eventually becomes separated from the
body cavity by the attachment of the liver to the abdominal
wall, and by a horizontal septum in which run the two ductus
Cuvieri (fig. 23 A, sv}. Two perforations in this septum (fig. 23
A) leave the cavities in permanent communication.

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

From the somatic layer are formed

(1) A considerable part of the voluntary muscular

system of the body.

(2) The dermis.

(3) A large part of the inter-muscular connective tissue.

(4) Part of the peritoneal epithelium.
From the splanchnic layer are formed

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

B. III. 4

THE MESOBLAST.

(2) Part of the inter-muscular connective tissue.

(3) The axial skeleton and surrounding connective

tissue.

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

alimentary tract.

(5) Part of the peritoneal epithelium.

In the region of the head the mesoblast does not at first
become divided into somites ; but on the formation of the gill
A. B.

sp.c

Ch-

ill:

FIG. 23. SECTIONS THROUGH THE TRUNK OF A SCYLLIUM EMBRYO SLIGHTLY
YOUNGER THAN 28 F.

Figure A shews the separation of the body cavity from the pericardial cavity by
a horizontal septum in which runs the ductus Cuvieri ; on the left side is seen the
narrow passage which remains connecting the two cavities. Fig. B through a
posterior part of the trunk shews the origin of the segmental tubes and of the primi-
tive ova.

sp.c. spinal canal ; W. white matter of spinal cord ; pr. commissure connecting
the posterior nerve-roots ; ch. notochord ; x. sub-notochordal rod ; ao. aorta ; sv. sinus
venosus ; cav. cardinal vein ; ht. heart ; //. body cavity ; pc. pericardial cavity ; <xs.
solid oesophagus ; /. liver ; mp. muscle-plate ; mp'. inner layer of muscle-plate ; Vr.
rudiment of vertebral body ; st. segmental tube ; sd. segmental duct ; sp.v. spiral
valve ; v. subintestinal vein.

clefts a division takes place, which is apparently equivalent to
the segmentation of the mesoblast in the trunk. This division
causes the body cavity of the head to be divided up into a series

ELASMOBRANCHII. 51

of separate segments, one of which is shewn in fig. 24, pp. The
walls of the segments eventually give rise to the main muscles of
the branchial clefts, and probably also to the muscles of the
mandibular arch, of the eye, and of other parts. The cephalic
sections of the body cavity will be spoken of as head cavities.

In addition to the parts
already mentioned the meso-
blast gives rise to the whole
of the vascular system, and
to the generative system.
The heart is formed from

part of the splanchnic meso- FlQ ^ HORIZONTAL SECTION THROUGH
blast, and the generative THE LAST VISCERAL ARCH BUT ONE OF AN

r r il_ EMBRYO OF PRISTIURUS.

system from a portion of the

mesoblast of the dorsal nart e?' ePiblast5 vc- Pouch of hypoblast
sai pare which win form the walls of a visceral cleft .

of the body cavity. pp- segment of body-cavity in visceral arch ;

mt- i-» TT aa. aortic arch.

The hypoblast. Very

shortly after the formation of the mesoblastic plates as lateral
differentiations of the lower layer cells, an axial differentiation
of the hypoblast appears, which gives rise to the notochord very-
much in the same way as in Amphioxus.

At first the hypoblast along the axial line forms a single
layer in contact with the epiblast. Along this line a rod-like
thickening of the hypoblast very soon appears (fig. 25, B and C,
Ch'} at the head end of the embryo, and gradually extends back-
wards. This is the rudiment of the notochord ; it remains
attached for some time to the hypoblast, and becomes separated
from it first at the head end of the embryo (fig. 25 A, ch} : the
separation is then carried backwards.

A series of sections taken through an embryo shortly after the first
differentiation of the notochord presents the following characters.

In the hindermost sections the hypoblast retains a perfectly normal
structure and uniform thickness throughout. In the next few sections (fig.
25 C, Ch'} a slight thickening is to be observed in it, immediately below the
medullary groove. The layer, which elsewhere is composed of a single row
of cells, here becomes two cells deep, but no sign of a division into two
layers is exhibited.

In the next few sections the thickening of the hypoblast becomes much
more pronounced ; we have, in fact, a ridge projecting from the hypoblast
towards the epiblast (fig. 25 B, Ch'}. This ridge is pressed firmly against

4—2

THE HYPOBLAST.

A &

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

In sections in front of
this a cylindrical rod, which
can at once be recognized
as the notochord, and is
continuous with the ridge
just described, begins to be
split off from the hypoblast
(fig. 25 A, Ch). It is diffi-
cult to say at what point
the separation of this rod
from the hypoblast is com-
pleted, since all intermedi-
ate gradations between
complete separation and
complete attachment are to
be seen.

Shortly after the separ-
ation takes place, a fairly

FIG. 25. THREE SECTIONS OF A PRISTIURUS
EMBRYO SLIGHTLY OLDER THAN FIG. 28 B.

The sections shew the development of the noto-
chord.

Ch. notochord ; Ch' . developing notochord ; mg.
medullary groove ; Ip. lateral plate of mesoblast ;
ep. epiblast ; Ay. hypoblast.

thick bridge is found con-
necting the two lateral
halves of the hypoblast,
but this bridge is anterior-
ly excessively delicate and
thin, and in some cases is
barely visible except with
high powers. In some sections I have observed possible indications of the
process like that described by Calberla for Petronyzon, by which the lateral
parts of the hypoblast grow in underneath the axial part, and so isolate it
bodily as the notochord.

It is not absolutely clear whether the notochord is to be
regarded as an axial differentiation of the hypoblast, or as an
axial differentiation of the lower layer cells.

The facts of development both in Amphioxus and Elasmo-
branchii tend towards the former view ; but the nearly simul-
taneous differentiation of the notochord and the mesoblastic
plates lends some support to the supposition that the notochord
may be merely a median plate of mesoblast developed slightly
later than the two lateral plates.

The alimentary canal or mesenteron was left as a space
between the hypoblast and the yolk, ending blindly in front, but

ELASMOBRANCHII.

53

FIG. 26 SECTION THROUGH
THE ANTERIOR PART OF A PRIS-
TIURUS EMBRYO TO SHEW THE
FORMATION OF THE ALIMENTARY

TRACT.

Ch. notochord ; hy. hypoblast ;
al, alimentary tract ; na. cells
passing in from the yolk to form
the ventral wall of the alimentary
tract.

opening behind by a widish aperture, the blastopore or anus of
Rusconi (vide fig. 19 B).

The conversion of this irregular cavity into a closed canal
commences first of all at the anterior
extremity. In this conversion two
distinct processes are concerned.
One of these is a process of folding
off of the embryo from the blasto-
derm. The other is a simple growth
of cells independent of any fold.
To the first of these processes the
depth and narrowness of the ali-
mentary cavity is due ; the second
is concerned in forming its ventral
wall. The process of the folding off
of the embryo from the blastoderm
resembles exactly the similar process
in the embryo bird. The fold is a perfectly continuous one round
the front end of the embryo, but may be
conveniently spoken of as composed of
a head-fold and two lateral folds.

Of far greater interest than the
nature of these folds is the formation of
the ventral wall of the alimentary canal.
This originates in a growth of cells from
the two sides to the middle line (fig. 26).
The cells for it are not however mainly
derived from pre-existing hypoblast
cells, but are formed de novo around the
nuclei of the yolk which have already
been spoken of (fig. 26, no). The ventral
wall of the mesenteron is in fact, to a
large extent at any rate, formed as a dif-
ferentiation of the primitive yolk floor.

The folding off and closing of the alimentary canal in the
anterior part of the body proceeds rapidly, and not only is a
considerable tract of the alimentary canal formed, but a great
part of the head is completely folded off from the yolk before
the medullary groove is closed.

FIG. 27. LONGITUDINAL
VERTICAL SECTION OF AN
EMBRYO SLIGHTLY YOUNGER
THAN THAT IN FIG. 26 D.

The section shews the
communication which exists
between the neural and ali-
mentary canals.

nc. neural canal ; al. ali-
mentary tract ; Ch. noto-
chord ; Ts. tail swelling.

54

THE HYPOBLAST.

The posterior part of the alimentary canal retains for a
longer time its primitive condition. Finally however it also
becomes closed in, by the lips of the blastopore at the hind end
of the embryo meeting and uniting. The peculiarity of the
closing in of the posterior part of the alimentary canal consists
in the fact that a similar continuity to that in Amphioxus
obtains between the neural and alimentary canals. This is due
to the medullary folds being continuous at the end of the tail
with the lips of the blastopore, which close in the hind end of
the alimentary canal ; so that, when the
medullary folds unite to form a canal, this
canal becomes continuous with the ali-
mentary canal, which is closed in at the
same time. In other words, the medullary
folds assist in enveloping the blastopore
which does not therefore become absolutely
closed, but opens into the floor of the
neural canal. It will afterwards be shewn
that it is only the posterior part of the
blastopore that becomes closed during the
above process, and that the anterior and
ventral part long remains open. The general
arrangement of the parts, at the time when
the hind end of the mesenteron is first
closed, is shewn in fig. 27. The same
points may be seen in the diagrammatic
longitudinal section fig. 19 C.

The middle portion of the alimentary
tract is the last to be closed in since it
remains till late in embryonic life as the
umbilical or vitelline canal, connecting the
yolk-sack with the alimentary cavity. The
umbilical canal falls into the alimentary
tract immediately behind the entrance of
the hepatic duct.

At a fairly early stage of development a rod is constricted off
from the dorsal wall of the alimentary canal (figs. 27* and 23 x],
which is known as the subnotochordal rod. It is placed immedi-
ately below the notochord, and disappears during embryonic life.

FIG. 27*. TRANSVERSE
SECTION THROUGH THE
TAIL REGION OF A PRIS-
TIURUS EMBRYO OF THE
SAME AGE AS FIG. 28 E.

df. dorsal fin ; s/>.c.
spinal cord ; pp. body
cavity ; sf. splanchnic
layer of mesoblast ; so.
somatic layer of meso-
blast ; mp. commencing
differentiation of mus-
cles; ch. notochord; x.
sub-notochordal rod aris-
ing as an outgrowth of
the dorsal wall of the
alimentary tract ; al. ali-
mentary tract.

ELASMOBRANCHII. 55

General features of tlie Elasnwbranch embryo at successive stages.

Shortly after the three germinal layers become definitely
established, the rudiment of the embryo, as visible from the
surface, consists of an oblong plate, which extends inwards from
the periphery of the blastoderm, and is bounded on its inner
side by a head-fold and two lateral folds (fig. 28 B). This plate
is the medullary plate ; along its axial line is a shallow groove
— the medullary groove (ing). The rudiment of the embryo
rapidly increases in length, and takes a spatula-like form
(fig. 28 C). The front part of it, turned away from the edge of
the blastoderm, soon becomes dilated into a broad plate, — the
cephalic plate (//) — while the tail end at the edge of the blasto-
derm is also enlarged, being formed of a pair of swellings — the
tail swellings (ts) — derived from the lateral parts of the original
embryonic rim. By this stage a certain number of mesoblastic
somites have become formed but are not shewn in my figure.
They are the foremost somites of the trunk, and those behind
them continue to be added, like the segments in Chaetopods.
between the last formed somite and the end of the body. The
increase in length of the body mainly takes place by growth in
the region between the last mesoblastic somite and the end of
the tail. The anterior part of the body is now completely folded
off from the blastoderm, and the medullary groove of the earlier
stage has become converted into a closed canal.

By the next stage (fig. 28 D) the embryo has become so
much folded off from the yolk both in front and behind that
the separate parts of it begin to be easily recognizable.

The embryo is attached to the yolk by a distinct stalk or
cord, which in the succeeding stages gradually narrows and
elongates, and is known as the umbilical cord (so. s.). The
medullary canal has now become completely closed. The anterior
region constitutes the brain ; and in this part slight constrictions,
not perceptible in views of the embryo as a transparent object,
mark off three vesicles. These vesicles are known as the fore,
mid, and hind brain. From the fore-brain there is an outgrowth
on each side, the first rudiment of the optic vesicles {op). The
tail swellings are still conspicuous.

GENERAL GROWTH OF THE EMBRYO.

The tissues of the body have now become fairly transparent,
and there may be seen at the sides of the body seventeen
mesoblastic somites. The notochord, which was formed long

—jug

FIG. 28. VIEWS OF ELASMOBRANCH EMBRYOS.
A — F. PRISTIURUS. G. and H. SCYLLIUM.

A. A blastoderm before the formation of the medullary plate, sc. segmentation
cavity ; cs. embryonic swelling.

B. A somewhat older blastoderm in which the medullary groove has been es-
tablished, mg. medullary groove.

C. An embryo from the dorsal surface, as an opaque object, after the medullary
groove has become posteriorly converted into a tube. mg. medullary groove : the
reference line points very nearly to the junction between the open medullary groove
with the medullary tube ; h. cephalic plate ; ts. tail swelling.

D. Side view of a somewhat older embryo as a transparent object, ch. notochord ;
op. optic vesicle ; I.v.c. ist visceral cleft; al. alimentary tract ; so.s. stalk connecting
the yolk-sack with the embryo.

E. Side view of an older embryo as a transparent object, mp. muscle-plates ;
au.v. auditory vesicle ; vc. visceral cleft ; lit. heart ; in. mouth invagination ; an. anal
diverticulum ; al.v. posterior vesicle of post-anal gut.

F. G. II. Older embryos as opaque objects.

ELASMOBRANCHII.

57

before the stage represented in figure 28 D, is now also distinctly
visible. It extends from almost the extreme posterior to the
anterior end of the embryo, and lies between the ventral wall of
the spinal canal and the dorsal wall of the intestine. Round its
posterior end the neural and alimentary tracts become continu-
ous with each other. Anteriorly the termination of the
notochord cannot be seen, it can only be traced into a mass of
mesoblast at the base of the brain, which there separates the
epiblast from the hypoblast. The alimentary canal (al) is
completely closed anteriorly and posteriorly, though still widely
open to the yolk-sack in the middle part of its course. In the
region of the head it exhibits on each side a slight bulging out-
wards, the rudiment of the first visceral cleft. This is
represented in the figure by two lines (l. v.c.}.

The embryo represented in fig. 28 E is far larger than the
one just described, but it has not been convenient to represent
this increase of size in the figure. Accompanying this increase
in size, the folding off from the yolk has considerably pro-
gressed, and the stalk which unites the embryo with the yolk is
proportionately narrower and longer than before.

The brain is now very distinctly divided into the three lobes,
the rudiments of which appeared during the last stage. From
the foremost of these the optic vesicles now present themselves
as well-marked lateral outgrowths, towards which there has
appeared an involution from the external skin (op) to form the
lens.

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

The mesoblastic somites have greatly increased in number
by the formation of fresh somites in the tail. Thirty-eight of
them were present in the embryo figured. The mesoblast at
the base of the brain is more bulky, and there is still a mass of
unsegmented mesoblast which forms the tail swellings. The
first rudiment of the heart (Jit) becomes visible during this stage
as a cavity between the mesoblast of the splanchnopleure and
the hypoblast.

CKNKKAI, CROWTII OF THE EMBRYO.

The fore and hind guts are now longer than they were. An
invagination from the exterior to form the mouth has appeared
(m) on the ventral side of the head close to the base of the
thalamencephalon. The upper end of this eventually becomes
constricted off as the pituitary body, and an indication of the
future position of the anus is afforded by a slight diverticulum
of the hind gut towards the exterior, some little distance from
the posterior end of the embryo (an}. The portion of the
alimentary canal behind this point, though at this stage large,
and even dilated into a vesicle at its posterior end (al.v), becomes
eventually completely
atrophied. It is known
as the post-anal gut.
In the region of the
throat the rudiment of
a second visceral cleft
has appeared behind
the first ; neither of
them is as yet open to
the exterior.

In a somewhat older
embryo the first spon-
taneous movements
take place, and consist
in somewhat rapid ex-
cursions of the embryo
from side to side, pro-
duced by a serpentine
motion of the body.

A ventral flexure
of the prae-oral part of
the head, known as the
cranial flexure, which commenced in earlier stages (fig. 28 D
and E), has now become very evident, and the mid-brain1 begins
to project in the same manner as in the embryo fowl on the

1 The part of the brain which I have here called mid-brain, and which unquestion-
ably corresponds to the part called mid-brain in the embryos of higher vertebrates,
becomes in the adult what Miklucho-Maclay and Gegenbaur called the vesicle of the
third ventricle or thalamencephalon.

cl. ul

FIG. 28*. FOUR SECTIONS THROUGH THK
POST-ANAL PART OF THE TAIL OF AN EMBRYO OF
THE SAME AGE AS FIG. 28 F.

A is the posterior section.

nc. neural canal ; al. post-anal gut ; alv. caudal
vesicle of post-anal gut ; x. sub-notochord rod ; inp.
muscle-plate; th. notochord; cl.al. cloaca; ao.
aorta ; v.cati. caudal vein.

ELASMOBRANCHII. 59

third day, and will soon form the anterior termination of the
long axis of the embryo. The fore-brain has increased in size
and distinctness, and the anterior part of it may now be looked
on as the unpaired rudiment of the cerebral hemispheres.

Further changes have taken place in the organs of sense,
especially in the eye, in which the involution for the lens has
made considerable progress. The number of the muscle-plates
has again increased, but there is still a region of unsegmented
mesoblast in the tail. The thickened portions of mesoblast,
which caused the tail swellings, are still to be seen, and would
seem to act as the reserve from which is drawn the matter for
the rapid growth of the tail, which occurs soon after this. The
mass of the mesoblast at the base of the brain has again
increased. No fresh features of interest are to be seen in the
notochord. The heart is very much more conspicuous than
before, and its commencing flexure is very apparent. It now
beats actively. The post-anal gut is much longer than during
the last stage ; and the point where the anus will appear is very
easily detected by a bulging out of the gut towards the external
skin. The alimentary vesicle at the end of the post-anal gut,
first observable during the last stage, is now a more conspicuous
organ. There are three visceral clefts, none of which are as yet
open to the exterior.

Figure 28 F represents a considerably older embryo viewed
as an opaque object, and fig. 29 A is a view of the head as a
transparent object. The stalk connecting it with the yolk is
now, comparatively speaking, quite narrow, and is of sufficient
length to permit the embryo to execute considerable move-
ments.

The tail has grown immensely, but is still dilated terminally.
The terminal dilatation is mainly due to the alimentary vesicle
(fig. 28* alv), but the post-anal section of the alimentary tract in
front of this is now a solid cord of cells. Both the alimentary
vesicle and this cord very soon disappear. Their relations are
shewn in section in fig. 28*.

The two pairs of limbs have appeared as differentiations of a
continuous but not very conspicuous epiblastic thickening, which
is probably the rudiment of a lateral fin. The anterior pair is
situated just at the front end of the umbilical stalk ; and the

6o

GENERAL GROWTH OK THE EMBRYO.

posterior pair, which is the later developed and less conspicuous
of the two, is situated
some little distance be-
hind the stalk.

The cranial flexure
has greatly increased,
and the angle between
the long axis of the
front part of the head

and of the body is less 0(

_, mb Jv^gi^.

than a right angle. The \^f* B^. iv.v

conspicuous mid-brain
(29 A, mb) forms the
anterior termination of
the long axis of the
body. The thin roof
of the fourth ventricle
(lib] may be noticed in
the figure behind the
mid-brain. The audi-
tory sack (au.V) is
nearly closed, and its
opening is not shewn
in the figure. In the
eye (op) the lens is
completely formed.
The olfactory pit (ol)
is seen a little in front
of the eye.

Owing to the opa-
city of the embryo, the
muscle-plates are only indistinctly indicated in fig. 28 F, and.no
other features of the mesoblast are to be seen.

The mouth is now a deep pit, the hind borders of which are
almost completely formed by a thickening in front of the first
branchial or visceral cleft, which may be called the first branch-
ial arch or mandibular arch.

Four branchial clefts are now visible, all of which are open
to the exterior, but in the embryo, viewed as a transparent

FlG. 29. VIEWS OF THE HEAD OF ELASMO-
HRANCH EMBRYOS AT TWO STAGES AS TRANS-
PARENT OBJECTS.

A. Frist iurus embryo of the same stage as fig.
28 F.

B. Somewhat older Scyllium embryo.

///. third nerve ; V. fifth nerve ; VII. seventh
nerve ; au.n. auditory nerve ; gl. glossopharyngeal
nerve ; Vg. vagus nerve ; ft. fore-brain ; pn. pineal
gland ; nib. mid-brain ; hb. hind-brain ; iv.v. fourth
ventricle ; cb. cerebellum ; ol. olfactory pit ; op.
eye ; au. V. auditory vesicle ; m. mesoblast at base
of brain ; ch. notochord ; ht. heart ; Vc. visceral
clefts ; eg. external gills ; //. sections of body cavity
in the head.

ELASMOBRANCHII.

61

object, two more, not open to the exterior, are visible behind the
last of these.

Between each of these and behind the last one there is
a thickening of the mesoblast which gives rise to a branchial
arch. The arch between the first and second cleft is known as
the hyoid arch.

Fig. 29 B is a representation of the head of a slightly older
embryo in which papillae may be seen in the front wall of the
second, third, and fourth branchial clefts : these papillae are the
commencements of filiform processes which grow out from the
gill-clefts and form external gills. The peculiar ventral curva-
ture of the anterior end of the notochord (cJi) both in this and in
the preceding figure deserves notice.

A peculiar feature in the anatomy makes its appearance at this period,
viz. the replacement of the original hollow oesophagus by a solid cord of
cells (fig. 23 A, ces) in which a lumen does not reappear till very much later.
I have found that in some Teleostei (the Salmon) long after they are
hatched a similar solidity in the oesophagus is present. It appears not
impossible that this feature in the oesophagus may be connected with the
fact that in the ancestors of the present types the oesophagus was perforated
by gill slits ; and that in the process of embryonic abbreviation the stage
with the perforated oesophagus became replaced by a stage with a cord of
indifferent cells (the oesophagus being in the embryo quite functionless) out
of which the non-perforated oesophagus was directly formed. In the higher
types the process of development appears to have become quite direct.

By this stage all the parts of the embryo have become
established, and in the succeeding stages the features character-
istic of the genus and species are gradually acquired.

Two embryos of Scyllium are represented in fig. 28 G
and H, the head and anterior part of the trunk being repre-
sented in fig. G, and the whole embryo at a much later stage in
fig. H.

In both of these, and especially in the second, an apparent
diminution of the cranial flexure is very marked. This diminu-
tion is due to the increase in the size of the cerebral hemispheres,
which grow upwards and forwards, and press the original fore-
brain against the mid-brain behind.

In fig. G the rudiments of the nasal sacks are clearly visible
as small open pits.

62 FORMATION OF THE YOLK-SACK.

The first cleft is no longer similar to the rest, but by the
closure of the lower part has commenced to be metamorphosed
into the spiracle.

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

In fig. H are seen the long filiform external gills which now
project out from all the visceral clefts, including the spiracle.
They are attached to the front wall of the spiracle, to both walls
of the next four clefts, and to the front wall of the last cleft.
They have very possibly become specially developed to facilitate
respiration within the egg ; and they disappear before the close
of larval life.

When the young of Scyllium and other Sharks are hatched
they have all the external characters of the adult. In Raja and
Torpedo the early stages, up to the acquirement of a shark-like
form, are similar to those in the Selachoidei, but during the
later embryonic stages the body gradually flattens out, and
assumes the adult form, which is thus clearly shewn to be a
secondary acquirement.

An embryonic gill cleft behind the last present in the adult
is found (Wyman, No. 54) in the embryo of Raja batis.

The unpaired fins are developed in Elasmobranchs as a fold
of skin on the dorsal side, which is continued round the end of
the tail along the ventral side to the anus. Local developments
of this give rise to the dorsal and anal fins. The caudal fin is at
first symmetrical, but a special lower lobe grows out and gives
to it a heterocercal character.

Enclosure of the yolk-sack and its relation to the embryo.

The blastoderm at the stage represented in fig. 28 A and B
forms a small and nearly circular patch on the surface of the
yolk, composed of epiblast and lower layer cells. While the
body of the embryo is gradually being moulded this patch
grows till it envelopes the yolk ; the growth is not uniform, but

ELASMOBRANCHTI.

is less rapid in the immediate neighbourhood of the embryonic
part of the blastoderm
than elsewhere. As a
consequence of this, that
part of the edge, to
which the embryo is at-
tached, forms a bay in
the otherwise regular
outline of the edge of
the blastoderm, and by
the time that about two-
thirds of the yolk is en-
closed this bay is very
conspicuous. It is shewn
in fig. 30 A, where bl
points to the blastoderm,
and yk to the part of the
yolk not yet covered by
the blastoderm. The em-
bryo at this time is only
connected with the yolk-
sack by a narrow umbili-
cal cord ; but, as shewn
in the figure, is still at-
tached to the edge of the

blastoderm.

Shortly subsequent to
this the bay in the blas-
toderm, at the head of
which the embryo is at-
tached, becomes oblitera-
ted by its two sides com-
ing together and coales-
cing. The embryo then
ceases to be attached at
the edge of the blasto-
derm. But a linear streak

FIG. 30. THREE VIEWS OF THE VITELLUS
OF AN ELASMOBRANCH, SHEWING THE EMBRYO,
THE BLASTODERM, AND THE VESSELS OF THE
YOLK-SACK.

The shaded part (bl) is the blastoderm; the
white part the uncovered yolk.

A. Young stage with the embryo still at-
tached at the edge of the blastoderm.

B. Older stage with the yolk not quite en-
closed by the blastoderm.

C. Stage after the complete enclosure of the
yolk.

yk. yolk ; bl. blastoderm ; v. venous trunks
of yolk-sack; a. arterial trunks of yolk-sack;
y. point of closure of the yolk blastopore ; x. por-
tion of the blastoderm outside the arterial sinus
terminalis.

formed by the coalesced

edges of the blastoderm is left connecting the embryo with the

64 FORMATION OF THE YOLK -SACK.

edge of the blastoderm. This streak is probably analogous to
(though not genetically related with) the primitive streak in the
Amniota.

This stage is represented in fig. 30 B. In this figure there is
only a small patch of yolk (yk] not yet enclosed, which is
situated at some little distance behind the embryo. Through-
out all this period the edge of the blastoderm has remained
thickened : a feature which persists till the complete investment
of the yolk, which takes place shortly after the stage last
described. In this thickened edge a circular vein arises which
brings back the blood from the yolk-sack to the embryo. The
opening in the blastoderm, exposing the portion of the yolk not
yet covered, may be conveniently called the yolk blastopore.
It is interesting to notice that, owing to the large size of the
yolk in Elasmobranchs, the posterior part of the primitive
blastopore becomes encircled by the medullary folds and tail-
swellings, and is so closed long before the anterior and more
ventral part, which is represented by the uncovered portion of
the yolk. It is also worth remarking that, owing to the embryo
becoming removed from the edge of the blastoderm, the final
closure of the yolk blastopore takes place at some little distance
from the embryo.

The blastoderm enclosing the yolk is formed of an external
layer of epiblast, a layer of mesoblast below in which the blood-
vessels are developed, and within this a layer of hypoblast,
which is especially well marked and ciliated (Leydig, No. 46) in
the umbilical stalk, where it lines the canal leading from the
yolk-sack to the intestine. In the region of the yolk-sack
proper the blastoderm is so thin that it is not easy to be quite
sure that a layer of hypoblast is throughout distinct. Both the
hypoblast and mesoblast of the yolk-sack are formed by a
differentiation of the primitive lower layer cells.

Nutriment from the yolk-sack is brought to the embryo
partly through the umbilical canal and so into the intestine, and
partly by means of blood-vessels in the mesoblast of the sack.
The blood-vessels arise before the blastoderm has completely
covered the yolk.

Fig. 30 A represents the earliest stage of the circulation of
the yolk-sack. At this stage there is visible a single arterial

ELASMOBRANCHII. 65

trunk (a) passing forwards from the embryo and dividing into
two branches. No venous trunk could be detected with the
simple microscope, but probably venous channels were present
in the thickened edge of the blastoderm.

In fig. 30 B the circulation is greatly advanced. The blasto-
derm has now nearly completely enveloped the yolk, and there
remains only a small circular space (yk] not enclosed by it.
The arterial trunk is present as before, and divides in front of
the embryo into two branches which turn backwards and form a
nearly complete ring round the embryo. In general appearance
this ring resembles the sinus terminalis of the area vasculosa
of the Bird, but in reality bears quite a different relation to the
circulation. It gives off branches on its inner side only.

A venous system of returning vessels is now fully developed,
and its relations are very remarkable. There is a main venous
ring in the thickened edge of the blastoderm, which is con-
nected with the embryo by a single stem running along the
seam where the edges of the blastoderm have coalesced. Since
the venous trunks are only developed behind the embryo, it
is only the posterior part of the arterial ring that gives off
branches.

The succeeding stage (fig. 30 C) is also one of considerable
interest. The arterial ring has greatly extended, and now
embraces nearly half the yolk, and sends off trunks on its inner
side along its whole circumference. More important changes
have taken place in the venous system. The blastoderm has
now completely enveloped the yolk, and the venous ring is
therefore reduced to a point. The small veins which originally
started from it may be observed diverging in a brush-like fashion
from the termination of the unpaired trunk, which originally
connected the venous ring with the heart.

At a still later stage the arterial ring embraces the whole
yolk, and, as a result of this, vanishes in its turn, as did the
venous ring before it. There is then present a single arterial
and a single venous trunk. The arterial trunk is a branch of
the dorsal aorta, and the venous trunk originally falls into the
heart together with the subintestinal or splanchnic vein. On
the formation of the liver the proximal end of the subintestinal
vein becomes the portal vein, and it is joined just as it enters
B. in. 5

66 BIBLIOGRAPHY.

the liver by the venous trunk from the yolk-sack. The venous
trunk leaves the body on the right side, and the arterial on the
left.

The yolk-sack persists during the whole of embryonic life,
and in the majority of Elasmobranch embryos there arises
within the body walls an outgrowth from the umbilical canal
into which a large ampunt of the yolk passes. This outgrowth
forms an internal yolk-sack. In Mustelus vulgaris the internal
yolk-sack is very small, and in Mustelus laevis it is absent.
The latter species, which is one of those in which development
takes place within the uterus, presents a remarkable peculiarity
in that the vascular surface of the yolk-sack becomes raised into
a number of folds, which fit into corresponding depressions in
the vascular walls of the uterus. The yolk-sack becomes in this
way firmly attached to the walls of the uterus, and the two
together constitute a kind of placenta. A similar placenta is
found in Carcharias.

After the embryo is hatched or born, as the case may be, the
yolk-sack becomes rapidly absorbed.

BIBLIOGRAPHY. •

(40) F. M. B a 1 f o u r. "A preliminary account of the development of the Elasmo-
branch Fishes." Quart. J. of Micr. Science, Vol. xrv. 1876.

(41) F. M. Balfour. "A Monograph on the development of Elasmobranch
Fishes." London, 1878. Reprinted from the Journal of Anat. and Physiol. for 1876,
1877, and 1878.

(42) Z. Gerbe. " Recherches sur la segmentation de la cicatrule et la formation
tits produits adventifs de Fceuf des Plagiostomes et particular em ent des Rates." Vide
also Journal de FAnatomie et de la Physiologie, 1872.

(43) W. His. " Ueb. d. Bildung v. Haifischenembryonen." Zeit.ftir Anat. u.
Entwick., Vol. n. 1877.

(44) A. Kowalevsky. "Development of Acanthias vulgaris and Mustelus
Isevis." (Russian.) Transactions of the Kieiv Society of Naturalists, Vol. I. 1870.

(45) R. Leuckart. " Ueber die allmahlige Bildung d. Korpergestalt bei d.
Kochen." Zeit. f. wiss. Zoo!., Bd. II., p. 258.

(46) Fr. Leydig. Rochen u. Haie. Leipzig, 1852.

(47) A. W. Malm. " Bidrag till kannedom om utvecklingen af Rajae." Kongl.
vctenskaps akademiens forhandlingar. Stockholm, 1876.

(48) Joh. M tiller. Clatter Haie des Aristoteles und iiber die Verschicdcnheitcn
unlcr den Haifischen und Rochen in der Entwicklung des Eies. Berlin, 1 840.

(49) S. L. Schenk. " Die Eier von Raja quadrimaculata innerhalb tier Eileiter."
Sitz. der k. Akad. IVien, Vol. LXXIII. 1873.

BIBLIOGRAPHY. 67

(50) Alex. Schultz. " Zur Entwicklungsgeschichte des Selachiereies. " Archiv
fiir micro. Anat., Vol. XI. 1875.

(51) Alex. Schultz. " Beitrag zur Entwicklungsgeschichte d. Knorpelfische."
Archiv fiir micro. Anat., Vol. xm. 1877.

(52) C. Semper. "Die Stammesverwandschaft d. Wirbelthiere u. Wirbello-
sen." Arbeit, a. d. zool.-zoot. Instit. Wurzburg, Vol. II. 1875.

(53) C. Semper. " Das Urogenitalsystem d. Plagiostomen, etc." Arbeit, a. d.
zool.-zoot. Instit. Wurzburg, Vol. II. 1875.

(54) Wyman. " Observations on the Development of Raja batis." Memoirs of
the American Academy of Arts and Sciences, Vol. IX. 1864.

5—2

CHAPTER IV.

TELEOSTEI.

THE majority of the Teleostei deposit their eggs before
impregnation, but some forms are viviparous, e.g. Blennius
viviparus. Not a few carry their eggs about ; but this operation
is with a few exceptions performed by the male. In Syngna-
thus the eggs are carried in a brood-pouch of the male situated
behind the anus. Amongst the Siluroids the male sometimes
carries the eggs in the throat above the gill clefts. Ostegenio-
sus militaris, Arius falcarius, and Arius fissus have this peculiar
habit.

The ovum when laid is usually invested in the zona radiata
only, though a vitelline membrane is sometimes present in
addition, e.g. in the Herring. It is in most cases formed of a
central yolk mass, which may either be composed of a single
large vitelline sphere, or of distinct yolk spherules. The yolk
mass is usually invested by a granular protoplasmic layer,
which is especially thickened at one pole to form the germinal
disc.

In the Herring's ovum the germinal disc is formed, as in
many Crustacea, at impregnation; the protoplasm which was
previously diffused through the egg becoming aggregated at the
germinal pole and round the periphery.

Impregnation is external, and on its occurrence a contraction
of the vitellus takes place, so that a space is formed between
the vitellus and the zona radiata, which becomes filled with
fluid.

The peculiarities in the development of the Teleostean ovum
can best be understood by regarding it as an Elasmobranch

TELEOSTEI. 69

ovum very much reduced in size. It seems in fact very probable
that the Teleostei are in reality derived from a type of Fish
with a much larger ovum. The occurrence of a meroblastic
segmentation, in spite of the ovum being usually smaller than
that of Amphibia and Acipenser, etc., in which the segment-
ation is complete, as well as the solid origin of many of the
organs, receives its most plausible explanation on this hypo-
thesis.

The proportion of the germinal disc to the whole ovum
varies considerably. In very small eggs, such as those of the
Herring, the disc may form as much as a fifth of the whole.

The segmentation, which is preceded by active movements
of the germinal disc, is meroblastic. There is nothing very
special to note with reference to its general features, but while in
large ova like those of the Salmon the first furrows only
penetrate for a certain depth through the germinal disc, in
small ova like those of the Herring they extend through the
whole thickness of the disc. During the segmentation a great
increase in the bulk of the blastoderm takes place.

In hardened specimens a small cavity amongst the segment-
ation spheres may be present at any early stage ; but it is
probably an artificial product, and in any case has nothing to do
with the true segmentation cavity, which does not appear till
near the close of segmentation. The peripheral layer of granu-
lar matter, continuous with the germinal disc, does not undergo
division, but it becomes during the segmentation specially
thickened and then spreads itself under the edge of the blasto-
derm ; and, while remaining thicker in this region, gradually
grows inwards so as to form a continuous sub-blastodermic
layer. In this layer nuclei appear, which are equivalent to those
in the Elasmobranch ovum. A considerable number of these
nuclei often become visible simultaneously (van Beneden, No. 60)
and they are usually believed to arise spontaneously, though this
is still doubtful1. Around these nuclei portions of protoplasm
are segmented off, and cells are thus formed, which enter the
blastoderm, and have nearly the same destination as the homo-
logous cells of the Elasmobranch ovum.

1 Fide Vol. II. p. 108.

70 SEGMENTATION.

During the later stages of segmentation one end of the
blastoderm becomes thickened and forms the embryonic swell-
ing ; and a cavity appears between the blastoderm and the yolk
which is excentrically situated near the non-embryonic part of
the blastoderm. This cavity is the true segmentation cavity.
Both the cavity and the embryonic swelling are seen in section
in fig. 31 A and B.

In Leuciscus rutilus Bambeke describes a cavity as appearing in the
middle of the blastoderm during the later stages of segmentation. From his
figures it might be supposed that this cavity was equivalent to the segment-
ation cavity of Elasmobranchs in its earliest condition, but Bambeke states
that it disappears and that it has no connection with the true segmentation
cavity. Bambeke and other investigators have failed to recognize the
homology of the segmentation cavity in Teleostei with that in Elasmo-
branchii, Amphibia, etc.

With the appearance of the segmentation cavity the portion
of the blastoderm which forms its roof becomes thinned out, so
that the whole blastoderm consists of (i) a thickened edge
especially prominent at one point where it forms the embryonic
swelling, and (2) a thinner central portion. The changes which
now take place result in the differentiation of the embryonic
layers, and in the rapid extension of the blastoderm round the
yolk, accompanied by a diminution in its thickness.

A

FIG. 31. LONGITUDINAL SECTIONS THROUGH THE BLASTODERM OF THE

TROUT AT AN EARLY STAGE OF DEVELOPMENT.

A. at the close of the segmentation; B. after the differentiation of the germinal layers.
ep' . epidermic layer of the epiblast; sc, segmentation cavity.

The first differentiation of the layers consists in a single row
of cells on the surface of the blastoderm becoming distinctly

TELEOSTEI. 71

marked off as a special layer (fig. 3 1 A) ; which however does
not constitute the whole epiblast but only a small part of it,
which will be spoken of as the epidermic layer. The
complete differentiation of the epiblast is effected by the cells of
the thickened edge of the blastoderm becoming divided into two
strata (fig. 31 B). The upper stratum constitutes the epiblast.
It is divided into two layers, viz., the external epidermic layer
already mentioned, and an internal layer known as the nervous
layer, formed of several rows of vertically arranged cells.
According to the unanimous testimony of investigators the roof
of the segmentation cavity is formed of epiblast cells only. The
lower stratum in the thickened rim of the blastoderm is several
rows of cells deep, and corresponds with the lower layer cells or
primitive hypoblast in Elasmobranchii. It is continuous at the
edge of the blastoderm with the nervous layer of the epiblast.

In smaller Teleostean eggs there is formed, before the blasto-
derm becomes differentiated into epiblast and lower layer cells,
a complete stratum of cells around the nuclei in the granular
layer underneath the blastoderm. This layer is the hypoblast ;
and in these forms the lower layer cells of the blastoderm are
stated to become converted into mesoblast only. In the larger
Teleostean eggs, such as those of the Salmonidae, the hypoblast,
as in Elasmobranchs, appears to be only partially formed from
the nuclei of the granular layer. In these forms however, as in
the smaller Teleostean ova and in Elasmobranchii, the cells
derived from the granular stratum give rise to a more or less
complete cellular floor for the segmentation cavity. The
segmentation cavity thus becomes enclosed between an hypo-
blastic floor and an epiblastic roof several cells deep. It
becomes obliterated shortly after the appearance of the medul-
lary plate.

At about the time when the three layers become established
the embryonic swelling takes a somewhat shield-like form
(fig- 33 A). Posteriorly it terminates in a caudal prominence
(ts) homologous with the pair of caudal swellings in Elasmo-
branchs. The homologue of the medullary groove very soon
appears as a shallow groove along the axial line of the shield.
After these changes there takes place in the embryonic layers a
series of differentiations leading to the establishment of the

72 FORMATION OF THE LAYERS.

definite organs. These changes are much more difficult to
follow in the Teleostei than in the Elasmobranchii, owing partly
to the similarity of the cells of the various layers, and partly to
the primitive solidity of all the organs.

The first changes in the epiblast give rise to the central
nervous system. The epiblast, consisting of the nervous and
epidermic strata already indicated, becomes thickened along the
axis of the embryo and forms a keel projecting towards the yolk
below : so great is the size of this keel in the front part of the
embryo that it influences the form of the whole body and causes
the outline of the surface adjoining the yolk to form a strong
ridge moulded on the keel of the epiblast (fig. 32 A and B).
Along the dorsal line of the epiblast keel is placed the shallow
medullary groove ; and according to Calberla (No. 61) the keel
is formed by the folding together of the two sides of the
primitively uniform epiblastic layer. The keel becomes gradu-
ally constricted off from the external epiblast and then forms a
solid cord below it. Subsequently there appears in this cord a
median slit-like canal, which forms the permanent central canal
of the cerebrospinal cord- The peculiarity in the formation of
the central nervous system of Teleostei consists in the fact that
it is not formed by the folding over of the sides of the medullary
groove into a canal, but by the separation, below the medullary
groove, of a solid cord of epiblast in which the central canal is
subsequently formed. Various views have been put forward to
explain the apparently startling difference between Teleostei,
with which Lepidosteus and Petromyzon agree, and other verte-
brate forms. The explanations of Gotte and Calberla appear to
me to contain between them the truth in this matter. The
groove above in part represents the medullary groove ; but the
closure of the groove is represented by the folding together
of the lateral parts of the epiblast plate to form the medullary
keel.

According to Gotte this is the whole explanation, but Calberla states for
Syngnathus and Salmo that the epidermic layer of the epiblast is carried
down into the keel as a double layer just as if it had been really folded in.
This ingrowth of the epidermic layer is shewn in fig. 32 A where it is just
commencing to pass into the keel ; and at a later stage in fig. 32 B where
the keel has reached its greatest depth.

TELEOSTEI.

73

Gotte maintains that Calberla's statements are not to be trusted, and I
have myself been unable to confirm them for Teleostei or Lepidosteus; but
if they could be accepted the difference in the formation of the medullary
canal in Teleostei and in other Vertebrata would become altogether unimpor-
tant and consist simply in the fact that the ordinary open medullary groove
is in Teleostei obliterated in its inner part by the two sides of the groove
coming together. Both layers of epiblast would thus have a share in the
formation of the central nervous
system ; the epidermic layer
giving rise to the lining epithe-
lial cells of the central canal,
and the nervous layer to the
true nervous tissue.

The separation of the
solid nervous system from
the epiblast takes place
relatively very late ; and,
before it has been com-
pleted, the first traces of
the auditory pits, of the
optic vesicles, and of the
olfactory pits are visible.
The auditory pit arises as
a solid thickening of the
nervous layer of the epi-
blast at its point of junc-
tion with the medullary
keel ; and the optic vesi-
cles spring as solid out-
growths from part of the
keel itself. The olfactory
pits are barely indicated
as thickenings of the ner-
vous layer of the epiblast.

FlG. 32. TWO TRANSVERSE SECTIONS OF

SYNGNATHUS. (After Calberla. )

A. Younger stage before the definite es-
tablishment of the notochord.

B. Older stage.

The epidermic layer of the epiblast is repre-
sented in black.

ep. epidermic layer of epiblast ; me. neural
cord ; hy. hypoblast ; me. mesoblast ; ch. noto-
chord.

At this early stage all the
organs of special sense are at-
tached to a layer continuous
with or forming part of the
central nervous system ; and

this fact has led Gotte (No. 63) to speak of a special- sense plate,
belonging to the central nervous system and not to the skin, from which

74 FORMATION OF THE LAYERS.

all the organs of special sense are developed ; and to conclude that a serial
homology exists between these organs in their development. A comparison
between Teleostei and other forms shews that this view cannot be upheld ;
even in Teleostei the auditory and olfactory rudiments arise rather from the
epiblast at the sides of the brain than from the brain itself, while the optic
vesicles spring from the first directly from the medullary keel, and are
therefore connected with the central nervous system rather than with the
external epiblast. In a slightly later stage the different connections of the
two sets of sense organs is conclusively shewn by the fact that, on the
separation of the central nervous system from the epiblast, the optic vesicles
remain attached to the former, while the auditory and olfactory vesicles are
continuous with the latter.

After its separation from the central nervous system the
remainder of the epiblast gives rise to the skin, etc., and most
probably the epidermic stratum develops into the outer layer of
the epidermis and the nervous stratum into the mucous layer.
The parts of the organs of special sense, which arise from the
epiblast, are developed from the nervous layer. In the Trout
(Oellacher, No. 72) both layers are continued over the yolk-
sack; but in Clupeus and Gasterosteus only the epidermic has
this extension. According to Gotte the distinction between the
two layers becomes lost in the later embryonic stages.

Although it is thoroughly established that the mesoblast
originates from the lower of the two layers of the thickened
embryonic rim, it is nevertheless not quite certain whether it is
a continuous layer between the epiblast and hypoblast, or
whether it forms two lateral masses as in Elasmobranchs. The
majority of observers take the former view, while Calberla is
inclined to adopt the latter. In the median line of the embryo
underneath the medullary groove there are undoubtedly from
the first certain cells which eventually give rise to the notochord ;
and it is these cells the nature of which is in doubt. They are
certainly at first very indistinctly separated from the mesoblast
on the two sides, and Calberla also finds that there is no sharp
line separating them from the secondary hypoblast (fig. 32 A).
Whatever may be the origin of the notochord the mesoblast
very soon forms two lateral plates, one on each side of the body,
and between them is placed the notochord (fig. 32 B). The
general fate of the two mesoblast plates is the same as in Elas-
mobranchs. They are at first quite solid and exhibit relatively

TELEOSTEI. 75

late a division into splanchnic and somatic layers, between
which is placed the primitive body cavity. The dorsal part of
the plates becomes transversely segmented in the region of the
trunk ; and thus gives rise to the mesoblastic somites, from
which the muscle plates and the perichordal parts of the
vertebral column are developed. The ventral or outer part
remains unsegmented. The cavity of the ventral section
becomes the permanent body cavity. It is continued forward
into the head (Oellacher), and part of it becomes separated off
from the remainder as the pericardial cavity.

The hypoblast forms a continuous layer below the mesoblast,
and, in harmony with the generally confined character of the
development of the organs in Teleostei, there is no space left
between it and the yolk to represent the primitive alimentary
cavity. The details of the formation of the true alimentary tube
have not been made out ; it is not however formed by a folding
in of the lateral parts of the hypoblast, but arises as a solid or
nearly solid cord in the a'xial line, between the notochord and
the yolk, in which a lumen is gradually established.

In the just hatched larva of an undetermined fresh-water fish with a very
small yolk-sack I found that the yolk extended along the ventral side of the
embryo from almost the mouth to the end of the gut. The gut had, except
in the hinder part, the form of a solid cord resting in a concavity of the yolk.
In the hinder part of the gut a lumen was present, and below this part the
amount of yolk was small and the yolk nuclei numerous. Near the limit
of its posterior extension the yolk broke up into a mass of cells, and the
gut ended behind by falling into this mass. These incomplete observations
appear to shew that the solid gut owes its origin in a large measure to nuclei
derived from the yolk.

When the yolk has become completely enveloped a postanal
section of gut undoubtedly becomes formed ; and although,
owing to the solid condition of the central nervous system, a
communication between the neural and alimentary canals
cannot at first take place, yet the terminal vesicle of the post-
anal gut of Elasmobranchii is represented by a large vesicle,
originally discovered by Kupffer (No. 68), which can easily be
seen in the embryos of most Teleostei, but the relations of which
have not been satisfactorily worked out (vide fig. 34, hyv). As
the tail end of the embryo becomes separated off from the yolk
the postanal vesicle atrophies.

76

GENERAL GROWTH OF THE EMBRYO.

General development of the Embryo. Attention has
already been called to the fact that the embryo first appears as a
thickening of the edge of the blastoderm which soon assumes a
somewhat shield-like form (fig. 33 A). The hinder end of the
embryo, which is placed at the edge of the blastoderm, is some-
what prominent, and forms the caudal swelling (ts). The axis
of the embryo is marked by a shallow groove.

The body now rapidly elongates, and at the same time

FIG.

33. THREE STAGES IN THE DEVELOPMENT OF THE SALMON.
His.)

(After

ts. tail-swelling; an.v. auditory vesicle; oc. optic vesicle; ce. cerebral rudiment;
m.b. mid-brain; ^.cerebellum; md. medulla oblongata ; m.so. mesoblastic somite.

becomes considerably narrower, while the groove along the axis
becomes shallower and disappears. The anterior, and at first
proportionately a very large part, soon becomes distinguished as
the cephalic region (fig. 33 B). The medullary cord in this
region becomes very early divided into three indistinctly sepa-
rated lobes, representing the fore, the mid, and the hind brains :
of these the anterior is the smallest. With it are connected the
optic vesicles (oc) — solid at first — which are pushed back into the
region of the mid-brain.

The trunk grows in the usual way by the addition of fresh
somites behind.

After the yolk has become completely enveloped by the
blastoderm the tail becomes folded off, and the same process
takes place at the front end of the embryo. The free tail end of

TELEOSTEI.

77

the embryo continues to grow, remaining however closely
applied to the yolk-sack, round which it curls itself to an extent
varying with the species (vide fig. 34).

The general growth of the embryo during the later stages
presents a few special features of interest. The head is remark-
able for the small apparent amount of the cranial flexure. This
is probably due to the late deve-
lopment of the cerebral hemi-
spheres. The flexure of the floor
of the brain is however quite as
considerable in the Teleostei as in
other types. The gill clefts deve-
lop from before backwards. The
first cleft is the hyomandibular,
and behind this there are the
hyobranchial and four branchial
clefts. Simultaneously with the
clefts there are developed the
branchial arches. The postoral
arches formed are the mandibular,
hyoid and five branchial arches. In the case of the Salmon all
of these appear before hatching.

The first cleft closes up very early (about the time of
hatching in the Salmon) ; and about the same time there springs
a membranous fold from the hyoid arch, which gradually grows
backwards over the arches following, and gives rise to the
operculum. There appear in the Salmon shortly before hatching
double rows of papillae on the four anterior arches behind the
hyoid. They are the rudiments of the branchiae. They reach
a considerable length before they are covered in by the opercu-
lar membrane. In Cobitis (Gotte, No. 64) they appear in young
larvae as filiform processes equivalent to the external gills of
Elasmobranchs. The extremities of these processes atrophy;
while the basal portions become the permanent gill lamellae.
The general relation of the clefts, after the closure of the
hyomandibular, is shewn in fig. 35.

The air-bladder is formed as a dorsal outgrowth of the alimentary tract
very slightly in front of the liver. It grows in between the two limbs of the
mesentery, in which it extends itself backwards. It appears in the Salmon,

FlG. 34. VIEW OF AN ADVANCED
EMBRYO OF A HERRING IN THE

EGG. (After Kupffer.)

oc. eye ; ht. heart ; hyv, post-anal
vesicle ; ch. notochord.

FORMATION OF THE TAIL.

Carp, and other types to originate rather on the right side of the median
dorsal line, but whether this fact has any special significance is rather
doubtful. In the Salmon and Trout it is formed considerably later than the
liver, but the two are stated by Von Baer to arise in the Carp nearly at the
same time. The absence of a pneumatic duct in the Physoclisti is due to a
post-larval atrophy. The region
of the stomach is reduced al-
most to nothing in the larva.

The oesophagus becomes
solid, like that of Elasmobranchs,
and remains so for a consider-
able period after hatching.

The liver, in the earliest
stage in which I have met with
it in the Trout (27 days after
impregnation), is a solid ventral
diverticulum of the intestine,
which in the region of the liver
is itself without a lumen.

The excretory system com-

FIG. 35. DIAGRAMMATIC VIEW OF THE
HEAD OF AN EMBRYO TELEOSTEAN, WITH THE
PRIMITIVE VASCULAR TRUNKS. (From Gegen-
baur.)

a. auricle ; v. ventricle ; abr. branchial
artery ; d . carotid ; ad. aorta ; s. branchial clefts ;
sv. sinus venosus ; dc. ductus Cuvieri ; n. nasal
pit.

mences with the formation of a segmental duct, formed by a constriction of
the parietal wall of the peritoneal cavity. The anterior end remains open to
the body cavity, and forms a pronephros (head kidney). On the inner side
of and opposite this opening a glomerulus is developed, and the part of the
body cavity containing both the glomerulus and the opening of the prone-
phros becomes shut off from the remainder of the body cavity, and forms a
completely closed Malpighian capsule.

The mesonephros (Wolffian body) is late in developing.

The unpaired fins arise as simple folds of the skin along the
dorsal and ventral edges, continuous with each other round the
end of the tail. The ventral fold ends anteriorly at the anus.

The dorsal and anal fins are developed from this fold by
local hypertrophy. The caudal fin1, however, undergoes a more
complicated metamorphosis. It is at first symmetrical or nearly
so on the dorsal and ventral sides of the hinder end of the
notochord. This symmetry is not long retained, but very soon
the ventral part of the fin with its fin rays becomes much more
developed than the dorsal part, and at the same time the
posterior part of the notochord bends up towards the dorsal
side.

1 In addition to the paper by Alex. Agassiz (No. 55) vide papers by Huxley,
Kolliker, Vogt, etc.

TELEOSTET.

79

In some few cases, e.g. Gadus, Salmo, owing to the simultane-
ous appearance of a number of fin rays on the dorsal and ventral
side of the notochord the external symmetry of the tail is not
interfered with in the above processes. In most instances this is
far from being the case.

In the Flounder, which may serve as a type, the primitive
symmetry is very soon destroyed by the appearance of fin rays
on the ventral side. The re-
gion where they are present
soon forms a lobe; and an
externally heterocercal tail is
produced (fig. 36 A). The
ventral lobe with its rays con-
tinues to grow more promi-
nent and causes the tail fin to
become bilobed (fig. 36 B) ;
there being a dorsal embry-
onic lobe without fin rays (c),
which contains the notochord,
and a ventral lobe with fin
rays, which will form the per-
manent caudal fin. In this
condition the tail fin resembles
the usual Elasmobranch form
or still more that of some
Ganoids, e.g. the Sturgeon.
The ventral lobe continues to
develop ; and soon projects
beyond the dorsal, which gra-
dually atrophies together with
the notochord contained in it,
and finally disappears, leaving
hardly a trace on the dorsal
side of the tail (fig. 36 C, c).
In the meantime the fin rays
of the ventral lobe gradually
become parallel to the axis of

THREE STAGES IN THE DE-
OF THE TAIL OF THE
(PLEURONECTES). (After

FIG. 36.

VELOPMENT

FLOUNDER

Agassiz.)

A. Stage in which the permanent
caudal fin has commenced to be visible as
an enlargement of the ventral side of the
embryonic caudal fin.

B. Ganoid-like stage in which there is
a tme external heterocercal tail.

C. Stage in which the embryonic
caudal fin has almost completely atro-
phied.

c. embryonic caudal fin ; f. permanent
caudal fin ; n. notochord ; it. urostyle.

the body ; and this lobe, to-
gether with a few accessory dorsal and ventral fin rays supported

80 FORMATION OF THE TAIL.

by neural and haemal processes, forms the permanent tail fin,
which though internally unsymmetrical, assumes an externally
symmetrical form. The upturned end of the notochord which
was originally continued into the primitive dorsal lobe becomes
enshcathed in a bone without a division into separate vertebrae.
This bone forms the urostyle (u). The haemal processes belong-
ing to it are represented by two cartilaginous masses, which
subsequently ossify, forming the hypural bones, and supporting
the primary fin rays of the tail (fig. 36 C). The ultimate
changes of the notochord and urostyle vary very considerably in
the different types of Teleostei. Teleostei may fairly be
described as passing through an Elasmobranch stage or a stage
like that of most pre-jurassic Ganoids or the Sturgeon as far as
concerns their caudal fin.

The anterior paired fins arise before the posterior ; and there
do not appear to be any such indications as in Elasmobranchii
of the paired fins arising as parts of a continuous lateral fin.

Most osseous fishes pass through more or less considerable post-embry-
onic changes, the most remarkable of which are those undergone by the
Pleuronectidae1. These fishes, which in the adult state have the eyes
unsymmetrically placed on one side of the head, leave the egg like normal
Teleostei. In the majority of cases as they become older the eye on the
side, which in the adult is without an eye, travels a little forward and then
gradually rotates over the dorsal side of the head, till finally it comes to lie
on the same side as the other eye. During this process the rotating eye
always remains at the surface and continues functional ; and on the two eyes
coming to the same side of the head the side of the body without an organ
of vision loses its pigment cells, and becomes colourless.

The dorsal fin, after the rotation of the eye, grows forward beyond the
level of the eyes. In the genus Plagusia (Steenstrup, Agassiz, No. 56) the
dorsal fin grows forward before the rotation of the eye (the right eye in this
form), and causes some modifications in the process. The eye in travelling
round gradually sinks into the tissues of the head, at the base of the fin
above the frontal bone ; and in this process the original large opening of the
orbit becomes much reduced. Soon a fresh opening on the opposite and
left side of the dorsal fin is formed ; so that the orbit has two external
openings, one on the left and one on the right side. The original one on the
right soon atrophies, and the eye passes through the tissues at the base of
the dorsal fin completely to the left side.

The rotating eye may be either the right or the left according to the
species.

1 Vide Agassiz (No. 56) and Steenstrup, Malm.

TELEOSTEI. 8 1

The most remarkable feature in which the young of a large number of
Teleostei differ from the adults is the possession of provisional spines, very
often formed as osseous spinous projections the spaces between which
become filled up in the adult. These processes are probably, as suggested
by Gunther, secondary developments acquired, like the Zocea spines of
larval Crustaceans, for purposes of defence.

The yolk-sack varies greatly in size in the different types of
Teleostei.

According as it is enclosed within the body-wall, or forms a distinct
ventral appendage, it is spoken of by Von Baer as an internal or external
yolk-sack. By Von Baer the yolk-sack is stated to remain in communication
with the intestine immediately behind the liver, while Lereboullet states that
there is a vitelline pedicle opening between the stomach and the liver which
persists till the absorption of the yolk-sack. My own observations do not
fully confirm either of these statements for the Salmon and Trout. So far
as I have been able to make out, all communication between the yolk-sack
and the alimentary tract is completely obliterated very early. In the Trout
the communication between the two is shut off before hatching, and in the
just-hatched Salmon I can find no trace of any vitelline pedicle. The
absorption of the yolk would seem therefore to be effected entirely by blood-
vessels.

The yolk-sack persists long after hatching, and is gradually
absorbed. There is during the stages either just before hatching
or shortly subsequent to hatching (Cyprinus) a rich vascular
development in the mesoblast of the yolk-sack. The blood is
at first contained in lacunar spaces, but subsequently it becomes
confined to definite channels. As to its exact relations to the
vascular system of the embryo more observations seem to be
required.

The following account is given by Rathke (No. 72*) and Lereboullet
(No. 71). At first a subintestinal vein (vide chapter on Circulation) falls into
the lacunae of the yolk-sack, and the blood from these is brought back direct
to the heart. At a later period, when the liver is developed, the subintes-
tinal vessel breaks up into capillaries in the liver, thence passes into the yolk-
sack, and from this to the heart. An artery arising from the aorta penetrates
the liver, and there breaks up into capillaries continuous with those of the
yolk-sack. This vessel is perhaps the equivalent of the artery which supplies
the yolk-sack in Elasmobranchii, but it seems possible that there is some
error in the above description.

BIBLIOGRAPHY.

(55) Al. Agassiz. " On the young Stages of some Osseous Fishes. I. Deve-
lopment of the Tail." Proceedings of the American Academy of Arts and Sciences,
Vol. xm. Presented Oct. u, 1877.

B. III. 6

82 BIBLIOGRAPHY.

(66) Al. Agassiz. "II. Development of the Flounders." Proceedings of the
American Acad. of Arts and Sciences, Vol. xiv. Presented June, 1878.

(57) K. E. v. Baer. Untersuchungen iiber die Entwicklungsgeschichte der Fische.
Leipzig, 1835.

(58) Ch. van Bamheke. "Premiers effets de la fecondation sur les ceufs de
Poissons: sur 1'origine et la signification du feuitlet muqueux ou glandulaire chez les
Poissons Osseux." Comptes Rendus des Stances de VAcademie des Sciences, Tome
i. xxiv. 1872.

(59) Ch. van Bambeke. " Recherches sur 1'Embryologie des Poissons
Osseux." Mtm. couronnes et Mem, de savants itrangers, de FAcademie roy. Belgique,

Vol. XL. 1875.

(60) E. v. Beneden. "A contribution to the history of the Embryonic deve-
lopment of the Teleosteans." Quart. J. of Micr. Set., Vol. xvm. 1878.

(61) E. Calberla. " Zur Entwicklung des Medullarrohres u. d. Chorda
dorsalis d. Teleostier u. d. Petromyzonten." Morphologisches Jahrbuch, Vol. III.
1877.

(62) A. Gbtte. "Beitrage zur Entwicklungsgeschichte der Wirbelthiere."
Archivf. mikr. Anat., Vol. IX. 1873.

(63) A. Gotte. " Ueber d. Entwicklung d. Central-Nervensystems der Teleos-
tier." Archivf. mikr. Anat., Vol. xv. 1878.

(64) A. Gotte. " Entwick. d. Teleostierkeime." Zoologischer Anzeiger, No. 3.
1878.

(65) W. His. " Untersuchungen iiber die Entwicklung von Knochenfischen, etc."
Zeit.f. Anat. u. Entwicklungsgeschichte, Vol. I. 1876.

(66) W. His. "Untersuchungen iiber die Bildung des Knochenfischembryo
(Salmen). " Archivf. Anat. u. Physiol., 1878.

(67) E. Klein. "Observations on the early Development of the Common
Trout." Quart. J. of Micr. Science, Vol. xvi. 1876.

(68) C. Kupffer. " Beobachtungen iiber die Entwicklung der Knochenfische."
Archivf. mikr. Anat., Bd. IV. 1868.

(69) C. Kupffer. Ueber Laichenu. Entwicklung des Ostsee-Herings. Berlin,
1878.

(70) M. Lereboullet. "Recherches sur le developpement du brochet de la
perche et de 1'ecrevisse." Annales des Sciences Nat., Vol. I., Series iv. 1854.

(71) M. Lereboullet. " Recherches d'Embryologie comparee sur le developpe-
ment de la Truite." An. Sci. Nat., quatrieme serie, Vol. xvi. 1861.

(72) T. Oellacher. " Beitrage zur Entwicklungsgeschichte der Knochenfische
nach Beobachtungen am Bachforellenei." Zeit. f. wiss. ZooL, Vol. xxn., 1872, and
Vol. xxni., 1873.

(72*) H. Rathke. Abh. z. Bildung u. Entwick. d. Menschenu. Thiere. Leipzig,
1832-3. Part II. Blennius.

(73) Reineck. " Ueber die Schichtung des Forellenkeims." Archiv f. mikr.
Anat., Bd. v. 1869.

(74) S. Strieker. "Untersuchungen iiber die Entwicklung der Bachforelle."
Sitzungsberuhte der Wiener k. Akad. d. Wiss., 1865. Vol. LI. Abth. 2.

(75) Carl Vogt. " Embryologie des Salmones." Histoire Naturelle des Poissons
de f Europe Centrale. L. Agassiz. 1842.

(76) C.Weil. " Beitrage zur Kenntniss der Knochenfische. " Sitzmtgsher. <1cr
Wiener kais. Akad. der Wins.. Bd. I. XVI. 1872.

CHAPTER V.
CYCLOSTOMATA1.

PETROMYZON is the only type of this degenerated but
primitive group of Fishes the development of which has been as
yet studied2.

The development does not however throw any light on the
relationships of the group. The similarity of the mouth and
other parts of Petromyzon to those of the Tadpole probably
indicates that there existed a common ancestral form for the
Cyclostomata and Amphibia. Embryology does not however
add anything to the anatomical evidence on this subject. The
fact of the segmentation being complete was at one time
supposed to indicate an affinity between the two groups ; but
the discovery that the segmentation is also complete in the
Ganoids deprives this feature in the development of any special
weight. In the formation of the layers and in most other
developmental characters there is nothing to imply a special
relationship with the Amphibia, and in the mode of formation
of the nervous system Petromyzon exhibits a peculiar modi-
fication, otherwise only known to occur in Teleostei and
Lepidosteus.

Dohrn3 was the first to bring into prominence the degenerate character
of the Cyclostomata. I cannot however assent to his view that they are

1 The following classification of the Cyclostomata is employed in the present
chapter :

I. Hyperoartia ex. Petromyzon.
II. Hyperotreta ex. Myxine, Bdellostoma.

2 The present chapter is in the main founded upon observations which I was able
to make in the spring of 1880 upon the development of Petromyzon Planeri. Mr
Scott very kindly looked over my proof-sheets and made a number of valuable
suggestions, and also sent me an early copy of his preliminary note (No. 87), which I
have been able to make use of in correcting my proof-sheets.

3 Der Ursprung d. Wirbelthiere, etc. Leipzig, 1875.

6—2

84

FORMATION OF THE LAYERS.

descended from a relatively highly-organized type of Fish. It appears to
me almost certain that they belong to a group of fishes in which a true
skeleton of branchial bars had not become developed, the branchial skeleton
they possess being simply an extra-branchial system; while I see no reason
to suppose that a true branchial skeleton has disappeared. If the primitive
Cyclostomata had not true branchial bars, they could not have had jaws,
because jaws are essentially developed from the mandibular branchial bar.
These considerations, which are supported by numerous other features of
their anatomy, such as the character of the axial skeleton, the straightness
of the intestinal tube, the presence of a subintestinal vein etc., all tend to
prove that these fishes are remnants of a primitive and praegnathostomatous
group. The few surviving members of the group have probably owed
their preservation to their parasitic or semiparasitic habits, while the
group as a whole probably disappeared on the appearance of gnathostoma-
tous Vertebrata.

The ripe ovum of Petromyzon Planeri is a slightly oval body
of about i mm. in diameter.
It is mainly formed of an

opaque nearly white yolk, TTI& ,

invested by a membrane
composed of an inner per-
forated layer, and an outer
structureless layer. There
appears to be a pore per-
forating the inner layer at
the formative pole, which
may be called a micropyle
(KupfTer and Benecke, No.
79). Enclosing the egg-
membranes there is present
a mucous envelope, which
causes the egg, when laid,
to adhere to stones or other objects.

Impregnation is effected by the male attaching itself by its
suctorial mouth to the female. The attached couple then shake
together ; and, as they do so, they respectively emit from their
abdominal pores ova and spermatozoa which pass into a hole
previously made1.

U

FIG. 37. LONGITUDINAL VERTICAL SEC-
TION THROUGH AN EMBRYO OF PETROMYZON

PLANERI OF 136 HOURS.

me. mesoblast ; yk. yolk-cells ; al. alimen-
tary tract ; bl. blastopore ; s.c. segmentation
cavity.

1 Artificial impregnation may be effected without difficulty by squeezing out into
the same vessel the ova and spermatozoa of a ripe female and male. The fertilized
eggs are easily reared. Petromyzon Planeri breeds during the second half of April.

CYCLOSTOMATA.

The segmentation is total and unequal, and closely resembles
that in the Frog's egg (Vol. II. p. 96). The upper pole is very
slightly whiter than the lower. A segmentation cavity is formed
very early, and is placed between the small cells of the upper
pole and the large cells of the lower pole. It is proportionately
larger than in the Frog ; and the roof eventually thins out so as
to be formed of a single row of small cells. At the sides of the
segmentation cavity there are always several rows of small cells,

FIG. 38. TRANSVERSE SECTION THROUGH A PETROMYZON EMBRYO 160 HOURS

AFTER IMPREGNATION.
ep. epiblast ; al. mesenteron ; yk. yolk-cells ; ms. mesoblast.

which gradually merge into the larger cells of the lower pole of
the egg. The segmentation is completed in about fifty hours.

The segmentation is followed by an asymmetrical invagina-
tion (fig. 37) which leads to a mode of formation of the hypo-
blast fundamentally similar to that in the Frog. The process
has been in the main correctly described by M. Schultze
(No. 81).

On the border between the -large and small cells of the
embryo, at a point slightly below the segmentation cavity,
a small circular pit appears ; the roof of which is formed by an
infolding of the small cells, while the floor is formed of the large
cells. This pit is the commencing mesenteron. It soon grows
deeper (fig. 37, al} and extends as a well-defined tube (shewn in
transverse section in fig. 38, al} in the direction of the segmenta-
tion cavity. In the course of the formation of the mesenteron
the segmentation cavity gradually becomes smaller, and is

86 FORMATION OF THE LAYERS.

finally (about the 2ooth hour) obliterated. The roof of the
mesenteron is formed by the continued invagination of small
cells, and its floor is composed of large yolk-cells. The wide
external opening is arched over dorsally by a somewhat promi-
nent lip — the homologue of the embryonic rim. The opening
persists till nearly the time of hatching ; but eventually becomes
closed, and is not converted into the permanent anus. On the
formation of the mesenteron the hypoblast is composed of two
groups of cells, (i) the yolk-cells, and (2) the cells forming the
roof of the mesenteron.

While the above changes are taking place, the small cells, or
as they may now be called the epiblast cells, gradually spread
over the large yolk-cells, as in normal types of epibolic invagi-
nation. The growth over the yolk-cells is not symmetrical, but
is most rapid in the meridian opposite the opening of the
alimentary cavity, so that the latter is left in a bay (cf. Elasmo-
branchii, p. 63). The epibolic invagination takes place as in
Molluscs and many other forms, not simply by the division of
pre-existing epiblast cells, but by the formation of fresh epiblast
cells from the yolk-cells (fig. 37) ; and till after the complete
enclosure of the yolk-cells there is never present a sharp line of
demarcation between the two groups of cells. By the time that
the segmentation cavity is obliterated the whole yolk is en-
closed by the epiblast. The yolk-cells adjoining the opening
of the mesenteron are the latest to be covered in, and on their
enclosure this opening constitutes the whole of the blastopore.
The epiblast is composed of a single row of columnar cells.

Mesoblast and notochord. During the above changes the
mesoblast becomes established. It arises, as in Elasmobranchs,
in the form of two plates derived from the primitive hypoblast.
During the invagination to form the mesenteron some of the
hypoblast cells on each side of the invaginated layer become
smaller, and marked off as two imperfect plates (fig. 38, ms).
It is difficult to say whether these plates are entirely derived
from invaginated cells, or are in part directly formed from the
pre-existing yolk-cells, but I am inclined to adopt the latter
view ; the ventral extension of the mesoblast plates undoubtedly
takes place at the expense of the yolk-cells. The mesoblast
plates soon become more definite, and form (fig. 39, ms) well-

CYCLOSTOMATA.

defined structures, triangular in section, on the two sides of the
middle line.

At the time the mesoblast is first formed the hypoblast cells,
which roof the mesenteron, are often imperfectly two layers
thick (fig. 38). They soon
however become constitu-
ted of a single layer only.
When the mesoblast is fair-
ly established, the lateral
parts of the hypoblast grow
inwards underneath the
axial part, so that the latter
(fig. 39, c/i) first becomes
isolated as an axial cord,
and is next inclosed be-
tween the medullary cord
(nc) (which has by this time
been formed) and a con-
tinuous sheet of hypoblast
below (fig. 40). Here its
cells divide and it becomes the notochord. The notochord is
thus bodily formed out of the axial portion of the primitive
hypoblast. Its mode of origin may be compared with that in
Amphioxus, in which an
axial fold of the archenteric
wall is constricted off as the
notochord. The above fea-
tures in the development of
the notochord were first es-
tablished by Calberla1 (No.
78).

General history of the de-
velopment. Up to about the
time when the enclosure of
the hypoblast by the epiblast is completed, no external traces
are visible of any of the organs of the embryo ; but about this
time, i.e. about 180 hours after impregnation, the rudiment of

FIG. 39. TRANSVERSE SECTION THROUGH
AN EMBRYO OF PETROMYZON PLANERI OF

208 HOURS.

The figure illustrates the formation of the
neural cord and of the notochord.

ms. mesoblast ; nc. neural cord ; ch. noto-
chord ; yk. yolk-cells ; al. alimentary canal.

m C.

FIG. 40. TRANSVERSE SECTION THROUGH
PART OF AN EMBRYO OF PETROMYZON PLA-
NERI OF 256 HOURS.

m.c. medullary cord ; ch. notochord ; al.
alimentary canal ; ms. mesoblastic plate.

1 In Calberla's figure, shewing the development of the notochord, the limits of
mesoblast and hypoblast are wrongly indicated.

88 GENERAL DEVELOPMENT.

the medullary plate becomes established, as a linear streak
extending forwards from the blastopore over fully one half the
circumference of the embryo. The medullary plate first con-
tains a shallow median groove, but it is converted into the
medullary cord, not in the usual vertebrate fashion, but, as first
shewn by Calberla, in a manner much more closely resembling
the formation of the medullary cord in Teleostei. Along the
line of the median groove the epiblast becomes thickened and
forms a kind of keel projecting inwards towards the hypoblast
(fig. 39, nc]. This keel is the rudiment of the medullary cord.
It soon becomes more prominent, the median groove in it
disappears, and it becomes separated from the epiblast as a solid
cord (fig. 40, me}.

By this time the whole embryo has become more elongated,
and on the dorsal surface is placed a ridge formed by the
projection of the medullary cord. At the lip of the blasto-
pore the medullary cord is continuous with the hypoblast, thus
forming the rudiment of a neurenteric canal.

Calberla gives a similar account of the formation of the neural canal to
that which he gives for the Teleostei (vide p. 72.)

He states that the epiblast becomes divided into two layers, of which the
outer is involuted into the neural cord, a median slit in the involution
representing the neural groove. The eventual neural canal is stated to be
lined by the involuted cells. Scott (No. 87) fully confirms Calberla on this
point, and, although my own sections do not clearly shew an involution of
the outer layer of epiblast cells, the testimony of these two observers must
no doubt be accepted on this point.

Shortly after the complete establishment of the neural cord
the elongation of the embryo proceeds with great rapidity.
The processes in this growth are shewn in fig. 41, A, B, and C.
The cephalic portion (A, c] first becomes distinct, forming an
anterior protuberance free from yolk. About the time it is
formed the mesoblastic plates begin to be divided into somites,
but the embryo is so opaque that this process can only be
studied in sections. Shortly afterwards an axial lumen appears
in the centre of the neural cord, in the same manner as in
Teleostei.

The general elongation of the embryo continues rapidly, and,
as shewn in my figures, the anterior end is applied to the

CYCLOSTOMATA.

89

ventral surface of the yolk (B). With the growth of the em-
bryo the yolk becomes entirely confined to the posterior part.
This part is accordingly greatly dilated, and might easily be
mistaken for the head. The position of the yolk gives to the
embryo a very peculiar appearance. The apparent difference
between it and the embryos of other Fishes in the position of
the yolk is due in the main to the fact that the post-anal portion
of the tail is late in developing, and always small. As the
embryo grows longer it becomes spirally coiled within the egg-
shell. Before hatching the mesoblastic somites become distinctly
marked (C).

The hatching takes place at between 13 — 21 days after
impregnation ; the period varying according to the temperature.

During the above changes in the external form of the

FlG. 41. FOUR STAGES IN THE DEVELOPMENT OF PETROMYZON.

(After Owsjannikoff.)

c. cephalic extremity ; bl. blastopore ; op. optic vesicle ; au.v. auditory vesicle ;
br.c. branchial clefts.

embryo, the development of the various organs makes great
progress. This is especially the case in the head. The brain
becomes distinct from the spinal cord, and the auditory sacks
and the optic vesicles of the eye become formed. The branchial
region of the mesenteron becomes established, and causes a

GENERAL DKVKLOl'MKNT.

dilatation of the anterior part of the body, and the branchial
pouches grow out from the throat. The anus becomes formed,
and a neurenteric canal is also established (Scott). The nature
of these and other changes will best be understood by a
description of the structure of the just-hatched larva. The
general appearance of the larva immediately after hatching
is shewn in fig. 41, D. The body is somewhat -curved ; the
posterior extremity being much dilated with yolk, while the
anterior is very thin. All the cells still contain yolk particles,
which render the embryo very opaque. The larva only exhibits
slow movements, and is not capable of swimming about.

The structure of the head is shewn in figs. 42 and 43. Fig.
42 is a section through a very young larva, while fig. 43 is taken
from a larva three days after hatching, and shews the parts with
considerably greater detail.

On the ventral side of the head is placed the oral opening
(fig. 43, m) leading into a large stomodaeum which is still with-
out a communication with the mesenteron. Ventrally the sto-
modaeum is prolonged for a considerable distance under the
anterior part of the mesenteron. Immediately behind the sto-

FIG. 42. DIAGRAMMATIC VERTICAL SECTION OF A JUST-HATCHED LARVA OF

PETROMYZON. (From Gegenbaur ; after Calberla.)

o. mouth ; o ' . olfactory pit ; v. septum between stomodaeum and mesenteron ;
h. thyroid involution ; n. spinal cord ; ch. notochord ; c. heart ; a. auditory vesicle.

modaeum is placed the branchial region of the mesenteron.
Laterally it is produced on each side into seven or perhaps eight
branchial pouches (fig. 43, br.c}, which extend outwards nearly
to the skin but are not yet open. Between the successive
pouches are placed mesoblastic segments, of the same nature
and structure as the walls of the head cavities in the embryos of
Elasmobranchs, and like them enclosing a central cavity. A

CYCLOSTOMATA. 91

similar structure is placed behind the last, and two similar
structures in front of the first persistent pouch. This pouch is
situated in the same vertical line as the auditory sack (au.v),
and would appear therefore to be the hyo-branchial cleft ; and
this identification is confirmed by the fact of two head cavities
being present in front of it At the front end of the branchial
region of the mesenteron is placed a thickened ridge of tissue,

pn

FIG. 43. DIAGRAMMATIC VERTICAL SECTION THROUGH THE HEAD OF A LARVA
OF PETROMYZON.

The larva had been hatched three days, and was 4 '8 mm. in length. The optic
and auditory vesicles are supposed to be seen through the tissues. The letter tv
pointing to the base of the velum is where Scott believes the hyomandibular cleft to be
situated.

c.h. cerebral hemisphere; th. optic thalamus; in, infundibulum; pn. pineal gland ;
mb. mid-brain; cl>. cerebellum; md. medulla oblongata ; au.v. auditory vesicle; op.
optic vesicle ; ol. olfactory pit ; m. mouth; br.c. branchial pouches; th. thyroid invo-
lution ; v.ao. ventral aorta ; ht. ventricle of heart ; ch. notochord.

which, on the opening of the passage between the stomodseum
and the mesenteron, forms a partial septum between the two,
and is known as the velum (fig. 43, tv).

According to Scott (No. 87) a hyomandibular pouch forming the eighth
pouch is formed in front of the pouch already defined as the hyobranchial.
It disappears early and does not acquire gill folds1. The tissue forming the

1 Scott informs me that he has been unable to find the hyomandibular pouch
in larvae larger than 4-8 mm. My material of the stages when it should be present is
somewhat scanty, but I have as yet, very likely owing to the imperfection of my
material, been unable to find Scott's hyomandibular pouch either in my sections or
surface-views. Huxley describes this pouch as present in the form of a cleft in later
stages; I have failed to find his cleft also. The vessel interpreted below as the
branchial artery of the mandibular arch was only imperfectly investigated by me, and
I was not sure of my interpretations about it. Scott however informs me by letter that
it is undoubtedly present.

92

GENERAL DEVELOPMENT.

line of insertion of the velum appears to me to represent the mandibular
arch. The grounds for this view are the following :

(1) The structure in question has exactly the position usually occupied
by the mandibular arch.

(2) There is present in late larvae (about 20 days after hatching) an
arterial vessel, continued from the ventral prolongation of the bulbus
arteriosus along the insertion of the velum towards the dorsal aorta, which
has the relations of a true branchial artery.

On the ventral aspect of the branchial region is placed a sack
(figs. 42, h, and 43, ///), which extends from the front end of the
branchial region to the fourth cleft. At first it constitutes a
groove opening into the throat above (fig. 44), but soon the
opening becomes narrowed to a pore placed between the second
and third of the permanent branchial pouches (fig. 43, tJi). In
Ammoccetes1 the simple tube becomes divided, and assumes a
very complicated form, though still retaining its opening into
the branchial region of the throat. In the adult it forms a
glandular mass underneath the branchial region of the throat
equivalent to the thyroid gland of higher Vertebrates.

On the ventral aspect of the head, and immediately in front
of the mouth, is placed the olfactory pit (fig. 43, of}. It is from
the first unpaired, and in just-hatched larvae simply forms a
shallow groove of thickened epiblast at the base of the front of
the brain. By the stage represented in fig. 43 the ventral part
of the original groove is prolonged into a pit, extending back-
wards beneath the brain nearly up to the infundibulum.

On the side of the head, nearly on a level with the front end
of the notochord, is placed
the eye (fig. 43, op}. It is
constituted (figs. 45 and 46)
of a very shallow optic cup
with a thick outer (retinal)
layer, and a thin inner cho-
roid layer. In contact with
the retinal layer is placed
the lens. The latter is form-
ed as an invagination of the

FIG. 44. DIAGRAMMATIC TRANSVERSE
SECTIONS THROUGH THE BRANCHIAL REGION

OF A YOUNG LARVA OK PETBOMYZON. (From
Gegenbaur ; after Calberla.)

d. branchial region of throat.

1 Schneider (No. 85) states that in the full-grown Ammoccetes the opening is situ-
ated between the third and fourth pouches. This is certainly not true for the young
larva.

CYCLOSTOMATA.

93

skin ; to which it is still attached in the just-hatched larva (fig.
45). The eye only differs at this stage from that of other
Vertebrata in its extraordinarily small size, and the rudimentary
character of its constituent parts.

The auditory sack is a large vesicle (fig. 43, au.v}, placed at
the side of the brain opposite the first persistent branchial
pouch.

The brain is formed of the usual vertebrate parts1, but is
characterized by the very slight cranial flexure. The fore-brain
consists (fig. 43) of a thalamencephalon (t/i) and an undivided
cerebral rudiment (ch}. To the roof
of the thalamencephalon is attached
a flattened sack (pn} which is prob-
ably the pineal gland. The floor is
prolonged into an infundibulum (in}
which contains a prolongation of the
third ventricle. The lateral walls of
the cerebral rudiment are much
thickened.

Behind the thalamencephalon
follows the mid-brain (mb], the sides
of which form the optic lobes, and
behind this again the hind-brain
(ind} ; the front border of the roof of
which is thickened to form the cere-
bellum (cb}. The medulla passes
without any marked line of demarca-
tion into the spinal cord.

The histological differentiation of the brain has already
proceeded to some extent ; and it has in the main the same
character as the spinal cord. Before the larva has been hatched
very long a lateral investment of white matter is present through-
out. The notochord (ck] is continued forwards in the head to
the hinder border of the infundibulum. It is slightly flexed
anteriorly.

From the hinder border of the auditory region to the end of
the branchial region the mesoblast is dorsally divided into

1 Max Schultze's statements as to the structure and histology of the brain are very
inadequate in the present state of our knowledge.

FIG. 45. HORIZONTAL SEC-
TION THROUGH THE HEAD OF A
JUST-HATCHED LARVA OF PETRO-
MYZON SHEWING THE DEVELOP-
MENT OF THE LENS OF THE EYE.

th.c. thalamencephalon ; op.v.
optic vesicle ; /. lens of eye ; h.c.
head cavity.

94

GENERAL DEVELOPMENT.

myotomes, which nearly, though
number with the branchial pouches.

not quite, correspond in

FIG. 46. EYE OF A LARVA
OF PETROMYZON NINE DAYS
AFTER HATCHING.

/. lens; r. retina.

The section passes through
one side of the lens.

The growth of the myotomes would seem,
as might be anticipated from their indepen-
dent innervation, not to be related to that of
the branchial pouches, so that there is a want
of correspondence between these parts, the
extent of which varies at different periods of
life. The relation between the two in an old
larva is shewn in fig. 47.

The head of the larva of Petromyzon
differs very strikingly in general ap-
pearance from that of the normal
Vertebrata. This is at once shewn
by a comparison of fig. 43 with fig. 29.
The most important difference between the two is due to the
absence of a pronounced cranial flexure in Petromyzon ; an
absence which is in its turn probably caused by the small
development of the fore-brain.

The stomodaeum of Petromyzon is surprisingly large, and its
size and structure in this type militate against the view of some
embryologists that the stomodaeum originated from the coa-
lescence of a pair of branchial pouches.

In the region of the trunk there is present an uninterrupted
dorsal fin continuous with a ventral fin round the end of the
tail.

There is a well-developed body cavity, which is especially
dilated in front, in the part which afterwards becomes the
pericardium. In this region is placed the nearly straight heart,
divided into an auricle and ventricle (figs. 42 and 43), the latter
continued forwards into a bulbus arteriosus.

The myotomes are now very numerous (about 57, including
those of the head, in a three days' larva). They are separated
by septa, but do not fill up the whole space between the septa,
and have a peculiar wavy outline. The notochord is provided
with a distinct sheath, and below it is placed a subnotochordal
rod.

The alimentary canal consists of a narrow anterior section
free from yolk, and a posterior region, the walls of which arc

CYCLOSTOMATA.

95

largely swollen with yolk. The anterior section corresponds to
the region of the oesophagus and stomach, but exhibits no dis-
tinction of parts. Immediately behind this point the alimentary
canal dilates considerably, and on the ventral side is placed the
opening of a single large sack, which forms the commencement
of the liver. The walls of the hepatic sack are posteriorly united
to the yolk-cells. At the region where the hepatic sack opens
into the alimentary tract the latter dilates considerably.

The posterior part of the alimentary tract still constitutes a
kind of yolk-sack, the ventral wall being enormously thick and
formed of several layers of yolk-cells. The dorsal wall is very
thin.

The excretory system is composed of two segmental ducts,
each connected in front with a well-developed pronephros (head-
kidney), with about five ciliated funnels opening into the peri-
cardial region of the body cavity. The segmental ducts in the
larvae open behind into the cloacal section of the alimentary
tract.

The development of the larva takes place with considerable
rapidity. The yolk becomes absorbed and the larva becomes
accordingly more transparent. It generally lies upon its side,
and resembles in general appearance and habit a minute Am-

tl

FIG. 47. HEAD OF A LARVA OF PETROMYZON six WEEKS OLD.

(Altered from Max Schultze.)

au.v. auditory vesicle ; op. optic vesicle ; ol. olfactory pit ; ul. upper lip ; //. lower
lip ; or.p. papillae at side of mouth ; v. velum ; br.s. extra branchial skeleton ; i — 7.
branchial clefts.

phioxus. It is soon able to swim with vigour, but usually, unless
disturbed, is during the day quite quiescent, and chooses by

96 GENERAL DEVELOPMENT.

preference the darkest situations. It soon straightens out, and,
with the disappearance of the yolk, the tail becomes narrower
than the head. A large caudal fin becomes developed.

When the larva is about twenty days old, it bears in most
anatomical features a close resemblance to an Ammoccetes ;
though the histological differences between my oldest larva
(29 days) and even very young Ammoccetes are considerable.

The mouth undergoes important changes. The upper lip becomes much
more prominent, forming of itself the anterior end of the body (fig. 47, «/).
The opening of the nasal pit is in this way relatively thrown back, and at
the same time is caused to assume a dorsal position. This will be at once
understood by a comparison of fig. 43 with fig. 47. On the inner side of the
oral cavity a ring of papillae is formed (fig. 47, or.p). Dorsally these papilla;
are continued forward as a linear streak on the under side of the upper lip.
A communication between the oral cavity and the branchial sack is very
soon established.

The gill pouches gradually become enlarged ; but it is some time before
their small external openings are established. Their walls, which are
entirely lined by hypoblast, become raised in folds, forming the branchial
lamellae. The walls of the head cavities between them become resolved into
the contractors and dilators of the branchial sacks. The extra-branchial
basketwork becomes established very early (it is present in the larva of 6
millimetres, about 9 days after hatching) and is shewn in an older larva in
fig. 47, br.s. It is not so complicated in these young larvae as in the
Ammoccetes, but in Max Schultze's figure, which I have reproduced, the
dorsal elements of the system are omitted. On the dorsal wall of the
branchial region a ciliated ridge is formed, which may be homologous with
the ridge on the dorsal wall of the branchial sack of Ascidians. It has been
described by Schneider in Ammoccetes.

With reference to the remainder of the alimentary canal there is but
little to notice. The primitive hepatic diverticulum rapidly sprouts out and
forms a tubular gland. The opening into the duodenum changes from a
ventral to a lateral or even dorsal position. The duct leads into a gall-
bladder imbedded in the substance of the liver. Ventrally the liver is united
with the abdominal wall, but laterally passages are left by which the
pericardial and body cavities continue to communicate.

The greater part of the yolk becomes employed in the formation of the
intestinal wall. This part of the intestine in a nine days' larva (67 mm.) has
the form of a cylindrical tube with very thick columnar cells entirely filled
with yolk particles. The dorsal wall is no longer appreciably thinner than
the ventral. In the later stages the cells of this part of the intestine become
gradually less columnar as the yolk is absorbed.

The fate of the yolk-cells in the Lamprey is different from that in most
other Vertebrata with an equally large amount of yolk. They no doubt

CYCLOSTOMATA. 97

supply nutriment for the growth of the embryo, and although in the anterior
part of the intestine they become to some extent enclosed in the alimentary
tract and break up, yet in the posterior part they become wholly transformed
into the regular epithelium of the intestine.

On the ninth day a slight fold filled with mesoblastic tissue is visible on
the dorsal wall of the intestine. This fold appears to travel towards the
ventral side ; at any rate a similar but better-marked fold is visible in a
ventro-lateral position at a slightly later period. This fold is the com-
mencement of the fold which in the adult makes a half spiral, and is no doubt
equivalent to the spiral valve of Elasmobranchs and Ganoids. It contains
a prolongation of the cceliac artery, which constitutes at first the vitelline
artery.

The nervous system does not undergo during the early larval period
changes which require a description.

The op-Mii.^ of the olfactory sack becomes narrowed and ciliated (fig. 47,
0/). It is carried by the process already mentioned to the dorsal surface of
the head. The lumen of the sack is well developed ; and lies in contact
with the base of the fore part of the brain.

The vascular system presents no very remarkable features. The heart is
two-chambered and straight. The ventricle is continued forwards as a
bulbus arteriosus, which divides into two arteries at the thyroid body. From
the bulbus and its continuations eight branches are given off to the gills ;
and, as mentioned above, a vessel, probably of the same nature, is given off
in the region of the velum. The blood from the branchial sacks is collected
into the dorsal aorta. Some of it is transmitted to the head, but the greater
part flows backwards under the notochord.

The venous system consists of the usual anterior and posterior cardinal
veins which unite on each side into a ductus Cuvieri, and of a great sub-
intestinal vessel of the same nature as that in embryo Elasmobranchs, which
persists however in the adult. It breaks up into capillaries in the liver, and
constitutes therefore the portal vein. From the liver the blood is brought
by the hepatic vein into the sinus venosus. In addition to these vessels
there is a remarkable unpaired sub-branchial vein, which brings back the
blood directly to the heart from the ventral part of the branchial region.

Metamorphosis. The larva just described does not grow
directly into the adult, but first becomes a larval form, known
as Ammoccetes, which was supposed to be a distinct species till
Aug. Miiller (No. 80) made the brilliant discovery of its nature.

The Ammoccetes does not differ to any marked extent from
the larva just described. The histological elements become more
differentiated, and a few organs reach a fuller development.

The branchial skeleton becomes more developed, and capsules for the
olfactory sack and auditory sacks are established.

B. III. 7

98

METAMORPHOSIS.

The olfactory sack is nearly divided into two by a ventral septum. The
eye (fig. 48) is much more fully developed, but lies a long way below the
surface. The optic cup forms a deep pit, in the mouth of which is placed
the lens. The retinal layers are well developed (cf. Langerhans), and the
outer layer of the optic cup or layer of retinal pigment (rp} contains
numerous pigment granules, especially on its dorsal side. At the edge of
the optic cup the two layers fall into each other. They constitute the com-
mencement of the pigment layer of the iris ; but at this stage they are not
pigmented. The mesoblast of the iris is hardly differentiated. The lens (/)
has the normal structure of the embryonic lens of Vertebrata. The inner
wall is thick and doubly convex, while the outer wall, which will form the
anterior epithelium, is very thin. There is a large space between the lens
and the retina containing the vitreous humour (v.h\ There is no aqueous
humour, and the tissues in front of the lens bear but little resemblance to
those in higher Vertebrata. The cornea is represented by (i) the epidermis
(<?/) ; (2) the dermis (d.c) ; (3) the sub-dermal connective tissue (s.d.c) which
passes without any sharp line of
demarcation into the dermis ; (4)
a thick membrane continuous with
the choroid which represents Des-
cemet's membrane. The sub-der-
mal connective tissue is continued
as an investment round the whole
eye. There is no specially differ-

s.d.c

cfm

entiated sclerotic, and a choroid
is only imperfectly indicated1.
The peculiar features of the eye
of the young larva of the Ammo-
ccetes are probably due to degen-
eration.

In the brain the two cerebral
hemispheres lie one on each side
of the anterior end of the thala-
mencephalon. There are well-
defined olfactory lobes, and two
distinct olfactory nerves are pre-
sent.

The excretory system has
undergone great changes. A series
of segmental tubes, which first
appear in a larva of about 9 mm.,

FIG. 48. EYE OF AN AMMOCCETES LYING
BENEATH THE SKIN.

ep. epidermis ; d.c. dermal connective
tissue continuous with the sub-dermal con-
nective tissue (s.d.c), which is also shaded.
There is no definite boundary to this tissue
where it surrounds the eye.

m. muscles ; dm. membrane of Desce-
met; /. lens; v.h. vitreous humour ; r. retina;
rp. retinal pigment.

1 Langerhans loc. cit. describes the eye of the Ammoccetes in some respects very
differently from the above. Very probably his description applies to an older
Ammoccetes. The most important points of difference appear to be (i) that the
vitreous humour is all but obliterated ; (2) that the iris is much better developed.

CYCLOSTOMATA.

99

becomes established behind the pronephros, and in an Ammoccetes of
65 mm. the pronephros has begun to atrophy. The generative organs are
formed in a larva of about 35 mm. Shortly before the metamorphosis the
portion of the cloaca into which the segmental tubes open becomes separated
off as a distinct urinogenital sinus, the walls of which become perforated by
the two abdominal pores.

The Ammoccetes of Petromyzon Planeri lives in the mud in
streams. Without undergoing any marked changes in structure
it gradually grows larger, and after three or four years undergoes
a metamorphosis. The full-grown larva may be as large or
even larger than the adult. The metamorphosis takes place
from August till January. The breeding season sets in during
the second half of April ; and shortly after depositing its
generative products the Lamprey dies. The changes which
take place in the metamorphosis are of a most striking
kind.

The dome-shaped mouth of the larva is replaced (fig. 47) by
a more definitely suctorial mouth with
horny cuticular teeth (fig. 49). The
eyes appear on the surface ; and the
dorsal fin becomes more prominent,
and is divided into two parts.

Besides these obvious external
changes very great modifications are
effected in almost all the organs, which
may be very briefly enumerated.

1. Very profound changes take
place in the skeleton. An elaborate
system of cartilages is developed in
connection with the mouth ; the
cranium itself undergoes important
modifications; and neural arches be-
come formed.

2. Considerable changes are effected in the gill pouches,
and, according to Schneider, whose statements must however be
received with some caution, the branchial sack becomes detached
posteriorly from the oesophagus, the oesophagus then sends
forwards a prolongation above the branchial sack which is at
first solid. This prolongation forms the anterior part of the

7—2

FIG. 49. MOUTH OF PE-
TROMYZON MARINUS WITH ITS
HORNY TEETH. (From Gegen-
baur; after Heckel and Kner.)

100 METAMORPHOSIS.

oesophagus of the adult, and joins the primitive oral cavity at
the velum. The so-called bronchus of the adult is thus the
whole branchial region of the Ammoccetes, and the anterior
part of the oesophagus of the adult is an entirely new forma-
tion.

3. The posterior part of the alimentary tract of the Ammo-
ccetes undergoes partial atrophy. The gall-bladder of the liver
is absorbed ; and the liver itself ceases to communicate with the
intestine.

4. The eye undergoes important changes in that it travels
to the surface, and acquires all the characters of the normal
vertebrate eye.

5. The brain becomes relatively larger but more compact,
and the optic lobes (corpora bigemina) become more distinct.

6. The pericardial cavity becomes completely separated
from the body cavity, and a distinct pericardium is formed.

7. The mesonephros of the larva disappears, and a fresh
posterior part is formed.

Myxine. The ovum of Myxine when ready to be laid is
inclosed, as shewn by Allen Thomson1, in an oval horny shell in
many respects similar to that of Elasmobranchii ; from its ends
there project a number of trumpet-shaped tubular processes, which
no doubt serve to attach it to marine objects. No observations
have been made on the development.

BIBLIOGRAPHY.

(77) E. Calberla. " Der Befruchtungsvorgang beim Petromyzon Planeri."
Zeit.f. wiss. Zool., Vol. xxx. 1877.

(78) E. Calberla. "Ueb. d. Entwicklung d. Medullarrohres u. d. Chorda
dorsalis d. Teleostier u. d. Petromyzonten." Morpholog. Jahrbuch, Vol. in. 1877.

(79) C. Kupffer u. B. Benecke. Der Vorgang d. Befruchtung am Ei d.
Nennangen. Konigsberg, 1878.

(80) Aug. Miiller. " Ueber die Entwicklung d. Neunaugen." Muller's
Archiv, 1856.

(81) Aug. Miiller. Beobachtungen iib. d. Befruchtungserscheinungen im Ei d.
Neunaugen, Konigsberg, 1864.

1 Cyclopedia of Anat. and Phys. Article 'Ovum.'

CYCLOSTOMATA. IOI

(82) W. Miiller. "Das Urogenitalsystem d. Amphioxus u. d. Cyclostomen."
Jenaische Zeitschrift, Vol. IX. 1875.

(83) Ph. Owsjannikoff. " Die Entwick. von d. Flussneunaugen." Vorlauf.
Mittheilung. Melanges Biologiques tires du Bulletin de VAcad. Imp. St Petersbourg,
Vol. vn. 1870.

(84) Ph. Owsjannikoff. On the development of Petromyzon fluviatilis
(Russian).

(85) Anton Schneider. Beitrdge z. vergleich. Anat. u. Entwick. d. Wirbel-
thiere. Quarto. Berlin, 1879.

(86) M. S. Schultze. "Die Entwickl. v. Petromyzon Planeri." Gekronte
Preisschrift. Haarlem, 1856.

(87) \V. B. Scott. " Vorlaufige Mittheilung lib. d. Entwicklungsgeschichte d.
Petromyzonten." Zoologischer Anzeiger, Nos. 63 and 64. ill. Jahrg. 1880.

CHAPTER VI.

GANOIDEI1.

IT is only within quite recent times that any investigations
have been made on the embryology of this heterogeneous, but
primitive group of fishes. Much still remains to be done, but we
now know the main outlines of the development of Acipenser
and Lepidosteus, which are representatives of the two important
sub-divisions of the Ganoids. Both types have a complete seg-
mentation, but Lepidosteus presents in its development some
striking approximations to the Teleostei. I have placed at the
end of the chapter a few remarks with reference to the affinities
indicated by the embryology.

ACIPENSER 2.

The freshly laid ovum is 2 mm. in diameter and is invested
by a two-layered shell, covered by a cellular layer derived from
the follicle3. The segmentation, though complete, approaches

The following classification of the Ganoidei is employed in the present chapter :

_ , , ., .

I. Selachoidei.

(Acipenseridse.

(Poiyodontidse. II. Teleostoidei.

Polypteridse.
Amiidas.

LepidosteicUe.

a Our knowledge of the development of Acipenser is in the main derived from
Salensky's valuable observations. His full memoir is unfortunately published in
Russian, and I have been obliged to satisfy myself with the abstract (No. 90), and
with what could be gathered from his plates. Prof. Salensky very kindly supplied me
with some embryos ; and I have therefore been able to some extent to work over the
subject myself. This is more especially true for the stages after hatching. The
embryos of the earlier stages were not sufficiently well preserved for me to observe
more than the external features and a few points with reference to the formation of the
layers.

3 Seven micropylar apertures, six of which form a circle round the seventh, are
stated by Kowalevsky, Wagner, and Owsjannikoff (No. 89) to be present at one of
the poles of the inner egg membrane. They are stated by Salensky to vary in number
from five to thirteen.

GANOIDEI.

103

the meroblastic type more nearly than the segmentation of the
frog's egg. The first furrow appears at the formative pole, at
which the germinal vesicle was situated. The earlier phases of
the segmentation are like those of meroblastic ova, in that the
furrows only penetrate for a certain distance into the egg. Eight
vertical furrows appear before the first equatorial furrow ; which
is somewhat irregular, and situated close to the formative pole.

In the later stages the vertical furrows extend through the
whole egg, and a segmentation cavity appears between the small
and the large spheres. The segmentation is thus in the main

JFb

FIG. 50. EMBRYOS OF ACIPENSER VIEWED FROM THE DORSAL SURFACE.
(After Salensky.)

A. Stage before the appearance of the mesoblastic somites.

B. Stage with five somites.

Mg. medullary groove; bl.p. blastopore ; s.d. segmental duct; Fb. fore- brain;
Hb. hind-brain; m.s. mesoblastic somite.

similar to that of a frog, from which it diverges in the fact that
there is a greater difference in size between the small and the
large segments.

In the final stages of the segmentation the cells become
distinctly divided into two layers. A layer of small cells is
placed at the formative pole, and constitutes the epiblast. The
cells composing it are divided, like those of Teleostei, etc., into a
superficial epidermic and a deeper nervous layer. The remaining
cells constitute the primitive hypoblast (the eventual hypoblast
and mesoblast) ; they form a great mass of yolk-cells at the
lower pole, and also spread along the roof of the segmentation
cavity, on the inner side of the epiblast.

A process of unsymmetrical invagination now takes place,
which is in its essential features exactly similar to that in the

104 ACIPENSER.

frog or the lamprey, and I must refer the reader for the details
of the process to the chapter on the Amphibia. The edge of the
cap of epiblast forms an equatorial line. For the greater extent
of this line the epiblast cells grow over the hypoblast, as in an
epibolic gastrula, but for a small arc they are inflected. At the
inflected edge an invagination of cells takes place, underneath
the epiblast, towards the segmentation cavity, and gives rise to
the dorsal wall of the mesenteron and the main part of the
dorsal mesoblast. The slit below the invaginated layer gradually
dilates to form the alimentary cavity ; the ventral wall of which
is at first formed of yolk-cells. The epiblast along the line of the
invaginated cells soon becomes thickened, and forms a medullary
plate, which is not very distinct in surface views. The cephalic
extremity of this plate, which is furthest removed from the edge,
dilates, and the medullary plate then assumes a spatula form
(fig. 50 A, Mg\

By the continued extension of the epiblast the uncovered
part of the hypoblast has in the meantime become reduced to a
small circular pore — the blastopore — and in surface views of the
embryo has the form represented in fig. 50 A, bl.p. The invagi-
nation of the mesenteron has in the meantime extended very far
forwards, and the segmentation cavity has become obliterated.
The lip of the blastopore has moreover become inflected for its
whole circumference.

The invaginated cells forming the dorsal wall of the mesen-
teron soon become divided into a pigmented hypoblastic epithe-
lium adjoining the lumen of the mesenteron (fig. 51, En) and a
mesoblastic layer (Sgp], between the hypoblast and the epiblast.
The mesoblastis divided into two plates, between which is placed
the notochord1 (Cli).

With the completion of the medullary plate and the germinal
layers, the first embryonic period may be considered to come to a
close. The second period ends with the hatching of the embryo.
During it the rudiments of the greater number of organs make
their appearance. The general form of the embryo during this
period is shewn in figs. 50 B and 52 A and B.

One of the first changes to take place is the conversion of the

1 Salensky believes that the notochord is derived from the mesoblast. I could
not satisfy myself on this point.

GANOIDEI. 105

medullary plate into the medullary canal. This, as shewn in fig.
51, is effected in the usual vertebrate fashion, by the establish-
ment of a medullary groove which is then converted into a closed
canal by the folding over of the sides.

The uncovered patch of yolk in the blastoporic area soon
becomes closed over ; and on the formation of the medullary
canal the usual neurenteric canal becomes established.

The further changes which take place are in the main similar
to those in other Ichthyopsida, but in some ways the appearance

FIG. 51. TRANSVERSE SECTION THROUGH THE ANTERIOR PART OF AN ACIPENSER

EMBRYO. (After Salensky.)

Rf. medullary groove; Mp. medullary plate; Wg. segmental duct; Ch. notochord;
En. hypoblast; Sgp. mesoblastic somite; Sp. parietal part of mesoblastic plate.

of the embryo is, as may be gathered from fig. 52, rather strange.
This is mainly due to the fact that the embryo does not become
folded off from the yolk in the manner usual in Vertebrates ; and
as will be shewn in the sequel, the relation of the yolk to the
embryo is unlike that in any other known Vertebrate. The
appearance of the embryo is something like that of an ordinary
embryo slit open along the ventral side and then flattened out.
Organs which properly belong to the ventral side appear on the
lateral parts of the dorsal surface. Owing to the great forward
extension of the yolk the heart (fig. 52 B) appears to be placed
directly in front of the head.

Even before the formation of the medullary canal the cephalic
portion of the nervous system becomes marked out. This part,
after the closure of the medullary groove, becomes divided into
two (fig. 50 B), and then three lobes — the fore-, the mid-, and the
hind-brain (fig. 52, A and B). From the lateral parts of the at
first undivided fore-brain the optic vesicles (fig. 52 B, Op} soon
sprout out ; and in the hind-brain a dilatation to form the fourth
ventricle appears in the usual fashion.

loo

AC1PENSKK.

The epiblast at the sides of the brain constitutes a more or
less well-defined structure, which may be spoken of as a cephalic
plate (fig. 52 A, cp~). From this plate are formed the essential
parts of the organs of special sense. Anteriorly the olfactory pits
arise (fig. 52 B, Olp] as invaginations of both layers of the

FIG. 52. EMBRYOS OF ACIPENSER BELONGING TO TWO STAGES VIEWED FROM THE
DORSAL SURFACE. (After Salensky.)

Fb. fore-brain; Mb. mid-brain; Hb. hind-brain; cp. cephalic plate; Op. optic
vesicle; Auv. auditory vesicle; Olp. olfactory pit ; Ht. heart; Md. mandibular arch;
Ha. hyoid arch ; Br1. first branchial arch ; Sd. segmental duct.

epiblast. The lens of the eye is formed as an ingrowth of the
nervous layer only, and opposite the hind-brain the auditory sack
(fig. 52 A and B, Auv} is similarly formed from the nervous
layer of the epiblast. At the sides of the cephalic plate the
visceral arches make their appearance; and in fig. 52 A and B
there are shewn the mandibular (Md}, hyoid (Ha) and first
branchial (Br'} arches, with the hyomandibular (spiracle) and
hyobranchial clefts between them. They constitute peculiar
concentric circles round the cephalic plate ; their shape being
due to the flattened form of the embryo, already alluded to.

While the above structures are being formed in the head the
changes in the trunk have also been considerable. The meso-
blastic plates at the junction of the head and trunk become very
early segmented, the segments being formed from before back-
wards (fig. 50 B). With their formation the trunk rapidly
increases in length. At their outer border the segmental duct
(fig. 50 B, and fig. 52 A, Sd} is very early established. It is
formed, as in Elasmobranchs, as a solid outgrowth of the meso-
blast (fig. 5 1, Wg) ; but its anterior extremity becomes converted
into a pronephros (fig. 57, pr.n}.

GANOIDEI.

ID/

Before hatching, the embryo has to a small extent become
folded off from the yolk both anteriorly and posteriorly ; and has
also become to some extent vertically compressed. As a result
of these changes, the general form of its body becomes much
more like that of an ordinary Teleostean embryo.

The general features of the larva after hatching are illustrated
by figs. 53, 54 and 55. Fig. 53 represents a larva of about 7 mm.
and fig. 54 a lateral and fig. 55 a ventral view of the head of a
larva of about 1 1 mm.

There are only a few points which call for special attention in
the general form of the body. In the youngest larva figured the
ventral part of the hyomandibular cleft is already closed : the
dorsal part of the cleft is destined to form the spiracle (sp). The
arch behind is the hyoid : on its posterior border is a mem-
branous outgrowth, which will develop into the operculum. In

FIG. 53. LARVA OF ACIPENSER OF 7 MM., SHORTLY AFTER HATCHING.
ol. olfactory pit ; op. optic vesicle ; sp. spiracle ; br.c. branchial clefts ; an. anus.

older larvae, a very rudimentary gill appears to be developed on
the front walls of the spiracular cleft (Parker), but I have not
succeeded in satisfying myself about its presence ; and rows of
gill papillae appear on the hyoid and the true branchial arches
(figs. 54 and 55, g). The biserially-arranged gill papillae of the
true branchial arches are of considerable length, and are not at
first covered by the operculum ; but they do not form elongated
thread-like external gills similar to those of the Elasmobranchii.
The oral cavity is placed on the ventral side of the head; it
has at first a more or less rhomboidal form. It soon however
(fig- 55) becomes narrowed to a slit with projecting lips, and
eventually becomes converted into the suctorial mouth of the
adult. The most remarkable feature connected with the mouth
is the development of provisional teeth (fig. 55) on both jaws.

io8

ACIPENSER.

These teeth were first discovered by Knock (No. 88). They do not
appear to be calcified, and might be supposed to be of the same nature as
the horny teeth of the Lamprey. They are however developed like true
teeth, as a deposit between a papilla of subepidermic tissue and an
epidermic cap. The substance of which they are formed corresponds
morphologically to the enamel of ordinary teeth. As they grow they pierce
the epidermis, and form hollow spine-like structures with a central axis
filled with subepidermic (mesoblastic) cells. They disappear after the third
month of larval life.

In front of the mouth two pairs of papillae grow out, which

appear to be of the same _-_-— _^_-^ -== — ^^i--- cp

nature as the papillae on
the suctorial disc in the
embryo of Lepidosteus
(wVfe p. 115). They are
very short in the embryo
represented in fig. 53;
soon however they grow
in length (figs. 54 and
55, st} ; and it is pro-
bable that they become

ol

FIG. 54. SIDE VIEW OF A LARVA OF ACIPEN-
SER OF II MILLIMETRES.

op. eye ; ol. olfactory pit ; st. suctorial (?) pro-
cesses ; m. mouth ; sp. spiracle ; g. gills.

the barbels, since these occupy a precisely similar position *.

The openings of the nasal pits are at first single ; but the
opening of each becomes
gradually divided into
two by the growth of a
flap on the outer side
(fig. 54, ol}. It is prob-
able that this flap is
equivalent to the fold of
the superior maxillary
process of the Amniota,
which by its growth roofs
over the open groove
which originally leads from the external to the internal nares ;
so that the two openings of each nasal sack, so established in
these and in other fishes, correspond to the external and
internal nares of higher Vertebrata.

1 If these identifications are correct the barbels of fishes must be phylogenetically
derived from the papilla? of a suctorial disc adjoining the mouth.

FIG. 55. VENTRAL VIEW OF A LARVA OF
ACIPENSER OF n MILLIMETRES.

in. mouth; st. suctorial (?) processes; <^>.'eye;
g. gills.

GANOIDEI.

109

At the time of hatching there is a continuous dorso-ventral
fin, which, by atrophy in some parts, and hypertrophy in other
parts, gives rise to all the unpaired fins of the adult, except the
first dorsal and the abdominal. The caudal part of the fin is at
first symmetrical, and the heterocercal tail is produced by the
special growth of the ventral part of the fin.

Of the internal features of development in the Sturgeon the most
important concern the relation of the yolk to the alimentary tract. In
most Vertebrata the yolk-cells form a protuberance of the part of the
alimentary canal, immediately behind the duodenum. The yolk may
either, as in the lamprey or frog, form a simple thickening of the alimentary
wall in this region, or it may constitute a well-developed yolk-sack as in
Elasmobranchii and the Amniota. In either case the liver is placed in
front of the yolk. In the Sturgeon on the contrary the yolk is placed
almost entirely in front of the liver, and the Sturgeon appears to be also
peculiar in that the yolk, instead of constituting an appendage of the

FIG. 56. DIAGRAMMATIC LONGITUDINAL SECTION THROUGH THE ANTERIOR
PART OF THE TRUNK OF A LARVA OF ACIPENSER TO SHEW THE POSITION OCCUPIED
BY THE YOLK.

in. intestine; st. stomach filled with yolk; as. oesophagus; /. liver; ht. heart;
ch. notochord ; sp.c. spinal cord.

alimentary tract, is completely enclosed in a dilated portion of the tract
which becomes the stomach (figs. 56 and 57). It dilates this portion
to such extent that it might be supposed to form a true external yolk-sack.
In the stages before hatching the glandular hypoblast, which was estab-
lished on the dorsal side of the primitive mesenteron, envelops the yolk-
cells, which fuse together into a yolk-mass, and lose all trace of their
original cellular structure.

The peculiar flattening out of the embryo over the yolk (vide p. 105)
is no doubt connected with the mode in which the yolk becomes enveloped
by the hypoblast.

no

ACIPENSER.

As the posterior part of the
trunk, containing the intestine,
becomes formed, the yolk is
gradually confined to the an-
terior part of the alimentary
tract, which, as before stated,
becomes the stomach. The
epithelial cells of the stomach,
as well as those of the intestine,
are enormously dilated with
food-yolk (fig. 57, sf). Behind
the stomach is formed the liver.
The subintestinal vein bring-
ing back the blood to the liver
appears to have the same course
as in Teleostei, in that the
blood, after passing through
the liver, is distributed to the
walls of the stomach and is
again collected into a venous
trunk which falls into the sinus
venosus. As the yolk becomes
absorbed, the liver grows for-
wards underneath the stomach
till it comes in close contact
with the heart. The relative
position of the parts at this
stage is shewn diagrammati-
cally in fig. 56. At the com-

Kf.C

ch —

pr.n

ft.

FIG. 57. TRANSVERSE SECTION THROUGH
THE REGION OF THE STOMACH OF A LARVA OF
ACIPENSER 5 MM. IN LENGTH.

it. epithelium of stomach ; yk. yolk ; ch.
notochord, below which is a subnotochordal
rod; pr.n. pronephros; ao. aorta; nip, muscle-
plate formed of large cells, the outer parts of
which are differentiated into contractile fibres ;
sp.c. spinal cord ; b.c. body cavity.

mencement of the intestine there arises in the larva of about 14 mm. a
great number of diverticula, which are destined to form the compact
glandular organ, which opens at this spot in the adult At this stage
there is also a fairly well developed pancreas opening into the duodenum
at the same level as the liver.

No trace of the air-bladder was present at the stage in question.

The spiral valve is formed, as in Elasmobranchii, as a simple fold in the
wall of the intestine.

There is a well developed subnotochordal rod (fig. 57) which, according
to Salensky, becomes the subvertebral ligament of the adult ; a statement
which confirms an earlier suggestion of Bridge. The pronephros (head-
kidney) resembles in the main that of Teleostei (fig. 57) ; while the front
end of the mesonephros, which is developed considerably later than the
pronephros, is placed some way behind it. In my oldest larva (14 mm.)
the mesonephros did not extend backwards into the posterior part of the
abdominal cavity.

GANOIDEI.

Ill

BIBLIOGRAPHY.

(88) Knock. "Die Beschr. d. Reise z. Wolga Behufs d. Sterlettbefruchtung. "
/>'////. Sac. Nat. Moscow, 1871.

(89) A. Kowalevsky, Ph. Owsjannikoff, and N. Wagner. "Die Entwick.
d. Store." Vorlauf. Mittheilung. Melanges Biologiques tire's du Bulletin d. PAcad.
Imp. St Petersbourg, Vol. vil. 1870.

(90) W.Salensky. "Development of the Sterlet (Acipenser ruthenus)." 2 Parts.
Proceedings of the Society of Naturalists in the imperial University of Kas an. 1878 and 9
(Russian). Part I., abstracted in Hoffmann and Schwalbe's Jahresbericht for 1878.

(91) W. Salensky. "Zur Embryologie d. Ganoiden (Acipenser)." Zoolo-
gischer Anzeiger, Vol. I. , Nos. u, 12, 13.

LEPIDOSTEUS1.

The ova of Lepidosteus are spherical bodies of about 3 mm.
in diameter. They are invested by a tough double membrane,
composed of (i) an outer
layer of somewhat pyriform
bodies, radiately arranged,
which appear to be the re-
mains of the follicular cells ;
and (2) of an inner zona radi-
ata, the outer part of which
is radiately striated, while the
inner part is homogeneous.

The segmentation, as in
the Sturgeon, is complete,
but approaches closely the
meroblastic type. It com-
mences with a vertical furrow
at the animal pole, extending

FIG. 58. SURFACE VIEW OF THE OVUM
OF LEPIDOSTEUS WITH THE MEMBRANES
REMOVED ON THE THIRD DAY AFTER IM-
PREGNATION.

through about one-fifth of the circumference. Before this furrow
has proceeded further a second furrow is formed at right angles

1 Alexander Agassiz was fortunate enough to succeed in procuring and rearing
a batch of eggs of this interesting form. He has given an adequate account of the
external characters of the post-embiyonic stages, and very liberally placed his
preserved material of the stages both before and after hatching at Prof. W. K. Parker's
and my disposal. The account of the stages prior to hatching is the result of
investigations carried on by Professor Parker's son, Mr W. N. Parker, and myself on
the material supplied to us by Agassiz. This material was not very satisfactorily
preserved, but I trust thar our results are not without some interest.

112

LEPIDOSTEUS.

to it. The next stages have not been observed, but on the third
day after impregnation (fig. 58), the animal pole is completely
divided into small segments, which form a disc similar to the
blastoderm of meroblastic ova ; while the vegetative pole, which
subsequently forms a large yolk-sack, is divided by a few
vertical furrows, four of which nearly meet at the pole opposite
the blastoderm. The majority of the vertical furrows extend
only a short way from the edge of the small spheres, and are
partially intercepted by imperfect equatorial furrows.

The stages immediately following the segmentation are still
unknown, and in the next stage satisfactorily observed, on the
fifth day after impregnation, the body of the embryo is distinctly
differentiated. The lower pole of the ovum is then formed of a

mass in which no traces of

j>

segments or segmentation fur- ;

rows can be detected.

The embryo (fig. 59) has
a dumbbell-shaped outline,
and is composed of (i) an
outer area, with some resem-
blance to the area pellucida
of an avian embryo, forming
the lateral part of the body ;
and (2) a central portion con-
sisting of the vertebral plates
and medullary plate. The
medullary plate is dilated in
front to form the brain (br).
Two lateral swellings in the
brain are the commencing
optic vesicles. The caudal
extremity of the embryo is somewhat swollen.

Sections of this stage (fig. 60) are interesting as shewing
a remarkable resemblance between Lepidosteus and Teleostei.

The three layers are fully established. The epiblast (ep} is
formed of a thicker inner nervous stratum, and an outer flat-
tened epidermic stratum. Along the axial line there is a solid
keel-like thickening of the nervous layer of the epidermis, which
projects towards the hypoblast. This thickening (MC) is the

FIG. 59. SURFACE VIEW OF A LEPI-
DOSTEUS EMBRYO ON THE FIFTH DAI-
AFTER IMPREGNATION.

br. dilated extremity of medullary plate
which forms the nidiment of the brain.

GANOIDEI. 113

medullary cord ; and there is no evidence of the epidermic layer
being at this or any subsequent period concerned in its form-
ation (vide chapter on Teleostei, p. 72). In the region of the
brain the medullary cord is so thick that it gives rise, as in
Teleostei, to a projection of the whole body of the embryo
towards the yolk. Posteriorly it is flatter. The mesoblast (Me)
in the trunk has the form of two plates, which thin out laterally.
The hypoblast (Jiy) is a single layer of cells, and is nowhere
folded in to form a closed alimentary canal. The hypoblast is
separated from the neural cord by the notochord (Ch], which
throughout the greater part of the embryo is a distinct structure.
In the region of the tail, the axial part of the hypoblast, the
notochord, and the neural cord fuse together, the fused part so

we.

FIG. 60. SECTION THROUGH AN EMBRYO OF LEPIDOSTEUS ON THE FIFTH DAY

AFTER IMPREGNATION.
MC. medullary cord; Ep. epiblast; Me. mesoblast; hy. hypoblast; Ch. notochord.

formed is the homologue of the neurenteric canal of other types.
Quite at the hinder end of the embryo the mesoblastic plates
cease to be separable from the axial structures between them.

In a somewhat later stage the embryo is considerably more
elongated, embracing half the circumference of the ovum. The
brain is divided into three distinct vesicles.

Anteriorly the neural cord has now become separated from
the epidermis. The whole of the thickened nervous layer of
the epiblast appears to remain united with the cerebro-spinal
cord, so that the latter organ is covered dorsally by the epider-
mic layer of the epiblast only. The nervous layer soon however
grows in again from the two sides.

Where the neural cord is separated from the epidermis, it is

15. TIL 8

114

LEPIDOSTEUS.

already provided with a well-developed lumen. Posteriorly it
remains in its earlier condition.

In the region of the hind-brain traces of the auditory vesicles
are present in the form of slightly involuted thickenings of the
nervous layer of the
epidermis.

The mesoblast of
the trunk is divided
anteriorly into splanch-
nic and somatic layers.

In the next stage,
on the sixth day after
impregnation (fig. 61),
there is a great advance
in development. The
embryo is considerably
longer, and a great num-
ber of mesoblastic so-
mites are visible. The
body is now laterally
compressed and raised
from the yolk.

The region of the head is more distinct, and laterally two
streaks are visible (br.c\, me

which, by comparison with
the Sturgeon, would seem to
be the two first visceral clefts1 :
they are not yet perforated.
In the lateral regions of the
trunk the two segmental ducts
are visible in surface views
(fig. 61, sd] occupying the
same situation as in the Stur-
geon. Their position in sec-
tion is shewn in fig. 62, sg.
With reference to the features

fc.t

FIG. 61. EMBRYO OF LEPIDOSTEUS ON THE
SIXTH DAY AFTER IMPREGNATION.

op. optic vesicles ; br.c. branchial clefts (?) ; s.d.
segmental duct.

N.B. The branchial clefts and segmental duct
are somewhat too prominent.

TOS

in development, visible in sections,
a few points may be alluded to.

FIG. 61. SECTION THROUGH THE TRUNK
OF A LEPIDOSTEUS EMBRYO ON THE SIXTH
DAY AFTER IMPREGNATION.

me. medullary cord ; ms. mesoblast ; sg.
segmental duct ; ch. notochord ; x. sub-noto-
chordal rod ; hy. hypoblast.

1 I have as yet been unable to make out these structures in section.

GANOIDEI.

The optic vesicles are very prominent outgrowths of the brain, but are
still solid, though the anterior cerebral vesicle has a well-developed lumen.
The auditory vesicles are now deep pits of the nervous layer of the
epiblast, the openings of which are covered by the epidermic layer. They
are shewn for a slightly later
stage in fig. 63 (au.v.}.

There is now present a sub-
notochordal rod, which develops
as in other types from a thick-
ening of the hypoblast (fig.
62, *•).

In an embryo of the
seventh day after impreg-
nation, the features of the
preceding stage become
generallymore pronounced.

FIG. 63. SECTION THROUGH THE HEAD
OF A LEPIDOSTEUS EMBRYO ON THE SIXTH
DAY AFTER IMPREGNATION.

au.v. auditory vesicle ; au.n. auditory
nerve ; ch. notochord ; hy. hypoblast.

fb

op

The optic vesicles are now
provided with a lumen (fig. 64), and have approached close to the epidermis.
Adjoining them a thickening (/) of the nervous layer of the epidermis has
appeared, which will form the lens.
The cephalic extremity of the
segmental duct, which, as shewn
in fig. 6 1, is bent inwards towards
the middle line, has now become
slightly convoluted, and forms the
rudiment of a pronephros (head-
kidney).

During the next few days
the folding off of the embryo
from the yolk commences,
and proceeds till the embryo
acquires the form represented
in fig. 65.

Both the head and tail
are quite free from the yolk ;
and the embryo presents a
general resemblance to that
of a Teleostean.

On the ventral surface of
the front of the head there is a disc (figs. 65, 66, sd), which is

8—2

FIG. 64. SECTION THROUGH THE FRONT
PART OF THE HEAD OF A LEPIDOSTEUS
EMBRYO ON THE SEVENTH DAY AFTER
IMPREGNATION.

al. alimentary tract ; fb. thalamencepha-
lon; /. lens of eye; op.v. optic vesicle. The
mesoblast is not represented.

u6

LEPIDOSTEUS.

beset with a number of processes, formed as thickenings of the
cpiblast. As shewn by Agassiz, these eventually become short
suctorial papiHae1. Immediately behind this disc is placed a
narrow depression which forms the rudiment of the mouth.

The olfactory pits are now developed, and are placed near
the front of the head.

A great advance has taken place in the development of the
visceral clefts and arches. The oral region is bounded behind
by a well-marked mandibular arch, which is separated by a
shallow depression from a still more prominent hyoid arch
(fig. 65, hy). Between the hyoid and mandibular arches a
double lamella of hypoblast, which represents the hyomandibu-
lar cleft, is continued from the throat to the external skin,
but does not, at this stage at any rate, contain a lumen.

The hyoid arch is prolonged backwards into a considerable
opercular fold, which to a great extent overshadows the branchial
clefts behind. The hyobranchial cleft is widely open.

Behind the hyobranchial cleft are four pouches of the throat
on each side, not yet open to the exterior. They are the
rudiments of the four branchial clefts of the adult.

The trunk has the usual compressed piscine form, and there
is a well-developed dorsal fin continuous round the end of the
tail, with a ventral fin. There is no trace of the paired fins.

The anterior and
posterior portions of
the alimentary tract ol
are closed in, but the s<1
middle region is still
open to the yolk.
The circulation is now
fully established, and
the vessels present
the usual vertebrate
arrangement. There
is a large subintesti- FIG. 65. EMBRYO OF LEPIDOSTEUS SHORTLY

, . BEFORE HATCHING.

ol. olfactory pit ; sd. suctorial disc ; hy. hyoid arch.

1 These papillae are very probably sensitive structures ; but I have not yet investi-
gated their histological characters.

GANQIDEI.

117

The first of Agassiz' embryos was hatched about ten days
after impregnation. The young fish on hatching immediately
used its suctorial disc to attach itself to the sides of the vessel in
which it was placed.

The general form of
Lepidosteus shortly after
hatching is shewn in fig.
67. On the ventral part
of the front of the head
is placed the large sucto-
rial disc. At the side of
the head are seen the
olfactory pit, the eye and
the auditory vesicle; while

the projecting vesicle of

op
the mid-brain is very pro-

FIG. 66. VENTRAL VIEW OF THE HEAD OF
A LEPIDOSTEUS EMBRYO SHORTLY BEFORE
HATCHING, TO SHEW THE LARGE SUCTORIAL

DISC.

m. mouth; op. eye; s.d. suctorial disc.

— -sd

minent above. Behind
the mouth follow the vis-
ceral arches. The man-
dibular arch (ind] is
placed on the hinder border of the mouth, and is separated by a
deep groove from the hyoid arch (hy}. This groove is connected
with the hyomandibular cleft, but I have not determined whether

FIG. 67. LARVA OF LEPIDOSTEUS SHORTLY AFTER HATCHING. (After Parker.)
ol. olfactory pit ; op. optic vesicle ; au.v. auditory vesicle ; mb. mid-brain ;
sd. suctorial disc; md. mandibular arch ; hy. hyoid arch with pperculum ; br. branchial
arches; an. anus.

it is now perforated. The posterior border of the hyoid arch is
prolonged into an opercular fold. Behind the hyoid arch are
seen the true branchial arches.

Il8 LEPIDOSTEUS.

There is still a continuous dorso-ventral fin, in which there
are as yet no fin-rays, and the anterior paired fins are present.

The yolk-sack is very large, but its communication with the
alimentary canal is confined to a narrow vitelline duct, which
opens into the commencement of the intestine immediately
behind the duct of the liver, which is now a compact gland. The
yolk in Lepidosteus thus behaves very differently from that in
the Sturgeon. In the first place it forms a special external
yolk-sack, instead of an internal dilatation of part of the
alimentary tract ; and in the second place it is placed behind
instead of in front of the liver.

I failed to find any trace of a pancreas. There is however,
opening' on the dorsal side of the throat, a well-developed append-
age continued backwards beyond the level of the commencement
of the intestine. This appendage is no doubt the air-bladder.

In the course of the further growth of the young Lepidosteus,
the yolk-sack is rapidly absorbed, and has all but disappeared
after three weeks. A rich development of pigment early takes
place; and the pigment is specially deposited on the parts of
the embryonic fin which will develop into the permanent fins.

The notochord in the tail bends slightly upwards, and by the
special development of a caudal lobe an externally heterocercal
tail like that of Acipenser is established. The ventral paired
fins are first visible after about the end of the third week, and by
this time the operculum has grown considerably, and the gills
have become well developed.

The most remarkable changes in the later periods are those
of the mouth.

The upper and lower
jaws become gradually
prolonged, till they event-
ually form a snout ; while
at the end of the upper

jaw is placed the sucto- fIG- 68- HEAD, °? AI1 ADVANCED LARVA
.... . . OF LEPIDOSTEUS. (After Parker.)

rial disc, which is now COn- oL openings of the olfactory pit ; sd. remains

siderably reduced in size of the larval suctorial disc.
(fig. 68, sd}. The " fleshy globular termination of the upper jaw
of the adult Lepidosteus is the remnant of this embryonic
sucking disc." (Agassiz, No. 92.)

GANOIDEI. 119

The fin-rays become formed as in Teleostei, and parts of
the continuous embryonic fin gradually undergo atrophy. The
dorsal limb of the embryonic tail, as has been shewn by Wilder,
is absorbed in precisely the same manner as in Teleostei, leaving
the ventral lobe to form the whole of the permanent tail-fin.

BIBLIOGRAPHY.

(92) Al. Agassiz. "The development of Lepidosteus." Proc. Amer. Acad. of
Arts and Sciences, Vol. xm. 1878.

General observations on the Embryology of the Ganoids.

The very heterogeneous character of the Ganoid group is clearly shewn
both in its embryology and its anatomy. The two known types of formation
of the central nervous system are exemplified in the two species which have
been studied, and these two species, though in accord in having a holoblastic
segmentation, yet differ in other important features of development, such as
the position of the yolk etc. Both types exhibit Teleostean affinities in the
character of the pronephros ; but as might have been anticipated Lepidosteus
presents in the origin of the nervous system, the relations of the hypoblast,
and other characters, closer approximations to the Teleostei than does
Acipenser. There are no very prominent Amphibian characters in the
development of either type, other than a general similarity in the segmenta-
tion and formation of the layers. In the young of Polypterus an interesting
amphibian and dipnoid character is found in the presence of a pair of true
external gills covered by epiblast. These gills are attached at the hinder
end of the operculum, and receive their blood from the hyoid arterial arch \ In
the peculiar suctorial disc of Lepidosteus, and in the more or less similar struc-
ture in the Sturgeon, these fishes retain, I believe, a very primitive vertebrate
organ, which has disappeared in the adult state of almost all the Vertebrata ;
but it is probable that further investigations will shew that the Teleostei, and
especially the Siluroids, are not without traces of a similar structure.

1 Vide Steindachner, Polypterus Lapradei, &c., and HyrtI, " Ueber d. Blutgefasse,
&c." Sitz. Wiener Akad., Vol. LX.

CHAPTER VII.

AMPHIBIA1.

THE eggs of most Amphibia2 are laid in water. They are
smallish nearly spherical bodies, and in the majority of known
Anura (all the European species), and in many Urodela (Am-
blystoma, Axolotl, though not in the common Newt) part of the
surface is dark or black, owing to the presence of a superficial
layer of pigment, while the remainder is unpigmented. The pig-
mented part is at the upper pole of the egg, and contains the
•germinal vesicle till the time of its atrophy ; and the yolk-
granules in it are smaller than those in the unpigmented part.
The ovum is closely surrounded by a vitelline membrane3, and
receives, in its passage down the oviduct, a gelatinous investment
of varying structure.

In the Anura the eggs are fertilized as they leave the oviduct.
In some of the Urodela the mode of fertilization is still imperfectly
understood. In Salamanders and probably Newts it is internal4;

1 The following classification of the Amphibia is employed in the present chapter:
fAGLOSSA.

I. Anura. {PHANEROGLOSSA.

( Trachystomata.
PERENNIBRANCHIATA \ Proteidce-

ii. Urodela.

CADUCIBRANCHIATA /AmphiumiiUv.

{Menopomidre.

( Amblystomidae .
MYCTODERA <,, , , .,

^Salamandndse.

III. Gymnophiona.

2 I am under great obligations to Mr Parker for having kindly supplied me, in
answer to my questions, with a large amount of valuable information on the develop-
ment of the Amphibia.

3 Within the vitelline membrane there appears to be present, in the Anura at any
rate, a very delicate membrane closely applied to the yolk.

4 Allen Thomson informs me that he has watched the process of fertilization in
the Newt, and that the male deposits the semen in the water close to the female.
From the water it seems to enter the female generative aperture. Von Siebold has
shewn that there is present in female Newts and Salamanders a spermatic bursa. In
this bursa the spermatozoa long (three months) retain their vitality in some Sala-
manders. Various peculiarities in the gestation are to be explained by this fact.

AMPHIBIA. 121

but in Amblystoma punctatum (Clark, No. 98), the male deposits
the semen in the water. The eggs are laid by the Anura in
masses or strings. By Newts they are deposited singly in the
angle of a bent blade of grass or leaf of a water-plant, and by
Amblystoma punctatum in masses containing from four eggs to
two hundred. Salamandra atra and Salamandra maculosa are
viviparous. The period of gestation for the latter species lasts a
whole year.

A good many exceptions to the above general statements have been
recorded1.

In Notodelphis ovipara the eggs are transported (by the male?) into a
peculiar dorsal pouch of the skin of the female, which has an anterior
opening, but is continued backwards into a pair of diverticula. The eggs
are very large, and in this pouch, which they enormously distend, they under-
go their development. A more or less similar pouch is found in Nototrema
marsupiatum.

In the Surinam toad (Pipa dorsigera) the eggs are placed by the male on
the back of the female. A peculiar pocket of skin becomes developed round
each egg, the open end of which is covered by a gelatinous operculum. The
larvae are hatched, and actually undergo their metamorphosis, in these
pockets. The female during this period lives in water. Pipa Americana (if
specifically distinct from P. dorsigera) presents nearly the same peculiarities.
The female of a tree frog of Ceylon (Polypedates reticulatus) carries the eggs
attached to the abdomen.

Rhinoderma Darwinii2 behaves like some of the Siluroid fishes, in that
the male carries the eggs during their development in an enormously
developed laryngeal pouch.

Some Anura do not lay their eggs in water. Chiromantis Guineensis
attaches them to the leaves of trees ; and Cystignathus mystacius lays them
in holes near ponds, which may become filled with water after heavy rains.

The eggs of Hylodes Martinicensis are laid under dead leaves in moist
situations.

Formation of the layers.

Anura. The formation of the germinal layers has so far
only been studied in some Anura and in the Newt. The
following description applies to the Anura, and I have called

1 For a summary of these and the literature of the subject vide "Amphibia," by
C. K. Hoffmann, in Bronn's Classen ^^nd Ordnungen d. Thier-reichs.

2 Vide Spengel, " Die Fortpflanzung des Rhinoderma Darwinii." Zeit. f. wiss.
Zool., Bd. XXIX., 1877. This paper contains a translation of a note by Jiminez de la
Espada on the development of the species.

122

FORMATION OF THE LAYERS.

attention, at the end of the section, to the points in which the
Newt is peculiar.

The segmentation of the Frog's ovum has already been
described (Vol. II. pp. 95-7), but I may remind the reader that the
segmentation (fig. 69) results in the formation of a vesicle, the
cavity of which is situated excentrically; the roof of the cavity
being much thinner than the floor. The cavity is the segmenta-
tion cavity. The roof is formed of two or three layers of smallish
pigmented cells, and the floor of large cells, which form the

FIG. 69. SEGMENTATION OF COMMON FROG. RANA TEMPORARIA.
(After Ecker.)

The numbers above the figures refer to the number of segments at the stage figured.

greater part of the ovum. These large cells, which are part of
the primitive hypoblast, will be spoken of in the sequel as yolk-
cells : they are equivalent to the food-yolk of the majority of
vertebrate ova.

The cells forming the roof of the cavity pass without any
sharp boundary into the yolk-cells, there being at the junction
of the two a number of cells of an intermediate character. The
cells both of the roof and the floor continue to increase in
number, and those of the roof become divided into two distinct
strata (fig. 70, ep}.

The upper of these is formed of a single row of somewhat
cubical cells, and the lower of several rows of more rounded
cells. Both of these strata eventually become the epiblast, of
which they form the epidermic and nervous layers. The roof of
the segmentation cavity appears therefore to be entirely consti-
tuted of epiblast.

The next changes which take place lead (i) to the formation

AMPHIBIA.

I23

of the mesenteron1, and (2) to the enclosure of the yolk-cells by
the epiblast.

The mesenteron is formed as in Petromyzon and Lepidosteus
by an unsymmetrical form of
invagination. The invagina-
tion first commences by an in-
flection of the epiblast-cells for
a small arc on the equatorial
line which marks the junction
between the epiblastic cells and
the yolk-cells (fig. 70, x].

The inflected cells become
continuous with the adjoining
cells ; and the region where
the inflection is formed consti-
tutes a kind of lip, below which
a slit-like cavity is soon es-
tablished. This lip is equiva-
lent to the embryonic rim of
the Elasmobranch blastoderm,
and the cavity beneath it is the
rudiment of the mesenteron.

The mesenteron now rapidly extends by the invagination of
the cells on its dorsal side. These cells grow inwards towards
the segmentation cavity as a layer of cells several rows deep.
At its inner end, this layer is continuous with the yolk-cells ;
and is divided into two strata (fig. 71 A), viz. (i) a stratum of
several rows of cells adjoining the epiblast, which becomes the
mesoblast (m), and (2) a stratum of a single row of more
columnar cells lining the cavity of the mesenteron, which forms
the hypoblast (Jiy). The growth inwards of the dorsal wall of
the mesenteron is no doubt in part a true invagination, but it
seems probable that it is also due in a large measure to an actual
differentiation of yolk-cells along the line of growth. The
mesenteron is at first a simple slit between the yolk and the
hypoblast (fig. 71 A), but as the involution of the hypoblast and

1 Since the body cavity is not developed as diverticula from the cavity of invagina-
tion, the latter cavity may conveniently be called the mesenteron and not the archen-
teron.

FIG. 70. SECTION THROUGH FROG'S
OVUM AT THE CLOSE OF SEGMENTATION.
(After Gotte.)

SS- segmentation cavity ; //. large yolk-
containing cells ; ep. small cells at forma-
tive pole (epiblast) ; x. point of inflection
of epiblast ; y. small cells close to junction
of the epiblast and yolk.

I24

FORMATION OF THE LAYERS.

mesoblast extends further inwards, this slit enlarges, especially
at its inner end, into a considerable cavity ; the blind end of
which is separated by a narrow layer of yolk-cells from the
segmentation cavity (fig. 71 B).

In the course of the involution, the segmentation cavity
becomes gradually pushed to one side and finally obliterated.
Before obliteration, it appears in some forms (Pelobates fuscus) to
become completely enclosed in the yolk-cells.

While the invagination to form the mesenteron takes place
as above described, the enclosure of the yolk has been rapidly
proceeding. It is effected by the epiblast growing over the yolk
at all points of its circumference. The nature of the growth is
however very different at the embryonic rim and elsewhere. At
the embryonic rim it takes place by the simple growth of the
rim, so that the point x in figs. 70 and 71 is carried further and
A B

FIG. 71. DIAGRAMMATIC LONGITUDINAL SECTIONS THROUGH THE EMBRYO OF
A FROG AT TWO STAGES, TO SHEW THE FORMATION OF THE GERMINAL LAYERS.
(Modified from Gotte.)

ep. epiblast; m, dorsal mesoblast; m'. ventral mesoblast; hy. hypoblast;
yk. yolk ; jr. point of junction of the epiblast and hypoblast at the dorsal side of the
blastopore ; al. mesenteron ; sg. segmentation cavity.

further over the surface of the yolk. Elsewhere the epiblast at
first extends over the yolk as in a typical epibolic gastrula, with-
out being inflected to form a definite lip. While a considerable
patch of yolk is still left uncovered, the whole of the edge of the
epiblast becomes however inflected, as at the embryonic rim
(fig. 71 A); and a circular blastopore is established, round the

AMPHIBIA. 125

whole edge of which the epiblast and intermediate cells are
continuous.

' From the ventral lip of the blastopore the mesoblast (fig. 71,
;#'), derived from the small intermediate cells, grows inwards till
it comes to the segmentation cavity ; the growth being not so
much due to an actual invagination of cells at the lip of the
blastopore, as to a differentiation of yolk-cells in situ. Shortly
after the stage represented in fig. 71 B, the plug of yolk, which
fills up the opening of the blastopore, disappears, and the mesen-
teron communicates freely with the exterior by a small circular
blastopore (fig. 73). The position of the blastopore is the same
as in other types, viz. at the hinder end of the embryo.

By this stage the three layers of the embryo are definitely
established. The epiblast, consisting from the first of two strata,
arises from the small cells forming the roof of the segmentation-
cavity. It becomes continuous at the lip of the blastopore with
cells intermediate in size between the cells of which it is formed
and the yolk-cells. These latter, increasing in number by
additions from the yolk-cells, give rise to the mesoblast and to
part of the hypoblast ; while to the latter layer the yolk-cells, as
mentioned above, must also be considered as appertaining.
Their history will be dealt with in treating of the general fate of
the hypoblast.

Urodela. The early stages of the development of the Newt have
been adequately investigated by Scott and Osborn (No. 114). The
segmentation and formation of the layers is in the main the same as in the
Frog. The ovum is without black pigment. There is a typical unsymmet-
rical invagination, but the dorsal lip of the blastopore is somewhat thickened.
The most striking feature in which the Newt differs from the Frog is the
fact that the epiblast is at first constituted of a single layer of cells (fig. 75, ep\
The roof of the segmentation cavity is constituted, during the later stages of
segmentation, of several rows of cells (Bambeke, No. 95), but subsequently it
would appear to be formed of a single row of cells only (Scott and Osborn,
No. 114).

General history of the layers.

Epiblast : Anura. At the completion of the invagination
the epiblast forms a continuous layer enclosing the whole ovum,
and constituted throughout of two strata. The formation of the
medullary canal commences by the nervous layer along the
axial dorsal line becoming thickened, and giving rise to a some-

126

EPIBLAST.

what pyriform medullary plate, the sides of which form the
projecting medullary folds (fig. 77 A). The medullary plate is
thickened at the two sides, and is grooved in the median line by
a delicate furrow (fig. 72, r). The dilated extremity of the
medullary plate, situated at the end of the embryo opposite the
blastopore, is the cerebral part of the plate, and the remainder

FIG. 72. TRANSVERSE SECTION THROUGH THE POSTERIOR CEPHALIC REGION OF

AN EARLY EMBRYO OF BOMBINATOR. (After Gotte.)

/. medullary groove; r. axial furrow in the medullary groove; h. nervous layer of
epidermis ; as. outer portion of vertebral plate ; is. inner portion of vertebral plate ;
s. lateral plate of mesoblast ; g. notochord ; e. hypoblast.

the spinal. The medullary folds bend upwards, and finally
meet above, enclosing a central cerebro-spinal canal (fig.
74). The point at which they first meet is nearly at the
junction of the brain and spinal cord, and from this point their
junction extends backwards and forwards; but the whole
process is so rapid that the closure of the medullary canal for its
whole length is effected nearly simultaneously. In front the
medullary canal ends blindly, but behind it opens freely into the
still persisting blastopore, with the lips of which the medullary
folds become, as in other types, continuous. Fig. 73 represents
a longitudinal section through an embryo, shortly after the
closure of the medullary canal (nc) ; the opening of which into
the blastopore (x) is clearly seen.

On the closure of the medullary canal, its walls become
separated from the external epiblast, which extends above it as
a continuous layer. In the formation of the central nervous
system both strata of the epiblast have a share, though the main
mass is derived from the nervous layer. After the central

AMPHIBIA.

I27

nervous tube has become separated from the external skin, the
two layers forming it fuse together ; but there can be but little
doubt that at a later period the epidermic layer separates itself
again as the central epithelium of the nervous system.

Both the nervous and epidermic strata have a share in form-
ing the general epiblast ; and though eventually they partially
fuse together yet the horny
layer of the adult epidermis,
where such can be distin-
guished, is probably derived
from the epidermic layer of
the embryo, and the mucous
layer of the epidermis from
the embryonic nervous layer.

In the formation of the
organs of sense the nervous
layer shews itself through-
out as the active layer. The

FIG. 73. DIAGRAMMATIC LONGITUDINAL

SECTION OF THE EMBRYO OF A FROG. (Modi-
fied from Gotte.)

nc. neural canal ; x. point of junction of
epiblast and hypoblast at the dorsal lip of the
blastopore ; al. alimentary tract ; yk. yolk-
cells ; m. mesoblast. For the sake of sim-
plicity the epiblast is represented as if com-
posed of a single row of cells.

lens of the eye and the audi-
tory sack are derived ex-
clusively from it, the latter
having no external opening.
The nervous layer also plays
the more important part in
the formation of the olfactory sack.

The outer layer of epiblast-cells becomes ciliated after the
close of the segmentation, but the cilia gradually disappear on
the formation of the internal gills. The cilia cause a slow
rotatory movement of the embryo within the egg, and probably
assist in the respiration after it is hatched. They are especially
developed on the external gills.

Urodela. In the Newt (Scott and Osborn, No. 114) the medullary
plate becomes established, while the epiblast is still formed of a single row
of cells ; and it is not till after the closure of the neural groove that any
distinction is observable between the epithelium of the central canal, and the
remaining cells of the cerebro-spinal cord (fig. 75).

Before the closure of the medullary folds the lateral epiblast becomes
divided into the two strata present from the first in the Frog ; and in the
subsequent development the inner layer behaves as the active layer, precisely
as in the Anura.

128

MESOBLAST AND NOTOCHORD.

The mesoblast and notochord : Anura. After the disap-
pearance of the segmentation cavity, the mesoblast is described
by most observers, including Gotte, as forming a continuous
sheet round the ovum, underneath the epiblast. The first
important differentiations in it take place, as in the case of the
epiblast, in the axial dorsal line. Along this line a central cord
of the mesoblast becomes separated from the two lateral sheets
to form the notochord. Calberla states, however, that when the
mesoblast is distinctly separated from the hypoblast it does not
form a continuous sheet, but two sheets one on each side,
between which is placed a ridge of cells continuous with the
hypoblastic sheet. This ridge subsequently becomes separated
from the hypoblast as the notochord. Against this view Gotte
has recently strongly protested, and given a series of careful
representations of his sections which certainly support his
original account. r

My own observations are in fa-
vour of Calberla's statement, and
so far as I can determine from my
sections the mesoblast never ap-
pears as a perfectly continuous
sheet, but is always deficient in the
dorsal median line. My observa-
tions are unfortunately not found-
ed on a sufficient series of sections
to settle the point definitely.

After the formation of the
notochord (fig. 72), the meso-
blast may be regarded as con-
sisting of two lateral plates,
continuous ventrally, but sepa-
rated in the median dorsal
line. By the division of the
dorsal parts of these plates
into segments, which com-
mences in the region of the
neck and thence extends back-
wards, the mesoblast of the
trunk becomes divided into

FIG. 74. SECTION THROUGH THE AN-
TERIOR PART OF THE TRUNK OF A YOUNG
EMBRYO OF BOMBINATOR. (After Gotte.)

as"', medulla oblongata ; is*, splanchno-
pleure ; as*, somatopleure in the vertebral
part of the mesoblastic plate ; s. lateral plate
of mesoblast ; f. throat ; e. passage of epi-
thelial cells into yolk-cells ; d. yolk-cells ;
r. dorsal groove along the line of junction of
the medullary folds.

AMPHIBIA.

129

a vertebral portion, cleft into separate somites, and a lateral un-
segmented portion (fig. 74).

The history of these two parts and of the mesoblast is
generally the same as in Elasmobranchs.

The mesoblast in the head becomes, according to Gotte, divided into
four segments, equivalent to the trunk somites. Owing to a confusion into
which Gotte has fallen from not recognizing the epiblastic origin of the
cranial nerves, his statements on this head must, I think, be accepted with
considerable reserve ; but some part of his segments appears to correspond
with the head-cavities of Elasmobranchii.

Urodela. Scott and Osborn (No. 114) have shewn that in the Newt
the mesoblast (fig. 75) is formed of two lateral plates, split off from the
hypoblast, and that^the ventral growth of these plates is largely effected by
the conversion of yolk-cells into mesoblast-cells. They have further shewn
that the notochord is formed of an axial portion of the hypoblast, as in the
types already considered (fig. 75). The body cavity is continued into the
region of the head ; and the mesoblast lining the cephalic section of the
body cavity is divided into the same number of head cavities as in Elasmo-
branchii, viz. one in front of the mouth, and one in the mandibular and one
in each of the following arches.

The hypoblast. There are no important points of difference
in the relations of the hypoblast between the Anura and
Urodela. The mesente-
ron, at the stage repre-
sented in fig. 73, forms a
wide cavity lined dorsally
by a layer of invaginated
hypoblast, and ventrally
by the yolk-cells. The
hypoblast is continuous
laterally and in front with
the yolk-cells (figs. 72,
74 and 75). At an earlier
stage, when the mesen-
teron has a less definite
form, such a continuity
between the true hypo-
blast and the yolk-cells does not exist at the sides of the cavity.

The definite closing in of the mesenteron by the true hypo-
blast-cells commences in front and behind, and takes place last
B. in. 9

0.1

FIG. 75. TRANSVERSE SECTION THROUGH
THE CEPHALIC REGION OF A YOUNG NEWT EM-
BBYO. (After Scott and Osborn.)

In.hy. invaginated hypoblast, the dorsal part
of which will form the notochord ; ep. epiblast
of neural plate; sp. splanchnopleure ; al. ali-
mentary tract ; yk. and Y.hy. yolk-cells.

130

HYPOBLAST.

of all in the middle (fig. 76). In front this process takes place
with the greatest rapidity. The cells of the yolk-floor become
continuously differentiated into hypoblast-cells, and very soon
the whole of the front end becomes completely lined by true
hypoblastic cells, while the yolk-cells become confined to the
floor of the middle part.

The front portion of the mesenteron gives rise to the oeso-
phagus, stomach and duodenum. Close to its hinder boundary
there appears a ventral outgrowth, which is the commencement
of the hepatic di-
verticulum (fig. 76,
/). The yolk is thus
post-hepatic, as in
Vertebrates gene-
rally.

The stomodae-
um is formed com-
paratively late by
an epiblastic inva-
gination (fig. 76, m).

It should be noticed
that the conversion of
the yolk-cells into hypo-
blast-cells to form the
ventral wall of the anterior region of the alimentary tract is a closely
similar occurrence to the formation of cells in the yolk-floor of the
anterior part of the alimentary tract in Elasmobranchii. This conversion
is apparently denied by Gotte, but since I find cells in all stages of
transition between yolk-cells and hypoblast-cells I cannot doubt the fact of
its occurrence.

At first, the mesenteron freely communicates with the exterior
by the opening of the blastopore. The lips of the blastopore
gradually approximate, and form a narrow passage on the dorsal
side of which the neural tube opens, as has already been described
(fig- 73)- The external opening of this passage finally becomes
obliterated, and the passage itself is left as a narrow diverticulum
leading from the hind end of the mesenteron into the neural
canal (fig. 76). It forms the post-anal gut, and gradually
narrows and finally atrophies. At its front border, on the
ventral side, there may be seen a slight ventrally directed

FIG. 76. LONGITUDINAL SECTION THROUGH AN

ADVANCED EMBRYO OF BOMBINATOR. (After Gotte.)

m. mouth ; an. anus ; /. liver ; ne. neurenteric
canal ; me. medullary canal ; ch. notochord ; pn. pineal
gland.

AMPHIBIA. 131

diverticulum of the alimentary tract, which first becomes visible
at a somewhat earlier stage (fig. 73). This diverticulum becomes
longer and meets an invagination of the skin (fig. 76, an), which
arises in Rana temporaria at a somewhat earlier period than
represented by Gotte in Bombinator. This epiblastic invagination
is the proctodaeum, and an anal perforation eventually appears
at its upper extremity.

The differentiation of the hinder end of the prseanal gut
proceeds in the same fashion as that of the front end, though
somewhat later. It gives rise to the cloacal and intestinal part
of the alimentary tract. From the ventral wall of the cloacal
section, there grows out the bifid allantoic bladder, which is
probably homologous with the allantois of the higher Vertebrata.
After the differentiation of the ventral wall of the fore and hind
ends of the alimentary tract has proceeded for a certain distance,
the yolk only forms a floor for a restricted median region of the
alimentary cavity, which corresponds to the umbilical canal of
the Amniota. The true hypoblastic epithelium then grows over
the outer side of the yolk, which thus constitutes a true, though
small, and internal yolk-sack. The yolk-cells enclosed in this
sack become gradually absorbed, and the walls of the sack form
part of the intestine.

General growth of the Embryo.

Anura. The pyriform medullary plate, already described,
is the first external indication of the embryo. This plate
appears about the stage represented in longitudinal section in
fig. 71 B. The feature most conspicuous in it at first is the
axial groove. It soon becomes more prominent (fig. 77 A), and
ends behind at the blastopore (bl\ the lips of which are con-
tinuous with the two medullary folds. As the sides of this plate
bend upwards to form the closed medullary canal, the embryo
elongates itself and assumes a somewhat oval form. At the
same time the cranial flexure becomes apparent (fig. 73), and
the blastopore shortly afterwards becomes shut off from the
exterior. The embryo now continues to grow in length (fig.
77 B), and the mesoblast becomes segmented. The somites are
first formed in the neck, and are added successively behind in

9—2

132

GENERAL GROWTH.

M

the unsegmented posterior region of the embryo. The hind end
of the embryo grows out

into a rounded prominence, __ A^ oc

which rapidly elongates, and
becomes a well-marked tail
entirely formed by the elon-
gation of the post-anal sec-
tion of the body. The whole
body has a very decided dor-
sal flexure, the ventral sur-
face being convex. Fig. 78
represents an embryo of
Bombinator in side view,
with the tail commencing to
project. The longitudinal
section (fig. 76) is taken
through an embryo of about
the same age. In the cephalic region important changes have
taken place. The cranial flexure has become more marked, but

FIG. 77. EMBRYOS OF THE COMMON FROG.
(After Remak.)

A. Young stage represented enclosed in
the egg-membrane. The medullary plate is
distinctly formed, but no part of the medullary
canal is closed, bl. blastopore.

B. Older embryo after the closure of the
medullary canal, oc, optic vesicle. Behind
the optic vesicle are seen two visceral arches.

FIG. 78. LATERAL VIEW OF AN ADVANCED EMBRYO OF BOMBINATOR.

(After Gotte.)

a, mid-brain, a', eye; b. hind-brain; d. mandibular arch; if. Gasserian ganglion;
e. hyoid arch ; e'. first branchial arch ; f, seventh nerve ; f, glossopharyngeal and
vagus nerve; g. auditory vesicle; i. boundary between liver and yolk-sack ; k. suctorial
disc; /. pericardial prominence; m, prominence formed by the pronephros.

is not so conspicuous a feature in the Amphibia as in most other
types, owing to the small size of the cerebral rudiment. The
mid-brain is shewn at fig. 78 a forming the termination of the

AMPHIBIA.

133

long axis of the body, and the optic vesicles (a'} are seen at its
sides.

The rudiments of the mandibular (d), hyoid (e), and first
branchial (e) arches project as folds at the side of the head, but
the visceral clefts are not yet open. Rudiments of the procto-
daeum and stomodaeum have appeared, but neither of them as
yet communicates with the mesenteron. Below the hyoid arch
is seen a peculiar disc (/£) which is an embryonic suctorial organ,
formed of a plate of thickened epiblast. There is a pair of these
discs, one on each side, but only one a,

of them is shewn in the figure. At a
later period they meet each other in the
middle line, though they separate again
before their final atrophy. They are
found in the majority of the Anura, but
are absent according to Parker in the
Aglossa(PipaandDactylethra(fig.83)).
They are probably remnants of the
same primitive organs as the suctorial
disc of Lepidosteus.

The embryo continues to grow in
length, while the tail becomes more
and more prominent, and becomes
bent round to the side owing to the
confinement of the larva within the
egg-membrane. At the front of the
head the olfactory pits become distinct.
The stomodaeum deepens, though still
remaining blind, and three fresh bran-
chial arches become formed ; the last
two being very imperfectly differenti-
ated, and not visible from the exterior.
There are thus six arches in all, viz.
the mandibular, the hyoid and four
branchial arches. Between the man-
dibular and the hyoid, and between
each of the following arches, pouches
of the mesenteron push their way
towards the external skin. Of these pouches there are five, there

FIG. 79. TRANSVERSE SEC-
TION THROUGH A VERY YOUNG
TADPOLE OF BOMBINATOR AT
THE LEVEL OF THE ANTERIOR
END OF THE YOLK-SACK. (After

Gotte.)

a. fold of epiblast continu-
ous with the dorsal fin ; is*.
neural cord ; m. lateral muscle ;
as*, outer layer of muscle-plate;
s. lateral plate of mesoblast ;
b. mesentery ; tt. fold of the
peritoneal epithelium which
forms the segmental duct ; f.
alimentary tract ; f. ventral
diverticulum which becomes
the liver; e. junction of yolk-
cells and hypoblast-cells ; d.
yolk -cells.

134 GENERAL GROWTH.

being no pouch behind the last branchial arch. The first of
these will form the hyomandibular cleft, the second the hyo-
branchial, and the third, fourth and fifth the three branchial
clefts.

Although the pouches of the throat meet the external skin,
an external opening is not formed in them till after the larva is
hatched. Before this takes place there grow, in the majority of
forms, from the outer side of the first and second branchial arches
small processes, each forming the rudiment of an external gill ;
a similar rudiment is formed, either before or after hatching,
on the third arch; but the fourth arch is without it (figs. 80
and 82).

These external gills, which differ fundamentally from the
external gills of Elasmobranchii in being covered by epiblast,
soon elongate and form branched ciliated processes floating
freely in the medium around the embryo (fig. 80).

Before hatching the excretory system begins to develop. The segmental
duct is formed as a fold of the somatic wall at the dorsal side of the body
cavity (fig. 79, u). Its anterior end alone remains open to the body cavity,
and gives rise to a pronephros with two or three peritoneal openings,
opposite to which a glomerulus is formed.

The mesonephros (permanent .kidney of Amphibia) is formed as a series
of segmental tubes much later than the pronephros, during late larval life.
Its anterior end is situated some distance behind the pronephros, and
during its formation the pronephros atrophies.

The period of hatching varies in different larvae, but in most
cases, at the time of its occurrence, the mouth has not yet
become perforated. The larva, familiarly known as a tadpole, is
at first enclosed in the detritus of the gelatinous egg envelopes.
The tail, by the development of a dorsal and ventral fin, very
soon becomes a powerful swimming organ. Growth, during the
period before the larva begins to feed, is no doubt carried on at
the expense of the yolk, which is at this time enclosed within the
mesenteron.

The mouth and anal perforations are not long in making
their appearance, and the tadpole is then able to feed. The gill
slits also become perforated, but the hyomandibular diverti-
culum in most species never actually opens to the exterior, and
in all cases becomes very soon closed.

AMPHIBIA.

135

There can be but little doubt that the hyomandibular diverticulum gives
rise, as in the Amniota, to the Eustachian tube and tympanic cavity, except
when these are absent (i.e. Bombinatoridye). Gotte holds however that
these parts are derived from the hyobranchial cleft, but his statements on
this head, which would involve us in great morphological difficulties, stand
in direct contradiction to the careful researches of Parker.

Shortly after hatching, there grows out from the hyoid arch
on each side an opercular fold of skin, which gradually covers
over the posterior branchial arches and the external gills (fig.
80 d}. It fuses with the skin at the upper part of the gill arches,
and also with that of the pericardial wall below them ; but is
free in the middle, and so assists in forming a cavity, known
as the branchial cavity, in which the gills are placed. Each
branchial cavity at first opens by a separate widish pore behind

A.

FIG. 80. TADPOLES WITH EXTERNAL BRANCHIAE. (From Huxley; after Ecker.)

A. Lateral view of a young tadpole.

B. Ventral view of a somewhat older tadpole.

kb. external branchiae; m. mouth; n. nasal sack; a. eye; o. auditory vesicle;
z. horny jaws ; s. ventral sucker; d. opercular fold.

C. More advanced larva, in which the opercular fold has nearly covered the
branchiae.

s. ventral sucker ; ks. external branchiae ; y. rudiment of hind limb.

(fig. 80), and in Dactylethra both branchial apertures are preserved
(Huxley). In the larva of Bombinator, and it would seem also
that of Alytes and Pelodytes, the original widish openings of the
two branchial chambers meet together in the ventral line, and

136

GENERAL GROWTH.

form a single branchial opening or spiracle. In most other
forms, i. e. Rana, Bufo, Pelobates, etc., the two branchial chambers
become united by a transverse canal, and the opening of the
right sack then vanishes, while that of the left remains as the
single unsymmetrical spiracle. In breathing the water is taken
in at the mouth, passes through the branchial clefts into the
branchial cavities, and is thence carried out by the spiracle.

Immediately after the formation of the branchial cavities, the
original external gills atrophy, but in their place fresh gills,
usually called internal gills, appear on the outer side of the
middle region of the four branchial arches.

There is a single row of these on the first and fourth branchial
arches, and two rows on the second and third. In addition to
these gills, which are vascular processes of the mesoblast, covered,
according to Gotte, with an epiblastic (?) epithelium, branchial
processes appear on the hypoblastic walls of the three branchial
clefts. The last-
named branchial
processes would ap-
pear to be homolo-
gous with the gills
of Lampreys. In
Dactylethra no
other gills but these
are formed (Parker).

The mouth, even

before the tadpole begins to feed, acquires a transversely oval
form (fig. 81), and becomes armed with provisional structures in
the form of a horny beak and teeth, which are in use during
larval life.

FIG. 81. TADPOLE OF BOMBINATOR 'FROM THE

VENTRAL SIDE, WITH THE ABDOMINAL WALL REMOVED.

(After Gotte.)

Behind the mouth are placed the two suckers, and
behind these are seen the gills projecting through the
spiracles.

The beak is formed of a pair of horny plates moulded on the upper and
lower pairs of labial cartilages. The upper valve of the beak is the larger
of the two, and covers the lower. The beak is surrounded by a projecting
lip formed of a circular fold of skin, the free edge of which is covered by
papillse. Between the papilla; and the beak rows of horny teeth are placed
on the inner surface of the lip. There are usually two rows of these on the
upper side, the inner one not continuous across the middle line, and three or
four rows on the lower side, the inner one or two divided into two lateral
parts.

AMPHIBIA. 137

As the tadpole attains its full development, the suctorial
organs behind the mouth gradually atrophy. The alimentary
canal, which is (fig. 81) at first short, rapidly elongates, and fills
up with its numerous coils the large body cavity. In the mean-
time, the lungs develop as outgrowths from the oesophagus.

Various features in the anatomy of the Tadpole point to its being a
repetition of a primitive vertebrate type. The nearest living representative
of this type appears to be the Lamprey.

The resemblance between the mouths of the Tadpole and Lamprey is
very striking, and many of the peculiarities of the larval skull of the Anura,
especially the position of the Meckelian cartilages and the subocular arch,
perhaps find their parallel in the skull of the Lamprey1. The internal
hypoblastic gill-sacks of the Frog, with their branchial processes, are
probably equivalent to the gill-sacks of the Lamprey2; and it is not
impossible that the common posterior openings of the gill-pouches in Myxine
are equivalent to the originally paired openings of the branchial sack of the
Tadpole.

The resemblances between the Lamprey and the Tadpole appear to me
to be sufficiently striking not to be merely the results of more or less similar
habits ; but at the same time there are no grounds for supposing that the
Lamprey itself is closely related to an ancestral form of the Amphibia. In
dealing with the Ganoids and other types arguments have been adduced to
shew that there was a primitive vertebrate stock provided with a perioral
suctorial disc ; and of this stock the Cyclostomata are the degraded, but at the
same time the nearest living representatives. The resemblances between the
Tadpole and the Lamprey are probably due to both of them being descended
from this stock. The Ganoids, as we have seen, also shew traces of a
similar descent ; and the resemblance between the larva of Dactylethra
(fig. 83), the Old Red Sandstone Ganoids3 and Chimasra, probably indicates
that an extension of our knowledge will bring to light further affinities
between the primitive Ganoid and Holocephalous stocks and the Amphibia.

Metamorphosis. The change undergone by the Tadpole in
its passage into the Frog is so considerable as to deserve the
name of a metamorphosis. This metamorphosis essentially
consists in the reduction and atrophy of a series of provisional
embryonic organs, and the appearance of adult organs in their

1 Vide Huxley, " Craniofacial apparatus of Petromyzon." Journ. of Anat. and
Phys. Vol. X. 1876. Huxley's views about the Meckelian arch, etc., are plausible,
but it seems probable from Scott's observations that true branchial bars are not
developed in the Lamprey. How far this fact necessarily disproves Huxley's views is
still doubtful.

" Conf. Huxley and Gotte. a Cf. Parker (No. 107).

138

METAMORPHOSIS.

place. The stages of this metamorphosis are shewn in fig. 82, 5,
0, 7, 8.

The two pairs of limbs appear nearly simultaneously as
small buds ; the hinder pair at the junction of the tail and body
(fig. 82, 5), and the anterior pair concealed under the opercular
membrane. The lungs acquire a greater and greater importance,
and both branchial and pulmonary respirations go on together
for some time.

FIG. 82. TADPOLES AND YOUNG OF THE COMMON FROG. (From Mivart. )
i. Recently-hatched Tadpoles twice the natural size. 2. Tadpole with external

gills. 20. Same enlarged. 3 and 4. Later stages after the enclosure of the gills

by the operailar membrane. 5. Stage with well-developed hind-limbs visible.

6. Stage after the ecdysis, with both pairs of limbs visible. 7. Stage after partial

atrophy of the tail. 8. Young Frog.

When the adult organs are sufficiently developed an ecdysis
takes place, in which the gills are completely lost, the provisional
horny beak is thrown off", and the mouth loses its suctorial form.

AMPHIBIA. 139

The eyes, hitherto concealed under the skin, become exposed
on the surface, and the front limbs appear (fig. 82, 6). With
these external changes important internal modifications of the
mouth, the vascular system, and the visceral arches take place.
A gradual atrophy of the tail, commencing at the apex, next
sets in, and results in the complete absorption of this organ.

The long alimentary canal becomes shortened, and the, Jn
the main, herbivorous Tadpole gradually becomes converted
into the carnivorous Frog (fig. 82, 6, 7, 8).

The above description of the metamorphosis of the Frog applies fairly to
the majority of the Anura, but it is necessary to notice a few of the more
instructive divergences from the general type.

In the first place, several forms are known, which are hatched in the
condition of the adult. The exact amount of metamorphosis which these
forms pass through in the egg is still a matter of some doubt. Hylodes
Martinicensis is one of these forms. The larva no doubt acquires within the
egg a long tail ; but while Bavay1 states that it is provided with external
gills, which however are not covered by an operculum, Peters2 was unable to
see any traces of such structures.

In Pipa Americana, and apparently in Pipa dorsigera also if a distinct
species, the larva leaves the cells on the back of the mother in a condition
closely resembling the adult. The embryos of both species develop a long
tail in the egg, which is absorbed before hatching, and according to Wyman3
P. Americana is also temporarily provided with gills, which atrophy early.

The larva of Rhinoderma Darwinii is stated by Jiminez de la Espada to
be without external gills, and it appears to be hatched while still in the
laryngeal pouch of the male. In Nototrema marsupiatum the larvae are also
stated to be without external gills.

Amongst the forms with remarkable developments Pseudis paradoxa
deserves especial mention, in that the tadpole of this form attains an
immensely greater bulk than the adult ; a peculiarity which may be simply a
question of nutrition, or may perhaps be explained by supposing that the
larva resembles a real ancestral form, which was much larger than the
existing Frog.

Another form of perhaps still greater morphological interest is the larva
of Dactylethra. The chief peculiarities of this larva (fig. 83) have been
summarized by Parker (No. 107, p. 626), from whom I quote the following
passage :

a. "The mouth is not inferior in position, suctorial and small, but is
very wide like that of the ' Siluroids and Lophius ;' has an underhung lower

1 AnnaL de Sciences Nat., 5th Series, Vol. xvn., 1873.

2 Berlin. Monatsbericht, 1876, p. 703, and Nature, April 5, 1877.

3 Proceed, of Boston Nat. Hist. Society, Vol. v., 1854.

140 METAMORPHOSIS.

jaw, an immensely long tentacle from each upper lip, and possesses no trace
of the primordial horny jaws of the ordinary kind.

b, "In conformity with these characters the head is extremely flat or
depressed, instead of being high and thick.

FIG. 83. LARVA OF DACTYLETHRA. (After Parker.)

c. " There are no claspers beneath the chin.

d. " The branchial orifice is not confined to the left side, but exists on
the right side also.

e. " The tail, like the skull, is remarkably chimaeroid ; it terminates in a
long thin pointed lash, and the whole caudal region is narrow and elongated
as compared with that of our ordinary Batrachian larvaa.

f. " The fore-limbs are not hidden beneath the opercular fold."
Although most Anurous embryos are not provided with a sufficient

amount of yolk to give rise to a yolk-sack as an external appendage of the
embryo, yet in some forms a yolk-sack, nearly as large as that of Teleostei,
is developed. One of these forms, Alytes obstetricans, belongs to a well-
known European genus allied to Pelobates. The embryos of Pipa dorsigera
(Parker) are also provided with a very large yolk-sack, round which they are
coiled like a Teleostean embryo. A large yolk-sack is also developed in the
embryo of Pseudophryne australis.

The actual complexity of the organization of different tadpoles, and their
relative size, as compared with the adult, vary considerably. The tadpoles
of Toads are the smallest, Pseudophryne australis excelling in this respect ;
those of Pseudis are the largest known.

The external gills reach in certain forms, which are hatched in late larval
stages, a very great development. It seems however that this development
is due to these gills being especially required in the stages before hatching.
Thus in Alytes, in which the larva leaves the egg in a stage after the loss of
the external gills, these structures reach in the egg a very great development.
In Notodelphis ovipara, in which the eggs are carried in a dorsal pouch of
the mother, the embryos are provided with long vesicular gills attached to
the neck by delicate threads. The fact (if confirmed) that some of the forms
which are not hatched till post-larval stages are without external gills,
probably indicates that there may be various contrivances for embryonic
respiration1 ; and that the external gills only attain a great development in

1 In confirmation of this view it may be mentioned that in Pipa Americana the
tail appears to function as a respiratory organ in the later stages of development
(Peters).

AMPHIBIA. 141

those instances in which respiration is mainly carried on by their means.
The external gills of Elasmobranchii are probably, as stated in a previous
chapter, examples of secondarily developed structures, which have been
produced by the same causes as the enlarged gills of Alytes, Notodelphis,
etc.

Urodela. Up to the present time complete observations on
the development of the Urodela are confined to the Myctodera1.

The early stages are in the main similar to those of the
Anura. The body of the embryo is, as pointed out by Scott
and Osborn, ventrally instead of dorsally flexed. The metamor-
phosis is much less complete than in the Anura. The larva of
Triton may be taken as typical. At hatching, it is provided
with a powerful swimming tail bearing a well-developed fin :
there are three pairs of gills placed on the three anterior of the
true branchial arches.

Between the hyoid and first branchial arch, and between the
other branchial arches, slits are developed, there being four slits
in all. At the period just before hatching, only three of these
have made their appearance. The hyomandibular cleft is not
perforated. Stalked suckers, of the same nature as the suckers
of the Anura, are formed on the ventral surface behind the
mouth. A small opercular fold, developed from the lower part
of the hyoid arch, covers over the bases of the gills. The
suctorial mouth and the provisional horny beak of the Anura
have no counterpart in these larvae. The skin is ciliated, and the
cilia cause a rotation in the egg. Even before hatching, a small
rudiment of the anterior pair of limbs is formed, but the hind-
limbs are not developed till a later stage, and the limbs do not
attain to any size till the larva is well advanced. In the course
of the subsequent metamorphosis lungs become developed, and
a pulmonary respiration takes the place of the branchial one.
The branchial slits at the same time close and the branchiae
atrophy.

The other types of Myctodera, so far investigated, agree fairly with the
Newt.

The larva of Amblystoma punctatum (fig. 84) is provided with two very

1 The recent observations on this subject are those of Scott and Osborn (No. 114)
on Triton, of Bambeke (No. 95) on various species of Triton and the Axolotl, and of
Clark (No. 98) on Amblystoma punctatum.

142

URODELA.

op

long processes (j), like the suctorial processes in Triton, placed on the throat
in front of the external gills. They are used to support the larva when it
sinks to the bottom, and have been called by Clarke (No. 98) balancers. On
the development of the limbs,
these processes drop off. The
external gills atrophy about one
hundred days after hatching.

It might have been anticipated
that the Axolotl, being a larval
form of Amblystoma, would agree
in development with Amblystoma
punctatum. The conspicuous suc-
torial processes of the latter form
are however represented by the
merest rudiments in the Axolotl.

The young of Salamandra
maculata leave the uterus with
external gills, but those of the
Alpine Salamander (Salamandra
atra) are born in the fully de-
veloped condition without gills.
In the uterus they pass through a
metamorphosis, and are provided
(in accordance with the principle
already laid down) with very long
gill-filaments1.

Salamandra atra has only two
embryos, but there are originally

a larger number of eggs (Von Sie-
,,,,,.,.,„,. f ., .

bold), of which all but two fail to

develop, whi,e their remains are

used as pabulum by the two which

survive. Both species of Sala-

mander have a sufficient quantity of food-yolk to give rise to a

sack.

Spelerpes only develops three post-hyoid arches, between which slits are
formed as in ordinary types. Menobranchus and Proteus agree with
Spelerpes in the number of post-hyoid arches.

One of the most remarkable recent discoveries with reference to the
metamorphosis of the Urodela was made by Dumeril2. He found that some
of the larvae of the Axolotl, bred in the Jardin des Plantes, left the water,
and in the course of about a fortnight underwent a similar metamorphosis to
that of the Newt, and became converted into a form agreeing in every

1 Allen Thomson informs me that the crested Newt, Triton cristatus, is in rare
instances viviparous.

- Comptes RenJits, 1870. 11.782.

FlG- 84. LARWE OF AMBLYSTOMA
PUNCTATUM. (After Clarke.)

yolk-

AMPHIBIA. 143

particular with the American genus Amblystoma. During this metamorpho-
sis a pulmonary respiration takes the place of a branchial one, the gills are
lost, and the gill slits close. The tail loses its fin and becomes rounded, the
colour changes, and alterations take place in the gums, teeth, and lower jaw.

Madame von Chauvin1 was able, by gradually accustoming Axolotl larvae
to breathe, artificially to cause them to undergo the above metamorphosis.

It seems very possible, as suggested by Weismann2, that the existing
Axolotls are really descendants of Amblystoma forms, which have reverted
to a lower stage. In favour of this possibility a very interesting discovery of
Filippi's3 may be cited. He found in a pond in a marsh near Andermat
some examples of Triton alpestris, which, though they had become sexually
mature, still retained the external gills and the other larval characters.
Similar sexually mature larval forms of Triton taeniatus have been described
by Jullien. These discoveries would seem to indicate that it might be
possible artificially to cause the Newt to revert to a perennibranchiate
condition.

Gymnophiona. The development of the Gymnophiona is
almost unknown, but it is certain that some larval forms are
provided with a single gill-cleft, while others have external gills.

A gill-cleft has been noticed in Epicrium glutinosum
(Miiller), and in Ccecilia oxyura. In Ccecilia compressicauda,
Peters (No. 108) was unable to find any trace of a gill-cleft, but
he observed in the larvae within the uterus two elongated
vesicular gills.

BIBLIOGRAPHY.
A mphibia.

(93) Ch. van Bambeke. " Recherches sur le developpement du Pelobate
brun." Memoires couronnes, etc. de F Acad. roy. de Belgique, 1868.

(94) Ch. van Bambeke. "Recherches sur 1'embryologie des Batraciens."
Bulletin de I'Acad. roy. de Belgique, 1875.

(95) Ch. van Bambeke. " Nouvelles recherches sur 1'embryologie des Batra-
ciens." Archives de Biologie, Vol. I. 1880.

(96) K. E. von Baer. " Die Metamorphose des Eies der Batrachier." Muller's
Archiv, 1834.

(97) B. Benecke. " Ueber die Entwicklung des Erdsalamanders. " Zoolo-
gischer Anzeiger, 1880.

1 Zeit.f. wiss. ZooI.,"Bd.. xxvn. 1876.

2 Zeit.f. wiss. Zool., Bd. xxv. sup. 1875.

3 Archivio per la Zoologia, /' Anatomia e la Fisiologia, Vol. I. Genoa, 1861. Conf.
also Von Siebold, " Ueber die geschlechtliche Entwicklung d. Urodelen-Larven."
Zeit.f. wiss. Zool., Bd. xxvm., 1877.

144 BIBLIOGRAPHY.

(98) S. F. Clarke. "Development of Amblystoma punctatum," Part I., Ex-
ternal. Studies from the Biological Laboratory- of the Johns Hopkins University,
No. II. 1880.

(99) H. Cramer. "Bemerkungen lib. d. Zellenleben in d. Entwick. d. Fros-
cheies." Mullcr's Archiv, 1848.

(100) A. Ecker. Icones Physiolog. 1851— 1859.

(101) A. Gotte. Die Entwicklungsgeschichte der Unke. Leipzig, 1875.

(102) C. K. Hoffmann. "Amphibia." Klassen u. Ordnungen d. T/iierreicks,
1873—1879.

(103) T. II. Huxley. Article "Amphibia" in the Encyclopedia Britannica.

(104) A. Moquin-Tandon. "Developpement des Batraciens anures." Annales
des Sciences Naturelles, III. 1875.

(105) G. Newport. " On the impregnation of the Ovum in Amphibia " (three
memoirs). Phil. Trans. 1851, 1853, and 1854.

(106) W. K. Parker. " On the structure and development of the Skull of the
common Frog." Phil. Trans., CLXI. 1871.

(107) W. K. Parker. "On the structure and development of the Skull of the
Batrachia." Phil. Trans., Vol. CXLVI., Part a. 1876.

(108) W. C. H. Peters. " Ueber die Entwicklung der Coecilien und besonders
von Coecilia compressicauda." Berlin Monatsbericht, p. 40, 1874.

(109) W. C. H. Peters. "Ueber die Entwicklung der Coecilien." Berl.
Monatsbericht , p. 483, 1875.

(110) J. L. Prevost and J. B. Dumas. " Deuxieme Mem. s. 1. generation.
Developpement de 1'ceuf d. Batraciens." Ann. Sci. Nat. II. 1824.

(111) R. Remak. Untersuchungen iiber die Entwicklung der Wirbelthiere,
1850—1858.

(112) M. Rusconi. Developpement de la grenouille commune depttis le moment de
sa naissance jusqu 'd son (tat parfait, 1826.

(113) M. Rusconi. Histoire naturelle, developpement et metamorphose de la
Salamandre terrestre, 1854.

(114) W. B. Scott and H. F. Osborn. "On the early development of the
common Newt." Quart. J. of Micr. Science, Vol. xxix. 1879.

(115) S. Strieker. "Entwicklungsgeschichte von Bufo cinereus." Sitzb. der
kaiserl. Acad. zu Wien, 1860.

(116) S. Strieker. "Untersuchungen liber die ersten Anlagen in Batrachier-
Eiern." Zeitschrift f. wiss. Zoologie, Bd. xi. 1861.

CHAPTER VIII.

AVES.

INTRODUCTION.

THE variations in the character of the embryonic development
of the Amniota are far less important than in the case of the
Ichthyopsida. There are, it is true, some very special features in
the early developmental history of the Mammalia, but apart from
these there is such a striking uniformity in the embryos of all the
groups that it would, in many cases, be difficult to assign a young
embryo to its proper class.

Amongst the Sauropsida the Aves have for obvious reasons
received a far fuller share of attention than any other group; and
an account of their embryology forms a suitable introduction to
this part of our subject. For the convenience of the student many
parts of their developmental history will be dealt with at greater
length than in the case of the previous groups.

The development of the Aves.

Comparatively few types of Birds have been studied embryo-
logically. The common Fowl has received a disproportionately
large share of attention ; although within quite recent times the

FIG. 85. YOLK ELEMENTS FROM THE EGG OF THE FOWL.
A. Yellow yolk. B. White yolk.

Duck, the Goose, the Pigeon, the Starling, and a Parrot (Melo-
psittacus undulatus) have also been studied. The result of these

B. III. 10

146 GERMINAL DISC.

investigations has been to shew that the variations in the early
development of different Birds are comparatively unimportant.
In the sequel the common Fowl will be employed as type, atten-
tion being called when necessary to the development of the other
forms.

The ovum of the Fowl, at the time when it is clasped by the
expanded extremity of the oviduct, is a large yellow body en-
closed in a vitelline membrane. It is mainly formed of spherules
of food-yolk. Of these there are two varieties ; one known as
yellow yolk, and the other as white. The white yolk spherules
form a small mass at the centre of the ovum, which is continued
to the surface by a narrow stalk, and there expands into a some-
what funnel-shaped disc, the edges of which are continued over
the surface of the ovum as a delicate layer. The major part of
the ovum is formed of yellow yolk. The yellow yolk consists of
large delicate spheres, filled with small granules (fig. 85 A) ;
while the white yolk is formed of vesicles of a smaller size than
the yellow yolk spheres, in which are a variable number of highly
refractive bodies (fig. 85 B).

In addition to the yolk there is present in the ovum a small
protoplasmic region, containing the remains of the germinal
vesicle, which forms the germinal disc (fig. 86). It overlies the

w.y.

FIG. 86. 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 ; b. epithelium of the capsule, 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 ; x. germinal vesicle enclosed in a distinct
membrane, but shrivelled up; y. space originally completely filled up by the germinal
vesicle, before the latter was shrivelled up.

funnel-shaped disc of white yolk, into which it is continued with-
out any marked line of demarcation. It contains numerous

AVES.

147

minute spherules of the same nature as the smallest white yolk
spherules.

Impregnation takes place at the upper extremity of the
oviduct.

In its passage outwards the ovum gradually receives its acces-
sory coverings in the form of albumen, shell-membrane, and shell
(fig. 87).

y-y-

c-fi.Z

FIG. 87. 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 concentrically arranged around it. 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.l. 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-membrane, i.s.m. internal layer of shell-membrane;
s.m. external layer of shell-membrane ; s. shell.

The segmentation commences in the lower part of the ovi-
duct, shortly before the shell has begun to be formed. It is
meroblastic, being confined to the germinal disc, through the
full depth of which however the earlier furrows do not extend.
It is mainly remarkable for being constantly somewhat unsym-
metrical (Kolliker) — a feature which is not represented in fig. 88,
copied from Coste. Owing to the absence of symmetry the cells
at one side of the germinal disc are larger than those at the
other, but the relations between the disc and the axis of the

10 — 2

148

SEGMENTATION.

embryo are not known. During the later stages the segmentation
is irregular, and not confined to the surface ; and towards its

ABC

FIG. 88. SURFACE VIEWS OF THE EARLY STAGES OF THE SEGMENTATION

IN A FOWL'S EGG. (After Coste.)

a. edge of germinal disc; b. vertical furrow; c. small central segment; d. larger
peripheral segment.

close the germinal disc becomes somewhat lenticular in shape ;
and is formed of segments, which are smallest in the centre and
increase in size to-
wards the periphery
(figs. 89 and 90). The
superficial segments
in the centre of the
germinal disc are
moreover smaller than
those below, and more
or less separated as
a distinct layer (fig.
90). As development
proceeds the segmen-
tation reaches its

limits in the centre,

, , , . , , , FIG. 89. SURFACE VIEW OF THE GERMINAL DISC

tne OF FOWL'S EGG DURING A LATE STAGE OF THE SEG-
periphery; and thus MENTATION.

c. small central segmentation spheres; b. larger
segments outside these ; a. large, imperfectly circum-
scribed, marginal segments; e. margin of germinal

disc.

eventually the masses
at the periphery be-
come of the same size
as those at the centre. At the time when the ovum is laid
(fig. 91) the uppermost layer of segments has given rise to a
distinct membrane, the epiblast, formed of a single row of colum-

AVES.

149

nar cells (ep). The lower or hypoblast segments are larger, in
some cases very much larger, than those of the epiblast, and are

FIG. 90. 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 the nucleus
is present in them 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. No segmentation cavity is present.

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.

so granular that their nuclei can only with difficulty be seen.
They form a somewhat irregular mass, several layers deep, and
thicker at the periphery than at the centre : they rest on a bed
of white yolk, from which they are in parts separated by a more
or less developed cavity, which is probably filled with fluid yolk
matter about to be absorbed. In the bed of white yolk nuclei
are present, which are of the same character, and have the same
general fate, as those in Elasmobranchii. They are generally
more numerous in the neighbourhood of the thickened periphery
of the blastoderm than elsewhere. Peculiar large spherical bodies
are to be found amongst the lower layer cells, which superficially
resemble the larger cells around them, and have been called
formative cells \vide Foster and Balfour (No. 126)]. Their real
nature is still very doubtful, and though some are no doubt true
cells, others are perhaps only nutritive masses of yolk. In a
surface view the blastoderm, as the segmented germinal disc may

ISO

FORMATION OF THE LAYERS.

now be called, appears as a circular disc ; the central part of
which is distinguished from the peripheral by its greater trans-
parency, and forms what is known in the later stages as the area
pellucida. The narrow darker ring of blastoderm, outside the
area pellucida, is the commencing area opaca.

FIG. 91. SECTION OF A BLASTODERM OF A FOWL'S EGG AT §> ^

THE COMMENCEMENT OF INCUBATION.

The thin epiblast ep composed of columnar cells rests on the
incomplete lower layer /, composed of larger and more granular
hypoblast 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 b, 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 to
the left ep is continued farther than /, so that at the extreme edge
it rests directly on the white yolk.

As a result of incubation the blastoderm under-
goes a series of changes, which end in the definite
formation of three germinal layers, and in the es-
tablishment of the chief systems of organs of the
embryo. The more important of these changes
are accomplished in the case of the common Fowl
during the first day and the early part of the second
day of incubation.

There is hardly any question in development which has
been the subject of so much controversy as the mode of
formation of the germinal layers in the common Fowl. The
differences in the views of authors have been caused to a
large extent by the difficulties of the investigation, but
perhaps still more by the fact that many of the observations
were made at a time when the methods of making sections
were very inferior to those of the present day. The subject
itself is by no means of an importance commensurate with
the attention it has received. The characters which belong
to the formation of the layers in the Sauropsida are second- ^^ -,
arily derived from those in the Ichthyopsida, and are of but
little importance for the general questions which concern
the nature and origin of the germinal layers. In the account
in the sequel I have avoided as much as possible discussion
of controverted points. My statements are founded in the
main on my own observations, more especially on a recent
investigation carried on in conjunction with my pupil, Mr
Deighton. It is to Kolliker (No. 135), and to Gasser (No. 127) that the most
important of the more recent advances in our knowledge are due. Kolliker,

AVES.

in his great work on Embryology, definitely established the essential con-
nection between the primitive streak and the formation of the mesoblast ;
but while confirming his statement on this head, I am obliged to differ from
him with reference to some other points.

Gasser's work, especially that part of it which relates to the passages
leading from the neural to the alimentary canal, which he was the first to
discover, is very valuable.

The blastoderm gradually grows in size, and extends itself
over the yolk ; the growth over the yolk being very largely
effected by an increase in the size of the area opaca, which
during this process becomes more distinctly marked off from the
area pellucida. The area pellucida gradually assumes an oval
form, and at the same time becomes divided into a posterior
opaque region and an anterior transparent region. The posterior
opacity is named by some authors the embryonic shield.

FIG, 92. 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.

During these changes the epiblast (fig. 92) becomes two
layers deep over the greater part of the area pellucida, though
still only one cell deep in the area opaca. The irregular hypo-
blast spheres of the unincubated blastoderm flatten themselves
out, and unite into a definite hypoblastic membrane (fig. 92).
Between this membrane and the epiblast there remain a number
of scattered cells (fig. 92) 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 give rise to the opacity of that part.

At the edge of the area pellucida the hypoblast becomes con-
tinuous 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,

152

FORMATION OF THE LAYERS.

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. 160 and 161.

The changes which next take place result in the complete
differentiation of the embryonic layers, a process which is

FIG. 93. DIAGRAMS ILLUSTRATING THE POSITION OF THE BLASTOPORE, AND
THE RELATION OF THE EMBRYO TO THE YOLK IN VARIOUS MEROBLASTIC VERTE-
BRATE OVA.

A. Type of Frog. B. Elasmobranch type. C. Amniotic Vertebrate.
mg. medullary plate ; ne. neurenteric canal ; bl. portion of blastopore adjoining the
neurenteric canal. In B this part of the blastopore is formed by the edges of the
blastoderm meeting and forming a linear streak behind the embryo ; and in C it forms
the structure known as the primitive streak, yk. part of yolk not yet enclosed by the
blastoderm.

intimately connected with the formation of the structure known
as the primitive streak. The meaning of the latter structure,
and its relation to the embryo, can only be understood by
comparison with the development of the forms already con-
sidered. The most striking peculiarity in the first formation of
the embryo Bird, as also in that of the embryos of all Amniota,
consists in the fact that they do not occupy a position at the edge

AVES.

153

of the blastoderm, but are placed near its centre. Behind the
embryo there is however a peculiar structure — the primitive
streak above mentioned — which is a linear body placed in the
posterior region of the blastoderm. This body, the nature of
which will be more fully explained in the chapter on the com-
parative development of Vertebrates, is really a rudimentary
part of the blastopore, of the same nature as the linear streak
behind the embryo in Elasmobranchii formed by the concrescence
of the edges of the blastoderm (vide p. 64) ; although there is no
ontogenetic process in the Amniota,
like the concrescence in Elasmo-
branchii. The relations of the
blastopore in Elasmobranchii and
Aves is shewn in figs. B and C of
the diagram (fig. 93).

In describing in detail the suc-
ceeding changes we may at first
confine our attention to the area
pellucida. As this gradually as-
sumes an oval form the posterior
opacity becomes replaced by a very
dark median streak, which extends
forwards some distance from the
posterior border of the area (fig.
94). This is the first rudiment of the primitive streak.

FIG. 94. AREA PELLUCIDA OF
A VERY YOUNG BLASTODERM OF
A CHICK, SHEWING THE PRIMITIVE
STREAK AT ITS FIRST APPEARANCE.

pr.s. primitive streak ; ap. area
pellucida ; a.op. area opaca.

In the

FIG. 95. TRANSVERSE SECTION THROUGH A BLASTODERM OF ABOUT THE AGE
REPRESENTED IN FIG. 94, SHEWING THE FIRST DIFFERENTIATION OF THE PRIMITIVE

STREAK.

The section passes through about the middle of the primitive streak, pvs. primitive
streak; ep. epiblast; hy. hypoblast; yk. yolk of the germinal wall.

region in front of it the blastoderm is still formed of two layers

154

THE PRIMITIVE STREAK.

only, but in the region of the streak itself the structure of the
blastoderm is greatly altered. The most important features in
it are represented in fig. 95. This figure shews that the median
portion of the blastoderm has become very much thickened (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. 95 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.

In addition to the cells clearly derived from the epiblast,
there are certain other cells (vide fig. 95), closely adjoining the
hypoblast, which appear to me to be the derivatives of the cells
interposed between the epiblast and hypoblast, which gave rise
to the posterior opacity in the blastoderm during the previous
stage. In my opinion these cells also have a share in forming
the future mesoblast

The number and distribution of these cells is subject to not inconsider-
able variations. In a fair number of cases they are entirely congregated
along the line of the primitive streak,
leaving the sides of the blastoderm quite
free. They then form a layer, which can
only with difficulty be distinguished from
the cells derived from the epiblast by
slight peculiarities of staining, and by the
presence of a considerable proportion of
large granular cells. It is, I believe,
by the study of such blastoderms that
Kolliker has been led to deny to the inter-
mediate cells of the previous stage any
share in the formation of the mesoblast.
In other instances, of which fig. 95 is a
fairly typical example, they are more wide-
ly scattered. To follow with absolute cer-
tainty the history of these cells, and to
prove that they join the mesoblast is not,
I believe, possible by means of sections,
and I must leave the reader to judge how
far the evidence given in the sequel is
sufficient to justify my opinions on this
subject.

FIG. 96. SURFACE VIEW OK
THE AREA PELLUCIDA OF A CHICK'S
BLASTODERM SHORTLY AFTER THE
FORMATION OF THE PRIMITIVE

GROOVE.

fr. primitive streak with primi-
tive groove ; of. amniotic fold.

The darker shading round the
primitive streak shews the exten-
sion of the mesoblast.

AVES. 155

In the course of further growth the area pellucida soon
becomes pyriform, the narrower extremity being the posterior.
The primitive streak (fig. 96) elongates considerably, so as to
occupy about two-thirds of the length of the area pellucida ; but
its hinder end in many instances does not extend to the posterior
border of the area pellucida. The median line of the primitive
streak becomes marked by a shallow groove, known as the
primitive groove.

During these changes in external appearance there grow
from the sides of the primitive streak two lateral wings of
mesoblast cells, which gradually extend till they reach the sides
of the area pellucida (fig. 97). The mesoblast still remains

FIG. 97. TRANSVERSE SECTION THROUGH THE FRONT END OF THE PRIMITIVE

STREAK OF A BLASTODERM OF THE SAME AGE AS FIG. 96.
pv. primitive groove; m. mesoblast; ep. epiblast; hy. hypoblast; yh. yolk of
germinal wall.

attached to the epiblast along the line of the primitive streak.
During this extension many sections through the primitive streak
give an impression of the mesoblast being involuted at the lips
of a fold, and so support the view above propounded, that the
primitive streak is the rudiment of the coalesced lips of the
blastopore. 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 I believe to be derived from the
primitive hypoblast, can generally be distinctly traced. In many
cases, especially at the front end of the primitive streak, it forms,
as in fig. 97, a distinct layer of stellate cells, quite unlike the

1 56

FORMATION OF MESOBLAST.

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 some-
what thicker. Soon, however, as shewn in longitudinal section
in fig. 98, the hypoblast along the axial line becomes continuous
behind with the front end of the primitive streak. Thus at this

FIG. 98. LONGITUDINAL SECTION THROUGH THE AXIAL LINE OF THE
PRIMITIVE STREAK, AND THE PART OF THE BLASTODERM IN FRONT OF IT, OF
AN EMBRYO CHICK SOMEWHAT YOUNGER THAN FIG. 99.

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

point, which is the future hind end of the embryo, the mesoblast,
the epiblast, and the hypoblast all unite together ; just as they
do in all the types of Ichthyopsida.

Shortly afterwards, at a slightly later stage than that repre-
sented in fig. 96, an important change takes place in the constitu-
tion of the hypoblast in front of the primitive streak. The
rounded cells, of which it is at first composed (fig. 98), break up
into (i) a layer formed of a single row of more or less flattened
elements below — the hypoblast — and (2) into a layer formed of
several rows of stellate elements, between the hypoblast and the
epiblast — the mesoblast (fig. 99). 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

AVES.

157

from the front end of the primitive streak, but stopping short at
a semicircular fold — the future head-fold — near the front end of
the area pellucida. In section (fig. 100) this opaque line is seen
to be due to a special concentration of cells in the form of a cord.

FIG. 99. TRANSVERSE SECTION THROUGH THE EMBRYONIC REGION OF THE
BLASTODERM OF A CHICK SHORTLY PRIOR TO THE FORMATION OF THE MEDULLARY
GROOVE AND NOTOCHORD.

m. median line of the section; ep. epiblast; //. lower layer cells (primitive hypo-
blast) not yet completely differentiated into mesoblast and hypoblast ; n. nuclei of
germinal wall.

This cord is the commencement of the notochord (ch\ In some
instances the commencing notochord remains attached to the
hypoblast, while the mesoblast is laterally quite distinct (vide
fig. 100), and is therefore formed in the same manner as in most
Ichthyopsida ; while in other instances, and always apparently
in the Goose (Gasser, No. 127), 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 hypoblast, and that the notochord originates as a
median plate, simultaneously with the mesoblast, with which it
may sometimes be at first continuous.

Kolliker holds that the mesoblast of the region of the embryo is derived
from a forward growth from the primitive streak. There is no theoretical
objection to this view, and I think it would be impossible to shew for certain
by sections whether or not there is a growth such as he describes ; but such
sections as that represented in fig. 99 (and I have series of similar sections
from several embryos) appear to me to be conclusive in favour of the view
that the mesoblast of the region of the embryo is to a large extent derived

158

FORMATION OF MESOBLAST.

from a differentiation of the primitive hypoblast. I am however inclined to
believe that some of the mesoblast cells of the embryonic region have the
derivation which Kolliker ascribes to all of them.

'e/l.

FIG. 100. 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; n. nuclei of the
germinal wall yk.

As regards the mesoblast of the primitive streak, in a purely objective
description like that given above, the greater part of it may fairly be de-
scribed as being derived from the epiblast. But if it is granted that the
primitive streak corresponds with the blastopore, it is obvious to the com-
parative embryologist that the mesoblast derived from it really originates
from the lips of the blastopore, as in so many other cases ; and that to
describe it, without explanation, as arising from the epiblast, would give an
erroneous impression of the real nature of the process.

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 formation of these structures

FIG. iot. TRANSVERSE SECTION OF A BLASTODERM INCUBATED FOR 18 HOURS.

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

A. epiblast. B. mesoblast. C. hypoblast.

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

the axial part of the epiblast, above the notochord and in
front of the primitive streak, which is somewhat thicker than

AVES.

159

the lateral parts, becomes differentiated into a distinct medul-
lary plate, the sides of which
form two folds — the medullary
folds — enclosing between them
a medullary groove (fig. 101).

In front the two medullary
folds meet, while posteriorly
they thin out and envelop be-
tween them the front end of the
primitive streak. On the form-
ation of the medullary folds
the embryo assumes a form not
unlike that of the embryos of
many Ichthyopsida at a corre-
sponding stage. The appear-
ance of the embryo, and its re-
lation to the surrounding parts
is somewhat diagrammatically
represented in fig. 102. The
primitive streak now ends with
an anterior swelling (not repre-
sented in the figure), and is
usually somewhat unsymmetri-
cal. In most cases its axis is
more nearly continuous with the
left, or sometimes the right,
medullary fold than with the
medullary groove. In sections
its front end appears as a ridge
on one side or on the middle of
the floor of the widened end of
the medullary groove.

The mesoblast and hypo-

FIG. 102. SURFACE VIEW OF THE
PELLUCID AREA OF A BLASTODERM OF l8

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 is seen
the primitive groove pr., with its nearly
parallel walls, fading away behind, but
curving round and meeting in front so
as to form a distinct anterior termination
to the groove, about halfway up the pel-
lucid area.

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

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

blast, within the area pellucida,
do not give rise to the whole of these two layers in the surrounding
area opaca ; but the whole of the hypoblast of the area opaca,
and a large portion of the mesoblast, and possibly even some of
the epiblast, take their origin from the peculiar material already
spoken of, which forms the germinal wall, and is continuous with

160 GERMINAL WALL.

the hypoblast at the edge of the area opaca (vide figs. 91, 94, 97,
98, 99, 100).

The exact nature of this material has been the subject of many contro-
versies. Into these controversies it is not my purpose to enter, but sub-
joined are the results of my 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 extends
itself over the yolk, partly, it appears, at the expense of the cells of the
germinal wall, and possibly even of cells formed around the nuclei in this
part. This mode of growth of the epiblast is very similar to that in the
epibolic gastrulas of many Invertebrata, of the Lamprey, etc. ; but how far
this process is continued in the subsequent extension of the epiblast I am
unable to say. The cells of the germinal wall, which are at first well
separated from the yolk below, become gradually absorbed in the growth of
the hypoblast, and the remaining cells and yolk then become mingled
together, and constitute a compound structure, continuous at its inner
border with the hypoblast. 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 largish cells imbedded amongst them. The
nuclei typically form a special layer immediately below the epiblast, some of
which are probably enclosed by a definite cell-body. A special mass of nuclei
(vide figs. 98 and 100, «) is usually present at the junction of the hypoblast
with the germinal wall.

The germinal wall at this stage corresponds in many respects with
the granular material, forming a ring below the edge of the blastoderm in
Teleostei.

It retains the characters above enumerated till near the close of the first
day of incubation, i.e. till several mesoblastic somites have become
established. It then becomes more distinctly separated from the subjacent
yolk, and its component parts change very considerably in character. The
whole wall becomes much less granular. It is then mainly formed of large
vesicles, which often assume a palisade-like arrangement, and contain
granular balls, spherules of white yolk, and in an early stage a good deal of
granular matter (vide fig. 115). These bodies have some resemblance to
cells, and have been regarded as such by Kolliker (No. 135) and Virchow
(No. 150) : they contain however nothing which can be considered as a
nucleus. Between them however nuclei1 may easily be seen in specimens
hardened in picric acid, and stained with hasmatoxylin (these nuclei are not
shewn in fig. 115). These nuclei are about the same size as those of the
hypoblast cells, and are surrounded by a thin layer of granular protoplasm,

1 The presence of numerous nuclei in the germinal wall was, I believe, first
clearly proved by His (No. 132). I cannot however accept the greater number of his
interpretations.

AVES. l6l

which is continuous with a meshwork of granular protoplasm enveloping the
above described vesicles. The germinal wall is still continuous with the
hypoblast at its edge ; and close to the junction of the two the hypoblast at
first forms a layer of moderately columnar cells, one or two deep and
directly continuous with the germinal wall, and at a later period usually
consists of a mass of rounder cells lying above the somewhat abrupt inner
edge of the germinal wall.

The germinal wall certainly gives rise to the hypoblast cells, which
mainly grow at its expense. They arise at the edge of the area pellucida,
and when first formed are markedly columnar, and enclose in their proto-
plasm one of the smaller vesicles of the germinal wall.

In the later stages (fourth day and onwards) the whole germinal wall is
stated to break up into columnar hypoblast cells, each of them mainly
formed of one of the vesicles just spoken of. After the commencing
formation of the embryo the mesoblast becomes established at the inner
edge of the area opaca, between the germinal wall and the epiblast ; and
gives rise to the tissue which eventually forms the area vasculosa. It seems
probable that the mesoblast in this situation is mainly derived from cells
formed around the nuclei of the germinal wall, which are usually specially
aggregated close below the epiblast. Disse (No. 122) has especially
brought evidence in favour of this view, and my own observations also
support it.

The mesoblastic somites begin to be formed in the lateral
plates of the mesoblast before the closure of the medullary
folds. The first somite arises close to the foremost extremity of
the primitive streak, but the next is stated to arise in front
of this, so that the first formed somite corresponds to the second
permanent vertebra1. The region of the embryo in front of the
second formed somite — at first the largest part of the embryo —
is the cephalic region. The somites following the second are
formed in the regular manner, from before backwards, out
of the unsegmented posterior part of the embryo, which rapidly
grows in length to supply the necessary material (fig. 103). As
the somites retain during the early stages of development an
approximately constant breadth, their number is a fair test
of the length of the trunk. With the growth of the embryo the
primitive streak is continually 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 Further investigations in confirmation of this widely accepted statement are very
desirable.

B. III. I *

1 62

FIRST FORMATION OF THE EMBRYO.

FIG. 103. DORSAL VIEW
OF THE HARDENED BLASTO-

process the primitive streak undergoes important changes both
in itself and in its relation to the
embryo. Its anterior thicker part,
which is enveloped in the diverging
medullary folds, soon becomes distin-
guished in structure from the part
behind this, and placed symmetrically
in relation to the axis of the embryo
(fig. 103, a.pr\ and 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
the front part of the primitive streak.
The region of the embryo bird, where

the medullary folds diverge, is known DERM OF A CHICK WITH FIVE
as the sinus rhomboidalis, though it MESOBLASTIC SOMITES. THE

.,,.,, MEDULLARY FOLDS HAVE MET

has no connection with the similarly FOR PART OF THEIR EXTENT,
named structure in the adult. By the BUT HAVE NOT UNITED.
time that ten somites are formed the a-Pr- anterior part of the

..... primitive streak ; p-pr. pos-

Sinus rhomboidalis IS completely CS- terior part of the primitive

tablished, and the medullary groove streak-
has become converted into a tube till close up to the front end
of the sinus. In the following stages the closure of the
medullary canal extends to the sinus rhomboidalis, and the
folding off of the hind end of the embryo from the yolk
commences. Coincidently with the last-named changes the
sides of the front part of the primitive streak become thickened,
and give rise to conspicuous caudal swellings ; in which
the layers of the embryo are indistinguishably fused. The
apparently hinder part of the primitive streak becomes, as more
particularly explained in the sequel, folded downwards .and
forwards on the ventral side.

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 tube into the alimentary canal. They vary somewhat in different
species of birds, and it appears that in the same species there may be
several openings of the kind, which appear one after the other and then

AVES. 163

close again. They were first discovered by Gasser (No. 127). In all cases1
they lead round the posterior end of the notochord, or through the point
where the notochord falls into the primitive streak.

If the primitive streak is, as I believe, formed of the lips of the blasto-
pore, there can be but little doubt that these structures are disappearing,
and functionless rudiments of the opening of the blastopore, and they thus
lend support to my view as to the nature of the primitive streak. That, in
part, they correspond with the neurenteric canal of the Ichthyopsida is clear
from the detailed statements below. Till their relations have been more
fully worked out it is not possible to give a more definite explanation of
them.

According to Braun (No. 120) three independent communications are to
be distinguished in Birds. These are best developed in the Duck. The first
of these is a small funnel-shaped diverticulum leading from the neural groove
through the hypoblast. It is visible when eight mesoblastic somites are
present, and soon disappears. The second, which is the only one I have
myself investigated, is present in the embryo duck with twenty-six meso-
blastic somites, and is represented in the series of sections (fig. 104). The
passage leads obliquely backwards and ventralwards from the hind end
of the neural tube into the notochord, where the latter joins the primitive
streak (B). A narrow diverticulum from this passage is continued forwards
for a short distance along the axis of the notochord (A, ch}. After tra-
versing the notochord, the passage is continued into a hypoblastic diver-
ticulum, which opens ventrally into the future lumen of the alimentary
tract (C). Shortly behind the point where the neurenteric passage com-
municates 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.

This passage does not long remain open, but after its closure, when the
tail-end of the embryo has become folded off from the yolk, a third passage
is established, and leads round the end of the notochord from the closed
medullary canal into the post-anal gut. It is shewn diagrammatically in
fig. 1 06, ne, and, as may be gathered from that figure, has the same relations
as the neurenteric canal of the Ichthyopsida.

In the goose a passage has been described by Gasser, which appears
when about fourteen or fifteen somites are present, and lasts till twenty-
three are formed. Behind its opening the medullary canal is continued
back as a small diverticulum, which follows the course of the primitive
groove and is apparently formed by the conversion of this groove into a
canal. It is at first open to the exterior, but soon becomes closed, and then
atrophies.

In the chick there is a perforation on the floor of the neural canal,

1 This does not appear to be the case with the anterior opening in Melopsittacus
undulatus, though its relations are not clear from Braun's description (No. 120).

II— 2

164

NEURENTERIC 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 noto-

FlG. 104. FOUR TRANSVERSE SECTIONS THROUGH THE NEURENTERIC PASSAGE
AND ADJOINING PARTS IN A DUCK EMBRYO WITH TWENTY-SIX MESOBLASTIC
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 neurenteric 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. epiblast; hy. hypoblast; ch. notochord; pr. primitive

streak.

chord. 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 when about twelve somites are present. It is regarded by Braun as
equivalent to the first formed passage in the duck, but I very much doubt
whether there is a very exact equivalence between the openings in different
types, and think it. more probable that they are variable remnants of a
primitive neurenteric canal, which in the ancestors of those forms persisted
through the whole period of the early development. The third passage is
formed in the chick (Kupffer) during the third day of incubation. In

AVES. 165

Melopsittacus undulatus the two first communications are stated by Braun
(No. 120) to be present at the same time, the one in front of the other.

It is probable, from the above description, that the front portion of the
primitive streak in the bird corresponds with that part of the lips of the
blastopore in Elasmobranchii which becomes converted into the tail-swelling
and the lining of the neurentic canal ; while the original groove of the
front part of the primitive streak appears to be converted into the posterior
diverticulum of the neural canal. The hinder part of the primitive streak
of the bird corresponds, in a very general way, with the part of the blasto-
pore in Elasmobranchii, which shuts off the embryo from the edge of the
blastoderm (vide p. 64), though there is of course no genetic relation between
the two structures. When the anterior part of the streak is becoming
converted 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 the ventral and anterior
part1.

It has generally been maintained that the primitive streak and groove
become wholly converted into the dorsal portion of the trunk of the embryo,
i. e. into the posterior part of the medullary plate and subjacent structures.
This view appears to me untenable in itself, and quite incompatible with
the interpretation of the primitive streak given above. To shew how im-
probable it is, apart from any theoretical considerations, I have compiled
two tables of the relative lengths of the primitive streak and the body of
the embryo, measured by the number of sections made through them, in a
series of examples from the data in Gasser's important memoir (No. 127).
In these tables each horizontal line relates to a single embryo. The first
column shews the number of somites, and the second the number of sections

1 This nomenclature may seem a little paradoxical. But on reflection it will
appear that so long as the embryo is simply extended on the yolk-sphere, the point
where the ventral surface begins has to be decided on purely morphological grounds.
That point may fairly be considered to be close to the junction of the medullary plate
and primitive streak. To use a mathematical expression the sign will change when
we pass from the dorsal to the ventral surface, so that in strict nomenclature we
ought in continuing round the egg in the same direction to speak of passing backwards
along the medullary, but forwards along the primitive streak. Thus the apparent
hind end of the primitive streak is really the front end, and vice versa. I have
avoided using this nomenclature to simplify my description, but it is of the utmost
importance that the morphological fact should be grasped. If any reader fails to
understand my point, a reference to fig. 52 B will, I trust, make everything quite
clear. The heart of Acipenser (At) is there seen apparently in front of the head. It
is of course really ventral, and its apparent position is due to the extension of the
embryo on a sphere. The apparent front end of the heart is really the hind end, and
•vice versd.

1 66

HISTORY OF THE GERMINAL LAYERS.

through the primitive streak. Where the primitive streak becomes divided
into two parts the sections through the two parts are given separately : the
left column (A) referring to the anterior part of the streak ; the right
column (P) to the posterior part. The third column gives the number of
sections through the embryo. The first table is for fowl embryos, the
second for goose embryos.

No. of
Somites.

No. of
sections
through
the
Primitive
Streak.

No. of
sections
through
the
Embryo.

0

29

7

o

45

10

o

39

23

2

3°

30

4

3°

3°

A P

5 or 6

10+17 = 27

8

12 + 20 = 32

48

12

13+10 = 23

14

9+12 = 21

18

10+ 7 = 17

70

8+ 4=12

8+ 3 = 11

No. of
Somites.

No. of
sections
through
the
Primitive
Streak.

No. of
sections
through
the
Embryo.

o

10

4

o 28

5

o 44

12

36

32

4

24

42

A P

9

10+ 10 = 20

61

H

8+10 = 18

68

17

8+ 5 = i3

22

9+ 6=15

26

6+ 5 = 11

An inspection of these two tables shews that an actual diminution in
the length of the primitive streak takes place just about the time when the
first somites are being formed, but there is no ground for thinking that
the primitive streak becomes then converted into the medullary plate.
Subsequently the primitive streak does not for a considerable time become
markedly shorter, and certainly its curtailment is not really sufficient to
account for the increased length of the embryo — an increase in length,
which (with the exception of the head) takes place entirely by additions at
the hind end. At the stage with fourteen somites the primitive streak is
still pretty long. In the later stages, as is clearly demonstrated by the
tables, the diminution in the length of the primitive streak mainly concerns
the posterior part and not that adjoining the embryo.

General history of tJie germinal layers.

The epiblast. The epiblast of the body of the embryo,
though several rows of cells deep, does not become divided into
two strata till late in embryonic life ; so that the organs of sense
formed from the epiblast, which are the same as in the types
already described, are not specially formed from an inner
nervous stratum. The medullary canal is closed in the same

AVES.

167

manner as in Elasmobranchii, the Frog, etc., by the simple
conversion of an open groove into a closed canal. The closure
commences first of all in the region of the mid-brain, and
extends rapidly backwards and more slowly forwards. It is
completed in the Fowl by about the time that twelve meso-
blastic somites are formed.

The mesoblast. The general changes of this layer do not
exhibit any features of special interest — the division into lateral
and vertebral plates, etc., being nearly the same as in the lower
forms.

The hypoblast. The closure of the alimentary canal is
entirely effected by a process of tucking in or folding off of the
embryo from the yolk-sack. The general nature of the process
is seen in the diagrams figs. 105 and 121. The folds by which
it is effected are usually distinguished as the head-, the tail- and
the lateral folds. The head-fold (fig. 105) is the first to appear ;

JVC.

FIG.

105.

DIAGRAMMATIC LONGITUDINAL SECTION THROUGH THE AXIS OF AN
EMBRYO BIRD.

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. head-fold of the somatopleure. F.Sp. head-fold of the splanchnopleure.

pp. pleuroperitoneal cavity; Am. commencing (head-) fold of the amnion; D.
alimentary tract; N.C. neural canal; Ch. notochord; A. epiblast ; B. mesoblast;
C. hypoblast.

and in combination with the lateral folds gives rise to the
anterior part of the mesenteron (D) (including the oesophagus,
stomach and duodenum), which by its mode of formation clearly
ends blindly in front. The tail-fold, in combination with the
two lateral folds, gives rise to the hinder part of the alimentary
tract, including the cloaca, which is a true part of the mesen-
teron. At the junction between the two folds there is present

1 68

HISTORY OF THE GERMINAL LAYERS.

a circular opening leading into the yolk-sack, which becomes
gradually narrowed as development proceeds. The opening is
completely closed long before the embryo is hatched. Certain
peculiarities in reference to the structure of the tail-fold are
caused by the formation of the allantois, and are described with
the embryonic appendages. The stomodaeum and proctodaeum
are formed by epiblastic invaginations. The communication
between the stomodaeum and the mesenteron is effected com-
paratively early (on the 4th day in the chick), while that
between the proctodaeum and mesenteron does not take place
till very late (i$th day in the chick). The proctodaeum gives
rise to the bursa Fabricii, as well as to the anus. Although the

FIG. 106. DIAGRAMMATIC LONGITUDINAL SECTION THROUGH THE POSTERIOR
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. hypo-
blast; 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. peri-
visceral cavity ; am. amnion ; so. somatopleure ; sp. splanchnopleure.

opening of the anus is so late in being formed, the proctodaeum
itself is very early apparent. Soon after the hinder part of the
primitive streak becomes tucked in on the ventral side of the
embryo, an invagination may be noticed where the tail of the
embryo is folded off. This gradually becomes deeper, and
finally comes into contact with the hypoblast at the front
(primitively the apparent hind) border of the posterior section of
the primitive streak. An early stage in the invagination is
shewn in the diagram (fig. 106, an}. It deserves to be noted
that the anus lies some way in front of the blind end of

AVES. 169

the mesenteron, so that there is in fact a well-developed post-
anal section of the gut (fig. 106, p.a.g), which corresponds with
that in the Ichthyopsida. For a short period, as mentioned
above (p. 163), a neurenteric canal is present connecting the
post-anal gut with the medullary tube in the duck, fowl, and
other birds. On the ventral wall of the post-anal gut there are
at first two prominences. The posterior of these is formed of
part of the tail-swelling, and is therefore derived from the
apparent anterior part of the primitive streak. The anterior is
formed from what was originally the apparent posterior part of
the primitive streak. The post-anal gut becomes gradually less
and less prominent, and finally atrophies.

General development of the Embryo.

It will be convenient to take the Fowl as a type for the
general development of the Sauropsida.

The embryo occupies a fairly constant position with reference
to the egg-shell. Its long axis is placed at right angles to that
of the egg, and the broad end of the egg is on the left side of the
embryo. The general history of the embryo has already been
traced up to the formation of the first formed mesoblastic
somites (fig. 107). This stage is usually reached at about the
close of the first day. After this stage the embryo rapidly
grows in length, and becomes, especially in front and to the
sides, more and more definitely folded off from the yolk-
sack.

The general appearance of the embryo between the 3Oth and
4Oth hours of incubation is shewn in fig. 108 from the upper
surface, and in fig. 109 from the lower. The outlines of the
embryo are far bolder than during the earlier stages. Fig. 109
shews the nature of the folding, by which the embryo is con-
stricted off from the yolk-sack. The folds are complicated by
the fact that the mesoblast has already become split into two
layers — a splanchnic layer adjoining the hypoblast and a somatic
layer adjoining the epiblast — and that the body cavity between
these two layers has already become pretty wide in the lateral
parts of the body of the embryo and the area pellucida. The
fold by which the embryo is constricted off from the yolk-

I/O

GENERAL DEVELOPMENT OF THE EMBRYO.

sack is -in consequence a double one, formed of two limbs or
laminae, an inner limb constituted by
the splanchnopleure, and an outer limb
by the somatopleure. The relation of
these two limbs is shewn in the dia-
grammatic longitudinal section (fig.
105), and in the surface view (fig. 109)
the splanchnic limb being shewn at sf
and the somatic at so. Between the
two limbs, and closely adjoining the
splanchnopleure, is seen the heart (hf).
At the stage figured the head is well
marked off from the trunk, but the first
separation between the two regions was
effected at an earlier period, on the
appearance of the foremost somite (fig.
107). Very shortly after the cephalic
region is established, and before the
closure of the medullary folds, the an-
terior part of the neural canal becomes
enlarged to form the first cerebral
vesicle, from which two lateral diver-
ticula — rudiments of the optic lobes — are almost at once given
off (fig. 108, op.v). By the stage figured the cephalic part of the
neural canal has become distinctly differentiated into a fore-
(/.£), a mid- (m.b] and a hind-brain (k.b} ; and the hind-brain is
often subdivided into successive lobes. In the region of the
hind-brain two shallow epiblastic invaginations form the rudi-
ments of the auditory pits (au. p}.

A section through the posterior part of the head of an embryo
of 30 hours is represented in fig. no. The enlarged part of the
neural tube, forming the hind-brain, is shewn at (hb). It is
still connected with the epidermis, and at its dorsal border an
outgrowth on each side forming the root of the vagus nerve is
present (vg). The notochord (ch) is seen below the brain, and
below this again the crescentic foregut (al). The commencing
heart (hi), formed at this stage of two distinct tubes, is attached
to the ventral side of the foregut.

On the dorsal side of the foregut immediately below the notochord is

FIG. 107. DORSAL VIEW
OF THE HARDENED BLASTO-
DERM OF A CHICK WITH FIVE
MESOBLASTIC SOMITES. THE
MEDULLARY FOLDS HAVE MET

FOR PART OF THEIR EXTENT,
BUT HAVE NOT UNITED.

a.pr. anterior part of the
primitive streak ; f-pf. pos-
terior part of the primitive
streak.

AVES.

171

ojt.v:

seen a small body (x) formed as a thickening of the hypoblast. This may
possibly be a rudiment of the subnotochordal rod of the Ichthyopsida.

In the trunk (fig. 108) the chief point to be noticed is the
complete closure of the neural canal, though in the posterior
part, where the open sinus rhomboidalis was situated at an
earlier stage, there may still be seen a dilatation of the canal
(fig. 1 08, s.r}, on each side of which are the tail-swellings ; while
the mesoblastic somites stop short somewhat in front of it.
Underneath the neural canal may be seen the notochord (fig.
109, cJi) extending into the head, as far as the base of the mid-
brain. At the sides of the trunk are seen the mesoblastic
somites (/. v), the outer edges of which mark the boundary
between the vertebral and lateral plates. A fainter line can be
seen marking off the part of the lateral plates which will become

FIG. 108. EMBRYO OF THE
CHICK BETWEEN 30 AND 36
HOURS VIEWED FROM ABOVE

AS AN OPAQUE OBJECT. (Chro-
mic acid preparation.)

f.b. front-brain; m.b. mid-
brain; h.b. hind-brain; op.v.
optic vesicle ; ati.p. auditory
pit; o.f. vitelline vein; p.v.
mesoblastic somite; m.f. line of
junction of the medullary folds
above the 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
distinctly marked off; but
neither the somatopleuric nor
splanchnopleuric folds are shewn
in the figure ; the latter diverge
at the level of o.f., the former
considerably nearer the front,
somewhere between the lines
m.b. and h.b. The optic vesi-
cles op.v. are seen bulging out
beneath the superficial cpiblast.
The heart lying underneath the opaque body cannot be seen. The tail-fold t. is just
indicated; no distinct 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 still visible,
being lost forwards over the cerebral vesicles, while behind may be seen the remains
of the sinus rhomboidalis, s.r.

P-r

172

DEVELOPMENT DURING THE SECOND DAY.

FIG. 109. 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 con-
stituted, the backward limit of the somatopleure
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 <z,
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 fold extends as far back
as sp. Along its diverging limbs are seen the
conspicuous venous roots of 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.

HB. hind-brain; MB. mid-brain; p.v. and
v.pl. mesoblastic somites ; ch. front end of noto-
chord; me. posterior part of notochord; £. parietal
mesoblast; //. outline of area pellucida; pv.
primitive streak.

hi,

FIG. 110. TRANSVERSE SECTION THROUGH THE POSTERIOR PART OF THE HEAD

OF AN EMBRYO CHICK OF THIRTY HOURS.

lib. hind-brain; vg. vagus nerve; ep. epiblast; ch. notochord; x. thickening of
hypoblast (possibly a rudiment of the subnotochordal rod); al. throat; hi. heart;
pp. body cavity ; so. somatic mesoblast ; s/. splanchnic mesoblast ; hy. hypoblast.

AVES.

173

part of the body-wall, from that which pertains to the yolk-
sack.

During the latter half of the second day, and during the
third day, great progress is made in the folding off of the

FlG. III. CHICK OF THE THIRD
DAY (54 HOURS) VIEWED FROM
UNDERNEATH AS A TRANSPARENT

IIB

OBJECT.

a', 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, t.

In the embryo of which this is a
drawing the head-fold of the amnion
reached a little farther backward
than the reference u, but its limit
cannot be distinctly seen through
the body of the embryo.

C.H. cerebral hemisphere; F.B.
vesicle of the third ventricle ; M.S.
mid-brain; H.B. hind-brain; Op.
eye; Ot. auditory vesicle.

OfV. 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 somites on either side, are not clearly
seen. Their branches however, Of. a, the vitelline arteries, are conspicuous and are
seen to curve round the commencing side- folds.

Pv. mesoblastic somites.

x is placed at the "point of divergence " of the splanchnopleure folds. The blind
foregut begins here and extends about up to near y, the more transparent space
marked by that letter is however mainly due to the presence there of investing mass
at the base of the brain, x marks the hind limit of the splanchnopleure folds. The
limit of the more transparent somatopleure folds cannot be seen.

It will be of course understood that all the body of the embryo above the level of
the reference x, is seen through the portion of the yolk-sack (vascular and pellucid area),
which has been removed with the embryo from the egg, as well as through the double
amniotic fold.

The view being from beiow, whatever is described in the natural position as being
to the right appears here to the left, and vice versft.

174 DEVELOPMENT DURING THE THIRD DAY.

embryo. Both the head- and tail-ends of the embryo become
quite distinct, and the side-folds make such considerable pro-
gress that the embryo is only connected with the yolk by a
broad stalk. This stalk is double, and consists of an inner
splanchnic stalk, continuous with the walls of the alimen-
tary canal, and an outer somatic stalk, continuous with the
body-walls of the embryo. The somatic stalk is very much
wider than the splanchnic. (Compare fig. 121 E and F, which
may be taken as diagrammatic longitudinal and transverse
sections of the embryo on the third day.) A change also takes
place in the position of the embryo. Up to the third day it is
placed symmetrically, on the yolk, with its ventral face down-
wards. During this day it turns so as partially to lie on its left
side. This rotation affects first the head (fig. in), but in the
course of the fourth day gradually extends to the rest of the
body (fig. 1 1 8). Coincidently with this change in position the
whole embryo undergoes a ventral and somewhat spiral flexure.

During the latter part of the second day and during the
third day important changes take place in the head. One
of these is the cranial flexure. This, which must not be con-
founded with the curvature of the body just referred to, com-
mences by the bending downwards of the front part of the head
round a point which may be considered as the extreme end
either of the notochord or of the alimentary canal.

The cranial 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, so completely has the front end of the neural canal
been folded over the end of the notochord. The commencement
of this cranial flexure gives the body of an embryo of the third
day somewhat the appearance of a chemist's 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.

The anterior part of the fore-brain has now become greatly

AVES.

175

dilated, and may be distinguished from the posterior part as the
unpaired rudiment of the cerebral hemispheres. It soon bulges
out laterally into two lobes, which do not however become
separated by a median partition till a much later period.

Owing to the development of the
cerebral rudiment the posterior part
of the fore-brain no longer occupies
the front position (fig. HI, and 112
FB], and ceases to be the conspicu-
ous object that it was. Inasmuch as
its walls will hereafter be developed
into the parts surrounding the so
called third ventricle of the brain, it
is known as the vesicle of the third
ventricle, or the thalamencephalon.

On the summit of the thalamen-
cephalon there may now be seen a
small conical projection, the rudi-
ment of the pineal gland, while the
centre of the floor is produced into a
funnel-shaped process, the infundi-
bulum, which, stretching towards
the extreme end of the alimentary
canal, joins the pituitary body.

Beyond an increase in size,
which it shares with nearly all parts
of the embryo, and the change of
position which has already been
referred to, the mid-brain undergoes
no great alterations during the third
day. Its sides will ultimately become developed into the
corpora bigemina 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 and two
diverticula leading from this into the optic lobes.

In the hind-brain, or third cerebral vesicle, the roof of the
part which lies nearest to the mid-brain, becomes during the
third day marked off from the rest by a slight constriction.
This distinction, which becomes much more evident later on by

FIG. 112. SIDE VIEW OF THE
HEAD OF AN EMBRYO CHICK OF
THE THIRD DAY AS AN OPAQUE
OBJECT. (Chromic acid prepa-
ration.)

CH. Cerebral hemispheres ;
F.B. Vesicle of third ventricle ;
M.B. Mid-brain; Cb. Cerebel-
lum; H.B. Medulla oblongata ;
N. Nasal pit ; ot. auditory vesicle
in the stage of a pit with the
opening not yet closed up ; op.
Optic vesicle, with /. lens and
ch.f. choroidal fissure. The cho-
roidal fissure, though formed en-
tirely underneath the superficial
epiblast, is distinctly visible from
the outside.

i F. The first visceral fold ;
above it is seen a slight indica-
tion of the superior maxillary
process.

•2, 3, 4 F. Second, third and
fourth visceral folds, with the
visceral clefts between them.

176 DEVELOPMENT DURING THE THIRD DAY.

a thickening of the walls and roof of the front portion, separates
the hind-brain into the cerebellum and the medulla oblongata
(fig. 1 1 2 Cb and HB\ While the walls of the cerebellar portion
of the hind-brain become very much thickened as well at the
roof as at the sides, the roof of the posterior portion or medulla
oblongata thins out into a mere membrane, forming a delicate
covering to the cavity of the vesicle (fig. 114 IV\ which here
becoming broad and shallow with greatly thickened floor and
sides, is known as the fourth ventricle, subsequently overhung
by the largely-developed posterior portion of the cerebellum.

FIG. 113. HEAD OF AN EMBRYO CHICK OF THE FOURTH DAY VIEWED
AS AN OPAQUE OBJECT : FROM THE FRONT IN A, AND FROM THE SIDE IN B.
(Chromic acid preparation.)

CH. cerebral hemispheres ; FB. vesicle of the third ventricle ; Op. eyeball ; nf.
naso-frontal process; M. cavity of mouth; SM. superior maxillary process of F. i,
the first visceral fold (inferior maxillary process) ; F. 2, F. 3, second and third
visceral folds ; N. nasal pit ; ot. otic vesicle.

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
a!, the neural canal n.c., the notochord cA, the dorsal aorta AO, and the vertebral
veins V.

The third day, therefore, marks the distinct differentiation of
the brain into five distinct parts : the cerebral hemispheres, the
central masses round the third ventricle, the corpora bigemina,
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

AVES.

177

third ventricle, the iter (with a prolongation into the optic lobe
on each side), and the fourth ventricle.

By the third day the lens of the eye has become formed by
an invagination of the epiblast, and other changes in the eye
have taken place. The external opening of the auditory pit is
closed before the completion of the third day (fig. 114, RL) ;
and the rudiments of the external parts of the organ of smell
have become formed as small pits on the under surface of the
fore-brain (fig. 112, N). Like the lens and the labyrinth of the
ear, they are formed as invaginations of the external epiblast ;
unlike them they are never closed up.

During the second and third days there are formed the
visceral or branchial clefts, homologous with those of the

cv

cc

FIG. 114. SECTION THROUGH THE HIND-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. Ch. Notochord; CV. Anterior cardinal vein; CC. Involuted auditory
vesicle ; CC points to the end which will form the cochlear canal ; RL . Recessus
labyrinth! (remains of passage connecting the vesicle with the exterior); hy. Hypoblast
lining the alimentary canal; AO., AOA. Aorta, and aortic arch.

Ichthyopsida, though never developing branchial processes from
their walls.

They are however real clefts or slits passing right through
the walls of the throat, and are placed in series on either side
B. in. 12

178 VISCERAL ARCHES.

across the axis of the alimentary canal, lying not quite at right
angles to that axis nor parallel to each other, but converging
somewhat to the middle of the throat in front (fig. 112 and

fig. US)-

Four in number on either side, the anterior is the first to be
formed, the other three following in succession. They originate
as pouches of the hypoblast, which meet the epiblast. At the
junction of the epiblast and hypoblast an absorption of the
tissue is effected, placing the pouches in communication with the
exterior.

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 (figs. 1 1 2
and 1 1 3). The last two folds however, and especially the last,
are not nearly so thick and prominent as the other three,
the second 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 (fig. 113 A).

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
mesoblast of the body.

The history of these most important visceral folds and clefts
will be dealt with in detail hereafter ; meanwhile I may say that
in the Chick and higher Vertebrates the first three pairs of folds
are those which call for most notice.

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 upper edge (fig. 113). This branch, starting from near
the outer end of the fold, runs forwards and upwards in front
of the stomodaeum, tending to meet the corresponding branch

AVES.

179

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
(fig. 1 1 3, sm}. 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 (fig. 120, /£). Between the main folds, which are
directed somewhat downwards and their branches which slant
upwards the somewhat lozenge-shaped stomodseum is placed,

M.c.

So.

So.

FIG. 115. TRANSVERSE SECTION THROUGH THE DORSAL REGION OF AN EMBRYO

CHICK OF 45 HOURS.

M.c . medullary canal ; P.v. mesoblastic somite ; W.d. Wolffian duct ; So. Somato-
pleure ; S.p. Splanchnopleure ; /./. pleuroperitoneal cavity ; ao. aorta ; v. blood-
vessels; iv. germinal wall; ch. notochord; op. junction between area opaca and area
pellucida.

which, as the folds become more and more prominent, grows
deeper and deeper (fig. 120 A). The main folds form the
mandibular arch, and their branches the maxillary processes,
and the descending process which helps to complete the anterior
margin of the stomodseum or oral cavity is called, from the parts
which will be formed out of it, t\\Qfronto-nasal process.

In two succeeding pairs of visceral folds, which correspond
with the hyoid and first branchial arches of the Ichthyopsida,
are developed the parts of the hyoid bone, which will be best

12 — 2

ISO SECTIONS DURING THE SECOND AND THIRD DAY.

considered in connection with the development of the skull.
The last two disappear in the Chick without giving rise to any
permanent structures. The external opening of the first visceral
i.e. hyomandibular cleft becomes closed1, but the inner part
of the cleft, opening into the mouth, gives rise to the Eustachian
tube and the tympanic cavity, the latter being formed as a
special diverticulum.

Part of the membranous mandibular and hyoid arches form a wall round
the dorsal part of the original opening of this cleft, and so give rise to the
meatus auditorius externus. At the bottom of this is placed the tympanic
membrane, which is probably derived from the tissue which grows over the
dorsal part of the opening of the first cleft. It is formed of an external
epiblast epithelium, a middle layer of mesoblast, and an internal hypoblastic
epithelium.

FIG. 1 16. 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. seg-
mental tube; ca.v. cardinal vein; ms. muscle-plate; sp.g. spinal ganglion; sp.c. spinal
cord ; c h. notochord ; ao. aorta ; hy. hypoblast.

1 Vide Moldenhauer, " Die Entwicklung des mittleren und des ausseren Ohres."
Morphologisches Jahrbuch, Vol. III. 1877.

AVES.

181

The general nature of the changes, which take place in the
trunk between the commencement of the second half of the
second day and the end of the third day, is illustrated by the
sections figs. 115, 116, 117.

tup

so

117.

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. Wolffian duct ; W.b. commencing differen-
tiation of the mesoblast cells to form the Wolffian body ; ep. epiblast ; So. somato-
pleure ; Sp. splanchnopleure; hy. hypoblast. The section passes through the point
where the digestive canal communicates with the yolk-sack, and is consequently still
open below.

In the earliest of these sections there is not a trace of a fold-
ing off of the embryo from the yolk, and the body walls are
quite horizontal. In the second section (fig. 116), from an
embryo of about two days, the body walls are already partially
inclined, and the splanchnopleure is very distinctly folded
inwards. There is a considerable space between the notochord
and the hypoblast, which forms the rudiment of the mesentery.

1 82 SECTIONS DURING THE SECOND AND THIRD DAY.

In the third section (fig. 117) the body walls have become
nearly vertical, the folding of the splanchnopleure is nearly
completed, and it is only for a small region that the alimentary
tract is open, by the vitelline duct, to the yolk-sack.

These three sections further illustrate (i) the gradual diffe-

ffJ3

FIG. 118. EMBRYO CHICK 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.H. cerebral hemisphere; F.B. vesicle of the third ventricle with the pineal
gland (Pn) projecting from its summit; M.B. mid-brain; Cb. cerebellum. IV. V.
fourth ventricle; Z. lens; ch.s. choroid slit. Owing to the growth of the optic cup
the two layers of which it is composed cannot any longer be seen from the surface,
but the retinal surface of the layer alone is visible. Cen. V. auditory vesicle; s.rn.
superior maxillary process; i f, if, etc. first, second, third and fourth visceral
arches; V. fifth nerve sending one branch to the eye, the ophthalmic branch, and
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. pneumo-
gastric nerve passing towards the fourth visceral arch; iv. investing mass. 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. notochord. 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;
//. /.. hind limb. Beneath the hind limb is seen the curved tail.

AVKS.

183

rentiation of the mesoblastic somites (fig. 115, P.v] into (a) the
muscle-plates (figs. 116, ms and 117, m.p), and (b} the tissue to
form the vertebral bodies and adjacent connective tissue ; (2) the
formation of a mass of tissue between the lateral plates and the
mesoblastic somites (fig. 115), known as the intermediate cell
mass, on the dorsal side of which the Wolffian duct is formed,
while the intermediate cell mass itself breaks up into the seg-
mental tubes (fig. 116, st) and connective tissue of the Wolffian
body.

W.R

FIG. 119. SECTION THROUGH THE LUMBAR REGION OF AN EMBRYO CHICK AT THE
END OF THE FOURTH DAY.

n.c. neural canal; p.r. posterior root of spinal nerve with ganglion; 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. Wolffian ridge;
A 0. dorsal aorta ; V.c.a. posterior cardinal vein ; W.d. Wolffian duct; W.b. Wolffian
body, consisting of tubules and Malpighian bodies ; g.e. germinal epithelium ; d. ali-
mentary canal ; M. commencing mesentery; SO. somatopleure ; SP. splanchnopleure ;
V. blood-vessels ; pp. pleuroperitoneal cavity.

1 84

DEVELOPMENT DURING THE FOURTH DAY.

Various other features in the development of the vascular
system, general mesoblast, etc., are also represented in these
sections. It may more especially be noted that there are at
first two widely separated dorsal aortae, which gradually approach
(figs. 115 and 116); and meeting first of all in front finally
coalesce (figs. 1 17 and 119) for their whole length.

The general appearance of the embryo of the fourth day may
be gathered from fig. 118.

FIG. 120. HEAD OF A CHICK FROM BELOW ON THE SIXTH AND SEVENTH DAYS
OF INCUBATION. (From Huxley.)

/". cerebral vesicles; a. eye, in which the remains of the choroid slit can still be
seen in A ; g . nasal pits ; k. fronto-nasal process ; /. superior maxillary process ;
i. inferior maxillary process or first visceral arch; ?.. second visceral arch; x. first
visceral cleft.

In A 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.

In B the external opening of the mouth has become much constricted, 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
nearly the whole length of the latter.

The changes which have taken place consist for the most
part in the further development of the parts already present, and
do not need to be specified in detail. The most important event
of the day is perhaps the formation of the limbs. They appear
as outgrowths from a slightly marked lateral ridge (fig. 1 19, WR],
which runs on the level of the lower end of the muscle-plates for

AVES. 185

nearly the whole length of the trunk. This ridge is known as
the Wolffian ridge. The first trace of the limbs can be seen
towards the end of the third day ; and their appearance at the
end of the fourth day is shewn in fig. 1 18, W and HL.

A section through the trunk of the embryo on the fourth day
is represented in fig. 119. The section passes through the region
of the trunk behind the vitelline duct. The mesentery (M) is
very much deeper and thinner than on the previous day. The
notochord has become invested by a condensed mesoblastic
tissue, which will give rise to the vertebral column. The two
dorsal aortae have now completely coalesced into the single
dorsal aorta, and the Wolffian body has reached a far more
complete development.

In the course of the fifth day the face begins to assume a less
embryonic character, and by the sixth and succeeding days
presents distinctive avian characters.

The general changes which take place between the sixth
day and the time of hatching do not require to be specified in
detail.

Fcetal Membranes.

The Reptilia, Aves and Mammalia are distinguished from
the Ichthyopsida by the possession of certain provisional fcetal
membranes, known as the amnion and allantois.

As the mode of development of these membranes may be
most conveniently studied in the Chick, I have selected this
type for their detailed description.

The Amnion. The amnion is a peculiar sack which en-
velopes and protects the embryo.

At the end of the first day of incubation, when the cleavage of
the mesoblast has somewhat advanced, there appears, a little way
in front of the semilunar head-fold, a second fold (fig. 102, also fig.
121 C, d/ and fig. 122, Am], running more or less parallel or rather
concentric with the first and not unlike it in general appearance,
though differing widely from it in nature. This second fold
gives rise to the amnion, and is limited entirely to the somato-
pleure. Rising up as a semilunar fold with its concavity directed
towards the embryo (fig. 121 C, of], as it increases in height it

1 86

HKTAI, MEMBRANES.

is gradually drawn backwards over the developing head of the
embryo. The fold thus covering the head is in due time
accompanied by similar folds of somatopleure, starting at some

A /

-—/—- pj*

FIG. 121.

Fig. 121 A to N forms a series of purely diagrammatic representations 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-sack, amnion, and allantois.

In all vt is the vitelline membrane, placed, for convenience sake, at some distance
from its contents, and represented as persisting in the later stages; in reality it is
in direct contact with the blastoderm or yolk, and early ceases to have a separate
existence. In all e indicates the embryo proper; pp the general pleuroperitoneal
space with its extension between the membranes; of the folds of the amnion; a the
amnion proper ; ae or ac the cavity holding the liquor amnii ; al the allantois ; a the
alimentary canal ; y or ys the yolk or yolk-sack.

A, which may be considered as a vertical section taken longitudinally 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 embryo e. The
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 distance 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.

AVES.

I87

Incidentally it shews the formation of the medullary groove by the rising up
of the lamina; 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 outwards again. At a little distance
from both the head and the tail it is raised up into a fold, af, of, that in front of the
head being the highest. These are the amniotic folds. Descending from either fold,

1-1

I 88 FCETAL iMEMBRANES.

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 somatopleure
and the splanchnopleure is shaded with dots, pp. Close 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 C), shewn as a bud from the splanchnopleure, stretching downwards into
the pleuroperitoneal cavity //. The clotted area representing as before the 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 of.

E, also a longitudinal section, represents a stage still farther advanced. Both
splanchnic and somatic stalks are much narrowed, especially the former, the cavity of
the alimentary canal being now connected with the cavity of the yolk 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 splanchnopleure.
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 splanchnopleure is obvious.
The splanchnopleure, more or less thickened, 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 of. 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.

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

In H a longitudinal, and I 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 membrane
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 simplicity.

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

In I the splanchnopleure is seen converging to complete the closure of the ali-
mentary canal a' even at the stalk (elsewhere the canal has of course long been closed

AVES.

189

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.

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-sack, 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, cleavage of the mesoblast has
been carried all round the yolk (ys) except at the very 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-sack (ys) is therefore quite loose in the
pleuroperitoneal cavity, being connected only with the alimentary canal (a1) by a
solid pedicle.

Lastly, in N the yolk-sack (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 juxtaposition.

little distance behind the tail, and at some little distance from
the side (fig. 121 C, D, E, F, and 116, am}. In this way the

FCETAL MEMBRANES.

embryo becomes surrounded by a series of folds of thin somato-
pleure, which form a continuous wall all round it. All are
drawn gradually over the body of the embryo, and at last meet
and completely coalesce (fig. 121, H, I, and 117, Am), all traces
of their junction being removed. Beneath these united folds
there is therefore a cavity, within which the embryo lies (fig. 121
H, ae). This cavity is the cavity of the amnion.

Each fold is necessarily formed 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 fig. 121, is really part of the
space between the somatopleure and splanchnopleure ; it is
therefore continuous with the general space, part of which
afterwards becomes the pleuroperitoneal cavity of the body,
shaded with dots in the figure and marked (//) ; so that it is
possible to pass from the cavity between the two limbs of the
amniotic folds 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
unite to form a continuous inner membrane or sack, and all

JVC.

f.So

FIG. 112. 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; D. fore-gut.

//. pleuroperitoneal cavity between somatopleure and splanchnopleure ; Am. com-
mencing (head) fold of the amnion. For remaining reference letters vide p. 167.

their outer limbs a similarly continuous outer membrane or sack.
The inner membrane thus built up forms a completely closed
sack round the body of the embryo, and is called the amniotic

AVES. Ipl

sack, or amnion proper (fig. 121, H, I, &c., a), and the fluid which
it afterwards contains is called the amniotic fluid, or liquor
amnii. The space between the inner and outer sack is, from
the mode of its formation, simply a part of the general cavity
found everywhere between somatopleure and splanchnopleure.
The outer sack over the embryo lies close under the vitelline
membrane, and the cavity between it and the true amnion is
gradually extended over the whole yolk-sack.'

The actual manner in which the amniotic folds meet is somewhat
peculiar (His and Kolliker). The head-fold of the amnion is the earliest
formed, and completely covers over the head before the end of the second
day. The side and tail folds are later in developing. The side-folds finally
meet in the 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. This also becomes closed early on the third day.

The allantois1 is essentially a diverticulum of the alimentary
tract into which it opens immediately in front of the anus. Its
walls are formed of splanchnic mesoblast with blood-vessels,
within which is a lining of hypoblast. 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 the mesen-
teron the splitting of the mesoblast into somatopleure and
splanchnopleure has extended up to the border of the hinder
division of the primitive streak. As has been already mentioned,
the ventral wall of the postanal section of the alimentary tract
is formed by the primitive streak. Immediately in front of this
is the involution which forms the proctodaeum ; while the wall
of the hindgut in front of the anus owes its origin to a folding in
of the splanchnopleure.

The allantois first appears as a protuberance of the splanchno-
pleure just in front of the anus. This protuberance arises, how-
ever, before the splanchnopleure has begun to be tucked in so as

1 For details on the development of the allantois the reader is referred to the works
of Kolliker (No. 135), Gasser (No. 127), and for a peculiar view on the subject Kupffer
(No. 136). In addition to these works he may refer to Dobrynin " Ueber die erste
Anlage der Allantois." Sitz. der k. Akad. Wien, Bd. 64, 1871. E. Gasser, Beitrdge
zur Entwicklungsgeschichte d. Allantois, etc.

192

ALLANTOIS.

to form the ventral wall of the hindgut ; and it then forms a
diverticulum (fig. 123 A, All} the open end of which is directed
forward, while its blind end points somewhat upwards and
towards the peritoneal space behind the embryo.

As the hindgut becomes folded in the allantois shifts its
position, and forms (figs. 123 B and 124) a rather wide vesicle

A.

S.O-

FlG. 123. TWO LONGITUDINAL SECTIONS OF THE TAIL-END OF AN EMBRYO

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.)

/. the tail; m. the mesoblast of the body, about to form the mesoblastic somites;
x'. the roof of x" . the neural canal ; Dd. the hind end of the hindgut ; So. somato-
pleure; Spl. splanchnopleure ; u. the mesoblast of the splanchnopleure carrying the
vessels of the yolk-sack ; pp. pleuroperitoneal cavity ; Df. the epithelium lining the
pleuroperitoneal cavity; All. the commencing allantois; r.'. projection formed by
anterior and posterior divisions of the primitive streak; y. hypoblast which will form
the ventral wall of the hindgut ; v. anal imagination ; G. cloaca.

lying immediately below 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 connected with the cloaca
by a narrow canal which forms its neck or stalk (fig. 1 2 1 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-sack. It does not during the third 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

AVES. 193

splanchnic and somatic 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. 121 K). In this space it 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

FIG. 124. DIAGRAMMATIC LONGITUDINAL SECTION THROUGH THE POSTERIOR
END OF AN EMBRYO BlRD AT THE TIME OF THE FORMATION OF THE ALLANTOIS.

ep. epiblast; Sp.c. spinal canal; ch. notochord; n.e. neurenteric canal ; hy. hypo-
blast ; 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. peri-
visceral cavity ; am. amnion; so. somatopleure; sp. splanchnopleure.

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. 118,
A I}. At the same time its blood-vessels become important. It
receives its supply of blood from two branches of the iliac arteries
known as the allantoic arteries1, and the blood is brought back
from it by two allantoic veins which run along in the body walls
(fig. 119) and after uniting into a single trunk fall into the
vitelline vein close behind the liver.

Before dealing with the later history of the fcetal membranes,
it will be convenient to complete the history of the yolk-sack.

Yolk-Sack. The origin of the area opaca has already
been described. It rapidly extends over the yolk underneath
the vitelline membrane ; and is composed of epiblast and of the

1 I propose to call these arteries and the corresponding veins the allantoic arteries
and veins, instead of using the confusing term ' umbilical.'

B. III. 13

194

YOLK-SACK.

hypoblast of the germinal wall continuous with that of the area
pellucida, which on the fourth day takes the form of a more or
less complete layer of columnar cells1. Between the epiblast
and hypoblast there is a layer of mesoblast, which does not
extend as far as the two other layers. The yolk is completely
surrounded by the seventh day.

Towards the end of the first day blood-vessels begin to be

£CaV.

L.O/.A

L.of

FIG. 125. 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; 51. 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 vein; V.Ca. inferior cardinal vein. The veins are marked
in outline and the arteries are black. The whole blastoderm has been removed from
the egg and is supposed to be viewed from below. Hence the left is seen on the light,
and vice vcrsA.

1 Further investigations are required as to the character of this layer.

AVES. 195

developed in the inner part of the mesoblast of the area opaca.
Their development is completed on the second day ; and the
region through which they extend is known as the area vascu-
losa. The area vasculosa also grows round the yolk, and
completely encloses it not long after the area opaca. The part
of the blastoderm which thus encloses the yolk forms the yolk-
sack. The splitting of the mesoblast gradually extends to the
mesoblast of the yolk-sack, and eventually the somatopleure of
the sack, which is continuous, it will be remembered, with the
outer limb of the amnion, separates completely from the
splanchnopleure ; and between the two the allantois inserts
itself. These features are represented in fig. 121 E, K,
and L.

The circulation of the yolk-sack is most important during
the third day of incubation. The arrangement of the vessels
during that day is shewn in fig. 125.

The blood leaving the body of the embryo by the vitelline
arteries (fig. 125, R.Of.A, L.Of.A], which are branches of the
dorsal aortae, is carried to the small vessels and capillaries of
the vascular area, a small portion only being appropriated by
the pellucid area.

From the vascular area part of the blood returns directly to
the sinus venosus by the main lateral trunks of the vitelline
veins (R.Of., L.Of), and so to the heart. During the second
day these venous trunks join the body of the embryo consider-
ably 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 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 a venous trunk bounding the
vascular area, which is known as the sinus terminalis, 5. T., and
there divides on each side into two streams. Of these, the two
which, one on either side, flow backward, meet at a point about
opposite 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 vitelline vein. The

13—2

196 FCETAL MEMBRANES.

two forward streams reaching a gap in the front part of the
sinus terminalis fall into either one, or in some cases two veins,
which run straight backwards 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 disap-
pears. After the third day, although the vascular area goes
on increasing in size until it finally all but encompasses the
yolk, the prominence of the sinus terminalis becomes less and
less.

The foetal membranes and the yolk-sack may conveniently
be treated of together in the description of their later changes
and final fate.

On the sixth and seventh days they exhibit changes of great
importance.

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 then 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. By the seventh day very obvious
movements begin to appear in the amnion itself; slow ver-
micular contractions creeping rhythmically over it. The amnion
in fact begins to pulsate slowly and rhythmically, and by its
pulsation the embryo is rocked to and fro in the egg. This
pulsation is probably due 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.
Similar movements are also seen in the allantois at a consider-
ably 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

AVES. 197

amnion, that is, between the amnion proper and the false amnion
or serous envelope. It is filled with fluid, so that in spite of its
flattened form its opposite walls are distinctly separated from
each other.

The vascular area has become still further extended than on
the fifth day, but with a corresponding loss in the definite cha-
racter 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, assuming 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 von Baer) increased. This can only be
due to its absorbing the white of the egg, which indeed is dimin-
ishing rapidly.

During the eighth, ninth, and tenth days, the amnion 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, i.e. over about three-quarters of the
yolk-sack. The somatopleure at this point is continuous (as can
be easily seen by reference to fig. 121) 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-sack as a flattened bag filled with fluid, it now serves as the
chief organ of respiration. It is indeed very vascular and a
marked difference may be observed between the colour of the
blood in the outgoing and the returning vessels.

The yolk now begins to diminish rapidly in bulk. The yolk-
sack becomes flaccid, and on the eleventh day is thrown into a
series of internal folds, abundantly supplied by large venous
trunks. By this means the surface of absorption is largely in-
creased, and the yolk is more and more rapidly taken up by the

198 FCETAL MEMBRANES.

blood-vessels, and in a partially assimilated condition transferred
to the body of the embryo1.

By the eleventh day the abdominal parietes, though 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 allantois and the solid
cord suspending the yolk-sack.

The cleavage of the mesoblast is still progressing, and the
yolk is completely invested by a splanchnopleural sack.

The allantois meanwhile spreads out rapidly, and lies over
the embryo close under the shell, being separated from the shell
membrane by nothing more than the attenuated serous envelope,
formed out of the outer primitive fold of the amnion and the
remains of the vitelline membrane. With this 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.

On about the sixteenth day, the white having entirely dis-
appeared, the cleavage of the mesoblast is carried right over the
pole of the yolk opposite the embryo, and is thus completed (fig.
121). The yolk-sack 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 pleuroperitoneal
cavity of the body of the embryo. Deposits of urates now become
abundant in the allantoic fluid.

The loose and flaccid walls of the abdomen enclose a space
which the empty intestines are far from filling, and on the nine-
teenth day the yolk-sack, diminished greatly in bulk but still of
some considerable size, is withdrawn through the somatic stalk
into the abdominal cavity, which it largely distends. Outside the
embryo there now remains nothing but the highly vascular

1 For details on this subject vide A. Courty, "Structure des Appendices Vitellins
chez le Poulet." An. Set. Nat. Ser. III. Vol. IX. 1848.

AVES. 199

allantois and the bloodless serous membrane and amnion. The
amnion, whose fluid during the later days of incubation rapidly
diminishes, is continuous 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 meso-
blast and the withdrawal of the yolk-sack, entirely separated
from the embryo. The cavity of the allantois, by means of its
stalk passing through the umbilicus, is of course continuous with
the cloaca.

When the chick is about to be hatched it thrusts its beak
through the egg-membranes and begins to breathe the air con-
tained in the air chamber. Thereupon the pulmonary circulation
becomes functionally active, and at the same time blood ceases
to flow through the allantoic 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 remains of allantois, amnion and serous
membrane, and steps out into the world.

BIBLIOGRAPHY.

(117) K. E. vonBaer. " Ueb. Enhvicklungsgeschichte d. Thiere." Konigsberg,
1828—1837.

(118) F. M. Balfour. "The development and growth of the layers of the
Blastoderm," and "On the disappearance of the Primitive Groove in the Embryo
Chick." Quart. J. of Micros. Science, Vol. XIII. 1873.

(119) M. Braun. " Die Entwicklung d. Wellenpapagei's." Part I. Arbeit, d.
zool.-zoot. Instit. Wiirzburg. Vol. v. 1879.

(120) M. Braun. "Aus d. Entwick. d. Papageien; I. Riickenmark; II.
Entwicklung d. Mesoderms; III. Die Verbindungen zwischen Riickenmark u. Darm
bei Vogeln." Verh. d. phys.-med. Ges. zu Wiirzburg. N. F. Bd. xiv. and xv. 1879
and 1880.

(121) J. Disse. " Die Entwicklung des mittleren Keimblattes im Hiihnerei."
Archivfiir mikr. Anat., Vol. xv. 1878.

(122) J. Disse. "Die Entstehung d. Blutesu. d. ersten Gefiisse im Hiihnerei."
Archivf. mikr. Anat., Vol. xvi. 1879.

(123) Fr. Durante. "Sulla struttura della macula germinativa delle uova di
Gallina." Ricerche nel Laboratorio di Anatomia della R. Universita di Roma.

(124) E. Dursy. Der Primitivstreif des Hilhnchens. 1867.

(125) M. Duval. "Etude sur la ligne primitive de 1'embryon de Poulet."
Annales des Sciences Natureiles, Vol. vil. 1879.

(126) M. Foster and F. M. Balfour. Elements of Embryology. Part I.
London, 1874.

(127) Gasser. "Der Primitivstreifen bei Vogelembryonen. " Schriften d.

200 BIBLIOGRAPHY.

Gestll. zur Beford. d. gesammtcn Naturwiss. zu Marburg, Vol. II. Supplement i.
1879.

(128) A. Gotte. " Beitrage zur Entwicklungsgeschichte d. Wirbelthiere. II.
Die Bildung d. Keimblatter u. d. Blutes im Hiihnerei." Archiv fiir mikr. Anal.,
Vol. x. 1874.

(129) V. Hensen. " Embryol. Mitth." Archiv f. mikr. Anat., Vol. in.
1867.

(130) W. His. Untersuch. iib. d. erste Anlage d. Wirbelthierleibes. Leipzig,
1868.

(131) W. His. Unsere Kbrperform und da s physiol. Problem ihrer Entstehung.
Leipzig, 1875.

(132) W. His. "Der Keimwall des Hiihnereies u. d. Entstehung d. parablas-
tischen Zellen." Zeit.f. Anat.u. Entwicklungsgeschichte. Bd. I. 1876.

(133) W. His. " Neue Untersuchungen iib. die Bildung des Hiihnerembryo I."
Archiv f. Anat. u. Phys. 1877.

(134) E. Klein. "Das mittlere Keimblatt in seiner Bezieh. z. Entwick. d. ers.
Blutgefasse und Blutkorp. im Hiihnerembryo." Sitzungsber. Wien. Akad., Vol. LXIII.
1871.

(135) A. Kb Hiker. Entwicklungsgeschichte d. Menschen u. d. hoheren Thiere.
Leipzig, 1879.

(136) C. Kupffer. " Die Entsteh. d. Allantois u. d. Gastrula d. Wirbelth."
Zoolog. Anzeiger, Vol. u. 1879, pp. 520, 593, 612.

(137) C. Kupffer and B. Benecke. " Photogramme z. Ontogenie d.
Vogel." Nov. Act. d. k. Leop.-Carol.-Deutschen Akad. d. Natiirforscher, Vol. XLI.
1879.

(138) J. Oellacher. "Untersuchungen iiber die Furchung u. Blatterbildung
im Hiihnerei. " Strieker's Studien. 1870.

(139) C. H. Pander. Beitrage z. Entwick. d. Hilnchens im Eie. Wurzburg,
1817.

(140) A. Rauber. " Ueber die Embryonalanlage des Hiihnchens." Centralblatt
fur d. medic. Wissenschaften. 1874 — 75.

(141) A. Rauber. Ueber die Stellung des Hiihnchens im Entwicklungsplan.
1876.

(142) A. Rauber. "Primitivrinne und Urmund. Beitrage zur Entwicklungs-
geschichte des Hiihnchens." Morphol. Jahrbuch, B. u. 1876.

(143) A. Rauber. Primitivstreifen und Netirula der Wirbelthiere in normaler
und pathologischer Beziehimg. 1877.

(144) R. Remak. Untersuch. iib. d. Entwickhing d. Wirbelthiere. Berlin,
'850—55.

(145) S. L. Schenk. "Beitrage z. Lehre v. d. Organanlage im motorischen
Keimblatt." Sitz. Wien. Akad., Vol. LVII. 1860.

(146) S. L. Schenk. " Beitrage z. Lehre v. Amnion." Archiv f. mikr. Anat.,
Vol. VII. 1871.

(147) S. L. Schenk. Lcluinuh d. vergleich. Embryol. d. Wirbelthiere. Wien,
1874.

(148) S. Strieker. " Mittheil. iib. d. selbststandigen Bewegungen embryonaler
Zellen." Sitz. Wien. Akad., Vol. XLIX. 1864.

(149) S. Strieker. "Beitrage zur Kenntniss des Hiihnereies." Wiener Sit
zitngsber., Vol. Liv. 1866.

BIBLIOGRAPHY. 2OI

(150) H. Virchow. Ueber d. Epithel d. Dottersackes im Hiihnerei. Inaug.
Diss. Berlin, 1875.

(151) W. Waldeyer. "Ueber die Keimblatter und den Primitivstreifen bei
der Entwicklung des Huhnerembryo." Zeitschrift fiir rationelle Medicin. 1869.

(152) C.F.Wolff. Theoria generationis. Hake, 1759.

(153) C. F. Wolff. Ueb. d. Bildung d. Darmcanals im bebriiteten Hiinchen.
Halle, 1812.

CHAPTER IX.
REPTILIA.

THE formation of the germinal layers in the Reptilia is very
imperfectly known. The Lizard has been studied in this respect
more completely than other types, and there are a few scattered
observations on Turtles and Snakes.

The ovum has in all Reptilia a very similar structure to that
in Birds. Impregnation is effected in the upper part of the
oviduct, and the early stages of development invariably take
place in the oviduct. A few forms are viviparous, viz. some of
the blindworms amongst Lizards (Anguis, Seps), and some of
the Viperidae and Hydrophidae amongst the Serpents. In the
majority of cases, however, the eggs are laid in moist earth,
sand, &c. Around the true ovum an egg-shell (of the same
general nature as that in birds, though usually soft), and a
variable quantity of albumen, are deposited in the oviduct. The
extent to which development has proceeded in the oviparous
forms before the eggs are laid varies greatly in different species.

The general features of the development (for a knowledge of
which we are mainly indebted to Rathke's beautiful memoirs),
the structure of the amnion and allantois, &c. are very much the
same as in Birds.

The Lizards will be taken as type of the class, and a few
noteworthy points in the development of other groups will be
dealt with at the close of the Chapter. The following descrip-
tion, taken in the main from my own observations, applies to
Lacerta muralis.

The segmentation is meroblastic, and similar to that in Birds.
At its close the resulting blastoderm becomes divided into two
layers, a superficial epiblast formed of a single row of cells, and

REPTILIA.

203

a layer below this several rows deep. Below this layer fresh
segments continue for some time to be added to the blastoderm
from the subjacent yolk.

The blastoderm, which is thickened at its edge, spreads
rapidly over the yolk. Shortly before the yolk is half enclosed
a small embryonic shield (area pellucida) makes its appearance
near the centre of the blastoderm. The embryonic shield is
mainly distinguished from the re-
mainder of the blastoderm by the
more columnar character of its con-
stituent epiblast cells. It is some-
what pyriform in shape, the narrower
end corresponding with the future
posterior end of the embryo. At
the hind end of the shield a some-
what triangular primitive streak is
formed, consisting of epiblast con-
tinuous below with a great mass of
rounded mesoblast cells, probably
mainly formed, as in the bird, by a
proliferation of the epiblast. To
this mass of cells the hypoblast is
also partially adherent. At the front
end of the streak an epiblastic invo-
lution appears, which soon becomes
extended into a passage open at
both extremities, leading obliquely
forwards through the epiblast to
the space below the hypoblast. The
walls of the passage are formed of a
layer of columnar cells continuous
both with epiblast and hypoblast.
In front of the primitive streak the
body of the embryo becomes first
differentiated by the formation of a
medullary plate; and at the same
time there grows out from the pri-
mitive streak a layer of mesoblast,

FIG. 126. SECTIONS THROUGH
AN EMBRYO OF LACERTA MURALIS
REPRESENTED IN FIG. I2Q.

m.g. medullary groove ; mep.
mesoblastic plate ; ep. epiblast ; hy.
hypoblast ; ch' , notochordal thick-
ening of hypoblast ; ch. notochord ;
ne. neurenteric canal (blastopore).
In E. ne points a diverticulum of
the neurenteric canal into the pri-
mitive streak.

which spreads out in all directions between the epiblast and

204 FORMATION OF THE LAYERS.

hypoblast. In the region of the embryo the mesoblast plate is
stated by Kupffer and Benecke to be continuous across the
middle line, but this appears very improbable. In a slightly
later stage the medullary plate becomes marked by a shallow
groove, and the mesoblast of the embryo is then undoubtedly
constituted of two lateral plates, one on each side of the median
line. In the median line the notochord arises as a ridge-like
thickening of the hypoblast, which is continued posteriorly into
the front wall of the passage mentioned above.

The notochord does not long remain attached to the hypo-
blast, and the separation between the two is already effected for
the greater part of the length of the embryo by the stage repre-
sented in fig. 129. Fig. 126 represents a series of sections
through this embryo.

In a section (A) through the trunk of the embryo a short
way in front of the primitive streak, there is a medullary plate
with a shallow groove (mg), well-developed mesoblastic plates
(mep), already divided into somatic and splanchnic layers, and a
completely formed notochord independent of the hypoblast (fiy).
In the next section (B), taken just in front of the primitive
streak, the notochord is attached to the hypoblast, and the
medullary groove is deeper ; while in the section following (C),
which passes through the front border of the primitive streak,

FIG. 127. DIAGRAMMATIC LONGITUDINAL SECTION OF AN EMBRYO OF LACERTA.
//. body cavity; am. amnion; ne. neurenteric canal; ch. notochord; hy. hypo-
blast ; ep. epiblast of the medullary plate ; pr. primitive streak. In the primitive
streak all the layers are partially fused.

the notochord and hypoblast have become fused with the
epiblast. The section behind (D) shews the neurenteric passage
leading through the floor of the medullary groove and through
the hypoblast (ne). On the right side the mesoblastic plate has
become continuous with the walls of the passage. The last
section (E) passes through the front part of the primitive streak

REPTILIA. 205

behind the passage. The mesoblast, epiblast, and to some
extent the hypoblast, are now fused together in the axial line,
and in the middle of the fused mass is seen a narrow diverticulum
(tie) which is probably equivalent to the posterior diverticulum
of the neural canal in Birds (vide p. 164).

The general features of the stage will best be understood by
an examination of the diagrammatic longitudinal section repre-
sented in fig. 127. In front is shewn the amnion (am), growing
over the head of the embryo. The notochord (ch) is seen as an
independent cord for the greater part of the length of the
embryo, but falls into the hypoblast shortly in front of the
neurenteric passage. The neurenteric passage is shewn at ne,
and behind it is the front part of the primitive streak.

It is interesting to notice the remarkable relations of the notochord
to the walls of the neurenteric passage. More or less similar relations are
also well marked in the case of the goose and the fowl, and support the con-
clusion, deducible from the lower forms of Vertebrata, that the notochord is
essentially hypoblastic.

The passage at the front end of the primitive streak forms the posterior
boundary of the medullary plate, though the medullary groove is not at
first continued back to it. The anterior wall of this passage connects
together the medullary plate and the notochordal ridge of the hypoblast.
In the stage represented in fig. 126 and 129 the medullary groove has
become continued back to the opening of the passage, which thus becomes
enclosed in the medullary folds, and forms a true neurenteric passage1.

It will be convenient at this point to say a few words as to what
is known of the further fate of the neurenteric canal, and the early develop-
ment of the allantois. According to Strahl, who has worked on Lacerta
vivipara, the canal gradually closes from below upwards, and is obliterated

1 Kupffer and Benecke (No. 154) give a very different account from the above of
the early Lacertilian development, more especially in what concerns the so-called
neurenteric passage. They believe this structure to be closed below, and to form
therefore a blind sack open externally. The open end of this sack they regard as the
blastopore — an interpretation which accords with my own, but they regard the sack as
the rudiment of the allantois, and hold that it is equivalent to the invaginated archen-
teron of Amphioxus. I need scarcely say that I believe Kupffer and Benecke to have
made a mistake in denying the existence of the ventral opening of this organ. Kupffer
in a subsequent paper (No. 155) states that my descriptions of the structure of this
organ do not correspond with the fact. I have perfect confidence in leaving the
decision of this point to future observers, and may say that my observations have
already been fully confirmed by Strahl (No. 160), who has also added some observa-
tions on the later stages to which I shall hereafter have occasion to allude.

206

NEURENTERIC CANAL.

before the completion of the neural canal. The hind end of the alimentary
tract appears also to become a closed canal before this stage.

In Lacerta muralis the history appears to be somewhat different, and
it is more especially to be noticed that in this species the hindgut does
not become closed till considerably after the completion of the neural
canal. In a stage shortly after that last described, the neurenteric passage
becomes narrower. The next stage which I have observed is considerably

me

FlG. 128. FOUR TRANSVERSE SECTIONS THROUGH THE HINDER END OF A YOUNG
EMBRYO OF LACERTA MURALIS.

Sections A and B pass through the whole embryo, while C and D only pass
through the allantois, which at this stage projects backwards into the section of the
body cavity behind the primitive streak.

ne. neurenteric canal ; pr. primitive streak ; kg. hind-gut ; hy. hypoblast ; //. body
cavity; am. amnion ; se. serous envelope (outer limb of the amnion fold not yet
separated from the inner limb or true amnion); al. allantois; me. mesoblastic wall of
the allantois; v. vessels passing to the allantois.

later. The neural canal has become completely closed, and the flexure of
the embryo has already made its appearance. There is still a well-deve-
loped, though somewhat slit-like, neurenteric passage, but from the analogy
of birds, it is not impossible that it may have in the meantime closed up

REPTILIA. 207

and opened again. It has, in any case, the same relations as in the previous
stage.

It leads from the end of the medullary canal (at the point where its
walls are continuous with the cells of the primitive streak) round the end
of the notochord, which here becomes continuous with the medullary cord,
and so through the hypoblast. The latter layer is still a flat sheet without
any lateral infolding ; but it gives rise, behind the neurenteric passage, to
a blind posteriorly directed diverticulum, placed in the body cavity behind
the embryo, and opening at the ventral face of the apparent hind end
of the primitive streak. There is very little doubt that this diverticulum
is the commencing allantois.

At a somewhat later stage the arrangement of these parts has undergone
some changes. Their relations are shewn in the sections represented in
fig. 128.

The foremost section (A) passes through the alimentary opening of the
neurenteric passage (ne). Above this opening the section passes through
the primitive streak (pr) close to its junction with the walls of the medullary
canal. The hypoblast is folded in laterally, but the gut is still open below.
The amnion is completely established. In the next section figured (B), the
fourth of my series, the gut is completely closed in ; and the mesoblast has
united laterally with the axial tissue of the primitive streak. Vessels to
supply the allantois are shewn at v.

The three following sections are not figured, but they present the same
features as B, except that the primitive streak gets rapidly smaller, and the
lumen of the gut narrower. The section following (C) represents, I believe,
only the stalk of the allantoic diverticulum; This diverticulum appears
to be formed as usual of hypoblast (hy) enveloped by splanchnic mesoblast
(me), and projects into the section of the body cavity present behind the
embryo. Its position in the body cavity is the cause of its somewhat
peculiar appearance in the figure. Had the whole section been represented
the allantois would have been enclosed in a space between the serous mem-
brane (se) and a layer of splanchnic mesoblast below which has also been
omitted in fig. B1. It still points directly backwards, as it primitively does
in the chick, vide fig. 123 A, and Gasser, No. 127, PI. v. figs, i and 2. I do
not understand the apparently double character of the lumen of the allantois.
In the next section (not figured) the lumen of the allantoic stalk is larger,
but still apparently double, while in the last section (D) the lumen is
considerably enlarged and single. The neurenteric canal appears to close
shortly after the stage last described, though its further history has not
been followed in detail.

1 Owing to the difficulty of procuring material I have only been able to prepare the
two sets of sections just described, and in the absence of a fuller series there are some
points in the interpretation of the sections which must remain doubtful.

208

GENERAL DEVELOPMENT.

FIG. 129. SURFACE VIEW

OF A YOUNG EMBRYO OF LA-
CERTA MURALIS.

am. amnion
streak.

fr. primitive

General development of the Embryo.

The formation of the embryo commences with the appear-
ance of the medullary plate, the sides
of which soon grow up to form the

medullary folds. The medullarygroove \jj$^a.m

is developed anteriorly before any
trace of it is visible behind. In a
general way the closure of the groove
takes place as in Birds, but the an-
terior part of the body is very early
folded off, sinks into the yolk, and
becomes covered over by the amnion
as by a hood (figs. 127 and 129). All
this takes place before the closure of
the medullary canal ; and the changes
of this part are quite concealed from
view.

The closure of the medullary canal
commences in the neck, and extends
forwards and backwards ; and the whole region of the brain
becomes closed in, while the groove is still largely open behind.

The later stages in the development of the Lacertilian
embryo do not require a detailed description, as they present
the closest analogy with those already described for Aves.
The embryo soon turns on to its left side ; and then, becoming
continuously folded off from the yolk, passes through the series
of changes of form with which the reader is already familiar.
An advanced embryo is represented in fig. 130. The early
development and great length of the tail, which is spirally
coiled on the ventral surface, is a special feature to which the
attention of the reader may be called.

Embryonic Membranes and Yolk-Sack.

The early development of the cephalic portion of the amnion
has already been alluded to. The first traces of it become
apparent while the medullary groove is still extremely shallow.
The medullary plate in the region of the head forms an axial
strip of a thickish plate of epiblast. The edge of this plate

REPTILTA.

209

coincides with the line of the amniotic fold, and as this fold
rises up the two sides of the plate become bent over the embryo
and give rise to the inner limb of the amnion or amnion proper.
The section (fig. 127), representing the origin of the amniotic
hood of the head, shews very well how the space between the
two limbs of the amnion is continuous with the body cavity.
The amnion very early completely encloses the embryo (fig. 128
A and B), and its external limb or serous membrane, after
separating from the true amnion, soon approaches and fuses
with the vitelline membrane.

The first development of the allantois as a diverticulum of
the hypoblast covered by splanchnic mesoblast, at the apparent
posterior end of the primitive streak, has been described on
p. 207. The allantois continues for some time to point di-
rectly backwards; but

gradually assumes a ^\^^^ , j>6

more ventral direction ;
and, as it increases in
size, extends into the
space between the se-
rous membrane and
amnion, eventually to
form a large, highly
vascular, flattened sack
immediately below the
serous membrane.

The Yolk - Sack.
The blastoderm spreads
in the Lizard with very
great rapidity over the
yolk to form the yolk-
sack. The early ap-
pearance of the area
pellucida, or as it has
been called by Kupffer
and Benecke the embryonic shield, has already been noted.
Outside this a vascular area, which has the same function as

FIG. 130. ADVANCED EMBRYO OF LACERTA
MURALIS AS AN OPAQUE OBJECT1.

The embryo was 7 mm. in length in the curled
up state.

fb. fore-brain ; mb. mid-brain ; cb. cerebellum ;
au. auditory vesicle (closed) ; ol. olfactory pit ;
md. mandible ; hy. hyoid arch ; br. branchial
arches ; //. fore-limb ; hi. hind-limb.

1 This figure was drawn for me by Professor Haddon.
B. III. 14

210 CHELONIA.

in the chick, is not long in making its appearance. In all
Reptilia the vascular channels which arise in the vascular area,
and the vessels carrying the blood to and from the vascular area,
are very similar to those in the chick. In the Snake the sinus
terminalis never attains so conspicuous a development and in
Chelonia the stage with a pair of vitelline arteries is preceded by
a stage in which the vascular area is supplied, as it permanently
is in many Mammals, by numerous transverse arterial trunks,
coming off from the dorsal aorta (Agassiz, No. 164). The
vascular area gradually envelops the whole yolk, although it
does so considerably more slowly than the general blastoderm.

Ophidia. There is, as might have been anticipated, a very
close correspondence in general development between the
Lacertilia and Ophidia. The embryos of all the Amniota are,
during part of their development, more or less spirally coiled
about their long axis. This is well marked in the chick of the
third day; it is still more pronounced in the Lizard (fig. 130) ;
but it reaches its maximum in the Snake. The whole Snake
embryo has at the time when most coiled (Dutrochet, Rathke)
somewhat the form of a Trochus. The base of the spiral is
formed by the head, while the majority of the coils are supplied
by the tail. There are in all at this stage seven coils, and the
spiral is right-handed.

Another point, which deserves notice in the Snake, is the
absence in the embryo of all external trace of the limbs. It
might have been anticipated, on the analogy of the branchial
arches, that rudiments of the limbs would be preserved in the
embryo even when limbs were absent in the adult. Such,
however, is not the case. It is however very possible that
rudiments of the branchial arches and clefts have been preserved
because these structures were functional in the larva (Amphibia)
after they ceased to have any importance in the adult ; and that
the limbs have disappeared even in the embryo because in the
course of their gradual atrophy there was no advantage to the
organism in their being specially preserved at any period of life1.

Chelonia2. In their early development the Chelonia re-

1 It is very probable that in those Ophidia in which traces of limbs are still
preserved, that more conspicuous traces would be found in the embryos than in the
adults.

- Vide Agassiz (No. 164), Kupffer and Benecke (No. 154), and Parker (No. 165).

REPTILIA.

211

semble, so far as is known, the Lacertilia. The amnion arises
early, and soon forms a great cephalic hood. Before develop-
ment has proceeded very far the embryo turns over on to its
left side. The tail in many species attains a very considerable

FIG. 131. CHELONE MIDAS, FIRST STAGE.

Au. auditory capsule; br, i and 2, branchial arches; C. carapace; E. eye;f.b.
fore-brain; /./. fore-limb; H. heart; h.b. hind-brain; h.l. hind-limb; hy. hyoid;
m.b. mid-brain; mn. mandible; mx.p. maxillo-palatine ; N. nostril; u. umbilicus.

Tit

FIG. 132. CHELONE MIDAS, SECOND STAGE.
Letters as in fig. 131.

14 — 2

212 CHELONIA.

development (fig. 133). The chief peculiarity in the form of the
embryo (figs. 131, 132, and 133) is caused by the development
of the carapace. The first rudiment of the carapace appears in
the form of two longitudinal folds, extending above the line of
insertion of the fore- and hind-limbs, which have already made
their appearance (fig. 131). These folds are subsequently
prolonged so as to mark out the area of the carapace on the
dorsal surface. On the surface of this area there are formed the
horny plates (tortoise shell), and in the mesoblast below the
bony elements of the carapace (figs. 132 and 133).

fb

FIG. 133. CHELONE MIDAS, THIRD STAGE.
Letters as in fig. 131. r. rostrum.

Immediately after hatching the yolk-sack becomes withdrawn
into the body ; while the external part of the allantois shrivels
up.

BIBLIOGRAPHY.
General.

(154) C. Kupffer and Benecke. Die erste Entwicklung am Ei d. Reptilien.
Konigsberg, 1878.

(155) C. Kupffer. "Die Entstehung d. Allantois u. d. Gastrula d. Wirbel-
thiere." Zoologischer Anzeiger, Vol. II. 1879, pp. 520, 593, 612.

Lacertilia.

(156) F. M. Balfour. " On the early Development of the Lacertilia, together
with some observations, etc." Quart. J. of Micr. Science, Vol. XIX. 1879.

BIBLIOGRAPHY. 213

(157) Emmert u. Hochstetter. " Untersuchung Ub. d. Entwick. d. Eidechsen
in ihren Eiern." Rail's Archiv, Vol. x. 1811.

(158) M. Lereboullet. " Developpement de la Truite, du Lizard et du
Limnee. II. Embryologie du Lezard." An. Sci. Nat., Ser. iv., Vol. xxvn.
1862.

(159) W. K. Parker. "Structure and Devel. of the Skull in Lacertilia."
Phil. Trans., Vol. 170, p. 2. 1879.

(160) H. Strahl. " Ueb. d. Canalis myeloentericus d. Eidechse." Schrift. d.
Gesell. z. Bejor. d. gesam. Naturwiss. Marburg. July 23, 1880.

Ophidia.

(161) H. Dutrochet. " Recherches s. 1. enveloppes du foetus. " Mem. d. Soc.
Med. d' Emulation, Paris, Vol. vm. 1816.

(162) W. K. Parker. "On the skull of the common Snake." Phil. Trans.,
Vol. 169, Part II. 1878.

(163) H. Rathke. Entwick. d. Natter. Konigsberg, 1839.

Chelonia.

(164) L. A gas si z. Contributions to the Natural History of the United States,
Vol. II. 1857. Embryology of the Turtle.

(165) W. K. Parker. "On the development of the skull and nerves in the
green Turtle." Proc. of the Roy. Soc., Vol. xxvm. 1879. Vide also Nature,
April 14, 1879, and Challenger Reports, Vol. I. 1880.

(166) H. Rathke. Ueb. d. Entwicklung d. Schildkroten. Braunschweig, 1848.

Crocodilia.

(167) H. Rathke. Ueber die Entwicklung d. Krokodile. Braunschweig, 1866.

CHAPTER X.

MAMMALIA.

THE classical researches of Bischoff on the embryology of
several mammalian types, as well as those of other observers,
have made us acquainted with the general form of the embryos of
the Placentalia, and have shewn that, except in the earliest stages
of development, there is a close agreement between them. More
recently Hensen, Schafer, Kolliker, Van Beneden and Lieber-
kiihn have shed a large amount of light on the obscurer points of
the earliest developmental periods, especially in the rabbit. For
the early stages the rabbit necessarily serves as type; but there
are grounds for thinking that not inconsiderable variations are
likely to be met with in other species, and it is not at present
easy to assign to some of the developmental features their true
value. We have no knowledge of the early development of the
Ornithodelphia or Marsupialia.

The ovum on leaving the ovary is received by the fimbriated
extremity of the Fallopian tube, down which it slowly travels.
It is still invested by the zona radiata, and in the rabbit an al-
buminous envelope is formed around it in its passage downwards.
Impregnation takes place in the upper part of the Fallopian
tube, and is shortly followed by the segmentation, which is re-
markable amongst the Amniota for being complete.

Although this process (the details of which have been made
known by the brilliant researches of Ed. van Beneden) has
already been shortly dealt with as it occurs in the rabbit (Vol. II.
p. 98) it will be convenient to describe it again with somewhat
greater detail.

The ovum first divides into two nearly equal spheres, of
which one is slightly larger and more transparent than the

MAMMALIA.

other. The larger sphere and its products will be spoken of as
the epiblastic spheres, and the smaller one and its products as
the hypoblastic spheres, in accordance with their different
destinations.

Both the spheres are soon divided into two, and each of the
four so formed into two again; and thus a stage with eight
spheres ensues. At the moment of their first separation these
spheres are spherical, and arranged in two layers, one of them
formed of the four epiblastic spheres, and the other of the four
hypoblastic. This position is not long retained, but one of the
hypoblastic spheres passes to the centre; and the whole ovum
again takes a spherical form.

In the next phase of segmentation each of the four epiblastic
spheres divides into two, and the ovum thus becomes consti-
tuted of twelve spheres, eight epiblastic and four hypoblastic.
The epiblastic spheres have now become markedly smaller than
the hypoblastic.

The four hypoblastic spheres next divide, giving rise, to-
gether with the eight epiblastic spheres, to sixteen spheres in
all; which are nearly uniform in size. Of the eight hypoblastic
spheres four soon pass to the centre, while the eight superficial
epiblastic spheres form a kind of cup partially enclosing the
hypoblastic spheres. The epiblastic spheres now divide in their
turn, giving rise to sixteen spheres which largely enclose the
hypoblastic spheres. The segmentation of both epiblastic and
hypoblastic spheres continues, and in the course of it the epi-
blastic spheres spread further and further over the hypoblastic,
so that at the close of segmentation the hypoblastic spheres con-
stitute a central solid mass almost entirely surrounded by the
epiblastic spheres. In a small circular area however the hypo-
blastic spheres remain for some time exposed at the surface (fig.
1 34 A).

The whole process of segmentation is completed in the rabbit
about seventy hours after impregnation. At its close the epi-
blast cells, as they may now be called, are clear, and have an
irregularly cubical form ; while the hypoblast cells are polygonal
and granular, and somewhat larger than the epiblast cells.

The opening in the epiblastic layer where the hypoblast cells
are exposed on the surface may for convenience be called with

2l6

THE SEGMENTATION.

Van Beneden the blastoporc, though it is highly improbable that
it in any way corresponds with the blastopore of other vertebrate

ova1.

R

FIG. 134. OPTICAL SECTIONS OF A RAKBIT'S OVUM AT TWO STAGES CLOSELY
FOLLOWING UPON THE SEGMENTATION. (After E. van Beneden.)

ep. epiblast ; hy. primary hypoblast ; bp. Van Beneden's blastopore.
The shading of the epiblast and hypoblast is diagrammatic.

After its segmentation the ovum passes into the uterus. The
epiblast cells soon grow over the blastopore and thus form a
complete superficial layer. A series of changes next take place
which result in the formation of what has been called the blas-
todermic vescicle. To Ed. van Beneden we owe the fullest
account of these changes ; to Hensen and Kolliker however we
are also indebted for valuable observations, especially on the
later stages in the development of this vesicle.

The succeeding changes commence with the appearance of
a narrow cavity between the epiblast and hypoblast, which ex-
tends so as completely to separate these two layers except in
the region adjoining the original site of the blastopore (fig. 134
B)a. The cavity so formed rapidly enlarges, and with it the
ovum also ; which soon takes the form of a thin-walled vesicle
with a large central cavity. This vesicle is the blastodermic

1 It is stated by Bischoff that shortly after impregnation, and before the com-
mencement of the segmentation, the ova of the rabbit and guinea-pig are covered with
cilia and exhibit the phenomenon of rotation. This has not been noticed by other
observers.

* Van Beneden regards it as probable that the blastopore is situated somewhat
excentrically in relation to the area of attachment of the hypoblastic mass to the
epiblast.

MAMMALIA.

217

vesicle. The greater
part of its walls are
formed of a single row
of flattened epiblast
cells; while the hypo-
blast cells form a small
lens -shaped mass at-
tached to the inner side
of the epiblast cells
(fig- 135).

In the Vespertilionidee
Van Beneden and Julin have
shewn that the ovum under-
goes at the close of seg-
mentation changes of a more
or less similar nature to
those in the rabbit ; the
blastopore would however
appear to be wider, and to
persist even after the cavity
of the blastodermic vesicle
has commenced to be de-
veloped.

FIG. 135. RABBIT'S OVUM BETWEEN 70—90

HOURS AFTER IMPREGNATION. (After E. van

Beneden.)

bv. cavity of blastodermic vesicle (yolk-sack) ;
ep. epiblast ; hy. primitive hypoblast ; Z/. mu-
cous envelope (zona pellucida).

Although by this stage, which occurs in the rabbit between
seventy and ninety hours after impregnation, the blastodermic
vesicle has by no means attained its greatest dimensions, it has
nevertheless grown from about 0x39 mm. — the size of the ovum
at the close of segmentation — to about 0*28. It is enclosed by a
membrane formed from the zona radiata and the mucous layer
around it. The blastodermic vesicle continues to enlarge rapidly,
and during the process the hypoblastic mass undergoes im-
portant changes. It spreads out on the inner side of the epi-
blast and at the same time loses its lens-like form and be-
comes flattened. The central part of it remains however thicker,
and is constituted of two rows of cells, while the peripheral part,
the outer boundary of which is irregular, is formed of an im-
perfect layer of amoeboid cells which continually spread further
and further within the epiblast. The central thickening of the
hypoblast forms an opaque circular spot on the blastoderm,
which constitutes the commencement of the embryonic area.

2l8 FORMATION OF THE LAYERS.

The history of the stages immediately following, from about
the commencement of the fifth day to the seventh day, when a
primitive streak makes its appearance, is imperfectly understood,
and has been interpreted very differently by Van Beneden
(No. 171) on the one hand and by Kolliker (184), Rauber (187)
and Lieberkiihn (186) on the other. I have myself in conjunc-
tion with my pupil, Mr Heape, also conducted some investiga-
tions on these stages, which have unfortunately not as yet led
me to a completely satisfactory reconciliation of the opposing
views.

Van Beneden states that about five days after impregnation the hypo-
blast cells in the embryonic area become divided into two distinct strata,
an upper stratum of small cells adjoining the epiblast and a lower stratum
of flattened cells which form the true hypoblast. At the edge of the em-
bryonic area the hypoblast is continuous with a peripheral ring of the
amoeboid cells of the earlier stage, which now form, except at the edge of
the ring, a continuous layer of flattened cells in contact with the epiblast.
During the sixth day the flattened epiblast cells are believed by Van
Beneden to become columnar. The embryonic area gradually extends
itself, and as it does so becomes oval. A central lighter portion next
becomes apparent, which gradually spreads, till eventually the darker part
of the embryonic area forms a crescent at the posterior part of the now
somewhat pyriform embryonic area. The lighter part is formed of columnar
epiblast and hypoblast only, while in the darker area a layer of the meso-
blast, derived from the intermediate layer of the fifth day, is also found.
In this darker area the primitive streak originates early on the seventh
day.

Kolliker, following the lines originally laid down by Rauber, has arrived
at very different results. He starts from the three-layered condition described
by Van Beneden for the fifth day, but does not give any investigations of
his own as to the origin of the middle layer. He holds the outer layer to be
a provisional layer of protective cells, forming part of the wall of the original
vesicle, the middle layer he regards as the true epiblast and the inner layer
as the hypoblast.

During the sixth day he finds that the cells of the outer layer gradually
cease to form a continuous layer and finally disappear ; while the cells of
the middle layer become columnar, and form the columnar epiblast present in
the embryonic area at the end of the sixth day. The mesoblast first takes
its origin in the region and on the formation of the primitive streak.

The investigations of Heape and myself do not extend to the first form-
ation of the intermediate layer found on the fifth day. We find on the
sixth day in germinal vesicles of about 2-2 — 2'5 millimetres in diameter
with embryonic areas of about '8 mm. that the embryonic area (fig. 136) is
throughout composed of

MAMMALIA.

(1) A layer of flattened hypoblast cells ;

(2) A somewhat irregular layer of more columnar elements, in some
places only a single row deep and in other places two or more rows deep.

(3) Flat elements on the surface, which do not, however, form a con-
tinuous layer, and are intimately attached to the columnar cells below.

Our results as to the structure of the blastoderm at this stage closely
correspond therefore with those of Kolliker, but on one important point we
have arrived at a different conclusion. Kolliker states that he has never
found the flattened elements in the act of becoming columnar. We believe
that we have in many instances been able to trace them in the act of
undergoing this change, and have attempted to shew this in our figure.

Our next oldest embryonic areas were somewhat pyriform measuring
about i '19 mm. in length and '85 in breadth. Of these we have several,
some from a rabbit in which we also met with younger still nearly circular
areas. All of them had a distinctly marked posterior opacity forming a com-
mencing primitive streak, though decidedly less advanced than in the blasto-
derm represented in fig. 140. In the younger specimens the epiblast in
front of the primitive streak was formed of a single row of columnar cells
(fig. 138 A), no mesoblast was present and the hypoblast formed a layer of
flattened cells. In the region immediately in front of the primitive streak,
an irregular layer of mesoblast cells was interposed between the epiblast and
hypoblast. In the anterior part of the primitive streak itself (fig. 138 B)
there was a layer of mesoblast with a considerable lateral extension, while in
the median line there was a distinct mesoblastic proliferation of epiblast cells.
In the posterior sections the lateral extension of the mesoblast was less, but
the mesoblast cells formed a thicker cord in the axial line.

Owing to the unsatisfactory character of our data the follow-
ing attempt to fill in the history of the fifth and sixth days must
be regarded as tentative1. At the commencement of the fifth
day the central thickening, of what has been called above the
primitive hypoblast, becomes divided into two layers: the lower
of these is continuous with the peripheral hypoblast and is
formed of flattened cells, while the upper one is formed of small
rounded elements. The superficial epiblast again is formed of
flattened cells.

During the fifth day remarkable changes take place in the
epiblast of the embryonic area. It is probable that its con-

1 The attempt made below to frame a consecutive history out of the contradictory
data at my disposal is not entirely satisfactory. Should Kolliker's view turn out to be
quite correct, the origin of the middle layer of the fifth day, which Kolliker believes
to become the permanent epiblast, will have to be worked out again, in order to
determine whether it really comes, as it is stated by Van Beneden to do, from the
primitive hypoblast.

220

FORMATION OF THE LAYERS.

stituent cells increase in number and become one by one colum-
nar; and that in the process they press against the layer of
rounded elements below them, so that the two layers cease to be
distinguishable, and the whole embryonic area acquires in section
the characters represented in fig. I361. Towards the end of the

FIG. 136. SECTION THROUGH THE NEARLY CIRCULAR EMBRYONIC AREA OF A
RABBIT'S OVUM OF six DAYS, NINE HOURS AND '8 MM. IN DIAMETER.

The section shews the peculiar character of the upper layer with a certain number
of superficial flattened cells ; and represents about half the breadth of the area.

sixth day the embryonic area becomes oval, but the changes
which next take place are not understood. In the front part of
the area only two layers of cells are found, (i) an hypoblast, and
(2) an epiblast of columnar cells probably derived from the
flattened epiblast cells of the earlier stages. In the posterior
part of the blastoderm a middle layer is present (Van Beneden)
in addition to the two other layers; and this layer probably
originates from the middle layer which extended throughout the
area at the beginning of the fifth day, and then became fused
with the epiblast. The middle layer does not give rise to the
whole of the eventual mesoblast, but only to part of it. From its
origin it may be called the hypoblastic mesoblast, and it is
probably equivalent to the hypoblastic mesoblast already de-
scribed in the chick (pp. 154 and 155). The stage just described
has only been met with by Van Beneden2.

A diagrammatic view of the whole blastodermic vesicle at
about the beginning of the seventh day is given in fig. 137. The
embryonic area is represented in white. The line ge in B shews
the extension of the hypoblast round the inner side of the vesicle.
The blastodermic vesicle is therefore formed of three areas, (i)

1 The section figured may perhaps hardly appear to justify this view; the exami-
nation of a larger number of sections is, however, more favourable to it, but it must
be admitted that the interpretation is by no means thoroughly satisfactory.

• Kolliker does not believe in the existence of this stage, having never met with it
himself. It appears to me, however, more probable that Kolliker has failed to obtain
it, than that Van Beneden has been guilty of such an extraordinary blunder as to have
described a stage which has no existence.

MAMMALIA.

221

the embryonic area with three layers: this area is placed where
the blastopore was originally situated. (2) The ring around the
embryonic area where the walls of the vesicle are formed of epi-
blast and hypoblast. (3) The area beyond this again where the
vesicle is formed of epiblast only1.

A.

B,

FlG. 137. VIEWS OF THE BLASTODERMIC VESICLE OF A RABBIT ON THE SEVENTH
DAY WITHOUT THE ZONA. A. from above, B. from the side. (From Kolliker.)

ag. embryonic area ; ge. boundary of the hypoblast.

The changes which next take place begin with the formation
of a primitive streak, homologous with, and in most respects
similar to, the primitive streak in Birds. The formation of the
streak is preceded by that of a clear spot near the middle of the
blastoderm, forming the nodal point of Hensen. This spot sub-
sequently constitutes the front end of the primitive streak.

The history of the primitive streak was first worked out in a
satisfactory manner by Hensen (No. 182), from whom however I
differ in admitting the existence of a certain part of the meso-
blast before its appearance.

Early on the seventh day the embryonic area becomes pyri-
form, and at its posterior and narrower end a primitive streak
makes its appearance, which is due to a proliferation of rounded
cells from the epiblast. At the time when this proliferation

1 Schafer describes the blastodermic vesicle of the cat as being throughout in a
bilaminar condition before the formation of a definite primitive streak or of the
mesoblast.

222

THE PRIMITIVE STREAK.

commences the layer of hypoblastic mesoblast is present, espe-
cially just in front of, and at the sides of, the anterior part of the
streak; but no mesoblast is found in the anterior part of the
embryonic area. These features are shewn in fig. 138 A and B.

A.

FlG. 138. TWO SECTIONS THROUGH OVAL BLASTODERMS OF A RABBIT ON
THE SEVENTH DAY. THE LENGTH OF THE AREA WAS ABOUT I '2 MM. AND ITS
BREADTH ABOUT '86 MM.

A. Through the region of the blastoderm in front of the primitive streak; B.
through the front part of the primitive streak ; ep. epiblast ; m. mesoblast ; hy. hypo-
blast ; /;-. primitive streak.

The mesoblast derived from the proliferation of the epiblast soon
joins the mesoblast already present; though in many sections it

FlG. 139. TWO TRANSVERSE SECTIONS THROUGH THE EMBRYONIC AREA OF AN
KMBRYO RABBIT OF SEVEN DAYS.

The embryo has nearly the structure represented in fig. 140.

A. is taken through the anterior part of the embryonic area. It represents about
half the breadth of the area, and there is no trace of a medullary groove or of the
mesoblast.

B. Is taken through the posterior part of the primitive streak.

ep. epiblast; hy. hypoblast.

MAMMALIA. 223

seems possible to trace a separation between the two parts (fig.
139 B) of the mesoblast.

During the seventh day the primitive streak becomes a more
pronounced structure, the mesoblast in its neighbourhood in-
creases in quantity, while an axial groove — the primitive groove
— is formed on its upper surface. The mesoblastic layer in
front of the primitive streak becomes thicker, and, in the two-
layered region in front, the epiblast becomes several rows deep
(fig. 139 A).

In the part of the embryonic area in front of the primitive
streak there arise during the eighth day two folds bounding a
shallow median groove, which meet in
front, but diverge behind, and enclose
between them the foremost end of the
primitive streak (fig. 141). These folds
are the medullary folds and they consti-
tute the first definite traces of the em-
bryo. The medullary plate bounded by
them rapidly grows in length, the primi-
tive streak always remaining at its hinder
end. While the lateral epiblast is formed
of several rows of cells, that of the me- FlG I40 EMBRYONIC

dullary plate is at first formed of but a AREA OF AN EIGHT DAYS'

, J J RABBIT. (After Kolliker.)

Single row (fig. 142, mg). The mesoblast, ^ embryonic area ;pr.

which appears to grow forward from the primitive streak.
primitive streak, is stated to be at first a continuous sheet be-
tween the epiblast and hypoblast (Hensen). The evidence on
this point does not however appear to me to be quite conclusive.
In any case, as soon as ever the medullary groove is formed, the
mesoblast becomes divided, exactly as in Lacerta and Elasmo-
branchii, into two independent lateral plates, which are not
continuous across the middle line (fig. 142, me]. The hypoblast
cells are flattened laterally, but become columnar beneath the
medullary plate (fig. 142).

In tracing the changes which take place in the relations of
the layers, in passing from the region of the embryo to that of
the primitive streak, it will be convenient to follow the account
given by Schafer for the guinea-pig (No. 190), which on this
point is far fuller and more satisfactory than that of other ob-

224

THE BLASTOPORE.

servers. In doing so I shall leave out of consideration the fact
(fully dealt with later in this chapter) that the layers in the
guinea-pig are inverted. Fig. 143 represents a series of sections
through this part in the guinea-pig. The anterior section (D)

FIG. 141. EMBRYONIC AREA OF A SEVEN DAYS' EMBRYO RABBIT. (From
Kolliker.)

o. place of future area vasculosa ; rf. medullary groove ; fir. primitive streak ;
ag. embryonic area.

FIG. 142. TRANSVERSE SECTION THROUGH AN EMBRYO RABBIT OF EIGHT DAYS.
ep. epiblast ; me. mesoblast ; hy. hypoblast ; mg. medullary groove.

passes through the medullary groove near its hinder end. The
commencement of the primitive streak is marked by a slight
prominence on the floor of the medullary groove between the
two diverging medullary folds (fig. 143 C, ae). Where this pro-
minence becomes first apparent the epiblast and hypoblast are
united together. The mesoblast plates at the two sides remain

MAMMALIA.

225

in the meantime quite free. Slightly further back, but before
the primitive groove is reached, the epiblast and hypoblast arc
connected together by a cord of cells (fig. 143 B, /), which in
the section next following becomes detached from the hypoblast
and forms a solid keel pro-
jecting from the epiblast.
In the following section the
hitherto independent mcso-
blast plates become united
with this keel (fig. 143 A);
and in the posterior sec-
tions, through the part of
the primitive streak with
the primitive groove, the
epiblast and mesoblast con-
tinue to be united in the
axial line, but the hypoblast
remains distinct. These pe-
culiar relations may shortly
be described by saying that
in the axial line the hypo-
blast becomes united with
the epiblast at the posterior
cud of the embryo; and
that the cells which con-
nect the hypoblast and epi-
blast are posteriorly con-
tinuous with the fused epi-
blast and mesoblast of the

primitive Streak, the hypo- , epiblast; ///. mesoblast; A. hypoblasl;

blast in the region of the ac- axial q>il>last <>f the primitive streak ;
. . , . all. axial hypoblast attached in 15. and C. to

primitive Streak having be- the epiblast at the rudimentary blaslopore ;

;/;'. medullary groove; / rudimentary bias-

topore.

FlG. 143. A SERIES OH TRANSVERSE SKC
TIONS THROUGH THE JUNCTION OK TIIK
I'RIMITIVK. STRKAK A.N'l) MKIMM.I.AK Y GROOVE

OK A YOUNG GuiNKA-i'io. (After Schiifcr.)
A. is the posterior section.

the

come distinct from
other layers.

The peculiar relations just described, which hold also for the
rabbit, receive their full explanation by a comparison of the
Mammal with the Bird and the Lizard, but before entering into
this comparison, it will be well to describe the next stage in the
rabbit, which is in many respects very instructive. In this stage
li in. 15

226 THE BLASTOPORE.

the thickened axial portion of the hypoblast in the region of the
embryo becomes separated from the lateral part as the notochord.
Very shortly after the formation of the notochord, the hypoblast
grows in from the two sides, and becomes quite continuous
across the middle line. The formation of the notochord takes
place from before backwards ; and at the hinder end of the
embryo the notochord is continued into the mass of cells which
forms the axis of the primitive streak, becoming therefore at
this point continuous with the epiblast. The notochord in
fact behaves exactly as did the axial hypoblast in the earlier
stage.

In comparison with Lacerta (pp. 203 — 205) it is obvious that the axial
hypoblast and the notochord derived from it have exactly the same relations
in Mammalia and Lacertilia. In both they are continued at the hind end of the
embryo into the epiblast ; and close to where they join it, the mesoblast and
epiblast fuse together to form the primitive streak. The difference between
the two types consists in the fact that in Reptilia there is formed a passage
connecting the neural and alimentary canals, the front wall of which is con-
stituted by the cells which form the above junction between the notochord
and epiblast ; and that in Mammalia this passage — which is only a rudi-
mentary structure in Reptilia — has either been overlooked or else is absent.
In any case the axial junction of the epiblast and hypoblast in Mammalia
is shewn by the above comparison with Lacertilia to represent the dorsal lip
of the true vertebrate blastopore. The presence of this blastopore seems to
render it clear that the blastopore discovered by Ed. van Beneden cannot
have the meaning he assigned to it in comparing it with the blastopore of
the frog.

Kolliker adduces the fact that the notochord is continuous with the axial
cells of the primitive streak as an argument against its hypoblastic origin.
The above comparison with Lacertilia altogether deprives this argument of
any force.

At the stage we have now reached the three layers are defi-
nitely established. The epiblast (on the view adopted above)
clearly originates from epiblastic segmentation cells. The hypo-
blast without doubt originates from the hypoblastic segmenta-
tion spheres which give rise to the lenticular mass within the
epiblast on the appearance of the cavity of the blastodermic
vesicle ; while, though the history of the mesoblast is still ob-
scure, part of it appears to originate from the hypoblastic mass,
and part is undoubtedly formed from the epiblast of the primi-
tive streak.

MAMMALIA. 227

While these changes have been taking place the rudiments
of a vascular area become formed, and it is very possible that
part of the hypoblastic mesoblast passes in between the epiblast
and hypoblast. immediately around the embryonic area, to give
rise to the area vasculosa. From Hensen's observation it seems
at any rate clear that the mesoblast of the vascular area arises
independently of the primitive streak: an observation which is
borne out by the analogy of Birds.

General growth of the Embryo.

We have seen that the blastodermic vesicle becomes divided
at an early stage of development into an embryonic area, and a
non-embryonic portion. The embryonic area gives rise to the
whole of the body of the embryo, while the non-embryonic part
forms an appendage, known as the umbilical vesicle, which
becomes gradually folded off from the embryo, and has precisely
the relations of the yolk-sack of the Sauropsida. It is almost
certain that the Placentalia are descended from ancestors, the em-
bryos of which had large yolk-sacks, but that the yolk has become
reduced in quantity owing to the nutriment received from the
wall of the uterus taking the place of that originally supplied by
the yolk. A rudiment of the yolk-sack being retained in the
umbilical vesicle, this structure may be called indifferently um-
bilical vesicle or yolk-sack.

The yolk which fills the yolk-sack in Birds is replaced in
Mammals by a coagulable fluid ; while the gradual extension of
the hypoblast round the wall of the blastodermic vesicle, which
has already been described, is of the same nature as the growth
of the hypoblast round the yolk-sack in Birds.

The whole embryonic area would seem to be employed in
the formation of the body of the embryo. Its long axis has no
very definite relation to that of the blastodermic vesicle. The
first external trace of the embryo to appear is the medullary
plate, bounded by the medullary folds, and occupying at first
the anterior half of the embryonic area (fig. 141). The two
medullary folds diverge behind and enclose the front end of the
primitive streak. As the embryo elongates, the medullary folds

15-2

228 GENERAL GROWTH OF THE EMBRYO.

nearly meet behind and so cut off the front portion of the primi-
tive streak, which then appears as a projection in the hind end
of the medullary groove. In an embryo rabbit, eight days after
impregnation, the medullary groove is about r8o mm. in length.
At this stage a division may be clearly seen in the lateral plates
of mesoblast into a vertebral zone adjoining the embryo and
a more peripheral lateral zone ; and in the vertebral zone indi-
cations of two somites, about O'37 mm. from the hinder end of
the embryo, become apparent. The foremost of these somites
marks the junction, or very nearly so, of the cephalic region and
trunk. The small size of the latter as compared with the former
is very striking, but is characteristic of Vertebrates generally.
The trunk gradually elongates relatively to the head, by the
addition behind of fresh somites. The embryo has not yet
begun to be folded off from the yolk-sack. In a slightly older
embryo of nine days there appears (Hensen, Kolliker) round the
embryonic area a delicate clear ring which is narrower in front
than behind (fig. 144 A, ap). This ring is regarded by these
authors as representing the peripheral part of the area pellucida
of Birds, which does not become converted into the body of the
embryo. Outside the area pellucida, an area vasculosa has
become very well defined. In the embryo itself (fig. 144 A) the
disproportion between head and trunk is less marked than be-
fore ; the medullary plate dilates anteriorly to form a spatula-
shaped cephalic enlargement ; and three or four somites are
established. In the lateral parts of the mesoblast of the head
there may be seen on each side a tube-like structure (Jiz). Each
of these is part of the heart, which arises as two independent
tubes. The remains of the primitive streak (pr) are still present
behind the medullary groove.

In somewhat older embryos (fig. 144 B) with about eight
somites, in which the trunk considerably exceeds the head in
length, the first distinct traces of the folding-off of the head end
of the embryo become apparent, and somewhat later a fold also
appears at the hind end. In the formation of the hind end of
the embryo the primitive streak gives rise to a tail swelling and
to part of the ventral wall of the post-anal gut. In the region
of the head the rudiments of the heart (//) are far more definite.
The medullary groove is still open for its whole length, but in

MAMMALIA..

229

the head it exhibits a series of well-marked dilatations. The
foremost of these (v/t) is the rudiment of the fore-brain, from the
sides of which there project the two optic vesicles (ab} ; the next

A.

ao

•fc

FIG. 144. EMBRYO RABBITS OF ABOUT NINE DAYS FROM THE DORSAL SIDE.

(From Kolliker.)

A. magnified 22 times, and B. 21 times.

ap. area pellucida ; rf. medullary groove ; h' . medullary plate in the region of the
future fore-brain; h". medullary plate in the region of the future mid-brain; vh. fore-
brain; ab. optic vesicle; mh. mid-brain; h!i. and h'" . hind-brain; tiw. mesoblastic
somite; stz. vertebral zone; pz. lateral zone; hz. and h. heart; ph. pericardial section
of body cavity ; vo. vitelline vein ; of. amnion fold.

is the mid-brain (ink), and the last is the hind-brain (/£//), which
is again divided into smaller lobes by successive constrictions.
The medullary groove behind the region of the somites dilates
into an embryonic sinus rhomboidalis like that of the Bird.
Traces of the amnion (of) are now apparent both in front of and
behind the embryo.

230

GENERAL GROWTH OF THE EMBRYO.

The structure of the head and the formation of the heart at
this age are illustrated in fig. 145. The widely-open medullary
groove (rf) is shewn in the centre. Below it the hypoblast is
thickened to form the notochord dcf ; and at the sides are seen
the two tubes, which, on the folding-in of the fore-gut, give rise
to the unpaired heart. Each of these is formed of an outer mus-
cular tube of splanchnic mesoblast (a/i/i), not quite closed towards
the hypoblast, and an inner epithelioid layer (ik/i) ; and is placed

A.

B.

FIG. 145. TRANSVERSE SECTION THROUGH THE HEAD OF A RABBIT OF THE SAME
AGE AS FIG. 144 B. (From Kolliker.)

B. is a more highly magnified representation of part of A.

rf. medullary groove ; mp. medullary plate ; rw. medullary fold ; h. epiblast ;
dd. hypoblast; dd' . notochordal thickening of hypoblast; sp. undivided mesoblast;
tip. somatic mesoblast ; dfp. splanchnic mesoblast ; ph. pericardial section of body
cavity; ahk. muscular wall of heart; ihh. epithelioid layer of heart; nies. lateral
undivided mesoblast ; s?v. fold of hypoblast which will form the ventral wall of the
pharynx ; sr. commencing throat.

in a special section of the body cavity (//^), which afterwards
forms the pericardial cavity.

Before the ninth day is completed great external changes are
usually effected. The medullary groove becomes closed for its
whole length with the exception of a small posterior portion.
The closure commences, as in Birds, in the region of the mid-
brain. Anteriorly the folding-off of the embryo proceeds so far

MAMMALIA. 231

that the head becomes quite free, and a considerable portion of
the throat, ending blindly in front, becomes established. In the
course of this folding the, at first widely separated, halves of the
heart are brought together, coalesce on the ventral side of the
throat, and so give rise to a median undivided heart. The fold
at the tail end of the embryo progresses considerably, and dur-
ing its advance the allantois is formed in the same way as in
Birds. The somites increase in number to about twelve. The
amniotic folds nearly meet above the embryo.

The later stages in the development proceed in the main in
the same manner as in the Bird. The cranial flexure soon be-
comes very marked, the mid-brain forming the end of the long
axis of the embryo (fig. 146). The sense organs have the usual
development. Under the fore-brain appears an epiblastic invo-
lution giving rise both to the mouth and to the pituitary body.
Behind the mouth are three well-marked pairs of visceral arches.
The first of these is the mandibular arch (fig. 146, md\ which
meets its fellow in the middle line, and forms the posterior
boundary of the mouth. It sends forward on each side a superior

01$

hy

Si

FIG. 146. ADVANCED EMBRYO OF A RABBIT (ABOUT TWELVE DAYS)1.
mb. mid-brain; th. thalamencephalon ; ce. cerebral hemisphere; op. eye; iv.v.
fourth ventricle; mx. maxillary process ; md. mandibular arch ; hy. hyoid arch;//,
fore-limb; hi. hind-limb; urn. umbilical stalk.

1 This figure was drawn for me by my pupil, Mr Weldon.

232 GENERAL GROWTH OF THE EMBRYO.

maxillary process (mx) which partially forms the anterior margin
of the mouth. Behind the mandibular arch are present a well-
developed hyoid (hy) and a first branchial arch (not shewn in
fig. 146). There are four clefts, as in other Amniota, but the
fourth is not bounded behind by a definite arch. Only the first
of these clefts persists as the tympanic cavity and Eustachian
tube.

At the time when the cranial flexure appears, the body also
develops a sharp flexure immediately behind the head, which is
thus bent forwards upon the posterior straight part of the body
(fig. 146). The amount of this flexure varies somewhat in differ-
ent forms. It is very marked in the dog (Bischoff). At a later
period, and in some species even before the stage figured, the tail
end of the body also becomes bent (fig. 146), so that the whole
dorsal side assumes a convex curvature, and the head and tail
become closely approximated. In most cases the embryo, on
the development of the tail, assumes a more or less definite spiral
curvature (fig. 146); which however never becomes nearly so
marked a feature as it commonly is in Lacertilia and Ophidia.
With the more complete development of the lower wall of the
body the ventral flexure partially disappears, but remains more
or less persistent till near the close of intra-uterine life. The
limbs are formed as simple buds in the same manner as in Birds.
The buds of the hind-limbs are directed somewhat forwards, and
those of the fore-limb backwards.

Embryonic membranes and yolk-sack.

The early stages in the development of the embryonic mem-
branes are nearly the same as in Aves ; but during the later
stages in the Placentalia the allantois enters into peculiar rela-
tions with the uterine walls, and the two, together with the
interposed portion of the subzonal membrane or false amnion,
give rise to a very characteristic Mammalian organ — the
placenta — into the structure of which it will be necessary to
enter at some length. The embryonic membranes vary so
considerably in the different forms that it will be advantageous
to commence with a description of their development in an ideal
case.

MAMMALIA. 233

We may commence with a blastodermic vesicle, closely
invested by the delicate remnant of the zona radiata, at the
stage in which the medullary groove is already established.
Around the embryonic area a layer of mesoblast would have
extended for a certain distance ; so as to give rise to an area
vasculosa, in which however the blood-vessels would not have
become definitely established. Such a vesicle is represented
diagrammatically in fig. 147, 1. Somewhat later the embryo
begins to be folded off, first in front and then behind (fig. 147,
2). These folds result in a constriction separating the embryo
and the yolk-sack (ds), or as it is known in Mammalian embryo-
logy, the umbilical vesicle. The splitting of the mesoblast
into a splanchnic and a somatic layer has taken place, and at
the front and hind end of the embryo a fold (ks) of the somatic
mesoblast and epiblast begins to rise up and grow over the head
and tail of the embryo. These two folds form the commence-
ment of the amnion. The head and tail folds of the amnion are
continued round the two sides of the embryo, till they meet and
unite into a continuous fold. This fold grows gradually up-
wards, but before it has completely enveloped the embryo, the
blood-vessels of the area vasculosa become fully developed.
They are arranged in a manner not very different from that in
the chick.

The following is a brief account of their arrangement in the
Rabbit :—

The outer boundary of the area, which is continually extending further
and further round the umbilical vesicle, is marked by a venous sinus
terminalis (fig. 147, st). The area is not, as in the chick, a nearly com-
plete circle, but is in front divided by a deep indentation extending inwards
to the level of the heart. In consequence of this indentation the sinus
terminalis ends in front in two branches, which bend inwards and fall
directly into the main vitelline veins. The blood is brought from the
dorsal aortas by a series of lateral vitelline arteries, and not by a single
pair as in the chick. These arteries break up into a more deeply situated
arterial network, from which the blood is continued partly into the sinus
terminalis, and partly into a superficial venous network. The hinder end
of the heart is continued into two vitelline veins, each of which divides
into an anterior and a posterior branch. The anterior branch is a limb
of the sinus terminalis, and the posterior and smaller branch is continued
towards the hind part of the sinus, near which it ends. On its way it
receives, on its outer side, numerous branches from the venous network,

234

FCETAL MEMBRANES.

FlG. 147. FIVE DIAGRAMMATIC FIGURES ILLUSTRATING THE FORMATION OF
THE FCETAL MEMBRANES OF A MAMMAL. (From Kolliker.)

In i, 2, 3, 4 the embryo is represented in longitudinal section.
i . Ovum with zona pellucida, blastodermic vesicle, and embryonic area.
i. Ovum with commencing formation of umbilical vesicle and amnion.

3. Ovum with amnion about to close, and commencing allantois.

4. Ovum with villous subzonal membrane, larger allantois, and mouth and anus.

5. Ovum in which the mesoblast of the allantois has extended round the inner

MAMMALIA. 235

surface of the subzonal membrane and united with it to form the chorion. The cavity
of the allantois is aborted. This fig. is a diagram of an early human ovum.

d. zona radiata; d' '. processes of zona; sh. subzonal membrane; ch. chorion; ch.z.
chorionic villi; am. amnion; ks. head-fold of amnion ; ss. tail-fold of amnion; a.
epiblast of embryo; a. epiblast of non-embryonic part of the blastodermic vesicle;
;«. embryonic mesoblast; m' . non-embryonic mesoblast; df. area vasculosa; st. sinus
terminalis; dd. embryonic hypoblast; i. non-embryonic hypoblast; kh. cavity of
blastodermic vesicle, the greater part of which becomes the cavity of the umbilical
vesicle ds. ; dg. stalk of umbilical vesicle ; al. allantois ; e. embryo ; r. space between
chorion and amnion containing albuminous fluid; vl. ventral body wall; hh. peri-
cardial cavity.

which connect by their anastomoses the posterior branch of the vitelline
vein and the sinus terminalis.

While the above changes have been taking place the whole
blastodermic vesicle, still enclosed in the zona, has become
attached to the walls of the uterus. In the case of the typical
uterus with two tubular horns, the position of each embryo,
when there are several, is marked by a swelling in the walls of
the uterus, preparatory to the changes which take place on
the formation of the placenta. In the region of each swelling
the zona around the blastodermic vesicle is closely embraced, in
a ring-like fashion, by the epithelium of the uterine wall. The
whole vesicle assumes an oval form, and it lies in the uterus
with its two ends free. The embryonic area is placed close to
the mesometric attachment of the uterus. In many cases
peculiar processes or villi grow out from the ovum (fig. 147, 4,
sz), which fit into the folds of the uterine epithelium. The
nature of these processes requires further elucidation, but in
some instances they appear to proceed from the zona (the
Rabbit) and in other instances from the subzonal membrane
(the Dog). In any case the attachment between the blasto-
dermic vesicle and the uterine wall becomes so close at the
time when the body of the embryo is first formed out of the
embryonic area, that it is hardly possible to separate them with-
out laceration ; and at this period — from the 8th to the pth day
in the Rabbit — it requires the greatest care to remove the ovum
from the uterus without injury. It will be understood of course
that the attachment above described is at first purely superficial
and not vascular.

Shortly after the establishment of the circulation of the yolk-

236

FCETAL MEMBRANES.

sack the folds of the amnion meet and coalesce above the
embryo (fig. 147, 3 and 4, am). After this the inner or true
amnion becomes severed from the outer or false amnion,
though the two sometimes remain connected by a narrow stalk.
Between the true and false amnion is a continuation of the body
cavity. The true amnion consists of a layer of epiblastic epithe-
lium and generally also of somatic mesoblast, while the false
amnion consists, as a rule, of epiblast only ; though it is possible
that in some cases (the Rabbit ?) the mesoblast may be con-
tinued along its inner face.

Before the two limbs of the amnion are completely severed,
the epiblast of the umbilical vesicle becomes separated from the
mesoblast and hypoblast of the vesicle (fig. 147, 3), and, to-

FIG. 147*. DIAGRAM OF THE FCETAL MEMBRANES OF A MAMMAL.

(From Turner.)

Structures which either are or have been at an earlier period of development
continuous with each other are represented by the same character of shading.

pc. zona with villi; ss. subzonal membrane; E. epiblast of embryo; am. amnion;
A C. amniotic cavity ; M. mesoblast of embryo ; H. hypoblast of embryo ; UV.
umbilical vesicle; al. allantois; ALC. allantoic cavity.

gether with the false amnion (s/i), with which it is continuous,
forms a complete lining for the inner face of the zona radiata.

MAMMALIA. 237

The space between this membrane and the umbilical vesicle
with the attached embryo is obviously continuous with the body
cavity (vide figs. 147, 4 and 147*). To this membrane Turner
has given the appropriate name of subzonal membrane: by
Von Baer it was called the serous envelope. It soon fuses with
the zona radiata, or at any rate the zona ceases to be dis-
tinguishable.

While the above changes are taking place in the amnion, the
allantois grows out from the hind gut as a vesicle lined by hypo-
blast, but covered externally by a layer of splanchnic mesoblast
(fig. 147, 3 and 4, a/)1. The allantois soon becomes a flat sack,
projecting into the now largely developed space between the
subzonal membrane and the amnion, on the dorsal side of the
embryo (fig. 147*, ALC). In some cases it extends so as to
cover the whole inner surface of the subzonal membrane; in
other cases again its extension is much more limited. Its
lumen may be retained or may become nearly or wholly
aborted. A fusion takes place between the subzonal membrane
and the adjoining mesoblastic wall of the allantois, and the two
together give rise to a secondary membrane round the ovum,
known as the chorion. Since however the allantois does not
always come in contact with the whole inner surface of the sub-
zonal membrane, the term chorion is apt to be somewhat vague ;
and in the rabbit, for instance, a considerable part of the
so-called chorion is formed by a fusion of the wall of the yolk-
sack with the subzonal membrane (fig. 148). The placental
region of the chorion may in such cases be distinguished as the
true chorion, from the remaining part which will be called the
false chorion.

The mesoblast of the allantois, especially that part of it
which assists in forming the chorion, becomes highly vascular ;
the blood being brought to it by two allantoic arteries continued
from the terminal bifurcation of the dorsal aorta, and returned
to the body by one, or rarely two, allantoic veins, which join the
vitelline veins from the yolk-sack. From the outer surface of
the true chorion (fig. 147, 5, d, 148) villi grow out and fit into
crypts or depressions which have in the meantime made their

1 The hypoblastic element in the allantois is sometimes very much reduced, so that
the allantois may he mainly formed of a vascular layer of mesoblast.

238 FCETAL MEMBRANES.

appearance in the walls of the uterus1. The villi of the.chorion
are covered by an epithelium derived from the subzonal mem-
brane, and are provided with a connective tissue core containing
an artery and vein and a capillary plexus connecting them. In
most cases they assume a more or less arborescent form, and
have a distribution on the surface of the chorion varying
characteristically in different species. The walls of the crypts
into which the villi are fitted also become highly vascular,
and a nutritive fluid passes from the maternal vessels of the
placenta to the fcetal vessels by a process of diffusion ; while
there is probably also a secretion by the epithelial lining of the
walls of the crypts, which becomes absorbed by the vessels of
the fcetal villi. The above maternal and fcetal structures con-
stitute together the organ known as the placenta. The mater-
nal portion consists essentially of the vascular crypts in the
uterine walls, and the fcetal portion of more or less arborescent
villi of the true chorion fitting into these crypts.

While the placenta is being developed, the folding-off of the
embryo from the yolk-sack becomes more complete ; and the
yolk-sack remains connected with the ileal region of the
intestine by a narrow stalk, the vitelline duct (fig. 147, 4 and 5
and fig. 147*), consisting of the same tissues as the yolk-sack,
viz. hypoblast and splanchnic mesoblast. While the true
splanchnic stalk of the yolk-sack is becoming narrow, a somatic
stalk connecting the amnion with the walls of the embryo is also
formed, and closely envelops the stalk both of the allantois
and the yolk-sack. The somatic stalk together with its contents
is known as the umbilical cord. The mesoblast of the
somatopleuric layer of the cord develops into a kind of gela-
tinous tissue, which cements together the whole of the contents.
The allantoic arteries in the cord wind in a spiral manner round
the allantoic vein. The yolk-sack in many cases atrophies
completely before the close of intra-uterine life, but in other
cases it is only removed with the other embryonic membranes
at birth. The intra-embryonic portion of the allantoic stalk
gives rise to two structures, viz. to (-1) the urinary bladder

1 These crypts have no connection with the openings of glands in the walls of the
uterus. They are believed by Ercolani to be formed to a large extent by a regene-
ration of the lining tissue of the uterine walls.

MAMMALIA. 239

formed by a dilatation of its proximal extremity, and to (2) a
cord known as the urachus connecting the bladder with the wall
of the body at the umbilicus. The urachus, in cases where the
cavity of the allantois persists till birth, remains as an open
passage connecting the intra- and extra-embryonic parts of the
allantois. In other cases it gradually closes, and becomes
nearly solid before birth, though a delicate but interrupted
lumen would appear to persist in it. It eventually gives rise to
the ligamentum vesicae medium.

At birth the foetal membranes, including the fcetal portion of
the placenta, are shed ; but in many forms the interlocking of
the fcetal villi with the uterine crypts is so close that the uterine
mucous membrane is carried away with the fcetal part of the
placenta. It thus comes about that in some placentae the
maternal and fcetal parts simply separate from each other at
birth, and in others the two remain intimately locked together,
and both are shed together as the after-birth. These two forms
of placenta are distinguished as non-deciduate and deciduate,
but it has been shewn by Ercolani and Turner that no sharp
line can be drawn between the two types ; moreover, a larger
part of the uterine mucous membrane than that forming the
maternal part of the placenta is often shed in the deciduate
Mammalia, and in the non-deciduate Mammalia it is probable
that the mucous membrane (not including vascular parts) of the
maternal placenta either peels or is absorbed.

Comparative history of the Mammalian foetal membranes.

Two groups of Mammalia — the Monotremata and the
Marsupialia — are believed not to be provided with a true
placenta.

The nature of the fcetal membranes in the Monotremata is
not known. Ova, presumably in an early stage of development,
have been found free in the uterus of Ornithorhyncus by Owen.
The lining membrane of the uterus was thickened and highly
vascular. The females in which these were found were killed
early in October1.

1 The following is Owen's account of the young after birth (Comp. Anat. of
Vertebrates, Vol. in. p. 717) : " On the eighth of December Dr Bennet discovered in
"the subterranean nest of Ornithorhyncus three living young, naked, not quite two

240 COMPARATIVE HISTORY OF FCETAL MEMBRANES.

Marsupialia. Our knowledge of the foetal membranes of
the Marsupialia is almost entirely due to Owen. In Macropus
major he found that birth took place thirty-eight days after
impregnation. A foetus at the twentieth day of gestation
measured eight lines from the mouth to the root of the tail.
The foetus was enveloped in a large subzonal membrane, with
folds fitting into uterine furrows, but not adhering to the uterus,
and witlwut villi. The embryo was enveloped in an amnion
reflected over the stalk of the yolk-sack, which was attached by
a filamentary pedicle to near the end of the ileum. The yolk-
sack was large and vascular, and was connected with the fostal
vascular system by a vitelline artery and two veins. The yolk-
sack was partially adherent, especially at one part, to the
subzonal membrane. No allantois was observed. In a some-
what older foetus of ten lines in length there was a small allantois
supplied by two allantoic arteries and one vein. The allantois
was quite free and not attached to the subzonal membrane. The
yolk-sack was more closely attached to the subzonal membrane
than in the younger embryo1.

All Mammalia, other than the Monotremata and Marsupialia,
have a true allantoic placenta. The placenta presents a great
variety of forms, and it will perhaps be most convenient first to
treat these varieties in succession, and then to give a general
exposition of their mutual affinities2.

Amongst the existing Mammals provided with a true placenta, the
most primitive type is probably retained by those forms in which the
placental part of the chorion is confined to a comparatively restricted area
on the dorsal side of the embryo ; while the false chorion is formed by the

"inches in length." On the i2th of August, 1864, "a female Echidna hystrix was
" captured .... having a young one with its head buried in a mammary or marsupial
" fossa. This young one was naked, of a bright reel colour, and one inch two lines in
"length."

1 Owen quotes in the Anatomy of Vertebrates* Vol. in. p. 721, a description from
Rengger of the development of Didelphis azarse, which would seem to imply that a
vascular adhesion arises between the uterine walls and the subzonal membrane, but
the description is too vague to be of any value in determining the nature of the fcetal
membranes.

2 Numerous contributions to our knowledge of the various types of placenta have
been made during the last few years, amongst which those of Turner and Ercolani
may be singled out, both from the variety of forms with which they deal, and the
important light they have thrown on the structure of the placenta.

MAMMALIA.

241

vascular yolk-sack fusing with the remainder of the subzonal membrane.
In all the existing forms with this arrangement of foetal membranes, the
placenta is deciduate. This, however, was probably not the case in more
primitive forms from which these are descended1. The placenta would
appear from Ercolani's description to be simpler in the mole (Talpa) than
in other species. The Insectivora, Cheiroptera, and Rodentia are the
groups with this type of placenta ; and since the rabbit, amongst the latter,
has been more fully worked out than other species, we may take it first.

The Rabbit. In the pregnant female Rabbit several ova are gene-
rally found in each horn of the uterus. The general condition of the egg-
membranes at the time of their full development is shewn in fig. 148.

The embryo is surrounded by the amnion, which is comparatively small.
The ;yolk-sack (ds) is large and attached to the embryo by a long stalk.
It has the form of a flattened sack closely applied to about two-thirds of the
surface of the subzonal membrane. The outer wall of this sack, adjoining
the subzonal membrane, is formed of hypoblast only ; but the inner wall is
covered by the mesoblast of
the area vasculosa, as indi-
cated by the thick black line
(fd}. The vascular area is
bordered by the sinus ter-
minalis (st}. In an earlier
stage of development the
yolk-sack had not the com-
pressed form represented in
the figure. It is, however,
remarkable that the vascular
area never extends over the
whole yolk-sack ; but the in-
ner vascular wall of the yolk-
sack fuses with the outer,
and with the subzonal mem-
brane, and so forms a false
chorion, which receives its
blood supply from the yolk-
sack. This part of the cho-
rion does not develop vas-
cular villi.

The allantois (al) is a
simple vascular sack with a
large cavity. Part of its wall
is applied to the subzonal
membrane, and gives rise to

1 Vide Ercolani, No. 197, and Harting, No. 201, and also Von Baer, Entivick-
lungsgeschichte table on p. 225, part I., where the importance of the limited area of
attachment of the allantois as compared with the yolk-sack is distinctly recognised.

B. III. l6

•sh.

FIG. 148. DIAGRAMMATIC LONGITUDINAL SEC-
TION OF A RABBIT'S OVUM AT AN ADVANCED STAGE
OF PREGNANCY. (From Kblliker after Bischoff.)

e. embryo ; a. amnion ; a. urachus ; al. allan-
tois with blood-vessels; s/i. subzonal membrane;
pi. placental villi ; fd. vascular layer of yolk-sack ;
ed. hypoblastic layer of yolk-sack; ed' '. inner por-
tion of hypoblast, and ed". outer portion of hypo-
blast lining the compressed cavity of the yolk-
sack ; ds. cavity of yolk-sack ; st. sinus terminalis ;
r. space filled with fluid between the amnion, the
allantois and the yolk-sack.

242

FCETAL MEMBRANES OF THE RODENTIA.

the true chorion, from which there project numerous vascular villi. These
fit into corresponding uterine crypts. It seems probable, from Bischoff's
and Kolliker's observations, that the subzonal membrane in the area of
the placenta becomes attached to the uterine wall, by means of villi, even
before its fusion with the allantois. In the later periods of gestation
the intermingling of the maternal and fcetal parts of the placenta becomes
very close, and the placenta is truly deciduate. The cavity of the allantois
persists till birth. Between the yolk-sack, the allantois, and the embryo,
there is left a large cavity filled with an albuminous fluid.

The Hare does not materially differ in the arrangement of its foetal
membranes from the Rabbit.

In the Rat (Mus decumanus) (fig. 149) the sack of the allantois com-
pletely atrophies before the close of fcetal life1, and there is developed, at

771

FIG. 149. SECTION THROUGH THE PLACENTA AND ADJACENT PARTS OF A RAT

ONE INCH AND A QUARTER LONG. (From Huxley.)

a. uterine vein ; b. uterine wall ; c. cavernous portion of uterine wall ; d. deciduous
portion of uterus with cavernous structure; i. large vein passing to the foetal portion of
the placenta ; f. false chorion supplied by vitelline vessels ; k. vitelline vessel ; /.
allantoic vessel; g. boundary of true placenta; e, m, m, e. line of junction of the
deciduate and non-deciduate parts of the uterine wall.

the junction of the maternal part of the placenta and the unaltered mucous
membrane of the uterus, a fold of the mucous membrane which completely
encapsules the whole chorion, and forms a separate chamber for it, distinct
from the general lumen of the uterus. Folds of this nature, which are
specially developed in Man and Apes, are known as a decidua reflexa.
The decidua reflexa of the Rat is reduced to extreme tenuity, or even
vanishes before the close of gestation.

Guinea-pig. The development of the Guinea-pig is dealt with else-
where, but, so far as its peculiarities permit a comparison with the Rabbit,
the agreement between the two types appears to be fairly close.

1 This is denied by Nasse ; vide Kolliker, No. 183, p. 361.

MAMMALIA.

243

The blastodermic vesicle of the Guinea-pig becomes completely en-
veloped in a capsule of the uterine wall (decidua reflexa) (fig. 150). The
epithelium of the blastodermic vesicle in contact with the uterine wall is not
epiblastic, but corresponds with the hypoblast of the yolk-sack of other
forms, and the mesoblast of the greater part of the inner side of this
becomes richly vascular (yk) ; the vascular area being bounded by a sinus
terminalis.

The blastodermic vesicle is so situated within its uterine capsule that the
embryo is attached to the part
of it adjoining the free side of
the uterus. From the opposite
side of the uterus, viz. that to
which the mesometrium is at-
tached, there grow into the wall

a 11-4

y*-4

of the blastodermic vesicle
numerous vascular processes
of the uterine wall, which es-
tablish at this point an organic
connection between the two
(pi). The blood-vessels of the
blastodermic vesicle (yolk-
sack) stop short immediately
around the area of attachment
to the uterus ; but at a late
period the allantois grows to-
wards, and fuses with this area.
The blood-vessels of the allan-
tois and of the uterus become
intertwined, and a disc-like
placenta more or less similar
to that in the Rabbit becomes formed (pi).
developed, vanishes completely.

In all the Rodentia the placenta appears to be situated on the mesome-
tric side of the uterus.

Insectivora. In the Mole (Talpa) and the Shrew (Sorex), the foetal
membranes are in the main similar to those in the rabbit, and a deciduate
discoidal placenta is always present. It may be situated anywhere in the
circumference of the uterine tube. The allantoic cavity persists (Owen), but
the allantois only covers the placental area of the chorion. The yolk-sack is
persistent, and fuses with the non-allantoic part of the subzonal membrane ;
which is rendered vascular by its blood-vessels. There would seem to be
(Owen) a small decidua reflexa. A similar arrangement is found in the
Hedgehog (Erinaceus Europaeus) (Rolleston), in which the placenta occupies
the typical dorsal position. It is not clear from Rolleston's description
whether the yolk-sack persists till the close of foetal life, but it seems
probable that it does so. There is a considerable reflexa which does not,

1 6 — 2

FIG. 150. DIAGRAMMATIC LONGITUDINAL
SECTION OF AN OVUM OF A GUINEA-PIG AND
THE ADJACENT UTERINE WALLS AT AN AD-
VANCED STAGE OF PREGNANCY. (After Bischoff.)

yk. yolk-sack (umbilical vesicle) formed of
an external hypoblastic layer (shaded) and an
internal mesoblastic vascular layer (black). At
the end of this layer is placed the sinus termi-
nalis ; all. allantois ; //. placenta.

The external shaded parts are the uterine
walls.

The cavity of the allantois, if

244 HUMAN PLACENTA.

however, cover the whole chorion. In the Tenrec (Centetes) the yolk-sack
and non-placental part of the chorion are described by Rolleston as being
absent, but it seems not impossible that this may have been owing to the
bad state of preservation of the specimen. The amnion is large. In the
Cheiroptera ( Vespertilio and Pteropus], the yolk-sack is large, and coalesces
with part of the chorion. The large yolk-sack has been observed in Ptero-
pus by Rolleston, and in Vespertilio by Owen. The allantoic vessels supply
the placenta only. The Cheiroptera are usually uniparous.

Simiadao and Anthropidae. The foetal membranes of Apes and
Man, though in their origin unlike those of the Rodentia and Insectivora,
are in their ultimate form similar to them, and may be conveniently dealt
with here. The early stages in the development of these membranes in the
human embryo have not been satisfactorily observed ; but it is known that
the ovum, shortly after its entrance into the uterus, becomes attached to the
uterine wall, which in the meantime has undergone considerable preparatory
changes. A fold of the uterine wall appears to grow round the blastodermic
vesicle, and to form a complete capsule for it, but the exact mode of forma-
tion of this capsule is a matter of inference and not of observation. During
the first fortnight of pregnancy villi grow out, according to Allen Thomson
over its whole surface, but according to Reichert in a ring-like fashion round
the edge of the somewhat flattened ovum, and attach it to the uterus. The
further history of the early stages is extremely obscure, and to a large extent
a matter of speculation : what is known with reference to it will be found in
a special section, but I shall here take up the history at about the fourth
week.

At this stage a complete chorion has become formed, and is probably
derived from a growth of the mesoblast of the allantois (unaccompanied by
the hypoblast) round the whole inner surface of the subzonal membrane.
From the whole surface of the chorion there project branched vascular pro-
cesses, covered by an epithelium. The allantois is without a cavity, but a
hypoblastic epithelium is present in the allantoic stalk, through which it
does not, however, form a continuous tube. The blood-vessels of the chorion
are derived from the usual allantoic arteries and vein. The general condi-
tion of the embryo and of its membranes at this period is shewn diagramma-
tically in fig. 147, 5. Around the embryo is seen the amnion, already sepa-
rated by a considerable interval from the embryo. The yolk-sack is shewn
at ds. Relatively to the other parts it is considerably smaller than it was at
an earlier stage. The allantoic stalk is shewn at al. Both it and the stalk
of the yolk-sack are enveloped by the amnion (ant). The chorion with its
vascular processes surrounds the whole embryo.

It may be noted that the condition of the chorion at this stage is very
similar to that of the normal diffused type of placenta, described in the
sequel.

While the above changes are taking place in the embryonic membranes,
the blastodermic vesicle greatly increases in size, and forms a considerable
projection from the upper wall of the uterus. Three regions of the uterine

MAMMALIA.

245

wall, in relation to the blastodermic vesicle, are usually distinguished ; and
since the superficial parts of all of these are thrown off with the afterbirth,
each of them is called a decidua. They are represented at a somewhat later
stage in fig. 151. There is (i) the part of the wall reflected over the blasto-
dermic vesicle, called the decidua reflexa (dr) ; (2) the part of the wall
forming the area round which the reflexa is inserted, called the decidua
serotina (<&) ; (3) the general wall of the uterus, not related to the embryo,
called the decidua vera (du).

The decidua reflexa and serotina together envelop the chorion, the
processes of which fit into crypts in them. At this period both of them are
highly and nearly uniformly vascular. The general cavity of the uterus is to a
large extent obliterated by the ovum, but still persists as a space filled with
mucus, between the decidua reflexa and the decidua vera.

The changes which ensue from this period onwards are fully known.
The amriion continues to dilate (its cavity being intensely filled with amnio-
tic fluid) till it comes very close to the chorion (fig. 151, am) ; from which,

FIG. 151. DIAGRAMMATIC SECTION OF PREGNANT HUMAN UTERUS WITH
CONTAINED FOETUS. (From Huxley after Longet.)

al. allantoic stalk; nb. umbilical vesicle; am. amnion; ch. chorion; ds. decidua
serotina; du. decidua vera; dr. decidua reflexa; /. Fallopian tube; c. cervix uteri;
n. uterus; z. fcetal villi of true placenta; z. villi of non-placental part of chorion.

however, it remains separated by a layer of gelatinous tissue. The villi of
the chorion in the region covered by the decidua reflexa, gradually cease to
be vascular, and partially atrophy, but in the region in contact with the
decidua serotina increase and become more vascular and more arborescent
(fig. 151, z). The former region becomes known as the chorion lasve, and
the latter as the chorion frondosum. The chorion f rondo sum, together
with the decidua serotina, gives rise to the placenta.

246 HUMAN PLACENTA.

Although the vascular supply is cut off from the chorion lasve, the
processes on its surface do not completely abort. It becomes, as the time
of birth approaches, more and more closely united with the reflexa, till the
union between the two is so close that their exact boundaries cannot be
made out. The umbilical vesicle (fig. 151, ti&), although it becomes greatly
reduced in size and flattened, persists in a recognisable form till the time of
birth.

As the embryo enlarges, the space between the decidua vera and
decidua reflexa becomes reduced, and finally the two parts unite together.
The decidua vera is mainly characterised by the presence of peculiar round-
ish cells in its subepithelial tissue, and by the disappearance of a distinct
lining of epithelial cells. During the whole of pregnancy it remains highly
vascular. The decidua reflexa, on the disappearance of the vessels in the
chorion lieve, becomes non-vascular. Its tissue undergoes changes in the
main similar to those of the decidua vera, and as has been already mention-
ed, it fuses on the one hand with the chorion, and on the other with the
decidua vera. The membrane resulting from its fusion with the latter struc-
ture becomes thinner and thinner as pregnancy advances, and is reduced to
a thin layer at the time of birth.

The placenta has a somewhat discoidal form, with a slightly convex
uterine surface and a concave embryonic surface. At its edge it is continu-
ous both with the decidua reflexa and decidua vera. Near the centre of the
embryonic surface is implanted the umbilical cord. As has already been
mentioned, the placenta is formed of the decidua serotina and the fcetal villi
of the chorion frondosum. The fcetal and maternal tissues are far more
closely united (fig. 152) than in the forms described above. The villi of the
chorion, which were originally comparatively simple, become more and
more complicated, and assume an extremely arborescent form. Each of
them contains a vein and an artery, which subdivide to enter the complicat-
ed ramifications ; and are connected together by a rich anastomosis. The
villi are formed mainly of connective tissue, but are covered by an epithelial
layer generally believed to be derived from the subzonal membrane ; but, as
was first stated by Goodsir, and has since been more fully shewn by Ercolani
and Turner, this epithelial layer is really a part of the cellular decidua
serotina of the uterine wall, which has become adherent to the villi in
the development of the placenta (fig. 161, g). The placenta is divided into
a number of lobes, usually called cotyledons, by septa which pass towards
the chorion. These septa, which belong to the serotina, lie between the
arborescent villi of the chorion. The cotyledons themselves consist of a net-
work of tissue permeated by large vascular spaces, formed by the dilatation
of the maternal blood-vessels of the serotina, into which the ramifications of
the fcetal villi project. In these spaces they partly float freely, and partly are
attached to delicate trabecuke of the maternal tissue (fig. 161, G). They are,
of course, separated from the maternal blood by the uterine epithelial layer
before mentioned. The blood is brought to the maternal part of the pla-
centa by spirally coiled arteries, which do not divide into capillaries, but

MAMMALIA.

247

open into the large blood-spaces already spoken of. From these spaces
there pass off oblique utero-placental veins, which pierce the serotina, and
form a system of large venous sinuses in the adjoining uterine wall (fig. 152,
F), and eventually fall into the general uterine venous system. At birth the

FIG. 151. SECTION OF THE HUMAN UTERUS AND PLACENTA AT THE THIRTIETH
WEEK OF PREGNANCY. (From Huxley after Ecker.)

A. umbilical cord; B. chorion; C. foetal villi separated by processes of the
decidua serotina, D ; E, F, G. walls of uterus.

whole placenta, together with the fused decidua vera, and reflexa, with
which it is continuous, is shed ; and the blood-vessels thus ruptured are
closed by the contraction of the uterine wall.

The fcetal membranes and the placenta of the Simiadas (Turner, No. 225)
are in most respects closely similar to those in Man ; but the placenta is, in
most cases, divided into two lobes, though in the Chimpanzee, Cynocephalus,
and the Apes of the New World, it appears to be single.

The types of deciduate placenta so far described, are usually classified by
anatomists as discoidal placentas, although it must be borne in mind that
they differ very widely. In the Rodentia, Insectivora, and Cheiroptera there
is a (usually) dorsal placenta, which is co-extensive with the area of contact
between the allantois and the subzonal membrane, while the yolk-sack ad-
heres to a large part of the subzonal membrane. In Apes and Man the allan-
tois spreads over the whole inner surface of the subzonal membrane ;
the placenta is on the ventral side of the embryo, and occupies only a small
part of the surface of the allantois. The placenta of Apes and Man might be

248 THE ZONARY PLACENTA.

called metadiscoidal, in order to distinguish it from the primitive discoidal
placenta of the Rodentia and Insectivora.

In the Armadilloes (Dasypus) the placenta is truly discoidal and decidu-
ate (Owen and Kolliker). Alf. Milne Edwards states that in Dasypus
novemcinctus the placenta is zonary, and both Kolliker and he found four
embryos in the uterus, each with its own amnion, but the placenta of all four
united together ; and all four enclosed in a common chorion. A reflexa does
not appear to be present. In the Sloths the placenta approaches the discoi-
dal type (Turner, No. 218). It occupies in Cholaspus Hoffmanni about four-
fifths of the surface of the chorion, and is composed of about thirty-four dis-
coid lobes. It is truly deciduate, and the maternal capillaries are replaced
by a system of sinuses (fig. 161). The amnion is close to the inner surface of
the chorion. A dome-shaped placenta is also found amongst the Edentata in
Myrmecophaga and Tamandua (Milne Edwards, No. 208).

Zonary Placenta. Another form of deciduate placenta is known
as the zonary. This form of placenta occupies a broad zone of the chorion,
leaving the two poles free. It is found in the Carnivora, Hyrax, Elephas, and
Orycteropus.

It is easy to understand how the zonary placenta may be derived
from the primitive arrangement of the membranes (vide p. 240) by the exten-
sion of a discoidal placental area to a zonary area, but it is possible that
some of the types of zonary placenta may have been evolved from the con-
centration of a diffused placenta (vide p. 261) to a zonary area. The
absence of the placenta at the extreme poles of the chorion is explained by
the fact of their not being covered by a reflection of the uterine mucous
membrane. In the later periods of pregnancy the placental area becomes,
however, in most forms much more restricted than the area of contact
between the uterus and chorion.

In the Dog1, which may be taken as type, there is a large vascular yolk-
sack formed in the usual way, which does not however fuse with the chorion.
It extends at first quite to the end of the citron-shaped ovum, and persists
till birth. The allantois first grows out on the dorsal side of the embryo,
where it coalesces with the subzonal membrane, over a small discoidal area.

Before the fusion of the allantois with the subzonal membrane, there
grow out from the whole surface of the external covering of the ovum, except
the poles, numerous non-vascular villi, which fit into uterine crypts. When
the allantois adheres to the subzonal membrane vascular processes grow
out from it into these villi. The vascular villi so formed are of course at
first confined to the disc-shaped area of adhesion between the allantois and
the subzonal membrane ; and there is thus formea a rudimentary discoidal
placenta, closely resembling that of the Rodentia. The view previously
stated, that the zonary placenta is derived from the discoidal one, receives
from this fact a strong support.

The cavity of the allantois is large, and its inner part is in contact with

1 Vide Bischoff, No. 175.

MAMMALIA. 249

the amnion. The area of adhesion between the outer part of the allantois
and subzonal membrane gradually spreads over the whole interior of the
subzonal membrane, and vascular villi are formed over the whole area of
adhesion except at the two extreme poles of the egg. The last part to be
covered is the ventral side where the yolk-sack adjoins the subzonal mem-
brane.

During the extension of the allantois its cavity persists, and its inner part
covers not only the amnion, but also the yolk-sack. It adheres to the am-
nion and supplies it with blood-vessels (Bischoff).

With the full growth of the allantois there is formed a broad placental
zone, with numerous branched villi, fitting into corresponding pits which be-
come developed in the uterine walls. The maternal and fcetal structures be-
come closely interlocked and highly vascular ; and at birth a large part of
the maternal part is carried away with the placenta ; some of it however still
remains attached to the muscular wall of the uterus. The villi of the chorion
do not fit into uterine glands. The zone of the placenta diminishes greatly
in proportion to the chorion as the latter elongates, and at the full time the
breadth of the zone is not more than about one-fifth of the whole length of
the chorion.

At the edge of the placental zone there is a very small portion of the
uterine mucous membrane reflected over the non-placental part of the
chorion, which forms a small reflexa analogous with the reflexa in Man.

The Carnivora generally closely resemble the Dog, but in the Cat the
whole of the maternal part of the placenta is carried away with the fcetal
parts, so that the placenta is more completely deciduate than in the Dog.
In the Grey Seal (Halichcerus gryphus, Turner, No. 219) the general
arrangement of the foetal membranes is the same as in the other groups
of the Carnivora, but there is a considerable reflexa developed at the edge
of the placenta. The fcetal part of the placenta is divided by a series of
primary fissures which give off secondary and tertiary fissures. Into the
fissures there pass vascular laminae of the uterine wall. The general sur-
face of the foetal part of the placenta between the fissures is covered by
a greyish membrane formed of the coalesced terminations of the fcetal villi.

The structure of the placenta in Hyrax is stated by Turner (No. 221)
to be very similar to that in the Felidae. The allantoic sack is large, and
covers the whole surface of the subzonal membrane. The amnion is also
large, but the yolk-sack would seem to disappear at an early stage, instead
of persisting, as in the Carnivora, till the close of fcetal life.

The Elephant (Owen, Turner, Chapman) is provided with a zonary
deciduate placenta, though- a villous patch is present near each pole of the
chorion.

Turner (No. 220) has shewn that in Orycteropus there is present a zonary
placenta, which differs however in several particulars from the normal
zonary placenta of the Carnivora ; and it is even doubtful whether it is
truly deciduate. There is a single embryo, which fills up the body of the
uterus and also projects into only one of the horns. The placenta forms a

2$0 PLACENTA OF THE UNGULATA.

broad median zone, leaving the two poles free. The breadth of the zone is
considerably greater than is usual in Carnivora, one-half or more of the
whole longitudinal diameter of the chorion being occupied by the placenta.
The chorionic villi are arborescent, and diffusely scattered, and though the
maternal and fcetal parts are closely interwoven, it has not been ascer-
tained whether the adhesion between them is sufficient to cause the ma-
ternal subepithelial tissue to be carried away with the fcetal part of the
placenta at birth. The allantois is adherent to the whole chorion, the non-
placental parts of which are vascular. In the umbilical cord a remnant of
the allantoic vesicle was present in the embryos observed by Turner, but in
the absence of a large allantoic cavity the Cape Ant-eater differs greatly
from the Carnivora. The amnion and allantois were in contact, but no
yolk sack was observed.

Non-deciduate placenta. The remaining Mammalia are charac-
terized by a non-deciduate placenta ; or at least by a placenta in which only
parts of the maternal epithelium and no vascular maternal structures are
carried away at parturition. The non-deciduate placentae are divided into
two groups : (i) The polycotyledonary placenta, characteristic of the true
Ruminantia (Cervidae, Antilopidae, Bovida?, Camelopardalidae) ; (2) the
diffused placenta found in the other non-deciduate Mammalia, viz. the
Perissodactyla, the Suidae, the Hippopotamidae, the Tylopoda, the Tragulidae,
the Sirenia, the Cetacea, Manis amongst the Edentata, and the Lemuridae.
The polycotyledonary form is the most differentiated ; and is probably a
modification of the diffused form. The diffused non-deciduate placenta is
very easily derived from the primitive type (p. 240) by an extension of the
allantoic portion of the chorion ; and the exclusion of the yolk-sack from any
participation in forming the chorion.

The possession in common of a diffused type of placenta is by no
means to be regarded as a necessary proof of affinity between two groups,
and there are often, even amongst animals possessing a diffused form of
placenta, considerable differences in the general arrangement of the em-
bryonic membranes.

Ungulata. Although the Ungulata include forms with both coty-
ledonary and diffused placentae, the general arrangement of the embryonic
membranes is so similar throughout the group, that it will be convenient to
commence with a description of them, which will fairly apply both to the
Ruminantia and to the other forms.

The blastodermic vesicle during the early stages of development lies
freely in the uterus ; and no non-vascular villi, similar to those of the
Dog or the Rabbit, are formed before the appearance of the allantois.
The blastodermic vesicle has at first the usual spherical form, but it grows
out at an early period, and with prodigious rapidity, into two immensely
long horns ; which in cases where there is only one embryo are eventually
prolonged for the whole length of the two horns of the uterus. The
embryonic area is formed in the usual way, and its long axis is placed at
right angles to that of the vesicle. On the formation of an amnion there

MAMMALIA. 251

is formed the usual subzonal membrane, which soon becomes separated by
a considerable space from the yolk-sack (fig. 153). The yolk-sack is, how-

FIG. 153. EMBRYO AND FOETAL MEMBRANES OF A YOUNG EMBRYO ROE-DEER.

(After Bischoff.)

yk. yolk-sack; all. allantois just sprouting as a bilobed sack.

ever, continued into two elongated processes (yk), which pass to the two
extremities of the subzonal membrane. It is supplied with the normal
blood-vessels. As soon as the allantois appears (fig. 153 all], it grows out
into a right and a left process, which rapidly fill the whole free space within
the subzonal membrane and in many cases, e.g. the Pig (Von Baer), break
through the ends of the membrane, from which they project as the diver-
ticula allantoidis. The cavity of the allantois remains large, but the
lining of hypoblast becomes separated from the mesoblast, owing to the
more rapid growth of the latter. The mesoblast of the allantois applies
itself externally to the subzonal membrane to form the chorion1, and in-
ternally to the amnion, the cavity of which remains very small. The
chorionic portion of the allantoic mesoblast is very vascular, and that
applied to the amnion also becomes vascular in the later developmental
periods.

The horns of the yolk-sack gradually atrophy, and the whole yolk-
sack disappears some time before birth.

Where two or more embryos are present in the uterus, the chorions of
the several embryos may unite where they are in contact.

From the chorion there grow out numerous vascular villi, which fit into
corresponding pits in the uterine walls. According to the distribution of
these villi, the allantois is either diffused or polycotyledonary.

The pig presents the simplest type of diffused placenta. The villi of

1 According to Bischoff the subzonal membrane atrophies, leaving the allantoic
mesoblast to constitute the whole chorion.

252

PLACENTA OF THE UNGULATA.

the surface of the chorion cover a broad zone, leaving only the two poles
free; their arrangement differs therefore from that in a zonary placenta
in the greater breadth of the zone covered by them. The villi have the
form of simple papilla;, arranged on a series of ridges, which are highly

Kit;. 154. PORTION OF THE INJECTED CHORION OF A PIG, SLIGHTLY MAGNIFIED.

(From Turner.)

The figure shews a minute circular spot (l>) (enclosed by a vascular ring) from
which villous ridges (r) radiate.

vascular as compared with the intervening valleys. If an injected chorion is
examined (fig. 154^ certain clear non-vascular spots are to be seen (b), from
which the ridges of villi radiate. The surface of the uterus adapts itself
exactly to the elevations of the chorion ; and the furrows which receive the

155. SURFACE-VIEW OF THE INJECTED UTERINE MUCOSA OF A GRAVID PIG.

(From Turner.)

The fig. shews a circular non-vascular spot where a gland opens (g ) surrounded by
numerous vascular crypts (cr).

MAMMALIA.

253

chorionic ridges are highly vascular (fig. 155). On the other hand, there are
non-vascular circular depressions corresponding to the non-vascular areas
on the chorion ; and in these areas, and in these alone, the glands of the
uterus open (fig. 155 g) (Turner). The maternal and foetal parts of the
placenta in the pig separate with very great ease.

FIG. 156. VERTICAL SECTION THROUGH THE INJECTED PLACENTA OF A MARE.

(From Turner.)

ch. chorion with its villi partly in situ and partly drawn out of the crypts (cr) ;
E. loose epithelial cells which formed the lining of the crypt; g. uterine glands;
v. blood-vessels.

In the mare (Turner), the foetal villi are arranged in a less definite
zonary band than in the pig, though still absent for a very small area at
both poles of the chorion, and also opposite the os uteri. The filiform villi,
though to the naked eye uniformly scattered, are, when magnified, found to
be clustered together in minute cotyledons, which fit into corresponding
uterine crypts (fig. 156). Surrounding the uterine crypts are reticulate
ridges on which are placed the openings of the uterine glands. The re-
maining Ungulata with diffused placentas do not differ in any important
particulars from those already described.

The polycotyledonary form of placenta is found in the Ruminantia
alone. Its essential character consists in the foetal villi not being uni-
formly distributed, but collected into patches or cotyledons which form as
it were so many small placentae (fig. 157). The foetal villi of these patches
fit into corresponding pits in thickened patches of the wall of the uterus
(figs. 158 and 159). In many cases (Turner), the interlocking of the
maternal and foetal structures is so close that large parts of the maternal

254

PLACENTA OF THE UNGULATA.

epithelium are carried away when the foetal villi are separated from the
uterus. The glands of the uterus open in the intervals between the
cotyledons. The character of the cotyledons differs greatly in different
types. The maternal parts are cup-shaped in the sheep, and mushroom-
shaped in the cow. There are from 60—100 in the cow and sheep, but

Ch

FIG. 157. UTERUS OF A Cow IN THE MIDDLE OF PREGNANCY LAID OPEN.

(From Huxley after Colin.)
V. vagina; U. uterus; Ch. chorion; C\ uterine cotyledons; C2. fcetal cotyledons.

FIG. 158. COTYLEDON OF A Cow, THE FCETAL AND MATERNAL PARTS HALF

SEPARATED. (From Huxley after Colin.)
u. uterus; Ch. chorion; C1. maternal part of cotyledon; C2. fetal part.

MAMMALIA.

255

only about five or six in the Roe-deer. In the Giraffe there are, in addition
to larger and smaller cotyledons, rows and clusters of short villi, so that the
placenta is more or less intermediate between the polycotyledonary and
diffused types (Turner). A similarly intermediate type of placenta is found
in Cervus mexicanus (Turner).

FIG. 159. SEMI-DIAGRAMMATIC VERTICAL SECTION THROUGH A PORTION OF A

MATERNAL COTYLEDON OF A SHEEP. (From Turner.)

cr. crypts ; e. epithelial lining of crypts ; v. veins and c. curling arteries of sub-
epithelial connective tissue.

The groups not belonging to the Ungulata which are characterized by
the possession of a diffused placenta are the Sirenia, the Cetacea, Manis,
and the Lemuridae.

Sirenia. Of the Sirenia, the placentation of the Dugong is known
from some observations of Harting (No. 201).

It is provided with a diffuse and non-deciduate placenta ; with the
villi generally scattered except at the poles. The umbilical vesicle vanishes
early.

Cetacea. In the Cetacea, if we may generalize from Turner's observa-
tions on Orca Gladiator and the Narwhal, and those of Anderson (No. 191)
on Platanista and Orcella, the blastodermic vesicle is very much elongated,
and prolonged unsymmetrically into two horns. The mesoblast (fig. 160)
of the allantois would appear to grow round the whole inner surface of the
subzonal membrane, but the cavity of the allantois only persists as a widish
sack on the ventral aspect of the embryo (al). The amnion (am) is enor-
mous, and is dorsally in apposition with, and apparently coalesces with
the chorion, and ventrally covers the inner wall of the persistent allantoic
sack. The chorion, except for a small area at the two poles and opposite
the os uteri, is nearly uniformly covered with villi, which are more nume-

256

DIFFUSED PLACENTA.

rous than in fig. 160. In the large size of the amnion, and small dimen-
sions of the persistent allantoic sack, the Cetacea differ considerably from
the Ungulata.

cli

FIG. 160. DIAGRAM OF THE FCETAL MEMBRANES IN ORCA GLADIATOR.

(From Turner.)
ck. chorion; am. amnion; al. allantois; E. embryo.

Manis. Manis amongst the Edentata presents a type of diffused pla-
centa1. The villi are arranged in ridges which radiate from a non-villous
longitudinal strip on the concave surface of the chorion.

Manis presents us with the third type of placenta found amongst the
Edentata. On this subject, I may quote the following sentence from Turner
(Journal of Anat. and Phys., vol. x., p. 706).

"The Armadilloes (Dasypus), according to Professor Owen, possess a
single, thin, oblong, disc-shaped placenta ; a specimen, probably Dasypus
gymnurus, recently described by Kolliker2, had a transversely oval placenta,
which occupied the upper §rds of the uterus. In Manis, as Dr Sharpey has
shewn, the placenta is diffused over the surfaces of the chorion and uterine
mucosa. In Myrmecophaga and Tamandua, as MM. Milne Edwards have
pointed out, the placenta is set on the chorion in a dome-like manner.
In the Sloths, as I have elsewhere described, the placenta is dome-like in its
general form, and consists of a number of aggregated, discoid lobes. In
Orycteropus, as I have now shewn, the placenta is broadly zonular. "

Lemuridae. The Lemurs in spite of their affinities with the Primates
and Insectivora have, as has been shewn by Milne Edwards and Turner, an
apparently very different form of placenta. There is only one embryo, which
occupies the body and one of the cornua of the uterus. The yolk-sack
disappears early, and the allantois (Turner) bulges out into a right and left
lobe, which meet above the back of the embryo. The cavity of the allantois
persists, and the mesoblast of the outer wall fuses with the subzonal
membrane (the hypoblastic epithelium remaining distinct) to give rise to the
chorion.

On the surface of the chorion are numerous vascular villi, which fit into
uterine crypts. They are generally distributed, though absent at the two

1 The observations on this head were made by Sharpey, and are quoted by Huxley
(No. 202) and with additional observations by Turner in his Memoir on the placenta-
lion of the Sloths. Anderson (No. 191) has also recently confirmed Sharpey's account
of the diffused character of the placenta of Manis.

* Entwicklungsgcschichte des Menschen, etc., 2nd ed., p. 362. Leipzig, 1876.

MAMMALIA. 257

ends of the chorion and opposite the os uteri. Their distribution accords
with Turner's diffused type. Patches bare of villi correspond with smooth
areas on the surface of the uterine mucosa in which numerous utricular
glands open. There is no reflexa.

Although the Lemurian type of placenta undoubtedly differs from that of
the Primates, it must be borne in mind that the placenta of the Primates
may easily be conceived to be derived from a Lemurian form of placenta.
It will be remembered that in Man, before the true placenta becomes deve-
loped, there is a condition with simple vascular villi scattered over the cho-
rion. It seems very probable that this is a repetition of the condition of the
placenta of the ancestors of the Primates which has probably been more or
less retained by the Lemurs. It was mentioned above that the resemblance
between the metadiscoidal placenta of Man and that of the Cheiroptera, In-
sectivora and Rodentia is rather physiological than morphological.

Comparative histology of the Placenta.

It does not fall within the province of this work to treat from a histologi-
cal standpoint the changes which take place in the uterine walls during
pregnancy. It will, however, be convenient to place before the reader a
short statement of the relations between the maternal and fetal tissues
in the different varieties of placenta. This subject has been admirably dealt
with by Turner (No. 222), from whose paper fig. 161 illustrating this subject
is taken.

The simplest known condition of the placenta is that found in the pig (B).
The papilla-like fcetal villi fit into the maternal crypts. The villi (v) are
formed of a connective tissue cone with capillaries, and are covered by
a layer of very flat epithelium (e) derived from the subzonal membrane.
The maternal crypts are lined by the uterine epithelium (e'\ immediately
below which is a capillary flexus. The maternal and fcetal vessels are here
separated by a double epithelial layer. The same general arrangement
holds good in the diffused placentae of other forms, and in the polycotyledo-
nary placenta of the Ruminantia, but the fcetal villi (C) in the latter acquire
an arborescent form. The maternal vessels retain the form of capillaries.

In the deciduate placenta a considerably more complicated arrangement
is usually found. In the typical zonary placenta of the fox and cat (D and
E), the maternal tissue is broken up into a complete trabecular meshwork,
and in the interior of the trabeculae there run dilated maternal capillaries
(</). The trabeculae are covered by a more or less columnar uterine epithe-
lium (<?'), and are in contact on every side with fcetal villi. The capillaries of
the fcetal villi preserve their normal size, and the villi are covered by a flat
epithelial layer (e).

In the sloth (F) the maternal capillaries become still more dilated, and
the epithelium covering them is formed of very flat polygonal cells.

In the human placenta (G), as in that of Apes, the greatest modification

B. III. 17

258

HISTOLOGY OF THE PLACENTA.

M£

1C

:gy?
^1

T^-Tr- ' ~

' ^v\v( r' /}&

M

K 5^

J*

^

IK;. iCn. DIA<;KAMMATIC REPRESENTATIONS OK THE MINUTE STRUCTURE OK
i m I'l \>-\.;\\.\. (From Turner.)

MAMMALIA. 259

F. the foetal ; M. the maternal placenta ; e. epithelium of chorion ; ^. epithelium
of maternal placenta; d. fcetal blood-vessels; d'. maternal blood-vessels; v. villus.

A. Placenta in its most generalized form.

B. Structure of placenta of a Pig.

C. Structure of placenta of a Cow.

D. Structure of placenta of a Fox.

E. Structure of placenta of a Cat.

F. Structure of placenta of a Sloth. On the right side of the figure the flat
maternal epithelial cells are shewn in situ. On the left side they are removed, and
the dilated maternal vessel with its blood-corpuscles is exposed.

G. Structure of Human placenta. In addition to the letters already referred to
ds, ds. represents the decidua serotina of the placenta; /, t. trabeculse of serotina
passing to the foetal villi; ca. curling artery ; up. utero-placental vein; x. a prolonga-
tion of maternal tissue on the exterior of the villus outside the cellular layer e', which
may represent either the endothelium of the maternal blood-vessel or delicate con-
nective tissue belonging to the serotina, or both. The layer e' represents maternal
cells derived from the serotina. The layer of fcetal epithelium cannot be seen on the
villi of the fully-formed human placenta.

is found in that the maternal vessels have completely lost their capillary
form, and have become expanded into large freely communicating sinuses
(d'). In these sinuses the fcetal villi hang for the most part freely, though
occasionally attached to their walls (/). In the late stages of fcetal life there
is only one epithelial layer (/) between the maternal and fcetal vessels, which
closely invests the fcetal villi, but, as shewn by Turner and Ercolani, is part
of the uterine tissue. In the fcetal villi the vessels retain their capillary
form.

Evolution of the Placenta.

From Owen's observations on the Marsupials it is clear that
the yolk-sack in this group plays an important, if not the most
important part, in absorbing the maternal nutriment destined
for the foetus. The fact that in Marsupials both the yolk-sack
and the allantois are functional in rendering the chorion
vascular makes it d priori probable that this was also the case in
the primitive types of the Placentalia, and this deduction is
supported by the fact that in the Rodentia, Insectivora and
Cheiroptera this peculiarity of the fcetal membranes is actually
found. In the primitive Placentalia there was probably present a
discoidal allantoic region of the chorion, from which simple fcetal
villi, like those of the pig (fig. 161 B), projected into uterine
crypts ; but it is not certain how far the umbilical part of the
chorion, which was no doubt vascular, may also have been

17—2

26O EVOLUTION OF THE PLACENTA.

villous. From such a primitive type of foetal membranes
divergences in various directions have given rise to the types of
foetal membranes now existing.

In a general way it may be laid down that variations in any
direction which tended to increase the absorbing capacities of
the chorion would be advantageous. There are two obvious
ways in which this might be done, viz. (i) by increasing the
complexity of the fcetal villi and maternal crypts over a limited
area, (2) by increasing the area of the part of the chorion
covered by placental villi. Various combinations of the two
processes would also of course be advantageous.

The most fundamental change which has taken place in all
the existing Placentalia is the exclusion of the umbilical vesicle
from any important function in the nutrition of the fcetus.

The arrangement of the fcetal parts in the Rodentia, In-
sectivora and Cheiroptera may be directly derived from the
primitive form by supposing the villi of the discoidal placental
area to have become more complex, so as to form a deciduate
discoidal placenta ; while the yolk-sack still plays a part, though
physiologically an unimportant part, in rendering the chorion
vascular.

In the Carnivora again we have to start from the discoidal
placenta, as shewn by the fact that the allantoic region of the
placenta is at first discoidal (p. 248). A zonary deciduate
placenta indicates an increase both in area and in complexity.
The relative diminution of the breadth of the placental zone in
late fcetal life in the zonary placenta of the Carnivora is probably
due to its being on the whole advantageous to secure the
nutrition of the fcetus by insuring a more intimate relation
between the fcetal and maternal parts, than by increasing their
area of contact. The reason of this is not obvious, but as
mentioned below, there are other cases where it can be shewn
that a diminution in the area of the placenta has taken place,
accompanied by an increase in the complexity of its villi.

The second type of differentiation from the primitive form of
discoidal placenta is illustrated by the Lemuridae, the Suidae,
and Manis. In all these cases the area of the placental villi
appears to have increased so as to cover nearly the whole
subzonal membrane, without the villi increasing to any great

MAMMALIA. 261

extent in complexity. From the diffused placenta covering the
whole surface of the chorion, differentiations appear to have
taken place in various directions. The metadiscoidal placenta of
Man and Apes, from its mode of ontogeny (p. 248), is clearly
derived from a diffused placenta — very probably similar to that
of Lemurs — by a concentration of the foetal villi, which are
originally spread over the whole chorion, to a disc-shaped area,
and by an increase in their arborescence.

The polycotyledonary forms of placenta are due to similar
concentrations of the foetal villi of an originally diffused placenta.

In the Edentata we have a group with very varying types of
placenta. Very probably these may all be differentiations
within the group itself from a diffused placenta, such as that
found in Manis. The zonary placenta of Orycteropus is capable
of being easily derived from that of Manis, by the disappearance
of the fcetal villi at the two poles of the ovum. The small size
of the umbilical vesicle in Orycteropus indicates that its discoidal
placenta is not, like that in Carnivora, directly derived from a
type with both allantoic and umbilical vascularization of the
chorion. The discoidal and dome-shaped placentae of the
Armadilloes, Myrmecophaga, and the Sloths may easily have
been formed from a diffused placenta, just as the discoidal
placenta of the Simiadae and Anthropidse appears to have been
formed from a diffused placenta like that of the Lemuridae.

The presence of zonary placentae in Hyrax and Elephas does
not necessarily afford any proof of affinity of these types with
the Carnivora. A zonary placenta may quite easily be derived
from a diffused placenta ; and the presence of two villous patches
at the poles of the chorion in Elephas indicates that this was
very probably the case with the placenta of this form.

Although it is clear from the above considerations that the
placenta is capable of being used to some extent in classification,
yet at the same time the striking resemblances which can exist
between such essentially different forms of placenta, as for
instance those of Man and the Rodentia, are likely to prevent it
being employed, except in conjunction with other characters.

262 DEVELOPMENT OF THE GUINEA-PIG.

Special types of development.

The Guinea-pig, Cavia cobaya. Many years ago Bischoff
(No. 176) shewed that the development of the guinea-pig was strikingly
different from that of other Mammalia. His statements, which were at first
received with some doubt, have been in the main fully confirmed by Hensen
(No. 182) and Schafer (No. 190), but we are still as far as ever from explain-
ing the mystery of the phenomenon.

The ovum, enclosed by the zona radiata, passes into the Fallopian tube
and undergoes a segmentation which has not been studied with great detail.
On the close of segmentation, about six days after impregnation, it assumes
(Hensen) a vesicular form not unlike that of other Mammalia. To the inner
side of one wall of this vesicle is attached a mass of granular cells similar to
the hypoblastic mass in the blastodermic vesicle of the rabbit. The egg still
lies freely in the uterus, and is invested by its zona radiata. The changes
which next take place are in spite of Bischoff's, Reichert's (No. 188) and
Hensen's observations still involved in great obscurity. It is certain, how-
ever, that during the course of the seventh day a ring-like thickening of the
uterine mucous membrane, on the free side of the uterus, gives rise to a kind
of diverticulum of the uterine cavity, in which the ovum becomes lodged.
Opposite the diverticulum the mucous membrane of the mesometric side of
the uterus also becomes thickened, and this thickening very soon (shortly
after the seventh day) unites with the wall of the diverticulum, and com-
pletely shuts off the ovum in a closed capsule.

The history of the ovum during the earlier period of its inclusion in the
diverticulum of the uterine wall is not satisfactorily elucidated. There
appears in the diverticulum during the eighth and succeeding days a cylin-
drical body, one end of which is attached to the uterine walls at the mouth
of the diverticulum. The opposite end of the cylinder is free, and contains
a solid body.

With reference to the nature of this cylinder two views have been put
forward. Reichert and Hensen regard it as an outgrowth of the uterine wall,
while the body within its free apex is regarded as the ovum. Bischoff and
Schafer maintain that the cylinder itself is the ovum attached to the uterine
wall. The observations of the latter authors, and especially those of Schafer,
appear to me to speak for the correctness of their view1.

The cylinder gradually elongates up to the twelfth day. Before this pe-
riod it becomes attached by its base to the mesometric thickening of the
uterus, and enters into vascular connection with it. During its elongation it

1 Schiifcr's and Hensen's statements are in more or less direct contradiction as to
the structure of the ovum after the formation of the embryo; and it is not possible to
decide between the two views about the ovum till these points of difference have been
cleared up.

MAMMALIA. 263

becomes hollow, and is filled with a fluid not coagulable in alcohol, while the
body within its apex remains unaltered till the tenth day.

On this day a cavity develops in the interior of this body which at the
same time enlarges itself. The greater part of its wall next attaches itself
to the free end of the cylinder, and becomes considerably thickened. The

FIG. 162. DIAGRAMMATIC LONGITUDINAL SECTION THROUGH THE EMBRYO OF

A GUINEA-PIG WITH ITS MEMBRANES. (After Schafer.)

e. epiblast ; h. hypoblast ; in', amniotic mesoblast ; in" . splanchnic mesoblast ;
am. amnion ; ev. cavity of amnion ; all. allantois ; f. rudimentary blastopore ; me.
cavity of vesicle continuous with body cavity; mm. mucous membrane of uterus;
m'm'. parts where vascular uterine tissue perforates hypoblast of blastodermic vesicle ;
vt. uterine vascular tissue ; /. limits of uterine tissue.

remainder of the wall adjoining the cavity of the cylinder becomes a com-
paratively thin membrane. At the free end of the cylinder there appears on
the thirteenth day an embryonic area similar to that of other Mammalia.
It is at first round but soon becomes pyriform, and in it there appear a
primitive streak and groove ; and on their appearance it becomes obvious
that the outer layer of the cylinder is the hypoblast^, instead of, as in all
other Mammalia, the epiblast ; and that the epiblast is formed by the wall of
the inner vesicle, i.e. the original solid body placed at the end of the cylinder.
Thus the dorsal surface of the embryo is turned inwards, and the ventral
surface outwards, and the ordinary position of the layers is completely
inverted.

1 According to Hensen the hypoblast grows round the inside of the wall of the
cylinder from the body which he regards as the ovum. The original wall of the
cylinder persists as a very thin layer separated from the hypoblast by a membrane.

264 DEVELOPMENT OF THE GUINEA-PIG.

The previously cylindrical egg next assumes a spherical form, and the
mesoblast arises in connection with the primitive streak in the manner
already described. A splanchnic layer of mesoblast attaches itself to the
inner side of the outer hypoblastic wall of the egg, a somatic layer to the
epiblast of the inner vesicle, and a mass of mesoblast grows out into the
cavity of the larger vesicle forming the commencement of the allantois.
The general structure of the ovum at this stage is represented on fig. 162,
copied from Schafer ; and the condition of the whole ovum will best be
understood by a description of this figure.

It is seen to consist of two vesicles, (i) an outer larger one (h] — the
original egg-cylinder — united to the mesometric wall of the uterus by n vas-
cular connection at ;«';«', and (2) an inner smaller one (ev) — the originally
solid body at the free end of the egg-cylinder. The outer vesicle is formed
of (i) an external lining of columnar hypoblast (h) which is either pierced or
invaginated at the area of vascular connection with the uterus, and (2) of an
inner layer of splanchnic mesoblast (in"} which covers without a break the
vascular uterine growth. At the upper pole of the ovum is placed the
smaller epiblastic vesicle, and where the two vesicles come together is
situated the embryonic area with the primitive streak (_/), and the medullary
plate seen in longitudinal section. The thinner wall of the inner vesicle is
formed of epiblast and somatic mesoblast, and covers over the dorsal face
of the embryo just like the amnion. It is in fact usually spoken of as the
amnion. The large cavity of the outer vesicle is continuous with the body
cavity, and into it projects the solid mesoblastic allantois («//), so far with-
out hypoblast1.

The outer vesicle corresponds exactly with the yolk-sack, and its meso-
blastic layer receives the ordinary vascular supply.

The embryo becomes folded off from the yolk-sack in the usual way, but
comes to lie not outside it as in the ordinary form, but in its interior, and is
connected with it by an umbilical stalk. The yolk-sack forms the substitute
for part of the subzonal membrane of other Mammalia. The so-called
amnion appears to me from its development and position rather to
correspond with the non-embryonic part of the epiblastic wall (true
subzonal membrane) of the blastodermic vesicle of the ordinary mammalian
forms than with the true amnion ; and a true amnion would seem not to be
developed.

The allantois meets the yolk-sack on about the seventeenth day at
the region of its vascular connection with the uterine wall, and gives rise to
the placenta. A diagrammatic representation of the structure of the embryo
at this stage is given in fig. 163.

The peculiar inversion of the layers in the Guinea-pig has naturally
excited the curiosity of embryologists, but as yet no satisfactory explanation
has been offered of it.

1 Hensen states that the hypoblast never grows into the allantois; while Bischoff,
though not very precise on the point, implies that it does ; he states however that it
soon disappears.

MAMMALIA.

265

At the time when the ovum first becomes fixed it will be remembered
that it resembles the early blastodermic vesicle of the Rabbit, and it is
natural to suppose that the apparently hypoblastic mass attached to
the inner wall of the vesicle
becomes the solid body at the
end of the egg-cylinder. This
appears to be Bischoff's view,
but, as shewn above, the solid
mass is really the epiblast !
Is it conceivable that the hypo-
blast in one species becomes
the epiblast in a closely allied
species? To my mind it is not
conceivable, and I am reduced
to the hypothesis, put forward
by Hensen, that in the course
of the attachment of the ovum
to the wall of the uterus a rup-
ture of walls of the blasto-
dermic vesicle takes place, and
that they become completely
turned inside out. It must be
admitted, however, that in the
present state of our knowledge
of the development of the o-
vum on the seventh and eighth

days it is not possible to frame a satisfactory explanation how such an
inversion can take place.

The Human Embryo. Our knowledge as to the early develop-
ment of the human embryo is in an unsatisfactory state. The positive facts
we know are comparatively few, and it is not possible to construct from
them a history of the development which is capable of satisfactory com-
parison with that in other forms, unless all the early embryos known are
to be regarded as abnormal. The most remarkable feature in the develop-
ment, which was first clearly brought to light by Allen Thomson in 1839, is
the very early appearance of branched villi. In the last few years several
ova, even younger than those described by Allen Thomson, have been met
with, which exhibit this peculiarity.

The best-preserved of these ova is one described by Reichert (No. 237).
This ovum, though probably not more than thirteen days old, was com-
pletely enclosed by a decidua reflexa. It had (fig. 164 A and B) a flattened
oval form, measuring in its two diameters 5*5 mm. and 3-5 mm. The edge
was covered with branched villi, while in the centre of each of the flattened
surfaces there was a spot free from villi. On the surface adjoining the
uterine wall was a darker area (e) formed of two layers of cells, which is
interpreted by Reichert as the embryonic area, while the membrane forming

FIG. 163. DIAGRAMMATIC LONGITUDINAL
SECTION OF AN OVUM OF A GUINEA-PIG AND THE
ADJACENT UTERINE WALLS AT AN ADVANCED
STAGE OF PREGNANCY. (After Bischoff.)

yk. inverted yolk-sack (umbilical vesicle)
formed of an external hypoblastic layer (shaded)
and an internal vascular layer (black). At the
end of this layer is placed the sinus terminalis ;
all. allantois ; //. placenta.

The external shaded parts are the uterine
walls.

266 HUMAN OVUM.

the remainder of the ovum, including the branched villi, was stated by
Reichert to be composed of a single row of epithelial cells.

Whether or no Reichert is correct in identifying his darker spot as the
embryonic area, it is fairly certain from the later observations of Beigel and
Lowe (No. 228), Ahlfeld (No. 227), and Kollmann (No. 234) on ova nearly
as young as that of Reichert, that the wall of very young ova has a more
complicated structure than Reichert is willing to admit. These authors do
not however agree amongst themselves, but from Kollmann's description,
which appears to me the most satisfactory, it is probable that it is composed
of an outer epithelial layer, and an inner layer of connective tissue, and that
the connective tissue extends at a very early period into the villi ; so that
the latter are not hollow, as Reichert supposed them to be.

FIG. 164. THE HUMAN OVA DURING EARLY STAGES OF DEVELOPMENT.

(From Quain's Anatomy.)

A. and B. Front and side view of an ovum figured by Reichert, supposed to be
about thirteen days. e. embryonic area.

C. An ovum of about four or five weeks shewing the general structure of the ovum
before the formation of the placenta. Part of the wall of the ovum is removed to shew
the embryo in situ, (After Allen Thomson.)

The villi, which at first leave the flattened poles free, seem soon to
extend first over one of the flat sides, and finally over the whole ovum
(fig. 164 C).

Unless the two-layered region of Reichert's ovum is the embryonic area,
nothing which can clearly be identified as an embryo has been detected in
these early ova. In an ovum described by Breus (No. 228), and in one
described long ago by Wharton- Jones a mass found in the interior of the
egg may perhaps be interpreted (His) as the remains of the yolk. It is,
however, very probable that all the early ova so far discovered are more or
less pathological.

The youngest ovum with a distinct embryo is one described by His
(No. 232). This ovum, which is diagrammatically represented in fig. 168 in
longitudinal section, had the form of an oval vesicle completely covered by
villi, and about 8'5 mm. and $'5 mm. in its two diameters, and flatter on
one side than on the other. An embryo with a yolk-sack was attached to
the inner side of the flatter wall of the vesicle by a stalk, which must be

MAMMALIA.

267

regarded as the allantoic stalk1, and the embryo and yolk-sack filled up
but a very small part of the whole cavity of the vesicle.

The embryo, which was probably not quite normal (fig. 165 A), was
very imperfectly developed ; a medullary plate was hardly indicated, and,

am..

ch-

FIG. 165. THREE EARLY HUMAN EMBRYOS. (Copied from His.)
An early embryo described by His from the side. am. amnion; urn. umbilical
ch. chorion, to which the embryo is attached by a stalk.
Embryo described by Allen Thomson about 12 — 14 days. urn. umbilical

A.

vesicle

B.
vesicle ; md. medullary groove.

C. Young embryo described by His.

•mil. umbilical vesicle.

though the mesoblast was unsegmented, the head fold, separating the
embryo from the yolk-sack (#;#), was already indicated. The amnion (am]
was completely formed, and vitelline vessels had made their appearance.

Two embryos described by Allen Thomson (No. 239) are but slightly
older than the above embryos of His. Both of them probably belong to the
first fortnight of pregnancy. In both cases the embryo was more or less
folded off from the yolk-sack, and in one of them the medullary groove was
still widely open, except in the region of the neck (fig. 165 B). The allantoic
stalk, if present, was not clearly made out, and the condition of the amnion
was also not fully studied. The smaller of the two ova was just 6 mm. in

1 Allen Thomson informs me that he is very confident that such a form of attach-
ment between the hind end of the embryo and the wall of the vesicle, as that described
and figured by His in this embryo, did not exist in any of the younger embryos
examined by him.

268

HUMAN OVUM.

its largest diameter, and was nearly completely covered with simple villi,
more developed on one side than on the other.

In a somewhat later period, about the stage of a chick at the end of the
second day, the medullary folds are completely closed, the region of the
brain already marked, and the cranial flexure commencing. The mesoblast
is divided up into numerous somites, and the mandibular and first two
branchial arches are indicated. The embryo is still but incompletely folded
off from the yolk-sack below.

In a still older stage the cranial flexure becomes still more pronounced,
placing the mid-brain at the end of the long axis of the body. The body
also begins to be ventrally curved (fig. 165 C).

Externally human embryos at this age are characterised by the small
size of the anterior end of the head.

The flexure goes on gradually increasing, and in the third week of
pregnancy in embryos of about 4 mm. the limbs make their appearance.
The embryo at this stage (fig. 166), which is about equivalent to that of a

FIG. 166. Two VIEWS OK A HUMAN EMBRYO OF BETWEEN THE THIRD AND

FOURTH WEEK.

A. Side view. (From Kolliker; after Allen Thomson.) a. amnion; b. umbilical
vesicle; c, mandibular arch; e. hyoid arch ; f. commencing anterior limb; g. primitive
auditory vesicle; h. eye; i. heart.

B. Dorsal view to shew the attachment of the dilated allantoic stalk to the
chorion. (From a sketch by Allen Thomson.) am- amnion; all. allantois; ys. yolk-
sack.

chick on the fourth day, resembles in almost every respect the normal
embryos of the Amniota. The cranial flexure is as pronounced as usual,
and the cerebral region has now fully the normal size. The whole body
soon becomes flexed ventrally, and also somewhat spirally. The yolk-
sack (b} forms a small spherical appendage with a long wide stalk, and the
embryo (B) is attached by an allantoic stalk with a slight swelling (all],
probably indicating the presence of a small hypoblastic diverticulum, to the
inner face of the chorion.

A remarkable exception to the embryos generally observed is afforded
by an embryo which has been described by Krause (No. 235). In this

MAMMALIA. 269

embryo, which probably belongs to the third week of pregnancy, the limbs
were just commencing to be indicated, and the embryo was completely
covered by an amnion, but instead of being attached to the chorion by an
allantoic cord, it was quite free, and was provided with a small spherical
sack-like allantois, very similar to that of a fourth-day chick, projected from
its hind end.

FIG. 167. FIGURES SHEWING THE EARLY CHANGES IN THE FORM OF THE
HUMAN HEAD. (From Quain's Anatomy.)

A. Head of an embryo of about four weeks. (After Allen Thomson.)

B. Head of an embryo of about six weeks. (After Ecker.)

C. Head of an embryo of about nine weeks.

i. mandibular arch; i'. persistent part of hyomandibular cleft; a. auditory vesicle.

No details are given as to the structure of the chorion or the presence of
villi upon it. The presence of such an allantois at this stage in a human
embryo is so unlike what is usually found that Krause's statements have been
received with considerable scepticism. His even holds that the embryo is a
chick embryo, and not a human one ; while Kolliker regards Krause's
allantois as a pathological structure. The significance to be attached to this
embryo is dealt with below.

A detailed history of the further development of the human embryo does
not fall within the province of this work ; while the later changes in the
embryonic membranes have already been dealt with (pp. 244 — 248).

For the changes which take place on the formation of the face I may
refer the reader to fig. 167.

The most obscure point connected with the early history of the human
ovum concerns the first formation of the allantois, and the nature of the villi
covering the surface of the ovum. The villi, if really formed of mesoblast
covered by epiblast, have the true structure of chorionic villi ; and can
hardly be compared to the early villi of the dog which are derived from the
subzonal membrane, and still less to those of the rabbit formed from the
zona radiata.

Unless all the early ova so far described are pathological, it seems to

2/0

HUMAN OVUM.

follow that the mesoblast of the chorion is formed before the embryo is
definitely established, and even if the pathological character of these ova is
admitted, it is nevertheless probable (leaving Krause's embryo out of
account), as shewn by the early embryos of Allen Thomson and His, that it
is formed before the closure of the medullary groove. In order to meet this
difficulty His supposes that the embryo never separates from the blasto-
dermic vesicle, but that the allantoic
stalk of the youngest embryo (fig. 168)
represents the persistent attachment be-
tween the two1. His' view has a good
deal to be said for it. I would venture,
however, to suggest that Reichert's em-
bryonic area is probably not in the two-
layered stage, but that a mesoblast has
already become established, and that it
has grown round the inner face of the FlG> l68> DIAGRAMMATIC LONGI-
blastodermic vesicle from the (apparent) TUDINAL SECTION OF THE OVUM TO
posterior end of the primitive streak, jnnot.-.ggy-. *SA)».

This growth I regard as a frecoci^ ^ amnion. A,. umhilical vesicle.

formation of the mesoblast of the allantois

— an exaggeration of the early formation of the allantoic mesoblast which is
characteristic of the Guinea-pig (vide p. 264). This mesoblast, together
with the epiblast, forms a true chorion, so that in fig. 168, and probably also
in fig. 164 A and B, a true chorion has already become established. The
stalk connecting the embryo with the chorion in His' earliest embryo
(fig. 168) is therefore a true allantoic stalk into which the hypoblastic
allantoic diverticulum grows in for some distance. How the yolk-sack
(umbilical vesicle) is formed is not clear. Perhaps, as suggested by His, it
arises from the conversion of a solid mass of primitive hypoblast directly into
a yolk-sack. The amnion is probably formed as a fold over the head end of
the embryo in the manner indicated in His' diagram (fig. 168 Am}.

These speculations have so far left Krause's embryo out of account.
How is this embryo to be treated ? Krause maintains that all the other
embryos shewing an allantoic stalk at an early age are pathological. This,
though not impossible, appears to me, to say the least of it, improbable ;
especially when it is borne in mind that embryos, which have every ap-
pearance of being normal, of about the same age and younger than Krause's,
have been frequently observed, and have always been found attached to the
chorion by an allantoic stalk.

We are thus provisionally reduced to suppose either that the structure
figured by Krause is not the allantois, or that it is a very abnormal
allantois. It is perhaps just possible that it maybe an abnormally developed
hypoblastic vesicle of the allantois artificially detached from the mesoblastic
layer, — the latter having given rise to the chorion at an earlier date.

1 For a fuller explanation of His' views I must refer the reader to his Memoir (No.
•j:V2), pp. 170. 171, and to the diagrams contained in it.

MAMMALIA. 2/1

BIBLIOGRAPHY.

General.

(168) K. E. von Baer. Ueb. Entwicklungsgeschichte d. Jhiere. Konigsberg,
1828-1837.

(169) Barry. "Researches on Embryology." First Series. Philosophical
Transactions, 1838, Part II. Second Series, Ibid. 1839, Part II. Third Series, Ibid.
1840.

(170) Ed. van Beneden. La maturation deTceuf, la fecondation et les premieres
phases du dcveloppement embryonaire d. Mammiferes. Bruxelles, 1875.

(171) Ed. van Beneden. " Recherches sur 1'embryologie des Mammiferes."
Archives de Biologie, Vol. I. 1880.

(172) Ed. v. Beneden and Ch. Julin. "Observations sur la maturation etc.
de 1'ceuf chez les Cheiropteres." Archives de Biologie, Vol. I. 1880.

(173) Th. L. W. Bischoff. Entwickhmgsgeschichte d. Sdugethiere u. des
Menschen. Leipzig, 1842.

(174) Th. L. W. Bischoff. Entwicklungsgeschichte des Kanincheneies. Braun-
schweig, 1842.

(175) Th. L. W. Bischoff. Entwickhmgsgeschichte des Hundeeies. Braun-
schweig, 1845.

(176) Th. L. W. Bischoff. Entwickhmgsgeschichte des Meerschweinchens.
Giessen. 1852.

(177) Th. L. W. Bischoff. Entwicklungsgeschichte des Rehes. Giessen, -1854.

(178) Th. L. W. Bischoff. " Neue Beobachtungen z. Entwicklungsgesch. des
Meerschweinchens." Abh. d. bayr. Akad., Cl. II. Vol. X. 1866.

(179) Th. L. W. Bischoff. Historisch-kritische Bemerkungen z. d. neuesten
Mittheilungen rib. d. erste Entwick. d. Sdugethiereier. Miinchen, 1877.

(180) M. Coste. Embryogenie comparee. Paris, 1837.

(181) E. Haeckel. Anthropogenic, Entwicklungsgeschichte des Menschen.
Leipzig, 1874.

(182) V. Hensen. "Beobachtungen lib. d. Befrucht. u. Entwick. d. Kaninchens
u. Meerschweinchens." Zeit.f. Anat. u. Entwick., Vol. I. 1876.

(183) A. Kolliker. Entwicklungsgeschichte d. Menschen u. d. hoheren Thiere.
Leipzig, 1879.

(184) A. Kolliker. "Die Entwick. d. Keimblatter des Kaninchens." Zoolo-
gischer Anzeiger, Nos. 61, 62, Vol. Hi. 1880.

(185) N. Lieberkiihn. Ueber d. Keimbltitter d. Siiugethiere. Doctor-Jnbelfeier
d. Herrn. H. Nasse. Marburg, 1879.

(186) N. Lieberkiihn. "Z. Lehre von d. Keimblattern d. Saugethiere." Sitz.
d. Gesell. z. Beford. d. gesam. Naturwiss. Marburg, No. 3. 1880.

(187) Rauber. "Die erste Entwicklung d. Kaninchens." Sitzungsber. d.
naturfor. Gesell. z. Leipzig. 1875.

(188) C. B. Reichert. "Entwicklung des Meerschweinchens." Abh. der.
Berl. Akad. 1862.

(189) E. A. S chafer. " Description of a Mammalian ovum in an early con-
dition of development." Proc. Roy. Soc., No. 168. 1876.

2/2 MAMMALIAN BIBLIOGRAPHY.

(190) E. A. Schiifer. " A contribution to the history of development of the
guinea-pig." Journal of Anal, and Phys., Vol. x. and xi. 1876 and 1877.

Foetal Membranes and Placenta.

(191) John Anderson. Anatomical and Zoological Researches in Western
Yunnan. London, 1878.

(192) K. E. von Baer. Untersuchungen ilber die Gefassverbindung swischen
Mutter und Frucht, 1828.

(193) C. G. Carus. Tabulae anatomiam comparalivam illustrantes. 1831,
1840.

(194) H. C. Chapman. "The placenta and generative apparatus of the
Elephant." Journ. Acad. Nat. Sc., Philadelphia. Vol. vin. 1880.

(195) C. Creighton. " On the formation of the placenta in the guinea-pig.'.'
Journal of Anat. and Phys. , Vol. XII. 1 878.

(196) Ecker. Icones Physiologicae. 1852-1859.

(197) G. B. Ercolani. The utricular glands of the uterus, etc., translated from
the Italian under the direction of H. O. Marcy. Boston, 1880. Contains translations
of memoirs published in the Mem. delf Accad. d. Scienze d. Bologna, and additional
matter written specially for the translation.

(198) G. B. Ercolani. Nuove ricerche sulla placenta nei pesci cartilaginosi e
net mammiferi. Bologna, 1880.

(199) Eschricht. De organis quae respirationi et nutritioni fcetus Mammalium
inservinnt. Hafniae, 1837.

(200) A. H. Garrod and W. Turner. "The gravid uterus and placenta of
Hyomoschus aquaticus." Proc. Zool. Soc., London, 1878.

(201) P. Hart ing. Het ei en de placenta van Halicore Dtigong. Inaug. diss.
Utrecht. "On the ovum and placenta of the Dugong." Abstract by Prof. Turner.
Joitrnal of Anat. and Phys., Vol. xm.

(202) Th. H. Huxley. The Elements of Comparative Anatomy. London,
1864.

(203) A. Kolliker. " Ueber die Placenta der Gattung Tragulus." Verh. der
Wiirzb. phys.-med. Gesellschaft, Bd. x.

(204) C. D. Meigs. "On the reproduction of the Opossum (Didelphis Vir-
giniana)." Amer. Phil. Soc. Trans., Vol. x. 1853.

(205) H.Milne-Edwards. " Sur la Classification Naturelle." Ann. Sciences
Nat., SeY. 3, Vol. i. 1844.

(206) Alf. Milne-Edwards. "Recherches sur la famille des Chevrotains."
Ann. des Sciences Nat., Series V., Vol. II. 1864.

(207) Alf. Milne-Edwards. " Observations sur quelques points de PEmbryo-
logie des Lemuriens, etc." Ann. Sci. Nat., Ser. v., Vol. xv. 1872.

(208) Alf. Milne- Edwards. " Sur la conformation du placenta chez le Ta-
mandua." Ann. des Sci. Nat., xv. 1872.

(209) Alf. Milne-Edwards. " Recherches s. 1. enveloppes foetales du Tatou a
neuf bandes." Ann. Sci. Nat., Ser. vi., Vol. vin. 1878.

(210) R. Owen. "On the generation of Marsupial animals, with a description
of the impregnated uterus of the Kangaroo." Phil. Trans., 1834.

(211) R. Owen. "Description of the membranes of the uterine foetus of the
Kangaroo." Mag. Nat. Hist., Vol. I. 1837.

MAMMALIA. 273

(212) R. Owen. "On the existence of an Allantois in a foetal Kangaroo
(Macropus major)." Zool. Soc. Proc., V. 1837.

(213) R. Owen. "Description of the foetal membranes and placenta of the
Elephant." Phil. Trans., 1857.

(214) R.Owen. On the Anatomy of Vertebrates, Vol. in. London, 1868.

(215) G. Rolleston. " Placental structure of the Tenrec, etc." Transactions
of the Zoological Society, Vol. v. 1866.

(216) W. Turner. "Observations on the structure of the human placenta."
Journal of Anat. and Phys., Vol. VII. 1868.

(217) W. Turner. "On the placentation of the Cetacea." Trans. Roy. Soc.
Edinb., Vol. XXVI. 1872.

(218) W. Turner. "On the placentation of Sloths (Cholcepus Hoffrnanni)."
Trans, of R. Society of Edinburgh, Vol. xxvii. 1875.

(219) W. Turner. "On the placentation of Seals (Halichcerus gryphus)."
Trans, of R. Society of Edinburgh, Vol. xxvii. 1875.

(220) W. Turner. "On the placentation of the Cape Ant-eater (Orycteropus
capensis)." Journal of Anat. and Phys., Vol. X. 1876.

(221) W. Turner. Lectures on the Anatomy of the Placenta. First Series.
Edinburgh, 1876.

(222) W.Turner. "Some general observations on the placenta, with special
reference to the theory of Evolution." Journal of Anat. and Phys., Vol. xi. 1877.

(223) W. Turner. "On the placentation of the Lemurs." Phil. Trans., Vol.
166, p. 2. 1877.

(224) W.Turner. " On the placentation of Apes." Phil. Trans., 1878.

(225) W. Turner. "The cotyledonary and diffused placenta of the Mexican
deer (Cervus Americanus). " Journal of Anat. and Phys., Vol. xiii. 1879.

Human Embryo.

(226) Fried. Ahlfeld. " Beschreibung eines sehr kleinen menschlichen Eies."
Archivf. Gynaekologie, Bd. xiii. 1878.

(227) Herm. Beigel und Ludwig Loewe. "Beschreibung eines menschlichen
Eichens aus der zweiten bis dritten Woche der Schwangerschaft." Archiv f. Gynaeko-
logie, Bd. xn. 1877.

(228) K. Breus. " Ueber ein menschliches Ei aus der zweiten Woche der
Graviditat." Wiener medicinische Wochenschrift, 1877.

(229) M. Coste. Histoire generale et particuliere du developpement des corps or-
ganises, 1847-59.

(230) A. Ecker. Icones Physiologicae. Leipzig, 1851-1859.

(231) V. Hensen. " Beitrag z. Morphologic d. Korperform u. d. Gehirns d.
menschlichen Embryos." Archivf. Anat. u. Phys., 1877.

(232) W. His. Anatomic menschticher Embryonen, Part I. Embryonen d.
ersten Monats. Leipzig, 1880.

(233) J. Kollmann. "Die menschlichen Eier von 6 MM. Grosse." Archivf.
Anat. und Phys., 1879.

(234) W. Krause. " Ueber d. Allantois d. Menschen." Archiv f. Anat. und
Phys., 1875.

(235) W. Krause. " Ueber zwei fruhzeitige menschliche Embryonen." Zeit.
f. wiss. Zool., Vol. xxxv. 1880.

B. III. 1 8

274 MAMMALIAN BIBLIOGRAPHY.

(236) L. Loewe. " Im Sachen cler Eihaute jiingster menschlicher Eier. "
Archiv for Gynaekologie, Bd. xiv. 1879.

(237) C. B. Reichert. " Beschreibung einer fruhzeitigen menschlichen Frucht
im blaschenformigen Bildungszustande (sackfdrmiger Keim von Baer) nebst vergleich-
enden Untersuchungen liber die blaschenformigen Friichte der Saugethiere und des
Menschen. " Abhandlitngen der konigL Akad. d. Wiss. zu Berlin, 1873.

(238) Allen Thomson. "Contributions to the history of the structure of the
human ovum and embryo before the third week after conception ; with a description
of some early ova." Edinburgh Med. Surg. Journal, Vol. Lll. 1839.

CHAPTER XI.

COMPARISON OF THE FORMATION OF THE GERMINAL
LAYERS AND OF THE EARLY STAGES IN THE
DEVELOPMENT OF VERTEBRATES.

ALTHOUGH the preceding chapters of this volume contain a
fairly detailed account of the early developmental stages of
different groups of the Chordata, it will nevertheless be advan-
tageous to give at this place a short comparative review of the
whole subject.

In this review only the most important points will be dwelt
upon, and the reader is referred for the details of the processes
to the sections on the development of the individual groups.

The subject may conveniently be treated under three heads.

(1) The formation of the gastrula and behaviour of the
blastopore : together with the origin of the hypoblast.

(2) The mesoblast and notochord.

(3) The epiblast.

At the close of the chapter is a short summary of the organs
derived from the several layers, together with some remarks on
the growth in length of the vertebrate embryo, and some
suggestions as to the origin of the allantois and amnion.

Formation of the gastrula. Amphioxus is the type in
which the developmental phenomena are least interfered with by
the presence of food-yolk.

In this form the segmentation results in a uniform, or nearly
uniform, blastosphere, one wall of which soon becomes thickened
and invaginated, giving rise to the hypoblast ; while the larva
takes the form of a gastrula, with an archenteric cavity opening
by a blastopore. The blastopore rapidly narrows, while the

1 8— 2

276

THE GASTRULA OF AMPHIOXUS.

embryo assumes an elongated cylindrical form with the blasto-
pore at its hinder extremity (fig. 169 A). The blastopore now
passes to the dorsal surface, and by the flattening of this surface
a medullary plate is formed extending forwards from the blasto-

FIG. 169. EMBRYOS OF AMPHIOXUS. (After Kowalevsky.)
The parts in black with white lines are epiblastic; the shaded parts are hypo-
blastic.

A. Gastrula stage in optical section.

B. Slightly later stage after the neural plate np has become differentiated, seen as
a transparent object from the dorsal side.

C. Lateral view of a slightly older larva in optical section.

D. Dorsal view of an older larva with the neural canal completely closed except
for a small pore (no) in front.

E. Older larva seen as a transparent object from the side.

bl. blastopore (which becomes in D the neurenteric canal) ; ne. neurenteric canal ;
;//. neural or medullary plate; no. anterior opening of neural canal; ch. notochord;
so1, so", first and second mesoblastic somites.

pore (fig. 169 B). On the formation of the medullary groove
and its conversion into a canal, the blastopore opens into this
canal, and gives rise to a neurenteric passage, leading from the
neural canal into the alimentary tract (fig. 169 C and E). At a
later period this canal closes, and the neural and alimentary
canals become separated.

Such is the simple history of the layers in Amphioxus. In
the simplest types of Ascidians the series of phenomena is
almost the same, but the blastopore assumes a more definitely
dorsal position.

COMPARISON OF THE GERMINAL LAYERS.

2/7

Here also the blastopore lies at the hinder end of the
medullary groove, and on the closure of the groove becomes
converted into a neurenteric passage.

In the true Vertebrates the types which most approach
Amphioxus are the Amphibia, Acipenser and Petromyzon.
We may take the first of these as typical (though Petromyzon is
perhaps still more so) and fig. 170 A B C D represents four
diagrammatic longitudinal vertical sections through a form

A C

FIG. 170. DIAGRAMMATIC LONGITUDINAL SECTIONS THROUGH THE EMBRYO OF
BOMBINATOR AT TWO STAGES, TO SHEW THE FORMATION OF THE GERMINAL LAYERS.
(Modified from Gotte.)

ep. epiblast ; m. dorsal mesoblast ; m'. ventral mesoblast ; hy. hypoblast ;
yk. yolk ; x. point of junction of the epiblast and hypoblast at the dorsal side of the
blastopore ; al. mesenteron ; sg. segmentation cavity.

378 THE GASTRULA OF AMPHIBIA.

belonging to this group (Bombinator). The food-yolk is here
concentrated in what I shall call the lower pole of the egg, which
becomes the ventral aspect of the future embryo. The part of
the .egg containing the stored-up food-yolk is, as has already
been explained in the chapter on segmentation (Vol. II. pp. 94
and 95), to be regarded as equivalent to part of those eggs
which do not contain food-yolk ; a fact which requires to be
borne in mind in any attempt to deal comparatively with the
formation of the layers in the Vertebrata. It may be laid down
as a general law, which holds very accurately for the Vertebrata,
that in eggs in which the distribution of food-yolk is not
uniform, the size of the cells resulting from segmentation is
proportional to the quantity of food-material they contain.
In accordance with this law the cells of the Amphibian ovum
are of unequal size even at the close of segmentation. They
may roughly be divided into two categories, viz. the smaller
cells of the upper pole and the larger of the lower (fig. 170 A).
The segmentation cavity (sg) lies between the two, but is
unsymmetrically placed near the upper pole of the egg, owing to
the large bulk of the ventrally placed yolk-segments. In the
inequality of the cells at the close of segmentation the Amphibia
stand in contrast with Amphioxus. The upper cells are mainly
destined to form the epiblast, and the lower the hypoblast and
mesoblast.

The next change which takes place is an invagination, the
earliest traces of which are observable in fig. 170 A. The
invagination is not however so simple as in Amphioxus. Owing
in fact to the presence of the food-yolk it is a mixture of invagi-
nation by epibole and by embole.

At the point marked x in fig. 170 A, which corresponds with
the future hind end of the embryo, and is placed on the
equatorial line marking the junction of the large and small cells,
there takes place a normal invagination, which gives rise solely
to the hypoblast of the dorsal wall of the alimentary tract and to
part of the dorsal mesoblast. The invaginated layer grows
inwards from the point x along what becomes the dorsal side of
the embryo ; and between it and the yolk-cells below is formed
a slit-like space (fig. 170 B and C). This space is the mesen-
teron. It is even better shewn in fig. 171 representing the

COMPARISON OF THE GERMINAL LAYERS. 279

process of invagination in Petromyzon. The point x in fig. 170
where epiblast, mesoblast and hypoblast are continuous, is
homologous with the dorsal lip of the blastopore in Amphioxus.
In the course of the invagination the segmentation cavity, as in
Amphioxus, becomes obliterated.

While the above invagination has been taking place, the
epiblast cells have been simply growing in an epibolic fashion
round the yolk; and by the stage represented in fig. 170 C
and D the exposed surface of yolk has become greatly di-
minished ; and an obvious blastopore is thus established. Along
the line of the growth a layer of mesoblast cells (iri\ continuous
at the sides with the invaginated mesoblast layer, has become
differentiated from the small cells (fig. 170 A) intermediate
between the epiblast cells and the yolk.

Owing to the nature of the above process of invagination the
mesenteron is at first only provided with an epithelial wall on
its dorsal side, its ventral wall being formed of yolk-cells
(fig. 170). At a later period some of the yolk-cells become
transformed into the epithelial cells of the ventral wall, while the
remainder become enclosed in the alimentary cavity and
employed as pabulum. The whole of the yolk-cells, after the
separation of the mesoblast, are however morphologically part of
the hypoblast.

The final fate of the blastopore is nearly the same as in
Amphioxus. It gradually narrows, and the yolk-cells which at
first plug it up disappear (fig. 170 C and D). The neural groove,
which becomes formed on the dorsal surface of the embryo, is
continued forwards from the point x in fig. 170 C. On the
conversion of this groove into a canal the canal freely opens
behind into the blastopore ; and a condition is reached in which
the blastopore still opens to the exterior and also into the
neural canal fig. 170 D. In a later stage (fig. 172) the external
opening of the blastopore becomes closed by the medullary folds
meeting behind it, but the passage connecting the neural and
alimentary canals is left. There is one small difference between
the Frog and Amphioxus in the relation of the neural canal to
the blastopore. In both types the medullary folds embrace and
meet behind it, so that it comes to occupy a position at the hind
extremity of the medullary groove. In Amphioxus the closure

280

THE GASTRULA OF AMPHIBIA.

of the medullary folds commences behind, so that the external
opening of the blastopore
is obliterated simultane-
ously with the commencing 7rl£/

formation of the medullary
canal ; but in the Frog the
closure of the medullary
folds commences anteriorly
and proceeds backwards, so
that the obliteration of the
external opening of the
blastopore is a late event
in the formation of the
medullary canal.

The anus is formed (vide
fig. 172) some way in front
of the blastopore, and a
post-anal gut, continuous
with the neurenteric canal, is thus established. Both the post-
anal gut and the neurenteric canal eventually disappear.

The two other types classed above with the Amphibia, viz.
Petromyzon and Acipenser, agree sufficiently closely with them

FIG. 171. LONGITUDINAL VERTICAL SEC-
TION THROUGH AN EMBRYO OF PETROMYZON
OF 136 HOURS.

me. mesoblast ; yk. yolk-cells ; al. alimen-
tary tract ; bl. blastopore ; s.c. segmentation
cavity.

FIG. 172. LONGITUDINAL SECTION THROUGH AN ADVANCED EMBRYO OF
BOMBINATOR. (After Gotte.)

;//. mouth ; an. anus ; /. liver ; ne. neurenteric canal ; me. medullary canal ;
ch. notochord ; pn. pineal gland.

to require no special mention ; but with reference to both types
it may be pointed out that the ovum contains relatively more
food-yolk than that of the Amphibian type just described, and

COMPARISON OF THE GERMINAL LAYERS. 28 1

that this leads amongst other things to the lower layer cells
extending up the sides of the segmentation cavity, and assisting
in forming its roof.

The next type to be considered is that of Elasmobranchii.
The yolk in the ovum of these forms is enormously bulky, and
the segmentation is in consequence a partial one. At first sight
the differences between their development and that of Amphibia
would appear to be very great. In order fully to bridge over
the gulf which separates them I have given three diagrammatic
longitudinal sections of an ideal form intermediate between
Amphibia and Elasmobranchii, which differs however mainly
from the latter in the smaller amount of food-yolk; and by
their aid I trust it will be made clear that the differences between
the Amphibia and Elasmobranchii are of an insignificant
character. In fig. 174 A B C are represented three diagram-
matic longitudinal sections of Elasmobranch embryos, and in
fig. 173 A B C three longitudinal sections of the ideal inter-
mediate form. The diagrams correspond with the Amphibian
diagrams already described (fig. 170). In the first stage figured
there is present in all of these forms a segmentation cavity (sg)
situated not centrally but near the surface of the egg. The roof
of the cavity is thin, being composed in the Amphibian embryo
of epiblast alone, and in the Elasmobranch of epiblast and lower
layer cells. The floor of the cavity is formed of so-called yolk,
which forms the main mass of the embryo. In Amphibia the
yolk is segmented. In Elasmobranchii there is at first a layer
of primitive hypoblast cells separating the segmentation cavity
from the yolk proper; this however soon disappears, and an
unsegmented yolk with free nuclei fills the place of the seg-
mented yolk of the Amphibia. The small cells at the sides of
the segmentation cavity in Amphibia correspond exactly in
function and position with the lower layer cells of the Elasmo-
branch blastoderm.

The relation of the yolk to the blastoderm in the Elasmo-
branch embryo at this stage of development very well suits the
view of its homology with the yolk-cells of the Amphibian
embryo. The only essential difference between the two embryos
arises from the roof of the segmentation cavity being formed in
the Elasmobranch embryo of lower layer cells, which are absent

282

THE GASTRULA OF ELASMOBRANCHIL

in the Amphibian embryo. This difference no doubt depends
upon the greater quantity of yolk in the Elasmobranch ovum,
and a similar distribution of the lower layer cells is found in
Acipenser and in Petromyzon.

In the next stage for the Elasmobranch (fig. 173 and 174 B)
and for the Amphibian (fig. 170 C) or better still Petromyzon

FIG. 173. THREE DIAGRAMMATIC LONGITUDINAL SECTIONS THROUGH AN
IDEAL TYPE OF VERTEBRATE EMBRYO INTERMEDIATE IN THE MODE OF FOR-
MATION OF ITS LAYERS BETWEEN AMPHIBIA OR PETROMYZON AND ELASMO-

BRANCH1I.

s.if. segmentation cavity; ep. epiblast; m. mesoblast; hy. hypoblast; nc. neural
canal; al. mesenteron; «. nuclei of the yolk.

(fig. 171) the agreement between the three types is again very
close. For a small arc (x) of the edge of the blastoderm the
epiblast and hypoblast become continuous, while at all other

COMPARISON OF THE GERMINAL LAYERS. 283

parts the epiblast, accompanied by lower layer cells, grows round
the yolk or round the large cells which correspond to it. The
yolk-cells of the Amphibian embryo form a comparatively small
mass, and are therefore rapidly enveloped ; while in the case of
the Elasmobranch embryo, owing to the greater mass of the
yolk, the same process occupies a long period. The portion of
the blastoderm, where epiblast and hypoblast become continuous,
forms the dorsal lip of an opening — the blastopore — which leads
into the alimentary cavity. This cavity has the same relation in
all the three cases. It is lined dorsally by lower layer cells, and
ventrally by yolk-cells or what corresponds with yolk-cells ; a
large part of the ventral epithelium of the alimentary canal
being in both cases eventually derived from the yolk. In
Amphibia this epithelium is formed directly from the existing
cells, while in Elasmobranchii it is derived from cells formed
around the nuclei of the yolk.

As in the earlier stage, so in the present one, the anatomical
relations of the yolk to the blastoderm in the one case (Elasmo-
branchii) are nearly identical with those of the yolk-cells to the
blastoderm in the other (Amphibia).

The main features in which the two embryos differ, during
the stage under consideration, arise from the same cause as the
solitary point of difference during the preceding stage.

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

In Elasmobranchs, owing probably to the larger bulk of the
lower layer cells, the primitive hypoblast cells arrange themselves
in their final position during segmentation, and no room is left
for a true invagination ; but instead of this there is formed a
simple space between the blastoderm and the yolk. The homo-
logy of this space with the primitive invagination cavity is never-
theless proved by the survival of a number of features belonging
to the ancestral condition in which a true invagination was
present. Amongst the more important of these are the following :
— (i) The continuity of epiblast and hypoblast at the dorsal lip
of the blastopore. (2) The continuous conversion of primitive

284 THE GASTRULA OF ELASMOBRANCHII.

hypoblast cells into permanent hypoblast, which gradually ex-
tends inwards towards the segmentation cavity, and exactly re-
presents the course of the invagination whereby in Amphibia
the dorsal wall of the alimentary cavity is formed. (3) The ob-
literation of the segmentation cavity during the period when the
pseudo-invagination is occurring.

In the next stage there appear more important differences
between the two types than in the preceding stages, though here
again the points of resemblance predominate.

Figs. 170 D and 174 C represent longitudinal sections through
embryos after the closure of the medullary canal. The neuren-
teric canal is established ; and in front and behind the epithelium
of the ventral wall of the mesenteron has begun to be formed.

The mesoblast is represented as having grown in between
the medullary canal and the superjacent epiblast.

There are at this stage two points in which the embryo Elas-
mobranch differs from the corresponding Amphibian embryo,
(i) In the formation of the neurenteric canal, there is no free
passage leading into the mesenteron from the exterior as in
Amphibia (fig. 170 D). (2) The whole yolk is not enclosed by
the epiblast, and therefore part of the blastopore is still open.

The difference between Amphibia and Elasmobranchii in the
first of these points is due to the fact that in Elasmobranchii, as
in Amphioxus, the neural canal becomes first closed behind ; and
simultaneously with its closure the lateral parts of the lips of the
blastopore, which are continuous with the medullary folds, meet
together and shut in the hindmost part of the alimentary tract.

The second point is of some importance for understanding
the relations of the formation of the layers in the amniotic and
the non-amniotic Vertebrates. Owing to its large size the whole
of the yolk in Elasmobranchii is not enclosed by the epiblast at
the time when the neurenteric canal is established ; in other words
a small posterior and dorsal portion of the blastopore is shut
off in the formation of the neurenteric canal. The remaining
ventral portion becomes closed at a later period. Its closure
takes place in a linear fashion, commencing at the hind end of
the embryo, and proceeding apparently backwards ; though, as
this part eventually becomes folded in to form the ventral wall
of the embryo, the closure of it really travels forwards. The

COMPARISON OF THE GERMINAL LAYERS.

285

process causes however the embryo to cease to lie at the edge of
the blastoderm, and while situated at some distance from the
edge, to be connected with it by a linear streak, representing the
coalesced lips of the blastopore. The above process is diagram-
matically represented in fig. 175 B; while as it actually occurs

FIG. 174.

DIAGRAMMATIC LONGITUDINAL SECTIONS OF AN ELASMOBRANCH

EMBRYO.

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

ep. epiblast; m. mesoblast; al. alimentary cavity; sg. segmentation cavity; nc.
neural canal; ch. notochord; x. point where epiblast and hypoblast become continuous
at the posterior end of the embryo ; n. nuclei of yolk.

A. Section of young blastoderm, with the segmentation cavity enclosed in the
lower layer cells (primitive hypoblast).

B. Older blastoderm with embryo in which hypoblast and mesoblast are distinctly
formed, and in which the alimentary cavity has appeared. The segmentation cavity
is still represented, though by this stage it has in reality disappeared.

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

it is shewn in fig. 30, p. 63. The whole closure of the blastopore
in Elasmobranchii is altogether unlike what takes place in Am-
phibia, where the blastopore remains as a circular opening which

286 THE GASTRULA OF THE SAUROPSIDA.

gradually narrows till it becomes completely enveloped in the
medullary folds (fig. 175 A).

On the formation of the neurenteric canal the body of the
embryo Elasmobranch becomes gradually folded off from the
yolk, which, owing to its great size, forms a large sack appended
to the ventral side of the body. The part of the somatopleure,
which grows round it, is to be regarded as a modified portion of
the ventral wall of the body. The splanchnopleure also enve-
lops it, so that, morphologically speaking, the yolk lies within
the mesenteron.

The Teleostei, so far as the first formation of the layers is
concerned, resemble in all essential features the Elasmobranchii,
but the neurenteric canal is apparently not developed (?), owing
to the obliteration of the neural canal ; and the roof of the seg-
mentation cavity is formed of epiblast only.

In the preceding pages I have attempted to shew that the
Amphibia, Acipenser, Petromyzon, the Elasmobranchii and the
Teleostei agree very closely in the mode of formation of the
gastrula. The unsymmetrical gastrula or pseudo-gastrula which
is common to them all is, I believe, to be explained by the form
of the vertebrate body. In Amphioxus, where the small amount
of food-yolk present is distributed uniformly, there is no reason
why the invagination and resulting gastrula should not be sym-
metrical. In true Vertebrates, where more food-yolk is present,
the shape and structure of the body render it necessary for the
food-yolk to be stored away on the ventral side of the alimen-
tary canal. It is this fact which causes the asymmetry of the
gastrula, since it is not possible for the part of the ovum, which
will become the ventral wall of the alimentary tract, and which
is loaded with food-yolk, to be invaginated in the same fashion
as the dorsal wall.

Sauropsida. The comparison of the different types of the
Ichthyopsida is fairly simple, but the comparison of the Sauro-
psida with the Ichthyopsida is a far more difficult matter. In all
the Sauropsida there is a large food-yolk, and the segmentation
agrees closely with that in the Elasmobranchii. It might have
been anticipated that the resemblance would continue in the
subsequent development. This however is far from being the

COMPARISON OF THE GERMINAL LAYERS. 287

case. The medullary plate, instead of lying at the edge of the
blastoderm, lies in the centre, and its formation is preceded by
that of a peculiar structure, the primitive streak, which, on the

FIG. 175. DIAGRAMS ILLUSTRATING THE POSITION OF THE BLASTOPORE, AND
THE RELATION OF THE EMBRYO TO THE YOLK IN VARIOUS MEROBLASTIC VERTE-
BRATE OVA.

A. Type of Frog. B. Elasmobranch type. C. Amniotic Vertebrate.
mg. medullary plate ; ne. neurenteric canal ; bl. portion of blastopore adjoining the
neurenteric canal. In B this part of the blastopore is formed by the edges of the
blastoderm meeting and forming a linear streak behind the embryo ; and in C it forms
the structure known as the primitive streak, yk. part of the yolk not yet enclosed by
the blastoderm.

formation of the medullary plate, is found to lie at the hinder
end of the latter and to connect it with the edge of the blasto-
derm.

The possibility of a comparison between the Sauropsida and
the Elasmobranchii depends upon the explanation being possible
of (i) the position of the embryo near the centre of the blasto-
derm, and (2) the nature of the primitive streak.

The answers to these two questions are, according to my view,
intimately bound together.

288 THE GASTRULA OF THE SAUROPSIDA.

I consider that the embryos of the Sauropsida have come to
occupy a central position in the blastoderm owing to the abbre-
viation of a process similar to that by which, in Elasmobranchii,
the embryo is removed from the edge of the blastoderm ; and
that the primitive streak represents the linear streak connecting
the Elasmobranch embryo with the edge of the blastoderm after
it has become removed from its previous peripheral position, as
well as the true neurenteric part of the Elasmobranch blastopore.

This view of the nature of the primitive streak, which is
diagrammatically illustrated in fig. 175, will be rendered more
clear by a brief review of the early developmental processes in
the Sauropsida.

After segmentation the blastoderm becomes divided, as in
Elasmobranchii, into two layers. It is doubtful whether there is
any true representative of the segmentation cavity. The first
structure to appear in the blastoderm is a linear streak placed at
the hind end of the blastoderm, known as the primitive streak
(figs. 175 C, /5/and 176, pr). At the front end of the primitive
streak the epiblast and hypoblast become continuous, just as
they do at the dorsal lip of the blastopore in Elasmobranchii.
Continued back from this point is a streak of fused mesoblast and
epiblast to the under side of which a linear thin layer of hypoblast
is more or less definitely attached.

A further structure, best developed in the Lacertilia, appears
in the form of a circular passage perforating the blastoderm at
the front end of the primitive streak (fig. 176, ne). This passage
is bounded anteriorly by the layer of cells forming the continu-
ation of the hypoblast into the epiblast.

In the next stage the medullary plate becomes formed in
front of the primitive streak (fig. 175 C), and the medullary folds
are continued backwards so as to enclose the upper opening of
the passage through the blastoderm. On the closure of the me-
dullary canal (fig. 177) this passage leads from the medullary
canal into the alimentary tract, and is therefore the neurenteric
canal ; and a post-anal gut also becomes formed. The latter
part of the above description applies especially to the Lizard:
but in Chelonia and most Birds distinct remnants (vide pp. 162
— 164) of the neurenteric canal are developed.

On the hypothesis that the Sauropsidan embryos have come

COMPARISON OF THE GERMINAL LAYERS. 289

to occupy their central position, owing to an abbreviation of a
process analogous to the linear closing of the blastopore behind
the embryos of Elasmobranchii, all the appearances above describ-
ed receive a satisfactory explanation. The passage at the front
end of the primitive streak is the dorsal part of the blastopore,
which in Elasmobranchii becomes converted into the neurenteric
canal. The remainder of the primitive streak represents, in a
rudimentary form, the linear streak in Elasmobranchii, formed by
the coalesced edges of the blastoderm, which connects the hinder
end of the embryo with the still open yolk blastopore. That it
is in later stages not continued to the edge of the blastoderm, as
in Elasmobranchii, is due to its being a rudimentary organ. The
more or less complete fusion of the layers in the primitive streak
is simply to be explained by this structure representing the co-
alesced edges of the blastopore ; and the growth outwards from
it of the mesoblast is probably a remnant of a primitive dorsal in-
vagination of the mesoblast and hypoblast like that in the Frog.

FIG. 176. DIAGRAMMATIC LONGITUDINAL SECTION OF AN EMBRYO OF LACERTA.
//. body cavity; am. amnion; ne. neurenteric canal; ch. notochord; hy. hypo-
blast; ep. epiblast; pr. primitive streak. In the primitive streak all the layers are
partially fused.

The final enclosure of the yolk in the Sauropsida takes place
at the pole of the yolk-sack opposite the embryo, so that the
blastopore is formed of three parts, (i) the neurenteric canal, (2)
the primitive streak behind this, (3) the blastopore at the pole of
the yolk-sack opposite the embryo.

Mammalia. The features of the development of the placen-
tal Mammalia receive their most satisfactory explanation on the
hypothesis that their ancestors were provided with a large-yolked
ovum like that of the Sauropsida. The food-yolk must be sup-
posed to have ceased to be developed on the establishment of a
maternal nutrition through the uterus.

On this hypothesis all the developmental phenomena subse-
B. in 19

290

MAMMALIAN GASTRULA.

quently to the formation of the blastodermic vesicle receive a
satisfactory explanation.

The whole of the blastodermic vesicle, except the embryonic
area, represents the yolk-sack, and the growth of the hypoblast
and then of the mesoblast round its inner wall represents the

Air-

FIG. 177. DIAGRAMMATIC LONGITUDINAL SECTION THROUGH THE POSTERIOR
END OF AN EMBRYO BlRD AT THE TIME OF THE FORMATION OF THE ALLANTOIS.

ep. epiblast ; Sp.c. spinal canal ; ch. notochord ; n.e. neurenteric canal ; hy. hypo-
blast ; 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. peri-
visceral cavity am. amnion; so. somatopleure ; sp. splanchnopleure.

corresponding growths in the Sauropsida. As in the Sauropsida
it becomes constricted off from the embryo, and the splanchno-
pleuric stalk of the sack opens into the ileum in the usual way.

R

FIG. 178. OPTICAL SECTIONS OF A RABBIT'S OVUM AT TWO STAGES CLOSELY

FOLLOWING UPON THE SEGMENTATION. (After E. van Beneden.)
ep. epiblast; hy. primary hypoblast; bp. Van Beneden's so-called blastopore.
The shading of the epiblast and hypoblast is diagrammatic.

COMPARISON OF THE GERMINAL LAYERS.

291

In the formation of the embryo out of the embryonic area the
phenomena which distinguish the Sauropsida from the Ichthyo-
psida are repeated. The embryo lies in the centre of the area ;
and before it is formed there appears a primitive streak, from
which there grows out the greater part of the mesoblast. At the
front end of the primitive streak the hypoblast and epiblast be-
come continuous, though a perforated neurenteric blastopore has
not yet been detected.

All these Sauropsidan features are so obvious that they need
not be insisted on further. The embryonic evidence of the com-
mon origin of Mammalia and Sauropsida, both as concerns the
formation of the layers and of the embryonic membranes, is as
clear as it can be. The only difficulty about the early develop-
ment of Mammalia is presented by the epibolic gastrula and the

FIG. 179. RABBIT'S OVUM BETWEEN 70 — 90 HOURS AFTER IMPREGNATION.

(After E. van Beneden.)

bv. cavity of blastodermic vesicle (yolk-sack) ; ep. epiblast ; hy. primitive hypo-
blast ; Zp. mucous envelope.

formation of the blastodermic vesicle (figs. 178 and 179). That
the segmentation is a complete one is no doubt a direct conse-
quence of the reduction of the food-yolk, but the growth of the
epiblast cells round the hypoblast and the final enclosure of the
latter, which I have spoken of as giving rise to the epibolic
gastrula, are not so easily explained.

19 — 2

292 MESOBLAST AND NOTOCHORD.

It might have been supposed that this process was equivalent
to the growth of the blastoderm round the yolk in the Sauro-
psida, but then the blastopore ought to be situated at the pole of
the egg opposite to the embryonic area, while, according to Van
Beneden, the embryonic area corresponds approximately to the
blastopore.

Van Beneden regards the Mammalian blastopore as equiva-
lent to that in the Amphibia, but if the position previously adopt-
ed about the primitive streak is to be maintained, Van Bene-
den's view must be abandoned. No satisfactory phylogenetic
explanation of the Mammalian gastrula by epibole has in my
opinion as yet been offered.

The formation of the blastodermic vesicle may perhaps be
explained on the view that in the Proto-mammalia the yolk-sack
was large, and that its blood-vessels took the place of the pla-
centa of higher forms. On this view a reduction in the bulk of
the ovarian ovum might easily have taken place at the same time
that the presence of a large yolk-sack was still necessary for the
purpose of affording surface of contact with the uterus.

The formation of the Mesoblast and of the Notochord.

Amphioxus. The mcsoblast originates in Amphioxus, as in
several primitive invertebrate types, from a pair of lateral

FIG. 180. SECTIONS OF AN AMPHIOXUS EMBRYO AT THREE STAGES.
(After Kowalevsky.)

A. Section at gastrula stage.

B. Section of an embryo slightly younger than that represented in fig. 169 D.

C. Section through the anterior part of an embryo at the stage represented in
fig. 169 !•:.

«/. neural plate ; nc. neural canal ; mes. archenteron in A and B, and mesenteron
in C; ch. notochord ; so. mesoblastic somite.

COMPARISON OF THE GERMINAL LAYERS. 293

diverticula, constricted off from the archenteron (fig. 180). Their
formation commences at the front end of the body and is thence
carried backwards, and each diverticulum contains a prolongation
of the cavity of the archenteron. After their separation from the
archenteron the dorsal parts of these diverticula become divided by
transverse septa into successive somites, the cavities of which
eventually disappear ; while the walls become mainly converted
into the muscle-plates, but also into the tissue around the
notochord which corresponds with the vertebral tissue of the
higher Chordata.

The ventral part of each diverticulum, which is prolonged
so as to meet its fellow in the middle ventral line, does not
become divided into somites, but contains a continuous cavity,
which becomes the body cavity of the adult. The inner layer of
this part forms the splanchnic mesoblast, and the outer layer the
somatic mesoblast.

The notochord would almost appear to arise as a third
median and dorsal diverticulum of the archenteron (fig. 1 80 ch).
At any rate it arises as a central fold
of the wall of this cavity, which is
gradually constricted off from before
backwards.

Urochorda. In simple Ascidians
the above processes undergo a slight
modification, which is mainly due (i)
to a general simplification of the FIG igj TRANSVERSE OPTI.
organization, and (2) to the non- CAL SECTION OF THE TAIL OF AN
continuation of the notochord into ^SSSSSSSST'

the trunk. The section is from an embryo

The whole dorsal wall of the of the same age as fig. 8 iv.
posterior part of the archenteron is „*
converted into the notochord (fig. blast of tail.
181 ck), and the lateral walls into the mesoblast (me) ; so that
the original lumen of the posterior part of the archenteron ceases
to be bounded by hypoblast cells, and disappears as such.
Part of the ventral wall remains as a solid cord of cells (al1)
The anterior part of the archenteron in front of the notochord
passes wholly into the permanent alimentary tract.

The derivation of the mesoblast from the lateral walls of the

294

MESOBLAST AND NOTOCHORD.

n.al

posterior part of the archenteron is clearly comparable with the
analogous process in Amphioxus.

Vertebrata. In turning from Amphioxus to the true
Vertebrata we find no form in which diverticula of the primi-
tive alimentary tract give rise to the mesoblast. There is
reason to think that the type
presented by the Elasmo-
branchii in the formation of
the mesoblast is as primitive
as that of any other group.
In this group the mesoblast
is formed, nearly coincidently
with the hypoblast of the
dorsal wall of the mesenteron,
as two lateral sheets, one on
each side of the middle line
(fig. 182 m). These two
sheets are at first solid
masses ; and their differen-
tiation commences in front
and is continued backwards.
After their formation the
notochord arises from the
axial portion of the hypo-

FlG. 182. TWO TRANSVERSE SECTIONS
OF AN EMBRYO PRISTIURUS OF THE SAME
AGE AS FIG. 17.

A. Anterior section.

B. Posterior section.

mg. medullary groove ; ep. epiblast ; hy.
hypoblast ; n.al cells formed round the nuclei
of the yolk which have entered the hypo-
blast ; m. mesoblast.

The sections shew the origin of the
mesoblast.

blast (which had no share in
giving rise to the two mesoblast plates) as a solid thickening
(fig. 183 £//), which is separated from it as a circular rod. Its
differentiation, like that of the mesoblastic plates, commences in
front. The mesoblast plates subsequently become divided for
their whole length into two layers, between which a cavity is
developed (fig. 184). The dorsal parts of the plates become
divided by transverse partitions into somites, and these somites
with their contained cavities are next separated from the more
ventral parts of the plates (fig. 185 mp). In the somites the
cavities become eventually obliterated, and from their inner
sides plates of tissue for the vertebral bodies (fig. 186 Vr) are
separated ; while the outer parts, consisting of two sheets,
containing the remains of the original cavity, form the muscle-
plates (mp).

COMPARISON OF THE GERMINAL LAYERS.

295

The undivided ventral portion gives rise to the general
A

FIG. 183. THREE SECTIONS OF A PRISTIURUS EMBRYO SLIGHTLY OLDER THAN

FIG. 18 B.

The sections shew the development of the notochord.

Ch. notochord; CK. developing notochord; mg. medullary groove; lp. lateral
plate of mesoblast ; ep. epiblast ; hy. hypoblast.

somatic and splanchnic
mesoblast (fig. 185),
and the cavity between
its two layers consti-
tutes the body cavity.
The originally separate
halves of the body
cavity eventually meet
and unite in the ventral
median line throughout
the greater part of the
body, though in the tail
they remain distinct
and are finally oblite-
rated. Dorsally they
are separated by the
mesentery. From the
mesoblast at the junc-
tion of the dorsal and

FIG. 184. TRANSVERSE SECTION THROUGH THE
TAIL-REGION OF A PRISTIURUS EMBRYO OF THE
SAME AGE AS FIG. 28 E.

df. dorsal fin; sp.c. spinal cord; pp. body cavity;
sp. splanchnic layer of mesoblast; so. somatic layer
of mesoblast; mp'. commencing differentiation of
muscles ; ch. notochord ; x. subnotochordal rod
arising as an outgrowth of the dorsal wall of the
alimentary tract ; a/, alimentary tract.

296

MESOBLAST AND NOTOCHORD.

ventral parts of the primitive plates is formed the urinogenital

system.

That the above mode of origin of the mesoblast and noto-

chord is to be regarded as a modifi-
cation of that observable in Am-

phioxus seems probable from the

following considerations : —

In the first place, the mesoblast is

split off from the hypoblast not as a

single mass but as a pair of distinct

masses, comparable with the paired di-

vcrticula in Amphioxus. Secondly,

the body cavity, when it appears in

the mesoblast p\a.tes,does not arise as a

single cavity, but as a pair of cavities,

one for each plate of mesoblast ; and

these cavities remain permanentlydis-

tinct in some parts of the body, and

nowhere unite till a comparatively

late period. Thirdly, the primitive
body cavity of the embryo is not
confined to the region in which a
body cavity exists in the adult, but
extends to the summit of tJie muscle-
plates, at first separating parts which
become completely fused in the
adult to form the great lateral muscles
of the body.

It is difficult to understand how
the body cavity could thus extend
into the muscle-plates on the supposition that it represents a
primitive split in the mesoblast between the wall of the gut and
the body-wall ; but its extension to this part is quite intelligible,
on the hypothesis that it represents the cavities of two diver-
ticula of the alimentary tract, from the muscular walls of which
the voluntary muscular system has been derived ; and it may be
pointed out that the derivation of part of the muscular system
from what is apparently splanchnic mesoblast is easily explained
on the above hypothesis, but not, so far as I see, on any other.

FIG. 185. SECTION THROUGH
THE TRUNK OF A SCYLLIUM
EMBRYO SLIGHTLY YOUNG KK

THAN 28 F.

sp.c. spinal canal; W. white
matter of spinal cord ; pr. poste-
rior nerve-roots ; ch. notochord ;
x. subnotochordal rod ; ao. aorta ;
vip. muscle-plate ; mp'. inner layer
of muscle-plate already converted
into muscles ; Vr. rudiment of
vertebral body ; si. segmental
tube ; sd. segmental duct ; sp.v.
spiral valve ; v. subintestinal vein ;
p.o. primitive generative cells.

COMPARISON OF THE GERMINAL LAYERS.

297

Such are the main features, presented by the mesoblast
in Elasmobranchii, which favour the view of its having originally
formed the walls of the alimentary diverticula. Against this
view of its nature are the facts (i) of the mesoblast plates being
at first solid, and (2) of the
body cavity as a consequence
of this never communicating
with the alimentary canal.
These points, in view of our
knowledge of embryological
modifications, cannot be re-
garded as great difficulties
in my hypothesis. We have
many examples of organs,
which, though in most cases
arising as involutions, yet
appear in other cases as
solid ingrowths. Such ex-
amples are afforded by the
optic vesicle, auditory vesicle,
and probably also by the
central nervous system of
Osseous Fishes. In most Vertebrates these organs are formed
as hollow involutions from the exterior ; in Osseous Fishes,
however, as solid involutions, in which a cavity is secondarily
established.

There are strong grounds for thinking that in all Vertebrates
the mesoblast plates on each side of the notochord originate
independently, much as in Elasmobranchii, and that the noto-
chord is derived from the axial hypoblast ; but there are some
difficulties in the application of this general statement to all
cases. In Amphibia, Ganoids, and Petromyzon, where the
dorsal hypoblast is formed by a process of invagtnation as
in Amphioxus, the dorsal mesoblast also owes its origin to this
invagination, in that the indifferent invaginated layer becomes
divided into hypoblast and mesoblast. Amongst these forms
the mesoblast sheet, when separated from the hypoblast, is
certainly not continuous across the middle line in Petromyzon
(Calberla) and the Newt (Scott and Osborn), and doubtfully so

FIG. 1 86. HORIZONTAL SECTION
THROUGH THE TRUNK OF AN EMBRYO OF
SCYLLIUM CONSIDERABLY YOUNGER THAN
28 F.

The section is taken at the level of the
notochord, and shews the separation of the
cells to form the vertebral bodies from the
muscle-plates.

ch. notochord ; ep. epiblast ; Vr. rudiment
of vertebral body ; mp. muscle-plate ; mp'.
portion of muscle-plate already differentiated
into longitudinal muscles.

298 MESOBLAST AND NOTOCHORD.

in the other forms. It arises, in fact, as in Elasmobranchii, as
two independent plates. The fact of these plates originating
from an invaginated layer can only be regarded in the light of
an approximation to the primitive type found in Amphioxus.

In Petromyzon and the Newt the whole axial plate of dorsal
hypoblast becomes separated off from the rest of the hypoblast
as the notochord, and this mode of origin for the notochord
resembles more closely that in Amphioxus than the mode of
origin in Elasmobranchii.

In Teleostei, there is reason to think that the processes in the
formation of the mesoblast accord closely with what has been
described as typical for the Ichthyopsida, but there are still
some points involved in obscurity.

Leaving the Ichthyopsida, we may pass to the consideration
of the Sauropsida and Mammalia. In both of these types there
is evidence to shew that a part of the mesoblast is formed in situ
at the same time as the hypoblast, from the lower strata of
segmentation spheres. This mesoblast is absent in the front
part of the area pellucida, and on the formation of the primitive
streak (blastopore), an outgrowth of mesoblast arises from it as

FIG. 187. TRANSVERSE SECTION THROUGH AN EMBRYO RABBIT OF EIGHT DAYS.
ep. epiblast ; me. mesoblast ; ky. hypoblast ; mg. medullary groove.

in Amphibia, etc. From this region the mesoblast spreads as a
continuous sheet to the sides and posterior part of the blasto-
derm. In the region of the embryo, its exact behaviour has not
in some cases been quite satisfactorily made out. There are
reasons for thinking that it appears as two sheets not tinited in
the axial line in both Lacertilia (fig. 126) and Mammalia (fig.
187), and this to some extent holds true for Aves (vide p. 156).
In Lacertilia (fig. 188) and Mammalia, the axial hypoblast
becomes wholly converted into the notochord, which at the
posterior end of the body is continued into the epiblast at the
dorsal lip of the blastopore ; while in Birds the notochord is
formed by a very similar (fig. 189 cfi) process.

COMPARISON OF THE GERMINAL LAYERS.

299

The above processes in the formation of the mesoblast are
for the most part easily explained by a comparison with the
lower types. The outgrowth of the mesoblast from the sides of
the primitive streak is a rudiment of the dorsal invagination of
hypoblast and mesoblast found in Amphibia ; and the apparent

FIG. 188. DIAGRAMMATIC LONGITUDINAL SECTION THROUGH AN EMBRYO
LIZARD TO SHEW THE RELATIONS OF THE NEURENTERIC CANAL (ne) AND OF

THE PRIMITIVE STREAK (pr).

am. amnion; ep. epiblast; hy. hypoblast; ch. notochord ; //. body cavity; ne.
neurenteric canal ; pr. primitive streak.

outgrowth of the mesoblast from the epiblast in the primitive
streak is no more to be taken as a proof of the epiblastic origin
of the mesoblast, than the continuity of the epiblast with the
invaginated hypoblast and mesoblast at the lips of the blasto-
pore in the Frog of the derivation of these layers from the
epiblast in this type.

The division of the mesoblast into two plates along the dorsal
line of the embryo, and the formation of the notochord from the

ky.

FIG. 189. 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; ky. hypoblast; ch. notochord; me. mesoblast; n. nuclei in the
yolk of the germinal wall yk.

axial hypoblast, are intelligible without further explanation.
The appearance of part of the mesoblast before the formation of
the primitive streak is a process of the same nature as the

300 THE EPIBLAST.

differentiation of hypoblast and mesoblast in Elasmobranchii
without an invagination.

In the Sauropsida, some of the mesoblast of the vascular area
would appear to be formed in situ out of the germinal wall, by
a process of cell-formation similar to that which takes place in
the yolk adjoining the blastoderm in Elasmobranchii and Tele-
ostei. The mesoblast so formed is to be compared with that
which arises on the ventral side of the embryo in the Frog, by a
direct differentiation of the yolk-cells.

What was stated for the Elasmobranchii with reference to
the general fate of the mesoblast holds approximately for all the
other forms.

The Epiblast.

The epiblast in a large number of Chordata arises as a single
row of more or less columnar cells. Since the epidermis, into
which it becomes converted, is formed of two more or less
distinct strata in all Chordata except Amphioxus and Asci-
dians, the primitive row of epiblast cells, when single, neces-
sarily becomes divided in the course of development into two
layers.

In some of the Vertebrata, viz. the Anurous Amphibia, Tele-
ostei, Acipenser, and Lepidosteus, the epiblast is from the first
formed of two distinct strata. The upper of these, formed of a
single row of cells, is known as the epidermic stratum, and the
lower, formed of several rows, as the nervous stratum. In these
cases the two original strata of the epiblast are equivalent to
those which appear at a later period in the other forms. Thus
Vertebrates may be divided into groups according to the primi-
tive condition of their epiblast, viz. a larger group with but a
single stratum of cells at first ; and a smaller group with two
strata.

While there is no great difficulty in determining the equiva-
lent parts of the epidermis in these two groups, it still remains
an open question in which of them the epiblast retains its primi-
tive condition.

Though it is not easy to bring conclusive proofs on the one
side or the other, the balance of argument appears to me to be

COMPARISON OF THE GERMINAL LAYERS. 301

decidedly in favour of regarding the condition of the epiblast in
the larger group as primitive, and its condition in the smaller
group as secondary, and due to the throwing back of the
differentiation of the epiblast to a very early period of de-
velopment.

In favour of this view may be urged (i) the fact that the
simple condition is retained in Amphioxus through life. (2)
The correlation in Amphibia, and the other forms belonging to
this group, between a closed auditory pit and the early division
of the epiblast into two strata; there being no doubt that the
auditory pit was at. first permanently open, a condition of the
epiblast which necessitates its never having an external opening
must clearly be secondary. (3) It appears more likely that a
particular genetic feature should be thrown back in develop-
ment, than that such an important feature, as a distinction
between two primary layers, should be absolutely lost during
an early period of development, and then re-appear in later
stages.

The fact of the epiblast of the neural canal being divided,
like the remainder of the layer, into nervous and epidermic
parts, cannot, I think, be used as an argument in favour of the
opposite view to that here maintained. It seems probable that
the central canal of the nervous system arose phylogenetically
as an involution from the exterior, and that the epidermis
lining it is merely part of the original epidermis, which has
retained its primitive structure as a simple stratum, but is
naturally distinguishable from the nervous structures adjacent
to it.

Where the epiblast is divided at an early period into two
strata, the nervous stratum is always the active one, and takes
the main share in forming all the organs derived from the
layer.

Formation of the central nervous system. In all
Chordata an axial strip of the dorsal epiblast, extending from
the lip of the blastopore to the anterior extremity of the head,
and known as the medullary plate, becomes isolated from the
remainder of the layer to give rise to the central nervous axis.

According to the manner in which this takes place, three
types may, however, be distinguished. In Amphioxus the axial

3O2

THE CENTRAL NERVOUS SYSTEM.

strip becomes first detached from the adjoining epiblast, which
then meets and forms a continuous layer above it (fig. 190 A
and B ;//). The sides of the medullary plate, which is thus shut
off from the surface, bend over and meet so as to convert the

FIG. 190. SECTIONS OF AN AMPHIOXUS EMBRYO AT THREE STAGES.
(After Kowalevsky. )

A. Section at gastrula stage.

B. Section of an embryo slightly younger than that represented in fig. 169 D.

C. Section through the anterior part of an embryo at the stage represented in
fig. 169 E.

;//. neural plate; nc. neural canal; mes. archenteron in A and B, and mesenteron
in C ; ch. notochord ; so. mesoblastic somite.

plate into a canal (fig. 190 C nc). In the second and ordinary
type the sides of the medullary plate fold over and meet so as to
form a canal before the plate becomes isolated from the external
epiblast.

The third type is characteristic of Lepidosteus, Teleostei, and
Petromyzon. Here the axial plate becomes narrowed in such a
way that it forms a solid keel-like projection towards the ventral
surface (fig. 191 Me). This keel subsequently becomes separated
from the remainder of the epidermis, and a central canal is after-
wards developed in it. Calberla and Scott hold that the epi-
dermic layer of the skin is involuted into this keel in Petromy-
zon, and Calberla maintains the same view for Teleostei (fig.
32), but further observations on this subject are required. In
the Teleostei a very shallow depression along the axis of the
keel is the only indication of the medullary groove of other
forms.

In Amphioxus (fig. 190), the Tunicata, Petromyzon (?), Elas-
mobranchii (fig. 182), the Urodela and Mammalia (fig. 187), the
epiblast of the medullary plate is only formed of a single row of
cells at the time when the formation of the central nervous
system commences; but, except in Amphioxus and the Tuni-

COMPARISON OF THE GERMINAL LAYERS. 303

cata, it becomes several cells deep before the completion of the
process. In other types the epiblast is several cells deep even
before the differentiation of a medullary plate. In the Anura,
the nervous layer of the epidermis alone is thickened in the

FIG. 191. SECTION THROUGH AN EMBRYO OF LEPIDOSTEUS ON THE FIFTH DAY

AFTER IMPREGNATION.
MC. medullary cord; Ep. epiblast; Me. mesoblast ; hy. hypoblast; Ch. notochord.

formation of the central nervous system (fig. 72) ; and after the
closure of the medullary canal, the epidermic layer fuses for a
period with the nervous layer, though on the subsequent forma-
tion of the central epithelium of the nervous canal, there can be
little doubt that it becomes again distinct.

It seems almost certain that the formation of the central
nervous system from a solid keel-like thickening of the epider-
mis is a derived and secondary mode ; and that the folding of
the medullary plate into a canal is primitive. Apart from its
greater frequency the latter mode of formation of the central
nervous system is shewn to be the primitive type by the fact
that it offers a simple explanation of the presence of the central
canal of the nervous system ; while the existence of such a canal
cannot easily be explained on the assumption that the central
nervous system was originally developed as a keel-like thicken-
ing of the epiblast.

It is remarkable that the primitive medullary plate rarely ex-
hibits any indication of being formed of two symmetrical halves.
Such indications are, however, found in the Amphibia (fig. 192
and fig. 72) ; and, since in the adult state the nervous cord
exhibits nearly as distinct traces of being formed of two united
strands as does the ventral nerve-cord of many Chaetopods, it is

304 ORGANS DERIVED FROM THE GERMINAL LAYERS.

quite possible that the structure of the medullary plate in
Amphibia may be more primitive than that in other types1.

Formation of the organs of special sense. The more
important parts of the organs of smell, sight, and hearing are
derived from the epi-
blast ; and it has been
asserted that the olfact-
ory pit, optic vesicles and
auditory pit take their
origin from a special
sense plate, continuous at
first with this medullary
plate. In my opinion
this view cannot be main-
tained.

In the case of the
group of forms in which
the epiblast is early divi-

al

FIG. 192. TRANSVERSE SECTION THROUGH
THE CEPHALIC REGION OF A YOUNG NEWT EM-
BRYO. (After Scott and Osborn.)

In.hy. invaginated hypoblast, the dorsal part
of which will form the notochord ; ep. epiblast
of neural plate ; sp. splanchnopleure ; al. ali-
mentary tract ; yk. and Y. hy. yolk-cells.

ded into nervous and epi-
dermic layers, the former layer alone becomes involuted in the
formation of the auditory pit and the lens, the external openings
of which are never developed, while it is also mainly concerned
in the formation of the olfactory pit.

Summary of the more important Organs derived from the three
germinal layers.

The epiblast primarily gives origin to 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 epi-
blast being directly transformed into the characteristic multi-
polar nerve cells. The whole of the sympathetic nervous system

1 A parallel to the unpaired medullary plate of most Chordata is supplied by the
embryologically unpaired ventral cord of most Gephyrea and some Crustacea. In
these forms there can be little doubt that the ventral cord has arisen from the fusion of
two originally independent strands, so that it is not an extremely improbable hypo-
thesis to suppose that the same may have been the case in the Chordata.

COMPARISON OF THE GERMINAL LAYERS. 305

and the peripheral nervous elements of the body, including both
the spinal and the 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, is the undifferentiated remnant of the
primitive epiblast.

The epiblast also forms the epidermis ; 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 and the dermis. From the epiblast are
formed all such tegumentary organs or parts of organs as are
epidermic in nature.

In addition to the above, 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 organs where
the sensory expansion is derived from the involuted epiblast of
the medullary canal. To this class belongs the retina, including
the pigment epithelium 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 an involution of the epiblast covering the
external surface of the embryo. These accordingly have no
primary connection with the brain. ' Taste bulbs ' and other
terminal nervous organs, such as those of the lateral line in
fishes, are also structures formed from the external epiblast.

In addition to these we have the crystalline lens formed of
involuted epiblast as well as the cavity of the mouth and anus,
and the glands derived from them. The pituitary body is also
epiblastic in origin.

From the hypoblast are derived the epithelium of the diges-
tive canal, the epithelium of the trachea, bronchial tubes and air
cells, the cylindrical epithelium of the ducts of the liver,
pancreas, thyroid body, and other glands of the alimentary
canal, as well as the hepatic cells constituting the parenchyma
of the liver, developed from the hypoblast cylinders given off
around the primary hepatic diverticula.

B. III. 20

306 GROWTH IN LENGTH OF THE EMBRYO.

Homologous probably with the hepatic cells, and equally of
hypoblastic origin, are the spheroidal 'secreting cells' of the
pancreas and other glands. The epithelium of the salivary
glands, though these so closely resemble the pancreas, is proba-
bly of epiblastic origin, inasmuch as the cavity of the mouth is
entirely lined by epiblast.

The hypoblast also lines the allantois. To these parts must
be added the notochord and subnotochordal rod. From the
mesoblast are formed all the remaining parts of the body.
The muscles, the bones, the connective tissue and the vessels,
both arteries, veins, capillaries and lymphatics with their appro-
priate epithelium, are entirely formed from the mesoblast.

The generative and urinary organs are entirely derived from
the mesoblast. It is worthy of notice that the epithelium of the
urinary glands, though resembling the hypoblastic epithelium of
the alimentary canal, is undoubtedly mesoblastic.

From the mesoblast are lastly derived all the muscular, con-
nective tissue, 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 epidermic moiety of the latter
which is derived from the epiblast, so it is only the epithelium
of the former which comes from the hypoblast.

Growth in length of the Vertebrate Embryo.

With reference to the formation and growth in length of the body of the
Vertebrate embryo two different views have been put forward, which can be
best explained by taking the Elasmobranch embryo as our type. One of
these views, generally held by embryologists and adopted in the previous
pages, is that the Elasmobranch embryo arises from a differentiation of the
edge of the blastoderm ; which extends inwards from the edge for some little
distance. This differentiation is supposed to contain within itself the rudi-
ments of the whole of the embryo with the exception of the yolk-sack ; and
the hinder extremity of it, at the edge of the blastoderm, is regarded as
corresponding with the hind end of the body of the adult. The growth in
length takes place by a process of intussusception, and, till there are formed
the full number of mesoblastic somites, it is effected, as in Chastopods, by
the continual addition of fresh somites between the last-formed somite and
the hind end of the body.

A second and somewhat paradoxical view has been recently brought into
prominence by His and Rauber. This view has moreover since been taken
up by many embryologists, and has led to strange comparisons between the

COMPARISON OF THE GERMINAL LAYERS. 307

formation of the mesoblastic plates of the Chastopods and the medullary folds
of Vertebrata. According to this view the embryo grows in length by the
coalescence of the two halves of the thickened edges of the blastoderm in the
dorsal median line. The groove between the coalescing edges is the medul-
lary groove, which increases in length by the continued coalescence of fresh
portions of the edge of the blastoderm.

The following is His' own statement of his view: "I have shewn that the
embryo of Osseous Fishes grows together in length from two symmetrically-
placed structures in the thickened edge of the blastoderm. Only the fore-
most end of the head and the hindermost end of the tail undergo no concre-
scence, since they are formed out of that part of the edge of the blastoderm
which, together with the two lateral halves, completes the ring. The whole
edge of the blastoderm is used in the formation of the embryo."

The edges of the blastoderm which meet to form the body of the embryo
are regarded as the blastopore, so that, on this view, the blastopore primi-
tively extends for the whole length of the dorsal side of the embryo, and
the groove between the coalesced lips becomes the medullary groove.

It is not possible for me to enter at any great length into the arguments
used to support this position.

They may be summarised as (i) The general appearance ; i.e. that the
thickened edge of the blastoderm is continuous with the medullary fold.

(2) Certain measurements (His) which mainly appear to me to prove
that the growth takes place by the addition of fresh somites between that last
formed and the end of the body.

(3) Some of the phenomena of double monsters (Rauber).

None of these arguments appear to be very forcible, but as the view of
His and Rauber, if true, would certainly be important, I shall attempt shortly
to state the arguments against it, employing as my type the Elasmobran-
chii, by the development of which, according to His, the view which he
adopts is more conclusively proved than by that of any other group.

(1) The general appearance of the thickened edge of the blastoderm be-
coming continuous with the medullary folds has been used as an argument
for the medullary folds being merely the coalesced thickened edges of the
blastoderm. Since, however, the medullary folds are merely parts of the
medullary plate, and since the medullary plate is continuous with the adjoin-
ing epiblast of the embryonic rim, the latter structure must be continuous
with the medullary folds however they are formed, and the mere fact of their
being so continuous cannot be used as an argument either way. Moreover,
were the concrescence theory true, the coalescing edges of the blastoderm
might be expected to form an acute angle with each other, which they are far
from doing.

(2) The medullary groove becomes closed behind earlier than in front,
and the closure commences while the embryo is still quite short, and before
the hind end has begun to project over the yolk. After the medullary canal
becomes closed, and is continued behind into the alimentary canal by the
neurenteric passage, it is clearly impossible for any further increase in length

20 — 2

308 GROWTH IN LENGTH OF THE EMBRYO.

to take place by concrescence. If therefore His' and Rauber's view is accept-
ed, it will have to be maintained that only a small part of the body is form-
ed by concrescence, while the larger posterior part grows by intussusception.
The difficulty involved in this supposition is much increased by the fact that
long after the growth by concrescence must have ceased the yolk blastopore
still remains open, and the embryo is still attached to the edge of the blas-
toderm ; so that it cannot be maintained that the growth by concrescence
has come to an end because the thickened edges of the blastoderm have
completely coalesced.

The above are arguments derived simply from a consideration of the
growth of the embryo ; and they prove (i) that the points adduced by His
and Rauber are not at all conclusive ; (2) that the growth in length of the
greater part of the body takes place by the addition of fresh somites behind,
as in Chaetopods, and it would therefore be extremely surprising that a small
middle part of the body should grow in quite a different way.

Many minor arguments used by His might be replied to, but it is hardly
necessary to do so, and some of them depend upon erroneous views as to the
course of development, such as an argument about the notochord, which
depends for its validity upon the assumption that the notochord ridge ap-
pears at the same time as the medullary plate, while, as a matter of fact, the
ridge does not appear till considerably later. In addition to the arguments
of the class hitherto used, there may be brought against the His-Rauber
view a series of arguments from comparative embryology.

(1) Were the vertebrate blastopore to be co- extensive with the dorsal
surface, as His and Rauber maintain, clear evidence of this ought to be ap-
parent in Amphioxus. In Amphioxus, however, the blastopore is at first
placed exactly at the hind end of the body, though later it passes up just on
to the dorsal side (vide p. 4). It nearly closes before the appearance of the
medullary groove or mesoblastic somites ; and the medullary folds have
nothing to do with its lips, except in so far as they are continuous with them
behind, just as in Elasmobranchii.

(2) The food-yolk in the Vertebrata is placed on the ventral side of the
body, and becomes enveloped by the blastoderm ; so that in all large-yolked
Vertebrates the ventral walls of the body are obviously completed by the
closure of the lips of the blastopore, on the ventral side.

If His and Rauber are right the dorsal walls are also completed by the
closure of the blastopore, so that the whole of the dorsal, as well as of the
ventral wall of the embryo, must be formed by the concrescence of the lips of
the blastopore ; which is clearly a reductio adabsurdum of the whole theory.
To my own arguments on the subject I may add those of Kupffer, who has
very justly criticised His' statements, and has shewn that growth of the
blastoderm in Clupea and Gasterosteus is absolutely inconsistent with the
concrescence theory.

The more the theory of His and Rauber is examined by the light of com-
parative embryology, the more does it appear quite untenable ; and it may
be laid down as a safe conclusion from a comparative study of vertebrate

COMPARISON OF THE GERMINAL LAYERS. 309

embryology that the blastopore of Vertebrates is primitively situated at the
hind end of the body, but that, owing to the development of a large food-yolk,
it also extends, in most cases, over a larger or smaller part of the ventral
side.

The origin of the Allantois and Amnion.

The development and structure of the allantois and amnion have already
been dealt with at sufficient length in the chapters on Aves and Mammalia ;
but a few words as to the origin of these parts will not be out of place here.

The Allantois. The relations of the allantois to the adjoining organs,
and the conversion of its stalk into the bladder, afford ample evidence that it
has taken its origin from a urinary bladder such as is found in Amphibia.
We have in tracing the origin of the allantois to deal with a case of what
Dohrn would call ' change of function.' The allantois is in fact a urinary
bladder which, precociously developed and enormously extended in the em-
bryo, has acquired respiratory (Sauropsida) and nutritive (Mammalia) func-
tions. No form is known to have been preserved with the allantois in a
transitional state between an ordinary bladder and a large vascular sack.

The advantage of secondary respiratory organs during fcetal life, in addi-
tion to the yolk-sack, is evinced by the fact that such organs are very widely
developed in the Ichthyopsida. Thus in Elasmobranchii we have the
external gills (cf. p. 62). Amongst Amphibia we have the tail modified to be
a respiratory organ in Pipa Americana ; and in Notodelphis, Alytes and
Cascilia compressicanda the external gills are modified and enlarged for re-
spiratory purposes within the egg (cf. pp. 140 and 143).

The Amnion. The origin of the amnion is more difficult to explain
than that of the allantois ; and it does not seem possible to derive it from
any pre-existing organ.

It appears to me, however, very probable that it was evolved part flassu
with the allantois, as a simple fold of the somatopleure round the embryo,
into which the allantois extended itself as it increased in size and became a
respiratory organ. It would be obviously advantageous for such a fold, hav-
ing once started, to become larger and larger in order to give more and more
room for the allantois to spread into.

The continued increase of this fold would lead to its edges meeting on
the dorsal side of the embryo, and it is easy to conceive that they might then
coalesce.

To afford room for the allantois close to the surface of the egg, where
respiration could most advantageously be carried on, it would be convenient
that the two laminae of the amnion— the true and false amnion— should then
separate and leave a free space above the embryo, and thus it may have
come about that a separation finally takes place between the true and false
amnion.

This explanation of the origin of the amnion, though of course hypothe-
tical, has the advantage of suiting itself in most points to the actual ontogeny

310 ORIGIN OF ALLANTOIS AND AMNION.

of the organ. The main difficulty is the early development of the head-fold
of the amnion, since, from the position of the allantois, it might have been
anticipated that the tail-fold would be the first formed and most important
fold of the amnion.

BIBLIOGRAPHY.

(239) F. M. Balfour. " A comparison of the early stages in the development
of Vertebrates." Q:tarf. J. of Micr. Science, Vol. xv. 1875.

(240) F. M. Balfour. "A monograph on the development of Elasmobranch
Fishes." London, 1878.

(241) F. M. Balfour. " On the early development of the Lacertilia together
with some observations, etc." Quart, y. of Micr. Science, Vol. xix. 1879.

(242) A. Gotte. Die Entwicklungsgeschichte d. Unke. Leipzig, 1875.

(243) W. His. "Ueb. d. Bildung d. Haifischembryonen." Zeit. f. Anat, u.
Entwick., Vol. u. 1877. Cf. also His' papers on Teleostei, Nos. 65 and 66.

(244) A. Kowalevsky. " Entwick. d. Amphioxus lanceolatus." Mem. Acad.
des Sciences St Petersbourg, Ser. vn. Tom. xi. 1867.

(245) A. Kowalevsky. " Weitere Studien lib. d. Entwick. d. Amphioxus lan-
ceolatus." Archivf. mikr. Anat., Vol. xiil. 1877.

(246) C. Kupffer. "Die Entstehung d. Allantois u. d. Gastrula d. Wirbel-
thiere." Zool. Anzeiger, Vol. II. 1879, PP- 52°' 593> 612.

(247) R. Remak. Untersuchimgen iib. d. Entwicklung d. Wirbelthiere, 1850—
1858.

(248) A. Rauber. Primitimtreifen u. Neurula d. Wirbelthiere. Leipzig,
1877.

CHAPTER XII.

OBSERVATIONS ON THE ANCESTRAL FORM OF
THE CHORDATA.

THE present section of this work would not be complete
without some attempt to reconstruct, from the materials recorded
in the previous chapters, and from those supplied by compara-
tive anatomy, the characters of the ancestors of the Chordata ;
and to trace as far as possible from what invertebrate stock this
ancestor was derived.

The second of these questions has been recently dealt with in
a very suggestive manner by both Dohrn (No. 250) and Semper
(Nos. 255 and 256), but it is still so obscure that I shall refrain
from any detailed discussion of it.

While differing very widely in many points both Dohrn and Semper
have arrived at the view, already tentatively put forward by earlier anato-
mists, that the nearest allies of the Chordata are to be sought for amongst
the Chaetopoda, and that the dorsal surface of the Chordata with the spinal
cord corresponds morphologically with the ventral surface of the Chaetopods
with the ventral ganglion chain. In discussing this subject some time ago x
I suggested that we must look for the ancestors of the Chordata, not in
allies of the present Chaetopoda, but in a stock of segmented forms descend-
ed from the same unsegmented types as the Chaetopoda, but in which two
lateral nerve-cords, like those of Nemertines, coalesced dorsally, instead of
ventrally to form a median nervous cord. This group of forms, if my sug-
gestion as to its existence is well founded, appears now to have perished.
The recent researches of Hubrecht on the anatomy of the Nemertines a have,
however, added somewhat to the probability of my views, in that they shew
that in some existing Nemertines the nerve-cords approach each other very
closely in the dorsal line.

With reference to the characters of the ancestor of the
Chordata the following pages contain a few tentative suggestions
rather than an attempt to deal with the whole subject ; while the

1 Monograph on the development of Elasmobranch Fishes, pp. 170 — 173.

2 Hubrecht, "Zur Anat. u. Phys. d. Nervensystems der Nemertinen. " Kon. Akad.
Wiss. Amsterdam; and "Researches on the Nervous System of Nemertines." Quart.
Journ. of Micr. Science, 1880.

312 THE PR^iORAL LOBE.

origin of certain of the organs is dealt with in a more special
manner in the chapters on organogeny which form the second
part of this work.

Before entering upon the more special subject of this chapter,
it will be convenient to clear the ground by insisting on a few
morphological conclusions to be drawn from the study of
Amphioxus, — a form which, although probably in some respects
degenerate, is nevertheless capable of furnishing on certain
points very valuable evidence.

(1) In the first place it is clear from Amphioxus that the
ancestors of the Chordata were segmented, and that their
mesoblast was divided into myotomes which extended even into
the region in front of the mouth. The mesoblast of the greater
part of what is called the head in the Vertebrata proper was
therefore segmented like that of the trunk.

(2) The only internal skeleton present was the unsegmented
notochord — a fact which demonstrates that the skeleton is of
comparatively little importance for the solution of a large
number of fundamental questions, as for example the point
which has been mooted recently as to whether gill-clefts existed
at one time in front of the present mouth ; and for this reason : —
that from the evidence of Amphioxus and the lower Vertebrata1
it is clear that such clefts, if they ever existed, had atrophied

1 The greater part of the branchial skeleton of Petromyzon appears clearly to
belong to an extra-branchial system much more superficially situated than the true
branchial bars of the higher forms. At the same time-there is no doubt that certain
parts of the skeleton of the adult Lamprey have, as pointed out by Huxley, striking
points of resemblance to parts of a true mandibular and hyoid arches. Further em-
bryological evidence is required on the subject, but the statements on this head on
p. 84 ought to be qualified.

Should Huxley's views on this subject be finally proved correct, it is probable that,
taking into consideration the resemblance of these skeletal parts in the Tadpole to
those in the Lamprey, the cartilaginous mandibular bar, before being in any way
modified to form true jaws, became secondarily adapted to support a suctorial mouth,
and that it subsequently became converted into the true jaws. Thus the evolution of
this bar in the Frog would be a true repetition of the ancestral history, while its
ontogeny in Elasmobranchii and other types would be much abbreviated. For a fuller
statement on this point I must refer the reader to the chapter on the skull.

It is difficult to believe that the posterior branchial bars could have coexisted with
such a highly developed branchial skeleton as that in Petromyzon, so that the absence
of the posterior branchial bars in Petromyzon receives by far its most plausible
explanation on the supposition that Petromyzon is descended from a vertebrate stock
in which true branchial bars had not been evolved.

ON THE ANCESTRAL FORM OF THE CHORDATA. 313

completely before the formation of cartilaginous branchial bars ;
so that any skeletal structures in front of the mouth, which have
been interpreted by morphologists as branchial bars, can never
have acted in supporting the walls of branchial clefts.

(3) The region which, in the Vertebrata, forms the oeso-
phagus and stomach, was, in the ancestors of the Chordata,
perforated by gill-clefts. This fact, which has been clearly
pointed out by Gegenbaur, is demonstrated by the arrangement
of the gill-clefts in Amphioxus, and by the distribution of the
vagus nerve in the Vertebrata1. On the other hand the
insertion of the liver, which was probably a very primitive organ,
appears to indicate with approximate certainty the posterior
limit of the branchial clefts.

With these few preliminary observations we may pass to the
main subject of this section. A fundamental question which
presents itself on the threshold of our enquiries is the differen-
tiation of the head.

In the Chaetopoda the head is formed of a praeoral lobe and
of the oral segment ; while in Arthropods a somewhat variable
number of segments are added behind to this primitive head, and
form with it what may be called a secondary compound head.
It is fairly clear that the section of the trunk, which, in
Amphioxus, is perforated by the visceral clefts, has become the
head in the Vertebrates proper, so that the latter forms are
provided with a secondary head like that of Arthropods. There
remain however difficult questions (i) as to the elements of
which this head is composed, and (2) as to the extent of its
differentiation in the ancestors of the Chordata.

In Arthropods and Chaetopods there is a very distinct
element in the head known as the procephalic lobe in the case of
Arthropods, and the praeoral lobe in that of Chaetopods ; and
this lobe is especially characterized by the fact that the supra-
cesophageal ganglia and optic organs are formed as differentia-

1 The extension forwards in the vertebrata of an uninterrupted body-cavity into
the region previously occupied by visceral clefts presents no difficulty. In Amphioxus
the true body cavity extends forwards, more or less divided by the branchial clefts, for
the whole length of the branchial region, and in embryos of the lower Vertebrata there
is a section of the body cavity — the so-called head-cavities — between each pair of
pouches. On the disappearance of the pouches all these parts would naturally coalesce
into a continuous whole.

314 THE PR/KORAL LOBE.

tions of part of the epiblast covering it. Is such an element to
be recognized in the head of the Chordata ? From a superficial
examination of Amphioxus the answer would undoubtedly be
no ; but then it has to be borne in mind that Amphioxus, in
correlation with its habit of burying itself in sand, is especially
degenerate in the development of its sense-organs ; so that it is
not difficult to believe that its praeoral lobe may have become so
reduced as not to be recognizable. In the true Vertebrata there
is a portion of the head which has undoubtedly many features of
the praeoral lobe in the types already alluded to, viz. the part
containing the cerebral hemispheres and the thalamencephalon.
If there is any part of the brain homologous with the supra-
cesophageal ganglia of the Invertebrates, and it is difficult to
believe there is not such a part, it must be part of, or contain,
the fore-brain. The fore-brain resembles the supraoesophageal
ganglia in being intimately connected in its development with
the optic organs, and in supplying with nerves only organs of
sense. Its connection with the olfactory organs is an argument
in the same direction. Even in Amphioxus there is a small
bulb at the end of the nervous tube supplying what is very
probably the homologue of the olfactory organ of the Vertebrata ;
and it is quite possible that this bulb is the reduced rudiment of
what forms the fore-brain in the Vertebrata.

The evidence at our disposal appears to me to indicate that
the third nerve belongs to the cranio-spinal series of segmental
nerves, while the optic and olfactory nerves appear to me
equally clearly not to belong to this series1. The mid-brain, as
giving origin to the third nerve, would appear not to have been
part of the ganglion of the prseoral lobe.

These considerations indicate with fair probability that the
part of the head containing the fore-brain is the equivalent of the
praeoral lobe of many Invertebrate forms ; and the primitive
position of the Vertebrate mouth on the ventral side of the head
affords a distinct support for this view. It must however be
admitted that this part of the head is not sharply separated in
development from that behind ; and, though the fore-brain is

1 Marshall, in his valuable paper on the development of the olfactory organ, takes
a very different view of this subject. For a discussion of this view I must refer the
reader to the chapter on the nervous system.

ON THE ANCESTRAL FORM OF THE CHORDATA. 315

usually differentiated very early as a distinct lobe of the
primitive nervous tube, yet that such differentiation is hardly
more marked than in the other parts of the brain. The termi-
nation of the notochord immediately behind the fore-brain is,
however, an argument in favour of the morphological distinctness
of the latter structure.

The evidence at our disposal appears to indicate that the
posterior part of the head was not differentiated from the trunk
in lower Chordata ; but that, as the Chordata rose in the scale
of development, more and more centralizing work became
thrown on the anterior part of the nervous cord, and part passu
this part became differentiated into the mid- and hind-brain.
An analogy for such a differentiation is supplied in the compound
subcesophageal ganglion of many Arthropods ; and, as will be
shewn in the chapter on the nervous system, there is strong
embryological evidence that the mid- and hind-brains had
primitively the same structure as the spinal cord. The head
appears however to have suffered in the course of its diffe-
rentiation a great concentration in its posterior part, which
becomes progressively more marked, even within the limits of
the surviving Vertebrata. This concentration is especially shewn
in the structure of the vagus nerve, which, as first pointed out
by Gegenbaur, bears evidence of having been originally composed
of a great series of nerves, each supplying a visceral cleft.
Rudiments of the posterior nerves still remain as the branches
to the oesophagus and stomach1.

The atrophy of the posterior visceral clefts seems to have
taken place simultaneously with the concentration of the neural
part of the head ; but the former process did not proceed so
rapidly as the latter, so that the visceral region of the head is
longer in the lower Vertebrata than the neural region, and is
dorsally overlapped by the anterior part of the spinal cord and
the anterior muscle-plates (vide fig. 47).

On the above view the posterior part of the head must have
been originally composed of a series of somites like those of the

1 The lateral branch of the vagus nerve probably became differentiated in
connection with the lateral line, which seems to have been first formed in the
head, and subsequently to have extended into the trunk (vide section on Lateral
Line).

3'6

Till. MEDULLARY CANAL.

trunk, but in existing Vertebrata all trace of these, except in so
far as they are indicated by the visceral clefts, has vanished in
the adult. The cranial nerves however, especially in the embryo,
still indicate the number of anterior somites ; and an embryonic
segmentation of the mesoblast has also been found in many
lower forms in the region of the head, giving rise to a series of
cavities known as head-cavities, enclosed by mesoblastic walls
which afterwards break up into muscles. These cavities corre-
spond with the nerves, and it appears that there is a praeman-
dibular cavity corresponding with the third nerve (fig. 193, \pp)
and a mandibular cavity (2pp) and a cavity in each of the
succeeding visceral arches. The fifth nerve, the seventh nerve,
the glossopharyngeal nerve, and
the successive elements of the
vagus nerve correspond with the
posterior head-cavities.

The medullary canal. The
general history of the medullary
plate seems to point to the con-
clusion that the central canal of the
nervous system has been formed by
a groove having appeared in the
ancestor of the Chordata along the
median dorsal line, which caused
the sides of the nervous plate,
which was placed immediately
below the skin, or may perhaps at
that stage not have been distinctly
differentiated from the skin, to be
bent upwards ; and that this groove
subsequently became converted into
a canal. This view is not only
supported by the actual develop-
ment of the central canal of the
nervous system (the types of Tele-
ostei, Lepidosteus and Petromyzon
being undoubtedly secondary), but
also (i) by the presence of cilia in the epithelium lining the canal,
probably inherited from cilia coating the external skin, and (2) by

FIG. 193. TRANSVERSE SECTION
THROUGH THE FRONT PART OF THE
HEAD OF A YOUNG PRISTIURUS

EMBRYO.

The section, owing to the cranial
flexure, cuts both the fore- and the
hind-brain. It shews the prseman-
dibular and mandibular head-cavities
ipp and ipp, etc.

fb. fore-brain; /. lens of eye; /«.
mouth ; pt. upper end of mouth,
forming pituitary involution; iao..
mandibular aortic arch; ipp. and
ipp. first and second head-cavities ;
ivc. first visceral cleft ; V. fifth
nerve ; aun. ganglion of auditory
nerve ; VII. seventh nerve ; aa, dor-
sal aorta ; acv. anterior cardinal
vein; ^..notochord.

ON THE ANCESTRAL FORM OF THE CHORDATA.

317

the posterior roots arising from the extreme dorsal line (fig. 194),
a position which can most easily be explained on the supposition
that the two sides of the plate, from which the nerves originally
proceeded have been folded up so as to meet each other in the
median dorsal line1.

The medullary plate, before becoming folded to form the
medullary groove, is (except in Amphibia) without any indication
of being composed of two halves. In both the embryo and
adult the walls of the tube have however a structure which points
to their having arisen from the coalescence of two lateral, and
most probably at one time inde-
pendent, cords ; and as already indi-
cated this is the view I am myself in-
clined to adopt ; vide pp. 303 and

304-

The origin and nature of the
mouth. The most obvious point
connected with the development of
the mouth is the fact that in all
vertebrate embryos it is placed
ventrally, at some little distance
from the front end of the body.
This feature is retained in the adult
stage in Elasmobranchii, the Myx-
inoids, and some Ganoids, but is lost
in other vertebrate forms. A mouth,
situated as is the embryonic verte-
brate mouth, is very ill adapted for
biting ; and though it acquires in
this position a distinctly biting cha-
racter in the Elasmobranchii, yet it
is almost certain that it had not such
a character in the ancestral Chordata,
and that its terminal position in
higher types indicates a step in advance of the Elasmo-
branchii.

On the structure of the primitive mouth there appears to me

al

FIG. 194. TRANSVERSE SEC-
TION THROUGH THE TRUNK OF
AN EMBRYO SLIGHTLY OLDER
THAN FIG. 28 E.

nc. neural canal ; pr. posterior
root of spinal nerve ; x. subnoto-
chordal rod ; ao. aorta ; sc. so-
matic mesoblast ; sp. splanchnic
mesoblast ; mp. muscle-plate ;
mp'. portion of muscle-plate con-
verted into muscle ; Vv. portion
of the vertebral plate which will
give rise to the vertebral bodies ;
al. alimentary tract.

1 Vidf for further details the chapter on the nervous system.

318 PRIMITIVE SUCTORIAL MOUTH.

to be some interesting embryological evidence, to which attention
has already been called in the preceding chapters. In a large
number of the larvae or embryos of the lower Vertebrates the
mouth has a more or less distinctly suctorial character, and is
connected with suctorial organs which may be placed either in
front of or behind it. The more important instances of this
kind are (i) the Tadpoles of the Anura, with their posteriorly
placed suctorial disc, (2) Lepidosteus larva (fig. 195) with
its anteriorly placed suctorial disc, (3) the adhesive papillae
of the larvae of the Tunicata. To these may be added the
suctorial mouth of the Myxinoid fishes1.

All these considerations point to the conclusion that
in the ancestral Chordata the mouth had a more or less
definitely suctorial character2, and was placed on the
ventral surface immediately behind the praeoral lobe;
and that this mouth has become in the higher types
gradually modified for biting purposes, and has been
carried to the front end of the head.

The mouth in Elasmobranchii and other Vertebrates is
originally a wide somewhat rhomboidal cavity (fig. 28 G) ; on
the development of the mandibular and its maxillary (pterygo-
quadrate) process the opening of the mouth becomes narrowed
to a slit. The wide condition of the mouth may not improbably
be interpreted as a remnant of the suctorial state. The fact
that no more definite remnants of the suctorial mouth are
found in so primitive a group as the Elasmobranchii is probably
to be explained by the fact that the members of this group
undergo an abbreviated development within the egg.

1 The existing Myxinoid P'ishes are no doubt degenerate types, as was first clearly
pointed out by Dohrn ; but at the same time (although Dohrn does not share this view)
it appears to me almost certain that they are the remnants of a large and very primitive
group, which have very likely been preserved owing to their parasitic or semiparasitic
habits ; much in the same way as many of the Insectivora have been preserved owing
to their subterranean habits. I am acquainted with no evidence, embryological or
otherwise, that they are degraded gnathostomatous forms, and the group probably
disappeared as a whole from its incapacity to compete successfully with Vertebrata in
which true jaws had become developed.

3 I do not conceive that the existence of suctorial structures necessarily implies
parasitic habits. They might be used for various purposes, especially by predaceous
forms not provided with jaws.

ON THE ANCESTRAL FORM OF THE CHORDATA. 319

While the embryological data appear to me to point to the existence of a
primitive suctorial mouth, very different conclusions have been put forward
by other embryologists, more especially by Dohrn, which are sufficiently
striking and suggestive to merit a further discussion.

As mentioned above, both Dohrn and Semper hold that the Vertebrata
are descended from Chastopod-like forms, in which the ventral surface has
become the dorsal. In consequence of this view Dohrn has arrived at the
following conclusions : (i) that primitively the alimentary canal perforated
the nervous system in the region of the original cesophageal nerve-ring ; (2)
that there was therefore an original dorsal mouth (the present ventral mouth
of the Cheetopoda) ; and (3) that the present mouth was secondary and
derived from two visceral clefts which have ventrally coalesced.

A full discussion of these views1 is not within the scope of this work ;
but, while recognizing that there is much to be said in favour of the inter-
change of the dorsal and ventral surfaces, I am still inclined to hold that the
difficulties involved in this view are so great that it must, provisionally at
least, be rejected; and that there are therefore no reasons against supposing

— -sd

op

FIG. 195. VENTRAL VIEW OF THE HEAD OF A LEPIDOSTEUS EMBRYO SHORTLY

BEFORE HATCHING, TO SHEW THE LARGE SUCTORIAL DISC.

m. mouth ; op. eye ; sd. suctorial disc.

the present vertebrate mouth to be the primitive mouth. There is no
embryological evidence in favour of the view adopted by Dohrn that the
present mouth was formed by the coalescence of two clefts.

If it is once admitted that the present mouth is the primitive mouth, and
is more or less nearly in its original situation, very strong evidence will be
required to shew that any structures originally situated in front of it are the
remnants of visceral clefts ; and if it should be proved that such remnants
of visceral clefts were present, the views so far arrived at in this section
would, I think, have to be to a large extent reconsidered.

The nasal pits have been supposed by Dohrn to be remnants of visceral

1 For a partial discussion of this subject I would refer the reader to my Monograph
on Elasmobranch Fishes, pp. 165 — 172.

320 FORMATION OF THE JAWS.

clefts, and this view has been maintained in a very able manner by Marshall.
The arguments of Marshall do not, however, appear to me to have any
great weight unless it is previously granted that there is an antecedent pro-
bability in favour of the presence of a pair of gill-clefts in the position of the
nasal pits ; and even then the development of the nasal pits as epiblastic
involutions, instead of hypoblastic outgrowths, is a serious difficulty which
has not in my opinion been successfully met. A further argument of
Marshall from the supposed segmental nature of the olfactory nerve has
already been spoken of.

While most of the structures supposed to be remains of gill-clefts in front
of the mouth do not appear to me to be of this nature, there is one organ
which stands in a more doubtful category. This organ is the so-called cho-
roid gland. The similarity of this organ to the pseudo-branch of the mandi-
bular or hyoid arch was pointed out to me by Dohrn, and the suggestion
was made by him that it is the remnant of a praemandibular gill which has
been retained owing to its functional connection with the eye1. Admitting
this explanation to be true (which however is by no means certain) are we
necessarily compelled to hold that the choroid gland is the remnant of a
gill-cleft originally situated in front of the mouth ? I believe not. It is easy
to conceive that there may originally have been a praemandibular cleft behind
the suctorial mouth, but that this cleft gradually atrophied (for the same
reasons that the mandibular cleft shews a tendency to atrophy in existing
fishes, &c.), the rudiment of the gill (choroid gland) alone remaining to mark
its situation. After the disappearance of this cleft the suctorial mouth may
have become relatively shifted backwards. In the meantime the branchial
bars became developed, and as the mouth was changed into a biting one, the

1 The probability of the choroid gland having the meaning attributed to it by
Dohrn is strengthened by the existence of a praemandibular segment as evidenced by
the presence of a pnemandibular head-cavity, the walls of which as shewn by Marshall
and myself give rise to the majority of the eye-muscles and of a nerve (the third nerve,
cf. Marshall) corresponding to it; so that these parts together with the choroid gland
may be rudiments belonging to the same segment. On the other hand the absence of
the choroid gland in Ganoidei and Elasmobranchii, where a mandibular pseudo-branch
is present, coupled with the absence of a mandibular pseudo-branch in Teleostei
where alone a choroid gland is present, renders the above view about the choroid
gland somewhat doubtful. A thorough investigation of the ontogeny of the choroid
gland might throw further light on this interesting question, but I think it not
impossible that the" choroid gland may be nothing else but the modified mandibtddr
pseudo-branch, a view which fits in very well with the relations of the vessels of
the Elasmobranch mandibular pseudo-branch to the choroid. For the relations
and structure of the choroid gland vide F. Miiller, Vergl. Anal. Myxinoiden, Part in.
p. 82.

It is possible that the fourth nerve and the superior oblique muscle of the eye which
it supplies may be the last remaining remnants of a second praemandibular segment
originally situated between the segment of the third nerve and that of the fifth nerve
(mandibular segment).

ON THE ANCESTRAL FORM OF THE CHORDATA. 321

bar (the mandibular arch) supporting the then first cleft became gradually
modified and converted into a supporting apparatus for the mouth, and final-
ly formed the skeleton of the jaws. In the hyostylic Vertebrata the hyoid
arch also became modified in connection with the formation of the jaws.

The conclusions arrived at may be summed up as follows :
The relations which exist in all jaw-bearing Vertebrates be-
tween the mandibular arch and the oral aperture are secondary,
and arose paripassu with the evolution of the jaws1.

The cranial flexure and the form of the head in verte-
brate embryos. All embryologists who have studied the embryos of the
various vertebrate groups have been struck with the remarkable similarity

Vgr

aur

vir

FIG. 196. THE HEADS OF ELASMOBRANCH EMBRYOS AT TWO STAGES VIEWED

AS TRANSPARENT OBJECTS.

A. Pristiurus embryo of the same stage as fig. 28 F. B. Somewhat older
Scyllium embryo.

///. third nerve; V. fifth nerve; VII. seventh nerve; au.n. auditory nerve; gl.
glossopharyngeal nerve; Vg. vagus nerve; fb. fore-brain; pn. pineal gland; mb. mid-
brain; hb. hind-brain; iv.v. fourth ventricle; cb. cerebellum; ol. olfactory pit; op.
eye; au.V. auditory vesicle; m. mesoblast at base of brain; ch. notochord; ht. heart;
Vc. visceral clefts ; eg. external gills ; //. sections of body cavity in the head.

1 I do not mean to exclude the possibility of the mandibular arch having supported
a suctorial mouth before it became converted into a pair of jaws.

B. III. 21

322 POST-ANAL GUT.

which exists between them, more especially as concerns the form of the head.
This similarity is closest between the members of the Amniota, but there is
also a very marked resemblance between the Amniota and the Elasmobran-
chii. The peculiarity in question, which is characteristically shewn in fig.
196, consists in the cerebral hemispheres and thalamencephalon being ven-
trally flexed to such an extent that the mid- brain forms the termination of
the long axis of the body. At a later period in development the cerebral
hemispheres come to be placed at the front end of the head ; but the ori-
ginal nick or bend of the floor of the brain is never got rid of.

It is obvious that in dealing with the light thrown by embryology on the
ancestral form of the Chordata the significance of this peculiar character of
the head of many vertebrate embryos must be discussed. Is the constancy
of this character to be explained by supposing that at one period vertebrate
ancestors had a head with the same features as the embryonic head of
existing Vertebrata ?

This is the most obvious explanation, but it does not at the same time
appear to me satisfactory. In the first place the mouth is so situated at the
time of the maximum cranial flexure that it could hardly have been func-
tional ; so that it is almost impossible to believe that an animal with a
head such as that of these embryos can have existed.

Then again, this type of embryonic head is especially characteristic of
the Amniota, all of which are developed in the egg. It is not generally so
marked in the Ichthyopsida. In Amphibia, Teleostei, Ganoidae and Petromy-
zontidae, the head never completely acquires the peculiar characteristic form
of the head of the Amniota, and all these forms are hatched at a relatively
much earlier phase of development, so that they are leading a free existence
at a stage when the embryos of the Amniota are not yet hatched. The only
Ichthyopsidan type with a head like that of the Amniota is the Elasmobran-
chii, and the Elasmobranchii are the only Ichthyopsida which undergo the
major part of their development within the egg.

These considerations appear to shew that the peculiar characters of the
embryonic head above alluded to are in some way connected with an em-
bryonic as opposed to a larval development ; and for reasons which are
explained in the section on larval forms, it is probable that a larval develop-
ment is a more faithful record of ancestral history than an embryonic deve-
lopment. The flexure at the base of the brain appears however to be a typi-
cal vertebrate character, but this flexure never led to a conformation of the
head in the adult state similar to that of the embryos of the Amniota. The
form of the head in these embryos is probably to be explained by supposing
that some advantage is gained by a relatively early development of the brain,
which appears to be its proximate cause ; and since these embryos had not
to lead a free existence (for which such a form of the head would have been
unsuited) there was nothing to interfere with the action of natural selection
in bringing about this form of head during fcetal life.

Post-anal gut and neurenteric canal. One of the most

ON THE ANCESTRAL FORM OF THE CHORDATA. 323

remarkable structures in the trunk is the post-anal gut (fig. 197).
Its structure is fully dealt with in the chapter on the alimentary
tract, but attention may here be called to the light which it appears
to throw on the characters of the ancestor of the Chordata.

In face of the facts which are known with reference to the
post-anal section of the alimentary tract, it can hardly be
doubted that this portion of the alimentary tract must have
been at one time functional. This seems to me to be shewn (i)
by the constancy and persistence of this obviously now function-
less rudiment, (2) by its greater development in the lower than
in the higher forms, (3) by its relation to the formation of the
notochord and subnotochordal rod.

If the above position be admitted, it is not permissible to
shirk the conclusions which seem necessarily to follow, however
great the difficulties may be which are involved in their accept-

FIG. 197. LONGITUDINAL SECTION THROUGH AN ADVANCED EMBRYO OF BOM-

BINATOR. (After Gotte.)

m. mouth; an. anus; /. liver; ne. neurenteric canal; me. medullary canal;
ch. notochord; pn. pineal gland.

ance. These conclusions have in part already been dealt with
by Dohrn in his suggestive tract (No. 250). In the first place
the alimentary canal must primitively have been continued to
the end of the tail ; and if so, it is hardly credible that the
existing anus can have been the original one. Although, there-
fore, it is far from easy, on the physiological principles involved
in the Darwinian theory, to understand the formation of a new
anus1 ; it is nevertheless necessary to believe that the present

1 Dohrn (No. 250, p. 25) gives an explanation of the origin of the new anus which
does not appear to me quite satisfactory.

21 — 2

324

POST-ANAL GUT.

vertebrate anus is a formation acquired within the group of the
Chordata, and not inherited from some older group. This
involves a series of further consequences. The opening of the
urinogenital ducts into the cloaca must also be secondary, and it
is probable that the segmental tubes were primitively continued
along the whole post-anal region of the vertebrate tail, opening
into the body cavity which embryology proves to have been
originally present there. They are in fact continued in many
existing forms for some distance behind the present anus. If
the present anus is secondary, there must have been a primitive
anus, which was probably situated behind the post-anal vesicle ;
and therefore in the region of the neurenteric canal. The neur-
enteric canal is, however, the remnant of the blastopore (vide
p. 277). It follows, therefore, that tJie vertebrate blastopore is
probably almost, if not exactly identical in position with the primi-
tive aims. This consideration may assist in explaining the
remarkable phenomenon of the existence of the neurenteric
canal. The attempt has already been made to shew that the
central canal of the nervous system is really a groove converted
into a tube and lined by the external epidermis. This tube (as
may be concluded from embryological considerations) was pro-
bably at first open posteriorly, and no doubt terminated at the
primitive anus. On the closure of the primitive anal opening,
the terminal portions of the post-anal gut and the neural tube,
may conceivably have been so placed that both of them opened
into a common cavity, which previously had communication with
the exterior by the anus. Such an arrangement would neces-
sarily result in the formation of a neurenteric canal. It seems
not impossible that a dilated vesicle, often present at the end of
the post-anal gut (vide fig. 28*, p. 58), may have been the com-
mon cavity into which both neural and alimentary tubes opened1.

1 As pointed out in Vol. II. p. 255, there is a striking similarity between the history
of the neurenteric canal in Vertebrates, and the history of the blastopore and ventral
groove as described by Kowalevsky in the larva of Chiton. Mr A. Sedgwick has
pointed out to me that the ciliated ventral groove in Protoneomenia, which contains
the anus, is probably the homologue of the groove found in the larva of Chiton, and
not, as usually supposed, simply the foot. Were this groove to be converted into a canal,
on the sides of which were placed the nervous cords, there would be formed a precisely
similar neurenteric canal to that in Vertebrata, though I do not mean to suggest that
there is any homology between the two (vide Hubrecht, Zool. Anzeigcr, 1880, p. 589).

ON THE ANCESTRAL FORM OF THE CHORDATA. 325

Till further light is thrown by fresh discoveries upon the primi-
tive condition of the posterior continuation of the vertebrate
alimentary tract, it is perhaps fruitless to attempt to work out
more in detail the 'above speculation.

Body cavity and mesoblastic somites. The Chordata, or
at least the most primitive existing members of the group, are
characterized by the fact that the body cavity arises as a pair of
outgrowths of the archenteric cavity. This feature1 in the de-
velopment is a nearly certain indication that the Chordata are a
very primitive stock. The most remarkable point with reference
to the development of the two outgrowths is, however, the fact
that the dorsal part of each outgrowth becomes separated from
the ventral. Its walls become segmented and form the meso-
blastic somites, which eventually, on the obliteration of their
cavity, give rise to the muscle-plates and to the tissue surround-
ing the notochord. It is not easy to understand the full
significance of the processes concerned in the formation of the
mesoblastic somites (vide p. 296). The mesoblastic somites
have no doubt a striking resemblance to the mesoblastic somites
of the Chsetopods, and most probably the segmentation of the
mesoblast in the two groups is a phenomenon of the same
nature ; but the difference in origin between the two types of
mesoblastic somites is so striking, and the development of the
muscular system from them is so dissimilar in the two groups, as
to render a direct descent of the Chordata from the Chsetopoda
very improbable. The ventral parts of the original outgrowth
give rise to the permanent body cavity, which appears originally
to have been divided into two parts by a dorsal and a ventral
mesentery.

The notochord. The most characteristic organ of the
Chordata is without doubt the notochord. The ontogenetic
development of this organ probably indicates that it arose as a
differentiation of the dorsal wall of the archenteron ; at the same
time it is not perhaps safe to lay too much stress upon its mode
of development. Embryological and anatomical evidence de-
monstrate, however, in the clearest manner that the early Chor-
data were provided with this organ as their sole axial skeleton ;

1 Vide the chapter on the Germinal Layers.

326 GILL-CLEFTS.

and no invertebrate group can fairly be regarded as genetically
related to the Chordata till it can be shewn to possess some
organ either derived from a notochord, or capable of having
become developed into a notochord. No such organ has as yet
been recognized in any invertebrate group1.

Gill-clefts. The gill-clefts, which are essentially pouches of
the throat opening externally, constitute extremely character-
istic organs of the Chordata, and have always been taken into
consideration in any comparison between the Chordata and the
Invertebrata.

Amongst the Invertebrata organs of undoubtedly the same
nature are, so far as I know, only found in Balanoglossus, where
they were discovered by Kowalevsky. The resemblance in this
case is very striking ; but although it is quite possible that the
gill-clefts in Balanoglossus are genetically connected with those
of the Chordata, yet the organization of Balanoglossus is as a
whole so different from that of the Chordata that no comparison
can be instituted between the two groups in the present state of
our knowledge.

Other organs of the Invertebrata have some resemblance to the gill-clefts.
The lateral pits of the Nemertines, which appear to grow out as a pair of
oesophageal diverticula, which are eventually placed in communication with
the exterior by a pair of ciliated canals (vide Vol. II. pp. 200 and 202), are
such organs.

Semper (No. 256) has made the interesting discovery that in the budding
of Nais and Chaetogaster two lateral masses of cells, in each of which a lumen
may be formed, unite with the oral invagination and primitive alimentary
canal to form the permanent cephalic gut. The lateral masses of cells are
regarded by him as branchial passages homologous in some way with those
in the Chordata. The somewhat scanty observations on this subject which
he has recorded do not appear to me to lend much support to this interpre-
tation.

It is probable that the part of the alimentary tract in which gill- clefts
are present was originally a simple unperforated tube provided with highly
vascular walls ; and that respiration was carried on in it by the alternate
introduction and expulsion of sea water. A more or less similar mode of
respiration has been recently shewn by Eisig2 to take place in the fore part

1 In the Chaetopods various organs have been interpreted as rudiments of a
notochord, but none of these interpretations will bear examination.

2 " Ueb. d. Vorkommen eines schwimmblasenahnlichen Organs bei Anneliden."
Mittheil. a. d. zoo!. Station zu Neapel, Vol. n. 1881.

ON THE ANCESTRAL FORM OF THE CHORDATA. 327

of the alimentary tract of many Chastopods. This part of the alimentary
tract was probably provided with paired cascal pouches with their blind ends
in contiguity with the skin.

Perforations placing these pouches in communication with the exterior
must be supposed to have been formed ; and the existence of openings into
the alimentary tract at the end of the tentacles of many Actinias and of the
hepatic diverticula of some nudibranchiate Molluscs (Eolis, &C.1) shews that
such perforations may easily be made. On the formation of such perfora-
tions the water taken in at the mouth would pass out by them ; and the
respiration would be localized in the walls of the pouches leading to them,
and thus the typical mode of respiration of the Chordata would be esta-
blished.

Phylogeny of the Chordata. It may be convenient to
shew in a definite way the bearing of the above speculations on
the phylogeny of the Chordata. For this purpose, I have drawn
up the subjoined table, which exhibits what I believe to be the
relationships of the existing groups of the Chordata. Such a
table cannot of course be constructed from embryological data
alone, and it does not fall within the scope of this work to defend
its parts in detail.

MAMMALIA SAUROPSIDA

L- T J

. PROTO-AMNIOTA AMPHIBIA

TELEOSTEI PROTO-PENTADACTYLOIDEI

I

GANOIDEI

-DIPNOI

PROTO-GANOIDEI

— HOLOCEPHALI
-ELASMOBRANCHII

PROTO-GNATHOSTOMATA

Cyclostomata PROTO-VERTEBRATA

Cephafochorda PROTOCHORDATA Uroc/iorda

In the above table the names printed in large capitals are hypothetical groups.
The other groups are all in existence at the present day, hut those printed in Italics
are probably degenerate.

The ancestral forms of the Chordata, which may be called
the Protochordata, must be supposed to have had (i) a

1 The openings of the hepatic diverticula through the sacks lined with thread cells
are described by Hancock and Embleton, Ann. and Mag. of Nat. History, Vol. xv.
1845, p. 82. Von Jhering has also recently described these openings (Zool. Anzeiger,
No. 23) and apparently attributes their discovery to himself.

328 PHYLOGENY OF THE CHORDATA.

notochord as their sole axial skeleton, (2) a ventral mouth,
surrounded by suctorial structures, and (3) very numerous
gill-slits. Two degenerate offshoots of this stock still persist
in Amphioxus (Cephalochorda), and the Ascidians (Urochorda).

The direct descendants of the ancestral Chordata, were pro-
bably a group which may be called the Proto-vertebrata, of
which there is no persisting representative. In this group,
imperfect neural arches were probably present ; and a ventral
suctorial mouth without a mandible and maxillae was still per-
sistent. The branchial clefts had, however, become reduced in
number, and were provided with gill-folds ; and a secondary
head (vide p. 313), with brain and organs of sense like those of
the higher Vertebrata, had become formed.

The Cyclostomata are probably a degenerate offshoot of this
group.

With the development of the branchial bars, and the
conversion of the mandibular bar into the skeleton of the jaws,
we come to the Proto-gnathostomata. The nearest living repre-
sentatives of this group are the Elasmobranchii, which still
retain in the adult state the ventrally placed mouth. Owing to
the development of food-yolk in the Elasmobranch ovum the
early stages of development are to some extent abbreviated, and
almost all trace of a stage with a suctorial mouth has become
lost.

We next come to an hypothetical group which we may call
the Proto-ganoidei. Bridge, in his memoir on Polyodon1,
which contains some very interesting speculations on the affini-
ties of the Ganoids, has called this group the Pneumatoccela,
from the fact that we find for the first time a full development of
the air-bladder, though it is possible that a rudiment of this
organ, in the form of a pouch opening on the dorsal side of the
stomachic extremity of the oesophagus, was present in the
earlier type.

Existing Ganoids are descendants of the Proto-ganoidei.
Some of them at all events retain in larval life the suctorial
mouth of the Proto-vertebrata ; and the mode of formation of
their germinal layers, resembling as it does that in the Lamprey

1 Phil. Trans. 1878. Part II.

ON THE ANCESTRAL FORM OF THE CHORDATA. 329

and the Amphibia, probably indicates that they are not de-
scended from forms with a large food-yolk like that of Elasmo-
branchii, and that the latter group is therefore a lateral offshoot
from the main line of descent.

Of the two groups into which the Ganoidei may be divided
it is clear that certain members of the one (Teleostoidei), viz.
Lepidosteus and Amia, shew approximations to the Teleostei,
which no doubt originated from the Ganoids ; while the other
(Selachoidei or Sturiones) is more nearly related to the Dipnoi.
Polypterus has also marked affinities in this direction, e.g. the
external gills of the larva (vide p. 1 18).

The Teleostei, which have in common a meroblastic segmen-
tation, had probably a Ganoid ancestor, the ova of which were
provided with a large amount of food-yolk. In most existing
Teleostei, the ovum has become again reduced in size, but the
meroblastic segmentation has been preserved. It is quite possi-
ble that Amia may also be a descendant of the Ganoid ancestor
of the Teleostei ; but Lepidosteus, as shewn by its complete
segmentation, is clearly not so.

The Dipnoi as well as all the higher Vertebrata are descen-
dants of the Proto-ganoidei.

The character of the limbs of higher Vertebrata indicates
that there was an ancestral group, which may be called the
Proto-pentadactyloidei, in which the pentadactyle limb became
established ; and that to this group the common ancestor of the
Amphibia and Amniota belonged.

It is possible that the Plesiosauri and Ichthyosauri of
Mesozoic times may have been more nearly related to this
group than either to the Amniota or the Amphibia. The
Proto-pentadactyloidei were probably much more closely related
to the Amphibia than to the Amniota. They certainly must
have been capable of living in water as well as on land, and had
of course persistent branchial clefts. It is also fairly certain
that they were not provided with large-yolked ova, otherwise
the mode of formation of the layers in Amphibia could not be
easily explained.

The Mammalia and Sauropsida are probably independent
offshoots from a common stem which may be called the Proto-
amniota.

330 BIBLIOGRAPHY.

BIBLIOGRAPHY.

(249) F. M. Balfour. A Monograph on the development of Elasmobranch Fishes,
London, 1878.

(250) A. Dohrn. Der (Jrsprung d. Wirbelthiere und d. Princip. d. Functions-
wechsel. Leipzig, 1875.

(251) E. Haeckel. Sch'dpfungsgeschichte. Leipzig. Vide also Translation.
The History of Creation. King and Co. , London. 1876.

(252) E. Haeckel. Anthropogenie. Leipzig. Vide also Translation. Anthro-
pogeny. Kegan Paul and Co., London, 1878.

(253) A. Kowalevsky. " Entwicklungsgeschichte d. Amphioxus lanceolatus."
Mem. Acad. d. Scien. St Petersbourg, Ser. VII. Tom. XI. 1867, and Archivf. mikr.
Anat., Vol. xin. 1877.

(254) A. Kowalevsky. "Weitere Stud. lib. d. Entwick. d. einfachen Ascidien."
Archivf. mikr. Anat., Vol. VII. 1871.

(255) C. Semper. "Die Stammesverwandschaft d. Wirbelthiere u. Wirbel-
losen." Arbeit, a. d. zool.-zoot. Instit. Wurzburg, Vol. II. 1875.

(256) C. Semper. "Die Verwandschaftbeziehungen d. gegliederten Thiere."
Arbeit, a. d. zool.-zoot. Instit. Wurzburg, Vol. III. 1876 — 1877.

CHAPTER XIII.

GENERAL CONCLUSIONS.

I. THE MODE OF ORIGIN AND HOMOLOGIES OF THE
GERMINAL LAYERS.

IT has already been shewn in the earlier chapters of the
work that during the first phases of development the history of
all the Metazoa is the same. They all originate from the coales-
cence of two cells, the ovum and spermatozoon. The coalesced
product of these cells — the fertilized ovum — then undergoes a
process known as the segmentation, in the course of which it
becomes divided in typical cases into a number of uniform cells.
An attempt was made from the point of view of evolution to
explain these processes. The ovum and spermatozoon were
regarded as representing phylogenetically two physiologically
differentiated forms of a Protozoon ; their coalescence was equi-
valent to conjugation : the subsequent segmentation of the
fertilized ovum was the multiplication by division of the organ-
ism resulting from the conjugation ; the resulting organisms,
remaining, however, united to form a fresh organism in a higher
state of aggregation.

In the systematic section of this work the embryological
history of the Metazoa has been treated. The present chapter
contains a review of the cardinal features of the various his-
tories, together with an attempt to determine how far there are
any points common to the whole of these histories ; and the
phylogenetic interpretation to be given to such points.

Some years ago it appeared probable that a definite answer

332 INVAGINATION.

would be given to the questions which must necessarily be
raised in the present chapter ; but the results of the extended
investigations made during the last few years have shewn that
these expectations were premature, and in spite of the numerous
recent valuable contributions to this branch of Embryology,
amongst which special attention may be called to those of
Kowalevsky (No. 277), Lankester (Nos. 278 and 279), and
Haeckel (No. 266), there are few embryologists who would ven-
ture to assert that any answers which can be given are more
than tentative gropings towards the truth.

In the following pages I aim more at summarising the
facts, and critically examining the different theories which can
be held, than at dogmatically supporting any definite views of
my own.

In all the Metazoa, the development of which has been in-
vestigated, the first process of differentiation, which follows
upon the segmentation, consists in the cells of the organism
becoming divided into two groups or layers, known respectively
as epiblast and hypoblast.

These two layers were first discovered in the young embryos of verte-
brated animals by Pander and Von Baer, and have been since known as
the germinal layers, though their cellular nature was not at first recog-
nised. They were shewn, together with a third layer, or mesoblast, which
subsequently appears between them, to bear throughout the Vertebrata
constant relations to the organs which became developed from them. A
very great step was subsequently made by Remak (No. 287), who success-
fully worked out the problem of vertebrate embryology on the cellular
theory.

Rathke in his memoir on the development of Astacus (No. 286) at-
tempted at a very early period to extend the doctrine of the derivation of
the organs from the germinal layers to the Invertebrata. In 1859 Huxley
made an important step towards the explanation of the nature of these
layers by comparing them with the ectoderm and endoderm of the Hydro-
zoa ; while the brilliant researches of Kowalevsky on the development of
a great variety of invertebrate forms formed the starting point of the current
views on this subject.

The differentiation of the epiblast and hypoblast may
commence during the later phases of the segmentation, but
is generally not completed till after its termination. Not
only do the cells of the blastoderm become differentiated

ORIGIN OF THE GERMINAL LAYERS.

333

into two layers, but these
very large number of

two layers, in the case of a
ova with but little food-yolk, con-

(fig. 198)— the
require further

FIG. 198. DIAGRAM
OF A GASTRULA.

(From Gegenbaur.)

a. mouth ; b. ar-
chenteron ; c. hypo-
blast ; d. epiblast.

stitute a double-walled sack — the gastrula
characters of which are too well known to
description. Following the lines of phylo-
genetic speculation above indicated, it may
be concluded that the two-layered condition
of the organism represents in a general way
the passage from the protozoon to the meta-
zoon condition. It is probable that we may
safely go further, and assert that the gastrula
reproduces, with more or less fidelity, a stage
in the evolution of the Metazoa, permanent
in the simpler Hydrozoa, during which the
organism was provided with (i) a fully deve-
loped digestive cavity (fig. 198 b) lined by
the hypoblast with digestive and assimilative
functions, (2) an oral opening (a), and (3) a
superficial epiblast (d}. These generalisa-
tions, which are now widely accepted, are no doubt very valuable,
but they leave unanswered the following important questions :

(1) By what steps did the compound Protozoon become
differentiated into a Metazoon ?

(2) Are there any grounds for thinking that there is more
than one line along which the Metazoa have become indepen-
dently evolved from the Protozoa ?

(3) To what extent is there a complete homology between
the two primary germinal layers throughout the Metazoa ?

Ontogenetically there is a great variety of processes by which
the passage from the segmented ovum to the two-layered or
diploblastic condition is arrived at.

These processes may be grouped under the following heads :
1. Invagination. Under this term a considerable number
of closely connected processes are included. When the segmen-
tation results in the formation of a blastosphere, one half of the
blastosphere may be pushed in towards the opposite half, and a
gastrula be thus produced (fig. 199, A and B). This process is
known as embolic invagination. Another process, known as epi-
bolic invagination, consists in epiblast cells growing round and en-

334

INVAGINATION.

closing the hypoblast (fig. 200). This process replaces the former
process when the hypoblast cells are so bulky from being distended
by food-yolk that their invagination is mechanically impossible.

FlG. 199. TWO STAGES IN THE DEVELOPMENT OF HOLOTHURIA TUBULOSA,

VIEWED IN OPTICAL SECTION. (After Selenka.)
A. Stage at the close of segmentation. B. Gastrula stage.

mr. micropyle ; fl. chorion ; s.c. segmentation cavity; bl. blastoderm; ep. epiblast;
hy. hypoblast ; ms. amoeboid cells derived from hypoblast ; a.e. archenteron.

There are various peculiar modifications of invagination
which cannot be dealt with in detail.

Invagination in one form or other occurs in some or all the
members of the following groups :

The Dicyemidae, Calci-
spongiae (after the amphiblastu-
la stage) and Silicispongiae, Coe-
lenterata, Turbellaria, Nemer-
tea, Rotifera, Mollusca, Polyzoa,
Brachiopoda, Chaetopoda, Dis-
cophora, Gephyrea, Chaeto-
gnatha, Nematelminthes, Crus-
tacea, Echinodermata, and
Chordata.

The gastrula of the Crus-
tacea is peculiar, as is also that
of many of the Chordata (Rep-
tilia, Aves, Mammalia), but
there is every reason to suppose

FIG. 200. TRANSVERSE SECTION
THROUGH THE OVUM OF EUAXES
DURING AN EARLY STAGE OF DEVELOP-
MENT, TO SHEW THE NATURE OF
EPiiiOLic INVAGINATION. (After Kowa-
levsky. )

ep. epiblast ; ms. mesoblastic band ;
hy. hypoblast.

ORIGIN OF THE GERMINAL LAYERS.

335

that the gastrulae of these groups are simply modifications of the
normal type.

2. Delamination. Three types of delamination may be
distinguished :

a. Delamination where the cells of a solid morula become
divided into a superficial epiblast, and a central solid mass
in which the digestive cavity is subsequently hollowed out
(fig. 201).

FlG. 201. TWO STAGES IN THE DEVELOPMENT OF STEPHANOMIA PICTUM,
TO ILLUSTRATE THE FORMATION OF THE LAYERS BY DELAMINATION. (After

Metschnikoff.)

A. Stage after the delamination; ep. epiblastic invagination to form pneuma-
tocyst.

B. Later stage after the formation of the gastric cavity in the solid hypoblast.
po. polypite ; /. tentacle ; pp. pneumatocyst ; ep. epiblast of pneumatocyst ; hy. hypo-
blast surrounding pneumatocyst.

b. Delamination where the segmented ovum has the form
of a blastosphere, the cells of which give rise by budding to
scattered cells in the interior of the vesicle, which, though they
may at first form a solid mass, finally arrange themselves in the
form of a definite layer around a central digestive cavity
(fig. 202).

c. Delamination where the segmented ovum has the form
of a blastosphere in the cells of which the protoplasm is diffe-
rentiated into an inner and an outer part. By a subsequent

336

DELAMINATION.

process the inner parts of the cells become separated from the
outer, and the walls of the blastosphere are so divided into two
distinct layers (fig. 205).

Although the third of these processes is usually regarded
as the type of delamination, it does not, so far as I know, occur
in nature, but is most nearly approached in Geryonia (fig. 203).

The first type of delamination is found in the Ceratospongiae,
some Silicispongiae (?), and in many Hydrozoa and Actinozoa,
and in Nemertea and Nematelminthes (Gordioidea ?). The
second type occurs in many Porifera \Calcispongi(e (A see t fa),
Myxospongice], and in some Coelenterata, and Brachiopoda
( Thecidium).

Delamination and invagination are undoubtedly the two
most frequent modes in which the layers are differentiated, but

C

FIG. 202. THREE LARVAL STAGES OF EUCOPE POLYSTYLA. (After Kowalevsky.)
A. Blastosphere stage with hypoblast spheres becoming budded off into central
cavity. B. Planula stage with solid hypoblast. C. Planula stage with a gastric
cavity, ep. epiblast ; hy. hypoblast ; al. gastric cavity.

there are in addition several others. In the first place the
whole of the Tracheata (with the apparent exception of the
Scorpion) develop, so far as is known, on a plan peculiar to
them, which approaches delamination. This consists in the
appearance of a superficial layer of cells enclosing a central
yolk mass, which corresponds to the hypoblast (figs. 204 and
214). This mode of development might be classed under
delamination, were it not for the fact that the early development

ORIGIN OF THE GERMINAL LAYERS.

337

of many Crustacea is almost the same, but is subsequently
followed by an invagination (fig. 208), which apparently corre-

FIG. 203. DIAGRAMMATIC FIGURES SHEWING THE DELAMINATION OF THE

EMBRYO OF GERYONIA. (After Fol.)

A. Stage at the commencement of the delamination ; the dotted lines x shew

the course of the next planes of division. B. Stage at the close of the delamination.

cs. segmentation cavity ; a. endoplasm ; b. ectoplasm ; ep. epiblast ; hy. hypoblast.

spends to the normal invagination of other types. There are
strong grounds for thinking that the tracheate type of forma-

FIG. 204. SEGMENTATION AND FORMATION OF THE BLASTODERM IN CHELIFER.

(After Metschnikoff. )

In A the ovum is divided into a number of separate segments. In B a number of
small cells have appeared (bl) which form a blastoderm enveloping the large yolk-
spheres. In C the blastoderm has become divided into two layers.

B. III. 22

338 ORIGIN OF THE GASTRULA.

tion of the epiblast and hypoblast is a secondary modification of
an invaginate type (vide Vol. II. p. 457).

The type of some Turbellaria (Stylochopsis ponticus) and that
of Nephelis amongst the Discophora is not capable of being
reduced to the invaginate type.

The development of almost all the parasitic groups, i.e. the
Trematoda, the Cestoda, the Acanthocephala, and the Lingua-
tulida, and also of the Tardigrada, Pycnogonida, and other
minor groups, is too imperfectly known to be classed with either
the delaminate or invaginate types.

It will, I think, be conceded on all sides that, if any of the
ontogenetic processes by which a gastrula form is reached are
repetitions of the process by which a simple two-layered gastrula
was actually evolved from a compound Protozoon, these pro-
cesses are most probably of the nature either of invagination or
of delamination.

The much disputed questions which have been raised about
the gastrula and planula theories, originally put forward by
Haeckel and Lankester, resolve themselves then into the simple
question, whether any, and if so which, of the ontogenetic
processes by which the gastrula is formed are repetitions of the
phylogenetic origin of the gastrula.

It is very difficult to bring forward arguments of a conclusive
kind in favour of either of these processes. The fact that
delaminate and invaginate gastrulse are in several instances
found coexisting in the same group renders it certain that there
are not two independent phyla 'of the Metazoa, derived respec-
tively from an invaginate and a delaminate gastrula1.

1 It is not difficult to picture a possible derivation of delamination from invagina-
tion ; while a comparison of the formation of the inner layers (mesoblast and hypo-
blast) in Ascetta (amongst the Sponges), and in the Echinodermata, shews a very
simple way in which it is possible to conceive of a passage of delamination into
invagination. In Ascetta the cells, which give rise to the mesoblast and hypoblast, are
budded off from the inner wall of the blastosphere, especially at one point ; while in
Echinodermata (fig. 199) there is a small invaginated sack which gives rise to the
hypoblast, while from the walls of this sack amoeboid cells are budded off which give
rise to a large part of the mesoblast. If we suppose the hypoblast cells budded off
at one point in Ascetta gradually to form an invaginated sack, while the mesoblast
cells continued to be budded off as before, we should pass from the delaminate type of
tta t<> the invaginate type of an Echinoderm.

ORIGIN OF THE GERMINAL LAYERS. 339

The four most important cases in which the two processes
coexist are the Porifera, the Coelenterata, the Nemertea, and the
Brachiopoda. In the cases of the Porifera and Ccelenterata,
there do not appear to me to be any means of deciding which of
these processes is derived from the other ; but in the Nemertea
and the Brachiopoda the case is different. In all the types of
Nemertea in which the development is relatively not abbre-
viated there is an invaginate gastrula, while in the types with a
greatly abbreviated development there is a delaminate gastrula.
It would seem to follow from this that a delaminate gastrula has
here been a secondary result of an abbreviation in the develop-
ment. In the Brachiopoda, again, the majority of types develop
by a process of invagination, while Thecidium appears to
develop by delamination ; here also the delaminate type would
appear to be secondarily derived from the invaginate.

If these considerations are justified, delamination must be in
some instances secondarily derived from invagination ; and this
fact is so far an argument in favour of the more primitive nature
of invagination ; though it by no means follows that in the
invaginate process the steps by which the Metazoa were derived
from the Protozoa are preserved.

It does not, therefore, seem possible to decide conclusively in
favour of either of these processes by a comparison of the cases
where they occur in the same groups.

The relative frequency of the two processes supplies us with
another possible means for deciding between them ; and there is
no doubt that here again the scale inclines towards invagination.
It must, however, be borne in mind that the frequency of the
process of invagination admits of another possible explanation.
There is a continual tendency for the processes of development
to be abbreviated and simplified, and it is quite possible that the
frequent occurrence of invagination is due to the fact of its
being, in most cases, the simplest means by which the two-
layered condition can be reached. But this argument can have
but little weight until it can be shewn in each case that invagi-
nation is a simpler process than delamination ; and it is rendered
improbable by the cases already mentioned in which delami-
nation has been secondarily derived from invagination.

If it were the case that the blastopore had ih all types the

22 — 2

340

BLASTOPORE.

same relation to the adult mouth, there would be strong grounds
for regarding the invaginate gastrula as an ancestral form ; but
the fact that this is by no means so is an argument of great
weight in favour of some other explanation of the frequency of
invagination.

The force of this consideration can best be displayed by a
short summary of the fate of the blastopore in different forms.

The fate of the blastopore is so variable that it is difficult
even to classify the cases which have been described.

(1) It becomes the permanent mouth in the following forms1:
Ccelenterata.— Pelagia, Cereanthus.

Turbellaria. — Leptoplana (?), Thysanozoon.

Nemertea. — Pilidium, larvae of the type of Desor.

Mollusca.— In numerous examples of most Molluscan groups, except the

Cephalopoda.

Chcetopoda. — Most Oligochaeta, and probably many Polychseta.
Gephyrea. — Phascolosoma, Phoronis.
Nematelminthes.— Cucullanus.

(2) It closes in the position where the mouth is subsequently formed.
Ccelenterata. — Ctenophora (?).

Mollusca. — In numerous examples of most Molluscan groups, except the

Cephalopoda.
Crustacea.— Cirripedia (?), some Cladocera (Moina) (?).

(3) It becomes the permanent anus.
Mollusca. — Paludina.

Chatopoda. — Serpula and some other types.
Echinodermata.—Mmosl universally, except amongst the Crinoidea.

(4) It closes in the position where the anus is subsequently formed.
Echinodermata. — Crinoidea.

(5) It closes in a position which does not correspond or is not known
to correspond2 either with the future mouth or anus. — Porifera — Sycandra.
Ccelenterata — Chrysaora*, Aurelia*. Nemertea*— Some larvae which develop without
a metamorphosis. Rolifera*. Mollusca — Cephalopoda. Polyzoa*. Brachiopoda —
Argiope, Terebratula, Terebratulina. Ch(Etopoda — Euaxes. Discophora — Clepsine.
Gephyrea — Bonellia*. Chatognatha. Crustacea — Decapoda. Chordata.

The forms which have been classed together under the last
heading vary considerably in the character of the blastopore.
In some cases the fact of its not coinciding either with the mouth

1 The above list is somewhat tentative ; and future investigations will probably
shew that many of the statements at present current about the position of the blasto-
pore are inaccurate.

2 The forms in which the position of the blastopore in relation to the mouth or
anus is not known <ire marked with an asterisk.

ORIGIN OF THE GERMINAL LAYERS. 341

or anus appears to be due simply to the presence of a large
amount of food-yolk. The cases of the Cephalopoda, of Euaxes,
and perhaps of Clepsine and Bonellia, are to be explained in
this way : in the case of all these forms, except Bonellia, the
blastopore has the form of an elongated slit along the ventral
surface. This type of blastopore is characteristic of the Mollusca
generally, of the Polyzoa, of the Nematelminthes, and very
possibly of the Chaetopoda and Discophora. In the Chaeto-
gnatha (fig. 209 B) the blastopore is situated, so far as can be
determined, behind the future anus. In many Decapoda the
blastopore is placed behind, but not far from, the anus. In the
Chordata it is also placed posteriorly to the anus, and,
remarkably enough, remains, in a large number of forms, for
some time in connection with the neural tube by a neurenteric
canal.

The great variations in the character of the gastrula, indicated
in the above summary, go far to shew that if the gastrulae, as we
find them in most types, have any ancestral characters, these
characters can only be of the most general kind. This may
best be shewn by the consideration of a few striking instances.
The blastopore in Mollusca has an elongated slit-like form,
extending along the ventral surface from the mouth to the
anus. In Echinodermata it is a narrow pore, remaining as the
anus. In most Chsetopoda it is a pore remaining as the mouth,
but in some as the anus. In Chordata it is a posteriorly-
placed pore, opening into both the archenteron and the neural
canal.

It is clearly out of the question to explain all these differences
as having connection with the characters of ancestral forms.
Many of them can only be accounted for as secondary adap-
tations for the convenience of development.

The epibolic gastrula of Mammalia (vide pp. 215 and 291) is
a still more striking case of a secondary embryonic process, and
is not directly derived from the gastrula of the lower Chordata.
It probably originated in connection with the loss of food-yolk
which took place on the establishment of a placental nutrition
for the foetus. The epibolic gastrula of the Scorpion, of Isopods,
and of other Arthropoda, seems also to be a derived gastrula.
These instances of secondary gastrulse are very probably by no

342 BLASTOPORE.

means isolated, and should serve as a warning against laying too
much stress upon the frequency of the occurrence of invagination.
The great influence of the food-yolk upon the early development
might be illustrated by numerous examples, especially amongst
the Chordata (vide Chapter XL).

If the descendants of a form with a large amount of food-yolk
in its ova were to produce ova with but little food-yolk, the type
of formation of the germinal layers which would thereby result
would be by no means the same as that of the ancestors of the
forms with much food-yolk, but would probably be something
very different, as in the case of Mammalia. Yet amongst the
countless generations of ancestors of most existing forms, such
oscillations in the amount of the food-yolk must have occurred
in a large number of instances.

The whole of the above considerations point towards the
view that the formation of the hypoblast by invagination, as it
occurs in most forms at the present day, can have in many
instances no special phylogenetic significance, and that the
argument from frequency, in favour of invagination as opposed
to delamination, is not of prime importance.

A third possible method of deciding between delamination
and invagination is to be found in the consideration as to which
of these processes occurs in the most primitive forms. If there
were any agreement amongst primitive forms as to the type of
their development this argument might have some weight. On
the whole, delamination is, no doubt, characteristic of many
primitive types, but the not infrequent occurrence of invagination
in both the Ccelenterata and the Porifera — the two groups which
would on all hands be admitted to be amongst the most
primitive — deprives this argument of much of the value it might
otherwise have.

To sum up — considering the almost indisputable fact that
both the processes above dealt with have in many instances had
a purely secondary origin, no valid arguments can be produced
to shew that either of them reproduces the mode of passage
between the Protozoa and the ancestral two-layered Metazoa.
These conclusions do not, however, throw any doubt upon the
fact that the gastrula, however evolved, was a primitive form of
the Metazoa ; since this conclusion is founded upon the actual

ORIGIN OF TIIH GERMINAL LAYERS.

343

existence of adult gastrula forms independently of their occur-
rence in development.

Though embryology does not at present furnish us with a definite
answer to the question how the Metazoa became developed from the Pro-
tozoa, it is nevertheless worth while reviewing some of the processes by
which this can be conceived to have occurred.

On purely a priori grounds there is in my opinion more to be said for
invagination than for any other view.

On this view we may suppose that the colony of Protozoa in the course
of conversion into Metazoa had the form of a blastosphere ; and that at
one pole of this a depression appeared. The cells lining this depression we

F

FIG. 205. DIAGRAM SHEWING THE FORMATION OF A GASTRULA IJY
DELAMINATION. (From Lankester.)

Fig. i, ovum; fig. 2, stage in segmentation; fig. 3, commencement of delamination
after the appearance of a central cavity ; fig. 4, delamination completed, mouth form-
ing at M. In figs, i, 2, and 3, EC, is ectoplasm, and En. is endoplasm. In fig. 4,
EC. is epiblast, and En. hypoblast. E. and F. food particles.

may suppose to have been amoeboid, and to have carried on the work of
digestion ; while the remaining cells were probably ciliated. The digestion
may be supposed to have been at first carried on in the interior of the cells,
as in the Protozoa; but, as the depression became deeper (in order to
increase the area of nutritive cells and to retain the food) a digestive
secretion probably became poured out from the cells lining it, and the mode
of digestion generally characteristic of the Metazoa was thereby inaugurated.
It may be noted that an intracellular protozoon type of digestion persists in
the Porifera, and appears also to occur in many Ccelenterata, Turbellaria,

344 PASSAGE FROM THE PROTOZOA TO THE METAZOA.

&c., though in most of these cases both kinds of digestion probably go on
simultaneously1.

Another hypothetical mode of passage, which fits in with delamination,
has been put forward by Lankester, and is illustrated by fig. 205. He
supposes that at the blastosphere stage the fluid in the centre of the colony
acquired special digestive properties ; the inner ends of the cells having at
this stage somewhat different properties from the outer, and the food being
still incepted by the surface of the cells (fig. 205, 3). In a later stage of the
process the inner portions of the cells became separated off as the hypo-
blast ; while the food, though still ingested in the form of solid particles by
the superficial cells, was carried through the protoplasm into the central
digestive cavity. Later (fig. 205, 4), the point where the food entered became
localised, and eventually a mouth became formed at this point.

The main objection which can be raised against Lankester's view is that
it presupposes a type of delamination which does not occur in nature except
in Geryonia.

Metschnikoff has propounded a third view with reference to delamina-
tion. He starts as before with a ciliated blastosphere. He next supposes
the cells from the walls of this to become budded off into the central cavity,
as in Eucope (fig. 202), and to lose their cilia. These cells give rise to
an internal parenchyma, which carries on an intracellular digestion. At a
later stage a central digestive cavity is supposed to be formed. This view
of the passage from the protozoon to the metazoon state, though to my
mind improbable in itself, fits in very well with the ontogeny of the lower
Hydrozoa.

Another view has been put forward by myself in the chapter on the
Porifera*, to the effect that the amphiblastula larva of Calcispongias may
be a transitional form between the Protozoa and the Metazoa, composed of
a hemisphere of nutritive amoeboid cells, and a hemisphere of ciliated cells.
The absence of such a larval form in the Ccelenterata and higher Metazoa
is opposed, however, to this larva being regarded as a transitional form,
except for the Porifera.

It is obvious that so long as there is complete uncertainty as
to the value to be attached to the early developmental processes,
it is not possible to decide from these processes whether there is
only a single metazoon phylum or whether there may not be two
or more such phyla. At the same time there appear to be strong

1 J. Parker, "On the Histology of Hydra fusca," Quart. Journ. Micr. Science,
vol. xx. 1880; and El. Metschnikoff, " Ueb. die intracellulare Verdauung bei Ccelen-
teraten," Zoologischer Anzeiger, No. 56, vol. in. 1880 and Lankester, " On the
intracellular digestion and endoderm of Limnocodium, " Quart. Journ. Micr. Science,
vol. xxi. 1881.

! Vol. n. p. 149.

ORIGIN OF THE GERMINAL LAYERS. 345

arguments for regarding the Porifera as a phylum of the
Metazoa derived independently from the Protozoa. This seems
to me to be shewn (i) by the striking larval peculiarities of the
Porifera ; (2) by the early development of the mesoblast in the
Porifera, which stands in strong contrast to the absence of this
layer in the embryos of most Ccelenterata ; and above all, (3) by
the remarkable characters of the system of digestive channels.
A further argument in the same direction is supplied by the fact
that the germinal layers of the Sponges very probably do not
correspond physiologically to the germinal layers of other types.
The embryological evidence is insufficient to decide whether the
amphiblastula larva is, as suggested above, to be regarded as the
larval ancestor of the Porifera.

Homologies of the germinal layers. The question as to
how far there is a complete homology between the two primary
germinal layers throughout the Metazoa was the third of the
questions proposed to be discussed here.

Since there are some Metazoa with only two germinal layers,
and other Metazoa with three, and since, as is shewn in the
following section, the third layer or mesoblast can only be
regarded as a derivative of one or both the primary layers, it is
clear that a complete homology between the two primary germinal
layers does not exist.

That there is a general homology appears on the other hand
hardly open to doubt.

The primary layers are usually continuous with each other,
near one or both (when both are present) the openings of the
alimentary tract.

As a rule an oral and anal section of the alimentary tract
— the stomodaeum and proctodaeum — are derived from the
epiblast ; but the limits of both these sections are so variable,
sometimes even in closely allied forms, that it is difficult to avoid
the conclusion that there is a border-land between the epiblast
and hypoblast, which appears by its development to belong in
some forms to the epiblast and in other forms to the hypoblast.
If this is not the case it is necessary to admit that there are
instances in which a very large portion of the alimentary canal
is phylogenetically an epiblastic structure. In some of the
Isopods, for example, the stomodaeum and proctodaeum give

346 ORIGIN OF THE MESOBLAST.

rise to almost the whole of the alimentary canal with its appen-
dages, except the liver.

The origin of the Mesoblast. A diploblastic condition of
the organism preceded, as we have seen, the triploblastic. The
epiblast during the diploblastic condition was, as appears from
such forms as Hydra, especially the sensory and protective
layer, while the hypoblast was the secretory and assimilating
layer; both layers giving rise to muscular elements. It must
not, however, be supposed that in the early diploblastic ancestors
there was a complete differentiation of function, but there is
reason to think that both the primary layers retained an
indefinite capacity for developing into any form of tissue1. The
fact of the triploblastic condition being later than the diploblastic
proves in a conclusive way that the mesoblast is a derivative of
one or both the primary layers. In the Ccelenterata we can
study the actual origin from the two primary layers of various
forms of tissue which in the higher types are derived from the
mesoblast2. This fact, as well as general a priori considerations,
conclusively prove that the mesoblast did not at first
originate as a mass of independent cells between the
two primary layers, but that in the first instance it
gradually arose as differentiations of the two layers,
and that its condition in the embryo as an indepen-
dent layer of undifferentiated cells is a secondary
condition, brought about by the general tendency

1 The Hertwigs (No. 270) have for instance shewn that nervous structures are
developed in the hypoblast in the Actinozoa and other Coelenterata.

2 There is considerable confusion in the use of the names for the embryonic layers.
In some cases various tissues formed by differentiations of the primary layers have
been called mesoblast. Schultze, and more recently the Hertwigs, have pointed out
the inconvenience of this nomenclature. In the case of the Coelenterata it is difficult
to decide in certain instances (e.g. Sympodium) whether the cells which give rise to a
particular tissue of the adult are to be regarded as forming a mesoblast, i.e. a middle
undifferentiated layer of cells, or whether they arise as already histologically differ-
entiated elements from one of the primary layers. The attempt to distinguish by a
special nomenclature the epiblast and hypoblast after and before the separation of the
mesoblast, which has been made by Allen Thomson (No. 1), appears incapable of
being consistently applied, though it is convenient to distinguish a primary and a
secondary hypoblast. A proposal of the Hertwigs to adopt special names for the
outer and inner limiting membranes of the adult, and for the interposed mass of
organs, appears to me unnecessary.

ORIGIN OF THE GERMINAL LAYERS. 347

towards a simplification of development, and a re-
tardation of histological differentiation1.

The Hertwigs have recently attempted (No. 271) to distin-
guish two types of differentiation of the mesoblast, viz. (i) a
direct differentiation from the primitive epithelial cells ; (2) a
differentiation from primitively indifferent cells budded off into
the gelatinous matter between the two primary layers.

It is quite possible that this distinction may be well founded, but no
conclusive evidence of the occurrence of the second process has yet been
adduced. The Ctenophora are the type upon which special stress is laid,
but the early passage of amoeboid cells into the gelatinous tissue, which
subsequently become muscular, is very probably an embryonic abbreviation ;
and it is quite possible that these cells may phylogenetically have origi-
nated from epithelial cells provided with contractile processes passing
through the gelatinous tissue.

The conversion of non-embryonic connective-tissue cells into muscle cells
in the higher types has been described, but very much more evidence is
required before it can be accepted as a common occurrence.

In addition to the probably degraded Dicyemida:: and Ortho-
nectidae, the Ccelenterata are the only group in which a true
mesoblast is not always present. In other words, the Ccelen-
terata are the only group in which there is not found in the
embryo an undifferentiated group of cells from which the
majority of the organs situated between the epidermis and the
alimentary epithelium are developed.

The organs invariably derived, in the triploblastic forms,
from the mesoblast, are the vascular and lymphatic systems, the
muscular system, and the greater part of the connective tissue
and the excretory and generative (?) systems. On the other
hand, the nervous systems (with a few possible exceptions) and
organs of sense, the epithelium of most glands, and a few
exceptional connective-tissue organs, as for example the noto-
chord, are developed from the two primary layers.

The fact of the first-named set of organs being invariably
derived from the mesoblast points to the establishment of the
two following propositions: — (i) That with the differenti-

1 The causes which give rise to a retardation of histological differentiation will be
dealt with in the second part of this chapter which deals with larval characters and
larval forms.

348 ORIGIN OF THE MESOBLAST.

ation of the mesoblast as a distinct layer by the process
already explained, the two primary layers lost for the
most part the capacity they primitively possessed of
giving rise to muscular and connective-tissue diffe-
rentiations1, to the epithelium of the excretory organs,
and to generative cells. (2) That the mesoblast through-
out the triploblastic Metazoa, in so far as these forms
have sprung from a common triploblastic ancestor, is
an homologous structure.

The second proposition follows from the first. The mesoblast
can only have ceased to be homologous throughout the triplo-
blastica by additions from the two primary layers, and the
existence of such additions is negatived by the first proposition.

These two propositions, which hang together, are possibly
only approximately true, since it is quite possible that future
investigations may shew that differentiations of the two primary
layers are not so rare as has been hitherto imagined.

Ranvier2 finds that the muscles of the sweat-glands are developed from
the inner part of the layer of epiblast cells, invaginated to form these
glands.

Gotte3 describes the epiblast cells of the larva of Comatula as being at a
certain stage contractile and compares them with the epithelio-muscular
cells of Hydra. These cells would appear subsequently to be converted
into a simple cuticular structure.

It is moreover quite possible that fresh differentiations from
the two primary layers may have arisen after the triploblastic
condition had been established, and by the process of simplifi-
cation of development and precocious segregation, as Lankester
calls it, have become indistinguishable from the normal meso-
blast. In spite of these exceptions it is probable that the major
part of the muscular system of all existing triploblastic forms
has been differentiated from the muscular system of the ancestor
or ancestors (if there is more than one phylum) of the triplo-

1 The connective-tissue test of the Tunicata, though derived from the epiblast, is
not really an example of such a differentiation.

1 M. L. Ranvier. " Sur la stricture des glandes sudoripares." Comptes Rendus,
Dec. 29, 1879.

1 A. Gotte, "Vergleich. Entwick. d. Comatula mediterranea." Archiv f. mikr.
Anat. vol. XI I. p. 597.

ORIGIN OF THE GERMINAL LAYERS.

349

blastica. In the case of other tissues there are a few instances
which might be regarded as examples of an organ primitively
developed in one of the two primary layers having become
secondarily carried into the mesoblast. The notochord has
sometimes been cited as such an organ, but, as indicated in a
previous chapter, it is probable that its hypoblastic origin can
always be demonstrated.

A

FIG. 206. EPIBOLIC GASTRULA OF BONELLIA. (After Spengel.)

A. Stage when the four hypoblast cells are nearly enclosed.

B. Stage after the formation of the mesoblast has commenced by an infolding of
the lips of the blastopore.

ep. epiblast; me. mesoblast; bl. blastopore.

The nervous system, although imbedded in mesoblastic
derivates in the adults of all the higher triploblastica, retains
with marvellous constancy its epiblastic origin (though it is
usually separated from the epiblast prior to its histogenic
differentiation) ; yet in the Cephalopoda, and some other
Mollusca, the evidence is in favour of its developing in the
mesoblast. Should future investigations confirm these conclu-
sions, a good example will be afforded of an organ changing the
layer from which it usually develops1. The explanation of such
a change would be precisely the same as that already given for
the mesoblast as a whole.

The actual mode of origin of various tissues, which in the
true triploblastic forms arise in mesoblast, can be traced in the

1 The Hertvvigs hold that there is a distinct part of the nervous system which was
at first differentiated in the mesoblast in many types, amongst others the Mollusca.
The evidence in favour of this view is extremely scanty and the view itself appears to
me highly improbable.

350 ORIGIN OF THE MESOBLAST.

Ccelenterata1. In this group the epiblast and hypoblast both
give rise to muscular and connective-tissue elements ; and
although the main part of the nervous system is formed in the
epiblast, it seems certain that in some types nerves may be
derived from the hypoblast'2. These facts are extremely interest-

FlG. 207. TWO TRANSVERSE SECTIONS THROUGH EMBRYOS OF HYDROPHILUS

PICEUS. (After Kowalevsky.)

A. Section through an embryo at the point where the two germinal folds most
approximate.

B. Section through an embryo, in the anterior region where the folds of the
amnion have not united.

gg. germinal groove; me. mesoblast; am. amnion; yk. yolk.

ing, but it is by no means certain that any conclusions can
be directly drawn from them as to the actual origin of the
mesoblast in the triploblastic forms, till we know from what
diploblastic forms the triploblastica originated. All that they
shew is that any of the constituents of the mesoblast may have
originated from either of the primitive layers.

1 The reader is referred for this subject to the valuable memoirs which have been
recently published by the Hertwigs, especially to No. 270. He will find a general
account of the subject written before the appearance of the Hertwigs' memoir in
pp. 180-182 of Volume II. of this treatise.

- It would be interesting to know the history of the various nervous structures
found in the walls of the alimentary tract in the higher forms. I have shewn
(Development of Elasmobranch Fishes, p. 172) that the central part of the sympa-
thetic system is derived from the epiblast. It would however be well to work over
the development of Auerbach's plexus.

ORIGIN OF THE GERMINAL LAYERS.

351

For further light as to the origin of the mesoblast, it is
necessary to turn to its actual development.

The following summary illustrates the more important
modes in which the mesoblast originates.
B

FIG. 208. FIGURES ILLUSTRATING THE DEVELOPMENT OF ASTACUS.
(From Parker; after Reichenbach. )

A. Section through part of the ovum during segmentation, n. nuclei ; w.y. white
yolk ; y.p. yolk pyramids ; c. central yolk mass.

B. and C. Longitudinal sections of the gastrula stage, a. archenteron ; b. blasto-
pore; ms. mesoblast; ec. epiblast; en. hypoblast, distinguished from epiblast by
shading.

D. Highly magnified view of anterior lip of blastopore, to shew the origin of the
primary mesoblast from the wall of the archenteron. /. ms. primary mesoblast ; ec.
epiblast ; en. hypoblast.

E. Two hypoblast cells to shew the amoeba-like absorption of yolk spheres.
y. yolk ; n. nucleus ; p. pseudopodial process.

F. Hypoblast cells giving rise endogenously to the secondary mesoblast (s.tns.) ;
n. nucleus.

I. It grows inwards from the lips of the blastopore as a pair
of bands. In these cases it may originate (a) from cells which
are clearly hypoblastic, (b} from cells which are clearly epiblastic,
(c) from cells which cannot be regarded as belonging to either
layer.

Mollusca. — Gasteropoda, Cephalopoda, and Lamellibranchiata. In
Gasteropoda and Lamellibranchiata the mesoblast sometimes originates

352

DEVELOPMENT OF THE MESOBLAST.

from a pair of cells at the lips of the blastopore, though very probably some
of the elements subsequently come from the epiblast ; and in Cephalopoda
it begins as a ring of cells round the edge of the blastoderm.

Polyzoa Entoprocta. — It originates from a pair of cells at the lips of the
blastopore.

Chaetopoda. — Euaxes. It arises as a ridge of cells at the lips of the
blastopore (fig. 200).

Gephyrea. — Bonellia. It arises (fig. 206) as an infolding of the epi-
blastic lips of the blastopore.

Nematelminthes. — Cucullanus. It grows backwards from the hypoblast
cells at the persistent oral opening of the blastopore.

Tracheata. — Insecta. It grows inwards from the lips of the germinal
groove (fig. 207), which probably represent the remains of a blastopore.
Part of the mesoblast is probably also derived from the yolk-cells. A similar
though more modified development of the mesoblast occurs in the Araneina
(fig. 214).

Crustacea. — Decapoda. It partly grows in from the hypoblastic lips of
the blastopore, and is partly derived from the yolk-cells (fig. 208).

FIG. tog. THREE STAGES IN THE DEVELOPMENT OF SAGITTA. (A. and C.

after Biitschli, and B. after Kowalevsky.)
The three embryos are represented in the same positions.

A. Represents the gastrula stage.

B. Represents a succeeding stage, in which the primitive archenteron is com-
mencing to be divided into three.

C. Represents a later stage, in which the mouth involution (m) has become con-
tinuous with the alimentary tract, and the blastopore has become closed.

m. mouth; al. alimentary canal; ae. archenteron; bl.p. blastopore; pv. peri-
visceral cavity; sp. splanchnic mesoblast; so. somatic mesoblast; ge. generative
organs.

2. The mesoblast is developed from the walls of hollow
outgrowths of the archenteron, the cavities of which become
the body cavity.

ORIGIN OF THE GERMINAL LAYERS.

353

Brachiopoda. — The walls of a pair of outgrowths form the whole of the
mesoblast.

Chaetognatha. — The mesoblast arises in the same manner as in the
Brachiopoda (fig. 209).

Echinodermata. — The lining of the peritoneal cavity is developed from
the walls of outgrowths of the archenteron, but the greater part of the meso-
blast is derived from the amoeboid cells budded off from the walls of the
archenteron (fig. 210).

ME

Mp. Pld.

FIG. 210. LONGITUDINAL SECTION THROUGH AN EMBRYO OF CUCUMARIA
DOL1OLUM AT THE -END OF THE FOURTH DAY.

Vpv. vaso-peritoneal vesicle; ME. mesenteron; Sip., Ptd. blastopore, proc-
todjeum.

Enteropneusta (Balanoglossus).— The body cavity is derived from two
pairs of alimentary diverticula, the walls of which give rise to the greater
part of the mesoblast.

Chordata. — Paired archenteric outgrowths give rise to the whole meso-
blast in Amphioxus (fig. 211), and the mode of formation of the mesoblast in
other Chordata is probably secondarily derived from this.

3. The cells which will form the mesoblast become marked
out very early, and cannot be regarded as definitely springing
from either of the primary layers.

Turbellaria. — Leptoplana (fig. 212), Planaria polychroa (?).

Chsetopoda. — Lumbricus, &c.

Discophora.

It is very possible that the cases quoted under this head ought more
properly to belong to group i.

4. The mesoblast cells are split off from the epiblast.

Nemertea. — Larva of Desor. The mesoblast is stated to be split off
from the four invaginated discs.

B. III. 23

354

DEVELOPMENT OF THE MESOBLAST.

5. The mcsoblast is split off from the hypoblast.

Nemertea. — Some of the types without a metamorphosis.

Mollusca. — Scaphopoda. It is derived from the lateral and ventral cells
of the hypoblast.

Oephyrea. — Phascolosoma.

Vertebrata. — In most of the Ichthyopsida the mesoblast is derived from
the hypoblast (fig. 213). In some types (i.e. most of the Amniota) the meso-
blast might be described as originating at the lips of the blastopore (primi-
tive streak).

6. The mesoblast is derived from both germinal layers.

Tracheata. — Araneina (fig. 214). It is derived partly from cells split off
from the epiblast and partly from the yolk-cells ; but it is probable that the
statement that the mesoblast is derived from both the germinal layers is
only formally accurate ; and that the derivation of part of the mesoblast
from the yolk-cells is not to be interpreted as a derivation from the
hypoblast.

Amniota. — The derivation of the mesoblast of the Amniota from both
the primary germinal layers is without doubt a secondary process.

The conclusions to be drawn from the above summary are by
no means such as might have been anticipated. The analogy
of the Ccelenterata would lead us to expect that the mesoblast

FIG. 211.

A.
B.

SECTIONS OF AN AMPH'IOXUS EMBRYO AT TIIRKE STAGES.

(After Kowalevsky.)
Section at gastrula stage.
Section of a somewhat older embryo.
C. Section through the anterior part of still older embryo.
np. neural plate; nc. neural canal; mcs. archenteron in A, and mesenteron in B
and C ; ch. notochord ; so. mesoblastic somite.

would be derived partly from the epiblast and partly from the
hypoblast. Such, however, is not for the most part the case,
though more complete investigations may shew that there are a
greater number of instances in which the mesoblast has a mixed
origin than might be supposed from the above summary.

ORIGIN OF THE GERMINAL LAYERS. 355

I have attempted to reduce the types of development of the
mesoblast to six ; but owing to the nature of the case it is not
always easy to distinguish the first of these from the last four-
Of the six types the second will on most hands be admitted to
be the most remarkable. The formation of hollow outgrowths
of the archenteron, the cavities of which give rise to the body
cavity, can only be explained on the supposition that the body
cavity of the types in which such outgrowths occur is derived
from diverticula cut off from the alimentary tract. The lining
epithelium of the diverticula — the peritoneal epithelium — is
clearly part of the primitive hypoblast, and this part of the
mesoblast is clearly hypoblastic in origin.

FIG. 112. SECTIONS THROUGH THE OVUM OF LEPTOPLANA TREMELLARIS IN

THREE STAGES OF DEVELOPMENT. (After Hallez.)
cp. epiblast ; ;;/. mesoblast; hy. yolk-cells (hypoblast); bl. blastopore.

In the case of the Chaetognatha (Sagitta), Brachiopoda, and
Amphioxus, the whole of the mesoblast originates from the walls
of the diverticula ; while in the Echinodermata the walls of the
diverticula only give rise to the vaso-peritoneal epithelium, the
remainder of the mesoblast being derived from amoeboid cells
which spring from the walls of the archenteron before the origin
of the vaso-peritoneal outgrowths (figs. 199 and 210).

Reserving for the moment the question as to what conclu-
sions can be deduced from the above facts as to the origin of the
mesoblast, it is important to determine how far the facts of
embryology warrant us in supposing that in the whole of the
triploblastic forms the body cavity originated from the alimentary
diverticula. There can be but little doubt that the mode of
origin of the mesoblast in many Vertebrata, as two solid plates
split off from the hypoblast, in which a cavity is secondarily
developed, is an abbreviation of the process observable in
Amphioxus ; but this process approaches in some forms of

23—2

356

ORIGIN OF THE MESOBLAST.

Vertebrata to the ingrowth of the mesoblast from the lips of the
blastopore.

It is, therefore, highly _ en A.

probable that the paired in-
growths of the mesoblast from
the lips of the blastopore may
have been in the first instance
derived from a pair of arch-
enteric diverticula. This pro-
cess of formation of the meso-
blast is, as may be seen by
reference to the summary,
the most frequent, including
as it does the Chaetopoda,

the Mollusca, the Arthro- FIG. 213. Two SECTIONS OF A YOUNG
101 ELASMOBRANCH EMBRYO, TO SHEW THE

pOda, &C. MESOBLAST SPLIT OFF AS TWO LATERAL

MASSES FROM THE HYPOBLAST.

While there is no difficulty in
the view that the body cavity may
have originated from a pair of en-
teric diverticula in the case of the
forms where a body cavity is present, there is a considerable difficulty in
holding this view, for forms in which there is no body cavity distinct from
the alimentary diverticula.

Of these types the Platyelminthes are the most striking. It is, no doubt,
possible that a body cavity may have existed in the Platyelminthes, and
become lost ; and the case of the Discophora, which in their muscular and
connective tissue systems as well as in the absence of a body cavity resemble
the Platyelminthes, may be cited in favour of this view, in that, being closely
related to the Chaetopoda, they are almost certainly descended from ancestors
with a true body cavity. The usual view of the primitive character of the

nig. medullary groove ; ep. epiblast ; ;;/.
mesoblast ; hy. hypoblast ; n.al. cells formed
around the nuclei of the yolk which have
entered the hypoblast.

1 The wide occurrence of this process was first pointed out by Rabl. He holds,
however, a peculiar modification of the gastrsea theory, for which I must refer the
reader to his paper (No. 284) ; according to this theory the mesoblast has sprung
from a zone of cells of the blastosphere, at the junction between the cells which will
be invaginated and the epiblast cells. In the bilateral blastosphere, from which he
holds that all the higher forms (Bilateralia) have originated, these cells had a
bilateral arrangement, and thus the bilateral origin of the mesoblast is explained.
The origin of the mesoblast from the lips of the blastopore is explained by the
position of its mother-cells in the blastosphere. It need scarcely be said that the
views already put forward as to the probable mode of origin of the mesoblast,
founded on the analogy of the Ccelenterata, are quite incompatible with Rabl's
theories.

ORIGIN OF THE GERMINAL LAYERS. 357

Platyelminthes, which has much to support it, is, however, opposed to the
idea that the body cavity has disappeared.

If Kowalevsky1 is right in stating that he has found a form intermediate
between the Ccelenterata and the Platyelminthes, there will be strong grounds
for holding that the Platyelminthes are, like the Ccelenterata, forms the
ancestors of which were not provided with a body cavity.

Perhaps the triploblastica are composed of two groups, viz. (i) a more
ancestral group (the Platyelminthes), in which there is no body cavity as dis-

FIG. 214. SECTION THROUGH AN EMBRYO OK AGEI.ENA LABYRINTHICA.
The section is represented with the ventral plate upwards. In the ventral plate
is seen a keel-like thickening, which gives rise to the main mass of the mesoblast.
yk. yolk divided into large polygonal cells, in several of which are nuclei.

tinct from the alimentary, and (2) a group descended from these, in which
two of the alimentary diverticula have become separated from the alimen-
tary tract to form a body cavity (remaining triploblastica). However this
may be, the above considerations are sufficient to shew how much there is
that is still obscure with reference even to the body cavity.

If embryology gives no certain sound as to the questions just
raised with reference to the body cavity, still less is it to be
hoped that the remaining questions with reference to the origin
of the mesoblast can be satisfactorily answered. It is clear, in
the first place, from an inspection of the summary given above,
that the process of development of the mesoblast is, in all the
higher forms, very much abbreviated and modified. Not only is
its differentiation relatively deferred, but it does not in most
cases originate, as it must have done to start with, as a more or

1 Zoologischer Anzeiger, No. 52, p. 140. This form has been named by Kowa-
levsky Cceloplana Metschnikowii. Kowalevsky's description appears, however, to be
quite compatible with the view that this form is a creeping Ctenophor, in no way
related to the Turbellarians.

358 EVOLUTION OK THE MESOBLAST.

less continuous sheet, split off from parts of one or both the
primary layers. It originates in most cases from the hypoblast,
and although the considerations already urged preclude us from
laying very great stress on this mode of origin, yet the deriva-
tion of the mesoblast from the walls of archenteric outgrowths
suggests the view that the whole, or at any rate the greater part,
of the mesoblast primitively arose by a process of histogenic
differentiation from the walls of the archenteron or rather from
diverticula of these walls. This view, which was originally put
forward by myself (No. 260), appears at first sight very
improbable, but if the statement of the Hertwigs (No. 270), that
there is a large development of a hypoblastic muscular system
in the Actinozoa, is well founded, it cannot be rejected as
impossible. Lankester (No. 279), on the other hand, has urged
that the mode of origin of the mesoblast in the Echinodermata
is more primitive ; and that the amoeboid cells which here give
rise to the muscular and connective tissues represent cells which
originally arose from the whole inner surface of the epiblast. It
is, however, to be noted that even in the Echinodermata the
amoeboid cells actually arise from the hypoblast, and their mode
of origin may, therefore, be used to support the view that the
main part of the muscular system of higher types is derived
from the primitive hypoblast.

The great changes which have taken place in the develop-
ment of the mesoblast would be more intelligible on this view
than on the view that the major part of the mesoblast primitively
originated from the epiblast. The presence of food-yolk is
much more frequent in the hypoblast than in the epiblast ; and
it is well known that a large number of the changes in early
development are caused by food-yolk. If, therefore, the meso-
blast has been derived from the hypoblast, many more changes
might be expected to have been introduced into its early
development than if it had been derived from the epiblast. At
the same time the hypoblastic origin of the mesoblast would
assist in explaining how it has come about that the development
of the nervous system is almost always much less modified than
that of the mesoblast, and that the nervous system is not, as
might, on the grounds of analogy, have been anticipated, as a
rule secondarily developed in the mesoblast.

ORIGIN OF THE GERMINAL LAYERS. 359

The Hertwigs have recently suggested in their very interesting memoir
(No. 271) that the Triploblastica are to be divided into two phyla, (i)
the Enteroccela, and (2) the Pseudocoela ; the former group containing the
Chaetopoda, Gephyrea, Brachiopoda, Nematoda, Arthropoda, Echinoder-
mata, Enteropneusta and Chordata ; and the latter the Mollusca, Polyzoa,
the Rotifera, and Platyelminthes.

The Enteroccela are forms in which the primitive alimentary diverticula
have given origin to the body cavity, while the major part of the muscular
system has originated from the epithelial walls of these diverticula, part
however being in many cases also derived from the amoeboid cells, called by
them mesenchyme, by the second process of mesoblastic differentiation men-
tioned on p. 347.

In the Pseudoccela the muscular system has become differentiated from
mesenchyme cells ; while the body cavity, where it exists, is merely a split in
the mesenchyme.

It is impossible for me to attempt in this place to state fully, or do
justice to, the original and suggestive views contained in this paper. The
general conclusion I cannot however accept. The views of the Hertwigs
depend to a large extent upon the supposition that it is possible to dis-
tinguish histologically muscle cells derived from epithelial cells, from those
derived from mesenchyme cells. That in many cases, and strikingly so in
the Chordata, the muscle cells retain clear indications of their primitive
origin from epithelial cells, I freely admit ; but I do not believe either that
its histological character can ever be conclusive as to the non-epithelial
origin of a muscle cell, or that its derivation in the embryo from an indiffer-
ent amoeboid cell is any proof that it did not, to start with, originate from an
epithelial cell.

I hold, as is clear from the preceding statements, that such immense
secondary modifications have taken place in the development of the meso-
blast, that no such definite conclusions can be deduced from its mode
of development as the Hertwigs suppose.

In support of the view that the early character of embryonic cells is no
safe index as to their phylogenetic origin, I would point to the few following
facts.

(1) In the Porifera and many of the Ccelenterata (Eucope polystyla,
Geryonia, &c.) the hypoblast (endoderm) originates from cells, which accord-
ing to the Hertwigs' views ought to be classed as mesenchyme.

(2) In numerous instances muscles which have, phylogenetically, an
undoubted epithelial origin, are ontogenetically derived from cells which
ought to be classed as mesenchyme. The muscles of the head in all the
higher Vertebrata, in which the head cavities have disappeared, are examples
of this kind ; the muscles of many of the Tracheata, notably the Araneina,
must also be placed in the same category.

(3) The Mollusca are considered by the Hertwigs to be typical Pseudo-
coela. A critical examination of the early development of the mesoblast in
these forms demonstrates however that with reference to the mesoblast they

360 FCETAL AND LARVAL DEVELOPMENT.

must be classed in the same group as the Cluetopoda. The mesoblast (Vol.
II. p. 227) clearly originates as two bands of cells which grow inwards from
the blastopore, and in some forms (Paludina, Vol. II. fig. 107) become divided
into a splanchnic and somatic layer, with a body cavity between them. All
these processes are such as are, in other instances, admitted to indicate
Enteroccclous affinities.

The subsequent conversion of the mesoblast elements into amoeboid cells,
out of which branched muscles are formed, is in my opinion simply due
to the envelopment of the soft Molluscan body within a hard shell.

In addition to these instances I may point out that the distinction be-
tween the Pseudocoela and Enteroccela utterly breaks down in the case
of the Discophora, and the Hertwigs have made no serious attempt to
discuss the characters of this group in the light of their theory, and that the
derivation of the Echinoderm muscles from mesenchyme cells is a difficulty
which is very slightly treated.

II. LARVAL FORMS: THEIR NATURE, ORIGIN AND AFFINITIES.

Preliminary considerations. In a general way two types
of development may be distinguished, viz. a foetal type and a
larval type. In the foetal type animals undergo the whole or
nearly the whole of their development within the egg or within
the body of the parent, and are hatched in a condition closely
resembling the adult ; and in the larval type they are born at an
earlier stage of development, in a condition differing to a greater
or less extent from the adult, and reach the adult state either
by a series of small steps, or by a more or less considerable
metamorphosis.

The satisfactory application of embryological data to mor-
phology depends upon a knowledge of the extent to which the
record of ancestral history has been preserved in development.
Unless secondary changes intervened this record would be com-
plete ; it becomes therefore of the first importance to the
cmbryologist to study the nature and extent of the secondary
changes likely to occur in the fcetal or the larval state.

The principles which govern the perpetuation of variations
which occur in either the larval or the fcetal state are the same
as those for the adult condition. Variations favourable to the
survival of the species are equally likely to be perpetuated, at
whatever period of life they occur, prior to the loss of the re-
productive powers. The possible nature and extent of the

LARVAL FORMS. 361

secondary changes which may have occurred in the develop-
mental history of forms, which have either a long larval existence,
or which are born in a nearly complete condition, is primarily
determined by the nature of the favourable variations which can
occur in each case.

Where the development is a fcetal one, the favourable varia-
tions which can most easily occur are — (i) abbreviations, (2) an
increase in the amount of food-yolk stored up for the use of the
developing embryo. Abbreviations take place because direct
development is always simpler, and therefore more advantageous;
and, owing to the fact of the foetus not being required to lead an
independent existence till birth, and of its being in the mean-
time nourished by food-yolk, or directly by the parent, there are
no physiological causes to prevent the characters of any stage of
the development, which are of functional importance during a
free but not during a fcetal existence, from, disappearing from the
developmental history. All organs of locomotion and nutrition
not required by the adult will, for this reason, obviously have a
tendency to disappear or to be reduced in foetal developments;
and a little consideration will shew that the ancestral stages in
the development of the nervous and muscular systems, organs
of sense, and digestive system will be liable to drop out or be
modified, when a simplification can thereby be effected. The
circulatory and excretory systems will not be modified to the
same extent, because both of them are usually functional during
fcetal life.

The mechanical effects of food-yolk are very considerable,
and numerous instances of its influence will be found in the
earlier chapters of this work1. It mainly affects the early stages
of development, i.e. the form of the gastrula, &c.

The favourable variations which may occur in the free larva
are much less limited than those which can occur in the fcetus.
Secondary characters are therefore very numerous in larvae, and
there may even be larvae with secondary characters only, as, for
instance, the larvae of Insects.

In spite of the liability of larvae to acquire secondary charac-
ters, there is a powerful counterbalancing influence tending

1 For numerous instances of this kind, vide Chapter XI. of Vol. in.

362 METAL AND LARVAL DEVELOPMENT.

towards the preservation of ancestral characters, in that larvae
are necessarily compelled at all stages of their growth to retain
in a functional state such systems of organs, at any rate, as are
essential for a free and independent existence. It thus comes
about that, in spite of the many causes tending to produce
secondary changes in larvae, there is always a better chance of
larvae repeating, in an unabbreviated form, their ancestral history,
than is the case with embryos, which undergo their development
within the egg.

It may be further noted as a fact which favours the relative
retention by larvae of ancestral characters, that a secondary
larval stage is less likely to be repeated in development than an
ancestral stage, because there is always a strong tendency for
the former, which is a secondarily intercalated link in the chain
of development, to drop out by the occurrence of a reversion to
the original type of development.

The relative chances of the ancestral history being preserved
in the foetus or the larva may be summed up in the following
way : — There is a greater chance of the ancestral history being
lost in forms which develop in the egg ; and of its being masked
in those which are hatched as larvae.

The evidence from existing forms undoubtedly confirms the
a priori considerations just urged1. This is well shewn by a
study of the development of Echinodermata, Nemertea, Mollusca,
Crustacea, and Tunicata. The free larvae of the four first groups
are more similar amongst themselves than the embryos which
develop directly, and since this similarity cannot be supposed to
be due to the larvae having been modified by living under pre-
cisely similar conditions, it must be due to their retaining
common ancestral characters. In the case of the Tunicata the
free larvae retain much more completely than the embryos
certain characters such as the notochord, the cerebrospinal
canal, etc., which are known to be ancestral.

1 It has long been known that land and freshwater forms develop without a
metamorphosis much more frequently than marine forms. This is probably to be
explained by the fact that there is not the same possibility of a land or freshwater
species extending itself over a wide area by the agency of free larvre, and there k,
therefore, much less advantage in the existence of such larva:; while the fact of such
larviu being more liable to be preyed upon than eggs, which are either concealed, or
carried about by the parent, might render a larval stage absolutely disadvantageous.

LARVAL FORMS. 363

Types of Larvae. — Although there is no reason to suppose
that all larval forms are ancestral, yet it seems reasonable to
anticipate that a certain number of the known types of larvae
would retain the characters of the ancestors of the more im-
portant phyla of the animal kingdom.

Before examining in detail the claims of various larvae to
such a character, it is necessary to consider somewhat more at
length the kind of variations which are most likely to occur in
larval forms.

It is probable a priori that there are two kinds of larvae,
which may be distinguished as primary and secondary larvae.
Primary larvae are more or less modified ancestral forms, which
have continued uninterruptedly to develop as free larvae from
the time when they constituted the adult form of the species.
Secondary larvae are those which have become introduced into
the ontogeny of species, the young of which were originally
hatched with all the characters of the adult; such secondary
larvae may have originated from a diminution of food-yolk in
the egg and a consequently earlier commencement of a free
existence, or from a simple adaptive modification in the just
hatched young. Secondary larval forms may resemble the
primary larval forms in cases where the ancestral characters were
retained by the embryo in its development within the egg; but
in other instances their characters are probably entirely adaptive.

Causes tending to produce secondary changes in larv<z. — The
modes of action of natural selection on larvae may probably be
divided more or less artificially into two classes.

1. The changes in development directly produced by the
existence of a larval stage.

2. The adaptive changes in a larva acquired in the ordinary
course of the struggle for existence.

The changes which come under the first head consist essen-
tially in a displacement in the order of development of certain
organs. There is always a tendency in development to throw
back the differentiation of the embryonic cells into definite
tissues to as late a date as possible. This takes place in order
to enable the changes of form, which every organ undergoes, in
repeating even in an abbreviated way its phylogenetic history,
to be effected with the least expenditure of energy. Owing to

364 CHANGES IN LARWE.

this tendency it comes about that when an organism is hatched
as a larva many of the organs are still in an undifferentiated
state, although the ancestral form which this larva represents
had all its organs fully differentiated. In order, however, that
the larva may be enabled to exist as an independent organism,
certain sets of organs, e.g. the muscular, nervous, and digestive
systems, have to be histologically differentiated. If the period
of foetal life is shortened, an earlier differentiation of certain
organs is a necessary consequence ; and in almost all cases the
existence of a larval stage causes a displacement in order of
development of organs, the complete differentiation of many
organs being retarded relatively to the muscular, nervous, and
digestive systems.

The possible changes under the second head appear to be
unlimited. There is, so far as I see, no possible reason why an
indefinite number of organs should not be developed in larvae to
protect them from their enemies, and to enable them to com-
pete with larvae of other species, and so on. The only limit to
such development appears to be the shortness of larval life,
which is not likely to be prolonged, since, ceteris paribus, the
more quickly maturity is reached the better it is for the species.

A very superficial examination of marine larvae shews that
there are certain peculiarities common to most of them, and it is
important to determine how far such peculiarities are to be
regarded as adaptive. Almost all marine larvae are provided
with well-developed organs of locomotion, and transparent
bodies. These two features are precisely those which it is most
essential for such larvae to have. Organs of locomotion are
important, in order that larvae may be scattered as widely as
possible, and so disseminate the species ; and transparency is
very important in rendering larvae invisible, and so less liable to
be preyed upon by their numerous enemies1.

These considerations, coupled with the fact that almost all
free-swimming animals, which have not other special means of
protection, are transparent, seem to shew that the transparency

1 The phosphorescence of many larvze is very peculiar. I should have anticipated
that phosphorescence would have rendered them much more liable to be captured by
the forms which feed upon them; and it is difficult to see of what advantage it can be
to them.

LARVAL FORMS. 365

of larvae at all events is adaptive ; and it is probable that organs
of locomotion are in many cases specially developed, and not
ancestral.

Various spinous processes on the larvae of Crustacea and
Teleostei are also examples of secondarily acquired protective
organs.

These general considerations are sufficient to form a basis for
the discussion of the characters of the known types of larvae.

The following table contains a list of the more important of
such larval forms :

DICYEMID^. — The Infusoriform larva (vol. n. fig. 62).

PORIFERA. — (a) The Amphiblastula larva (fig. 215), with one-half of the body
ciliated, and the other half without cilia; (b) an oval uniformly ciliated larva, which
may be either solid or have the form of a vesicle.

CCELENTERATA. — The planula (fig. 216).

TURBELLARIA. — (a) The eight-lobed larva of Miiller (fig. 222); (b) the larvae of
Gotte and Metschnikoff, with some Pilidium characters.

NEMERTEA. — The Pilidium (fig. 221).

TREMATODA. — The Cercaria.

ROTIFERA. — The Trochosphere-like larvae of Brachionus (fig. 217) and Laci-
nularia.

MOLLUSCA. — The Trochosphere larva (fig. 218), and the subsequent Veliger larva
(fig. 219).

BRACHIOPODA. — The three-lobed larva, with a postoral ring of cilia (fig. 220).

POLYZOA. — A larval form with a single ciliated ring surrounding the mouth, and
an aboral ciliated ring or disc (fig. 228).

CH/ETOPODA. — Various larval forms with many characters like those of the
molluscan Trochosphere, frequently with distinct transverse bands of cilia. They are
classified as Atrochoe, Mesotrochse, Telotrochse (fig. 225 A and fig. 226), Polytrochae,
and Monotrochae (fig. 225 B).

GEPHYREA NUDA. — Larval forms like those of preceding groups. A specially
characteristic larva is that of Echiurus (fig. 227).

GEPHYREA TUBICOLA. — Actinotrocha (fig. 230), with a postoral ciliated ring of
arms.

MYRIAPODA.— A functionally hexapodous larval form is common to all the
Chilognatha (vol. n. fig. 174).

INSECTA. — Various secondary larval forms.

CRUSTACEA. — The Nauplius (vol. n. fig. 208) and the Zosea (vol. II. fig. 210).

ECHINODERMATA. — The Auricularia (fig. 223 A), the Bipinnaria (fig. 223 B), and
the Pluteus (fig. 224), and the transversely-ringed larvae of Crinoidea (vol. II. fig. 268).
The three first of which can be reduced to a common type (fig. 231 c).

ENTEROPNEUSTA. — Tornaria (fig. 229).

UROCHORDA (TUNICATA). — The tadpole-like larva (vol. in. fig. 8).

GANOIDEI. — A larva with a disc with adhesive papillae in front of the mouth
(vol. in. fig. 67).

ANUROUS AMPHIBIA. — The tadpole (vol. in. fig. 80).

366

TYPES OF LARVAE.

Of the larval forms included in the above list a certain

en

e.g.

FlG. 21=1. TWO FREE STAGES IN THE DEVELOPMENT OF SYCANDRA RAPHANUS.

(After Schultze. )

A. Amphiblastula stage.

B. Stage after the ciliated cells have commenced to be invaginated.

c.s. segmentation cavity; ec. granular epiblast cells; en. ciliated hypoblast cells.

number are probably without affinities outside the group to
which they belong. This is the case with the larvae of the

C

FIG. 216. THREE LARVAL STAGES OF EUCOPE POLYSTYI.A. (After Kowalevsky.)

A. Blastosphere stage with hypoblast spheres becoming budded into the central
cavity.

B. Planula stage with solid hypoblast.

C. I'lanula stage with a gastric cavity.

<•/>. epiblast: hy. hypoblast; al. gastric cavity.

LARVAL FORMS.

367

Myriapoda, the Crustacean lame, and with the larval forms of
the Chordata. I shall leave these forms out of consideration.

There are, again, some larval forms which may possibly turn
out hereafter to be of importance, but from which, in the present
state of our knowledge, we cannot draw any conclusions. The
infusoriform larva of the Dicyemidse, and the Cercaria of the
Trematodes, are such forms.

Excluding these and certain other forms, we have finally left
for consideration the larvae of the Ccelenterata, the Turbellaria,
the Rotifera, the Nemertea, the Mollusca, the Polyzoa, the
Brachiopoda, the Chaetopoda, the Gephyrea, the Echinodermata,
and the Enteropneusta.

The larvae of these forms can be divided into two groups.
The one group contains the larva of the Ccelenterata or Planula,
the other group the larvae of all the other forms.

The Planula (fig. 216) is characterised by its extreme sim-
plicity. It is a two-layered
organism, with a form varying
from cylindrical to oval, and
usually a radial symmetry. So
long as it remains free it is not
usually provided with a mouth,
and it is as yet uncertain whether
or no the absence of a mouth is
to be regarded as an ancestral
character. The Planula is very
probably the ancestral form of
the Ccelenterata.

The larvae of almost all the
other groups, although they may
be subdivided into a series of
very distinct types, yet agree in
the possession of certain common
characters1. There is a more or
less dome-shaped dorsal surface,
and a flattened or concave ventral surface, containing the open-

1 The larva of the Brachiopoda does not possess most of the characters mentioned
below. It is probably, all the same, a highly differentiated larval form belonging to
this group.

Id

ov

FIG. 217. EMBRYO OF BRACHI-
ONUS URCEOLARIS, SHORTLY BEFORE
IT is HATCHED. (After Salensky.)

m. mouth ; ms. masticatory appa-
ratus ; me. mesenteron ; an. anus ;
Id. lateral gland ; ov. ovary ; t. tail
(foot) ; tr. trochal disc ; sg. supra-
oesophageal ganglion.

368 LARWE OF THE TRIPLOBLASTICA.

ing of the mouth, and usually extending posteriorly to the
opening of the anus, when such is present.

The dorsal dome is continued in front of the mouth to form
a large prceoral lobe.

There is usually present at first an uniform covering of cilia ;
but in the later larval stages there are almost always formed
definite bands or rings of long cilia, by which locomotion is
effected. These bands are often produced into arm-like pro-
cesses.

The alimentary canal has, typically, the form of a bent tube
with a ventral concavity, constituted (when an anus is present)

FIG. 218. DIAGRAM OF AN EMBRYO OF PLEUROBRANCHIDIUM.

(From Lankester.)

/. foot; ol. otocyst; m. mouth; v. velum; ng. nerve ganglion; ry. residual yolk
spheres; sAs. shell-gland; i. intestine.

of three sections, viz. an oesophagus, a stomach, and a rectum.
The oesophagus and sometimes the rectum are epiblastic in
origin, while the stomach always and the rectum usually are
derived from the hypoblast1.

To the above characters may be added a glass-like trans-
parency ; and the presence of a widish space possibly filled with
gelatinous tissue, and often traversed by contractile cells,
between the alimentary tract and the body wall.

1 There is some uncertainty as to the development of the oesophagus in the
Echinodermata, but recent researches appear to indicate that it is developed from the
hypoblast.

LARVAL FORMS.

369

Considering the very profound differences which exist
between many of these larvae, it may seem that the characters
just enumerated are hardly sufficient to justify my grouping
them together. It is, however, to be borne in mind that my
grounds for doing so depend quite as much upon the fact that
A B

FIG. 219. LARVAE OF CEPHALOPHOROUS MOLLUSCA IN THE VELIGER STAGE.
(From Gegenbaur.)

A. and B. Earlier and later stage of Gasteropod. C. Pteropod (Cymbulia).
v. velum; c. shell; /. foot; op. operculum ; t. tentacle.

they constitute a series without any great breaks in it, as upon
the existence of characters common to
the whole of them. It is also worth
noting that most of the characters which
have been enumerated as common to the
whole of these larvae are not such second-
ary characters as (in accordance with the
considerations used above) might be ex-
pected to arise from the fact of their
being subjected to nearly similar con-
ditions of life. Their transparency is, no
doubt, such a secondary character, and it
is not impossible that the existence of
ciliated bands may be so also ; but it is
quite possible that if, as I suppose, these
larvae reproduce the characters of some
ancestral form, this form may have
existed at a time when all marine
animals were free-swimming, and that it
may, therefore, have been provided with at least one ciliated
band.

FIG. 220. LARVA OF
ARGIOPE. (From Gegen-
baur ; after Kowalevsky.)

m. mantle ; b. setre ;
d. archenteron.

B. III.

24

370 THE ECHINODERM GROUP.

The detailed consideration of the characters of these larvae,
given below, supports this view.

This great class of larvae may, as already stated, be divided
into a series of minor subdivisions. These subdivisions are the
following :

1. The Pilidium Group. — This group is characterised by
the mouth being situated nearly in the centre of the ventral
surface, and by the absence of an anus. It includes the Pilidium

FlG. 221. TWO STAGES IN THE DEVELOPMENT OF PlLIDIUM.

(After Metschnikoff.)

ae. archenteron; oe. oesophagus; st. stomach; am. amnion; pr.d. prostomial
disc ; pod. metastomial disc ; c.s. cephalic sack (lateral pit).

of the Nemertines (fig. 221), and the various larvae of marine
Dendrocoela (fig. 222). At the apex of the praeoral lobe a
thickening of epiblast may be present, from which (fig. 232) a
contractile cord sometimes passes to the oesophagus.

2. The Echinoderm Group. — This group (figs. 223, 224
and 231 C) is characterised by the presence of a longitudinal
pastoral band of cilia, by the absence of special sense organs in
the praeoral region, and by the development of the body cavity
as an outgrowth of the alimentary tract. The three typical
divisions of the alimentary tract are present, and there is a more
or less developed praeoral lobe. This group only includes the
larvae of the Echinodermata.

LARVAL FORMS. 371

3. The Trochosphere Group. — This group (figs. 225, 226)
is characterised by the presence of a praeoral ring of long cilia,
the region in front of which forms a great part of the praeoral
lobe. The mouth opens immediately behind the praeoral ring
of cilia, and there is very often a second ring of short cilia
parallel to the main ring, immediately behind the mouth. The

B.

FIG. 222. A. LARVA OF EURYLEPTA AURICULATA IMMEDIATELY AFTER
HATCHING. VIEWED FROM THE SIDE. (After Hallez.) m. mouth.

B. MULLER'S TURBELLARIAN LARVA (PROBABLY THYSANOZOON). VIEWED
FROM THE VENTRAL SURFACE. (After Muller.) The ciliated band is represented by
the black line. m. mouth ; u.l. upper lip.

function of the ring of short cilia is nutritive, in that its cilia are
employed in bringing food to the mouth ; while the function of
the main ring is locomotive. A perianal patch or ring of cilia is
often present (fig. 225 A), and in many forms intermediate rings
are developed between the praeoral and perianal rings.

The praeoral lobe is usually the seat of a special thickening
of epiblast, which gives rise to the supra-cesophageal ganglion of
the adult. On this lobe optic organs are very often developed
in connection with the supra-oesophageal ganglion, and a con-
tractile band frequently passes from this region to the oesophagus.

The alimentary tract is formed of the three typical divisions.

The body cavity is not developed directly as an outgrowth
of the alimentary tract, though the process by which it originates
is very probably secondarily modified from a pair of alimentary
outgrowths.

24 — 2

372

TORNARIA.

Paired excretory organs, opening to the exterior and into the
body cavity, are often present (fig. 226 nph}.

This type of larva is found in the Rotifera (fig. 217) (in which
it is preserved in the adult state), the Chaetopoda (figs. 225 and
226), the Mollusca (fig. 218), the Gephyrea nuda (fig. 227), and
the Polyzoa (fig. 228)'.

FIG. 223. A. THE LARVA OF A HOLOTHUROID.
B. THE LARVA OF AN ASTEROID.

m. mouth; si. stomach; a. anus; I.e. primitive longitudinal ciliated band; pr.c.
pneoral ciliated band.

4. Tornaria.— This larva (fig. 229) is intermediate in most
of its characters between the larvae of the Echinodermata (more
especially the Bipinnaria) and
the Trochosphere. It resembles
Echinoderm larvae in the posses-
sion of a longitudinal ciliated
band (divided into a praeoral
and a postoral ring), and in the
derivation of the body cavity
and water-vascular vesicle from
alimentary diverticula ; and it
resembles the Trochosphere in
the presence of sense organs on
the praeoral lobe, in the existence
of a perianal ring of cilia, and in
the possession of a contractile
band passing from the praeoral lobe to the oesophagus.

FIG. 224.
LOCENTRUS.

m. mouth :
d. stomach ;

A LARVA OF STROXGY-
(From Agassiz.)

a. anus ; o. oesophagus ;
c. intestine ; v ' . and v.

ciliated ridges ; w. water- vascular tube ;
r. calcareous rods.

1 For a discussion as to the structure of the Polyzoon larva, vide Vol. II. p. 305.

LARVAL FORMS. 373

5. Actinotrocha. — The remarkable larva of Phoronis (fig.
230), known as Actinotrocha, is characterised by the presence of
(i) a postoral and somewhat longitudinal ciliated ring produced
into tentacles, and (2) a perianal ring. It is provided with a
prseoral lobe, and a terminal or somewhat dorsal anus.

6. The larva of the Brachiopoda articulata (fig. 220).
The relationships of the six types of larval forms thus briefly

characterised have been the subject of a considerable amount of
controversy, and the following suggestions on their affinities
must be viewed as somewhat speculative. The Pilidium type of
larva is in some important respects less highly differentiated

FIG. 225. Two CH^TOPOD LARWE. (From Gegenhaur.)

o. mouth ; i. intestine ; a. anus ; v. praeoral ciliated band ; w. perianal ciliated
band.

than the larvae of the five other groups. It is, in the first place,
without an anus ; and there are no grounds for supposing that
the anus has become lost by retrogressive changes. If for the
moment it is granted that the Pilidium larva represents more
nearly than the larvae of the other groups the ancestral type of
larva, what characters are we led to assign to the ancestral form
which this larva repeats ?

In the first place, this ancestral form, of which fig. 231 A is
an ideal representation, would appear to have had a dome-shaped
body, with a flattened oral surface and a rounded aboral surface.
Its symmetry was radial, and in the centre of the flattened oral
surface was placed the mouth, and round its edge was a ring of
cilia. The passage of a Pilidium-like larva into the vermiform
bilateral Platyelminth form, and therefore it may be presumed
of the ancestral form which this larva repeats, is effected by the

374 ORIGIN OF PILIDIUM LARVA.

larva becoming more elongated, and by the region between the
mouth and one end of the body becoming the pneoral region,
and by an outgrowth between the mouth and the opposite end
developing into the trunk, an anus
becoming placed at its extremity in
the higher forms.

If what has been so far postulated
is correct, it is clear that this primitive
larval form bears a very close resem-
blance to a simplified free-swimming
Ccelenterate (Medusa), and that the
conversion of such a radiate form into

..... , , , , . , , i FlG. 226. POLYGORDIUS

the bilateral took place, not by the LARVA- ( After Hatschek.)
elongation of the aboral surface, and ;;/ mouth; ^ ^.^

the formation of an anus there, but by phageal ganglion ; nph. nephri-
, , , . r . 1 i r dion ; me.p. mesoblastic band :

the unequal elongation of the oral face, aw< anus f oL stomach.

an anterior part, together with the dome

above it, forming a praeoral lobe, and a posterior outgrowth the

trunk (figs. 226 and 233) ; while the aboral surface became the

dorsal surface.

This view fits in very well with the anatomical resemblances
between the Coelenterata and the Turbellaria1, and shews, if true,
that the ventral and median position of the mouth in many
Turbellaria is the primitive one.

The above suggestion as to the mode of passage from the radial into the
bilateral form differs largely from that usually held. Lankester2, for
instance, gives the following account of this passage :

" It has been recognised by various writers, but notably by Gegenbaur
and Haeckel, that a condition of radiate symmetry must have preceded the
condition of bilateral symmetry in animal evolution. The Diblastula may
be conceived to have been at first absolutely spherical with spherical
symmetry. The establishment of a mouth led necessarily to the establish-
ment of a structural axis passing through the mouth, around which axis the
body was arranged with radial symmetry. This condition is more or less
perfectly maintained by many Ccelenterates, and is reassumed by degrada-

1 Vide Vol. II. pp. 179 and 191. In this connection attention may be called
to Cceloplana Mdschnikowii, a form described by Kowalevsky, Zoologischer Anzeiger,
No. 52, p. 140, as being intermediate between the Ctenophora and the Turbellaria.
As already mentioned, there does not appear to me to be sufficient evidence to prove
that this form is not merely a creeping Ctenophor.

• Qiiart. Journ. of Micr. Science, Vol. XVH. pp. 422-3.

LARVAL FORMS.

375

tion of higher forms (Echinoderms, some Cirrhipedes, some Tunicates).
The next step is the differentiation of an upper and a lower surface in

FIG. 227. LARVA OF ECHIURUS. (After Salensky.)
;#. mouth ; an. anus ; sg. supra-oesophageal ganglion (?).

relation to the horizontal position, with mouth placed anteriorly, assumed by
the organism in locomotion. With the differentiation of a superior and
inferior surface, a right and a left side, complementary one to the other, are
necessarily also differentiated. Thus the organism
becomes bilaterally symmetrical. The Ccelentera
are not wanting in indications of this bilateral
symmetry, but for all other higher groups of animals
it is a fundamental character. Probably the de-
velopment of a region in front of, and dorsal to the
mouth, forming the Prattomium, was accomplished
pari passu with the development of bilateral sym-
metry. In the radially symmetrical Ccelentera we
find very commonly a series of lobes of the body-
wall or tentacles produced equally — with radial sym-
metry, that is to say — all round the mouth, the
mouth terminating the main axis of the body — that
is to say, the organism being ' telostomiate.' The
later fundamental form, common to all animals above the Ccelentera, is
attained by shifting what was the main axis of the body — so that it may be
described now as the ' enteric ' axis ; whilst the new main axis, that parallel
with the plane of progression, passes through the dorsal region of the body
running obliquely in relation to the enteric axis. Only one lobe or outgrowth
of those radially disposed in the telostomiate organisms now persists. This
lobe lies dorsally to the mouth, and through it runs the new main axis. This
lobe is the Prostomium, and all the organisms which thus develop a new
main axis, oblique to the old main axis, may be called prostomiate."

FIG. 228. DIAGRAM
OF A LARVA OF THE

POLYZOA.

m. mouth ; an. anus ;
st. stomach; s. ciliated
disc.

376

COMPARISON -BETWEEN TYPES OF LARVAE.

It will be seen from this quotation that the aboral part of the body is sup-
posed to elongate to form the trunk, while the prasoral region is derived from
one of the tentacles.

Before proceeding to further considerations as to the origin
of the Bilateralia, suggested by the Pilidium type of larva, it is
necessary to enter into a more detailed comparison between our
larval forms.

A very superficial consideration of the characters of these
forms brings to light two important features in which they differ,
viz. :

(l) In the presence or absence of sense organs on the prasoral
lobe.

FlG. 229. TWO STAGES IN THE DEVELOPMENT OF TORNARIA.

(After Metschnikoff.)

The black lines represent the ciliated bands.

in. mouth; an. anus; br. branchial cleft; ht. heart; c. body cavity between
splanchnic and somatic mesoblast layers ; w. so-called water-vascular vesicle ; v.
circular blood-vessel.

(2) In the presence or absence of outgrowths from the
alimentary tract to form the body cavity.

The larvae of the Echinodermata and Actinotrocha (?) are
without sense organs on the praeoral lobe, while the other types

LARVAL FORMS.

377

of larvae are provided with them. Alimentary diverticula are
characteristic of the larvae of the Echinodermata and of Tornaria.

If the conclusion already arrived at to the effect that the
prototype of the six larval groups was descended from a radiate
ancestor is correct, it appears to follow that the nervous system,
in so far as it was differentiated, had primitively a radiate form ;
and it is also probably true that there were alimentary diverticula
in the form of radial pouches, two of which may have given
origin to the paired diverticula which become the body cavity in
such types as the Echinodermata, Sagitta, etc. If these two
points are granted, the further conclusions seem to follow — (i)
that the ganglion and sense organs of
the praeoral lobe were secondary struc-
tures, which arose (perhaps as diffe-
rentiations of an original circular
nerve ring) after the assumption of a
bilateral form; and (2) that the absence
of these organs in the larvae of the
Echinodermata and Actinotrocha (?)
implies that these larvae retain, so
far, more primitive characters than the
Pilidium. The same may be said of
the alimentary diverticula. There are
thus indications that in two important
points the Echinoderm larvae are more
primitive than the Pilidium.

The above conclusions with refer-
ence to the Pilidium and Echinoderm
larvae involve some not inconsiderable
difficulties, and suggest certain points for further discussion.

In the first place it is to be noted that the above speculations
render it probable that the type of nervous system from which
that found in the adults of the Echinodermata, Platyelminthes,
Chsetopoda, Mollusca, etc., is derived, was a circumoral ring,
like that of Medusae, with which radially arranged sense organs
may have been connected ; and that in the Echinodermata this
form of nervous system has been retained, while in the other types
it has been modified. Its anterior part may have given rise to
supra-cesophageal ganglia and organs of vision ; these being

FIG. 230. ACTINOTROCHA.
(After Metschnikoff.)

/«. mouth ; an. anus.

378

PRIMITIVE TYPE OF NERVOUS SYSTEM.

developed on the assumption of a bilaterally symmetrical form,
and the consequent necessity arising for the sense organs to
be situated at the anterior end of the body. If this view is
correct, the question presents itself as to how far the posterior
part of the nervous system of the Bilateralia can be regarded as
derived from the primitive radiate ring.

FIG. 231. THREE DIAGRAMS REPRESENTING THE IDEAL EVOLUTION OF VARIOUS

LARVAL FORMS.

A. Ideal ancestral larval form.

B. Larval form from which the Trochosphere larva may have been derived.

C. Larval form from which the typical Echinoderm larva may have been
derived.

m. mouth ; an. anus ; st. stomach ; s.g. supra-cesophageal ganglion.
The black lines represent the ciliated bands.

A circumoral nerve-ring, if longitudinally extended, might
give rise to a pair of nerve-cords united in front and behind —
exactly such a nervous system, in fact, as is present in many
Nemertines1 (the Enopla and Pelagonemertes), in Peripatus2,
and in primitive molluscan types (Chiton, Fissurella, etc.).
From the lateral parts of this ring it would be easy to derive the
ventral cord of the Chaetopoda and Arthropoda. It is especially
deserving of notice in connection with the nervous system of the

1 Vute Hubrecht, "Zur Anat. und Phys. d. Nerven-System. d. Nemertinen," Kbn.
Akad. Wiss., Amsterdam ; and " Researches on the Nervous System of Nemertines,"
Quart. Journ. of Micr. Science, 1880.

* Vide F. M. Balfour, " On some points in the Anat. of Peripatus capensis," Quart.
Jourt:. of Micr. Science, Vol. xix. 1879.

LARVAL FORMS. 379

above-mentioned Nemertines and Peripatus, that the commissure
connecting the two nerve-cords behind is placed on the dorsal
side of the intestine. As is at once obvious, by referring to the
diagram (fig. 231 B), this is the position this commissure ought,
undoubtedly, to occupy if derived from part of a nerve-ring which
originally followed more or less closely the ciliated edge of the
body of the supposed radiate ancestor.

The fact of this arrangement of the nervous system being
found in so primitive a type as the Nemertines tends to establish
the views for which I am arguing ; the absence or imperfect
development of the two longitudinal cords in Turbellarians may
very probably be due to the posterior part of the nerve-ring
having atrophied in this group.

It is by no means certain that this arrangement of the nervous
system in some Mollusca and in Peripatus is primitive, though it
may be so.

In the larvae of the Turbellaria the development of sense organs in the
praeoral region is very clear (fig. 222 B) ; but this is by no means so obvious
in the case of the true Pilidium. There is in Pilidium (fig. 232 A) a thicken-
ing of epiblast at the summit of the dorsal dome, which might seem, from
the analogy of Mitraria, etc. (fig. 233), to correspond to the thickening of the
praaoral lobe, which gives rise to the supra-cesophageal ganglion ; but, as a
matter of fact, this part of the larva does not apparently enter into the
formation of the young Nemertine (fig. 232). The peculiar metamorphosis,
which takes place in the development of the Nemertine out of the Pilidium1,
may, perhaps, eventually supply an explanation of this fact ; but at present
it remains as a still unsolved difficulty.

The position of the flagellum in Pilidium, and of the supra-cesophageal
ganglion in Mitraria, suggests a different view of the origin of the supra-
oesophageal ganglion from that adopted above. The position of the ganglion
in Mitraria corresponds closely with that of the auditory organ in Cteno-
phora ; and it is not impossible that the two structures may have had
a common origin. If this view is correct, we must suppose that the apex of
the aboral lobe has become the centre of the praeoral field of the Pilidium
and Trochosphere larval forms2 — a view which fits in very well with their
structure (figs. 226 and 233). The whole of the questions concerning the
nervous system are still very obscure, and until further facts are brought to
light no definite conclusions can be arrived at.

1 Vide Vol. ii. p. 204.

2 The independent development of the supra-cesophageal ganglion and ventral
nerve-cord in Chaetopoda (vide Kleinenberg, Development of Lumbricus trapezoides)
agrees very satisfactorily with this view.

380 PRIMITIVE RADIAL SYMMETRY OF ECHINODERMATA.

The absence of sense organs on the praeoral lobe of larval
Echinodermata, coupled with the structure of the nervous system
of the adult, points to the conclusion that the adult Echinoder-

FlG. 232. A. PlLIDIUM WITH AN ADVANCED NEMERTINE WORM. B. RlPE
EMBRYO OF NEMERTES IN THE POSITION IT OCCUPIES IN PlLIDIUM. (Both after
Biitschli.)

ft. oesophagus ; st. stomach ; i. intestine ; fr. proboscis ; lp. lateral pit (cephalic
sack) ; a«. amnion ; n. nervous system.

mata have retained, and not, as is now usually held, secondarily
acquired, their radial symmetry; and if this is admitted it follows
that the obvious bilateral symmetry of Echinoderm larvae is a
secondary character.

The bilateral symmetry of many Ccelenterate larvae (the
larva of ,/Eginopsis, of many Acraspeda, of Actinia, &c.), coupled
with the fact that a bilateral symmetry is obviously advanta-

LARVAL FORMS. 381

geous to a free-swimming form, is sufficient to shew that this
supposition is by no means extravagant ; while the presence of
only two alimentary diverticula in Echinoderm larvae is quite in
accord with the presence of a single pair of perigastric chambers
in the early larva of Actinia, though it must be admitted that
the derivation of the water-vascular system from the left
diverticulum is not easy to understand on this view.

A difficulty in the above speculation is presented by the fact
of the anus of the Echinodermata being the permanent blastopore,
and arising prior to the mouth. If this fact has any special
significance, it becomes difficult to regard the larva of Echino-
derms and that of the other types as in any way related ; but if
the views already urged, in a previous section on the germinal
layers, as to the unimportance of the blastopore, are admitted,
the fact of the anus coinciding with the blastopore ceases to be
a difficulty. As may be seen, by referring to fig. 231 C, the
anus is placed on the dorsal side of the ciliated band. This
position for the anus adapts itself to the view that the Echino-
derm larva had originally a radial symmetry, with the anus
placed at the aboral apex, and that, with the elongation of the
larva on the attainment of a bilateral symmetry, the aboral apex
became shifted to the present position of the anus.

It may be noticed that the obscure points connected with the absence of
a body cavity in most adult Platyelminthes, which have already been dealt
with in the section of this chapter devoted to the germinal layers, present
themselves again here ; and that it is necessary to assume either that ali-
mentary diverticula, like those in the Echinodermata, were primitively
present in the Platyelminthes, but have now disappeared from the ontogeny
of this group, or that the alimentary diverticula have not become separated
from the alimentary tract.

So far the conclusion has been reached that the archetype of
the six types of larvae had a radiate form, and that amongst
existing larvae it is most nearly approached in general shape
and in the form of the alimentary canal by the Pilidium group,
and in certain other particulars by the Echinoderm larvae.

The edge of the oral disc of the larval archetype was probably
armed with a ciliated ring, from which the ciliated ring of the
Pilidium type and of the Echinodermata was most likely derived.
The ciliated ring of the Pilidium varies greatly in its characters,

382 PRIMITIVE RADIAL SYMMETRY OF ECHINODERMATA.

and has not always the form of a complete ring. In Pilidium
proper (fig. 232 A) it is a simple ring surrounding the edge of
the oral disc. In Muller's larva of Thysanozoon (fig. 222 B) it is

FlG. 233. TWO STAGES IN THE DEVELOPMENT OF MlTRARIA. (After Metschnikoff.)
m. mouth; an. anus; sg. supra-cesophageal ganglion; br. and b. provisional
bristles ; pr.b. prasoral ciliated band.

inclined at an axis to the oral disc, and might be called praeoral, but
such a term cannot be properly used in the absence of an anus.

FIG. 234. CYPHONAUTES (LARVA OF MEMBRANIPORA). (After Hatschek.)

m. mouth ; a '. anus ; f.g. foot gland ; x. problematical body (probably a bud).

The aboral apex is turned downwards.

LARVAL FORMS. 383

The Echinoderm ring is oblique to the axis of the body, and,
owing to the fact of its passing ventrally in front of the anus,
must be called postoral.

The next point to be considered is that of the affinities of the
other larval types to these two types.

The most important of all the larval types is the Trochosphere,
and this type is undoubtedly more closely related to the Pilidium
than to the Echinoderm larva. Mitraria amongst the Chaetopods
(fig. 233) has, indeed, nearly the form of a Pilidium, and mainly
differs from a Pilidium in the possession of an anus and of
provisional bristles ; the same may be said of Cyphonautes (fig.
234) amongst the Polyzoa.

The existence of these two forms appears to shew that the
praeoral ciliated ring of the Trochosphere may very probably be
derived directly from the circumoral ciliated ring of the Pilidium;
the other ciliated rings or patches of the Trochosphere having a
secondary origin.

The larva of the Brachiopoda (fig. 220), in spite of its peculiar
characters, is, in all probability, more closely related to the
Chaetopod Trochosphere than to any other larval type. The
most conspicuous point of agreement between them is, however,
the possession in common of provisional setae.

Echinoderm larvae differ from the Trochosphere, not only in
the points already alluded to, but in the character of the ciliated
band. The Echinoderm band is longitudinal and postoral. As
just stated, there is reason to think that the praeoral band of
the Trochosphere and the postoral band of the Echinoderm
larva are both derived from a ciliated ring surrounding the oral
disc of the prototype of these larvae (vide fig. 231). In the case
of the Echinodermata the anus must have been formed on the
dorsal side of this ring, and in the case of the Trochosphere on
the ventral side ; and so the difference in position between the
two rings was brought about. Another view with reference to
these rings has been put forward by Gegenbaur and Lankester,
to the effect that the praeoral ring of the Trochosphere is derived
from the breaking up of the single band of most Echinoderm
larvae into the two bands found in Bipinnaria (vide fig. 223) and
the atrophy of the posterior band. There is no doubt a good
deal to be said for this origin of the praeoral ring, and it is

384 PHYLOGENETIC CONCLUSIONS.

strengthened by the case of Tornaria ; but the view adopted
above appears to me more probable.

Actinotrocha (fig. 230) undoubtedly resembles more closely
Echinoderm larvae than the Trochosphere. Its ciliated ring has
Echinoderm characters, and the growth along the line of the
ciliated ring of a series of arms is very similar to what takes
place in many Echinoderms. It also agrees with the Echinoderm
larvae in the absence of sense organs on the praeoral lobe.

Tornaria (fig. 229) cannot be definitely united either with
the Trochosphere or with the Echinoderm larval type. It has
important characters in common with both of these groups, and
the mixture of these characters renders it a very striking and
well-defined larval form.

Phylogenetic conclusions. The phylogenetic conclusions
which follow from the above views remain to be dealt with.
The fact that all the larvae of the groups above the Ccelenterata
can be reduced to a common type seems to indicate that all the
higher groups are descended from a single stem.

Considering that the larvae of comparatively few groups have
persisted, no conclusions as to affinities can be drawn from the
absence of a larva in any group; and the presence in two groups
of a common larval form may be taken as proving a common
descent, but does not necessarily shew any close affinity.

There is every reason to believe that the types with a
Trochosphere larva, viz. the Rotifera, the Mollusca, the Chaeto-
poda, the Gephyrea, and the Polyzoa, are descended from a
common ancestral form ; and it is also fairly certain there was a
remote ancestor common to these forms and to the Platyelminthes.
A general affinity of the Brachiopoda with the Chaetopoda is
more than probable. All these types, together with various
other types which are nearly related to them, but have not
preserved an early larval form, are descended from a bilateral
ancestor. The Echinodermata, on the other hand, are probably
directly descended from a radial ancestor, and have more or less
completely retained their radial symmetry. How far Actino-
trocha1 is related to the Echinoderm larvae cannot be settled.
Its characters may possibly be secondary, like those of the

1 It is quite possible that Phoronis is in no way related to the other Gephyrea.

LARVAL FORMS. 385

mesotrochal larvae of Chaetopods, or they may be due to its
having branched off very early from the stock common to the
whole of the forms above the Ccelenterata. The position of
Tornaria is still more obscure. It is difficult, in the face of the
peculiar water-vascular vesicle with a dorsal pore, to avoid the
conclusion that it has some affinities with the Echinoderm larvae.
Such affinities would seem, on the lines of speculation adopted
in this section, to prove that its affinities to the Trochosphere,
striking as they appear to be, are secondary and adaptive. From
this conclusion, if justified, it would follow that the Echinodermata
and Enteropneusta have a remote ancestor in common, but not
that the two groups are in any other way related.

General conclusions and summary. Starting from the
demonstrated fact that the larval forms of a number of widely
separated types above the Ccelenterata have certain characters
in common, it has \&&\ provisionally assumed that the characters
have been inherited from a common ancestor ; and an attempt
has been made to determine (i) the characters of the prototype
of all these larvae, and (2) the mutual relations of the larval
forms in question. This attempt started with certain more or
less plausible suggestions, the truth of which can only be tested
by the coherence of the results which follow from them, and
their capacity to explain all the facts.

The results arrived at may be summarised as follows :

1. The larval forms above the Ccelenterata may be divided
into six groups enumerated on pages 370 to 373.

2. The prototype of all these groups was an organism
something like a Medusa, with a radial symmetry. The mouth
was placed in the centre of a flattened ventral surface. The
aboral surface was dome-shaped. Round the edge of the oral
surface was a ciliated ring, and probably a nervous ring provided
with sense organs. The alimentary canal was prolonged into
two or more diverticula, and there was no anus.

3. The bilaterally symmetrical types were derived from
this larval form by the larva becoming oval, and the region in
front of the mouth forming a praeoral lobe, and that behind the
mouth growing out to form the trunk. The aboral dome became
the dorsal surface.

On the establishment of a bilateral symmetry the anterior

15. in. 25

386 GENERAL CONCLUSIONS.

part of the nervous ring gave rise (?) to the supra-cesophageal
ganglia, and the optic organs connected with them ; while the
posterior part of the nerve-ring formed (?) the ventral nerve-cords.
The body cavity was developed from two of the primitive
alimentary diverticula.

The usual view that radiate forms have become bilateral by
the elongation of the aboral dome into the trunk is probably
erroneous.

4. Pilidium is the larval form which most nearly reproduces
the characters of the larval prototype in the course of its
conversion into a bilateral form.

5. The Trochosphere is a completely differentiated bilateral
form, in which an anus has become developed. The praeoral
ciliated ring of the Trochosphere is probably directly derived
from the ciliated ring of Pilidium, which is itself the original ring
of the prototype of all these larval forms.

6. Echinoderm larvae, in the absence of a nerve-ganglion or
special organs of sense on the prseoral lobe, and in the presence
of alimentary diverticula, which give rise to the body cavity,
retain some characters of the prototype larva which have been
lost in Pilidium. The ciliated ring of Echinoderm larvae is
probably derived directly from that of the prototype by the
formation of an anus on the dorsal side of the ring. The anus
was very probably originally situated at the aboral apex.

Adult Echinoderms have probably retained the radial sym-
metry of the forms from which they are descended, their nervous
ring being directly derived from the circular nervous ring of their
ancestors. They have not, as is usually supposed, secondarily
acquired their radial symmetry. The bilateral symmetry of the
larva is, on this view, secondary, like that of so many Coelenterate
larvae.

7. The points of similarity between Tornaria and (i) the
Trochosphere and (2) the Echinoderm larvae are probably
adaptive in the one case or the other ; and, while there is no
difficulty in believing that those to the Trochosphere are
adaptive, the presence of a water- vascular vesicle with a dorsal
pore renders probable a real affinity with Echinoderm larvae.

8. It is not possible in the present state of our knowledge
to decide how far the resemblances between Actinotrocha and
Echinoderm larvae are adaptive or primary.

LARVAL FORMS. 387

BIBLIOGRAPHY.

(257) Allen Thomson. British Association Address, 1877.

(258) A. Agassiz. " Embryology of the Ctenophorae." Mem. Amer. Acad. of
Arts and Sciences, Vol. X. 1874.

(259) K. E. von Baer. Ueb. Entivicklungsgeschichte d. Thiere. Konigsberg,
1828—1837.

(260) F. M. Balfour. "A Comparison of the Early Stages in the Development
of Vertebrates." Quart. Joum. of Micr. Set., Vol. XV. 1875.

(261) C. Glaus. Die Typenlehre u. E. HaeckeFs sg. Gastraa-tlieorie. Wien,
1874.

'(262) C. Glaus. Grundziige d. Zoologie. Marburg und Leipzig, 1879.

(263) A. Dohrn. Der Ursprung d. Wirbelthiere u. d. Princip des Functions-
ivechsels. Leipzig, 1875.

(264) C. Gegenbaur. Grttndriss d. vergleichenden Anatomic. Leipzig, 1878.
Vide also Translation. Elements of Comparative Anatomy. Macmillan & Co.
1878.

(265) A. Gotte. Entwicklungsgeschichte d. Unke. Leipzig, 1874.

(266) E. Haeckel. Studien z. Gastraa-theorie, Jena, 1877; and also jtenaisc/ic
Zeitschrift, Vols. vin. and IX. 1874-5.

(267) E. Haeckel. Schopfungsgeschichte. Leipzig. Vide also Translation,
The History of Creation. King & Co., London, 1878.

(268) E. Haeckel. Anthropogenic. Leipzig. Vide also Translation, Anthro-
pogeny. Kegan Paul & Co., London, 1878.

(269) B. Hatschek. "Studien lib. Entwicklungsgeschichte d. Anneliden."
Arbeit, a. d. zool. Instit. d. Univer. Wien. 1878.

(270) O. and R. Hertwig. "Die Actinien." Jenaische Zeitschrift, Vols. xm.
and xiv. 1879.

(271) O. and R. Hertwig. Die Ccelomtheorie. Jena, 1881'.

(272) O. Hertwig. Die Chatognathen. Jena, 1880.

(273) R. Hertwig. Ueb. d. Bau d. Ctenophoren. Jena, 1880.

(274) T. H. Huxley. The Anatomy of Invertebrated Animals. Churchill,
1877.

(274*) T. H. Huxley. "On the Classification of the Animal Kingdom."
Quart. J. of Micr. Science, Vol. xv. 1875.

(275) N. Kleinenberg. Hydra, eine anatomisch-cntwickhingsgeschichtiiche Un-
tersuchung. Leipzig, 1872.

(276) A. Kolliker. Entwicklungsgeschichte d. Menschen it, d. hoh. Thiere.
Leipzig, 1879.

(277) A. Kowale vsky. " Embryologische Studien an Wiirmern u. Arthropoden."
Mem. Acad. Petersbourg, Series vil. Vol. xvi. 1871.

(278) E. R. Lankester. "On the Germinal Layers of the Embryo as the
Basis of the Genealogical Classification of Animals." Ann. and Mag. of Nat. Hist.
1873-

1 This important memoir only came into my hands after this chapter was already
in type.

25 — 2

388 BIBLIOGRAPHY.

(279) E. R. Lankester. "Notes on Embryology and Classification." Quart.
Jonrn. of Micr. Set., Vol. XVII. 1877.

(280) E. Metschnikoff. "Zur Entwicklungsgeschichte d. Kalkschwamme."
Zeit.f. wiss. Zool., Vol. xxiv. 1874.

(281) E. Metschnikoff. " Spongiologische Stuclien." Zeit.f, wiss. Zool.,
Vol. xxxn. 1879.

(282) A. S. P. Packard. Life Histories of Animals, including Man, or Outlines
of Comparative Embryology. Holt and Co., New York, 1876.

(283) C. Rabl. " Ueb. d. Entwick. d. Malermuschel. " Jenaische Zeitsch., Vol.
x. 1876.

(284) C. Rabl. "Ueb. d. Entwicklung. d. Tellerschnecke (Planorbis)." Morph.
Jahrbuch, Vol. v. 1879.

(285) H. Rathke. Abhandlungen 2. Bildung und Entwicklungsgesch. d. Menschen
«. d. Thiere. Leipzig, 1833.

(286) H. Rathke. Ueber die Bildung u. Entwicklungs. d. Flusskrebses. Leipzig,
1829.

(287) R. Remak. Untersuch. iib. d. Entwick. d. Wirbelthiere. Berlin, 1855.

(288) Salensky. " Bemerkungen iib. Haeckels Gastrsea-theorie." Archiv f.
Na turgesch ich te, 1874.

(289) E. Schafer. "Some Teachings of Development." Quart. Jonnt. of Micr.
Science, Vol. xx. 1880.

(290) C. Semper. "Die Verwandtschaftbeziehungen d. gegliederten Thiere.
Arbeiten a. d. zool.-zoot. Instit. Wiirzburg, Vol. III. 1876-7.

PART II.

ORGANOGENY.

PART II.
ORGANOGENV.

INTRODUCTION.

OUR knowledge of the development of the organs in most of
the Invertebrate groups is so meagre that it would not be profit-
able to attempt to treat systematically the organogeny of the
whole animal kingdom.

For this reason the plan adopted in this section of the work
has been to treat somewhat fully the organogeny of the Chor-
data, which is comparatively well known ; and merely to indicate
a few salient facts with reference to the organogeny of other
groups. In the case of the nervous system, and of some other
organs which especially lend themselves to this treatment, such
as the organs of special sense and the excretory system, a wider
view of the subject has been taken ; and certain general princi-
ples underlying the development of other organs have also been
noticed.

The classification of the organs is a matter of some difficulty.
Considering the character of this treatise it seemed desirable to
arrange the organs according to the layers from which they are
developed. The compound nature of many organs, e.g. the eye
and ear, renders it, however, impossible to carry out consistently
such a mode of treatment. I have accordingly adopted a rough
classification of the organs according to the layers, dropping the
principle where convenient, as, for instance, in the case of the
stomodaeum and proctodseum.

The organs which may be regarded as mainly derived from

392 INTRODUCTION.

the epiblast are (i) the skin; (2) the nervous system; (3) the
organs of special sense.

Those from the mesoblast are (i) the general connective
tissue and skeleton ; (2) the vascular system and body cavity ;
(3) the muscular system ; (4) the urinogenital system.

Those from the hypoblast are the alimentary tract and its
derivates ; with which the stomodaeum and proctodaeum and
their respective derivates are also dealt with.

BIBLIOGRAPHY.

General works dealing with the development of the organs of the

Chordata.

(291) K. E. von Baer. Ueber Entwicklungsgeschichte d. Thiere. Konigsberg,
1828—1837.

(292) F. M. Balfour. A Monograph on tlic development of Elasmobrancli Fishes.
London, 1878.

(293) Th. C. W. Bischoff. Entwicklungsgesch. d. Sdtigethiere ti. d. Menschen.
Leipzig, 1842.

(294) C. Gegenbaur. Gnindriss d. vergleichenden Anatomic. Leipzig, 1878.
Vide also English translation, Elements of Comp. Anatomy. London, 1878.

(295) M. Foster and F. M. Balfour. The Elements of Embryology. Part I.
London, 1874.

(296) Alex. Gotte. Entwicklungsgeschichte d. Unke. Leipzig, 1875.

(297) W. His. Untersitch. iib. d. erste Anlage d. Wirbelthierleibcs . Leipzig,
1868.

(298) A. Ko Hiker. Entwicklungsgeschichte d. Menschen u. der hoheren Thiere.
Leipzig, 1879.

(299) H. Rathke. Abhandlungen it. Bildung mid Entwicklungsgeschichle d.
Menschen it. d. Thiere. Leipzig, 1838.

(300) H. Rathke. Entwicklungs. d. Natter. Kbnigsberg, 1839.

(301) H. Rathke. Entwicklungs. d. Wirbelthiere. Leipzig, 1861.

(302) R. Remak. Untersuchnngen iib. d. Entwicklung d. Wirbelthiere. Berlin,
1850—1855.

(303) S. L. Schenk. Lehrbuch d. vei'gleich. Embryologie d. Wirbeltliicre.
Wien, 1874.

.

CHAPTER XIV.
THE EPIDERMIS AND ITS DERIVATIVES.

IN many of the Ccelenterata the outermost layer of the blas-
toderm is converted as a whole into the skin or ectoderm.
The cells composing it become no doubt in part differentiated
into muscular elements and in part into nervous elements, &c. ;
but still it may remain through life as a simple external
membrane. This membrane contains in itself indefinite poten-
tialities for developing into various organs, and in all the true
Triploblastica these potentialities are more or less realized.
The embryonic epiblast ceases in fact, in the higher forms, to
become converted as a whole into the epidermis, but first gives
rise to parts of the nervous system, organs of special sense, and
other parts.

After the formation of these parts the remnant of the
epiblast gives rise to the epidermis, and often unites more or
less intimately with a subjacent layer of mesoblast, known as
the dermis, to form with it the skin.

Various differentiations may arise in the epidermis forming
protective or skeletal structures, terminal sense organs, or
glands. The structure of the epidermis itself varies greatly, and
for Vertebrates its general modifications have been already
sufficiently dealt with in chapter XII. Of its special differenti-
ations those of a protective or skeletal nature and those of a
glandular nature may be considered in this place.

Protective epidermal structures. These structures con-
stitute a general cuticle or an exoskeleton of scales, hairs,
feathers, nails, hoofs, &c. They may be entirely formed from

394 THE EXOSKELETON.

the epidermis either as (i) a cuticular deposit, or as (2) a
chitinization, a cornification, or calcification of its constituent
cells. These two processes run into each other, and are in many
cases not easily distinguished. The protective structures of the
epidermis may be divided into two groups according as they are
formed on the outer or the inner side of the epidermis. Dermal
skeletal structures are in many cases added to them. Amongst
the Invertebrata the most widely distributed type of exoskeleton
is a cuticle formed on the outer surface of the epidermis, which
reaches its highest development in the Arthropoda. In the same
class with this cuticle must be placed the molluscan and brachi-
opod shells, which are developed as cuticular plates on special
regions of the epidermis. They differ, however, from the more
usual form of cuticle in their slighter adhesion to the subjacent
epidermis, and in their more complicated structure. The test of
Ascidians is an abnormal form of exoskeleton belonging to this
type. It is originally formed (Hertvvig and Semper) as a
cuticle on the surface of the epidermis ; but subsequently
epidermic cells migrate into it, and it then constitutes a tissue
similar to connective tissue, but differing from ordinary epidermic
cuticles in that the cells which deposit it do so over their whole
surface, instead of one surface, as is usually the case with
epithelial cells.

In the Vertebrata the two types of exoskeleton mentioned
above are both found, but that developed on the inner surface of
the epidermis is always associated with a dermal skeleton, and
that on the outer side frequently so. The type of exoskeleton
developed on the inner side of the general epidermis is confined
to the Pisces, where it appears as the scales; but a primitive
form of these structures persists as the teeth in the Amphibia
and Amniota. The type developed on the outer side of the
epidermis is almost entirely1 confined to the Amphibia and Am-
niota, where it appears as scales, feathers, hairs, claws, nails, &c.
For the histological details as to the formation of these various
organs I must refer the reader to treatises on histology, confining
my attention here to the general embryological processes which
take place in their development.

1 The horny teeth of the Cyclostomala are structures belonging to this group.

THE EPIDERMIS AND ITS DERIVATIVES.

395

The most primitive form of the first type of dermal structures
is that of the placoid scales of Elasmobranchii1. These consist,
when fully formed, of a plate bearing a spinous projection.
They are constituted of an outer enamel layer on the projecting
part, developed as a cuticular deposit of the epidermis (epiblast),
and an underlying basis of dentine (the lower part of which may
be osseous) with a vascular pulp in its axis. The development
(fig. 235) is as follows (Hertwig, No. 306). A papilla of the
dermis makes its appearance, the outer layer of which gradually
calcifies to form the dentine and osseous tissue. This papilla is
covered by the columnar mucous layer of the epidermis (e), from
which it is separated by a basement membrane, itself a product
of the epidermis. This membrane gradually thickens and calci-
fies, and so gives rise to the enamel cap (o). The spinous point
gradually forces its way through the epidermis, so as to project
freely at the surface.

The scales of other forms of fishes are to be derived from those of
Elasmobranchii. The great dermal plates of many fishes have been formed
by the concrescence of groups of such scales. The dentine in many cases
partially or completely atrophies, leaving the major part of the scale formed
of osseous tissue ; such plates often become parts of the internal skeleton.

d

5\

FIG. 235. VERTICAL SECTION THROUGH THE SKIN OF AN EMBRYONIC SHARK,
TO SHEW A DEVELOPING PLACOID SCALE. (From Gegenbaur ; after O. Hertwig.)

E. epidermis ; C. layers of dermis ; d. uppermost layer of dermis ; p. papilla of
dermis ; e. mucous layer of epidermis ; o. enamel layer.

1 For the most important contributions on this subject from which the facts and
views here expressed are largely derived, vide O. Hertwig, Nos. 306 — 808.

396 THK KXOSKELETON.

The teeth, as will be more particularly described in the section on the
alimentary tract, are formed by a modification of the same process as the
placoid scales, in which a ridge of the epithelium grows inwards to meet
a connective tissue papilla, so that the development of the teeth takes place
entirely below the superficial layer of epidermis.

In most Teleostei the enamel and dentine layers have disappeared, and
the scales are entirely formed of a peculiar calcified tissue developed in the
dermis.

The cuticle covering the scales of Reptiles is the simplest
type of protective structure formed on the outer surface of the
epidermis. The scales consist of papillae of the dermis and
epidermis ; and are covered by a thickened portion of a two-
layered cuticle, formed over the whole surface of the body
from a cornification of the superficial part of the epidermis.
Dermal osseous plates may be formed in connection with these
scales, but are never of course united with the superficial
cuticle.

Feathers are probably special modifications of such scales. They arise
rom an induration of the epidermis of papillae containing a vascular core.
The provisional down, usually present at the time of hatching, is formed by
the cornification of longitudinal ridges of the mucous layer of the epidermis
of the papillee ; each cornified ridge giving rise to a barb of the feather. The
horny layer of the epidermis forms a provisional sheath for the developing
feather below. When the barbs are fully formed this sheath is thrown- off,
the vascular core dries up, and the barbs become free except at their base.

Without entering into the somewhat complicated details of the formation
of the permanent feathers, it may be mentioned that the calamus or quill is
formed by a cornification in the form of a tube of both layers of the epidermis
at the base of the papilla. The quill is open at both ends, and to it is
attached the vexillum or plume of the feather. In a typical feather this
is formed at the apex of the papilla from ridge-like thickenings of the mucous
layer of the epidermis, arranged in the form of a longitudinal axis, con-
tinuous with the cornified mucous layer of the quill, and from lateral ridges.
These subsequently become converted into the axis and barbs of the plume.
The external epidermic layer becomes converted into a provisional horny
sheath for the true feather beneath.

On the completion of the plume of the feather the external sheath is
thrown off, leaving it quite free, and the vascular core belonging to it shrivels
up. The papilla in which the feather is formed becomes at a very early
period secondarily enveloped in a pit or follicle which gradually deepens as
the development of the feather is continued.

Hairs (Kolliker, No. 298) are formed in solid processes of
the mucous layer of the epidermis, which project into the

THE p;PIDERMIS AND ITS DERIVATIVES. 397

subjacent dermis. The hair itself arises from a cornification of
the cells of the axis of one of the above processes ; and is
invested by a sheath similarly formed from the more superficial
epidermic cells. A small papilla of the dermis grows into the
inner end of the epidermic process when the hair is first formed.
The first trace of the hair appears close to this papilla, but soon
increases in length, and when the end of the hair projects from
the surface, the original solid process of the epidermis becomes
converted into an open pit, the lumen of which is filled by the
root of the hair. Hairs differ in their mode of formation from
scales in a manner analogous to that in which the teeth differ
from ordinary placoid scales ; i.e. they are formed in inwardly
directed projections of the epidermis instead of upon free
papillae at the surface.

Nails (Kolliker, No. 298) are developed on special regions of the epider-
mis, 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.

Although the nail at first arises in the interior of the epidermis, yet its
position on the outer side of the mucous layer clearly indicates with which
group of epidermic structures it should be classified.

Dermal skeletal structures. We have seen that in the
Chordata skeletal structures, which were primitively formed of
both an epidermic and dermic element, may lose the former
element and be entirely developed in the dermis. Amongst the
Invertebrata there are certain dermal skeletal structures which
are evolved wholly independently of the epidermis. The most
important of these structures are the skeletal plates of the
Echinodermata.

Glands. The secretory part of the various glandular struc-
tures belonging to the skin is invariably formed from the
epidermis. In Mammalia it appears that these glands are
always formed as solid ingrowths of the mucous layer (Kolliker,
No. 298). The ends of these ingrowths dilate to form the true
glandular part of the organs, while the stalks connecting the
glandular portions with the surface form the ducts. In the case
of the sweat-glands the lumen of the duct becomes first
established. Its formation is inaugurated by the appearance of

398 THE EXOSKELETON.

the cuticle, and appears first at the inner end of the duct and
thence extends outwards (Ranvier, No. 311). In the sebaceous
glands the first secretion is formed by a fatty modification of the
whole of the central cells of the gland.

The muscular layer of the secreting part of the sweat-glands
is formed, according to Ranvier (No. 311), from a modification
of the deeper layer of the epidermic cells.

The Mammary Glands arise in essentially the same man-
ner as the other glands of the skin1. The glands of each side
are formed as a solid bud of the mucous layer of the epidermis.
From this bud processes sprout out, each of which gives rise to
one of the numerous glands of which the whole organ is formed.
Two very distinct types in the relation of the ducts of the
glands to the nipple are found (Gegenbaur, No. 313).

BIBLIOGRAPHY OF EPIDERMIS.
General.

(304) T. H. Huxley. " Tegumentary organs." Tocld's Cyclopaedia of Anat.
and Physiol.

(305) P. Z. Unna. " Histol. u. Entwick. d. Oberhaut." Archiv f. mikr. Anat.
Vol. xv. 1876. FzV&also Kolliker (No. 298).

Scales of tJic Pisces.

(306) O. Her twig. " Ueber Bau u. Entwicklung d. Placoidschuppen u. d.
Zahne d. Selachier." Jenaische Zeitschrift, Vol. vin. 1874.

(307) O. Hertwig. " Ueber d. Hautskelet d. Fische." Morphol. Jahrln<ch,
Vol. n. 1876. (Siluroiden u. Acipenseridre.)

(308) O. Hertwig. "Ueber d. Hautskelet d. Fische (Lepidosteus u. Polypte-
rus)." Alorph. Jahrbuch, Vol. v. 1879.

FeatJiers.

(309) Th. Studer. Die Entwick. d. Federn. Inaug. Diss. Bern, 1873.

(310) Th. Studer. "Beitrage z. Entwick. d. Feder." Zeit. f. wiss. Zool., Vol.
xxx. 1878.

Sweat-glands.

(311) M. S. Ranvier. " Sur la structure des glandes sudoripares." Comptes
A'f/iiiits, Dec. 29, 1879.

1 For a very different view on this subject vide Creighton (No. 312).

BIBLIOGRAPHY OF EPIDERMIS. 399

Mammary glands.

(312) C. Creighton. "On the development of the Mamma and the Mammary
function." Jour, of Anat. and Phys. , Vol. XI. 1877.

(313) C. Gegenbaur. " Bemerkungen iib. d. Milchdriisen-Papillen d. Sauge-
thiere." Jenaische Zeit., Vol. vn. 1873.

(314) M. Huss. "Beitr. z. Entwick. d. Milchdriisen b. Menschen u. b. Wieder-
kauern." Jenaische Zeit., Vol. vil. 1873.

(315) C. Langer. " Ueber d. Ban u. d. Entwicklung d. Milchdriisen." Denk.
d. k. Akad. Wiss. Wien, Vol. III. 1851.

CHAPTER XV.

NERVOUS SYSTEM.

Origin of the Nervous System.

ONE of the most important recent embryological discoveries
is the fact that the central nervous system, in all the Metazoa in
which it is fully established, is (with a few doubtful exceptions)
derived from the primitive epiblast1. As we have already seen
that the epiblast represents to a large extent the primitive
epidermis, the fact of the nervous system being derived from the
epiblast implies that the functions of the central nervous system,
which were originally taken by the whole skin, became gradu-
ally concentrated in a special part of the skin which was step by
step removed from the surface, and has finally become in the
higher types a well-defined organ imbedded in the subdermal
tissues.

Before considering in detail the comparative development of
the nervous system, it will be convenient shortly to review the
present state of our knowledge on the general process of its
evolution.

This process may be studied either embryologically, or by a
comparison of the various stages in its evolution preserved in
living forms. Both the methods have led to important results.

1 Whether there is any part of it in many types not so derived requires further
investigation, now that it has been shewn by the Hertwigs that part of the system
develops from the endoderm in some Coelenterata. O. Hertwig holds that part of it
has a mesoblastic origin in Sagitta, but his observations on this point appear to me
very inconclusive. It would be very advantageous to investigate the origin of
. \ucrl >ach's plexus in Mammalia.

NERVOUS SYSTEM. 401

The embryological evidence shews that the ganglion-cells of
the central part of the nervous system are originally derived
from the simple undifferentiated epithelial cells of the surface of
the body, while the central nervous system itself has arisen from
the concentration of such cells in special tracts. In the Chor-
data at any rate the nerves arise as outgrowths of the central
organ.

Another important fact shewn by embryology is that the
central nervous system, and percipient portions of the organs of
special sense, especially of optic organs, are often formed from
the same part of the primitive epidermis. Thus the retina of
the Vertebrate eye is formed from the two lateral lobes of the
primitive fore-brain.

The same is true for the compound eyes of some Crustacea.
The supracesophageal ganglia of these animals are formed in the
embryo from two thickened patches of the epiblast of the pro-
cephalic lobes. These thickened patches become gradually
detached from the surface, remaining covered by a layer of
epidermis. They then constitute the supraoesophageal ganglia ;
but they form not only the ganglia, but also the retinulae of the
eye — the parts in fact which correspond to the rods and cones in
our own retina. The accessory parts of these organs of special
sense, viz. the crystalline lens of the Vertebrate eye, and the
corneal lenses and crystalline cones of the Crustacean eye, are
independently formed from the epiblast after the separation of
the part which becomes the central nervous system.

In the Acraspedote Medusae the rudimentary central nervous
system has the form of isolated rings, composed of sense-cells
prolonged into nervous fibres, surrounding the stalks of tentacle-
like organs, at the ends of which are placed the sense-organs.

This close connection between certain organs of special sense
and ganglia is probably to be explained by supposing that the
two sets of structures actually originated part passu.

We may picture the process as being somewhat as. follows : —

It is probable that in simple ancestral organisms the whole body was
sensitive to light, but that with the appearance of pigment-cells in certain
parts of the body, the sensitiveness to light became localised to the areas
where the pigment-cells were present. Since, however, it was necessary
that stimuli received by such organs should be communicated to other parts

B. III. 26

402 EVOLUTION OF THE NERVOUS SYSTEM.

of the body, some of the epidermic cells in the neighbourhood of the
pigment-spots, which were at first only sensitive in the same manner as
other cells of the epidermis, became gradually differentiated into special
nerve-cells. As to the details of this differentiation embryology does not as
yet throw any great light ; but from the study of comparative anatomy there
are grounds for thinking that it was somewhat as follows: — Cells placed on
the surface sent protoplasmic processes of a nervous nature inwards, which
came into connection with nervous processes from similar cells placed
in other parts of the body. The cells with such processes then became
removed from the surface, forming a deeper layer of the epidermis below
the sensitive cells of the organ of vision. With the latter cells they remained
connected by protoplasmic filaments, and thus they came to form a thicken-
ing of the epidermis underneath the organ of vision, the cells of which
received their stimuli from those of the organ of vision, and transmitted the
stimuli so received to other parts of the body. Such a thickening would
obviously be the rudiment of a central nervous system, and is in fact very
similar to the rudimentary ganglia of the Acraspeda mentioned above. It
is easy to see by what steps it might become larger and more important,
and might gradually travel inwards, remaining connected with the sense-
organ at the surface by protoplasmic filaments, which would then constitute
nerves. The rudimentary eye would at first merely consist of cells sensitive
to light, and of ganglion-cells connected with them ; while at a later period
optical structures, constituting a lens capable of throwing an image of
external objects upon it, would be developed, and so convert the whole
structure into a true organ of vision. It has thus come about that, in the
development of the individual, the retina is often first formed in connection
with the central nervous system, while the lenses of the eye are indepen-
dently evolved from the epidermis at a later period.

A series of forms of the Ccelenterata and Platyelminthes
affords us examples of various stages in the differentiation of a
central nervous system1.

In sea-anemones (Hertwigs, No. 321) there are, for instance, no organs
of special sense, and no definite central nervous system. There are, however,
scattered throughout the skin, and also throughout the lining of the digestive
tract, a number of specially modified epithelial cells, which are no doubt
delicate organs of sense. They are provided at their free extremity with a
long hair, and are prolonged on their inner side into fine processes which
penetrate into the deeper part of the epithelial layer of the skin or digestive
wall. They eventually join a fine network of protoplasmic fibres which forms
a special layer immediately within the epithelium. The fibres of this net-
work are no doubt essentially nervous. In addition to fibres there are,

1 Our knowledge on this subject is especially due to the brothers Hertwig (Nos.
320 and 321), Eimer (No. 318), Claus (No. 317), Schafer (No. 326), and Hubrecht
(No. 323).

NERVOUS SYSTEM.

403

FIG. 236. NEURO-
EPITHELIALSENSE-
CELLS OFAURELIA

AURITA. (From

Lankester ; after
Schafer.)

moreover, present in the network cells of the same character as the multipolar
ganglion-cells in the nervous system of Vertebrates, and some of these cells
are characterised by sending a process into the superjacent epithelium.
Such cells are obviously intermediate between neuro-
epithelial cells and ganglion-cells ; and it is probable
that the nerve-cells are, in fact, sense-cells which have
travelled inwards and lost their epithelial character.

In the Craspedote Medusae (Hertwigs, No. 320)
the differentiation of the nervous system is carried
somewhat further. There is here a definite double
ring, placed at the insertion of the velum, and usually
connected with sense-organs. The two parts of the
ring belong respectively to the epithelial layers on
the upper and lower surfaces of the velum, and are not
separated from these layers ; they are formed of fine
nerve-fibres and ganglion-cells. The epithelium above
the nerve rings contains sense-cells (fig. 237) with a
stiff hair at their free extremity, and a nervous pro-
longation at the opposite end, which joins the nerve-
fibres of the ring. Between such cells and true ganglion-
cells an intermediate type of cell has been found (fig.
237 B) which sends a process upwards amongst the
epithelial cells, but does not reach the surface. Such cells, as the Hertwigs
have pointed out, are clearly sense-cells partially transformed into ganglion-
cells.

A still higher type of nervous system has been met with amongst some
primitive Nemertines (Hubrecht, No. 323), consisting of a pair of large
cephalic ganglia, and two well-developed lateral ganglionic cords placed
close beneath the epidermis. These cords, instead of giving off definite
nerves, as in animals with a fully differentiated nervous system, are con-
nected with a continuous subdermal nervous plexus.

The features of the embryology and the anatomy of the
nervous system, to which attention has just been called, point to
the following general conclusions as to the evolution of the
nervous system.

(1) The nervous system of the higher Metazoa appears to
have been evolved in the course of a long series of generations
from a differentiation of some of the superficial epithelial cells of
the body, though it is possible that some parts of the system
may have been formed by a differentiation of the alimentary
epithelium.

(2) An early feature in the differentiation consisted in the
growth of a series of delicate processes of the inner ends of

26 — 2

404

EVOLUTION OF THE NERVOUS SYSTEM.

certain epithelial cells, which became at the'same time especially
differentiated as sense-cells (figs. 236 and 237).

FIG. 237. ISOLATED CELLS BELONGING TO THE UPPER NERVE-RING OF CARMARINA
HASTATA. (After O. and R. Hertwig.)

A. Neuro-epithelial sense-cell, c. sense-hair.

B. Transitional cell between a neuro-epithelial cell and a ganglion -cell.

(3) These processes gave rise to a subepithelial nervous
plexus, in which ganglion-cells, formed from sense-cells which
travelled inwards and lost their epithelial character (fig. 237 B),
soon formed an important part.

(4) Local differentiations of the nervous network, which was
no doubt distributed over the whole body, took place partly in
the formation of organs of special sense, and partly in other
ways, and such differentiations gave rise to a central nervous
system. The central nervous system was at first continuous
with the epidermis, but became separated from it and travelled
inwards.

(5) Nerves, such as we find them in the higher types,
originated from special differentiations of the nervous network,
radiating from the parts of the central nervous system.

The following points amongst others are still very obscure : —

(1) The steps by which the protoplasmic processes from the primitive
epidermic cells became united together so as to form a network of nerve-
fibres, placing the various parts of the body in nervous communication.

(2) The process by which nerves became connected with muscles, so
that a stimulus received by a nerve-cell could be communicated to and
cause a contraction in a muscle.

It is probable, as stated in the above summary, that the nervous net-

NERVOUS SYSTEM. 405

work took its origin from processes of the sense-cells. The processes of the
different cells probably first met and then fused together, and, becoming
more arborescent, finally gave rise to a complicated network.

The primitive relations between the
nervous network and the muscular system
are matters of pure speculation. The
primitive muscular cells consist of epithe-
lial cells with muscular processes (fig. 238),
but the branches of the nervous network
have not been traced into connection with FIG. 238. MYO-EPITHELIAL

the muscles in any Ccelenterata except CELLS OF HYDRA. (From Gegen-
the Ctenophora. In the higher types a baur5 after Kleinenberg.)
continuity between nerves and muscles '«• contractile fibres; processes

in the form of motorial end plates has

been widely observed. Even in the case of the Ccelenterata it is quite
clear from Romanes' experiments that stimuli received by the nerves are
capable of being transmitted to the muscles, and that there must therefore
be some connection between nerves and muscles. How did this connection
originate?

Epithelial cells with muscular processes (fig. 238) were discovered by
Kleinenberg (No. 324) in Hydra before epithelial cells with nervous pro-
cesses were known, and Kleinenberg pointed out that Hydra shewed the
possibility of nervous and muscular tissues existing without a central nervous
system, and suggested that the epithelial part of the myo-epithelial cells was
a sense-organ, and that the connecting part between this and the contractile
processes was a rudimentary nerve. He further supposed that in the subse-
quent evolution of these elements the epithelial part of the cell became a
ganglion-cell, while the part connecting this with the muscular tail became
prolonged so as to form a true nerve. The discovery of neuro-epithelial
cells existing side by side with myo-epithelial cells demonstrates that this
theory must in part be abandoned, and that some other explanation must be
given of the continuity between nerves and muscles. The hypothetical
explanation which most obviously suggests itself is that of fusion.

It seems quite possible that many of the epithelial cells of the epidermis
and walls of the alimentary tract were originally provided with processes,
the protoplasm of which, like that of the Protozoa, carried on the functions
of nerves and muscles at the same time, and that these processes united
amongst themselves into a network. Such cells would be very similar to
Kleinenberg's neuro-muscular cells. By a subsequent differentiation some
of the cells forming this network may have become specially contractile, the
epithelial parts of the cells ceasing to have a nervous function, and other
cells may have lost their contractility and become solely nervous. In this way
we should get neuro-epithelial cells and myo-epithelial cells both differen-
tiated from the primitive network, and the connection between the two would
also be explained. This hypothesis fits in moreover very well with the
condition of the neuro-muscular system as we find it in the Coelenterata.

406 INVERTEBRATA.

BIBLIOGRAPHY.
Origin of the Nervous System,

(316) F. M. Balfour. " Address to the Department of Anat. and Physiol. of the
British Association." 1880.

(317) C. Claus. "Studien lib. Polypen u. Quallen d. Adria. I. Acalephen,
Discomedusen." Denk. d. math.-naturwiss. Classe d. k. Akad. Wiss. Wien, Vol.
xxxvin. 1877.

(318) Th. Eimer. Zoologische Studien a, Capri. I. Ueber Beroe ovatus, Ein
Beitrag 2. Anat. d. Rippenquallen. Leipzig, 1873.

(319) V. Hen sen. " Zur Entwicklung d. Nervensystems. " Virchmifs Archiv,
Vol. xxx. 1864.

(320) O. and R. Hertwig. Das Nerveiisystem u. d. Sinnesorgane d. Medusen.
Leipzig, 1878.

(321) O. and R. Hertwig. "Die Actinien anat. u. histol. mit besond. Beriick-
sichtigung d. Nervenmuskelsystem untersucht." Jenaische Zeit., Vol. xin. 1879.

(322) R. Hertwig. "Ueb. d. Bau d. Ctenophoren." Jenaische Zeitschrift,
Vol. xiv. 1880.

(323) A. W. Hubrecht. "The Peripheral Nervous System in Palaeo- and
Schizonemertini, one of the layers of the body- wall." Quart. J. of After. Science,
Vol. xx. 1880.

(324) N. Kleinenberg. Hydra, eine anatomisch-entwicklungsgeschichtliche Un-
tersuchung. Leipzig, 1872.

(325) A. Kowalevsky. " Embryologische Studien an Wurmern u. Arthropo-
den." Mem. Acad. Petersbourg, Series VII., Vol. XVI. 1871.

(326) E. A. Schafer. "Observations on the nervous system of Aurelia aurita."
Phil. Trans. 1878.

Nervous system of the Invertebrata. Our knowledge of
the development of the central nervous system is still very
imperfect in the case of many Invertebrate groups. In the
Echinodermata and some of the Ghaetopoda it is never detached
from the epidermis, and in such cases its origin is clear without
embryological evidence.

In the majority of groups the central nervous system may be
reduced to the type of a pair of cephalic ganglia, continued pos-
teriorly into two cords provided with nerve-cells, which may
coalesce ventrally or be' more or less widely separated, and be
unsegmented or segmented. Various additional visceral ganglia
may be added, and in different instances parts of the system
may be much reduced, or peculiarly modified. The nervous
system of the Platyelminthes (when present), of the Rotifera,
Brachiopoda, Polyzoa (?), the Mollusca, the Chaetopoda, the

NERVOUS SYSTEM. 407

Discophora, the Gephyrea, the Tracheata, and the Crustacea,
the various small Arthropodan phyla (Pcecilopoda, Pycnognida,
Tardigrada, &c.), the Chaetognatha (?), and the Myzostomea,
probably belongs to this type.

The nervous system of the Echinodermata cannot be reduced
to this form ; nor in the present state of our knowledge can that
of the Nematelminthes or Enteropneusta.

It is only in the case of members of the former set of groups
that any adequate observations have yet been made on the
development of the nervous system, and even in the case of
these groups observations which have any claim to completeness
are confined to certain members of the Chaetopoda, the Arthro-
poda and the Mollusca. An account of imperfect observations
on other forms, where such have been made, will be found in the
systematic part of this work.

Chaetopoda. We are indebted to Kleinenberg (No. 329) for
the most detailed account which we have
of the development of the central nervous
system in the Chaetopoda.

The supracesophageal ganglion with
the cesophageal commissure developes in-
dependently of the ventral cord. It arises
as an unpaired thickening of the epiblast, pIG- 239. SECTION
close to the dorsal side of the oesophagus THROUGH THE HEAD OF

A 'YOUNG EMBRYO OF

at the front end of the head (fig. 239), LUMBRICUS TRAPEZOIDES.
which becomes separated from the epi- <After Kleinenbers-)

, e.g. cephalic ganglion ;

blast, and extends obliquely backwards CCi cephalic portion of the
and downwards in a somewhat arched body cavity ;*. oesophagus.
form ; its lower extremities being somewhat swollen. The
inner portion of this curved rudiment becomes converted into
commissural nerve-fibres, while the cells of the outer and upper
portion assume the characters of ganglion-cells. The com-
missural fibres are continued downwards to meet the ventral
chord, but their junction with the latter structure is not effected
till late in embryonic life.

The ventral cord is formed by the coalescence of a pair of
linear cords, the development of which takes place from before
backwards, so that when their anterior part is well developed their
posterior part is hardly differentiated. These cords arise, one on

408

CH^TOPODA.

FIG. 240. SECTION THROUGH
PART OF THE VENTRAL WALL OF THK
TRUNK OF AN EMBRYO OF LUMBRI-
cus TRAPEZOIDES. (After Kleinen-
berg.)

m. longitudinal muscles ; so. so-
matic mesoblast ; sp. splanchnic me-
soblast; hy. hypoblast; Vg- ventral
nerve-cord; w. ventral vessel.

each side of a ventral ciliated furrow, first as a single row of epi-
blast cells, and subsequently as several rows (fig. 240, Vg). While
still united to the external epi-
blast, they extend themselves be-
low the cells lining the ventral
furrow, and unite into a single
nervous band, which however
exhibits its double origin by its
bilobed section. Before the two
cords unite, the groove between
them becomes somewhat deep,
but subsequently shallows out
and disappears. The nervous
band, before separating from the
epiblast, exhibits, in correspond-
ence with the mesoblastic segments, alternate swellings and
constrictions. The former become the ganglia, and the latter the
connecting trunks.

As soon as the cord becomes free from the epiblast, it
becomes surrounded by a sheath, formed of somatic mesoblast.
In each of the ganglionic enlargements there next appears on
the dorsal surface a pair of areas of punctiform material, the
substance of which soon differentiates itself into .nerve-fibres.
These areas, by uniting from side to side, give rise to the
transverse commissures, and also by a linear coalescence to the
longitudinal commissures of the cord. The cellular parts of the
band surrounding them become converted into a ganglionic
covering of the cord.

In each ganglion the cells of this ganglionic investment
penetrate as a median septum into the cord. A fissure is next
formed, dividing this septum into two ; it is subsequently
continued for the whole length of the cord.

Arthropoda. In the Tracheata and the Crustacea the
development of the ventral cord is in the main similar to that in
the Chaetopods, while that of the supracesophageal ganglia is as
a rule somewhat more complicated. No such clear evidence of
an independent development of these two parts, as in the case
of the Chaetopods, has as yet been produced.

The most primitive type of nervous system amongst the

NERVOUS SYSTEM. 409

Tracheata is that of Peripatus, where it consists of large supra-
oesophageal ganglia, continuous with a pair of widely separated
but large ventral cords united posteriorly above the anus. These
cords have an investment of ganglion- cells for their whole length,
and are imperfectly divided into ganglia corresponding in
number with the feet.

The ventral cords are formed as two separate epiblastic
ridges (fig. 241, v.n], continued in front into a pair of thickenings

FIG. 241. SECTION THROUGH THE TRUNK OF AN EMBRYO OF PERIPATUS.
The embryo from which the section is taken was somewhat younger than that of
fig. 242.

sp.m. splanchnic mesoblast ; s.m. somatic mesoblast ; me. median section of body
cavity ; Ic. lateral section of body cavity ; -v. 11. ventral nerve cord ; me. mesenteron.

of the procephalic lobes, which are at first independent of each
other, and from which a large part of the supracesophageal
ganglia takes its origin. . After the latter have become separated
from the epiblast an invagination of the epiblast covering them
grows into each lobe (fig. 242), and becoming constricted from
the superficial epiblast, which remains as the epidermis, forms a
not unimportant part of the permanent supracesophageal ganglia.

In the Arachnida the mode of development of the nervous
system is essentially the same, and the reader will find a
detailed account of it for Spiders in Vol. II. pp. 447 — 451. The
ventral cords are here formed as independent and at first widely
separated strands (fig. 243, vii), which for a long time remain far
apart ; they are subsequently divided into ganglia and become
united by transverse commissures.

The supracesophageal ganglia are formed as two independent

4io

ARTHROPODA.

thickenings of the procephalic lobes (fig. 244), which eventually
separate from the superficial skin. There is formed however in

FIG. 242. HEAD OF AN EMBRYO PERIPATUS. (From Moseley.)
The figure shews the jaws (mandibles), and close to them epiblastic involutions,
which grow into the supracesophageal ganglia. The antennas, oral cavity, and oral
papillae are also shewn.

each of them a semicircular groove (fig. 244, gr) lined by the
superficial epiblast, which becomes detached from the skin, and
is involuted to form part of the ganglia.

A similar mode of formation of both the ventral cords and
the supraoesophageal ganglia obtains in Insects (fig. 245). The

FIG. 243. TRANSVERSE SECTION THROUGH THE VENTRAL PLATE OF AGELENA

LABYRINTHICA.

The ventral cords have begun to be formed as thickenings of the epiblast, and the
limbs are established.

me.s. mesoblastic somite; vn. ventral nerve-cord; yk. yolk.

ventral cords are however much less widely separated than in
Spiders, and early unite in the median line. In the supraoeso-
phageal ganglia the invaginated epiblast has in Lepidoptera
(Hatschek) the form of a pit on the dorsal border of the
antennae.

NERVOUS SYSTEM.

Hatschek states that there takes place an invagination of a median part
of the skin between the two ventral cords, for the details of which I must
refer the reader to Vol. II. p. 410. He has made more or less similar
statements for the earthworm, but his observations in both instances are
open to serious doubt.

ce.s

FIG. 244. SECTION THROUGH THE PROCEPHALIC LOBES OF AN EMBRYO OF

AGELENA LABYRINTHICA.

st. stomadaeum; gr. section through semi-circular groove in procephalic lobe;
ce.s. cephalic section of body cavity.

Full details as to the development of the nervous system in
the Crustacea are still wanting ; a fairly complete account of

nie.s

FlG. 245. TWO TRANSVERSE SECTIONS THROUGH THE EMBRYO OF HYDROPHILUS.

(After Kowalevsky.)

A. Transverse section through an embryo in the region of one of the stigmata.

B. Transverse section through an older embryo.

vn. ventral nerve-cord ; am. amnion and serous membrane ; me. mesoblast ; me.s.
somatic mesoblast; hy. hypoblast (?) ; yk. yolk-cells (true hypoblast); st. stigma of
trachea.

412 GEPHYREA.

what is known on the subject is given in Vol. n. pp. 521 — 2. It
appears that the ventral cord may either arise as an unpaired
thickening of the epiblast (Isopoda), marked however by a
shallow median furrow, or from two cords which eventually
coalesce1. It is not certain how far the supracesophageal
ganglia are usually in the first instance continuous with the
ventral cord. In Astacus, the early stages of which have been
elaborately investigated by Reichenbach (No. 331), they are
stated to be so ; the supracesophageal ganglia are moreover de-
scribed by this author as having a somewhat complicated origin.
Five elements enter into their composition. There is first
formed a pair of pits on the procephalic lobes, which become
very deep during the Nauplius stage, and are continuous with a
pair of epiblastic ridges which pass round the mouth, and join
the ventral cords just described. The walls of the pits are
believed to form a part of the embryonic ganglia which gives
rise to the retina as well as to the optic ganglia. The ridges
form the remainder of the ganglia and the cesophageal com-
missures ; while the fifth element is supplied by a median
invagination in front of the mouth, which appears at a much
later date than the other parts.

In the Isopoda supracesophageal ganglia are stated to arise
as thickenings of the procephalic lobes, which become eventually
detached from the epidermis.

The ventral cord is at first unsegmented, but soon becomes
partially divided by a series of constrictions into a number of
ganglia, corresponding with the segments. The development of
the commissural and ganglionic portions takes place much as in
the Chaetopoda.

The Gephyrea approach closely the types so far dealt with, but the
ventral cord in the Inermia is formed as an unpaired thickening of the
epiblast. In Echiurus, as has been shewn by Hatschek in an interesting
paper on the larva of this species, published since the appearance of the first
volume, there is a pair of ventral cords2. In correspondence with a general
segmentation of the body, which is subsequently lost, these cords become

1 Reichenbach (No. 331) holds that the walls of the groove between the two
strands of the ventral cords become invaginated and assist in the formation of the
ventral cord.

8 " Ueber Entwicklungsgeschichte d. Echiurus." Arbeit, a. d. zool. Instit. Wien
Vol. ill. 1880.

NERVOUS SYSTEM.

4'3

segmented. The two cords unite in the median line, and Hatschek, in
accordance with his general view on this subject, states that their junction is
effected by means of a median cord of invaginated epiblast. The segmenta-
tion of the cords subsequently becomes lost. The supracesophageal ganglia
arise as an unpaired median thickening of the procephalic lobe. No traces
of segmentation in the ventral cord have been observed by Spengel in
Bonellia, and the supracesophageal ganglion is formed in this genus as an
unpaired band.

In all the groups above considered the nervous system
clearly presents the same type of development with various
modifications.

It is formed of two parts, viz. (i) the supracesophageal
ganglia, and (2) the ventral cord.

In the simpler forms, Chaetopoda and Gephyrea, the supra-
cesophageal ganglia are usually stated to be formed as an
unpaired thickening at the apex of the praeoral lobe, which in
most cases becomes subsequently bilobed.

In the Arthropoda the unpaired praeoral lobe of the Chaeto-
poda is replaced by the so-called procephalic lobes, which are
themselves bilobed ; and the supracesophageal ganglia are
formed of two independent halves ; further complications in
development are also generally found.

There is not as yet sufficient evidence to decide whether the
supracesophageal ganglia were primitively developed continu-
ously with, or independently of, the ventral cords.

The ventral cord appears in the embryo as two independent
unsegmented strands, although in a few cases (some Crustacea
and Gephyrea) these cords, by an abbreviation in development,
arise as an unpaired median thickening of the epiblast.

The form of nervous system of the Chaetopoda, Arthropoda,
and Gephyrea is clearly therefore to be derived, as was first
pointed out by Gegenbaur, from a more or less similar type to
that now found in the Nemertines ; and as suggested in the
chapter on larval forms (vide p. 378) may perhaps be derived
from the elongation of a circular ring, of which the anterior end
has become developed into the supracesophageal ganglia, the
lateral parts into the two lateral strands, while the posterior
part persists in some forms in the junction of the ventral cords
above the anus (Enopla and Peripatus).

414 MOLLUSCA.

Mollusca. While study of the anatomy of the nervous system of the
Mollusca, especially of certain primitive genera (Chiton, Haliotis, Fissurella,
&c.) leaves little doubt that it is formed on the same type as that of the
groups just spoken of, the development, so far as our imperfect knowledge
enables us to make definite statements on the subject, is somewhat ab-
normal1.

In the Gasteropoda and Pteropoda the supracesophageal ganglia appear
most probably to be developed either as paired thickenings of the epiblast
of the velar area, or as invaginated pits of the velar area, which become
detached from the surface, and then become solid (Hyaleacea and Limax).
In either case the supracesophageal ganglia appear to be developed quite
independently of the pedal ganglia. The latter, as might be anticipated, are
earlier in their development and more constant than the various visceral
ganglia ; and, if the views above expressed are correct, are homologous
with the ventral cord of the Chaetopods and Arthropods. Their actual
development is very imperfectly known.

The most precise statements on the subject, viz. those of Bobretzky and
Fol, would lead us to suppose that they arise in the mesoblast, but it seems
more probable that they are formed as thickenings of the sides of the foot.

In the Cephalopods all the ganglia are stated to be differentiated in the
mesoblast (Lankester, Bobretzky).

Hatschek2 has recently given a detailed description of the development
of the supracesophageal and pedal ganglia of Teredo. He finds that the
former ganglia arise as an unpaired thickening of the epiblast in the centre
of the velar area, and the latter as an unpaired thickening of the epiblast
of the ventral side of the body between the mouth and the anus. The two
ganglia would thus seem to be disconnected with each other in their
development.

(327) F. M. Balfour. "Notes on the development of the Araneina." Quart.
J. of Micr. Science, Vol. XX. 1880.

(328) B. Hatschek. " Beitr. z. Entwicklung d. Lepidopteren." Jenaische
Zeitschrift, Vol. xi. 1877.

(329) N. Kleinenberg. "The development of the Earthworm, Lumbricus
Trapezoides." Quart. J. of Micr. Science, Vol. XIX. 1879.

(330) A. Kowalevsky. " Embryologische Studien an Wiirmem u. Arthro-
poden." Mem. Acad. Petersbourg, Series vin., Vol. XVI. 1871.

(331) H. Reichenbach. " Die Embryonalanlage u. erste Entwick. d. Fluss-
krebses." Zeit.f. wiss. Zool., Vol. xxix. 1877.

1 Vide Vol. ii., pp. 273, 274.

2 " Ueber Entwicklungsgeschichte von Teredo." Arbeit, a. d. zool. Instit, IVieit,
Vol. in. 1880.

NERVOUS SYSTEM OF THE VERTEBRATA. 415

THE CENTRAL NERVOUS SYSTEM OF THE VERTEBRATA1.

The formation of the cerebro-spinal axis of the Chordata
from the medullary plate has already been treated at length
(pp. 301 — 304). Before entering into the consideration of the
morphological value of the various parts of this cord, it will be
convenient to describe the more important features of its
ontogeny. For this purpose the two parts into which the
nervous axis becomes at an early period divided, viz. the spinal
cord and the brain, may be dealt with separately.

The Spinal Cord, shortly after the closure of the medullary
canal, has, in all the true Vertebrata, the form of an oval tube ;
the walls of which are of a fairly uniform thickness, and are
composed of several rows of elongated cells. This cord, as
development proceeds, usually becomes vertically prolonged in
transverse section, and the central canal which it contains also
becomes vertically elongated. The variations in shape of the
spinal canal are very great at different periods and in different
parts of the body, and an attempt to chronicle them would
appear, in the present state of our knowledge, to be quite
valueless'2. Fig. 117, in which the spinal cord of the chick
of the third day is shewn in transverse section, illustrates the
character of the cord at the stage just described. Up to this
time the walls of the spinal canal have exhibited an uniform
structure. A series of changes now however takes place, which
results in the differentiation (i) of the epithelium of the central
canal, (2) of the grey matter of the cord, and (3) of the external
coating of white matter.

The relative time at which each of these parts becomes
developed is not constant in the different forms.

The white matter is apparently the result of a differentiation
of the outermost parts of the superficial cells of the cord into

1 For the development of the central nervous system in Amphioxus and the
Tunicata the reader is referred to the chapters dealing with those two groups.

2 Lowe (No. 341) holds that at an early stage of development three regions can
always be distinguished in any section of the central canal, viz. (i) a ventral narrow
slit, (2) a median enlargement, and (3) a dorsal slit. Such a form can no doubt often
be observed, but my own observations do not lead me to attach any special importance
to it.

41 6 SPINAL CORD.

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 and would seem to contain neither nuclei nor
cells1. The white matter may from the first form only two
masses, one on each side, forming a layer on the ventral and
lateral parts of the spinal cord but not extending to the dorsal
surface (Elasmobranchii, fig. 185, W) ; or it may form 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

»c

FIG. 246. SECTION THROUGH THE SPINAL CORD OF A SEVEN DAYS' CHICK.

pew. dorsal white column ; lew. lateral white column ; acw. ventral 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.

posterior nerve-roots (the Fowl, Human embryo, etc.). In
whichever of these forms the white matter appears, it is always,
at first, a layer of extreme tenuity, which rapidly increases

1 This holds true at first for Elasmobranchii, but at a later stage there are present
numerous nerve-cells in the white matter, so that the distinction between the white
and grey matter becomes much less marked than in higher types; in this respect Elas-
mobranchii present an approximation to Amphioxus.

NERVOUS SYSTEM OF THE VERTEBRATA. 417

in thickness in the subsequent stages, and extends so as
gradually to cover the whole cord (fig. 246).

The anterior white commissure is formed very shortly after
the first appearance of the white matter. The grey matter and
the central epithelium are formed by a differentiation of the
main mass of the spinal cord. The outer cells lose their
epithelial-like arrangement, and, becoming prolonged into fibres,
give rise to the grey matter, while the innermost cells retain
their primitive arrangement, and constitute 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.

As has already been mentioned, the central epithelium of
the nervous system probably 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: — (i) 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 no trace of the ventral or dorsal
fissure, and the shape of the central canal is not very different to
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 owes its origin to a downgrowth of the
anterior horns of the cord on each side of the middle line. The
two downgrowths enclose between them a somewhat linear
space — the anterior fissure — which increases in depth in the
succeeding stages (fig. 246, af}.

The dorsal or posterior fissure is formed at a later period
than the anterior, and accompanies the atrophy of the dorsal
section of the embryonically large canal of the spinal cord.
B. III. 27

41 8 SPINAL CORD.

The exact mode of its formation appears to me to be still
involved in some obscurity.

In the Elements of Embryology the development of the posterior fissure
was described in the following way :

" On the seventh day the most important event is the formation of the
posterior fissure,

" This is brought about by the absorption of the roof of the posterior of
the two parts into which the neural canal has become divided.

"Between the posterior horns of the cord, the epithelium forming the
roof of the, so to speak, posterior canal is along the middle line covered
neither by grey nor by white matter, and on the seventh day is partially
absorbed, thus transforming the canal into a wedge-shaped fissure, whose
mouth however is seen in section to be partially closed by a triangular
clump of elongated cells (fig. 246, c]. Below this mass of cells the fissure
is open. It is separated from the 'true spinal canal' by a very narrow space
along which the side walls have coalesced. In the lumbar and sacral regions
the two still communicate.

"We thus find, as was first pointed out by Lockhart Clarke, that the
anterior and posterior fissures of the spinal cord are, morphologically speak-
ing, entirely different. The anterior fissure is merely the space left between
two lateral downward growths of the cord, while the posterior fissure is part
of the original neural canal separated from the rest of the cavity (which goes
to form the true spinal canal) by a median coalescence of the side walls."

I confess that I have some doubts as to the complete accuracy of the
above statement.

Kolliker gives a full account of the gradual atrophy of the central canal ;
but I do not fully understand his statements with reference to the formation
of the posterior fissure, which in fact appears to be only incidentally
mentioned. It would seem from his account that a shallow and somewhat
wide dorsal fissure is formed to start with, in the human embryo, by two
projections of the posterior white horns. On the atrophy of the central
canal this furrow becomes narrowed, but Kolliker does not definitely state
how it becomes deepened so as to give rise to the permanent dorsal fissure.

It seems to me 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 the canal coalesce dorsally, and the coalescence
gradually extends ventralwards, 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

NERVOUS SYSTEM OF THE VERTEBRATA. 419

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, which is not improbably
derived from part of the epithelium of the original central
canal.

Development of the Brain.

The brain is formed from the anterior portion of the me-
dullary plate. When the medullary plate first becomes diffe-
rentiated it is not possible to distinguish between the region of
the brain and that of the spinal cord. The brain region is
however usually very early indicated by a widening of the
medullary plate, but does not become sharply marked off from
the region of the spinal cord. In many Ichthyopsida (Elasmo-
branchii (fig. 28, C) and Amphibia (fig. 77, A)) the anterior
dilatation gives to the medullary plate, before its sides meet to
form a canal, a spatula-like form ; which is either not present or
less marked in Reptilia, Aves and Mammalia.

The length of the brain as compared to the spinal cord is
always very great in the embryo, and in the earliest develop-
mental periods the disproportion in the size of the brain is
specially marked, owing to the full number of the somites of
the trunk not having been formed. In Elasmobranchii the
brain is about one-third of the whole length of the embryo at
the stage immediately following the closure of the medullary
canal.

The first differentiation of the brain into distinct parts is
a very early occurrence, and may take place before (Mammalia)
or during the closure of the medullary folds. The brain first
becomes divided into two successive lobes or vesicles by a
single transverse constriction, and subsequently the posterior of
these again 'becomes divided into two, so that three lobes

27—2

42O

THE BRAIN.

are formed — known as the fore- the mid- and the hind-brain ; of
these the hind-brain is usually the longest. In some instances a
bilobed stage can hardly be recognised. This primitive division
of the brain is shewn in many of the figures already given.
The reader may perhaps best refer to fig. 108. On the closure
of the medullary groove the lumen of the medullary canal
is continued uninterruptedly through the brain, but dilates
considerably in each of the cerebral vesicles.

The anterior lobe of the brain becomes converted into the
cerebral hemispheres, the thalamencephalon, the primary optic
vesicles, and the parts connected with them. The middle lobe
becomes the optic lobes (corpora bigemina or corpora quadri-
gemina in Mammalia) and the crura cerebri ; while the posterior
lobe becomes converted into the cerebellum and medulla
oblongata.

Before describing in detail the changes by which the primary
vesicles of the brain become con-
verted into the above parts, it will
be convenient to say a few words
about the general development of
the brain.

The most striking peculiarity
with reference to the general develop-
ment of the brain is a curvature
which appears in its axis, known as
the cranial flexure. The flexure
takes place through the mid-brain;
and causes the fore-brain to be
gradually bent downwards so that
the axis of its floor forms, first, a
right angle with that of the hinder
part of the brain, and subsequently,
as a rule, an acute angle.

During these changes the brain,
in most Amniota at any rate, becomes in the first instance
retort-shaped, the cerebral vesicle forming the swollen part of
the retort, but subsequently the retort-shape is lost owing to the
great development of the vesicle of the mid-brain, which forms
the termination of the long axis of the embryo. Figs. 29, 76,

FIG. 247. LONGITUDINAL
SECTION THROUGH THE BRAIN
OF A YOUNG PRISTIURUS EMBRYO.

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

NERVOUS SYSTEM OF THE VERTEBRATA.

421

mb

pn.

and 1 1 8, are representative figures of embryos of various
vertebrate forms at a period when the mid-brain forms the
termination of the long axis of the body.

It is generally stated that the
cranial flexure is at its maximum
at the stage represented in these
figures, and there can be no
doubt that viewed from the ex-
terior the cranial flexure ceases
to be so marked a feature, and
finally disappears as the embryo
gradually grows older ; but
though the mid-brain ceases to
form the termination of the long
axis of the embryo, the flexure
of the brain becomes in many
forms absolutely more marked ;
while in other forms, though
stated to diminish, it does not
entirely vanish.

The general nature of the
changes which take place will
perhaps best be understood by a
comparison of figs. 247 and 248
representing longitudinal sec-
tions at two stages through the brain of an embryo Elasmo-
branch. The actual cranial flexure, i.e. flexure of the floor of
the brain, is obviously greater in the older of the two brains,
though viewed from the exterior the axis of this brain appears
to be quite straight. In the younger stage, fig. 247, the mid-
brain (mb) forms the end of the long axis of the body, while in
the older one the cerebral hemispheres (cer) have grown very
greatly, especially forwards and dorsalwards. They have thus
come to lie in front of the mid-brain, and to form the end of the
long axis of the body, and have at the same time compressed
the originally large thalamencephalon against the mid-brain.
The same general features may be seen in fig. 250 representing
a longitudinal section of the brain of an embryo fowl, and fig. 255
representing a longitudinal section of the brain of a Mammal.

FIG. 248. LONGITUDINAL SECTION
THROUGH THE BRAIN OF SCYLLIUM
CANICULA AT AN ADVANCED STAGE OF
DEVELOPMENT.

cer. cerebral hemisphere ; pn. pi-
neal gland ; op.th. optic thalamus, con-
nected with its fellow by a commissure
(the middle commissure). In front of
it is seen a fold of the roof of the fore-
brain, which is connected with the cho-
roid plexus of the third ventricle ; op.
optic chiasma ; //. pituitary body ; in.
infundibulum ; cb. cerebellum ; ati.v.
passage leading from the auditory vesi-
cle to the exterior ; mel. medulla ob-
longata ; c.in. internal carotid artery.

422 HISTOGENESIS OF THE BRAIN.

The infundibulum or perhaps rather the point of origin
of the optic nerves is to be regarded as the anterior termination
of the axis of the base of the brain.

The cranial flexure is least marked in Cyclostomata (fig. 253), Teleostei,
Ganoidei, and Amphibia, while it is very pronounced in Elasmobranchii,
Reptilia, Aves, and Mammalia. In Teleostei, and still more in Cyclostomata,
it permanently remains slight, owing to the small development of the
cerebral hemispheres.

In addition to the cranial flexures, two other flexures make their
appearance in the base of the brain. A posterior at the junction of the
brain and spinal cord, and an anterior at the boundary between the
cerebellum and medulla oblongata, just at the point where the pons Varolii
is formed in Mammalia. The anterior of these is the most marked and
constant ; it is shewn in fig. 250. It arises considerably later than the main
cranial flexure, and since it is turned the opposite way it assists to a con-
siderable extent in causing the apparent straightening of the cranial axis.

Histogenetic changes1. The walls of the brain are at first
very thin and, like those of the spinal cord, are formed of a
number of ranges of spindle-shaped cells. The processes of
each of these cells are stated to be continued through the whole
thickness of the wall. In the floor of the hind- and mid-brain a
superficial layer of delicate nerve-fibres is formed at an early
period. This layer appears in the first instance on the floor and
sides of the hind-brain, and very slightly, if at all, later on the
floor and the sides of the mid-brain. The cells internal to the
nerve-fibres become differentiated into an innermost epithelial
layer lining the cavities of the ventricles, and an outer layer of
grey matter.

The similarity of the primitive arrangement and histological
character of the parts of the brain behind the cerebral hemi-
spheres to that of the spinal cord is very conclusively shewn by
the examination of any good series of sections. In both brain
and spinal cord the white matter forms a cap on the ventral and
lateral parts considerably before it extends to the dorsal surface.
In the medulla the white matter does not eventually extend to
the roof owing to the peculiar degeneration which that part
undergoes.

1 It is not within the scope of this work to give an account of the histogenesis of
the brain; in the statement in the text only a few points, of some morphological
importance, are touched on.

NERVOUS SYSTEM OF THE VERTEBRATA. 423

In the case of the fore-brain the earliest histological changes,
except possibly in Mammals, take place on the same general
plan as those of the remainder of the central nervous system1;
but though the general plan is the same, yet the early histologi-
cal distinction between the fore-brain, and the mid- and hind-
brain is more marked than the distinction between the latter
and the spinal cord.

On the floor and sides of the thalamencephalon, and appa-
rently the whole of the hemispheres of the lower types, there is
formed, somewhat later than in the remainder of the brain, a
very delicate layer of white matter. The inner part of the wall,
which still remains comparatively thin, is not at first clearly
divided into an epithelial and nervous layer. This distinction
soon however becomes more or less apparent, though it is not so
marked as in most other parts of the brain ; and it appears that
in the subsequent growth the greater part of the original
epithelial layer becomes converted into nervous tissue.

In Mammals the same plan of differentiation would seem to
be followed, though somewhat less obviously than in the lower
types. The walls of the hemispheres become first divided
(Kolliker) into a superficial thinner layer of rounded elements,
and a deeper and thicker epithelial layer, and between these the
fibres of the crura cerebri soon interpose themselves. At a
slightly later period a thin superficial layer of white matter,
homologous with that of the remainder of the brain, becomes
established.

The inner layer, together with the fibres from the crura
cerebri, gives rise to the major part of the white matter of the
hemispheres and to the epithelium lining the lateral ventricles.

The outer layer of rounded cells becomes divided into
(i) a superficial part with comparatively few cells, which,
together with its coating of white matter, forms the cortical
part of the grey matter, and (2) a deeper layer with numerous
cells which forms the main mass of the grey matter of the
hemispheres.

The development of the several parts of the brain will now
be described.

1 I have worked out these changes in Elasmobranchii, Amphibia (Salamandra)
and Aves.

424

THE HIND-BRAIN.

The hind-brain. The hind-brain is at first an elongated,
funnel-shaped tube, the walls of which are of a nearly uniform
thickness, though the roof and floor are somewhat thinner than
the sides. It forms a direct continuation of the spinal cord, into
which it passes without any sharp line of demarcation. The
ventricle it contains is known as the fourth ventricle.

The sides become in the chick marked by a series of transverse con-
strictions, dividing it into lobes, which are somewhat indefinite in number.
The first of these remains permanent, and its roof gives rise to the cerebellum.
It is uncertain whether the other constrictions have any morphological
significance. More or less similar constrictions are present in Teleostei.
In Elasmobranchii the medulla presents on its inner face at a late period a
series of lobes corresponding with the roots of the vagus and glossopharyngeal
nerves, and it is possible that the earlier constrictions may potentially
correspond to so many nerve-roots.

Throughout the Vertebrata an anterior lobe of the hind-
brain becomes very early marked off, so that the primitive
hind-brain becomes divided into two regions which may be

cc

AOA

FIG. 249. SECTION THROUGH THE HIND -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. Ch. Notochord ; CV. Anterior cardinal vein; CC. Involuted auditory
vesicle ; CC points to the end which will form the cochlear canal ; RL. Recessus
labyrinth! (remains of passage connecting the vesicle with the exterior) ; hy. Hypoblast
lining the alimentary canal; AO., AOA. Aorta, and aortic arch.

NERVOUS SYSTEM OF THE VERTEBKATA. 425

conveniently spoken of as the cerebellum (figs. 247 and 248, cb)
and medulla oblongata. The floor of these regions is quite
continuous and is also prolonged without any break into the
floor of the mid-brain.

The posterior section of the hind-brain, which forms the
medulla, undergoes changes of a somewhat complicated charac-
ter. In the first place its roof becomes in front very much
extended and thinned out. At the raphe, where the two lateral
halves of the brain originally united, a separation, as it were,
takes place, and the two sides of the brain become pushed apart,
remaining united by only a very thin layer of nervous matter,
consisting of a single row of flattened cells (fig. 249). As a
result of this peculiar growth in the brain, the roots of the nerves
of the two sides, which were originally in contact at the dorsal
summit of the brain, become carried away from one another, and
appear to arise at the sides of the brain.

The thin roof of the fourth ventricle is triangular, or, in
Mammalia, somewhat rhomboidal in shape. The apex of the
triangle is directed backwards.

At a later period the blood-vessels of the pia mater form a
rich plexus over the anterior part of the thin roof of the medulla,
which becomes at the same time somewhat folded. The whole
structure is known as the tela vasculosa, or choroid plexus
of the fourth ventricle (fig. 250, chd 4). The floor of the
whole hind-brain becomes thickened, and there very soon
appears on its outer surface a layer of non-medullated nerve-
fibres, similar to those which first appear on the spinal cord.
They are continuous with a similar layer of fibres on the floor
of the mid-brain, where they constitute the crura cerebri. On
the ventral floor of the medulla is a shallow continuation of the
anterior fissure of the spinal cord.

In Elasmobranchii and many Teleostei the restiform tracts are well
developed, and are anteriorly continued into the cerebellum, of which they
form the peduncles. Near their junction with the cerebellum they form
prominent bodies, which are regarded by Miklucho-Maclay as representing
the true cerebellum of Elasmobranchii.

In Elasmobranchii a dorsal pair of ridges projects into the cavity of the
fourth ventricle, corresponding apparently with the fasciculi teretes of the
Mammalia.

In Mammalia there develop, subsequently to the longitudinal fibres

426 THE HIND-BRAIN.

already spoken of, first the olivary bodies of the ventral side of the medulla,
and at a still later period the pyramids. The fasciculi teretes in the cavity of
the fourth ventricle are developed shortly before the pyramids.

When the hind-brain becomes divided into two regions the
roof of the anterior part does not become thinned out like that
of the posterior, but on the contrary, becomes somewhat thick-
ened and forms a band-like structure roofing over the anterior
part of the fourth ventricle (fig. 247 and fig. 253, cb).

This is a rudiment of the cerebellum, and in all Craniate Ver-
tebrates it at first presents this simple structure and insig-
nificant size. In Cyclostomata, Amphibia and many Reptilia
this condition is permanent. In Elasmobranchii, on the other
hand, the cerebellum assumes in the course of development a
greater and greater prominence (fig. 248, cb), and eventually
overlaps both the optic lobes in front and the medulla behind.
In the later embryonic stages it exhibits in surface-views the
appearance of a median constriction, and the portion of the
ventricle contained in it is prolonged into two lateral out-
growths.

Miklucho-Maclay, from his observations on the brains of adult Elasmo-
branchii, was led to regard what is here called the cerebellum as identical
with the mid-brain, and the true mid-brain as part of the thalamencephalon.
Miklucho-Maclay was no doubt misled by the large size of the cerebellum,
but, as we have seen, this body does not begin to be conspicuous till late in
embryonic life.

The mid-brain and thalamencephalon (according to the ordinary inter-
pretations) have in the embryo of Elasmobranchs exactly the same relations
as in the embryos of other Vertebrates ; so that the embryological evidence
appears to me to be conclusive against Miklucho-Maclay's view.

In Birds the cerebellum attains a very considerable develop-
ment (fig. 250, cbl\ consisting of a folded central lobe with an
arbor vitae, into which the fourth ventricle is prolonged. There
are two small lateral lobes, apparently equivalent to the flocculi.
Anteriorly the cerebellum is connected with the roof of the mid-
brain by a delicate membrane, the velum medullas anterius,
or valve of Vieussens (fig. 250, vtna). The pons Varolii of
Mammalia is represented by a small number of transverse
fibres on the floor of the hind-brain immediately below the
cerebellum.

In Mammalia the cerebellum attains a still greater develop-

NERVOUS SYSTEM OF THE VERTEBRATA. 427

ment The median lobe or vermiform process is first developed.
In the higher Mammalia the lateral parts forming the hemi-

fXJ^ cmfl l,.n

vnut

cU

ats inS fo s

FIG. 250. LONGITUDINAL SECTION THROUGH THE BRAIN OF A CHICK OF TEN

DAYS. (After Mihalkovics.)

Jims, cerebral hemispheres; alf. olfactory lobe; alf^. olfactory nerve; ggt. corpus
striatum ; oma. anterior commissure; chd-$. choroid plexus of the third ventricle;
pin. pineal gland; cmp. posterior commissure; trm. lamina terminalis; chm. optic
chiasma; inf. infundibulum; hph. pituitary body; bgm. commissure of Sylvius (roof
of iter a tertio ad quartum ventriculum) ; vma. velum medullae anterius (valve of
Vieussens); cbl. cerebellum; chd 4. choroid plexus of the fourth ventricle; obt 4. roof
of fourth ventricle ; obi. medulla oblongata ; pns. commissural part of medulla ; inv.
sheath of brain ; bis. basilar artery ; crts. internal carotid.

spheres of the cerebellum become formed as swellings at the
sides at a considerably later period, and are hardly developed in
the Monotremata and Marsupialia.

The cerebellum is connected with the roof of the mid-brain in front and
with the choroid plexus of the fourth ventricle behind by delicate membranous
structures, known as the velum medullae anterius (valve of Vieussens) and
the velum medullae posterius.

The pons Varolii is formed on the ventral side of the floor of the
cerebellar region as a bundle of transverse fibres at about the same time as
the olivary bodies.

The mid-brain. The changes undergone by the mid-brain
are simpler than those of any other part of the brain. We
have already seen that the rnid-brain, on the appearance of the
cranial flexure, forms an impaired vesicle with a vaulted roof and
curved floor, at the front end of the long axis of the body (fig.
1 1 8, MB}. It is at this period in most Vertebrates relatively
much larger than in the adult ; and it is only in the Teleostei
that it more or less retains in the adult its embryonic pro-
portions.

428 THE FORE-BRAIN.

The cavity of the mid-brain, greatly reduced in size in the
higher forms, is known as the iter a tertio ad quartum ventricu-
lum, or aqueductus Sylvii.

The roof of the mid-brain is sharply constricted off from the
divisions of the brain in front of and behind it, but these
constrictions do not extend to the floor.

In some Vertebrates the region of the mid-brain is stated to
undergo hardly any further development. In the Axolotl it
remains according to Stieda1 as a simple tube with nearly uni-
formly thick walls. In the majority of forms it undergoes, how-
ever, a more complicated development.

In Elasmobranchs the sides become thickened to form the optic lobes,
which are soon separated by a median longitudinal groove. The floor
becomes thickened to form the crura cerebri. The primitive simple median
cavity becomes imperfectly divided into a median portion below, and two
lateral diverticula in the optic lobes.

In Teleostei the changes, resulting in the formation of (i) a pair of
longitudinal ridges projecting from the roof into the cavity of the iter,
constituting the fornix of Gottsche, and (2) of the two swellings on the floor,
forming the tori semicirculares, are more complicated, but have not been
satisfactorily worked out. In Bombinator and the Anura generally the
changes are of the same nature as those in Elasmobranchii, except that the
prolongations of the ventricle into the optic lobes are still further constricted
off from the median portion, which forms the true iter.

In Reptilia and Aves the development of the mid-brain takes place on
the same type as in Elasmobranchii and the Anura. In Birds the optic
lobes are pushed very much aside, and the roof of the iter is greatly thinned
out. In Mammalia the sides of the mid-brain give rise to two pairs of
prominences — the corpora quadrigemina — instead of the two optic lobes of
other Vertebrata. The prominences, which do not contain prolongations of
the iter, become first visible on the appearance of an oblique transverse furrow,
while the anterior pair alone are separated by a longitudinal furrow. In the
later stages of development the longitudinal furrow is continued so as to
bisect the posterior pair.

The floor, which is bounded posteriorly by the pons Varolii, becomes the
crura cerebri. The corpora geniculata interna also belong to this division
of the brain.

Fore-brain. In its earliest condition the fore-brain forms a
single vesicle without a trace of separate divisions, but very
early it buds off the optic vesicles, whose history is described
with that of the eye.

1 " Ueb. d. Bau d. centralen Nervensystem d. Axolotl." Zdt.f. wlss. Zool., Vol.
xxv. 1875.

NERVOUS SYSTEM OF THE VERTEBRATA.

429

The optic vesicles become gradually constricted off from the
fore-brain in a direction obliquely backwards and downwards.
They remain, however, attached to it at the anterior extremity
of the base of the fore-brain (fig. 251, op.v.). While the above
changes are taking place in the optic vesicles the anterior part

opy

FIG. 251. SECTION THROUGH THE
FRONT PART OF THE HEAD OF A LEPIDOS-
TEUS EMBRYO ON THE SEVENTH DAY AFTER
IMPREGNATION.

al. alimentary tract ; fb. thalamencepha-
lon ; /. lens of eye ; op.v. optic vesicle. The
mesoblast is not represented.

FIG. 252. LONGITUDINAL
SECTION THROUGH THE BRAIN
OF A YOUNG PRISTIURUS EM-
BRYO.

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.
notochord; al. alimentary tract;
laa. artery of mandibular arch.

of the fore-brain becomes prolonged, and at the same time
somewhat dilated. At first there is no sharp boundary between
the primitive fore-brain and its anterior prolongation, but there
shortly appears a constriction which passes from above obliquely
forwards and downwards. This constriction is shallow at first,
but soon becomes much deeper, leaving however the cavities of
the two divisions of the fore-brain united ventrally by a some-
what wide canal (fig. 252).

Of these two divisions the posterior becomes the thalamen-
cephalon, while the anterior and larger division (cer) forms the
rudiment of the cerebral hemispheres and olfactory lobes. For
a considerable period this rudiment remains perfectly simple, and
exhibits no signs, either externally or internally, of a longitudinal
constriction dividing it into two lobes.

From the above description it may be concluded that the

430 THE THALAMENCEPHALON.

rudiment of the cerebral hemispheres is contained in the
original fore-brain. In spite however of their great importance
in all the Craniata, it is probable that the hemispheres were
either not present as distinct structures, or only imperfectly
separated from the thalamencephalon, in the primitive vertebrate
stock.

The thalamencephalon. The thalamencephalon varies so
slightly in structure throughout the Vertebrate series that a
general description will suffice for all the types.

It forms at first a simple vesicle, the walls of which are of
a nearly uniform thickness and formed of the usual spindle-
shaped cells.

md.

FIG. 253. DIAGRAMMATIC VERTICAL SECTION THROUGH THE HEAD OF A
LARVA OF PETROMYZON.

The larva had been hatched three days, and was 4*8 mm. in length. The optic
and auditory vesicles are supposed to be seen through the tissues.

c.h. cerebral hemisphere ; th. optic thalamus; in. infundibulum ; pn. pineal gland ;
mb. mid-brain ; cb. cerebellum ; md. medulla oblongata ; au.v. auditory vesicle ; op.
optic vesicle; ol. olfactory pit; m. mouth; br.c. branchial pouches; th. thyroid
involution; v.ao. ventral aorta; ht. ventricle of heart ; ch. notochord.

The cavity it contains is known as the third ventricle. An-
teriorly it opens widely into the cerebral rudiment, and posteriorly
into the ventricle of the mid-brain. The opening into the
cerebral rudiment becomes the foramen of Munro.

For convenience of description I shall divide it into three
regions, viz. (i) the floor, (2) the sides, and (3) the roof.

The floor becomes divided into two parts, an anterior part,
giving origin to the optic nerves, in which is formed the optic
chiasma ; and a posterior part, which becomes produced into an

NERVOUS SYSTEM OF THE VERTEBRATA.

431

at first inconspicuous prominence — the rudiment of the infundi-
bulum (fig. 252, In}. This comes in contact with an involution
from the mouth, which gives rise to the pituitary body (fig. 252,
//), the development of which will be dealt with separately.

In the later stages of development the infundibulum becomes
gradually prolonged, and forms an elongated diverticulum of
the third ventricle, the apex of which is in contact with the
pituitary body (figs. 252, 254, in, and figs. 250 and 255, inf}.

Along the sides of the infundibulum run the commissural
fibres connecting the floor of the mid-brain with the cerebrum.

In its later stages the infundibular region presents considerable variations
in the different vertebrate types. In Fishes it generally remains very large,
and permanently forms a marked diverticulum of the floor of the thalamen-
cephalon. In Elasmobranchii the distal end becomes divided into three
lobes — a median and two lateral. The lateral lobes appear to become the
sacci vasculosi of the adult.

In Teleostei peculiar bodies known as the lobi inferiores (hypoaria) make
their appearance at the sides of the a2r,. y

infundibulum. They appear to corre-
spond in position with the tuber cine-
reum of Mammalia1. In Birds, Rep-
tiles, and Amphibia the lower part of
the embryonic infundibulum becomes
atrophied and reduced to a mere finger-
like process — the processus infundibuli.

In Mammalia the posterior part of
the primitive infundibulum becomes the
corpus albicans, which is double in
Man and the higher Apes ; the ventral
part of the posterior wall forms the
tuber cinereum. Laterally, at the junc-
tion of the optic thalami and infundi-
bulum, there are placed the fibres of
the crura cerebri, which are probably
derived from the walls of the infundi-
bulum. A special process grows out
from the base of the infundibulum,
which undergoes peculiar changes, and
becomes intimately united with the
pituitary body ; in which connection it
will be more fully described.

rncl

c.in
'Pt

FIG. 254. LONGITUDINAL SECTION
THROUGH THE BRAIN OF SCYLLIUM
CANICULA AT AN ADVANCED STAGE OF
DEVELOPMENT.

cer. cerebral hemisphere ; pn. pi-
neal gland ; op. th. optic thalamus, con-
nected with its fellow by a commissure
(the middle commissure). In front of
it is seen a fold of the roof of the fore-
brain, which is the choroid plexus of
the third ventricle ; op. optic chiasma ;
ft. pituitary body ; in. infundibulum ;
cb. cerebellum ; au.v. passage leading
from the auditory vesicle to the ex-
terior ; mel. medulla oblongata ; c . in.
internal carotid artery.

1 For the relations of these bodies, vide L. Stieda, "Stud. lib. d. centrale Nerven-
system d. Knochenfische." Zeit. f. wiss. Zool. Vol. xvni. 1868.

432 THE PINEAL GLAND.

The sides of the thalamencephalon become very early
thickened to form the optic thalami, which constitute the most
important section of the thalamencephalon. They are separated,
in Mammalia at all events, on their inner aspect from the
infundibular region by a somewhat S-shaped groove, known as
the sulcus of Munro, which ends in the foramen of Munro. They
also become in Mammalia secondarily united by a transverse
commissure, the grey or middle commissure, which passes
across the cavity of the third ventricle. This commissure is
probably homologous with, and derived from, a commissural
band in the roof of the thalamencephalon, placed immediately
in front of the pineal gland which is well developed in Elasmo-
branchii (fig. 254).

The roof undergoes more complicated changes. It becomes
divided, on the appearance of the pineal gland as a small
papilliform outgrowth (the development of which is dealt with
separately), into two regions — a longer anterior in front of the
pineal gland and a shorter posterior. The anterior region
becomes at an early period excessively thin, and at a later
period, when the roof of the thalamencephalon is shortened by
the approach of the cerebral hemispheres to the mid-brain, it
becomes (vide figs. 250 and 255, chd 3, and 254) considerably
folded, while at the same time a vascular plexus is formed in
the pia mater above it. On the accomplishment of these
changes it is known as the tela choroidea of the third ventricle.

In the roof of the third ventricle behind the pineal gland
there appear in Elasmobranchii, the Sauropsida and Mammalia
transverse commissural fibres, forming a structure known as the
posterior commissure, which connects together the two optic
thalami.

The most remarkable organ in the roof of the thalamen-
cephalon is the pineal gland, which is developed in most Verte-
brates as a simple papilliform outgrowth of the roof, and is at
first composed of cells similar to those of the other parts of the
central nervous system (figs. 250, 252, 254 and 255, pn or pin}.
In the lower Vertebrata it is directed forwards, but in Mammalia,
and to some extent in Aves, it is directed backwards.

In Amphibia it is described by Gotte (No. 296) as being a product of the
point where the roof of the brain remains latest attached to the external skin.

NERVOUS SYSTEM OF THE VERTEBRATA. 433

The figure which Gotte gives to prove this does not appear to me fully to
bear out his conclusion ; which if true is very important. Although I
directed my attention specially to this point, I could find no indication in
Elasmobranchii of a process similar to that described by Gotte, and his
observations have not as yet been confirmed for other Vertebrates. Gotte
compares the pineal gland to the long-persis.ting pore which leads into the
cavity of the brain in the embryo of Amphioxus, and we might add the
Ascidians, and, should his facts be confirmed, the conclusion he draws from
them would appear to be well founded.

The later stages in the development of the pineal gland in
different Vertebrates have not in all cases been fully worked
out1.

In Elasmobranchii the pineal gland becomes in time very
long, and extends far forwards over the roof of the cerebral

FIG. -255. LONGITUDINAL VERTICAL SECTION THROUGH THE ANTERIOR PART
OF THE BRAIN OF AN EMBRYO RABBIT OF FOUR CENTIMETRES. (After
Mihalkovics.)

The section passes through the median line so that the cerebral hemispheres are
not cut ; their position is however indicated in outline.

spt. septum lucidum formed by the coalescence of the inner walls of part of the
cerebral hemispheres; cna. anterior commissure; frx. vertical pillars of the fornix;
cal, genu of corpus callosum; trm. lamina terminalis; hms. cerebral hemispheres;
olf. olfactory lobes; acl. artery of corpus callosum; fmr. position of foramen of
Munro; chdi,. choroid plexus of third ventricle ; pin. pineal gland; cmp. posterior
commissure; bgm. lamina uniting the lobes of the mid-brain; chm, optic chiasma ;
hph. pituitary body; inf. infundibulum ; pns. pons Varolii; pde. cerebral peduncles;
agd. iter.

1 For a full account of this subject vide Ehlers (No. 337).
B. Ill, 28

434 THE PINEAL GLAND.

hemispheres (fig. 254/w). Its distal extremity dilates somewhat,
and in the adult the whole organ forms (Ehlers, No. 337) an
elongated tube, enlarged at its free extremity, and opening at
its base into the brain. The enlarged extremity may either be
lodged in a cavity in the cartilage of the cranium (Acanthias), or
be placed outside the cranium (Raja).

In Petromyzon its form is very different. It arises (fig.
253 Pn) as a sack-like diverticulum of the thalamencephalon
extending at first both backwards«and forwards. In the Ammo-
ccete the walls of this sack are deeply infolded.

The embryonic form of the pineal gland in Amphibia is very
much like that which remains permanent in Elasmobranchii ;
the stalk connecting the enlarged terminal portion with the
brain soon however becomes solid and very thin except at its
proximal extremity. The enlarged portion also becomes solid,
and is placed in the adult externally to the skull, where it forms
a mass originally described by Stieda as the cerebral gland.

In Birds the primitive outgrowth to form the pineal gland
becomes, according to Mihalkovics, deeply indented by vascular
connective tissue ingrowths, so that it assumes a dendritic
structure (fig. 250 pin).

The proximal extremity attached to the roof of the thala-
mencephalon forms a special section, known as the infra -pineal
process. The central lumen of the free part of the gland finally
atrophies, but the branches still remain hollow. The infra-pineal
process becomes reduced to a narrow stalk, connecting the
branched portion of the body with the brain. The branched
terminal portion and the stalk obviously correspond with the
vesicle and distal part of the stalk of the types already described.
In Mammalia the development of the pineal gland is, according
to Mihalkovics, generally similar to that of Birds. The original
outgrowth becomes branched, but the follicles or lobes to which
the branching gives rise eventually become solid (fig. 255 pin).
An infra-pineal process is developed comparatively late, and is
not sharply separated from the roof of the brain.

No satisfactory suggestions have yet been offered as to the
nature of the pineal gland, unless the view of Gotte be regarded
as such. It appears to possess in all forms an epithelial structure,
but, except at the base of the stalk (infra-pineal process) in

NERVOUS SYSTEM OF THE VERTEBRATA. 435

Mammalia, in the wall of which there are nerve-fibres, no
nervous structures are present in it in the adult state.

The pituitary body. Although the pituitary body is not
properly a nervous structure, yet from its intimate connection
with the brain it will be convenient to describe its development
here. The pituitary body is in fact an organ derived from the
epiblast of the stomodaeum. This fact has been demonstrated
for Mammalia, Aves, Amphibia and Elasmobranchii, and may
be accepted as holding good for all the Craniata1. The epiblast
in the angle formed by the cranial flexure becomes involuted to
form the cavity of the mouth. This cavity is bordered on its
posterior surface by the front wall of the alimentary tract, and
on its anterior by the base of the fore-brain. Its uppermost end
does not at first become markedly constricted off from the
remainder, but is nevertheless the rudiment of the pituitary
body.

Fig. 256 represents a transverse section through the head of
an Elasmobranch embryo, in which, owing to the cranial flexure,
the fore part of the head is cut longitudinally and horizontally,
and the section passes through both the fore-brain (fb) and the
hind-brain. Close to the base of the fore-brain are seen the
mouth (in), and the pituitary involution from this (pf). In
contact with the pituitary involution is the blind anterior
termination of the throat («/) which a little way back opens to
the exterior by the first visceral cleft (l. v.c.}. This figure alone
suffices to demonstrate the correctness of the above account of
the pituitary body; but its truth is still further confirmed by
fig. 252; in which the mouth involution (pt) is in contact with,
but still separated from, the front end of the alimentary tract.
Very shortly after the septum between the mouth and throat
becomes pierced, and the two are placed in communication, the
pituitary involution becomes very partially constricted off from
the mouth involution, though still in direct communication with
it. In later stages the pituitary involution becomes longer and

1 Scott states that in the larva of Petromyzon the pituitary body is derived from
the walls of the nasal pit; Quart, jf. of Micr. Science^ Vol. xxi. p. 750. I have not
myself completely followed its development in Petromyzon, but I have observed a
slight diverticulum of the stomodaeum which I believe gives origin to it. Fuller
details are in any case required before we can admit so great a divergence from the
normal development as is indicated by Scott's statements.

28—3

436

PITUITARY BODY.

Ky-

is dilated terminally ; while the passage connecting it with the
mouth becomes narrower and narrower, and is finally reduced to
a solid cord, which in its turn disappears.

Before the connection between the pituitary vesicle and the
mouth is obliterated the cartilaginous cranium becomes developed,
and it may then be seen that the infundibulum projects through
the pituitary space to come into close juxtaposition with the
pituitary body.

After the pituitary vesicle has lost its connection with the
mouth it lies just in front of the infundibulum (figs. 250 and
255 hph and fig. 254 pf) ; and soon becomes surrounded by
vascular mesoblast, which grows in
and divides it into a number of
branching tubes. In many forms
the cavity of the vesicle completely
disappears, and the branches become
for the most part solid [Cyclostomata
and some Mammalia (the rabbit),
Elasmobranchii, Teleostei and Am-
phibia]. In Reptilia, Aves and most
Mammalia the lumen of the organ is
more or less retained (W. Miiller, No.
344).

Although in the majority of the
Vertebrata there is a close connection
between the pituitary body and the
infundibulum, there is no actual fusion
between the two. In Mammalia the
case is different. The part of the in-
fundibulum which lies at the hinder
end of the pituitary body is at first a
simple finger-like process of the brain
(fig. 255 inf), but its end becomes
swollen, and the lumen in this part
becomes obliterated. Its cells, origi-
nally similar to those of the other
parts of the nervous system and even
(Kolliker) containing differentiated nerve-fibres, partly atrophy,
and partly assume an indifferent form, while at the same time

FIG. 256. TRANSVERSE SEC-
TION THROUGH THE FRONT PART
OF THE HEAD OF A YOUNG PR1S-
TIURUS EMBRYO.

The section, owing to the cran-
ial flexure, cuts both the fore- and
the hind-brain. It shews the pre-
mandibular and mandibular head
cavities \pp and 2//, etc. The
section is moreover somewhat
oblique from side to side.

fb. fore-brain; /. lens of eye;
m. mouth ;pt. upper end of mouth,
forming pituitary involution ; lao.
mandibular aortic arch ; \pp. and
ipp. first and second head cavities ;
\vc. first visceral cleft; V. fifth
nerve ; aim. auditory nerve ; VII.
seventh nerve; aa. roots of dorsal
aorta ; acv. anterior cardinal vein ;
ch. notochord.

NERVOUS SYSTEM OF THE VERTEBRATA. 437

there grow in amongst them numerous vascular and connective-
tissue elements. The process of the infundibulum thus meta-
morphosed becomes inseparably connected with the true pituitary
body, of which it is usually described as the posterior lobe. The
part of the infundibulum which undergoes this change is very
probably homologous with the saccus vasculosus of Fishes.

The true nature of the pituitary body has not yet been made out. It is
clearly a rudimentary organ in existing craniate Vertebrates, and its
development indicates that when functional it was probably a sense organ
opening into the mouth, its blind end reaching to the base of the brain. No
similar organ has as yet been found in Amphioxus, but it seems possible
perhaps to identify it with the peculiar ciliated sack placed at the opening
of the pharynx in the Tunicata, the development of which was described at
p. 1 8. If the suggestion is correct, the division of the body into lobes in
existing Vertebrata must be regarded as a step towards a retrogressive
metamorphosis.

Another possible view is to regard the pituitary body as a glandular
structure which originally opened into the mouth in the lower Chordata, but
which has in all existing forms ceased to be functional. The intimate relation
of the organ to the brain appears to me opposed to this view of its nature,
while on the other hand its permanent structure is more easily explained on
this view than on that previously stated. In the Ascidians a glandular
organ has been described by Lacaze Duthiers^n juxtaposition to the ciliated
sack, and it is possible that this organ as well as the ciliated sack may be
related to the pituitary body. In view of this possibility further investigations
ought to be carried out in order to determine whether the whole pituitary
body is derived from the oral involution, or whether there may not be a
nervous part and a glandular part of the organ.

The Cerebral Hemispheres. It will be convenient to treat
separately the development of the cerebral hemispheres proper,
and that of the olfactory lobes.

Although the cerebral hemispheres vary more than any other
part of the brain, they are nevertheless developed from the
unpaired cerebral rudiment in a nearly similar manner through-
out the series of Vertebrata.

In the cerebral rudiment two parts may be distinguished, viz.
the floor and the roof. The former gives rise to the ganglia at
the base of the hemispheres — corpora striata, etc. — the latter to
the hemispheres proper.

1 " Les Ascidies simples des Cotes de France." Archives de Biologie exper. et
generate, Vol. III. 1874, p. 329.

43»

THE CEREBRAL HEMISPHERES.

The two lobes

— cc

The first change which takes place consists in the roof
growing out into two lobes, between which a shallow median
constriction makes its appearance (fig. 257).
thus formed are the rudi-
ments of the two hemi-
spheres. The cavity of each
of them opens by a widish
aperture into the vestibule
at the base of the cerebral
rudiment, which again opens
directly into the cavity of
the third ventricle (3 v).
The Y-shaped aperture thus
formed, which leads from
the cerebral hemispheres
into the third ventricle, is
the foramen of Munro. The
cavity (lv) in each of the
rudimentary hemispheres is
a lateral ventricle. The part of the cerebrum which lies between
the two hemispheres, and passes forwards from the roof of the
third ventricle round the end of the brain to the optic chiasma,
is the rudiment of the lamina terminalis (figs. 257 It and 255 trm}.
Up to this point the development of the cerebrum is similar in
all Vertebrata, but in some forms it practically does not proceed
much further.

In Elasmobranchii, although the cerebrum reaches a con-
siderable size (fig. 254 cer\ and grows some way backwards over
the thalamencephalon, yet it is not in many forms divided into
two distinct lobes, but its paired nature is only marked by
a shallow constriction on the surface. The lamina terminalis in
the later stages of development grows backwards as a thick
median septum which completely separates the two lateral
ventricles1 (fig. 263).

There are, it may be mentioned, considerable variations in

op.t/t

FIG. 257. DIAGRAMMATIC LONGITUDI-
NAL HORIZONTAL SECTION THROUGH THE
FORE-BRAIN.

j>.v. third ventricle ; lv. lateral ventricle ;
//. lamina terminalis ; ce, cerebral hemi-
sphere ; op.th. optic thalamus.

1 A comparison of the mode of development of this septum with that of the septum
lucidum with its contained commissures in Mammalia clearly shews that the two
structures are not homologous, and that Miklucho-Maclay is in error in attempting to
treat them as being so.

NERVOUS SYSTEM OF THE VERTEBRATA. 439

the structure of the cerebrum in Elasmobranchii into which it is
not however within the scope of this work to enter.

In the Teleostei the vesicles of the cerebral hemispheres
appear at first to have a wide lumen, but it subsequently
becomes almost or quite obliterated, and the cerebral rudiment
forms a small bilobed nearly solid body. In Petromyzon (fig.
253 c/i) the cerebral rudiment is at first an unpaired anterior
vesicle, which subsequently becomes bilobed in the normal
manner. The walls of the hemispheres become much thickened,
but the lateral ventricles persist

In all the higher Vertebrates the division of the cerebral
rudiment into two distinct hemispheres is quite complete, and
with the deepening of the furrow between the two hemispheres
the lamina terminalis is carried backwards till it forms a thin
layer bounding the third ventricle anteriorly, while the lateral
ventricles open directly into the third ventricle.

In Amphibians the two hemispheres become united together
immediately in front of the lamina terminalis by commissural
fibres, forming the anterior commissure. They also send out
anteriorly two solid prolongations, usually spoken of as the
olfactory lobes, which subsequently fuse together.

In all Reptilia and Aves there is formed an anterior commis-
sure, and in the higher members of the group, especially Aves
(fig. 250), the hemispheres may obtain a considerable develop-
ment. Their outer walls are much thickened, while their inner
walls become very thin ; and a well-developed ganglionic
mass, equivalent to the corpus striatum, is formed at their
base.

The cerebral hemispheres undergo in Mammalia the most
complicated development. The primitive unpaired cerebral
rudiment becomes, as in lower Vertebrates, bilobed, and at the
same time divided by the ingrowth of a septum of connective
tissue into two distinct hemispheres (figs. 260 and 26 \f and
258 I). From this septum is formed the falx cerebri and
other parts.

The hemispheres contain at first very large cavities, com-
municating by a wide foramen of Munro with the third ventricle
(fig. 260). They grow rapidly in size, and extend, especially
backwards, and gradually cover the thalamencephalon and the

440

THE CEREBRAL HEMISPHERES.

mid-brain (fig. 258 I,/). The foramen of Munro becomes very
much narrowed and reduced to a mere slit.

The walls are originally , ^

nearly uniformly thick, but
the floor becomes thickened
on each side, and gives rise
to the corpus striatum (figs.
260 and 261 st). The corpus
striatum projects upwards
into each lateral ventricle,
giving to it a somewhat
semilunar form, the two
horns of which constitute
the permanent anterior and
descending cornua of the
lateral ventricles (fig. 262 st).

FIG. 258. BRAIN OF A THREE MONTHS'
HUMAN EMBRYO: NATURAL SIZE. (From
Kolliker.)

i. From above with the dorsal part of
hemispheres and mid-brain removed ; i.
From below, f. anterior part of cut wall of
the hemisphere ; f ' . cornu ammonis ; f/io.
optic thalamus ; cst. corpus striatum ; to.
optic tract ; cm. corpora mammillaria ; /.
pons Varolii.

With the further growth of the hemisphere the corpus

CftZ

Ams

spt.

FIG. 259. TRANSVERSE SECTION THROUGH THE BRAIN OF A RABBIT OF FIVE
CENTIMETRES. (After Mihalkovics.)

The section passes through nearly the posterior border of the septum lucidum,
immediately in front of the foramen of Munro.

hms. cerebral hemispheres ; cal. corpus callosum ; amm. cornu ammonis (hippo-
campus major) ; cms. superior commissure of the cornua ammonis ; spt. septum
lucidum ; frx i. vertical fibres of the fornix; ana. anterior commissure ; trm. lamina
terminalis; str. corpus striatum; Iff. nucleus lenticularis of corpus striatum; vtr i.
lateral ventricle; vtr 3. third ventricle; ipl. slit between cerebral hemispheres.

NERVOUS SYSTEM OF THE VERTEBRATA. 44!

striatum loses its primitive relations to the descending cornu.
The reduction in size of the foramen of Munro above mentioned
is, to a large extent, caused by the growth of the corpora striata.
The corpora striata are united at their posterior border with
the optic thalami. In the later stages of development the area
of contact between these two pairs of ganglia increases to an
immense extent (fig. 261), and the boundary between them
becomes somewhat obscure, so that the sharp distinction which
exists in the embryo between the thalamencephalon and cerebral
hemispheres becomes lost. This change is usually (Mihalkovics,

FIG. 260. TRANSVERSE SECTION THROUGH THE BRAIN OF A SHEEP'S EMBRYO

OF 27 CM. IN LENGTH. (From Kolliker.)
The section passes through the level of the foramen of Munro.
st. corpus striatum ; m. foramen of Munro ; t. third ventricle ; pi. choroid plexus
of lateral ventricle; f. falx cerebri; th. anterior part of optic thalamus; ch. optic
chiasma; o. optic nerve; c. fibres of the cerebral peduncles; h. cornu ammonis;
/. pharynx; sa. pre-sphenoid bone; a. orbito-sphenoid bone; s. points to part of the
roof of the brain at the junction between the roof of the third ventricle and the lamina
terminalis ; /. lateral ventricle.

Kolliker) attributed to a fusion between the corpora striata and
optic thalami, but it has recently been attributed by Schwalbe
(No. 349), with more probability, to a growth of the original
surface of contact, and an accompanying change in the relations
of the parts.

442 THE CEREBRAL HEMISPHERES.

The outer wall of the hemispheres gradually thickens, while
the inner wall becomes thinner. In the latter, two curved folds,
projecting towards the interior of the lateral ventricle, become
formed. These folds extend from the foramen of Munro along
nearly the whole of what afterwards becomes the descending
cornu of the lateral ventricle.

The upper fold becomes the hippocampus major (cornu
ammonis) (figs. 259 amm, 260 and 261 /i, and 262 am). When

P'IG. 261. TRANSVERSE SECTION THROUGH THE BRAIN OF A SHEEP'S EMBRYO
OF 27 CM. IN LENGTH. (From Kolliker.)

The section is taken a short distance behind the section represented in fig. 260, and
passes through the posterior part of the hemispheres and the third ventricle.

st. corpus striatum ; th. optic thalamus; to. optic tract; t. third ventricle; d. roof
of third ventricle; c. fibres of cerebral peduncles; c' '. divergence of these fibres into
the walls of the hemispheres ; e. lateral ventricle with choroid plexus //; h. cornu
ammonis; f. primitive falx; am. alisphenoid; a. orbito-sphenoid ; sa. presphenoid; /.
pharynx; mk. Meckel's cartilage.

the rudiment of the descending cornu has become transformed
into a simple process of the lateral ventricle the hippocampus
major forms a prominence upon its floor.

The wall of the lower fold becomes very thin, and a vascular
plexus, derived from the connective-tissue septum between the
hemispheres, and similar to that of the roof of the third ventricle,

NERVOUS SYSTEM OF THE VKRTEBRATA. 443

is formed outside it. It constitutes a fold projecting far into the
cavity of the lateral ventricle, and together with the vascular
connective tissue in it gives rise to the choroid plexus of the
lateral ventricle (figs. 260 and 261 //).

It is clear from the above description that a marginal fissure
leading into the cavity of the lateral ventricle does not exist in
the sense often implied in works on human anatomy, in that the
epithelium covering the choroid plexus, which forms the true
wall of the brain, is a continuous membrane. The epit/ielium of
the choroid plexus of the lateral ventricle is quite independent
of that of the choroid plexus of the third ventricle, though at the
foramen of Munro the roof of the third ventricle is of course con-
tinuous with the inner wall of the lateral ventricle (fig. 260 s).
The vascular elements of the two plexuses form however a con-
tinuous structure.

The most characteristic parts of the Mammalian cerebrum
are the commissures connecting the two hemispheres. These
commissures are (i) the anterior commissure, (2) the fornix, and
(3) the corpus callosum, the two latter being peculiar to Mam-
malia.

By the fusion of the inner walls of the hemispheres in front
of the lamina terminalis a solid septum is formed, known as the
septum lucidum, continuous behind with the lamina terminalis,
and below with the corpora striata (figs. 255 and 259 spt). It is
by a series of differentiations within this septum that the above
commissures originate. In Man there is a closed cavity left in
the septum known as the fifth ventricle, which has however no
communication with the true ventricles of the brain.

In the septum lucidum there become first formed, below, the
transverse fibres of the anterior commissure (fig. 255 and fig.
259 cma), and in the upper part the vertical fibres of the fornix
(fig. 255 and fig. 259 frx 2). The vertical fibres meet above
the foramen of Munro, and thence diverge backwards, as the
posterior pillars, to lose themselves in the cornu ammonis (fig.
259 amm}. Ventrally they are continued, as the descending or
anterior pillars of the fornix, into the corpus albicans, and thence
into the optic thalami.

The corpus callosum is not formed till after the anterior
commissure and fornix. It arises in the upper part of the region

444

THE OLFACTORY LOBES.

(septum lucidum) formed by the fusion of the lateral walls of the
hemispheres (figs. 255 and 259 cal), and at first only its curved
anterior portion — the genu
or rostrum — is developed. ^
This portion is alone found
in Monotremes and Marsu-
pials. The posteriorportion,
which is present in all the
Monodelphia, is gradually
formed as the hemispheres
are prolonged further back-
wards.

Primitively the Mam-
malian cerebrum, like that
of the lower Vertebrata, is
quite smooth. In many of
the Mammalia, Monotre-
mata, Insectivora, etc., this
condition is nearly retained
through life, while in the
majority of Mammalia a
more or less complicated system of fissures is developed on the
surface. The most important, and first formed, of these is
the Sylvian fissure. It arises at the time when the hemi-
spheres, owing to their growth in front of and behind the
corpora striata, have assumed a somewhat bean-shaped form.
At the root of the hemispheres — the hilus of the bean — there
is formed a shallow depression, which constitutes the first trace
of the Sylvian fissure. The part of the brain lying in this fissure
is known as the island of Reil.

The olfactory lobes. The olfactory lobes, or rhinencephala,
are secondary outgrowths of the cerebral hemispheres, and con-
tain prolongations of the lateral ventricles, but may however be
solid in the adult state. According to Marshall they develop in
Birds and Elasmobranchs and presumably other forms later
than the olfactory nerves, so that the olfactory region of the
hemispheres is indicated before the appearance of the olfactory
lobes.

In most Vertebrates the olfactory lobes arise at a fairly early

FIG. 262. LATERAL VIEW or THE BRAIN
OF A CALF EMBRYO OF 5 CM. (After Mihal-
kovics.)

The outer wall of the hemisphere is re-
moved, so as to give a view of the interior of
the left lateral ventricle.

hs. cut wall of hemisphere ; st. corpus
striatum; am. hippocampus major (cornu am-
monis) ; d. choroid plexus of lateral ventricle ;
fm. foramen of Munro; op. optic tract; in.
infundibulum ; mb. mid-brain ; cb. cerebellum ;
IV. V. roof of fourth ventricle ; ps. pons Va-
rolii, close to which is the fifth nerve with
Gasserian ganglion.

NERVOUS SYSTEM OF THE VKRTEBRATA.

445

stage of development from the under and anterior part of the
hemispheres (fig. .250 olf}. In Elasmobranchs they arise, not

FIG. 263. SECTION THROUGH THE BRAIN AND OLFACTORY ORGAN OF AN
EMBRYO OF SCYLLIUM. (Modified from figures by Marshall and myself.)

ch. cerebral hemispheres ; ol.v. olfactory vesicle ; olf. olfactory pit ; Sch. Schnei-
derian folds ; I. olfactory nerve. The reference line has been accidentally taken through
the nerve to the brain ; pn. anterior prolongation of pineal gland.

from the base, but from the lateral parts of the brain (fig. 263),
and become subsequently divided into a bulbous portion and a
stalk. They vary considerably in their structure in the adult.

In Amphibia the solid anterior prolongations of the cerebral
hemispheres already spoken of are usually regarded as the
olfactory lobes, but according to Gotte, whose view appears to
me well founded, small papillae, situated at the base of these
prolongations, from which olfactory nerves spring, and which
contain a process of the lateral ventricle, should properly be
regarded as the olfactory lobes. These papillse arise prior to the
solid anterior prolongations of the hemispheres.

In Birds the olfactory lobes are small. In the chick they
arise (Marshall) on the seventh day of incubation.

General conclusions as to the Central Nervous System.

It has been shewn above that both the brain and spinal cord
are primitively composed of a uniform wall of epithelial cells,
and that the first differentiation results in the formation of an
external layer of white matter, a middle layer of grey matter
(ganglion cells), and an inner epithelial layer. This primitive

446 GENERAL CONCLUSIONS.

histological arrangement, which in many parts of the brain at
any rate, is only to be observed in the early developmental
stages, has a simple phylogenetic explanation.

As has been already explained in an earlier part of this chapter
the central nervous system was originally a differentiated part of
the superficial epidermis.

This differentiation (as may be concluded from the character
of the nervous system in the Ccelenterata and Echinodermata)
consisted in the conversion of the inner ends of the epithelial
cells into nerve-fibres ; that is to say, that the first differentiation
resulted in the formation of a layer of white matter on the inner
side of the epidermis. The next stage was the separation of a
deeper layer of the epidermis as a layer of ganglion cells from
the superficial epithelial layer, i.e. the formation of a middle
layer of ganglion cells and an outer epithelial layer. Thus,
phylogenetically, the same three layers as those which first make
their appearan-ce in the ontog'eny of the vertebrate nervous system
became successively differentiated, and in both cases they are
clearly placed in the same positions, because the central canal of
the vertebrate nervous system, as formed by an involution, is at
the true outer surface, and the external part of the cord is at the
true inner surface.

It is probable that a very sharp distinction between the white
and grey matter is a feature acquired in the higher Vertebrata,
since in Amphioxus there is no such sharp separation ; though
the nerve-fibres are mainly situated externally and the nerve-cells
internally.

As already stated in Chapter Xll. the primitive division of
the nervous axis was probably not into brain and spinal cord,
but into (i) a fore-brain, representing the ganglion of the prae-
oral lobe, and (2) the posterior part of the nervous axis, consist-
ing of the mid- and hind-brains and the spinal cord. This view
of the division of the central nervous system fits in fairly satis-
factorily with the facts of development. The fore-brain is, histo-
logically, more distinct from the posterior part of the nervous
system than the posterior parts are from each other ; the front
end of the notochord forms the boundary between these two parts
of the central nervous system (vide fig. 253), ending as it does at
the front termination of the floor of the mid-brain, and finally,

NERVOUS SYSTEM OF THE VERTEBRATA. 447

the nerves of the fore-brain have a different character to those of
the mid- and hind-brain.

This primitive division of the central nervous system is lost
in all the true Vertebrata, and in its place there is a secondary
division — corresponding with the secondary vertebrate head —
into a brain and spinal cord. The brain, as it is established in
these forms, is again divided into a fore-brain, a mid-brain and a
hind-brain. The fore-brain is, as we have already seen, the
original ganglion of the praeoral lobe. The mid-brain appears
to be the lobe, or ganglion, of the third pair of nerves (first pair
of segmental nerves), while the hind-brain is a more complex
structure, each section of which (perhaps indicated by the con-
strictions which often appear at an early stage of development)
giving rise to a pair of segmental nerves is, roughly speaking,
homologous with the whole mid-brain.

The type of differentiation of each of the primitively simple
vesicles forming the fore-, the mid- and the hind-brains is very
uniform throughout the Vertebrate series, but it is highly instruc-
tive to notice the great variations in the relative importance of
the parts of the brain in the different types. This is especially
striking in the case of the fore-brain, where the cerebral hemi-
spheres, which on embryological grounds we may conclude to
have been hardly differentiated as distinct parts of the fore-brain
in the most primitive types now extinct, gradually become more
and more prominent, till in the highest Mammalia they constitute
a more important section of the brain than the whole of the
remaining parts put together.

The little that is known with reference to the significance of
the more or less corresponding outgrowths of the floor and roof
of the thalamencephalon, constituting the infundibulunv and
pineal gland, has already been mentioned in connection with the
development of these parts.

(332) C. J. Cams. Vcrsnch einer Darstellnng d. Nervensy stems, etc. Leipzig,

1814.

(333) J. L. Clark. " Researches on the development of the spinal cord in Man,
Mammalia and Birds." Phil. Trans., 1862. .

448 BIBLIOGRAPHY.

(334) E. Dursy. " Beitrage zur Entwicklungsgeschichte des Hirnanhanges. "
Centralblatt f. d, med. Wissenschaften, 1868. Nr. 8.

(335) E. Dursy. Zur Entwicklungsgeschichte des Kopfes des Menschen and der
hoheren Wirbelthiere. Tiibingen, 1869.

(336) A. Ecker. "Zur Entwicklungsgeschichte der Furchen und Windungen
der Grosshirn-Hemispharen im Foetus des Menschen." Archiv f. Anthropologie, v.
Ecker und Lindenschmidt. Vol. ill. 1868.

(337) E. Ehlers. "Die Epiphyse am Gehirn d. Plagiostomen." Zeit. f. wiss.
Zool. Vol. xxx., suppl. 1878.

(338) P. Flechsig. Die Leitungsbahnen im Gehirn und Riickenmark des
Menschen. Auf Grund cntwicklungsgeschichtlicher Untersucfumgen. Leipzig, 1876.

(339) V. Hensen. "Zur Entwicklung des Nervensystems." Virchoitfs Archiv,
Bd. xxx. 1864.

(340) L. Lowe. "Beitrage z. Anat. u. z. Entwick. d. Nervensystems d. Sauge-
thiere u. d. Menschen." Berlin, 1880.

(341) L. Lowe. " Beitrage z. vergleich. Morphogenesis d. centralen Nerven-
systems d. Wirbelthiere." Mittheil. a. d. embryo!. Instit. Wien, Vol. II. 1880.

(342) A. M. Marshall. "The Morphology of the Vertebrate Olfactory organ."
Quart. J. of Micr. Science, Vol. XIX. 1879.

(343) V. v. Mihalkovics. Entwicklungsgeschichte d. Gehirns. Leipzig, 1877.

(344) W. Mil Her. " Ueber Entwicklung und Bau der Hypophysis und des
Processus infundibuli cerebri. " yenaische Zeitschrift. Bd. VI. 1871.

(345) H. Rahl-Riickhard. "Die gegenseitigen Verhaltnisse d. Chorda,
Hypophysis etc. bei Haifischembryonen, nebst Bemerkungen lib. d. Deutung d.
einzelnen Theile d. Fischgehirns." Morphol. Jahrbttch, Vol. vi. 1880.

(348) H. Rathke. " Ueber die Entstehung der glandula pituitaria." Mutter's
Archiv f. Anat. und Physiol., Bd. V. 1838.

(347) C. B. Reichert. Der Bau des mcnschlichen Gehirns. Leipzig, 1859 u-
1861.

(348) F. Schmidt. "Beitrage zur Entwicklungsgeschichte des Gehirns."
Zeitschrift f. wiss. Zoologie, 1862. Bd. xi.

(349) G. Schwalbe. "Beitrag z. Entwick. d. Zwischenhirns. " Sitz. d.
Jenaischcn Gesell.f. Med. u. Naturwiss. Jan. 23, 1880.

(350) F'ried. Tiedemann. Anatomic und Bildtmgsgeschichte des Gehirns im
Foetus des Menschen. Niirnberg, 1816.

THE DEVELOPMENT OF THE CRANIAL AND SPINAL NERVES1.

All the nerves are outgrowths of the central nervous system,
but the differences in development between the cranial and
spinal nerves are sufficiently great to make it convenient to
treat them separately.

1 Remak derived the posterior ganglia from the tissue of the mesoblastic somites,
and following in Remak's steps most authors believed the peripheral nervous system
to have a mesoblastic origin. This view, which had however been rejected on
theoretical grounds by Hensen and others, was finally attacked on the ground of
observation by His (No. 297). His (No. 352, p. 458) found that in the Fowl " the

NERVOUS SYSTEM OF THE VERTEBRATA. 449

Spinal nerves. The posterior roots of the spinal nerves, as
well as certain of the cranial nerves, arise in the same manner,
and from the same structure, and are formed considerably before
the anterior roots. Elasmobranch fishes may be taken as the
type to illustrate the mode of formation of the spinal nerves.

The whole of the nerves in question arise as outgrowths of a
median ridge of cells, which makes its appearance on the dorsal
side of the spinal cord (fig. 264 A, pr). This ridge has been
called by Marshall the neural crest. At each point, where a
pair of nerves will be formed, two pear-shaped outgrowths
project from it, one on each side ; and apply themselves closely
to the walls of the spinal cord (fig. 264 B, pr). These out-
growths are the rudiments of the posterior nerves. While still
remaining attached to the dorsal summit of the neural cord they
grow to a considerable size (fig. 264 B, pr).

The attachment to the dorsal summit is not permanent, but

spinal ganglia of the head and trunk arose from a small band of matter which is
placed between the medullary plate and epiblast, and the material of which he called
the 'intermediate cord'." He further states that: "Before the closure of the
medullary tube this band forms a special groove — the 'intermediate groove' — placed
close to the border of the medullary plate. As the closure of the medullary plate
into a tube is completed, the earlier intermediate groove becomes a compact cord.
In the head of the embryo a longitudinal ridge arises in this way, which separates the
suture of the brain from that of the epiblast. In the parts of the neck and in the
remaining region of the neck the intermediate cord does not lie over the line of
junction of the medullary tube, but laterally from this and forms a ridge, triangular
in section, with a slight indrawing." This intermediate ridge gives rise to four
ganglia in the head, viz. the g. trigemini, g. acousticum, g. glossopharyngei, and
g. vagi, and in the trunk to the spinal ganglia. In both cases it unites first with the
spinal cord.

I have given in the above account, as far as possible, a literal translation of His'
own words, because the reader will thus be enabled fairly to appreciate his meaning.

Subsequently to His' memoir (No. 297) I gave an account of some researches of
my own on this subject (No. 351), stating the whole of the nerves to be formed as
cellular outgrowths of the spinal cord. I failed fully to appreciate that some of the
stages I spoke of had been already accurately described by His, though interpreted by
him very differently. Marshall, and afterwards Kolliker, arrived at results in the main
similar to my own, and Hensen, independently of and nearly simultaneously with
myself, published briefly some observations on the nerves of Mammals in harmony
with my results.

His has since worked over the subject again (No. 352), and has reaffirmed as a
result of his work his original statements. I cannot, however, accept his interpreta-
tions on the subject, and must refer the reader who is anxious to study them more
fully, to His' own paper.

B. III. 29

450

SPINAL NERVES.

FIG. -264 A. TRANSVERSE SEC-
TION THROUGH A PRISTIURUS EM-
BRYO SHEWING THE PROLIFERATION
OF CELLS TO FORM THE NEURAL
CREST.

pr. neural crest ; nc. neural canal ;
ch. notochord ; ao. aorta.

FIG. 2646. TRANSVERSE SEC-
TION THROUGH THE TRUNK OK
AN EMBRYO SLIGHTLY OLDER
THAN FIG. 28 E.

nc. neural canal ; pr. posterior
root of spinal nerve ; x. subnoto-
chordal rod ; ao. aorta ; sc . so-
matic mesoblast ; sp. splanchnic
mesoblast ; mp. muscle-plate ;
mp'. portion of muscle-plate con-
verted into muscle ; Vv. portion
of the vertebral plate which will
give rise to the vertebr.il bodies ;
al. alimentary tract.

before describing the further fate
of the nerve-rudiments it is ne-
cessary to say a few words as to
the neural crest. At the period
when the nerves have begun to
shift their attachment to the
spinal cord, there makes its ap-
pearance, in Elashiobranchii, a
longitudinal commissure con-
necting the dorsal ends of all
the spinal nerves (figs. 265, 266
com}, as well as those of the
vagus and glosso-pharyngeal
nerves. This commissure has
as yet only been found in a com-
plete form in Elasmobranchii ;

FIG. 265. VERTICAL LONGITUDINAL
SECTION THROUGH PART OF THETRUNK
OF A YOUNG SCYLLIUM EMBRYO.

com. commissure uniting the dorsal
ends of the posterior nerve-roots ; pr.
ganglia of posterior roots; ar. anterior
roots; st. segmental tubes; sd. segmental
duct; g.c. epithelium lining the body
cavity in the region of the future germinal
ridge.

NERVOUS SYSTEM OF THE VERTEBRATA.

451

but it is nevertheless to be regarded as a very important morpho-
logical structure.

FIG. 266. SPINAL NERVES OF SCYLLIUM IN LONGITUDINAL SECTION TO SHEW
THE COMMISSURE CONNECTING THEM.

A. Section through a series of nerves.

B. Highly magnified view of the dorsal part of a single nerve, and of the
commissure connected with it.

com. commissure; sp.g. ganglion of posterior root; ar. anterior root.

It is probable, though the point has not yet been definitely
made out, that this commissure is derived from the neural crest,
which appears therefore to separate into two cords, one connected
with each set of dorsal roots.

7'r

FIG. 267. SECTION THROUGH THE DORSAL PART OF THE TRUNK OF A

TORPEDO EMBRYO.

pr. posterior root of spinal nerve ; g . spinal ganglion ; n. nerve ; ar. anterior root
of spinal nerve; ch. notochord; nc. neural canal; mp. muscle-plate.

29 — 2

452 SPINAL NERVES.

Returning to the original attachment of the nerve-rudiments
to the medullary wall, it has been already stated that this
attachment is not permanent. It becomes, in fact, at about the
time of the appearance of the above commissure, either extremely
delicate or absolutely interrupted.

The nerve-rudiment now becomes divided into three parts
(figs. 267 and 268), (i) a proximal rounded portion, to which is
attached the longitudinal commissure (pr) \ (2) an enlarged
portion, forming the rudiment of a ganglion (g and sp g}\ (3) a
distal portion, forming the commencement of the nerve (#).
The proximal portion may very soon be observed to be united
with the side of the spinal cord at a very considerable distance
from its original point of attachment. Moreover the proximal
portion of the nerve is attached, not by its extremity, but by its
side, to the spinal cord (fig. 268 x\ The dorsal extremities of
the posterior roots are therefore free.

This attachment of the posterior nerve-root to the spinal cord is, on
account of its small size, very difficult to observe. In favourable specimens
there may however be seen a distinct cellular prominence from the spinal
cord, which becomes continuous with a small prominence on the lateral
border of the nerve root near its proximal extremity. The proximal ex-
tremity of the nerve is composed of cells, which, by their small size and
circular form, are easily distinguished from those which form the succeeding
or ganglionic portion of the nerve. This part has a swollen configuration,
and is composed of large elongated cells with oval nuclei. The remainder
of the rudiment forms the commencement of the true nerve. This also is, at
first, composed of elongated cells1.

1 The cellular structure of embryonic nerves is a point on which I should have
anticipated that a difference of opinion was impossible, had it not been for the fact
that His and Kolliker, following Remak and other older embryologists, absolutely
deny the fact. I feel quite sure that no one studying the development of the nerves in
Elasmobranchii with well-preserved specimens could for a moment be doubtful on
this point, and I can only explain His' denial on the supposition that his specimens
were utterly unsuited to the investigation of the nerves. I do not propose in this
work entering into the histogenesis of nerves, but may say that for the earlier stages
of their growth, at any rate, my observations have led me in many respects to the
same results as Gotte (Entwick. d. Unke, pp. 482 — 483), except that I hold that
adequate proof is supplied by my investigations to demonstrate that the nerves are
for their whole length originally formed as outgrowths of the central nervous system.
As the nerve-fibres become differentiated from the primitive spindle-shaped cells, the
nuclei become relatively more sparse, and this fact has probably misled Kolliker.
Lowe, while admitting the existence of nuclei in the nerves, states that they belong to
mesoblastic cells which have wandered into the nerves. This is a purely gratuitous
assumption, not supported by observation of the development.

NERVOUS SYSTEM OF THE VERTEBRATA.

453

It is extremely difficult to decide whether the permanent 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. I am inclined
to adopt the former view, which is also held by Marshall and His, but may
refer to fig. 269, shewing the roots after they have become attached to the
side, as distinct evidence in favour of the view that the attachment simply
becomes shifted, a process which might perhaps be explained by a growth of
the dorsal part of the spinal cord. The change of position in the case of
some of the cranial nerves is, however, so great that I do not think that it is
possible to account for it without admitting the formation of a new attach-
ment.

The anterior roots of the spinal nerves appear somewhat
later than the posterior roots, but while the latter are still quite
small. Each of them (fig. 269 ar) arises as a small but distinct
conical outgrowth from a ventral corner of the spinal cord,
before the latter has acquired its covering of white matter.
From the very first the rudiments of the anterior roots have a
somewhat fibrous appearance and an indistinct form of peripheral

FIG. 268. SECTION THROUGH THE DORSAL REGION OF A PRISTIURUS EMBRYO.
pr. posterior root; sp.g. spinal ganglion; n. nerve; x. attachment of ganglion to
spinal cord ; nc. neural canal ; mp. muscle-plate ; ch. notochord ; i. investment of
spinal cord.

termination, while the protoplasm of which they are composed
becomes attenuated towards its end. They differ from the
posterior roots in never shifting their point of attachment to the
spinal cord, in not being united with each other by a commissure,
and in never developing a ganglion.

454

SPINAL NERVES.

The anterior roots grow rapidly, and soon form elongated
cords of spindle-shaped cells with wide attachments to the spinal
cord (fig. 267). At first they pass obliquely and nearly hori-
zontally outwards, but, before reaching the muscle-plates, they
take a bend downwards.

One feature of some interest with reference to the anterior
roots is the fact that they arise not vertically below, but
alternately with the posterior roots : a condition which persists
in the adult. They are at first quite separate from the posterior
roots ; but about the stage represented in fig. 267 a junction is
effected between each posterior root and the corresponding
anterior root. The anterior root joins the posterior at some
little distance below its ganglion (figs. 265 and 266).

Although I have made some efforts to
determine the eventual fate of the commis-
sure uniting the dorsal roots, I have not
hitherto met with success. It grows thinner
and thinner, becoming at the same time
composed of fibrous protoplasm with im-
bedded nuclei, and finally ceases to be re-
cognisable. I can only conclude that it
gradually atrophies, and ultimately vanishes.

After the junction of the posterior and
anterior roots the compound nerve extends
downwards, and may easily be traced for
a considerable distance. A special dorsal
branch is given off from the ganglion on
the posterior root (fig. 275 dn\ According
to Lowe the fibres of the anterior and pos-
terior roots can easily be distinguished in
the higher types by their structure and
behaviour towards colouring reagents, and
can be separately traced in the compound

FIG. 269. TRANSVERSE SKI -
TION THROUGH THE DORSAL RE-
GION OF A YOUNG TORPEDO EM-
BRYO TO SHEW THE ORIGIN OF
THE ANTERIOR AND POSTERIOR
ROOTS OF THE SPINAL NERVES.

pr. posterior root of spinal
nerve ; ar. anterior root of spinal
nerve; mp. muscle-plate; ch. noto-
chord; vr. mesoblast cells which
will form the vertebral bodies.

nerve.

So far as has been made out, the development of the spinal
nerves of other Vertebrates agrees in the main with that in
Elasmobranchii, but no dorsal commissure has yet been discovered,
except in the case of the first two or three spinal nerves of the
Chick.

In the Chick (Marshall, No. 353) the posterior roots, during their early
stages, closely resemble those in Elasmobranchii, though their relatively
smaller size makes them difficult to observe. They at first extend more or

NERVOUS SYSTEM OF THE VERTEBRATA.

455

less horizontally outwards above the muscle-plates (as a few of the nerves
also do to some extent in Elasmobranchii), but subsequently lie close to the
sides of the neural canal. They are shewn in this position in fig. 116 sp.g.
There does not appear to be a continuous crest connecting the roots of the
posterior nerves. The later stages of the development are precisely like
those in Elasmobranchii.

The anterior roots have not been so satisfactorily investigated as the
posterior, but they grow out, possibly by several roots for each nerve, from
the ventral corners of the spinal cord, and subsequently become attached
to the posterior nerves.

I have observed the development of the posterior roots in Lepidosteus, in
which they appear as projections from the dorsal angles of the spinal cord,
extending laterally outwards and, at first, having their extremities placed
dorsally to the muscle-plates.

The cranial nerves1. The earliest stages in the develop-
ment of the cranial nerves have been most satisfactorily studied,
especially by Marshall (No. 354), in the Chick, while the later
stages have been more fully worked out in Elasmobranchii,
where, moreover, they present a very primitive arrangement.

hi,

fy

FIG. 270.

TRANSVERSE SECTION THROUGH THE POSTERIOR PART OF THE

HEAD OF AN EMBRYO CHICK OF THIRTY HOURS.

hb. hind-brain; vg. vagus nerve; cp. epiblast; ch. notochord; x. thickening of
hypoblast (possibly a rudiment of the subnotochordal rod) ; al. throat ; ht. heart ;
//. body cavity ; so. somatic mesoblast ; sf. splanchnic mesoblast ; hy. hypoblast.

1 The optic nerves are for obvious reasons dealt with in connection with the
development of the eye.

456 CRANIAL NERVES.

In the Chick certain of the cranial nerves arise before the
complete closure of the neural groove. These nerves are formed
as paired outgrowths of a continuous band composed of two
laminae, connecting the dorsal end of the incompletely closed
medullary canal with the external epiblast. This mode of
development will best be understood by an examination of fig.
270, where the two roots of the vagus nerve (vg) are shewn
growing out from the neural band. Shortly after this stage the
neural band, becoming separated from the epiblast, constitutes
a crest attached to the roof of the brain, while its two laminae
become fused. The relation of the cranial nerves to the brain
then becomes exactly the same as that of the posterior roots of
the spinal nerves to the spinal cord.

It does not appear possible to decide whether the mode of development
of the cranial nerves in the Chick, or that of the posterior roots of the spinal
nerves, is the more primitive. The difference in development between the
two sets of nerves probably depends upon the relative time of the closure of
the neural canal. The neural crest clearly belongs to the brain, from the
fact of its remaining connected with the latter when the medullary tube
separates from the external epiblast.

It is not known whether the cranial nerves originate before the closure of
the neural canal in other forms besides the Chick.

The neural crest of the brain is continuous with that of the
spinal cord, and on its separation from the central nervous axis
forms on each side a commissure, uniting the posterior cranial
nerves with the spinal nerves, and continuous with the com-
missure connecting together the latter nerves.

Anteriorly, the neural crest extends as far as the roof of the
mid-brain1. The pairs of nerves which undoubtedly grow out
from it are the third pair (Marshall), the fifth, the seventh and
auditory (as a single root), the glossopharyngeal, and the various
elements of the vagus (as separate roots in Elasmobranchii, but
as a single root in Aves). Marshall holds that the olfactory

1 Marshall holds that the neural crest extends in front of the region of the optic
vesicle. I have been unable completely to satisfy myself of the correctness of this
statement. In my specimens the epiblast along the line of infolding of this part of
the roof of the brain is much thickened, but what Marshall represents as a pair of out-
growths from it like those of a true nerve (No. 354, PI. n. fig. 6) appears to me in my
specimens to be part of the external epiblast ; and I believe that they remain connected
with the external epiblast on the complete separation of the brain from it.

NERVOUS SYSTEM OF THE VERTEBRATA. 457

nerve probably also originates from this crest. It will however
be convenient to deal separately with this nerve, after treating of
the other nerves which undoubtedly arise from the neural crest.

The cranial nerves just enumerated present in their further
development many points of similarity ; and the glossopha-
ryngeal nerve, as it develops in Elasmobranchii, may perhaps be
taken as typical. This nerve is connected by a commissure with
those behind, but this fact may for the moment be left out of
consideration. Springing at first from the dorsal line of the
hind-brain immediately behind the level of the auditory capsule,
it apparently loses this primitive attachment and acquires a
secondary attachment about half-way down the side of the
hind-brain. The primitive undifferentiated rudiment soon be-
comes divided, exactly like a true posterior root of a spinal
nerve, into a root, a ganglion and a nerve. The main branch of
the nerve passes ventralwards, and supplies the^ first branchial
arch (fig. 271 gl}. Shortly afterwards it sends forwards a
smaller branch, which passes to the hyoid arch in front ; so that
the nerve forks over the hyobranchial cleft. A typical cranial
nerve appears therefore, except as concerns its relations to the
clefts, to develop precisely like the posterior root of the spinal
nerve.

Most of the cranial nerves of the above group, in correlation
with the highly differentiated character of the head, acquire
secondary differentiations, and render necessary a brief descrip-
tion of what is known with reference to their individual develop-
ment.

The Glossopharyngeal and Vagus Nerves. Behind the ear
there are formed, in Scyllium, a series of five nerves which pass down to
respectively the first, second, third, fourth and fifth branchial arches.

For each arch there is thus one nerve, whose course lies close to the
posterior margin of the preceding cleft ; a second anterior branch, forking
over the cleft and passing to the arch in front, being developed later. These
nerves are connected with the brain by roots at first attached to the dorsal
summit, but eventually situated about half-way down the sides. The
foremost of them is the glossopharyngeal. The next four are, as has been
shewn by Gegenbaur1, equivalent to four independent nerves, but form
together a compound nerve, which we may briefly call the vagus.

1 "Ueber d. Kopfnerven von Hexanchus," etc., Jenaische Zeitschrift, Vol. VI.
1871.

CRANIAL NERVES.

This compound nerve together with the glossopharyngeal soon attains a
very complicated structure, and presents several remarkable features. There
are present five branches (fig. 271 B), viz. the glossopharyngeal (gl) and
four branches of the vagus, the latter probably arising by a considerably
greater number of strands from the brain1. All the strands from the
brain are united together by a thin commissure (fig. 271 B, vg)} continuous
with the commissure of the posterior roots of the spinal nerves, and from
this commissure the five branches are continued obliquely ventralwards and
backwards, and each of them dilates into a ganglionic swelling. They all
become again united together by a second thick commissure, which is
continued backwards as the intestinal branch of the vagus nerve. The
nerves, however, are continued ventralwards each to its respective arch.

A6

t'A

FlG. 271. VIEWS OF THE HEAD OF El.ASMOBRANCH EMBRYOS AT TWO STAGES
AS TRANSPARENT OBJECTS.

A. Pristiurus embryo of the same stage as fig. 28 F.

B. Somewhat older Scyllium embryo.

///. third nerve ; V. fifth nerve ; VII. seventh nerve ; au.n. auditory nerve ; gl.
glossopharyngeal nerve; Vg. vagus nerve; fb. fore-brain; pn. pineal gland ; mb. mid-
brain; hb. hind-brain; iv.v. fourth ventricle; cb. cerebellum; ol. olfactory pit; op.
eye; au.V. auditory vesicle; m. mesohlast at base of brain; t/i. notochord; /it. heart;
Vc. visceral clefts; eg. external gills; //. sections of body cavity in the head.

1 " Ueber d. Kopfnerven von Hexanchus," etc., Jenaische Zeitschrift, Vol. vi. i S; i .

NERVOUS SYSTEM OF THE VERTEBRATA. 459

From the lower commissure springs the lateral nerve, at a point whose
relations to the branches of the vagus I have not certainly determined.

With reference to the dorsal commissure, which is almost certainly
derived from the original neural crest, it is to be noted that there is a
longish stretch of it between the last branch of the vagus and the first
spinal nerve, which is probably the remains of a part of the commissure
which connected the posterior branches of the vagus, at a stage in the
evolution of the Vertebrata, when the posterior visceral clefts were still
present. These branches of the vagus are probably partially preserved in
the ramifications of the intestinal stem of the vagus (Gegenbaur). The
origin of the ventral commissure, continued as the intestinal branch of the
vagus, has not been embryologically worked out.

The lateral nerve may very probably be a dorsal sensory branch of
the vagus, whose extension into the posterior part of the trunk has been
due to the gradual backward elongation of the lateral line1, causing
the nerve supplying it to elongate at the same time (vide Section on lateral
line).

In the Chick the common rudiment for the vagus and glossopharyngeal
nerves (Marshall), which has already been spoken of, subsequently divides
into two parts, an anterior forming the glossopharyngeal nerve, and a
posterior forming the vagus nerve.

The seventh and auditory nerves. As shewn by Marshall's
and my own observations 'there is a common rudiment for the seventh and
auditory nerves. This rudiment divides almost at once into two branches.
The anterior of these pursues a straight course to the hyoid arch (fig.
271 A, VII.} and forms the rudiment of the facial nerve ; the second of the
two (fig. 271 A, au.ti), which is the rudiment of the auditory nerve, develops
a ganglionic enlargement and, turning backwards, closely hugs the ventral
wall of the auditory involution (fig. 272).

The seventh or facial nerve soon becomes more complicated. It early
develops, like the glossopharyngeal and vagus nerves, a branch, which
forks over the cleft in front (spiracle), and supplies the mandibular arch
(fig. 27 1 B). This branch forms the praespiracular nerve of the adult, and
is homologous with the chorda tympani of Mammalia. Besides however
giving rise to this typical branch it gives origin, at a very early period,
to two other rather remarkable branches ; one of these, arising from its
dorsal anterior border, passes forwards to the front part of the head, im-
mediately dorsal to the ophthalmic branch of the fifth to be described
directly. This nerve is the portio major or superficialis of the nerve usually
known as the ramus ophthalmicus superficialis in the adult2.

1 The peculiar distribution of branches of the fifth and seventh nerves to the
lateral line, which is not uncommon, is to be explained in the same manner.

2 The two branches of the ramus ophthalmicus superficialis were spoken of as the
ram. opth. superficialis and ram. opth. profundus in my Monograph on Elasmobranch
Fishes. The nomenclature in the text is Schwalbe's, which is probably more correct
than mine.

460 CRANIAL NERVES.

The other branch of the seventh is the palatine branch — superficial
petrosal of Mammalia — the course of which has been more fully investigated
by Marshall than by myself. He has shewn that it arises "just below the
root of the ophthalmic branch," and " runs downwards and forwards, lying
parallel and immediately superficial to the maxillary branch of the fifth
nerve." This branch of the seventh nerve appears to bear the same sort of
relation to the superior maxillary branch of the fifth nerve, that the
ophthalmic branch of the seventh does to the ophthalmic branch of the fifth.

Both the root of the seventh and its main branches are gangliated.

The auditory nerve is probably to be regarded as a specially differen-
tiated part of a dorsal branch of the seventh, while the ophthalmic branch
may not improbably be a dorsal branch comparable to a dorsal branch of
one of the spinal nerves.

The fifth nerve. 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 becomes developed close to the root, which forms the
rudiment of the Gasserian ganglion. The main branch of the nerve grows
into the mandibular arch (fig. 271 A, V), maintaining towards it similar
relations to those of the posterior nerves to their respective arches.

Two other branches very soon become developed, which were not
properly distinguished in my original account. The dorsal one takes a
course parallel to the ophthalmic branch of the seventh nerve, and forms,
according to the nomenclature already adopted, the portio profunda of the
ophthalmicus superficialis of the adult.

The second nerve (fig. 271 A) passes forwards, above the mandibular
head cavity, and is directed straight towards the eye, near which it meets
and unites with the third nerve, where the ciliary ganglion is developed
(Marshall). This branch is usually called the ophthalmic branch of the
fifth nerve, but Marshall rightly prefers to call it the communicating branch
between the fifth and third nerves1.

Later than these two branches there is developed a third branch, passing
to the front of the mouth, and forming the superior maxillary branch of the
adult (fig. 271 B).

Of the branches of the fifth nerve the main mandibular branch is
obviously comparable to the main branch of the posterior nerves. The
superficial ophthalmic branch is clearly equivalent to the ophthalmic branch
of the seventh. The superior maxillary is usually held to be equivalent to
that branch of the posterior nerves which forms the anterior limb of the fork
over a cleft. The similarity between the course of this nerve and that
of the palatine branch of the seventh, resembling as it does the similar
course of the ophthalmic branches of the two nerves, suggests that it may
perhaps really be the homologue of the palatine branch of the seventh, there

1 Marshall thinks that this nerve may be the remains of the commissure originally
connecting the roots of the third and fifth nerves. This suggestion can only be tested
by further observations.

NERVOUS SYSTEM OF THE VERTEBRATA. 461

being no homologue of the typical anterior branch of the other cranial
nerves.

The third nerve. Our knowledge of the development of the third
nerve is entirely due to Marshall. He has shewn that in the Chick there is
developed 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
outgrowth, the presence of which I can confirm, he believes to be the third
nerve, but although he is probably right in this view, 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 satisfactorily 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, and
which Marshall supposes to be the above rudiment, which has shifted its
position. It is shewn in Scyllium in fig. 271 B, ///. A few intermediate
stages between this and the earliest condition of the nerve have been
imperfectly traced by Marshall.

The nerve at the stage represented in fig. 271 B arises from a ganglionic
root, and " runs as a long slender stem almost horizontally backwards, then
turns slightly outwards to reach the interval between the dorsal ends of the
first and second head cavities, where it expands into a small ganglion."
This ganglion, as first suggested by Schwalbe (No. 359), and subsequently
proved embryologically by Marshall, is the ciliary ganglion. From the
ciliary ganglion two branches arise ; one branch continuing the main stem
of the nerve, and obviously homologous with the main branch of the other
nerves, and the other passing directly forwards " along the top of the first
head cavity, then along the inner side of the eye, and finally terminating at
the anterior extremity of the head, just dorsal of the olfactory pit."

The partial separation, in many forms, of the ciliary ganglion from the
stem of the third nerve has led to the erroneous view (disproved by the
researches of Marshall and Schwalbe) that the ciliary ganglion belongs to
the fifth nerve. The connecting branch of the fifth nerve often becomes
directly continuous with the anterior branch of the third nerve, and the two
together probably constitute the nerve known as the ramus ophthalmicus
profundus (Marshall). Further embryological investigations will be required
to shew whether this nerve is homologous with the nasal branch of the fifth
nerve in Mammalia.

Relations of the nerves to the head-cavities. The cranial
nerves, whose development has just been given, bear certain very definite
relations to the mesoblastic structures in the head, of the nature of somites,
which are known as the head-cavities. Each cranial nerve is typically
placed immediately behind the head-cavity of its somite. Thus the main
branch of the fifth nerve lies in contact with the posterior wall of the
mandibular cavity, as shewn in section in fig. 272 V. ipp and in surface view
in fig. 271 ; the main branch of the seventh nerve occupies a similar position
in relation to the hyoid cavity ; and, as Marshall has recently shewn, the
main branch of the third nerve adjoins the posterior border of the front

462

CRANIAL NERVES.

cavity, described by me as the preman-
dibular cavity. Owing to the early con-
version of the walls of the posterior head-
cavities into muscles, their relations to the
nerves are not quite so clear as in the
case of the anterior cavities, though, as
far as is known, they are precisely the
same.

Anterior nerve-roots in the brain.

During my investigations on the de-
velopment of the cranial nerves I was
unable to find any roots comparable with
the anterior roots of the spinal nerves,
and propounded an hypothesis (suggested
by the absence of anterior spinal roots
in Amphioxus1) that the head and trunk
had become differentiated from each other
at a stage when mixed motor and sensory
posterior roots were the only roots pre-
sent, and I supposed the cranial and
spinal nerves to have been independently
evolved from a common ground form,
the resulting types of nerves being so
different that no roots strictly comparable
with the anterior roots of spinal nerves
were to be found in the cranial nerves.

The views put forward by me on this
subject, though accepted by Schwalbe

Vll

FIG. 272. TRANSVERSE SECTION
THROUGH THE FRONT PART OF THE
HEAD OF A YOUNG PRISTIURUS

EMBRYO.

The section, owing to the cranial
flexure, cuts both the fore- and the
hind-brain. It shews the praman-
dibular and mandibular head-cavities
\pp and ipp, etc.

fb. fore-brain; /. lens of eye; m.
mouth ; pt. upper end of mouth,
forming pituitary involution; \ao.
mandibular aortic arch; ipp. and
ipp. first and second head-cavities ;
ivc. first visceral cleft ; V. fifth
nerve ; aun. ganglion of auditory
nerve ; VII. seventh nerve ; aa. dor-
sal aorta ; acv. anterior cardinal
vein ; ch. notochord.

(No. 357), have in other quarters not
met with much favour. Wiedersheim holds that it is impossible to believe
that the cranial nerves are simpler than the spinal nerves. Such simplicity,
which is clearly not found, I have never asserted to exist ; I have only
stated that the cranial nerves, in acquiring the complicated character they
have in the adult, do not develop anterior roots comparable with those
of the spinal nerves. Marshall also strongly objects to my views, and has
made some observations for the purpose of testing them, leading to some
very interesting results, which I proceed to state, and I will then explain my
opinion concerning them.

The most important observation of Marshall on this subject concerns
the sixth nerve. In both the Chick and Scy Ilium he has detected a nerve
(the first development of which has unfortunately not been made out) arising
by a series of roots from the base of the hind-brain. By tracing this nerve
to the external rectus muscle of the eye he has satisfactorily identified

1 Schneider holds that anterior roots are present in Amphioxus, but I have been
unable to satisfy myself of their presence.

NERVOUS SYSTEM OF THE VERTEBRATA. 463

it as the sixth nerve. " Neither in the nerve nor in its roots are there any
ganglion cells." This nerve he finds to be placed vertically below the roots
of the seventh nerve ; and it is not visible till much later than the cranial
nerves above described.

In addition to this nerve Marshall has found, both in the third nerve
and in the fifth nerve, a series of non-gangliated roots, which arise in a
manner not yet satisfactorily elucidated, considerably later than, and in front
of, the main roots. These roots join the gangliated roots on the proximal
side of the ganglion or in the ganglion1; and Marshall believes them to be
homologous with the anterior roots of spinal nerves, while he holds the
sixth nerve to be an anterior root of the seventh nerve.

In addition to these nerves Marshall holds certain ventral roots, which
occur in Elasmobranchs close to the boundary of the spinal cord and
medulla, and which probably form the hypoglossal nerve of higher types, to
be anterior roots of the vagus. It is very difficult to prove anything
definitely about these nerves, but, for reasons stated in my work on
Elasmobranch Fishes, I am inclined to regard them as anterior roots of one
or more spinal nerves.

Before attempting to decide how far Marshall's views about the so-called
anterior roots of the seventh, the fifth and the third nerves are well founded
it will conduce to clearness to state the characters and relations of the two
roots of spinal nerves.

The posterior root is (i) always purely sensory ; (2) it always develops a
ganglion. The anterior root is (i) always purely motor ; (2) it always joins
the posterior root below the ganglion, except in Petromyzon (though not in
Myxine) where the two roots are stated to be independent.

How far do Marshall's anterior and posterior roots of the cranial nerves
exhibit these respective peculiarities ?

With reference to the sixth and seventh nerves he states " we must
regard the sixth nerve as having the same relation to the seventh that the
anterior root of a spinal nerve has to the posterior root." On this I would
remark (i) that the posterior root of this nerve is a mixed sensory and
motor nerve and therefore differs in a very fundamental point from that of
a spinal nerve ; (2) the sixth nerve though resembling the anterior root of
a spinal nerve in being motor and without a ganglion, differs from the
nearly universal arrangement of spinal nerves in not uniting with the
seventh.

With reference to the fifth nerve it is to be observed that it is by no
means certain that the whole of the motor fibres are supplied by the so-
called anterior roots, and that these roots differ again in the most marked
manner from the anterior roots of spinal nerves in joining the main root of
the nerve above (nearer the brain), and not as in a spinal nerve below the

1 These non-gangliated roots of the fifth nerve are not to be confounded with the
motor root of the fifth nerve in higher types. They appear to form the anterior root
of the adult which gives origin to the ramus ophthalmicus.

464 CRANIAL NERVES.

ganglion. The gangliated root of the third nerve is purely motor 1, and its
so-called anterior roots again differ from the anterior roots of spinal nerves,
in the same manner as those of the fifth nerve.

With reference to the glossopharyngeal and vagus nerves I would
merely remark that no anterior root has even been suggested for the
glossopharyngeal nerve and that the posterior roots of both these nerves
contain a mixture of sensory and motor fibres.

In view of these facts, my original hypothesis appears to me to be
confirmed by Marshall's observations.

The fact of all the posterior roots of the above cranial nerves (except
the third which may be purely motor) being mixed motor and sensory roots
appears to me to demonstrate that the starting-point of their differentiation
was a mixed nerve with a single dorsal root ; and that they did not therefore
become differentiated from nerves built on the same type as the spinal
nerves with dorsal sensory and ventral motor roots. The presence of such
non-gangliated roots as those of the third and fifth nerves is not a difficulty
to this view. Considering that the cranial nerves are more highly differen-
tiated than the spinal nerves, and have more complicated functions to
perform, it would be surprising if there had not been developed non-
ganglionated roots analogous to, but not of course homologous with, the
anterior roots of the spinal nerves2.

As to the sixth nerve further embryological investigations are requisite
before its true position in the series can be determined ; but it appears to
me very probable that it is a product of the differentiation of the seventh
nerve.

The fourth nerve. No embryological investigations have been
made with reference to the fourth nerve. It is possible that it is a segmental
nerve comparable with the third nerve, and that the only remnant still left
of the segment to which it belongs is the superior oblique muscle of the eye.
If this is the case there must have been two praemandibular segments, viz.
that belonging to the third nerve, and that belonging to the fourth nerve.
Against this view of the fourth nerve is the fact, urged with great force by
Marshall, that the superior oblique muscle is in front of the other eye
muscles, and that the fourth nerve therefore crosses the third nerve to
reach its destination.

The Olfactory nerve. It was shewn in my monograph on Elas-
mobranch Fishes that the olfactory nerve grew out from the brain in the

1 If Marshall's view about the ramus ophthalmicus profundus (p. 461) is correct,
the third must still be, as it no doubt was primitively, a mixed motor and sensory
nerve.

2 In the higher types, as is well known, the fifth nerve has its roots formed on the
same type as a spinal nerve. The fact that this is not the case in the lower types,
either in the embryo or the adult, is a clear indication, to my mind, that the mam-
malian arrangement of the roots of the fifth nerve has been secondarily acquired, a
fact which is a most striking confirmation of my views as to the differences between
the cranial and spinal nerves.

NERVOUS SYSTEM OF THE VERTEBRATA. 465

same manner as other nerves ; and Marshall (No. 355), to whom we are
indebted for the greater part of our knowledge on the development of this
nerve, has proved that it arises prior to the differentiation of the olfactory
lobes.

The earliest stages in the development of the nerve have not been
made out. Marshall, as already stated, finds that in the Chick the neural
crest is continued in front of the optic vesicles, and holds that this fact is
strong a priori evidence in favour of the nerve growing out from it. As
mentioned above, note on p. 456, I cannot without further evidence accept
Marshall's statements on this point. In any case Marshall has not yet been

FIG. 273. SECTION THROUGH THE BRAIN AND OLFACTORY ORGAN OF AN
EMBRYO OF ScvLLiUM. (Modified from figures by Marshall and myself.)

c.h. cerebral hemispheres; ol.v. olfactory vesicle ; olf. olfactory pit; Sch. Schnei-
derian folds; /. olfactory nerve. The reference line has been accidentally taken
through the nerve to the brain; pn. pineal gland.

able again to find an olfactory nerve till long after the disappearance of the
neural crest. The olfactory nerve at the next stage observed forms an out-
growth of fusiform cells springing on either side from near the summit of
the fore-brain ; and at fifty hours it ends close to a slight thickening of the
epiblast forming the first rudiment of the olfactory pit, with the walls of
which it soon becomes united.

The growth of the cerebral hemispheres causes its point of insertion in
the brain to be relatively shifted ; and on the development of the olfactory
lobes (vide pp. 444, 445) it arises from them (fig. 273). In Elasmobranchs
there is a large development of ganglion cells near its root. From Marshall's
figures these appear also to be present in the Chick, but they do not seem to
have been found in other forms. In both Teleostei and Amphibia the
olfactory nerves are at first extremely short.

Marshall holds that the olfactory nerve is a segmental nerve equivalent
to the third, fifth, seventh etc. nerves. It has been already stated that in my
opinion the origin of the olfactory nerves from the fore-brain, which I hold
to be the ganglion of the prseoral lobe, negatives this view. The mere fact

B. HI- 30

466 SYMPATHETIC NERVOUS SYSTEM.

of these nerves originating as an outgrowth from the central nervous
system is no argument in favour of Marshall's view of their nature ; and
even if Marshall's opinion that they arise from the neural crest should turn
out to be well founded, this fact would not prove their segmental nature,
because their origin from this crest would, as indicated in the next
paragraph, merely seem to imply that they primitively arose from the
lateral borders of the nerve-plate from which the cerebro-spinal tube has
been formed.

Situation of the dorsal roots of the cranial and spinal
nerves. The probable explanation of the origin of nerves from the neural
crest has already been briefly given (p. 316). It is that the neural crest
represents the original lateral borders of the nervous plate, and that, in the
mechanical folding of the nervous plate to form the cerebro-spinal canal, its
two lateral borders have become approximated in the median dorsal line to
form the neural crest. The subsequent shifting of the nerves I am unable
to explain, and the meaning of the transient longitudinal commissure
connecting the nerves is also unknown. The folding of the neural plate
must have extended to the region of the origin of the olfactory nerves, so
that, as just stated, there would be no special probability of the olfactory
nerves belonging to the same category as the other dorsal nerves from the
fact of their springing from the neural crest.

BIBLIOGRAPHY OF THE PERIPHERAL NERVOUS SYSTEM.

(351) F. M. Balfour. "On the development of the spinal nerves in Elasmo-
branch Fishes." Philosophical Transactions, Vol. CLXVI. 1876; vide also, A mono-
graph on the development of Elasmobranch Fishes. London, 1878, pp. 191 — 216.

(352) W. His. " Ueb. d. Anfange d. peripherischen Nervensystems." Archiv
f. Anat. it. Physiol., 1879.

(353) A. M. Marshall. " On the early stages of development of the nerves in
Birds." Journal of Anat. and P/iys.,No\. xi. 1877.

(354) A. M. Marshall. "The development of the cranial nerves in the Chick."
Quart, y. of Micr. Science, Vol. xvm. 1878.

(355) A. M> Marshall. "The morphology of the vertebrate olfactory organ."
Quart. J. of Micr. Science, Vol. xix. 1879.

(356) A. M. Marshall. " On the head-cavities and associated nerves in Elasmo-
branchs." Quart. J. of Micr. Science, Vol. xxi. 1881.

(357) C. Schwalbe. "Das Ganglion oculomotorii." Jenaische Zeitschrift,
Vol. xili. 1879.

Sympathetic nervous system.

The discovery that the spinal and cranial nerves together
with their ganglia were formed from the epiblast was shortly
afterwards extended to the sympathetic nervous system, which
has now been shewn to arise in connection with the spinal and

NERVOUS SYSTEM OF THE VERTEBRATA.

467

cranial nerves. The earliest observations on this subject were
those contained in my Monograph on Elasmobranck Fishes
(P- T73)> while Schenk and Birdsell (No. 361) have since
arrived at the same result for Aves and Mammalia.

In my account of the development of these ganglia, it is
stated that they were first met with as small masses situated at
the ends of short branches of the spinal nerves (fig. 275 sy.g).
More recent investigations have shewn me that the sympathetic
ganglia are at first simply swellings on the main branches of the
spinal nerves some way below the ganglia. Their situation
may be understood from fig. 274, sy.g,
which belongs however to a somewhat
later stage. Subsequently the sympath-
etic ganglia become removed from the
main stem of their respective nerves,
remaining however connected with those
stems by a short branch (fig. 275, sy.g).
I have been unable to find a longitudinal
commissure connecting them in their
early stages; and I presume that they
are at first independent, and become sub-
sequently united into a continuous cord
on each side.

The observations of Schenk and
Birdsell on the Mammalia seem to in-
dicate that the main parts of the sym-
pathetic system arise in continuity with
the posterior spinal ganglia : they also shew that in the neck
and other parts the sympathetic cords arise as a continuous
ganglionic chain. The observations on the topographical
features of the development of the sympathetic system in
higher types are however as yet very imperfect.

The later history of the sympathetic ganglia is intimately
bound up with that of the so-called supra-renal bodies, which
are dealt with in another chapter.

FIG. 274. LONGITUDI-
NAL VERTICAL SECTION
THROUGH PART OF THE
BODY WALL OF AN ELASMO-
BRANCH EMBRYO SHEWING
PARTOFTWOSPINAL NERVES
AND THESYMPATHETICGAN-
GLIA BELONGING TO THEM.

ar. anterior root ; pr. pos-
terior root ; sy.g. sympathetic
ganglion ; tnp. part of mus-
cle-plate.

30—2

468

SYMPATHETIC NERVOUS SYSTEM.

time.

FIG. 275. TRANSVERSE SECTION THROUGH THE ANTERIOR PART OF THE TRUNK
OF AN EMBRYO OF SCYLLIUM SLIGHTLY OLDER THAN FIG. 29 B.

The section is diagrammatic in the fact that the anterior nerve-roots have been
inserted for their whole length ; whereas they join the spinal cord half-way between
two posterior roots.

sp.c. spinal cord; sp.g. ganglion of posterior root; ar, anterior root; d.n. dorsally
directed nerve springing from posterior root; mp. muscle plate; mp'. part of muscle
plate already converted into muscles ; mp. /. part of muscle plate which gives rise to
the muscles of the limbs; «/. nervus lateralis; ao. aorta; ch. notochord; sy.g. sym-
pathetic ganglion; ca.v. cardinal vein; sp.n. spinal nerve; sd. segmental (archinephric)
duct; st. segmental tube; dn. duodenum; pan. pancreas; hp.d. point of junction of
hepatic duct with duodenum; nmc. umbilical canal.

NERVOUS SYSTEM OF THE VEKTEBRATA. 469

BIBLIOGRAPHY OF THE SYMPATHETIC NERVOUS SYSTEM.

(360) F. M. Balfour. Monograph on the development of Elasmobranch Fishes.
London, 1878, p. 173.

(361) S. L. Schenk and W. R. Birdsell. "Ueb. d. Lehre vond. Entwicklung
d. Ganglien d. Sympatheticus. " Mittheil. a. d. embryologischen histit. Wien, Heft
in. 1879.

CHAPTER XVI.
ORGANS OF VISION.

IN the lowest forms of animal life the whole surface is sensitive
to light, and organs of vision have no doubt arisen in the first
instance from limited areas becoming especially sensitive to light
in conjunction with a deposit of pigment. Lens-like structures,
formed either as a thickening of the cuticle, or as a mass of cells,
were subsequently formed ; but their function was not, in the first
instance, to throw an image of external objects on the perceptive
part of the eye, but to concentrate the light on it. From such a
simple form of visual organ it is easy to pass by a series of steps
to an eye capable of true vision.

There are but few groups of the Metazoa which are not pro-
vided with optic organs of greater or less complexity.

In a large number of instances these organs are placed on the
anterior part of the head, and are innervated from the anterior
ganglia. It is possible that many of the eyes so situated may
be modifications of a common prototype. In other instances
organs of vision are situated in different regions of the body, and
it is clear that such eyes have been independently evolved in each
instance.

The percipient elements of the eye would invariably appear
to be cells, one end of each of which is continuous with a
nerve, while the other terminates in a cuticular structure, or
indurated part of the cell forming what is known as the rod or
cone.

The presence of such percipient elements in various eyes is
therefore no proof of genetic relationship between these eyes*
but merely of similarity of function.

Embryological data as to the development of the eye do not

ORGANS OF VISION.

471

exist except in the case of the Arthropoda, Mollusca and Chor-
data. From such data as there are, combined with study of the
adult structure of the eye, it can be shewn that two types of
development are found. In one of these the percipient elements
are formed from the central nervous system, in the other from
the epidermis. The former may be called cerebral eyes. It is
probable however that this distinction is not, in all cases at
any rate, so fundamental as might be supposed ; but that in
both instances the eye may have taken its origin from the
epidermis. In the eyes in which the retina is continuous with
the central nervous system, these two organs were probably
evolved simultaneously as differentiations of the epidermis, and
continue to develop together in the ontogenetic growth of the eye.

Some of the eyes in which the retina is formed from the epi-
dermis have also probably arisen simultaneously with part of the
central nervous system, while in other instances they have arisen
as later formations subsequently to the complete establishment
of a central nervous system.

Coelenterata. The actual evolution of the eye is best
shewn in the Hydrozoa. The simplest types
are those found in Oceania and Lizzia1. In
"Lizzia. the eye is placed at the base of a
tentacle and consists of (fig. 276) a lens (/)
and a percipient bulb (oc). The lens is a
simple thickening of the cuticle, while the
percipient part of the eye is formed of three
kinds of elements: — (i) pigment cells; (2)
sense cells, forming the true retinal elements,
and consisting of a central swelling with the
nucleus, a peripheral process representing a
hardly differentiated rod, and a central pro-
cess continuous with (3) ganglion cells at
the base of the eye. In this eye there is
present a commencing differentiation of a
ganglion as well as of a retina.

The eye of Oceania is simpler than that of Lizzia
in the absence of a lens. Claus has shewn that in

oc.

(From Lankester; after
Hertwig.)

/. lens; oc. percep-
tive part of eye.

1 O. and R. Hertwig. Das Nei~uen system #. Sinnesorgane d. Medtisen.
1878.

Leipzig,

472

MOLLUSCA.

Charybdea amongst the Acraspeda a more highly differentiated eye is
present, with a lens formed of cells like the vertebrate eye.

Mollusca. In a large number of the odontophorous Mollusca
eyes, innervated by the supracesophageal ganglia, are present
on the dorsal side of the head. These eyes exhibit very various
degrees of complexity, but are shewn both by their structure and
development to be modifications of a common prototype.

The simplest type of eye is that found in the Nautilus, and
although the possibility of this eye being degenerated must be
borne in mind, it is at the same time very interesting to note
(Hensen) that it retains permanently the early embryonic struc-
ture of the eyes of the other groups.

It has (fig. 277 A) the form of a vesicle, with a small opening
in the outer wall, placing the cavity of the vesicle in free com-
munication with the exterior. The cells lining the posterior face
of the vesicle form a retina (7?); and are continuous with the
fibres of the optic nerve (N.op). We have no knowledge of the
development of this eye.

In the Gasteropods the eye (fig. 277 B) has the form of a
closed vesicle: the cells lining the inner side form the retina,
while the outer wall of the vesicle constitutes the cornea. A

N.op

G.op

FIG. 277. THREE DIAGRAMMATIC SECTIONS OF THE EYES OF MOLLUSCA.

(After Grenacher.)
A. Nautilus. B. Gasteropod (Limax or Helix). C. Dibranchiate Cephalopod.

Pal. eyelid; Co. cornea; Co.ep. epithelium of ciliary body ; Ir. iris; Int, Int"1...
Int*. different parts of the integument; /. lens; I1, outer segment of lens; R. retina;
N.op. optic nerve; G.op. optic ganglion; x. inner layer of retina; N.S. nervous
stratum of retina.

ORGANS OF VISION. 473

cuticular lens is placed in the cavity, on the side adjoining the
cornea. This eye originates from the ectoderm, within the velar
area, and close to the supra-cesophageal ganglia, usually at the
base of the tentacles. According to Rabl (Vol. II. No. 268) it is
formed as an invagination, the opening of which soon closes ;
while according to Bobretzky (Vol. II. No. 242) and Fol it arises
as a thickening of the epiblast, which becoming detached takes
the form of a vesicle. It is quite possible that both types of
development may occur, the second being no doubt abbreviated.
The vesicle, however formed, soon acquires a covering of pigment,
except for a small area of its outer wall, where the lens becomes
formed as a small body projecting into the lumen of the vesicle.
The lens seems to commence as a cuticular deposit, and to grow
by the addition of concentric layers. The inner wall of the vesicle
gives rise to the retina.

The most highly differentiated molluscan eye is that of the
Dibranchiate Cephalopoda, which is in fact more highly organized
than any other invertebrate eye.

A brief description of its adult structure l will perhaps render more clear
my account of the development. The most important features of the eye
are shewn in fig. 277 C. The outermost layer of the optic bulb forms a kind
of capsule, which may be called the sclerotic. Posteriorly the sclerotic abuts
on the cartilaginous orbit, which encloses the optic ganglion (G. op~) ; and in
front it becomes transparent and forms the cornea Co, which may be either
completely closed, or (as represented in the diagram) perforated by a larger
or smaller opening. Behind the cornea is a chamber known as the anterior
optic chamber. This chamber is continued back on each side round a
great part of the circumference of the eye, and separates the sclerotic from a
layer internal to it.

In the anterior optic chamber there are placed (i) the anterior part of
the lens (71) and (2) the folds of the iris (Ir). The whole chamber, except
the part formed by the lens, is lined by the epidermis (InP and Infi}.
Bounding the inner side of the anterior optic chamber is a layer which is
called the choroid (Int1} which is continued anteriorly into the fold of the
iris (Ir). The most superficial layer of the choroid is the epithelium already
mentioned, next comes a layer of obliquely placed plates known as the
argentea externa, then a layer of muscles, and finally the argentea interna.
The argentea interna abuts on a cartilaginous capsule, which completely
invests the inner part of the eye.

The lens is a nearly spherical body composed of concentric lamellae of a
structureless material. It is formed of a small outer (71) and large inner

1 Vide Hensen, Zeit. f. wiss. Zool. Bd. XV.

474 CEPHALOPODA.

(/) segment, the two being separated by a thin membrane. It is supported
by a peculiar projection of the wall of the optic cup, known as the ciliary
body (Co.ep), inserted at the base of the iris, and mainly formed of a
continuation of the retina. This body is however muscular, and presents a
series of folds on its outer and inner surfaces, which are especially developed
on the latter.

The membrane dividing the lens into two parts is continuous with the
ciliary body. Within the lens is the inner optic chamber, bounded in front
by the lens and the ciliary body, and behind by the retina.

The retina is formed of two main divisions, an anterior division adjoining
the inner optic chamber, and a posterior division (N.S) adjoining the
cartilage of the choroid. The two layers are separated by a membrane.
Passing from within outwards the following layers in the retina may be
distinguished :

(1) Homogeneous membrane. | Anterior division of

(2) Layer of rods. retina

(3) Layer of granules imbedded in pigment. J

(4) Cellular layer.

(5) Connective tissue layer.

Posterior layer of retina.

(6) Layer of nerve-fibres.

At the side of the optic ganglion is a peculiar body, known as the white
body (not shewn in the figure), which has the histological characters of
glandular tissue.

The first satisfactory account of the development of the eye
is due to Lankester (No. 365). The more important features in
it were also independently worked out by Grenacher (No. 363),
and are beautifully illustrated in Bobretzky's paper (No. 362).
The eye first appears as an oval pit of the epiblast, the edge of
which is formed by a projecting rim (fig. 278 A). The epiblast

A

FlG. 278. TWO SECTIONS THROUGH THE DEVELOPING EYE OF A CEI'HALUl'OD

TO SHEW THE FORMATION OF THE OPTIC CUP. (After Lankester.)

layer lining the floor of the pit soon becomes considerably thick-
ened. By the growth inwards of the rim the mouth of the pit

ORGANS OF VISION.

475

is gradually narrowed (fig. 278 B), resembling at this stage the
eye of Nautilus, and finally closed. There is thus formed a
flattened sack, lined by epiblast, which may be called the primary
optic vesicle. Its cavity eventually forms the inner optic chamber.
The anterior wall of the sack is lined by a much less columnar
layer than the posterior, the former giving rise to the epithelium
on the inner side of the ciliary processes, the latter to the retina.
The cavity of the sack rapidly enlarges, and assumes a
spherical form. At the same time a layer of mesoblast grows
in between the walls of the sack and the external epiblast.

FIG. 279. TRANSVERSE SECTION THROUGH THE HEAD OF AN ADVANCED
EMBRYO OF LoLlGO. (After Bobretzky.)

gls. salivary gland; g.vs. visceral ganglion; gc. cerebral ganglion; g.op. optic gan-
glion; adk. optic cartilage; ak. and_y. lateral cartilage or (?) white body; rt. retina;
gm. limiting membrane of retina ; vk, ciliary region of eye ; cc. iris ; ac. auditory sack
(the epithelium lining the auditory sacks is not represented) ; vc. vena cava ; ff. folds
of funnel ; x, epithelium of funnel.

Two new structures soon arise nearly simultaneously (fig. 279),
— which become in the adult eye the iris (cc) and the posterior
segment of the lens. The iris is formed as a circular fold of the
skin in front of the optic vesicle. It consists both of epiblast
and mesoblast, and gives rise to a pit lined by epiblast. The
posterior segment of the lens arises as a structureless rod-like
body, which is shewn in fig. 279 depending from the inner side

476 CEPHALOPODA.

of the anterior wall of the optic vesicle. Its exact mode of origin
is somewhat obscure. The following is Lankester's account of
it1: "It is formed entirely within the primitive optic chamber,
and at first depends as a short cylindrical rod from the middle
point of the anterior wall of that chamber, that is to say, from
the point at which the chamber finally closed up. It grows sub-
sequently by the deposition of concentric layers of a horny material
round this cone. No cells appear to be immediately concerned
in effecting the deposition, and it must be looked upon as an
organic concretion, formed from the liquid contained in the
primitive optic chamber."

The lens would thus appear to be a cuticular structure. It
gradually assumes a nearly spherical form ; and is then composed
of concentrically arranged layers (fig. 280, /if).

While the lens is being formed, the ciliary epithelium of the
optic vesicle becomes divided into two layers, an outer layer of
large cells and an inner of small cells. Both layers are at first
continuous across the anterior wall of the optic chamber in front
of the lens, but soon become confined to the sides (fig. 280 A,
cc and gz). The inner layer is stated by Lankester to give rise
to the muscles present in the adult. The mesoblast cells also
disappear from the region in front of the lens, and the outer
epithelium is converted into a kind of cuticular membrane. By
these changes the original layers of cells in front of the lens
become reduced to mere membranes, — a change which appears
to be preparatory to the appearance of the anterior segment of
the lens. The formation of the latter has not been fully followed
out by any investigator except Bobretzky. His figures would
seem to indicate that it is formed as a cuticular deposit in
front of the membrane already spoken of (fig. 280 B, vl). The
two segments of the lens appear at any rate to be separated by
a membrane continuous with the ciliary region of the optic
vesicle.

Grenacher believes that the front part of the lens is formed in a pocket-
like depression of the epiblastic layer covering the outer side of the optic
cup ; and Lankester thinks that the lens " pushes its way through the median
anterior area of the primitive optic chamber, and projects into the second or
anterior optic chamber where the iridian folds lie closely upon it."

1 "Devel. of Cephalopoda." Q. J. Micro. Scien. 1875, p. 44.

ORGANS OF VISION.

477

While the lens is attaining its complete development there
appears a fresh fold round the circumference of the eye, which
gradually grows inwards so as to form a chamber outside the
parts already present. This chamber is the anterior optic
chamber of the adult. In most Cephalopods (fig. 277 C) the
edges of the fold do not quite meet, but leave a larger or smaller
aperture leading into the chamber containing the iris, outer
segment of the lens, etc. In some forms however they meet
and coalesce, and so shut off this chamber from communication
with the exterior. The edge of the fold constitutes the cornea
while the remainder of it gives rise to the sclerotic.

The retina is at first a thick layer of numerous rows of oval

<>

FIG. 280. SECTIONS THROUGH THE DEVELOPING EYE OF LOLIGO

AT TWO STAGES. (After Bobretzky.)

///. inner segment of lens ; vl. outer segment of lens ; a and a. epithelium lining
the anterior optic chamber; gz. large epiblast cells of ciliary body; cc. small epi-
blast cells of ciliary body ; ms . layer of mesoblast between the two epiblastic layers
of the ciliary body; of. and if. fold of iris; rt. retina; rt". inner layer of retina;
st. rods ; aq. cartilage of the choroid.

478 ONCHIDIUM.

cells (fig. 279). When the inner segment of the lens is far
advanced towards its complete formation pigment becomes
deposited in the anterior part of the retina, and a layer of rods
grows out from the surface turned towards the cavity of the
optic vesicle (fig. 280 A, st). At a slightly later stage the retina
becomes divided into two layers (Bobretzky), a thicker anterior
layer, and a thinner posterior layer (fig. 280, rt and rf}. The
former is composed of two strata, (i) the rods and (2) a stratum
with numerous rows of nuclei which becomes in the adult the
granular layer with its pigment. The posterior layer gives rise
to the cellular part of the posterior division of the retina, while
layers of connective tissue around it give rise to the connective
tissue of this portion of the retina (layer 6 in the scheme on
p. 474). The nervous layer is derived from the optic ganglion
which attaches itself to the inner side of the connective tissue layer.

The greater part of the choroid is formed from the mesoblast
adjoining the retina, but the epithelium covering its outer wall
is of epiblastic origin.

It is difficult to decide from development whether the Mollus-
can eyes, so far dealt with, originated in the first instance part
passu with the supra-cesophageal ganglia or independently at a
later period. On purely a priori ground I should be inclined to
adopt the former alternative.

In addition to the above eyes there occur amongst Mollusca highly
complicated eyes, of a very different kind, in two widely separated groups,
viz. certain species of a genus of slug (Onchidium), and certain Lamelli-
branchiata. These eyes, though they have no doubt been evolved indepen-
dently of each other, present certain remarkable points of agreement. In
both of them the rods of the retina are turned away from the surface, and
the nerve-fibres are placed, as in the Vertebrate eye, on the side of the retina
which faces outwards.

The peculiar eyes of Onchidium, investigated by Semper1, are scattered
on the dorsal surface, there being normal eyes in the usual situation on the
head. The eyes on the dorsal surface are formed of a cornea, a lens
composed of i — 7 cells, and a retina surrounded by pigment ; which is
perforated in the centre by an optic nerve, the retinal elements being in the
inverted position above mentioned.

The development of these eyes has been somewhat imperfectly studied
in the adult, in which they continue to be formed anew. They arise by a

1 Ueber Sehorgane von Typus d. Wirbdthieraugen, etc., Wiesbaden, 1877, anfl
Archiv f. mikr. Anat. Vol. xiv. pp. 118 — 122.

ORGANS OF VISION. 479

differentiation of the epidermis at the end of a papilla. At first a few
glandular cells appear in the epidermis in the situation where an eye is
about to be formed. Then, by a further process of growth, an irregular mass
of epidermic cells becomes developed, which pushes the glandular cells to
one side, and constitutes the rudiment of the eye. This mass, becoming
surrounded by pigment, unites with the optic nerve, and its cells then differ-
entiate themselves, in situ, into the various elements of the eye. No
explanation is offered by Semper of the inverted position of the rods, nor is
any suggested by his account of the development. As pointed out by
Semper these eyes are no doubt modifications of the sensory epithelium of
the papillce.

The eyes of Pecten and Spondylus1 are placed on short stalks at the
edge of the mantle, and are probably modifications of the tentacular
processes of the mantle edge. They are provided with a cornea, a cellular
lens, a vitreous chamber, and a retina. The retinal elements are inverted,
and the optic nerve passes in at the side, but occupies, in reference to its
ramifications, the same relative situation as the optic nerve in the Vertebrate
eye. The development has unfortunately not yet been studied.

Our knowledge of the structure or still more of the development of the
organ of vision of the Platyelminthes, Rotifera, and Echinodermata is too
scanty to be of any general interest.

Chaetopoda. Amongst the Chaetopoda the cephalic eyes of Alciope
(fig. 281) have been adequately investigated as to their anatomy by Greeff.
These are provided with a large cuticular lens (/), separated from the retina
by a wide cavity containing the vitreous humour. The retina is formed of a
single row of cells, with rods at their free extremities, continuous at their
opposite ends with nerve-fibres. The development of this eye has not been
worked out. Eyes not situated on the head are found in Polyophthalmus,
and have probably been evolved from the more indifferent type of sense-
organ found by Eisig in the allied Capitellidas.

Chaetognatha2. The paired cephalic eyes of Sagitta are spherical
bodies imbedded in the epidermis. They are formed of a central mass of
pigment with three lenses partially imbedded in it. The outer covering of
the eye is the retina, which is mainly composed of rod-bearing cells ; the
rods being placed in contact with the outer surface of each of the lenses. In
the presence of three lenses the eye of Sagitta approaches in some respects
the eye of the Arthropoda.

Arthropodan eye. A satisfactory elucidation of the phylo-
geny of Arthropodan eyes has not yet been given.

All the types of eyes found in the group (with exception of

1 Vide Hensen (No. 364) and S. J. Hickson, "The Eye of Pecten," Quart. J. of
Micr. Science, Vol. xx. 1 880.

2 O. Hertwig. " Die Chaetognathen." Jenaischc Zcitschrift, Vol. XTV. 1880.

480 ARTHROPODA.

that of Peripatus)1 present marked features of similarity, but I
am inclined to view this similarity as due rather to the character
of the exoskeleton modifying in a more or less similar way all
the forms of visual organs, than to the descent of all these eyes
from a common prototype. In none of these eyes is there
present a chamber filled with fluid between the lens and the

FIG. 281. EYE OF AN ALCIOPID (NEOPHANTA CELOX). (From Gegenbaur;

after Greef.)

i. cuticle; c. continuation of cuticle in front of eye; /. lens; h. vitreous humour;
o. optic nerve; o. expansion of the optic nerve; b. layer of rods; /. pigment layer.

retina, but the space in question is filled with cells. This
character sharply distinguishes them from such eyes as those of
Alciope (fig. 281). The types of eyes which are found in the
Arthropoda are briefly the following :

(i) Simple eyes. In all simple eyes the corneal lens is
formed by a thickening of the cuticle. Such eyes are confined
to the Tracheata.

There are three types of simple eyes, (a) A type in which
the retinal cells are placed immediately behind the lens, found

1 The eye of Peripatus is similar neither to the eye of the Arthropoda, nor to that
of the Choetopoda, but resembles much more closely the Molluscan eye. The hypo-
dermis and cuticle form together a highly convex cornea, within which is a large optic
chamber, the posterior wall of which is formed by the retina. The optic chamber
would appear to contain a structureless lens, but it is possible that what I regard as a
lens may, on fuller investigation, turn out to be only a coagulum.

ORGANS OF VISION.

481

(Lowne) in the larvae of some Diptera (Eristalis), and also in
some Chilognatha.

(b] A type of simple eye found in some Chilopoda, and in
some Insect larvae (Dytiscus, etc.) (fig. 282), the parts of which
are entirely derived from the epidermis. There is present a
lens (/) formed as a thickening of the cuticle, a so-called vitreous
humour (gl] formed of modified hypodermis cells, and a retina
(r) derived from the same source.

The outer ends of the retinal cells
terminate in rods, and their inner
ends are continuous with nerve-
fibres.

(c) A type of simple eye found
in the Arachnida, and apparently
some Chilopoda, and forming the
simple eyes of most Insects, which
differs from type (a) in the cells
of the retina forming a distinct
layer beneath the hypodermis ; the
latter only obviously giving rise to
the vitreous humour.

The development of the simple eyes has not yet been
studied.

The simple eyes so far described are always placed on the
head, and are usually rather numerous.

(2) Compound eyes. Compound eyes are almost always
present in the Crustacea, and are usually found in adult Insects.
In both groups they are paired, though in the Crustacea a median
much simplified compound eye may either take the place of the
paired eyes in the Nauplius larva and lower forms, or be present
together with them during a period in the development of higher
forms.

The typical compound eye is formed (fig. 283) of a series of
corneal lenses (c) developed from the cuticle; below which
are placed bodies known as the crystalline cones, one to each
corneal lens ; and below the crystalline cones are placed bodies
known as the retinulae (r) constituting the percipient elements
of the eye, each of them being formed of an axial rod, the
rhabdom, and a number of cells surrounding it.

B. in. 31

FIG. 282. SECTION THROUGH
THE SIMPLE EYE OF A YOUNG DYTIS-
CUS LARVA. (From Gegenbaur ; after
Grenadier.)

/. corneal lens ; g. vitreous hu
mour ; r. retina ; o. optic nerve ; h.
hypodermis.

482

ARTHROPODA.

The crystalline cones are formed from the coalescence of cuticular
deposits in several cells, the nuclei of which usually remain as Semper's
nuclei. These cells are probably simple hypodermis cells, but in some
forms, e.g. Phronima, there may be a continuous layer of hypodermis cells
between them and the cuticle. In various Insect eyes the cells which
usually give rise to a crystalline cone may remain distinct, and such eyes
have been called by Grenacher aconouseyes, while eyes with incompletely
formed crystalline cones are called by him pseudoconouseyes.

The rhabdom of the retinulae is, like the crystalline cone, developed by
the coalescence of a series of parts, which are primitively separate rods
placed each in its own cell : this condition of the retinulas is permanently
retained in the eyes of the Tipulidae.

The development of the compound eye has so far only been
satisfactorily studied in some Crustacea by Bobretzky (No. 367) ;
by whom it has been worked out in Palaemon and Astacus, but
more fully in the latter, to which the following account refers :

The eye of Astacus takes its
origin from two distinct parts, (i)
the external epidermis of the pro-
cephalic lobes which will be spoken
of as the epidermic layer of the
eye, (2) a portion of the supra-
cesophageal ganglia, which will be
spoken of as the neural layer of
the eye. The mesoblast is more-
over the source of some of the
pigment between the two above
layers. The epidermic layer gives
rise to the corneal lenses, the
crystalline cones, and the pigment
around the latter. The neural
layer on the other hand seems to
give rise to the retinulae with their rhabdoms, and to the optic
ganglion.

After the separation of the supra-cesophageal ganglia from the superficial
epiblast, the cells of the epidermis in the region of the future eye become
columnar, and so form the above-mentioned epidermic layer of the eye.
This layer soon becomes two or three cells deep. At the same time the
most superficial part of the adjoining supra- oesophageal ganglion becomes
partially constricted off from the remainder as the neural layer of the eye,
but is separated by a small space from the thickened patch of epidermis.

FIG. 283. DIAGRAMMATIC RE-
PRESENTATIONS OF PARTS OF A COM-
POUND ARTHROPOD EYE. (From
Gegenbaur.)

A. Section through the eye.

B. Corneal facets.

C. Two segments of the eye.

c. corneal (cuticular) lenses ; r.
retinulae with rhabdoms ; n. optic
nerve ; g. ganglionic swelling of optic
nerve.

ORGANS OF VISION. 483

Into this space some mesoblast cells penetrate at a slightly later period.
Both the epidermic and neural layers next become divided into two strata.
The outer stratum of the epidermic layer gives rise to the crystalline cones
and Semper's nuclei ; each crystalline cone being formed from four coalesced
rods, developed as cuticular differentiations of four cells, the nuclei of which
may be seen in the embryo on its outer side. The lower ends of the cones
pass through the inner stratum of the epidermic disc, the cells of which
become pigmented, and constitute the pigment cells surrounding the lower
part of the crystalline cones in the adult. The outer end of each of the
crystalline cones is surrounded by four cells, believed by Bobretzky to be
identical with Semper's nuclei1. These cells give rise in a later stage (not
worked out in Astacus) to the cuticular corneal lenses.

Of the two strata of the neural layer the outer is several cells deep, while
the inner is formed of elongated rod-like cells. Unfortunately however the
fate of the two neural layers has not been worked out, though there can be
but little doubt that the retinuke originate from the outer layer.

The mesoblast which grows in between the neural and epidermic layers
becomes a pigment layer, and probably also forms the perforated membrane
between the crystalline cones and the retinulas.

The above observations of Bobretzky would appear to
indicate that the paired compound eyes of Crustacea belong to
the type of cerebral eyes. How far this is also the case with the
compound eyes of Insects is uncertain, in that it is quite possible
that the latter eyes may have had an independent origin.

The relation between the paired and median eye of the
Crustacea is also uncertain.

In the genus Euphausia amongst the Schizopods there is present a series
of eyes placed on the sides of some of the thoracic legs and on the sides of
the abdomen. The structure of these eyes, though not as yet satisfactorily
made out, would appear to be very different from that of other Arthropodan
visual organs.

The Eye of the Vertebrata. In view of the various
structures which unite to form it, the eye is undoubtedly the
most complicated organ of the Vertebrata ; and though its
mode of development is fairly constant throughout the group,
it will be convenient shortly to describe what may be regarded
as its typical development, and then to proceed to a comparative
view of the origin of its various parts, and to enter into greater
detail with reference to some of them. At the end of the section

1 There would appear to be some confusion as to the nomenclature of these parts
in Bobretzky's account,

31

484

PRIMARY OPTIC VESICLE.

there is an account of the accessory structures connected with
the eye.

The formation of the eye commences with the appearance of
a pair of hollow outgrowths from the anterior cerebral vesicle or
thalamencephalon, which arise in many instances, even before
the closure of the medullary canal. These outgrowths, known
as the optic vesicles, at first open freely into the cavity of the
anterior cerebral vesicle. From this they soon however become
partially constricted, and form vesicles (fig. 284, a], united to the
base of the brain by compara-
tively narrow hollow stalks, the
rudiments of the optic nerves.
The constriction to which the
stalk or optic 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. 284, ff).

After the establishment of the
optic nerves, there take place (i)
the formation of the lens, and (2)
the formation of the optic cup
from the walls of the primary optic vesicle.

The external or superficial epiblast which covers, and is in
most forms in 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. 285 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. 285 B).

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

FIG. 284. SECTION THROUGH
THE HEAD OF AN EMBRYO TELEOS-
TEAN, 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.

ORGANS OF VISION OF THE VERTEBRATA.

485

always remains widely open in front. Of its double walls
the inner or anterior (fig. 285 .

B, r) is formed from the front
portion, the outer or posterior
(fig. 285 B, u] from the hind por-
tion of the wall of the primary
optic vesicle. The inner or ante-
rior (r), which very speedily be-
comes thicker than the other, is
converted into the retina : in
the outer or posterior («), which
remains thin, pigment is even-
tually deposited, and it ultimately
becomes the tesselated pigment-
layer of the choroid.

By the closure of its mouth
the pit of the involuted epiblast
becomes a completely closed sac
with thick walls and a small
central cavity (fig. 285 B, /). At
the same time it breaks away
from the external epiblast, 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. 285 B). In
a short time however, the lens is seen to lie in the mouth of the
cup (fig. 288 D), 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

FIG. 285. DIAGRAMMATIC SEC-
TIONS ILLUSTRATING THE FORMATION
OF THE EYE. (After Remak.)

In A the thin superficial epiblast h
is seen to be thickened at x, 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. Accompany-
ing 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 B 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 xh.

486

CHOROID FISSURE.

slants away from the vesicle. Hence, when the involution of
the lens takes place, the direction 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. 285), but forms an obtuse angle with that axis, after the
manner of fig. 286, where / represents the cavity of the stalk
leading away from the almost obliterated cavity of the primary
vesicle.

Fig. 286 represents the early stage at which the lens fills the
whole cup of the secondary vesicle. The subsequent condition
is brought about through the rapid
growth of the walls of the cup. This
growth however does not take place
equally in all parts of the cup. The
walls of the cup rise up all round except
that point 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 choroidal
fissure, a way is open from the meso-
blastic 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 recognised in the embryo
seen as a transparent object (fig. 1 18, chs).

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 is viewed from underneath as a transparent

FIG. 286. DIAGRAMMATIC
SECTION OF THE EYE AND
THE OPTIC NERVE AT AN
EARLY STAGE. (From Lie-
berkiihn.)

To shew the lens / occu-
pying 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.

ORGANS OF VISION OF THE VERTEBRATA.

487

object the eye presents very much the appearance represented in
the diagram (fig. 287).

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. 288 D. The lens,
the cavity and double walls of the
secondary vesicle, the remains of the
primary cavity, would all be repre-
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 obtained as
that shewn in fig. 288 E. Here the
fissure f is obvious, and the com-
munication 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. Lastly, a sec-
tion, taken perpendicular to the
plane of the paper along the line z,
i.e. through the fissure itself, would
present the appearances of fig. 288 F,
where the wall of the vesicle is
entirely wanting in the region of
the fissure marked by the position
of the letter f. The external epi-
blast has been omitted in this figure.

With reference to the above description, taken with very slight alterations
from the Elements of Embryology, Pt. I., 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 an inequality in the growth of the walls of the secondary optic
cup, but is partly due to a doubling up of the primary vesicle from the side

FIG. 287. DIAGRAMMATIC RE-
PRESENTATION OF THE EYE OF
THE CHICK OF ABOUT THE THIRD
DAY AS SEEN WHEN THE HEAD IS
VIEWED FROM UNDERNEATH AS A
TRANSPARENT OBJECT.

/. the lens ; /'. 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 in
some places thicker than r, it
should have been thinner through-
out, 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 in-
dicates the continuity of the cavity
of the stalk with that of the primary
vesicle.

The line z z, through which the
section shewn in fig. 288 F is sup-
posed to be taken, passes through
the choroidal fissure.

488 SECONDARY OPTIC CUP.

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. And in other forms a partial phenomenon of the
same kind is usually observable, as is more particularly described in the
sequel.

Before describing the development of the cornea, aqueous
humour, etc. we may consider the further .growth of the parts,
whose first development has just been described, commencing
with the optic cup.

During the above changes the mesoblast surrounding the
optic cup assumes the character of a distinct investment, whereby
the outline of the eye-ball 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.

The cavity of the original optic vesicle was left as a nearly
obliterated space 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 con-
trast 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. 289).

In the front portion however, along, so to speak, the lip of
the cup, anterior to a line which afterwards becomes the ora
serrata, both layers cease to take part in the increased thickening,
accompanied by peculiar histological changes, which the rest of
the cup is undergoing. Thus a hind portion or true retina is
marked off from a front portion.

The front portion, accompanied by the mesoblast which
immediately overlies it, is behind the lens thrown into folds, the

ORGANS OF VISION OF THE VERTEBRATA. 489

ciliary ridges ; while further forward 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 of the cup, is
now inclosed in its cavity. While in the hind portion of the
cup or retina proper no deposit of black pigment takes place in

D

E

FIG. 288.

D. Diagrammatic section taken perpendicular to the plane of the paper, along
the linejjy, fig. 287. 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. 285 B.
(After Remak.)

E. Section taken parallel to the plane of the paper through fig. 287, so far behind
the front surface of the eye as to shave off a small portion of the posterior surface of
the lens /, but not so far behind as to be carried at all through the stalk. Letters as
before ; f. the choroidal fissure.

F. Section along the line zz, 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 one side of the line zs,
the wall of the optic cup would have extended up to the lens below as well as above.
Letters as before. The external epiblast is omitted in this section.

the layer formed out of the inner or anterior wall of the vesicle ;
in the front portion forming the region of the iris, pigment is
largely deposited throughout both layers, though first of all in
the outer one, so that eventually this portion seems to become
nothing more than a forward prolongation of the pigment epi-
thelium of the choroid.

Thus, while the hind moiety of the optic cup becomes 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

490

THE RETINA.

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 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 flattened

c.t

p.Ch

FIG. 289. SECTION OF THE EYE OF CHICK AT THE FOURTH DAY.

e.p. superficial epiblast of the side of the head ; /?. true retina : anterior wall of the
optic cup; p.Ch. pigment-epithelium of the choroid: posterior wall of the optic cup.
b is placed at the extreme lip of the optic cup at what will become the margin of the
iris. /. the lens. The hind wall, the nuclei of whose elongated cells are shewn at «/,
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.

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

ORGANS OF VISION OF THE VERTEBRATA. 491

cells (fig. 289, p.C/i). At about the 8oth 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.

On the fourth day, the inner (anterior) wall of the optic cup
(fig. 289, R) has a perfectly uniform 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 becomes 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.
According to the usual view, which is not however fully sup-
ported by the development, the retina becomes divided in the
subsequent growth into (i) an outer part, corresponding morpho-
logically to the epithelial lining of the cerebro-spinal canal,
composed of what may be called the visual cells 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 walls of the brain. According to Lowe,
however, 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 actual development of the retina is not thoroughly understood.
According to the usual statements (Kolliker, No. 298, p. 693) 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 and cones are formed as prolongations (Kolliker,
Babuchin), or cuticularizations (Schultze, W. Miiller) 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.

492 THE OPTIC NERVE.

Lowe's account of the development of the retina in the Rabbit is in many
points different from the above. He finds that three stages in the differen-
tiation of the layers of the retina may be distinguished.

In the first stage, in an embryo of four or five millimetres, the following
layers are present, commencing at the outer side, adjoining the external wall
of the secondary optic cup.

(1) A membrane, which does not however, as usually believed,
become the membrana limitans externa.

(2) A layer of clear elements, derived from metamorphosed cells,
constituting the outer limbs of the rods and cones.

(3) A layer of dark rounded elements.

(4) An indistinctly striated layer, the future layer of nerve-fibres.
The third of these layers gives rise to all the eventual strata of the

retina proper, except the outer limbs of the rods and cones.

In the next stage, when the embryo has reached a length of 2 cm., this
layer becomes divided into three strata : viz. an outer and inner layer of
dark elements and a middle one of clearer elements. The two inner of these
layers become respectively the inner molecular layer and the layer of gan-
glion cells, while the outer layer gives rise to the parts of the retina external
to the inner molecular layer.

In the newly born animal the outer darker layer of the previous stage
has become considerably subdivided. Its outermost part forms a stratum
of darkly coloured elements, which develop into the inner limbs of the rods
and cones. It is bounded internally by a membrane — the true membrana
elastica externa. The part of the layer within this is soon divided into the
outer and inner granular layers, separated from each other by the delicate
outer molecular layer. Thus, shortly after birth, all the layers of the retina
are established in the Rabbit. It is important to notice that, according to
Lowe's views, the outer and inner limbs of the rods and cones are metamor-
phosed cells. The outer limbs at first form a continuous layer, in which
separate elements cannot be recognised.

At a very early period there appears a membrane on the side of the
retina adjoining the vitreous humour. This membrane is the hyaloid mem-
brane. The investigations of Kessler and myself lead to the conclusion that
it may be formed at a time when there is no trace of mesoblastic structures
in the cavity of the vitreous humour, and that it is therefore necessarily
developed as a cuticular deposit of the cells of the optic cup. Lieberkiihn,
Arnold, Lowe, and other authors regard it however as a mesoblastic
product ; and Kolliker believes that a primitive membrane is developed
from the cells of the optic cup, and that a true hyaloid membrane is
developed much later as a product of the mesoblast.

For fuller information on this subject the reader is referred to the
authors quoted above.

The optic nerve. The optic nerves are derived, as we have
said, from the at first hollow stalks of the optic vesicles. Their

ORGANS OF VISION OF THE VERTEBRATA. 493

cavities gradually become obliterated 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
from fibres 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 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 continuous with the cavity of the
stalk. When the cavity within the optic nerve vanishes, and
the fibres of the optic nerve appear, all connection is ruptured
between the outer wall of the optic cup and the optic nerve, and
the optic nerve simply perforates the outer wall, and becomes
continuous with the inner one.

There does not appear to me any ground for doubting (as has
been done by His and Kolliker) that the fibres of the optic nerve
are derived from a differentiation of the epithelial cells of which
the nerve is at first formed.

Choroid Fissure. With reference to the choroid fissure we
may state that its behaviour varies somewhat in the different
types. It becomes for the greater part of its extent closed,
though its proximal end is always perforated by the optic nerve,
and in many forms by a mesoblastic process also.

The lens when first formed is an oval vesicle with a small
central cavity, the front and hind walls being of nearly equal
thickness, and each consisting of a single layer of elongated
columnar cells. In the subsequent stages the mode of growth
of the hind wall is of precisely an opposite character 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
contrary becomes thinner and thinner and its cells flattened.

These modes of growth continue until, as shewn in fig. 289,
the hind wall / is in absolute contact with the front wall el, and
the cavity thus becomes entirely obliterated. The cells of the
hind wall have by this time become veritable fibres, which, when

494 THE VITREOUS HUMOUR.

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, somewhat thickened at either side where it becomes
continuous 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 the
latter it becomes the epithelium.

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

During their multiplication they become arranged in the
manner characteristic of the adult lens of the various forms. The
lens-capsule, as was originally stated by Kolliker, appears to be
formed as a cuticular membrane deposited by the epithelial cells
of the lens.

The views of Lieberkiihn, Arnold, Lowe and others, according to
which the lens-capsule is a mesoblastic structure, do not appear to be well
founded. The contrary view, held by Kolliker, Kessler, etc., is supported
mainly by the fact that at the time when the lens-capsule first appears
there are no mesoblast cells to give rise to it. It should however be stated
that W. Miiller has actually found cellular elements in what he believes to
be the lens-capsule of the Ammoccete lens. Considering the degraded
character of the Ammoccete eye, evidence derived from its structure must
be accepted with caution.

The vitreous humour. The vitreous humour is derived
(except in Cyclostomata) from a vascular ingrowth, which differs
considerably in different types, through the choroid slit. Its
real nature is very much disputed. According to Kessler's view,
it is of the nature of a fluid transudation, but the occasional
presence in it of ordinary embryonic mesoblast cells, in addition
to more numerous blood-corpuscles, gives it a claim to be regarded
as intercellular substance. The number of cells in it is however
at best extremely small and in many cases there is no trace of
them. In Mammals there appear to be some mesoblast cells in-
vaginated with the lens, which are not improbably employed in
the formation of the vessels of the so-called membrana capsulo-
pupillaris. In the Ammoccete the vitreous humour originates
from a distinct mesoblastic ingrowth, though the cells which give
rise to it subsequently disappear.

ORGANS OF VISION OF THE VERTEBRATA. 495

The development of the zonula of Zinn in Mammalia, which ought to
throw some light on the nature of the vitreous humour, has not been fully
investigated. According to Lieberkiihn (No. 373, p. 43), this structure
appears in half-grown embryos of the sheep and calf.

He says "At the point where the ciliary processes and the ciliary
part of the retina are entirely removed, one sees in the meridian bundles
of fine fibres, which correspond to the valleys between the ciliary pro-
cesses and fill them ; also between these bundles there extend, as a thin
layer, similar finely striated masses, and these would have been on the
top of the ciliary processes." He further states that these fibres may be
traced to the anterior and posterior limb of the lens-capsule, and that
amongst them are numerous cells. Kolliker confirms Lieberkiihn's state-
ments. There can be little doubt that the fibres of the zonula are of the
nature of connective tissue : they are stated to be elastic. By Lowe they
are believed to be developed out of the substance of the vitreous humour,
but this does not appear to me to follow from the observations hitherto
made. It seems quite possible that they arise from mesoblast cells which
have grown into the cavity of the vitreous humour, solely in connection
with their production.

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 described, and there remain to be
dealt with the cornea, and the cavity containing the aqueous
humour.

The cornea. The cornea is formed by the coalescence 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 epidermis
its outer wall is directly in contact with the external epiblast
(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. In the chick, which we may take as typical,
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
epiblast. 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 off to

496 THE CORNEA.

nothing towards the centre. It soon however becomes broader,
and finally forms a continuous stratum of considerable 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 surround-
ing the optic cup (fig. 290, dm). This layer is the epithelioid
layer of the membrane of Descemet. After it1 has become

FIG. 290. SECTION THROUGH THE EYE OF A FOWL ON THE EIGHTH DAY
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 struc-
tureless 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.

completely established, the mesoblast around the edge of the
cornea becomes divided into two strata ; an inner one (fig. 290,
cb) destined to form the mesoblastic tissue of the iris already
described, and an outer one (fig. 290, 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 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 them alone remains as the membrana elastica anterior
and posterior (Descemet's membrane) of the cornea. The corneal

1 It appears to me 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 correction. From the structure of the eye in the Ammocoete it
seems probable that Descemet's membrane is continuous with the choroid.

ORGANS OF VISION OF THE VERTEBRATA. 497

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
cellular elements have made their way into the cornea, the latter
becomes continuous at its edge with the mesoblast 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 to me more
probable that Kolliker is right in regarding it as derived from the meso-
blast. The grounds for this view are, (i) the fact of its growth inwards
from the border of the mesoblast 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 Lieberkiihn.
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 (No. 374) and Arnold
(No. 370), and are not entirely accepted by Kolliker. It is especially on
the development of these parts in Mammalia (to be spoken of in the sequel)
that the above authors found their objections. I have had through Kessler's
kindness an opportunity of looking through some of his beautiful prepara-
tions, and have no hesitation in generally accepting his conclusions, though
as mentioned above I cannot agree with all his interpretations.

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

Comparative view of the development of the Vertebrate Eye.

The organ of vision, when not secondarily aborted, contains in all
Vertebrata the essential parts above described. The most interesting cases
of partial degeneration are those of Myxine and the Ammoccete. The
development of such aborted eyes has as yet been studied only in the

B. III. 32

498

THE AMMOCCETE EYE.

Ammocoete1, in which it resembles in most important features that of other
Vertebrata.

Eye of Ammoccetes. The optic vesicle arises as an outgrowth of
the fore-brain, but the secondary optic cup is remarkable in the young larva
for its small size (fig. 291, opv). The thicker outer wall gives rise to the
retina, and the thinner inner wall to the choroid pigment. The lens is formed
as an invagination of the single-layered epidermis (fig. 291, /). As develop-
ment proceeds the parts of the eye gradually enlarge, and the mesoblast
around the hinder and dorsal part of the optic cup becomes pigmented.
There is at first no cavity for the vitreous humour, but eventually the
growth of the optic cup gives rise to a space, into which a cellular process
of mesoblast grows at a slight notch in the ventral edge of the optic cup
(W. Muller, No. 377). This notch is the only rudiment of the choroid
fissure of other types. The mesoblastic process
is probably the homologue of the processus
falciformis and pecten, and appears to give rise
to the vitreous humour ; for a long time it
retains its connection with the surrounding
mesoblast. Its cells eventually disappear, and
it never contains any vascular structures.

The lens for a long time remains as an oval
vesicle with a central cavity. In a later stage,
when the Ammoccete is fully developed, the
secondary optic cup forms a deep pit (fig. 292, r) ;
in the mouth of which is placed the lens (/).
The two walls of the retina have now the normal
vertebrate structure, though the pigment is as
yet imperfectly present in the choroid layer.
The lens has the embryonic forms of higher
types (cf. fig. 289), consisting of an inner thicker
segment, the true lens, and an outer layer form-
ing the epithelium of the lens capsule. The
edge of the optic cup, which forms the rudiment
of the epiblast of the iris, is imperfectly separated
from the remainder of the optic cup ; and a
mesoblastic element of the iris, distinct from
Descemet's membrane (dm\ can hardly be spoken of.

There is no cavity for the aqueous humour in front of the lens ; and
there is no cornea as distinct from the epidermis and subepidermic tissues.
The elements in front of the lens are (i) the epidermis (ep} ; (2) the dermis
(dc) ; (3) the subdermal connective tissue (sdc) which passes without any
sharp line of demarcation into the dermis ; (4) a thick membrane, con-
tinuous with the mesoblastic part of the choroid, which appears to represent
Descemet's membrane. The subdermal connective tissue is continued as an

FIG. 291. HORIZONTAL

SECTION THROUGH THE
HEAD OF A JUST HATCHED
LARVA OF PETROMYZON
SHEWING THE DEVELOP-
MENT OF THE LENS OF THE

EYE.

th.c. thalamencephalon ;
op.v. optic vesicle ; /. lens of
eye ; h.c. head cavity.

The most detailed account is that of W. Muller (No. 377).

ORGANS OF VISION OF THE VERTEBRATA.

499

investment round the whole eye ; and there is no differentiated sclerotic and
only an imperfect choroid.

In a still later stage a distinct mesoblastic element for the iris is formed.
When the Ammoccete is becoming a Lamprey, the eye approaches the
surface ; an anterior chamber is established ; and the eye differs from that
of the higher types mainly in the fact that the cornea is hardly distinguished
from the remainder of the skin, and that a sclerotic is very imperfectly
represented.

Optic vesicles. The development of the primitive optic vesicles, so
far as is known, is very constant throughout the Vertebrata. In Teleostei
and Lepidosteus alone is there an important deviation from the ordinary
type, dependent however upon the mode of formation of the medullary keel,
the optic vesicles arising while the medullary keel is still solid, and being at
first also solid. They subsequently acquire a lumen and undergo the
ordinary changes.

The lens. In the majority of groups, viz. Elasmobranchii, Reptilia,
Aves, and Mammalia, the lens is formed by an open invagination of the
epiblast, but in Amphibia, Teleostei and Lepidosteus, where the nervous

S.d.c

FIG. 292. EYE OF AN AMMOCCETES LYING BENEATH THE SKIN.

ep. epidermis; d.c. dermal connective tissue continuous with the sub-dermal
connective tissue (s.d.c}, which is also shaded. There is no definite boundary to this
tissue where it surrounds the eye.

m. muscles; dm. membrane of Descemet ; /.lens; v.h. vitreous humour ; r. retina;
rp. retinal pigment.

layer of the skin is early established, this layer alone takes part in the
formation of the lens (fig. 293, /). The lens is however formed even in
these types as a hollow body by an invagination ; but its opening remains
permanently shut off from communication with the exterior by the epidermic

32—2

500 THE CORNEA.

layer of the epiblast. Gotte describes the lens as formed by a solid
thickening of the nervous layer in Bombinator. This is probably a mistake.

The cornea. The mode of formation of the cornea already described
appears to be characteristic of most Vertebrata except the Ammocoete. It
has been found by Kessler in Aves, Reptilia and Amphibia, and probably
also occurs in Pisces. In Mammals it is not however so easy to establish.
There are at first no mesoblast cells between the lens and the epiblast (fig.
295) but in many Mammals (vide Kessler, No. 372, pp. 91 — 94) a layer of
rounded mesoblast cells, which forms Descemet's membrane, grows in
between the two, at a time when it is not easy to recognise a corneal
lamina, as distinct from a simple coagulum.

After the formation of this layer the mesoblast cells grow into the
corneal lamina from the sides, and becoming flattened arrange themselves
in rows between the laminae of the cornea. The cornea continues to
increase in thickness by the addition of laminae on the side adjoining the
epiblast.

We have already seen that in the Lamprey the cornea is nothing else
but the slightly modified and more transparent epidermis and dermis.

The optic nerve and the choroid fissure. It will be con-
venient to consider together the above structures, and with them the
vascular and other processes which pass into the cavity of the optic cup
through the choroid fissure. These parts present on the whole a greater
amount of variation than any other parts of the eye.

I commence with the Fowl which is both a very convenient general type
for comparison, and also that in which these structures have been most
fully worked out.

During the third day of incubation there passes in through the choroid
slit a vascular 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 pec ten. 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 one
is placed near the ciliary edge of the slit, and is traversed by the efferent
branch of the above-mentioned vascular loop. This vessel soon atrophies,
and with it the distal opening in the choroid slit completely vanishes. In
some varieties of domestic Fowl (Lieberkiihn) the opening however persists.
The seam which marks the original site of the choroid slit is at first

ORGANS OF VISION OF THE VERTEBRATA. 501

conspicuous 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 puckerings

FIG. 293. SECTION THROUGH THE FRONT PART OF THE HEAD OF A LEPIDOS-

TEUS EMBRYO ON THE SEVENTH DAY AFTER IMPREGNATION.
al. alimentary tract ; fb. thalamencephalon ; /. lens of eye ; op.v. optic vesicle.
The mesoblast is not represented.

have become nearly as numerous as in the adult, there being in all seventeen
or eighteen. The pecten is 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
becomes 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.

In the formation of the optic cup the extreme peripheral part of the optic
nerve, which is in immediate proximity with the artery of the pecten,
becomes folded. 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 radiating from it as
from a centre to all parts of the retina.

In the Lizard the choroid slit closes considerably earlier than in the
Fowl. The vascular loop in the vitreous humour is however more developed.
The pecten long remains without vessels, and does not in fact become at all

502 THE CHOROID FISSURE.

vascular till after the very late disappearance of the distal part of the
vascular loop of the vitreous humour.

The arrangement of the ingrowth through the choroid slit in Elasmo-
branchii (Scyllium) has been partially worked out, and so far as is at present
known the agreement between the Avian and Elasmobranch type is fairly
close.

At the time when the cavity between the lens and the secondary optic
cup is just commencing to be formed, a process of mesoblast accompanied
by a vascular loop passes into the vitreous humour, through the choroid slit,
close to the optic nerve. The vessel in this process is no doubt equivalent
to the vascular loop in the Avian eye, but I have not made out that it pro-
jects beyond the mesoblastic process accompanying it. As the cavity of the
vitreous humour enlarges and the choroid slit elongates, the process through
it takes the form of a lamina with a somewhat swollen border, and projects
for some distance into the cavity of the vitreous humour.

At a later stage, after the outer layer of the optic cup has become pig-
mented, the distal part of the choroid slit adjoining the border of the lens
closes up ; but along the line where it was present the walls of the optic cup
remain very thin and are thrown into three folds, two lateral and one
median, projecting into the cavity of the vitreous humour. The median
fold is in contact with the lens, and the vascular mesoblast surrounding the
eye projects into the space between the two laminae of which it is formed.
In passing from the region of the lens to that of the optic nerve the lateral
folds of the optic cup disappear, and the median fold forms a considerable
projection into the cavity of the vitreous humour. It consists of a core of
mesoblast covered by a delicate layer derived from both strata of the optic
cup. Still nearer the optic nerve the choroid slit is no longer closed, and
the mesoblast, which in the neighbourhood of the lens only extended into the
folds of the wall of the optic cup, now projects freely into the cavity of the
vitreous humour, and forms the lamina already described. It is not very
vascular, but close to the optic nerve there passes into it a considerable
artery.

In the young animal the choroid slit is no longer perforated by a meso-
blastic lamina. At its inner end it remains open to allow of the passage of
the optic nerve. The line of the slit can easily be traced along the lower
side of the retina ; and close to the lens the retinal wall continues, as in the
embryo, to be raised into a projecting fold. Traces of these structures are
visible even in the fully grown examples of Scyllium.

As has been pointed out by Bergmeister the mesoblastic lamina pro-
jecting into the vitreous humour resembles the pecten at an early stage of
development, and is without doubt homologous with it. The artery which
supplies it is certainly equivalent to the artery of the pecten.

There can be no doubt that the mesoblastic lamina projecting into the
vitreous humour is equivalent to the processus falciformis of Teleostei, and
it seems probable that the whole of it, including the free part as well as that
covered by epiblast, ought to be spoken of under this title. The optic nerve

ORGANS OF VISION OF THE VERTEJ5RATA.

503

in Elasmobranchii is not included in the folding to which the secondary
optic vesicle owes its origin, and would seem to perforate the walls of the
optic cup only at the distal end of the processus falciformis.

In Teleostei there is at first a vascular loop like that in Birds, passing
through the choroid fissure. This has been noticed by Kessler in the Pike,
and by Schenk in the Trout. At a later period a mesoblastic ingrowth with
a blood-vessel makes its way in many forms into the cavity of the vitreous
humour, accompanied by two folds in the walls of the free edges of the
choroid fissure (fig. 294). These structures, which constitute the processus
falciformis, clearly resemble very closely the
mesoblastic process and folds of the optic cup
in Elasmobranchii. The processus falciformis
comes in contact with, and perhaps becomes
attached to the wall of the lens ; and persists
through life.

In Triton there is no vascular ingrowth
through the choroid fissure, but a few meso-
blastic cells pass in which represent the vascular
ingrowth of other types. The optic nerve per-
forates the proximal extremity of the original
choroid slit.

The absence of an embryonic blood-vessel
does not however hold good for all Amphibia,
as there is present in the embryo Alytes (Lieber-
kiihn) an artery, which breaks up into a capillary
system on the retinal border of the vitreous
humour.

In the Ammoccete the choroid slit is merely represented by a slight
notch on the ventral edge of the optic cup, and the mesoblastic process
which passes through the choroid slit in most types is represented by a
large cellular process, from which the vitreous humour would appear to be
derived.

Mammalia differ from all the types already described in the immense
fcetal development of the blood-vessels of the vitreous humour. There are
however some points in connection with the development of these vessels
which are still uncertain. The most important of these points concerns
the presence of a prolongation of the mesoblast around the eye into the
cavity of the vitreous humour. It is maintained by Lieberkiihn, Arnold,
Kolliker, etc., that in the invagination of the lens a thin layer of mesoblast
is carried before it ; and is thus transported into the cavity of the vitreous
humour. This is denied by Kessler, but the layer is so clearly figured by
the above embryologists, that the existence of it in some Mammalia (the
Rabbit, etc.) must I think be accepted.

In the folding in of the optic vesicle, which accompanies the formation
of the lens, the optic nerve becomes included, and on the development of
the cavity of the vitreous humour an artery, running in the fold of the optic

FIG. 294. HORIZONTAL
SECTION THROUGH THE EYE
OF A TELEOSTEAN EMBRYO.
(From Gegenbaur ; after
Schenk.)

s. choroid fissure, with
two folds forming part of the
processus falciformis ; a. cho-
roid layer of optic cup ; b.
retinal layer of optic cup ; c.
cavity of vitreous humour ; d.
lens.

504

THE CIIOROID FISSURE.

nerve, passes through the choroid slit into the cavity of the vitreous humour
(fig. 295, acr). The sides of the optic nerve subsequently bend over, and
completely envelope this artery, which at a later period gives off branches to
the retina, and becomes known as the arteria centralis retinas. It is
homologous with the arterial limb of the vascular loop projecting into the
vitreous humour in Birds, Lizards, Teleostei, etc.

Before becoming enveloped in the optic nerve this artery is continued
through the vitreous humour (fig. 295), and when it comes in close proximity

a. c.

,m, e o

FIG. 295. SECTION THROUGH THE EYE OF A RABBIT EMBRYO OF

ABOUT TWELVE DAYS.

c. epithelium of cornea ; /. lens ; mec. mesoblast growing in from the side to form
the cornea: rt. retina ; a.c.r. arteria centralis retinae; of.n. optic nerve.

The figure shews (i) the absence at this stage of mesoblast between the lens and
the epiblast : the interval between the two has however been made too great ; (2) the
arteria centralis retinae forming the vascular capsule of the lens and continuous with
vascular structures round the edges of the optic cup.

to the lens it divides into a number of radiating branches, which pass round
the edge of the lens, and form a vascular sheath which is prolonged so as to
cover the anterior wall of the lens. In front of the lens they anastomose
with vessels, coming from the iris, many of which are venous (fig. 295)— and
the whole of the blood from the arteria centralis is carried away by these
veins. The vascular sheath surrounding the lens receives the name of the
membrana capsulo-pupillaris. The posterior part of it appears (Kessler,
No. 372) to be formed of vessels without the addition of any other structures
and is either formed simply by branches of the arteria centralis, or out of

ORGANS OF VISION OF THE VERTEBRATA. 505

the mesoblast cells involuted with the lens. The anterior part of the
vascular sheath is however inclosed in a very delicate membrane, the
membrana pupillaris, continuous at the sides with the epithelium of
Descemet's membrane. On the formation of the iris this membrane lies
superficially to it, and forms a kind of continuation of the mesoblast of the
iris over the front of the lens.

The origin of this membrane is much disputed. By Kessler, whose
statements have been in the main followed, it is believed to appear
comparatively late as an ingrowth of the stroma of the iris ; while Kolliker
believes it to be derived from a mesoblastic ingrowth between the front wall
of the lens and the epiblast. According to Kolliker this ingrowth subse-
quently becomes split into two laminae, one of which forms the cornea, and
the other the anterior part of the vascular sheath of the lens with its mem-
brana pupillaris. Between the two appears the aqueous humour.

The membrana capsulo-pupillaris is simply a provisional embryonic
structure, subserving the nutrition of the lens. The time of its disappear-
ance varies somewhat for the different Mammalia in which this point has
been investigated. In the human embryo it lasts from the second to the
seventh month and sometimes longer. As a rule it is completely absorbed
at the time of birth. The absorption of the anterior part commences in the
centre and proceeds outwards.

In addition to the vessels of the vascular capsule round the lens, there
arise from the arteria centralis retinas, just after its exit from the optic nerve,
in many forms (Dog, Cat, Calf, Sheep, Rabbit, Man) provisional vascular
branches which extend themselves in the posterior part of the vitreous
humour. Near the ciliary end of the vitreous humour they anastomose with
the vessels of the membrana capsulo-pupillaris.

In Mammals the choroid slit closes very early, and is not perforated
by any structure homologous with the pecten. The only part of the slit
which remains open is that perforated by the optic nerve ; and in the centre
of the latter is situated the arteria centralis retinas as explained above.
From this artery there grow out the vessels to supply the retina, which
have however nothing to do with the provisional vessels of the vitreous
humour just described (Kessler). On the atrophy of the provisional
vessels the whole of the blood of the arteria centralis passes into the
retina.

It is interesting to notice (Kessler, No. 372, p. 78) that there seems to be
a blood-vessel supplying the vitreous humour in the embryos of nearly all
vertebrate types, which is homologous throughout the Vertebrata. This
vessel often exhibits a persisting and a provisional part. The latter in
Mammalia is the membrana capsulo-pupillaris and other vessels of the
vitreous humour ; in Birds and Lizards it is the part of the original vascular
loop, not included in the pecten, and in Osseous Fishes that part (?)
not involved in the processus falciformis. The permanent part is formed by
the retinal vessels of Mammalia, by the vessels of the pecten in Birds and
Lizards, and by those of the processus falciformis in Fishes.

506 THE IRIS.

The Iris and Ciliary processes. The walls of the edge of the
optic cup become very much thinner than those of the true retinal part. In
many Vertebrates (Mammalia, Aves, Reptilia, Elasmobranchii, etc.) the
thinner part, together with the mesoblast covering it, becomes divided into
two regions, viz. that of the iris, and that of the ciliary processes. In the
Newt and Lamprey this differentiation does not take place, but the part in
question simply becomes the iris.

Accessory Organs connected wit/i t/te Eye.

Eyelids. The most important accessory structures connected with
the eye are the eyelids. They are developed as simple folds of the integu-
ment with a mesoblastic prolongation between their two laminas. They
may be 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.

In Teleostei and Ganoidei eyelids are either not present or at most
very rudimentary. In Elasmobranchii they are better developed, and the
nictitating membrane is frequently present. The latter is also usually found
in Amphibia. In the Sauropsida all three eyelids are usually present, but in
Mammalia the nictitating membrane is rudimentary.

In many Mammalia the two eyelids meet together during a period of
embryonic life, and unite in front of the eye. A similar arrangement
is permanent through life in Ophidia and some Lacertilia ; and there is a
chamber formed between the coalesced eyelids and the surface of the cornea,
into which the lacrymal ducts open.

Lacrymal glands. Lacrymal glands are found in the Sauropsida
and Mammalia. They arise (Remak, Kdlliker) as solid ingrowths of the
conjunctival epithelium. They appear in the chick on the eighth day.

Lacrymal duct. The lacrymal duct first appears in Amphibia, and
is present in all the higher Vertebrates. Its mode of development in the
Amphibia, Lacertilia and Aves has recently been very thoroughly worked
out by Born (Nos. 380 and 381).

In Amphibia he finds that the lacrymal duct arises as a solid ridge of
the mucous layer of the epidermis, continued from the external opening
of the nasal cavity backwards towards the eye. It usually appears at
about the time when the nasal capsule is beginning to be chondrified. As
this ridge is gradually prolonged backwards towards the eye its anterior
end becomes separated from the epidermis, and grows inwards in the
mesoblast to become continuous with the posterior part of the nasal sack.
The posterior end which joins the eye becomes divided into the two
collecting branches of the adult. Finally the whole structure becomes
separated from the skin except at the external opening, and develops a
lumen.

ORGANS OF VISION OF THE VERTEBRATA. 507

In Lacertilia the lacrymal duct arises very much in the same manner as
in Amphibia, though its subsequent growth is somewhat different. It
appears as an internal ridge of the epithelium, at the junction of the superior
maxillary process and the fold which gives rise to the lower eyelid. A solid
process of this ridge makes its way through the mesoblast on the upper
border of the maxillary process till it meets the wall of the nasal cavity, with
the epithelium of which it becomes continuous. At a subsequent stage
a second solid growth from the upper part of the epithelial ridge makes its
way through the lower eyelid, and unites with the inner epithelium of the
eyelid ; and at a still later date a third growth from the lower part of the
structure forms a second junction with the epithelium of the eyelid. The
two latter outgrowths form the two upper branches of the duct. The
ridge now loses its connection with the external skin, and, becoming
hollow, forms the lacrymal duct. It opens at two points on the inner
surface of the eyelid, and terminates at its opposite extremity by opening
into the nasal cavity. It is remarkable, as pointed out by Born, that the
original epithelial ridge gives rise directly to a comparatively small part of
the whole duct.

In the Fowl the lacrymal duct is formed as a solid ridge of the epidermis,
extending along the line of the so-called lacrymal groove from the eye to the
nasal pit (fig. 120). At the end of the sixth day it begins to be separated
from the epidermis, remaining however united with it on the inner side of
the lower eyelid. After its separation from the epidermis it forms a solid
cord, the lower end of which unites with the wall of the nasal cavity. The
cord so formed gives rise 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 this cord. A lumen begins to be formed on
the twelfth day of incubation, and first appears at the nasal end. It arises
by the formation of a space between the cells of the cord, and not by
an absorption of the central cells.

In Mammalia Kolliker states that he has been unable to observe
anything similar to that described by Born in the Sauropsida and Amphibia,
and holds to the old view, originally put forward by Coste, that the duct is
formed by the closure of a groove leading from the eye to the nose between
the outer nasal process and the superior maxillary process. The upper
extremity of the duct dilates to form a sack, from which two branches pass
off to open on the lacrymal papillae. In view of Born's discoveries Kolliker's
statements must be received with some caution.

The Eye of tJte Tunicata.

The unpaired eye of the larva of simple Ascidians is situated
somewhat to the right side of the posterior part of the dorsal
wall of the anterior cephalic vesicle (fig. 296, O\ It consists of
a refractive portion, turned towards the cavity of the vesicle of

508 THE EYE OF THE TUNICATA.

the brain, and a retinal portion forming part of the wall of the
brain. The refractive parts consist of a convex-concave menis-
cus in front, and a spherical lens behind, adjoining the concave
side of the meniscus. The posterior part of this lens is im-

FIG. 296. LARVA OF ASCIDIA MENTULA. (From Gegenbaur ; after Kupffer. )
Only the anterior part of the tail is represented.

IV'. anterior swelling of neural tube; N. anterior swelling of spinal portion of
neural tube ; n. hinder part of neural tube ; ch. notochord ; K. branchial region of
alimentary tract; d. oesophageal and gastric region of alimentary tract; 0. eye;
a. otolith ; o. mouth ; s. papilla for attachment.

bedded in a layer of pigment The retina is formed of columnar
cells, with their inner ends imbedded in the pigment which
encloses the posterior part of the lens. The retinal part of the
eye arises in the first instance as a prominence of the wall of
the cerebral vesicle : its cells become very columnar and pig-
mented at their inner extremities (fig. 8, V, a). The lens is
developed at a later period, after the larva has become hatched,
but the mode of its formation has not been made out.

General considerations on the Eye of the Chordata.

There can be but little doubt that the eye of the Tunicata belongs to the
same phylum as that of the true Vertebrata, different as the two eyes are.
The same may also be said with reference to the degenerate and very
rudimentary eye of Amphioxus.

The peculiarity of the eye of all the Chordata consists in the retina being
developed from part of the wall of the brain. How is this remarkable feature
of the eye of the Chordata to be explained ?

Lankester, interpreting the eye in the light of the Tunicata, has made
the interesting suggestion1 "that the original Vertebrate must have been a
transparent animal, and had an eye or pair of eyes inside the brain, like that
of the Ascidian Tadpole."

1 Degeneration, London, 1880, p. 49.

ORGANS OF VISION. 509

This explanation may possibly be correct, but another explanation appears
to me possible, and I am inclined to think that the vertebrate eyes have not
been derived from eyes like those of Ascidians, but that the latter is a
degenerate form of vertebrate eye.

The fact of the retina being derived from the fore-brain may perhaps be
explained in the same way as has already been attempted in the case of the
retina of the Crustacea ; i.e. by supposing that the eye was evolved simulta-
neously with the fore part of the brain.

The peculiar processes which occur in the formation of the optic vesicle
are more difficult to elucidate ; and I can only suggest that the development
of a primary optic vesicle, and its conversion into an optic cup, is due to the
retinal part of the eye having been involved in the infolding which gave rise
to the canal of the central nervous system. The position of the rods and
cones on the posterior side of the retina is satisfactorily explained by this
hypothesis, because, as may be easily seen from figure 285, the posterior face
of the retina is the original external surface of the epidermis, which is
infolded in the formation of the brain ; so that the rods and cones are, as
might be anticipated, situated on what is morphologically the external surface
of the epiblast of the retina.

The difficulty of this view arises in attempting to make out how the eye
can have continued to be employed during the gradual change of position
which the retina must have undergone in being infolded with the brain in
the manner suggested. If however the successive steps in this infolding
were sufficiently small, it seems to me not impossible that the eye might have
continued to be used throughout the whole period of change, and a trans-
parency of the tissues, such as Lankester suggests, may have assisted in
rendering this possible.

The difficulty of the eye continuing to be in use when undergoing
striking changes in form is also involved in Lankester's view, in that if, as I
suppose, he starts from the eye of the Ascidian Tadpole with its lenses
turned towards the cavity of the brain ; it is necessary for him to admit that
a fresh lens and other optical parts of the eye became developed on the
opposite side of the eye to the original lens ; and it is difficult to understand
such a change, unless we can believe that the refractive media on the two
sides were in operation simultaneously. It may be noted that the same
difficulty is involved in supposing, as I have done, that the eye of the
Ascidian Tadpole was developed from that of a Vertebrate. I should
however be inclined to suggest that the eye had in this case ceased for a
period to be employed ; and that it has been re-developed again in some of
the larval forms. Its characters in the Tunicata are by no means constant.

Accessory eyes in the Vertebrata.

In addition to the paired eyes of the Vertebrata certain organs are
found in the skin of a few Teleostei living in very deep water, which, though
clearly not organs of true vision, yet present characters which indicate that

510 ACCESSORY EYES IN THE VERTEBRATA.

they may be used in the perception of light. The most important of such
organs are those found in Chauliodus, Stomias, etc., the significance of which
was first pointed out by Leuckart, while the details of their structure have
been recently worked out by Leydig1 and Ussow. They are distributed not
only in the skin, but are also present in the mouth and respiratory cavity, a
fact which appears to indicate that their main function must be something
else than the perception of light. It has been suggested that they have the
function of producing phosphorescence.

Another organ, probably of the same nature, is found on the head of
Scopelus.

The organs in Chauliodus are spherical or nearly spherical bodies
invested in a special tunic. The larger of them, which alone can have any
relation to vision, are covered with pigment except on their outer surface.
The interior is filled with two masses, named by Leuckart the lens and
vitreous humour. According to Leydig each of them is cellular and receives
a nerve, the ultimate destination of which has not however been made out.
According to Ussow the anterior mass is structureless, but serves to support
a lens, placed in the centre of the eye, and formed of a series of crystalline
cones prolonged into fibres, which in the posterior part of the eye diverge
and terminate by uniting with the processes of multipolar cells, placed near
the pigmented sheath. These cells, together with the fibres of the crystalline
cones which pass to them, are held by Ussow to constitute a retina.

Eye of the Mollusca.

(362) N. Bobretzky. " Observations on the development of the Cephalopoda "
(Russian). Nachrichten d. kaiserlichen Gesell.d. Freundcder Natunviss. Anthropolog.
Ethnogr. bei d. Universitiit Moskau.

(363) H. Grenacher. " Zur Entwicklungsgeschichte d. Cephalopoden." Zeit.
f. wiss. Zool., Bd. xxiv. 1874.

(364) V. Hensen. " Ueber d. Auge einiger Cephalopoden." Zeit. f. wiss.
Zool., Vol. xv. 1865.

(365) E. R. Lankester. " Observations on the development of the Cephalo-
poda." Quart, y. of Micr. Science, Vol. xv. 1875.

(366) C. Semper. Ueber Sehorgane von Typus d. Wirbelthieratigen. Wiesbaden,
1877-

Eye of the Arthropoda.

(367) N. Bobretzky. Development of Astacus and Palaemon. Kiew, 1873.

(368) A. Dohrn. " Untersuchungen lib. Bau u. Entwicklung d. Arthropoden.
Palinurus nnd Scyllarus. " Zeit. f. wiss. Zool., Bd. xx. 1870, p. 264 et seq.

1 F. Leydig. "Ueber Nebenaugen d. Chauliodus Sloani." Archiv f. Anal,
und Phys., 1879. M. Ussow. " Ueb. d. Bau d. augenahnlichen Flicken einiger
Knochenfische." Bui. d. la Soc. d. Naturalistes de Moscon, Vol. i.iv. 1879. Vide
for general description and further literature, Giinther, The Study of Fish>-st Edinburgh,
1880.

ORGANS OF VISION. 51 1

(369) E. Claparede. " Morphologic d. zusammengesetzten Auges bei den Ar-
thropoden." Zeit. f. wiss. Zool., Bd. x. 1860.

(370) H. Grenacher. Untersuchungen iib. d. Sehorgane d. Arthropoden.
Gottingen, 1879.

Vertebrate Eye.

(371) J.Arnold. Beitrage zur Entwicklungsgeschichte des Auges. Heidelberg,
1874.

(372) Babuchin. "Beitrage zur Entwicklungsgeschichte des Auges." Wilrz-
burger natiinuissenschaftliche Zeitschrift, Bd. 8.

(373) L. Kessler. Zur Entwicklung d. Attges d. Wirbelthiere. Leipzig, 1877.

(374) N. Lieberkiihn. Ueber das Auge des Wirbelthierembryo. Cassel, 1872.

(375) N. Lieberkiihn. "Beitrage z. Anat. d. embryonalen Auges." Archiv
f. Anat. imd Phys., 1879.

(376) L. Lowe. "Beitrage zur Anatomic des Auges" and "Die Histogenese
der Retina." Archiv f. mikr. Anat., Vol. xv. 1878.

(377) V. Mihalkowics. " Untersuchungen iiber den Kamm des Vogelauges."
Archiv f. mikr. Anat., Vol. ix. 1873.

(378) W. Miiller. " Ueber die Stammesentwickelung des Sehorgans der Wir-
belthiere." Festgabe Carl Ltidwig. Leipzig, 1874.

(379) S. L. Schenk. "Zur Entwickelungsgeschichte des Auges der Fische."
Wiener Sitzungsberichte, Bd. LV. 1867.

Accessory organs of the Vertebrate Eye.

(380) G. Born. "Die Nasenhohlen u. d. Thranennasengang d. Amphibien.''
Morphologisches Jahrbuch, Bd. II. 1876.

(381) G. Born. " Die Nasenhohlen u. d. Thranennasengang d. amnioten Wir-
belthiere. I. Lacertilia. II. Aves." Morphologisches Jahrbuch, Bd. v. 1879.

Eye of the Tunicata.

(382) A. Kowalevsky. "Weitere Studien lib. d. Entwicklung d. einfachen
Ascidien." Archiv f. mikr. Anat., Vol. vil. 1871.

(383) C. Kupffer. "Zur Entwicklung d. einfachen Ascidien." Archiv f.
mikr. Anat., Vol. vii. 1872.

CHAPTER XVII.

AUDITORY ORGAN, OLFACTORY ORGAN AND SENSE
ORGANS OF THE LATERAL LINE.

Auditory Organs.

A GREAT variety of organs, very widely distributed amongst
aquatic forms, and also found, though less universally, in land
forms, are usually classed together as auditory organs.

In the case of all aquatic forms, or of forms which have
directly inherited their auditory organs from aquatic forms,
these organs are built upon a common type ; although in the
majority of instances the auditory organs of the several groups
have no genetic relations. All the organs have their origin in
specialized portions of the epidermis. Some of the cells of a
special region become provided at their free extremities with
peculiar hairs, known as auditory hairs; while in other cells
concretions, known as otoliths, are formed, which appear often
to be sufficiently free to be acted upon by vibrations of the
surrounding medium, and to be so placed as to be able in their
turn to transmit their vibrations to the cells with auditory hairs1.
The auditory regions of the epidermis are usually shut off from
the surface in special sacks.

The actual function of these organs is no doubt correctly
described, in the majority of instances, as being auditory; but it
appears to me very possible that in some cases their function
may be to enable the animals provided with them to detect the
presence of other animals in their neighbourhood, through the

1 The function of the otoliths is not always clear. There is evidence to shew that
they sometimes act as dampers.

AUDITORY ORGANS. 513

unclulatory movements in the water, caused by the swimming of
the latter.

Auditory organs with the above characters, sometimes freely
open to the external medium, but more often closed, are found
in various Ccelenterata, Vermes and Crustacea, and universally
or all but universally in the Mollusca and Vertebrata.

In many terrestrial Insects a different type of auditory organ
has been met with, consisting of a portion of the integument
modified to form a tympanum or drum, and supported at its
edge by a chitinous ring. The vibrations set up in the mem-
branous tympanum stimulate terminal nerve organs at the ends
of chitinous processes, placed in a cavity bounded externally by
the tympanic membrane.

The tympanum of Amphibia and Amniota is an accessory
organ added, in terrestrial Vertebrata, to an organ of hearing
primitively adapted to an aquatic mode of life ; and it is interest-
ing to notice the presence of a more or less similar membrane
in the two great groups of terrestrial forms, i.e. terrestrial Verte-
brata and Insecta.

Nothing is known with reference to the mode of develop-
ment or evolution of the tympanic type of auditory organ found
in Insects, and, except in the case of Vertebrates, but little is
known with reference to the development of what may be called
the vesicular type of auditory organ found in aquatic forms.
Some very interesting facts with reference to the evolution of
such organs have however been brought to light by the brothers
Hertwig in their investigations on the Ccelenterata; and I
propose to commence my account of the development of the
auditory organs in the animal kingdom by a short statement of
the results of their researches.

Ccelenterata. Three distinct types of auditory organ have
been recognised in the Medusae ; two of them resulting from
the differentiation of a tentacle-like organ, and one from ecto-
derm cells on the under surface of the velum. We may com-
mence with the latter as the simplest. It is found in the
Medusae known as the Vesiculata. The least differentiated
form of this organ, so far discovered, is present in Mitrotrocha,
Tiaropsis and other genera. It has the form of an open pit ;
and a series of such organs are situated along the attached edge

B. in, 33

514 AUDITORY ORGANS OF THE CCELENTERATA.

of the velum with their apertures directed downwards. The
majority of the cells lining the outer, i.e. peripheral side of the

FIG. 297. AUDITORY VESICLE OF PHIALIDIUM AFTER TREATMENT WITH
DILUTE OSMIC ACID. (From Lankester; after O. and R. Hertwig.)

dl. epithelium of the upper surface of the velum; d2. epithelium of the under
surface of the velum ; r. circular canal at the edge of the velum ; nrl. upper nerve-
ring ; h. auditory cells ; hh. auditory hairs ; np. nervous cushion formed of a
prolongation of the lower nerve-ring. Close to the nerve-ring is seen a cell, shewn as
black, containing an otolith.

pit, contain an otolith, while a row of the cells on the inner, i.e.
central side, are modified as auditory cells. The auditory cells
are somewhat strap-shaped, their inner ends being continuous
with the fibres of the lower nerve-ring, and their free ends being
provided with bent auditory hairs, which lie in contact with the
convex surfaces of the cells containing the otoliths.

By the conversion of such open pits into closed sacks a more
complicated type of auditory organ, which is present in many of
the Vesiculata, viz. ^Equorea, Octorchis, Phialidium, &c., is
produced. A closed vesicle of this type is shewn in fig. 297.
Such organs form projections on the upper surface of the velum.
They are covered by a layer of the epithelium (d1} of the upper
surface of the velum, but the lining of the vesicle (d*} is derived
from what was originally part of the epithelium of the lower
surface of the velum, homologous with that lining the open pits
in the type already described. The general arrangement of the
cells lining such vesicles is the same as that of the cells lining
the open pits.

A second type of auditory organ, found in the Trachyme-
clusa,", appears in its simplest condition as a modified tentacle.

AUDITORY ORGANS. 515

It is formed of a basal portion, covered by auditory cells with
long stiff auditory hairs, supporting at its apex a club-shaped
body, attached to it by a delicate stalk. An endodermal axis is
continued through the whole structure, and in one or more of
the endoderm cells of the club-shaped body otoliths are always
present. The tails of the auditory cells are directly continued
into the upper nerve-ring.

In more complicated forms of this organ the tentacle becomes
enclosed in a kind of cup, by a wall-like upgrowth of the

FIG. 298. AUDITORY ORGAN OF RHOPALONEMA. (From Lankester; after O.
and R. Hertwig.)

The organ consists of a modified tentacle (hk) with auditory cells and con-
cretions, partially enclosed in a cup.

surrounding parts (fig. 298) ; and in some forms, e.g. Geryonia,
by the closure of the cup, the whole structure takes the form of
a completely closed vesicle, in the cavity of which the original
tentacle forms an otolith-bearing projection.

The auditory organs found in the Acraspedote Medusae
approach in many respects to the type of organ found in
the Trachymedusse. They consist of tentacular organs placed
in grooves on the under surface of the disc. They have a
swollen extremity, and are provided with an endodermal axis
for half the length of which there is a diverticulum of the gastro-
vascular canal system. The terminal portion of the endoderm
is solid, and contains calcareous concretions. The ectodermal
cells at the base of these organs have the form of auditory cells.

Mollusca. Auditory vesicles are found in almost all Mol-
lusca on the ventral side of the body in close juxtaposition to
the pedal ganglia. Except possibly in some Cephalopods, these

33—2

516 AUDITORY ORGANS OF THE VERTEBRATA.

vesicles are closed. They are provided with free otoliths,
supported by the cilia of the walls of the sack, but in addition
some of the cells of the sack are provided with stiff auditory
hairs.

In many forms these sacks have been observed to originate
by an invagination of the epiblast of the foot (Pahtdina, Nassa,
Heteropoda, Limax, Clio, Cephalopoda and Lamellibranchiata).
In other instances (some Pteropods, Lymnaeus, &c.) they appear,
by a secondary modification in the development, to originate by
a differentiation of a solid mass of epiblast.

According to Fol the otocysts in Gasteropods are formed by
cells of the wall of the auditory sacks ; and the same appears to
hold good for Cephalopoda (Grenacher)1 shewing that free otoliths
have in these instances originated from otoliths originally placed
in cells.

Crustacea. In the decapodous Crustacea organs, which have been
experimentally proved to be true organs of hearing, are usually present on
the basal joint of the anterior antennae. They may have (Hensen, No. 384)
the form either of closed or of open sacks, lined by an invagination of the
epidermis. They are provided with chitinous auditory hairs and free otoliths.
In the case of the open sacks the otoliths appear to be simply stones trans-
ported into the interior of the sacks, but in the closed sacks the otoliths,
though free, are no doubt developed within the sacks.

The Schizopods, which, as mentioned in the last chapter, are remarkable
as containing a genus (Euphausia) with abnormally situated eyes, distinguish
themselves again with reference to their auditory organs, in that another
genus (Mysis) is characterized by the presence of a pair of auditory sacks in
the inner plates of the tail. These sacks have curved auditory hairs support-
ing an otolith at their extremity.

The development of the auditory organs in the Crustacea has not been
investigated.

The Vertebrata. The Cephalochorda are without organs
of hearing, and the auditory organ of the Urochorda is constructed
on a special type of its own. The primitive auditory organs of
the true Vertebrata have the same fundamental characters as
those of the majority of aquatic invertebrate forms. They consist
of a vesicle, formed by the invagination of a patch of epiblast,
and usually shut off from the exterior, but occasionally (Elasmo-

1 For the somewhat complicated details as to the development of the auditory
sacks of Cephalopods I must refer the reader to Vol. II., pp. 278, 279, and to
Grenacher (Vol. i., No. 280).

AUDITORY ORGANS.

517

branchii) remaining open. The walls of this vesicle are always
much complicated and otoliths of various forms are present in its
cavity. To this vesicle accessory structures, derived from the
walls of the hyomandibular cleft, are added in the majority of
terrestrial Vertebrata.

The development of the true auditory vesicle will be considered
separately from that of the accessory structures derived from the
hyomandibular cleft.

In all Vertebrata the development of the auditory vesicle
commences with the formation of a thickened patch of epiblast,
at the side of the hind-brain, on the
level of the second visceral cleft.

•t.v.v

This patch soon becomes invaginated
in the form of a pit (fig. 299, aup), to
the inner side of which the ganglion
of the auditory nerve (ami), which as
shewn in a previous chapter is primi-
tively a branch of the seventh nerve,
closely applies itself.

In those Vertebrata (viz. Teleostei, Le-
pidosteus and Amphibia) in which the epi-
blast is early divided into a nervous and
epidermic stratum, the auditory pit arises
as an invagination of the nervous stratum
only, and the mouth of the auditory pit is
always closed -(fig. 300) by the epidermic
stratum of the skin. Since the opening of
the pit is retained through life in Elasmo-
branchii the closed form of pit in the above
forms is clearly secondary.

In Teleostei the auditory pit arises as a
solid invagination of the epiblast.

T/t,

FIG. 299. SECTION THROUGH
THE HEAD OF AN ELASMOBRANCH
EMBRYO, AT THE LEVEL OF THE
AUDITORY INVOLUTION.

aup. auditory pit; aun. gan-
glion of auditory nerve ; iv.v. roof
of fourth ventricle; a.c.v. anterior
cardinal vein; aa. aorta; I.aa.
aortic trunk of mandibular arch ;
pp. head cavity of mandibular
arch ; Ivc. alimentary pouch which
will form the first visceral cleft;
77?. rudiment of thyroid body.

The mouth of the auditory vesi-
cle gradually narrows, and in most

forms soon becomes closed, though in Elasmobranchii it remains
permanently open. In any case the vesicle is gradually removed
from the surface, remaining connected with it by an elongated
duct, either opening on the dorsal aspect of the head (Elasmo-
branchii), or ending blindly close beneath the skin.

In all Vertebrata the auditory vesicle undergoes further

5i8

AUDITORY ORGANS OF THE VERTEBRATA.

changes of a complicated kind. In the Cyclostomata these
changes are less complicated than in other forms, though whether
this is due to degeneration, or to the retention of a primitive

FIG. 300. SECTION THROUGH THE HEAD OK A LEPIUOSTEUS EMBRYO ON

THE SIXTH DAY AFTER IMPREGNATION.
au.v. auditory vesicle ; au.n. auditory nerve ; ch. notochord ; hy. hypoblast.

state of the auditory organ, is not known. In the Lamprey the
auditory vesicle is formed in the usual way by an invagination

cv

cc

AOA

FIG. 301. SECTION THROUGH THE HIND-BRAIN OK 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. Ch. notochord ; C V. anterior cardinal vein; CC. involuted auditory
vesicle (CC points to the end which will form the cochlear canal) ; RL. recessus
labyrinthi (remains of passage connecting the vesicle with the exterior) ; hy. hypoblast
lining the alimentary canal; AO., AO.A. aorta, and aortic arch.

AUDITORY ORGANS. 519

of the epiblast, which soon becomes vesicular, and for a consider-
able period retains a simple character. As pointed out by Max
Schultze, a number of otoliths appears in the vesicle during
larval life, and, although such otoliths are stated by J. Miiller to
be absent both in the full-grown Ammoccete and in the adult,
they have since been found by Ketel (No. 387). The formation
of the two semicircular canals has not been investigated.

In all the higher Vertebrates the changes of the auditory
sacks are more complicated. The ventral end of the sack is
produced into a short process (fig. 301, CC}\ while at the dorsal
end there is the canal-like prolongation of the lumen of the sack
(RL}, derived from the duct which primitively opened to the
exterior, and which in most cases persists as a blind diverticulum
of the auditory sack, known as the recessus labyrinthi or
aqueductus vestibuli. The parts thus indicated give rise to
the whole of the membranous labyrinth of the ear. The main
body of the vesicle becomes the utriculus and semicircular canals,
while the ventral process forms the sacculus hemisphericus and
cochlear canal.

The growth of these parts has been most fully studied in
Mammalia, where they reach their greatest complexity, and it
will be convenient to describe their development in this group,
pointing out how they present, during some of the stages in their
growth, a form permanently retained in lower types.

The auditory vesicle in Mammalia is at first nearly spherical,
and is imbedded in the mesoblast at the side of the hind-brain.
It soon becomes triangular in section, with the apex of the tri-
angle pointing inwards and downwards. This apex gradually
elongates to form the rudiment of the cochlear canal and sacculus
hemisphericus (fig. 302, CC). At the same time the recessus
labyrinthi (R.L) becomes distinctly marked, and the outer wall
of the main body of the vesicle grows out into two protuberances,
which form the rudiments of the vertical semicircular canals
( V.B}. In the lower forms (fig. 305) the cochlear process of the
vestibule hardly reaches a higher stage of development than that
found at this stage in Mammalia.

The parts of the auditory labyrinth thus established soon
increase in distinctness (fig. 303) ; the cochlear canal (CC}
becomes longer and curved ; its inner and concave surface being

520

AUDITORY ORGANS OF THE MAMMALIA.

lined by a thick layer of columnar epiblast. The recessus laby-
rinthi also increases in length, and just below the point where
the bulgings to form the vertical semicircular canals are situated,
there is formed a fresh protuberance for the horizontal semi-

V.B

FIG. 302. TRANSVERSE SECTION OF THE HEAD OF A FCETAL SHEEP (16 MM. IN
LENGTH) IN THE REGION OF THE HIND-BRAIN. (After Bottcher.)

HB. the hind -brain.

The section is somewhat oblique, hence while on the right side the connections of
the recessus vestibuli R.L., and of the commencing vertical semicircular canal V.B.,
and of the ductus cochlearis CC., with the cavity of the primary otic vesicle are seen :
on the left side, only the extreme end of the ductus cochlearis CC, and of the semi-
circular canal V.B. are shewn.

Lying close to the inner side of the otic vesicle is seen the cochlear ganglion GC ;
on the left side the auditory nerve G and its connection N with the hind-brain are also
shewn.

Below the otic vesicle on either side lies the jugular vein.

circular canal. At the same time the central parts of the walls
of the flat bulgings of the vertical canals grow together, oblit-
erating this part of the lumen, but leaving a canal round the
periphery ; and, on the absorption of their central parts, each of
the original simple bulgings of the wall of the vesicle becomes
converted into a true semicircular canal, opening at its two
extremities into the auditory vesicle. The vertical canals are
first established and then the horizontal canal.

AUDITORY ORGANS.

521

Shortly after the formation of the rudiment of the horizontal
semicircular canal a slight protuberance becomes apparent on the

FIG. 303. SECTION OF THE HEAD OF A FCETAL SHEEP 20 MM. IN LENGTH.

(After Bottcher.)

R. V. recessus labyrinthi ; V.B. vertical semicircular canal ; H.B. horizontal semi-
circular canal; C.C. cochlear canal ; G. cochlear ganglion.

inner commencement of the cochlear canal. A constriction arises
on each side of the protuberance, converting it into a prominent
hemispherical projection, the sacculus hemisphericus (fig. 304,
S.R\

The constrictions are so deep that the sacculus is only con-
nected with the cochlear canal on the one hand, and with the
general cavity of the auditory vesicle on the other, by, in each
case, a narrow though short canal.

The former of these canals (fig. 304, b) is known as the canalis
reuniens. At this stage we may call the remaining cavity of the
original otic vesicle, into which all the above parts open, the utri-
culus.

Soon after the formation of the sacculus hemisphericus, the

522 AUDITORY ORGANS OF THE MAMMALIA.

cochlear canal and the semicircular canals become invested with
cartilage. The recessus labyrinthi remains however still enclosed
in undifferentiated mesoblast

Between the cartilage and the parts which it surrounds there
remains a certain amount of indifferent connective tissue, which
is more abundant around the cochlear canal than around the
semicircular canals.

As soon as they have acquired a distinct connective-tissue
coat, the semicircular canals begin to be dilated at one of their
terminations to form the ampullae. At about the same time a
constriction appears opposite the mouth of the recessus labyrinthi,
which causes its opening to be divided into two branches — one
towards the utriculus and the other towards the sacculus hemi-
sphericus ; and the relations of the parts become so altered that
communication between the sacculus and utriculus can only take
place through the mouth of the recessus labyrinthi (fig. 305).

When the cochlear canal has come to consist of two and a
half coils, the thickened epithelium which lines the lower surface
of the canal forms a double ridge from which the organ of Corti
is subsequently developed. Above the ridge there appears a
delicate cuticular membrane, the membrane of Corti or mem-
brana tectoria.

The epithelial walls of the utricle, the recessus labyrinthi, the
semicircular canals, and the cochlear canal constitute together the
highly complicated product of the original auditory vesicle. The
whole structure forms a closed cavity, the various parts of which
are in free communication. In the adult the fluid present in this
cavity is known as the endolymph.

In the mesoblast lying between these parts and the cartilage,
which at this period envelopes them, lymphatic spaces become
established, which are partially developed in the Sauropsida, but
become in Mammals very important structures.

They consist in Mammals partly of a space surrounding the
utricle and semicircular canals, and partly of two very definite
channels, which largely embrace between them the cochlear canal.
The latter channels form the scala vestibuli on the upper side
of the cochlear canal and the scala tympani on the lower. The
scala vestibuli is in free communication with the lymphatic cavity
surrounding the vestibule, and opens at the apex of the cochlea

AUDITORY ORGANS.

523

into the scala tympani. The latter ends blindly at the fenestra
rotunda.

The fluid contained in the two scalae, and in the remaining
lymphatic cavities of the auditory labyrinth, is known as peri-
lymph.

The cavities just spoken of are formed by an absorption of

Ch.-

JUB

C.C

FIG. 304. SECTION THROUGH THE INTERNAL EAR OF AN EMBRYONIC SHEEP
28 MM. IN LENGTH. (After Bottcher.)

D.M. dura mater; R. V. recessus labyrinthi ; H.V.B. posterior vertical semi-
circular canal ; U. utriculus ; H.B. horizontal semicircular canal; b. canalis reuniens ;
a. constriction by means of which the sacculus hemisphericus S.R. is formed ; f.
narrowed opening between sacculus hemisphericus and utriculus ; C. C. cochlea ;
C.C. lumen of cochlea; K.K. cartilaginous capsule of cochlea; K.B. basilar plate;
Ch. notochord.

524 ORGAN OF CORTI.

parts of the embryonic mucous tissue between the perichondrium
and the walls of the membranous labyrinth.

The scala vestibuli is formed before the scala tympani, and
both scalae begin to be developed at the basal end of the cochlea :
the cavity of each is continually being carried forwards towards
the apex of the cochlear canal by a progressive absorption of the
mesoblast. At first both scalae are somewhat narrow, but they
soon increase in size and distinctness.

The cochlear canal, which is often known as the scala media
of the cochlea, becomes compressed on the formation of the
scalae so as to be triangular in section, with the base of the triangle
outwards. This base is only separated from the surrounding
cartilage by a narrow strip of firm mesoblast, which becomes the
stria vascularis, etc. At the angle opposite the base the canal
is joined to the cartilage by a narrow isthmus of firm material,
which contains nerves and vessels. This isthmus subsequently
forms the lamina spiralis, separating the scala vestibuli from
the scala tympani.

The scala vestibuli lies on the upper border of the cochlear
canal, and is separated from it by a very thin layer of mesoblast,
bordered on the cochlear aspect by flat epiblast cells. This mem-
brane is called the membrane ofReissner. The scala tympani
is separated from the cochlear canal by a thicker sheet of meso-
blast, called the basilar membrane, which supports the organ
of Corti and the epithelium adjoining it. The upper extremity
of the cochlear canal ends in a blind extremity called the cupola,
to which the two scalae do not for some time extend. This
condition is permanent in Birds, where the cupola is represented
by a structure known as the lagena (fig. 305, II. L}. Subse-
quently the two scalae join at the extremity of the cochlear canal ;
the point of the cupola still however remains in contact with the
bone, which has now replaced the cartilage, but at a still later
period the scala vestibuli, growing further round, separates the
cupola from the adjoining osseous tissue.

The ossification around the internal ear is at first confined to the
cartilage, but afterwards extends into the thick periosteum between the
cartilage and the internal ear, and thus eventually makes its way into the
lamina spiralis, etc.

The organ of Corti. In Mammalia there is formed from the

AUDITORY ORGANS.

525

epithelium of the cochlear canal a very remarkable organ known as the organ
of Corti, the development of which is of sufficient importance to merit a
brief description. A short account of this organ in the adult state may
facilitate the understanding of its development.

The cochlear canal is bounded by three walls, the outer one being the
osseous wall of the cochlea. The membrane of Reissner bounds it towards

— U

FIG. 305. DIAGRAMS OF THE MEMBRANOUS LABYRINTH. (From Gegenbaur.)

I. Fish. II. Bird. III. Mammal.

U. utriculus ; S. sacculus ; US. utriculus and sacculus ; Cr. canalis reuniens ;
R. recessus labyrinthi ; UC. commencement of cochlea ; C. cochlear canal ; L. lagena ;
PC. cupola at apex of cochlear canal; V. csecal sack of the vestibulum of the cochlear
canal.

the scala vestibuli, and the basilar membrane towards the scala tympani.
This membrane stretches from the margin of the lamina spiralis to the
ligamentum spirale ; the latter being merely an expanded portion of the
connective tissue lining the osseous cochlea.

The lamina spiralis is produced into two lips, called respectively the
labium tympanicum and labium vestibulare ; it is to the former and
longer of these that the basilar membrane is attached. At the margin of the
junction of the labium tympanicum with the basilar membrane the former is
perforated for the passage of the nervous fibres, and this region is called the
habenula perforata.

The labium vestibulare, so called from its position, is shorter than the
labium tympanicum and is raised above into numerous blunt teeth. Partly
springing out from the labium vestibulare, and passing from near the inner
attachment of the membrane of Reissner towards the outer wall of the
cochlea, is an elastic membrane, the membrana tectoria. Resting on the
basilar membrane is the organ of Corti.

Considering for the moment that a transverse section of the cochlear

$26 ORGAN OF CORTI.

canal only one cell deep is being dealt with, the organ of Corti will be found
to consist of a central part composed of two peculiarly shaped rods widely
separated below, but in contact above. These are the rods or fibres of
Corti. On their outer side, i.e. on the side towards the osseous wall of the
canal, is a reticulate membrane which passes from the inner rod of Corti
towards the osseous wall of the canal. With their upper extremities fixed in
that membrane, and their lower resting on the basilar membrane are three
(four in man) cells with auditory hairs known as the outer 'hair cells,'
which alternate with three other cells known as Deiters' cells. Between
these and the outer attachment of the basilar membrane is a series of cells
gradually diminishing in height in passing outwards. On the inner side of
the rods of Corti is one hair cell, and then a number of peculiarly modified
cells which fill up the space between the two lips of the lamina spiralis.

It will not be necessary to say much in reference to the development of
the labium tympanicum and the labium vestibulare.

The labium vestibulare is formed by a growth of the connective tissue
which fuses with and passes up between the epithelial cells. The epithelial
cells which line its upper (vestibular) border become modified, and remain
as its teeth.

The labium tympanicum is formed by the coalescence of the connective
tissue layer separating the scala tympani from the cochlear canal with part
of the connective tissue of the lamina spiralis. At first these two layers are
separate, and the nerve fibres to the organ of Corti pass between them.
Subsequently however they coalesce, and the region where they are
penetrated by the nervous fibres becomes the habenula perforata.

The organ of Corti itself is derived from the epiblast cells lining the
cochlear canal, and consists in the first instance of two epithelial ridges or
projections. The larger of them forms the cells on the inner side of the
organ of Corti, and the smaller the rods of Corti together with the inner and
outer hair cells and Deiters' cells.

At first both these ridges are composed of simple elongated epithelial
cells one row deep. The smaller ridge is the first to shew any change. The
cells adjoining the larger ridge acquire auditory hairs at their free extremities,
and form the row of inner hair cells ; the next row of cells acquires a broad
attachment to the basilar membrane, and gives origin to the inner and outer
rods of Corti.

Outside the latter come several rows of cells adhering together so as to
form a compact mass which is quadrilateral in section. This mass is
composed of three upper cells with nuclei at the same level, which form the
outer hair cells, each of them ending above in auditory hairs, and three
lower cells which form the cells of Deiters. Beyond this the cells gradually
pass into ordinary cubical epithelial cells.

As just mentioned, the cells of the second row, resting with their broad
bases on the basilar membrane, give rise to the rods of Corti. The breadth
of the bases of these cells rapidly increases, and important changes take
place in the structure of the cells themselves.

AUDITORY ORGANS. 527

The nucleus of each cell divides ; so that there come to be two nuclei or
sometimes three which lie close together near the base of the cell. Outside
the nuclei on each side a fibrous cuticular band appears. The two bands
pass from the base of the cell to its apex, and there meet though widely
separated below. The remaining contents of the cell, between the two
fibrous bands, become granular, and are soon to a great extent absorbed ;
leaving at first a round, and then a triangular space between the two fibres.
The two nuclei, surrounded by a small amount of granular matter, come to
lie, each at one of the angles between the fibrous bands and the basilar
membrane.

The two fibrous bands become, by changes which need not be described
in detail, converted into the rods of Corti — each of their upper ends growing
outwards into the processes which the adult rods possess.

Each pair of rods of Corti is thus (Bottcher) to be considered as the
product of one cell ; and the nuclei embedded in the granular mass between
them are merely the remains of the two nuclei formed by the division of the
original nucleus of that cell1. The larger ridge is for the most part not
permanent, and from being the most conspicuous part of the organ of Corti
comes to be far less important than the smaller ridge. Its cells undergo a
partial degeneration ; so that the epithelium in the hollow between the two
lips of the lamina spiralis, which is derived from the larger ridge, comes to
be composed of a single row of short and broad cells. In the immediate
neighbourhood however of the inner hair cell, one or two of the cells derived
from the larger ridge are very much elongated.

The membrana reticularis is a cuticular structure derived from the parts
to which it is attached. .

Accessory structures connected with the organ of hearing- in
Terrestrial Vertebrata.

In all the Amphibia, Sauropsida and Mammalia, except the
Urodela and a few Anura and Reptilia, the first visceral or hyo-
mandibular cleft enters into intimate relations with the organs
of hearing, and from it and the adjoining parts are formed the
tympanic cavity, the Eustachian tube, the tympanic membrane
and the meatus auditorius externus. The tympanic membrane
serves to receive from the air the sound vibrations, which are
communicated to fluids contained in the true auditory labyrinth
by one ossicle or by a chain of auditory ossicles.

The addition to the organ of hearing of a tympanic membrane
to receive aerial sound vibrations is an interesting case of the

1 It is not clear from Bottcher's description how it comes about that the inner rods
of Corti are more numerous than the outer.

528 THE TYMPANIC CAVITY.

adaptation of a structure, originally required for hearing in
water, to serve for hearing in air ; and as already pointed out,
the similarity of this membrane to the tympanic membrane of
some Insects is also striking.

There is much that is obscure with reference to' the actual
development of the above parts of the ear, which has moreover
only been carefully studied in Birds and Mammals.

The Eustachian tube and tympanic cavity seem to be derived
from the inner part of the first visceral or hyomandibular cleft,
the external opening of which becomes soon obliterated. Kolli-
ker holds that the tympanic cavity is simply a dorsally and
posteriorly directed outgrowth of the median part of the inner
section of this cleft; while Moldenhauer (No. 392) holds, if I
understand him rightly, that it is formed as an outgrowth of a
cavity called by him the sulcus tubo-tympanicus, derived from
the inner aperture of the first visceral cleft together with the
groove of the pharynx into which it opens ; and Moldenhauer is
of opinion that the greater part of the original cleft atrophies.

The meatus auditorius externus is formed at the region of a
shallow depression 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, and Moldenhauer
believes that this is the whole process. Kolliker states however
that the blind end of the meatus 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 constituted 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 fcetal life it
is relatively very thick, and presents a structure bearing but
little resemblance to that in the adult.

A proliferation of the connective tissue-cells in the vicinity of
the tympanic cavity causes in Mammalia the complete or nearly
complete obliteration of the cavity during fcetal life.

The tympanic cavity is bounded on its inner aspect by the
osseous investment of the internal ear, but at one point, known

AUDITORY ORGANS. 529

as the fenestra ovalis, the bone is deficient in the Amphibia,
Sauropsida and Mammalia, and its place is taken by a mem-
brane ; while in Mammalia and Sauropsida a second opening,
the fenestra rotunda, is also present.

These two fenestrae appear early, but whether they are
formed by an absorption of the cartilage, or by the nonchondri-
fication of a small area, is not certainly known. The upper of
the two, or fenestra ovalis, contains the base of a bone, known
in the Sauropsida and Amphibia as the columella. The main
part of the columella is formed of a stalk which is held by
Parker to be derived from part of the skeleton of the visceral
arches, but its nature is discussed in connection with the skeleton,
while the base, forming the stapes, appears to be derived from
the wall of the periotic cartilage.

In all Amphibia and Sauropsida with a tympanic cavity, 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. In Mammalia there is a stapes not directly
attached to the tympanic membrane by a stalk, and two addi-
tional auditory ossicles, derived from parts of the skeleton of the
visceral arches, are placed between the stapes and the tympanic
membrane. These ossicles are known as the malleus and incus,
and the chain of the three ossicles replaces physiologically the
single ossicle of the lower forms.

These ossicles are at first imbedded in the connective tissue
in the neighbourhood of the tympanic cavity, but on the full
development of this cavity, become apparently placed within
it ; though really enveloped in the mucous membrane lining it.

The fenestra ovalis is in immediate contiguity with the walls
of the utricle, while the fenestra rotunda adjoins the scala
tympani.

Hunt (No. 391) holds, from his investigations on the embryology of
the pig, that " the Eustachian tube is an involution of the pharyngeal
mucous membrane ;" and that "the meatus is an involution of the integu-
ment " while " the drum is formed by the Eustachian tube overlapping the
extremity of the meatus." Urbantschitsch also holds that the first visceral
cleft has nothing to do with the formation of the tympanic cavity and
Eustachian tube, and that these parts are derived from lateral outgrowths
of the oral cavity.

B. III. 34

530 THE TYMPANIC CAVITY.

The evolution of the accessory parts of the ear would be very difficult
to explain on Darwinian principles if the views of Hunt and Urbantschitsch
were correct ; and the accepted doctrine, originally proposed by Huschke
(No. 389), according to which these structures have originated by a ' change
of function' of the parts of the first visceral cleft, may fairly be held till
more conclusive evidence has been brought against it than has yet been
done.

Tunicata. The auditory organ of the Tunicata (fig. 306) is
placed on the under surface of the anterior vesicle of the brain.

FIG. 306. LARVA OF ASCIDIA MENTULA. (From Gegenbaur ; after Kupffer.)

Only the anterior part of the tail is represented.

N'. anterior swelling of neural tube ; IV. anterior swelling of spinal portion of
neural tube; n. hinder part of neural tube; ch. notochord ; A", branchial region
of alimentary tract ; d. cesophageal and gastric region of alimentary tract ; O. eye ;
a, otolith ; o. mouth ; s. papilla for attachment.

It consists of two parts (i) a prominence of the cells of the floor
of the brain forming a crista acustica, and (2) an otolith pro-
jecting into the cavity of the brain, and attached to the crista by
delicate hairs.

The crista acustica is formed of very delicate cylindrical
cells, and in its most projecting part is placed a vesicle with
clear contents. The otolith is an oval body with its dorsal half
pigmented, and its ventral half clear and highly refractive. It
is balanced on the highest point of the crista.

The crista acustica would seem to be developed from the
cells of the lower part of the front vesicle of the brain. The
otolith however is developed from a single cell on the dorsal and
right side of the brain. This cell commences to project into the
cavity of the brain and its free end becomes pigmented. It
gradually grows inwards till it forms a spherical prominence in
the cavity of the brain, to the wall of which it is attached by a

AUDITORY ORGANS. 531

stalk. At the same time it travels round the right side of the
vesicle of the brain (in a way not fully explained) till it reaches
the summit of the crista, which has become in the meantime
established.

The auditory organ of the simple Ascidians can hardly be
brought into relation with that of the other Chordata, and has
most probably been evolved within the Tunicate phylum.

BIBLIOGRAPHY.

Invertebrata.

(384) V. Hensen. "Studien lib. d. Gehororgan d. Decapoden." Zeit.f. wiss.
ZooL, Vol. xm. 1863.

(385) O. and R. Hertwig. Das Nervensystem u. d. Sinnesorgane d. Medusen.
Leipzig, 1878.

Vertebrata.

(386) A. Boettcher. "Bau u. Entwicklung d. Schnecke." Denkschriften d.
kaiserl. Leop. Carol. Akad. d. Wissenschaft., Vol. xxxv.

(387) C. H asse. Die vergleich. Morphologic u. Histologied. hiiutigen Gehororgane
d. Wirbelthiere. Leipzig, 1873.

(388) V. Hensen. "Zur Morphologic d. Schnecke." Zeit. f. wiss. ZooL, Vol.
XIII. 1863.

(389) E. Huschke. "Ueb. d. erste Bildungsgeschichte d. Auges u. Ohres beim
bebriiteten Kiichlein." Isis von Oken, 1831, and Meckel's Archiv, Vol. vi.

(390) Reissner. De Auris internes formatione. Inaug. Diss. Dorpat, 1851.

Accessory parts of Vertebrate Ear.

(391) David Hunt. "A comparative sketch of the development of the ear and
eye in the Pig. " Transactions of the International Otological Congress, \ 876.

(392) W. Moldenhaueir. "Zur Entwick. d. mittleren u. ausseren Ohres."
Morphol. Jahrbuch) Vol. III. 1877.

(393) V. Urbantschitsch. " Ueb. d. erste Anlage d. Mittelohres u. d. Trom-
melfelles." Mittheil. a. d. embryol. Instit. Wien, Heft i. 1877.

Olfactory organ.

Amongst the Invertebrata numerous sense organs have been
described under the title of olfactory organs. In aquatic animals
they often have the form of ciliated pits or grooves, while in the
Insects and Crustacea delicate hairs and other structures present
on the antennae are usually believed to be organs of smell. Our
knowledge of all these organs is however so vague that it

34—2

532

OLFACTORY PIT.

would not be profitable to deal with them more fully in this
place. Amongst the Chordata there are usually well developed
olfactory organs.

Amongst the Urochorda (Tunicata) it is still uncertain what
organs (if any) deserve this appellation. The organ on the
dorsal side of the opening of the respiratory pharynx may very
possibly have an olfactory function, but it is certainly not homo-
logous with the olfactory pits of the true Vertebrata, and as
mentioned above (pp. 436 and 437), may perhaps be homologous
with the pituitary body.

In the Cephalochorda (Amphioxus) there is a shallow ciliated
pit, discovered by Kolliker, which is situated on the left side of
the head, and is closely connected with a special process of the

FlG. 307. VIEWS OF THE HEAD OF ELASMOBRANCH EMBRYOS AT TWO STAGES
AS TRANSPARENT OBJECTS.

A. Pristiurus embryo of the same stage as fig. 28 F.

B. Somewhat older Scyllium embryo.

///. third nerve ; V. fifth nerve ; VII. seventh nerve ; au.n. auditory nerve ; gl.
glossopharyngeal nerve; Vg. vagus nerve; fb. fore-brain; pn. pineal gland; mb. mid-
brain; hb. hind-brain; iv.v. fourth ventricle; cb. cerebellum; ol. olfactory pit; op.
eye; au.V. auditory vesicle; m. mesoblast at base of brain; ch. notochord; kt. heart;
Vc. visceral clefts; eg. external gills; //. sections of body cavity in the head.

OLFACTORY ORGANS. 533

front end of the brain. It is most probably the homologue of
the olfactory pits of the true Vertebrata.

In the true Vertebrata the olfactory organ has usually the
form of a pair of pits, though in the Cyclostomata the organ is
unpaired.

In all the Vertebrata with two olfactory pits these organs
are formed from a pair of thickened patches of the epiblast, on
the under side of the fore-brain, immediately in front of the
mouth (fig. 307, ol). Each thickened patch of epiblast soon
becomes involuted as a pit (fig. 308, N), the lining cells of
which become the olfactory or Schneiderian epithelium. The
surface of this epithelium is usually much increased by various
foldings, which in the Elasmobranchii arise very early, and are
bilaterally symmetrical, diverging on each side like the barbs of
a feather from the median line. They subsequently become
very pronounced (fig. 309), serving greatly to increase the
surface of the olfactory epithelium. At a very early stage the
olfactory nerve attaches itself to the olfactory epithelium.

In Petromyzon the olfactory organ arises as an unpaired thickening of
the epiblast, which in the just hatched larva forms a shallow pit, on the
ventral side of the head, immediately in front of the mouth. This pit
rapidly deepens, and soon extends itself backwards nearly as far as the
infundibulum (fig. 310, 0!}. By the development of the upper lip the opening
of the olfactory pit is gradually carried to the dorsal surface of the head, and
becomes at the same time narrowed and ciliated (fig. 47, ol). The whole
organ forms an elongated sack, and in later stages becomes nearly divided
by a median fold into two halves.

It is probable that the unpaired condition of the olfactory organ in the
Lamprey has arisen from the fusion of two pits into one ; there is however
no evidence of this in the early development ; but the division of the sack
into two halves by a median fold may be regarded as an indication of such a
paired character in the later stages.

In Myxine the olfactory organ communicates with the mouth through
the palate, but the meaning of this communication, which does not appear
to be of the same nature as the communication between the olfactory pits
and the mouth by the posterior nares in the higher types, is not known.

The opening of the olfactory pit does not retain its em-
bryonic characters. In Elasmobranchii and Chimaera it becomes
enclosed by a wall of integument, often deficient on the side of
the mouth, so that there is formed a groove leading from the
nasal pit towards the angle of the mouth. This groove is

534

EXTERNAL AND INTERNAL NARES.

MB.

:u

usually constricted in the middle, and the original single
opening of the nasal sack thus becomes nearly divided into two.
In Teleostei and Ganoids the division of the nasal opening into
two parts becomes complete, but the ventral opening is generally
carried off some distance from the mouth, and placed, by the
growth of the snout, on the upper surface of the head (figs. 54
and 68). In all these instances it is

/ tftM

probable that the dorsal opening of
the nasal sack is homologous with
the external nares, and the ventral
opening with the posterior nares of
higher types. Thus the posterior
nares would in fact seem to be re-
presented in all Fishes by a ventral
part of the opening of the original
nasal pit which either adjoins the
border of the mouth (many Elasmo-
branchii) or is quite separate from
the mouth (Teleostei and Ganoidei).
In the Dipnoi, Amphibia and all the
higher types the oral region becomes
extended so as to enclose the pos-
terior nares, and then each nasal pit
acquires two openings ; viz. one out-
side the mouth, the external nares,
and one within the mouth, the in-
ternal or posterior nares. In the
Dipnoi the two nasal openings are very similar to those in
Ganoidei and Teleostei, but both are placed on the under surface
of the head, the inner one being within the mouth, and the
external one is so close to the outer border of the upper lip that
it also has been considered by some anatomists to lie within the
mouth.

In all the higher types the nasal pits have originally only a
single opening, and the ontogenetic process by which the
posterior nasal opening is formed has been studied in the
Amniota and Amphibia. Amongst the Amniota we may take
the Chick as representing the process in a very simple form. The
general history of the process was first made out by Kolliker.

FIG. 308. SIDE VIEW OF THE
HEAD OF AN EMBRYO CHICK OF
THE THIRD DAY AS AN OPAQUE
OBJECT. (Chromic acid prepara-
tion.)

C.H. cerebral hemispheres ;
F.B. vesicle of third ventricle;
M.B. mid-brain; Cb. cerebellum;
H.B. medulla oblongata; N. na-
sal pit ; ot. auditory vesicle in the
stage of a pit with the opening not
yet closed up; op. optic vesicle,
with /. lens and ch.f. choroidal
fissure.

i F. The first visceral fold ;
above it is seen the superior max-
illary process.

2, 3, 4 F. Second, third and
fourth visceral folds, with the
visceral clefts between them.

OLFACTORY ORGANS.

535

The opening of the nasal pit becomes surrounded by a ridge
except on its oral side. The deficiency of this ridge on the side
of the mouth gives rise to a kind of shallow groove leading from

FIG. 309. SECTION THROUGH THE BRAIN AND OLFACTORY ORGAN OF AN
EMBRYO OF SCYLLIUM. (Modified from figures by Marshall and myself.)

c.h. cerebral hemispheres; oLv. olfactory vesicle; olf, olfactory pit ; Seh. Schnei-
derian folds ; /. olfactory nerve. The reference line has been accidentally taken
through the nerve to the brain.

the nasal pit to the mouth. The ridge enveloping the opening
of the nasal pit next becomes prolonged along the sides of this
groove, especially on its inner one; and at the same time the
superior maxillary process grows forwards so as to bound the lower

ma

FIG. 310. DIAGRAMMATIC VERTICAL SECTION THROUGH THE HEAD OF A
LARVA OF PETROMYZON.

The larva had been hatched three days, and was 4-8 mm. in length. The optic
and auditory vesicles are supposed to be seen through the tissues.

c.h. cerebral hemisphere; th. optic thalamus; in. infundibulum ; pn. pineal gland;
mb. mid-brain; cb. cerebellum; md. medulla oblongata; au.v. auditory vesicle; op.
optic vesicle; ol. olfactory pit; m. mouth; br.c. branchial pouches; th. thyroid
involution; v. ao. ventral aorta ; ht. ventricle of heart ; ch. notochord.

536 EXTERNAL AND INTERNAL NARES.

part of its outer side. The inner and outer ridges, together
with the superior maxillary process, enclose a deep groove, con-
necting the original opening of the nasal pit with the mouth.
The process just described is illustrated by fig. 311 A, and it
may be seen that the ridge on the inner side of the groove
forms the edge of the fronto-nasal process (k).

On the sixth day (Born, 394) the sides of this groove unite
together in the middle, and convert it into a canal open at both
ends — the ventral openings of the canals of the two sides being
placed just within the border of the mouth, and forming the
posterior nares ; while the external openings form the anterior
nares. The upper part of the canal, together with the original

FIG. 311. HEAD OF A CHICK FROM BELOW ON THE SIXTH AND SEVENTH DAYS
OF INCUBATION. (From Huxley.)

/". cerebral vesicles ; a. eye, in which the remains of the choroid slit can still be
seen in A ; g. nasal pits ; k. fronto-nasal process ; /. superior maxillary process ;
i. inferior maxillary process or first visceral arch; 2. second visceral arch; x. first
visceral cleft.

In A 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.

In B the external opening of the mouth has become much constricted, 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
nearly the whole length of the latter.

nasal pit, is alone lined by olfactory epithelium ; the remaining
epithelium of the nasal cavity being indifferent epiblastic epi-

OLFACTORY ORGANS.

537

thelium. Further changes subsequently take place in connection
with the posterior nares, but these are described in the section
dealing with the mouth.

In Mammalia the general formation of the anterior and
posterior nares is the same as in Birds ; but, as shewn by Dursy
and Kolliker, an outgrowth from the inner side of the canal
between the two openings arises at an early period ; and
becoming separate from the posterior nares and provided with a
special opening into the mouth, forms the organ of Jacobson.
The general relations of this organ when fully formed are shewn
in fig. 312.

In Lacertilia the formation of the posterior nares differs in some
particulars from that in Birds (Born). A groove is formed leading from
the primitive nasal pit to the mouth, bordered on its inner side by the
swollen edge of the fronto-nasal process, and on its outer by an outer-
nasal process ; while the superior maxillary process does not assist in
bounding it. On the inner side of the narrowest part of this groove
there is formed a large lateral diverticulum, which is lined by a con-
tinuation of the Schneiderian epithelium, and forms the rudiment of
Jacobson's organ. The nasal groove continues to grow in length, but
soon becomes converted into a canal by the junction of the outer-nasal
process with the fronto-nasal process. This canal is open at both ends :
at its dorsal end is placed the original opening
of the nasal pit, and its ventral opening is
situated within the cavity of the mouth. The
latter forms the primitive posterior nares. The
superior maxillary process soon grows inwards
on the under side of the posterior part of the
nasal passage, and assists in forming its under
wall. This ingrowth of the superior maxillary
process is the rudiment of the hard palate.

On the conversion of the nasal groove into
a closed passage, the opening of Jacobson's
organ into the groove becomes concealed ; and
at a later period Jacobson's organ becomes
completely shut off from the nasal cavity, and
opens into the mouth at the front end of an
elongated groove leading back to the posterior
nares.

In Amphibia the posterior nares are formed
in a manner very different from that of the
Amniota. At an early stage a shallow groove
is formed leading from the nasal pit to the mouth ; but this groove instead

J

FIG. 312. SECTION THROUGH

THE NASAL CAVITY AND JA-

COBSON'S ORGAN. (From
Gegenbaur.)

sn. septum nasi ; en. nasal
cavity ; y. Jacobson's organ ;
d, edge of upper jaw.

538 ORGANS OF THE LATERAL LINE.

of forming the posterior nares soon vanishes, and by the growth of the front
of the head the nasal pits are carried farther away from the mouth.

The actual posterior nares are formed by a perforation in the palate,
opening into the blind end of the original nasal pit.

Considering that the various stages in the formation of the posterior nares
of the Amniota are so many repetitions of the adult states of lower forms, it
may probably be assumed that the mode of formation of the posterior nares
in Amphibia is secondary, as compared with that in the Amniota.

A diverticulum of the front part of the nasal cavity of the Anura is
probably to be regarded as a rudimentary form of Jacobson's organ.

BIBLIOGRAPHY.

(394) G. Born. "Die Nasenhohlen u. d. Thranennasengang d. amnioten
Wirbelthiere." Parts I. and II. Morphologisches Jahrbuch, Bd. V., 1879.

(395) A. Kollicker. " Ueber die Jacobson'schen Organe des Menschen."
Festschrift f. Rienecker, 1877.

(396) A. M. Marshall. "Morphology of the Vertebrate Olfactory Organ."
Quart. Journ. of Micr. Science, Vol. xix., 1879.

Sense organs of the lateral line.

Although I do not propose dealing with the general development of
various sense organs of the skin, there is one set of organs, viz. that of the
lateral line, which, both from its wide extension amongst the Ichthyopsida
and from the similarity of some of its parts to certain organs found amongst
the Chastopoda1, has a great morphological importance.

The organs of the lateral line consist as a rule of canals, partly situated
in the head, and partly in the trunk. These canals open at intervals on
the surface, and their walls contain a series of nerve-endings. The
branches of the canal in the head are innervated for the most part by
the fifth pair, and those of the trunk by the nervus lateralis of the vagus
nerve. There is typically but a single canal in the trunk, the openings
and nerve-endings of which are segmentally arranged.

Two types of development of these organs have been found. One of
these is characteristic of Teleostei ; the other of Elasmobranchii.

In just hatched Teleostei, Schulze (No. 402) found that instead of the
normal canals there was present a series of sense bulbs, projecting freely
on the surface and partly composed of cells with stiff hairs. In most

1 The organs which resemble those of the lateral line are the remarkable sense
organs found by Eisig in the Capitellidse (Mittheil. a. d. ZooL Station zu Neapel,
Vol. I.) ; but I am not inclined to think that there is a true homology between these
organs and the lateral line of Vertebrata. It seems to me probable that the
segmentally arranged optic organs of Polyophthalmus are a special modification of the
more indifferent sense organs of the Capitellidse. The close affinity of these two
types of Chsetopods is favourable to this view.

SENSE ORGANS. 539

cases each bulb is enclosed in a delicate tube open at its free extremity ;
while the bulbs correspond in number with the myotomes. In some
Teleostei (Gobius, Esox, etc.) such sense organs persist through life ; in
most forms however each organ becomes covered by a pair of lobes of the
adjacent tissue, one formed above and the other below it. The two lobes
of each pair then unite and form a tube open at both ends. The linear
series of tubes so formed is the commencement of the adult canal ; while
the primitive sense bulbs form the sensory organs of the tubes. The
adjacent tubes partially unite into a continuous canal, but at their points
of apposition pores are left, which place the canal in communication with
the exterior.

Besides these parts, I have found that there is present in the just hatched
Salmon a linear streak of modified epidermis on the level of the lateral
nerve, and from the analogy of the process described below for Elasmo-
branchii it appears to me probable that these streaks play some part in the
formation of the canal of the lateral line.

In Elasmobranchii (Scyllium) the lateral line is formed as a linear
thickening of the mucous layer of the epidermis. This thickening is at
first very short, but gradually grows backwards, its hinder end forming a
kind of enlarged growing point. The lateral nerve is formed shortly after
the lateral line, and by the time that the lateral line has reached the level
of the anus the lateral nerve has grown back for about two-thirds of that
distance. The lateral nerve would seem to be formed as a branch of the
vagus, but is at first half enclosed in the modified cells of the lateral line
(fig. 275, nl)1, though it soon assumes a deeper position.

A permanent stage, more or less corresponding to the stage just described
in Elasmobranchii, is retained in Chimasra, and Echinorhinus spinosus,
where the lateral line has the form of an open groove (Solger, No. 404).

The epidermic thickening, which forms the lateral line, is converted
into a canal, not as in Teleostei by the folding over of the sides, but by
the formation of a cavity between the mucous and epidermic layers of the
epiblast, and the subsequent enclosure of this cavity by the modified cells
of the mucous layer of the epiblast which constitute the lateral line.
The cavity first appears at the hind end of the organ, and thence extends
forwards.

After its conversion into a canal the lateral line gradually recedes from
the surface ; remaining however connected with the epidermis at a series
of points corresponding with the segments, and at these points perforations
are eventually formed to constitute the segmental apertures of the system.

The manner in which the lumen of the canal is formed in Elasmo-
branchs bears the same relation to the ordinary process of conversion
of a groove into a canal that the formation of the auditory involution

1 Gotte and Semper both hold that the lateral nerve, instead of growing in a
centrifugal manner like other nerves, is directly derived from the epiblast of the
lateral line. For the reasons which prevent me accepting this view I must refer the
reader to my Monograph on Elasmobranch Fishes, pp. 141 — 146.

540 ORGANS OF THE LATERAL LINE.

in Amphibia does to the same process in Birds. In both Elasmobranchii
and Amphibia the mucous layer of the epiblast behaves exactly as does
the whole epiblast in the other types, but is shut off from the surface by
the passive epidermic layer of the epiblast.

The mucous canals of the head and the ampullae are formed from the
mucous layer of the epidermis in a manner very similar to the lateral line ;
but the nerves to them arise as simple branches of the fifth and seventh
nerves, which unite with them at a series of points, but do not follow
their course like the lateral nerve.

It is clear that the canal of the lateral line is secondary, as compared
with the open groove of Chimaera or the segmentally arranged sense bulbs
of young Teleostei ; and it is also clear that the phylogenetic mode of
formation of the canal consisted in the closure of a primitively open groove.
The abbreviation of this process in Elasmobranchii was probably acquired
after the appearance of food-yolk in the egg, and the consequent dis-
appearance of a free larval stage.

While the above points are fairly obvious it does not seem easy to
decide a priori whether a continuous sense groove or isolated sense bulbs
were the primitive structures from which the canals of the lateral line
took their origin. It is equally easy to picture the evolution of the canal of
the lateral line either from (i) a continuous unsegmented sense line, certain
points of which became segmentally differentiated into special sense bulbs,
while the whole subsequently formed a groove and then a canal ; or from
(2) a series of isolated sense bulbs, for each of which a protective groove
was developed ; and from the linear fusion of which a continuous canal
became formed.

From the presence however of a linear streak of modified epidermis
in larval Teleostei, as well as in Elasmobranchii, it appears to me more
probable that a linear sense streak was the primitive structure from which
all the modifications of the lateral line took their origin, and that the
segmentally arranged sense bulbs of Teleostei are secondary differentiations
of this primitive structure.

The, at first sight remarkable, distribution of the vagus nerve to the
lateral line is probably to be explained in connection with the evolution
of this organ. As is indicated both by its innervation from the vagus,
as also from the region where it first becomes developed, the lateral line
was probably originally restricted to the anterior part of the body. As it
became prolonged backwards it naturally carried with it the vagus nerve,
and thus a sensory branch of this nerve has come to innervate a region
which is far beyond the limits of its original distribution.

BIBLIOGRAPHY.

(397) F. M. Balfour. A Monograph onthe development of Elasnwbranch Fishes,
pp. 141 — 146. London, 1878.

(398) H. Eisig. "Die Segmentalorgane cl. Capitelliden." Mitthcil. a. d. zool.
Station zu Neapel> Vol. I. 1879.

BIBLIOGRAPHY. 541

(399) A. Gotte. Entwicklungsgeschichte d. Unke. Leipzig, 1875.

(400) Fr. Leydig. Lehrbuch d. Histologie des Memchen u. d. Thiere. Hamm.

1857-

(401) Fr. Leydig. Neue Beitrdge z. anat. Kenntniss d. Hautdecke u. Haut-
sinnesorgane d. Fische. Halle, 1879.

(402) F. E. Schulze. " Ueb. d. Sinnesorgane d. Seitenlinie bei Fischen und
Amphibien." Archiv f. mikr. Anat., Vol. vi. 1870.

(403) C. Semper. "Das Urogenitalsy stem d. Selachier." Arbeit, a. d. zoo!.-
zoot. Instil. Wiirzburg, Vol. II.

(404) B. Solger. "Neue Untersuchungen zur Anat. d. Seitenorgane d. Fische."
Archiv f. mikr. Anat., Vol. xvn. and xvm. 1879 an<* !88o.

CHAPTER XVIII.

THE NOTOCHORD, THE VERTEBRAL COLUMN, THE
RIBS AND THE STERNUM.

INTRODUCTION.

AMONGST the products of that part of the mesoblast which
constitutes the connective tissue of the body special prominence
must be given to the skeleton of the Vertebrata, from its impor-
tance in relation to numerous phylogenetic and morphological
problems.

The development of the skeleton is however so large a
subject that it cannot be satisfactorily dealt with except in a
special treatise devoted to it ; and the following description must
be regarded as a mere sketch, from which detail has been as far
as possible excluded.

In the lowest Chordata the sole structure present, which
deserves to be called a skeleton, is the notochord. Although
the notochord often persists as an important organ in the true
Vertebrata, yet there are always added to it various skeletal
structures developed in the mesoblast. Before entering into a
systematic description of these, it will be convenient to say a
few words as to the general characters of the skeleton.

Two elements, distinct both in their genesis and structure,
are to be recognized in the skeleton. The one, forming the true
primitive internal skeleton or endoskeleton, is imbedded within
the muscles and is originally formed in cartilage. In many
instances it retains a cartilaginous consistency through life, but
in the majority of cases it becomes gradually ossified, and

NOTOCHORD AND VERTEBRAL COLUMN. 543

converted into true bone. Bones so formed are known as
cartilage bones.

The other element is originally formed by the fusion of the
ossified bases of the dermal placoid scales already described in
Chapter xiv., or by the fusion of the ossified bases of teeth
situated in the mucous membrane of the mouth. In both
instances the plates of bone so formed may lose the teeth or
spines with which they were in the first instance covered, either
by absorption in the individual, or phylogenetically by their
gradually ceasing to be developed. The plates of bone, which
originated by the above process, become in higher types directly
developed in the connective tissue beneath the skin ; and
gradually acquire a deeper situation, and are finally so inti-
mately interlocked with parts of the true internal skeleton, that
the two sets of elements can only be distinguished by the fact
of the one set ossifying in cartilage and the other in membrane.

It seems probable that in the Reptilia, and possibly the
extinct Amphibia, dermal bones have originated in the skin
without the intervention of superjacent spinous structures.

In cases where a membra nebone, as the dermal ossifica-
tions are usually called, overlies a part of the cartilage, it may
set up ossification in the latter, and the cartilage bone and mem-
brane bone may become so intimately fused as to be quite in-
separable. It seems probable that in cases of this kind the
compound bone may in the course of further evolution entirely
lose either its cartilaginous element or its membranous element ;
so that cases occasionally occur where the development of a
bone ceases to be an absolutely safe guide to its evolution.

As to the processes which take place in the ossification of
cartilage there is still much to be made out. Two processes are
often distinguished, viz. (i) a process known as ectostosis, in
which the ossification takes place in the perichondrium, and
either simply surrounds or gradually replaces the cartilage, and
(2) a process known as endostosis, where the ossification actually
takes place between the cartilage cells. It seems probable
however (Gegenbaur, Vrolik) that there is no sharp line to be
drawn between these two processes ; but that the ossification
almost always starts from the perichondrium. In the higher
types, as a rule, the vessels of the perichondrium extend into

544 MEMBRANE BONES AND CARTILAGE BONES.

the cartilage, and the ossification takes place around these
vessels within the cartilage; but in the lower types (Pisces, Am-
phibia) ossification is often entirely confined to the perichon-
drium ; and the cartilage is simply absorbed.

The regions where ossification first sets in are known as
centres of ossification; and from these centres the ossification
spreads outwards. There may be one or more centres for a
bone.

The actual causes which in the first instance gave rise to
particular centres of ossification, or to the ossification of par-
ticular parts of the cartilage, are but little understood ; nor have
we as yet any satisfactory criterion for determining the value to
be attached to the number and position of centres of ossification.
In some instances such centres appear to have an important
morphological significance, and in other instances they would
seem to be determined by the size of the cartilage about to be
ossified.

There is no doubt that the membrane bones and cartilage bones can
as a rule be easily distinguished by their mode of development ; but it is
by no means certain that this is always the case. It is necessarily very
difficult to establish the homology between bones, which develop in one
type from membrane and in another type from cartilage ; but there are
without doubt certain instances in. which the homology between two bones
would be unhesitatingly admitted were it not for the difference in their
development. The most difficult cases of this kind are connected with the
shoulder-girdle.

The possible sources of confusion in the development of bones are
obviously two. (i) A cartilage bone by origin may directly ossify in mem-
brane, without the previous development of cartilage, and (2) a membrane
bone may in the first instance be formed in cartilage.

The occurrence of the first of these is much more easy to admit than
that of the second ; and there can be little doubt that it sometimes takes
place. In a large number of cases it would moreover cause no serious
difficulty to the morphologist.

BIBLIOGRAPHY of the origin of the Skeleton.

(405) C. Gegenbaur. " Ueb. primare u. secundare Knochenbildung mit be-
sonderer Beziehung auf d. Lehre von dem Primordialcranium." Jcnaischc Zcit-
schrifl, Vol. III. 1867.

(406) O. Hertwig. " Ueber Ban u. Entwicklung d. Placoidschuppen u. d.
Ziihne d. Selachicr." Jenaische Zeitsckrift, Vol. viu. 1874.

NOTOCHORD AND VERTEBRAL COLUMN.

545

(407) O. Hertwig. " Ueb. cl. Zahnsystem d. Amphibien u. seine Bedeutung
f. d. Genese d. Skelets d. Mundhohle." Archiv f. mikr. Anat., Vol. xi. Supple-
mentheft, 1874.

(408) O. Hertwig. " Ueber d. Hautskelet cl. Fische." Morphol. Jahrbmh,
Vol. II. 1876. (Siluroiden u. Acipenseriden.)

(409) O. Hertwig. "Ueber d. Hautskelet d. Fische (Lepidosteus u. Polyp-
terus)." Morph. Jahrbnch, Vol. v. 1879.

(410) A. Kolliker. " Allgemeine Betrachtungen iib. die Entstehung d. knocher-
nen Schadels d. Wirbelthiere. " Berichte r. d. kijnigl. zoot. Anstalt z. Wiirzburg,
1849.

(411) Fr. Leydig. " Histologische Bemerkungen lib. d. Polypterus bichir."
Zeit.f. wiss. Zool., Vol. v. 1858.

(412) H. Miiller. " Ueber d. Entwick. d. Knochensubstanz nebst Bemerkun-
gen, etc." Zeit. f. wiss. ZooL, Vol. ix. 1859.

(413) Williamson. "On the structure and development of the Scales and
Bones of Fishes." Phil. Trans., 1851.

(414) Vrolik. " Studien lib. d. Verknocherung u. die Knochen d. Schadels d.
Teleostier. " Niederliindisches Archiv f. Zoologie, Vol. I.

NotocJtord and Vertebral column.

The primitive axial skeleton of the Chordata consists of the
notochord and its sheath. It persists as such in the adult in
Amphioxus, and constitutes, in embryos of all Vertebrata, for a
considerable period of their early embryonic life, the sole repre-
sentative of the axial skeleton.

The Notochord. The early formation of the notochord
has already been described in
detail (pp. 292 — 300). It is
developed, in most if not all
cases, as an axial differ-
entiation of the hypoblast,
and forms at first a solid
cord of cells, without a
sheath, placed between the
nervous system and the dor-
sal wall of the alimentary
tract, and extending from
the base of the front of the

mid-brain to the end of the
tail. The section in the
region of the brain will be
dealt with by itself. That
H. HI.

FIG. 313. HORIZONTAL SECTION THROUGH
THE TRUNK OF AN EMBRYO OF SCYLLIUM
CONSIDERABLY YOUNGER THAN F IN FIG. 28.

The section is taken at the level of the
notochord, and shews the separation of the
cells to form the vertebral bodies from the
muscle-plates.

ch. notochord ; ep. epiblast ; Vr. rudiment
of vertebral body; ;;//. muscle-plate; mp'.
portion of muscle-plate already differentiated
into longitudinal muscles.

35

546

NOTOCHORD.

in the trunk forms the basis round which the vertebral column
is moulded.

The early histological changes in the cells of the notochord
are approximately the same in all the Craniata. There is
formed by the superficial cells of the notochord a delicate sheath,
which soon thickens, and becomes a well-
defined structure. Vacuoles (one or more to
each cell) are formed in the cells of the
notochord, which enlarge till the whole noto-
chord becomes almost entirely formed of
large vacuoles separated by membranous
septa which form a complete sponge-like
reticulum 'fig. 313). In the Ichthyopsida
most of the protoplasm with the nuclei is
carried to the periphery, where it forms a
special nucleated layer sometimes divided
into definite epithelial-like cells (fig. 314),
while in the meshes of the reticulum a few
nuclei surrounded by a little protoplasm still
remain. In the Amniotic Vertebrata, pro-
bably owing to the early atrophy of the
notochord, the distribution of the nuclei in
the spaces of the mesh-work remains fairly
uniform.

FIG. 314. SECTION
THROUGH THE SPINAL
COLUMN OF A YOUNG
SALMON. (From Ge-
genbaur.)

cs. sheath of noto-
chord ; k. neural arch ;
k'. haemal arch; m.
spinal cord; a. dorsal
aorta ; z'. cardinal
veins.

In the early stages of development the spaces in the notochordal sponge-
work, each containing a nucleus and protoplasm, probably represent cells.
In the types in which the notochord persists in the adult the mesh-work
becomes highly complicated, and then forms a peculiar reticulum filled with
gelatinous material, the spaces in which do not indicate the outlines of
definite cells (figs. 315 and 318).

Around the sheath of the notochord there is formed in the
Cyclostomata, Ganoidei, Elasmobranchii and Teleostei an elastic
membrane usually known as the membrana elastica externa.

In most Vertebrates the notochord and its sheath either
atrophy completely or become a relatively unimportant part of
the axial skeleton; but in the Cyclostomata (fig. 315) and in the
Selachioidean Ganoids (Acipenser, etc.) they persist as the
sole representative of the true vertebral axis. The sheath becomes
very much thickened; and on the membrana elastica covering

NOTOCHORD AND VERTEBRAL COLUMN.

547

Ch

it the vertebral arches directly rest. In Klasmobranchii the
sheath of the notochord undergoes a more complicated series of
changes, which result first of all in the formation of a definite
unsegmented cartilaginous tube1
round the notochord, and subse-
quently (in most forms) in the
formation of true vertebral bodies.
Between the membrana elastica
externa and the sheath of the
notochord a layer of cells becomes
interposed (fig. 316, n}, which lie
in a matrix not sharply separated
from the sheath of the notochord.
The cells which form this layer
appear to be derived from a special
investment of the notochord, and
to have penetrated through the
membrana elastica externa to
reach their final situation. The
layer with these cells soon increases 7/> cardmal vems-
in thickness, and forms a continuous unsegmented tube of
fibrous tissue with flattened concentrically arranged nuclei (fig.
317, Vb}. Externally is placed cf,
the membrana elastica externa
(met}, while within is the cuticular
sheath of the notochord. This
tube is the cartilaginous tube
spoken of above and is known as
the cartilaginous sheath of
the notochord.

FIG. 315. SECTION THROUGH
THE VERTEBRAL COLUMN OF AMMO-
CCETES. (From Gegenbaur.)

Ch. notochord ; c s. notochordal
sheath ; m. spinal cord ; a. aorta ;

^ \

FIG. 316. LONGITUDINAL SEC-
TION THROUGH A SMALL PART OF
THE NOTOCHORD AND ADJOINING
PARTS OF A SCYLLIUM EMBRYO, AT
THE TIME OF THE FIRST FORMA-
TION OF THE CARTILAGINOUS
SHEATH.

ch. notochord; sc. sheath of noto-
chord; n. nuclei of cartilaginous
sheath; me.e. membrana elastica
externa.

The exact origin of the cartilaginous
tube just described is a question of fun-
damental importance with reference to
the origin of the vertebral column and
the homologies of its constituent parts ;
but is by no means easy to settle. In the account of the subject in my
memoir on Elasmobranch Fishes I held with Gegenbaur that it arose from

1 This tube consists of a peculiar form of fibrous tissue rather than true cartilage,
though part of it subsequently becomes hyaline cartilage.

35—2

548

SHEATH OF THE NOTOCHORD.

a layer of cells outside the sheath of the notochord, on the exterior of which
the membrana elastica externa was subsequently formed. To this view
Gotte (No. 419) also gave his adhesion. Schneider has since (No. 429)
stated that this is not the case, but that, as described above, the membrana
elastica externa is formed before the layer of cartilage. I have since worked
over this subject again, and am on the whole inclined to adopt Schneider's
correction.

It follows from the above description that the cartilaginous
tube in question is an essential part of the sheath of the noto-
chord, and that it is to some extent homologous with the noto-
chordal sheath of the Sturgeon and the Lamprey, and not an
entirely new formation.

This sheath forms the basis of the centra of the future
vertebrae. In a few adult forms, i.e. Chimaera and the Dipnoi, it

FIG. 317. TRANSVERSE SECTION THROUGH THE VENTRAL PART OF THE
NOTOCHORD AND ADJOINING STRUCTURES OF AN ADVANCED SCYLLIUM EMBRYO
AT THE ROOT OF THE TAIL.

Vb. cartilaginous sheath of the notochord ; ha. hasmal arch ; vp. process to which
the rib is articulated ; mcl. membrana elastica externa ; ch. notochord ; ao. aorta ;
. caudal vein.

retains its primitive condition, except that in Chimaera there
are present delicate ossified rings more numerous than the
arches ; while in the Notidani, Laemargi and Echinorhini the

NOTOCHORD AND VERTEBRAL COLUMN. 549

indications of vertebrae are imperfectly marked out. The further
history of this sheath in the forms in which true vertebrae are
formed can only be dealt with in connection with the formation
of the vertebral arches.

In Teleostei there is present, as in Elasmobranchii, an elastica externa,
and an inner notochordal sheath. The elastica externa contains, according
to Gotte, cells. These cells, if present, are however very difficult to make
out, but in any case the so-called elastica externa appears to correspond with
the cartilaginous sheath of Elasmobranchii together with its enveloping
elastica, since ossification, when it sets in, occurs in this layer. The sheath
within becomes unusually thick.

In the Amphibia and in the Amniota no membrane is
present which can be identified with the membrana elastica
externa of the Elasmobranchii, Teleostei, etc. In Amphibia
(Gotte) there is formed round the notochord a cellular sheath,
which has very much the relations of the cartilaginous tube
around the notochord of Elasmobranchii, and is developed in
the same way from the perichordal connective tissue cells. It
is only necessary to suppose that the rnembrana elastica externa
has ceased to be developed (which in view of its extreme delicacy
and unimportant function in Elasmobranchii is not difficult to
do) and this cellular sheath would then obviously be homologous
with the cartilaginous tube in question. In the Amniota an
external sheath of the notochord cannot be traced as a distinct
structure, but the connective tissue surrounding the notochord
and spinal cord is simply differentiated into the vertebral bodies
and vertebral arches.

Vertebral arches and Vertebral bodies.

Cyclostomata. The Cyclostomata are the most primitive
forms in which true vertebral arches are present. Their ontogeny
in this group has not been satisfactorily worked out. It is
however noticeable in connection with them that they form for
the most part isolated pieces of cartilage, the segmental
arrangement of which is only imperfect.

Elasmobranchii. In the Elasmobranchii the cells forming
the vertebral arches are derived from the splanchnic layer of the
mesoblastic somites. They have at first the same segmentation

55O NEURAL AND H^MAL ARCHES.

as the somites (fig. 313, Vr), but this segmentation is soon lost,
and there is formed round the notochord a continuous sheath of
embryonic connective tissue cells, which gives rise to the arches
of the vertebrae, the tissue forming the dura mater, the perichon-
drium, and the general investing connective tissue.

The changes which next follow result in what has been
known since Remak as the secondary segmentation of the
vertebral column. This segmentation, which occurs in all
Vertebrata with true vertebrae, is essentially the segmentation
of the continuous investment of the notochord and spinal cord
into vertebral bodies and vertebral arches. It does not however
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 myotome shall be
capable of acting on two vertebrae ; and this condition can only be fulfilled
when the myotomes are opposite the intervals between the vertebrae. For
this reason, when the vertebrae became formed, their centres were opposite
not the middle of the myotomes 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 (fig. 313) 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 segmented 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.

During the stage represented in fig. 28 g, and somewhat
before the cartilaginous sheath of the notochord is formed, there
appear four special concentrations of the mesoblastic tissue
adjoining the notochord, two of them dorsal (neural) and two of
them ventral (haemal). They are not segmented, and form four
ridges, seated on the sides of the notochord. They are united

NOTOCHORD AND VERTEBRAL COLUMN.

551

with each other by a delicate layer of tissue, and constitute the
substance in which the neural and haemal arches subsequently
become differentiated.

At about the time when the first traces of the cartilaginous
sheath of the notochord arise, dif-
ferentiations take place in the
neural and haemal ridges. In the
neural ridge two sets of arches are
formed for each myotome, one
resting on the cartilaginous sheath
of the notochord in the region
which will afterwards form the cen-
trum of a vertebra, and constituting
a true neural arch ; and a second
separate from the cartilaginous
sheath, forming an intercalated
piece1. Both of them soon become
hyaline cartilage.

There is a considerable portion
of the original tissue of the neural
ridge, especially in the immediate
neighbourhood of the notochord,
which is not employed in the for-
mation of the neural arches. This
tissue has a fibrous character and
becomes converted into the peri-
chondrium and other parts.

The haemal arches are formed
from the haemal ridge in precisely
the same way as the neural arches,
but interhsemal intercalated pieces
are often present. In the region
of the tail the haemal arches are
continued into ventral processes
which meet below, enclosing the aorta and caudal veins.

1 The presence of intercalated pieces in the neural arch system of Elasmobranchii,
Chimaera, etc. is probably not the indication of an highly differentiated type of
neural arch, but of a transitional type between an imperfect investment of the spinal
cord by isolated cartilaginous bars, and a complete system of neural arches like that
in the higher Vertebrata.

FIG. 318. SECTION THROUGH
THE VERTEBRAL COLUMN OF AN
ADVANCED EMBRYO OF SCYLLIUM
IN THE REGION OF THE TAIL.

na. neural arch ; ha. haemal
arch ; ch. notochord ; sh. inner
sheath of notochord ; ne. membrana
elastica externa.

552 NEURAL AND H^iMAL ARCIIKS.

Since primitively the postanal gut was placed between the
aorta and the caudal vein, the haemal arches potentially invest
a caudal section of the body cavity. In the trunk region they
do not meet ventrally, but give support to the ribs. The
structures just described are shewn in section in fig. 318, in
which the neural (110) and haemal (ha) arches are shewn resting
upon the cartilaginous sheath of the notochord.

While these changes are being effected in the arches the
cartilaginous sheath of the notochord undergoes important differ-
entiations. In the vertebral regions opposite the origin of the
neural and haemal arches (fig. 318) its outer part becomes
hyaline cartilage, while the inner parts adjoining the notochord
undergo a somewhat different development, the notochord in this
part becomes at the same time somewhat constricted. In the
intervertebral regions the cartilaginous sheath of the notochord
becomes more definitely fibrous, while the notochord is in no
way constricted. A diagrammatic longitudinal section through
the vertebral column, while these changes are being effected, is
shewn in fig. 320 B.

These processes are soon carried further. The notochord
within the vertebral body becomes gradually constricted, espe-
cially in the median plane, till it is here reduced to a fibrous
band, which gradually enlarges in either direction till it reaches
its maximum thickness in the median plane of the intervertebral
region. The hyaline cartilage of the vertebral region forms a
vertebral body in which calcification may to some extent take
place. The cartilage of the base of the arches gradually spreads
over it, and on the absorption of the membrana elastica externa,
which usually takes place long before the adult state is reached,
the arch tissue becomes indistinguishably fused with that of the
vertebral bodies, so that the latter are compound structures,
partly formed of the primitive cartilaginous sheath, and partly
of the tissue of the bases of the neural and haemal arches.
Owing to the beaded structure of the notochord the verte-
bral bodies take of necessity a biconcave hourglass-shaped
form.

The intervertebral regions of the primitive sheath of the noto-
chord form fibrous intervertebral ligaments enclosing the uncon-
stricted intervertebral sections of the notochord.

NOTOCHORD AND VERTKBKAL COLUMN. 553

A peculiar fact may here be noticed with reference to the formation
of the vertebral bodies in the tail of Scyllium, Raja, and possibly other
forms, viz. that there are double as many -vertebral bodies as there are
myotomes and spinal nerves. This is not due to a secondary segmentation
of the vertebras but, as I have satisfied myself by a study of the develop-
ment, takes place when the vertebral bodies first become differentiated.
The possibility of such a relation of parts is probably to be explained by
the fact that the segmentation of the vertebral column arose subsequently
to that of the nerves and myotomes.

Ganoidei. In Acipenser and other cartilaginous Ganoids
the haemal and neural arches are formed as in Elasmobranchii,
and rest upon the outer sheath of the notochord. Since however
the sheath of the notochord is never differentiated into distinct
vertebrae, this primitive condition is retained through life.

Teleostei. In Teleostei the formation of the vertebral arches and
bodies takes place in a manner, which can be reduced, except in certain
minor points, to the same type as that of Elasmobranchii.

There are early formed (fig. 314 k and k] neural and haemal arches
resting upon the outer sheath of the notochord. The latter structure,
which, as mentioned on p. 549, corresponds to the cartilaginous sheath
of the notochord of Elasmobranchii, soon becomes divided into vertebral
and intervertebral regions. In the former ossification directly sets in
without the sheath acquiring the character of hyaline cartilage (Gotte, 419).
The latter forms the fibrous intervertebral ligaments. The notochord
exhibits vertebral constrictions.

The ossified outer sheath of the notochord forms but a small part of
the permanent vertebrae. The remainder is derived partly from an ossifi-
cation of the connective tissue surrounding the sheath, and partly from
the bases of the arches, which do not spread round the primitive vertebral
bodies as in Elasmobranchii. The ossifications in the tissue surrounding
the sheath usually (fig. 319) take the form of a cross, while the bases of
the arches (k and k'} remain as four cartilaginous radii between the limbs
of the osseous cross. In some instances the bases of the arches also become
ossified, and are then with difficulty distinguishable from the other parts
of the secondary vertebral body. The parts of the arches outside the
vertebral bodies are for the most part ossified (fig. 319). In correlation
with the vertebral constrictions of the notochord the vertebral bodies are
biconcave.

Amphibia. Of the forms of Amphibia so far studied
embryologically the Salamandridae present the most primitive
type of formation of the vertebral column.

It has already been stated that in Amphibia there is present

554

VERTEBRAL COLUMN OF AMPHIBIA.

around the notochord a cellular sheath, equivalent to the
cartilaginous sheath of Elasmobranchii. In the tissue on the
dorsal side of this sheath a series of cartilaginous processes
becomes formed. These processes are the commencing neural
arches ; and they rest on the cellular sheath of the notochord
opposite the middle of the vertebral regions.

A superficial osseous layer becomes very early formed in
each vertebral region of the cellular
sheath ; while in each of the inter-
vertebral regions, which are con-
siderably shorter than the vertebral,
there is developed a ring-like carti-
laginous thickening of the sheath,
which projects inwards so as to
constrict the notochord. At a
period before this thickening has
attained considerable dimensions
the notochord becomes sufficiently

constricted in the centre of each FlG- 319- VERTICAL SECTION-
THROUGH THE MIDDLE OF A VER-

vertebral region to give a biconcave TEBRA OF Esox LUCIUS (PIKE).
form to the vertebrae for a very <From Gegenbaur.)
short period of fcetal life.

The stage with biconcave vertebrae is
retained through life in the Perennibran-
chiata and Gymnophiona.

ch. notochord ; cs. notochordal
sheath; /•. and K. cartilaginous
tissue of the neural and haemal
arches ; h. osseous hremal process ;
n. spinal canal.

The chief peculiarity which distinguishes the later history of
their vertebral column from that of fishes consists in the
immense development of the intervertebral thickenings just
mentioned, which increase to such an extent as to reduce the
notochord, where it passes through them, to a mere band ; while
the cartilage of which they are composed becomes differentiated
into two regions, one belonging to the vertebra in front, the
other to that behind, the hinder one being convex, and the
anterior concave. The two parts are not however absolutely
separated from each other.

By these changes each vertebra comes to be composed of (i) a
thin osseous somewhat hourglass-shaped cylinder with a dilated
portion of the notochord in its centre, and (2 and 3) of two

NOTOCHORD AND VERTEBRAL COLUMN.

555

halves of two intervertebral cartilages, viz. an anterior convex
half and a posterior concave half. The vertebrae thus come to
be opisthoccelous. A longitudinal section through the vertebral
column at this stage is diagrammatically shewn in fig. 320 C.

To the centre of each of these vertebrae the neural arches,
the origin of which was described above, become in the
meantime firmly attached ; and grow obliquely upwards and
A B CUE

FIG. 320. DIAGRAM REPRESENTING THE MODE OF DEVELOPMENT OK THE

VERTEBRA IN THE DIFFERENT TYPES. (From Gegenbaur.)

A. Ideal type in which distinct vertebrae are not established.

B. Type of Pisces with vertebral constrictions of the notochord.

C. Amphibian type, with intervertebral constrictions of the notochord by the
intervertebral parts of the cellular sheath.

D. Intervertebral constriction of the notochord as effected in Reptilia and Aves.

E. Vertebral constriction of the notochord as effected in Mammalia, the interver-
tebral parts of the cartilaginous sheath being converted into intervertebral ligaments.

c . notochord ; cs. cuticular sheath of notochord ; s. cartilaginous sheath ; v. verte-
bral regions ; iv. intervertebral regions ; g, intervertebral joints.

backwards, so as to meet and unite above the spinal cord. The
transverse processes of the vertebrae would seem (Pick) to be
developed independently of the arches, though they very soon
fuse with them. According to Gotte the transverse processes
are double in the trunk, there being two pairs, one vertically
above the other for each vertebra. The pair on each side
eventually fuse together.

In the tail haemal arches are formed, which are similar in
their mode of development to the neural arches.

The unconstricted portion of the notochord, which persists in
each vertebra, becomes in part converted into cartilage.

556 VKRTKIJRAL COLUMN OF THE AMNIOTA.

Anura. In the Anura the process of formation of the vertebral
column is essentially the same as that in the Salamandridte. Two types may
however be observed. One of these occurs in the majority of the Anura,
and mainly differs from that in Salamandra in (i) the earlier fusion of the
arches with the cellular sheath of the notochord ; (2) the more rapid growth
of the intervertebral thickenings of the cellular sheath, which results in the
early and complete obliteration of the intervertebral parts of the notochord ;
(3) the complete division of these intervertebral thickenings into anterior
and posterior portions, which unite with and form the articular surfaces of
two contiguous vertebras. The vertebrae are moreover proccelous instead
of being opisthoccelous.

The unconstricted vertebral sections of the notochord always persist till
the ossification of the vertebras has taken place. In some forms they
remain through life (Rana), while in other cases they eventually either
wholly or partially disappear.

The second type of vertebral development is found in Bombinator,
Pseudis, Pipa, and Pelobates. In these genera the formation of the
vertebra takes place almost entirely on the dorsal side of the notochord ;
so that the latter forms a band on the ventral side of the vertebral column.
In other respects the history of the vertebral column is the same in the two
cases ; the vertebral unconstricted parts of the notochord appear however
to become in part converted into cartilage. The type of formation of
the vertebral column in these genera has been distinguished as epichordal
in contradistinction to the more normal or perichordal type.

Amniota. In the Amniota all trace of a distinction between
a cellular notochord sheath and an arch tissue is lost, and the
two are developed together as a continuous whole forming an
unsegmented tube round the notochord, with a neural ridge
which does not at first nearly invest the neural cord. This tube
becomes differentiated, in the manner already described for other
types, into (i) vertebral regions with true arches, and (2) inter-
vertebral regions.

Reptilia. In Reptilia (Gegenbaur, No. 416) a cartilaginous
tube is formed round the notochord, which is continuous with
the cartilaginous neural arches. The latter are placed in the
vertebral regions, and in these regions ossification very early sets
in, while the notochord remains relatively unconstricted. In the
intervertebral regions the cartilage becomes thickened, as in
Amphibia, and gradually constricts the notochord. The carti-
lage in each of the intervertebral regions soon becomes divided
into two parts which form the articular faces of two contiguous
vertebrae.

NOTOCHORD AND VERTEBRAL COLUMN. 557

The general character of the vertebral column on the comple-
tion of these changes is shewn in fig. 320 D. The later changes
are relatively unimportant. The constricted intervertebral
sections of the notochord rapidly disappear, while the vertebral
sections become partially converted into cartilage, and only
cease to be distinguishable at a considerably later period.

The ossification extends from the bodies of the vertebrae into
the arches and into the articular surfaces, so that the whole
vertebrae eventually become ossified.

The Ascalabotae (Geckos) present an exceptional type of vertebral
column which has many of the characters of a developmental stage in
other Lizards. The body of the vertebra is formed of a slightly hourglass-
shaped osseous tube, united with adjoining vertebras by a short inter-
vertebral cartilage. There is a persistent and continuous notochord which,
owing to the small development of the intervertebral cartilages, is narrower
in the vertebral than in the intervertebral regions.

Aves. In Birds the cellular tube formed round the notochord
is far thicker than in the Reptilia. It is continuous in the
regions of the future vertebrae with neural arches, which do not
at first nearly enclose the spinal cord.

On about the fifth day, in the case of the chick, it becomes
differentiated into vertebral regions opposite the attachments of
the neural arches, and intervertebral regions between them ; the
two sets of regions being only distinguished by their histological
characters. Very shortly afterwards each intervertebral region
becomes segmented into two parts, which respectively attach
themselves to the contiguous vertebral regions. A part of each
intervertebral region, immediately adjoining the notochord, does
not however undergo this division, and afterwards gives rise to
the ligamentum suspensorium.

The notochord during these changes at first remains
indifferent, but subsequently, on about the seventh day in the
chick, a slight constriction of each vertebral region takes place ;
so that the vertebrae have temporarily, as they have also in
Amphibia, a biconcave form which repeats the permanent
condition of most fishes. By the ninth and tenth days, however,
this condition has completely disappeared, and in all the inter-
vertebral portions the notochord has become distinctly con-
stricted, and at the same time in each vertebral portion there

558

VERTEBRAL COLUMN OF MAMMALIA.

have also appeared two constrictions of the notochord giving
rise to a central and to two terminal enlargements.

On the twelfth day the ossification of the cartilaginous centra
commences.

The first vertebra to ossify is the second or third cervical,
and the ossification gradually extends to those behind. It does
not commence in the arches till somewhat later than in the
bodies. 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-fcetal 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 intervertebral
portions of the notochord may
long be detected in the ligamenta
suspensoria.

Schwarck (No. 420) states that in both
the intervertebral and the vertebral regions,
though less conspicuously in the former,
the cartilage is divided into two layers, an
inner and an outer. He holds that the
inner layer corresponds to the cartilaginous
notochordal sheath of the lower types,
and the outer to the arch tissue. Ossi-
fication (Gegenbaur) of the centra appears
in a special inner layer of cartilage, which
is probably the same as the inner layer of
the earlier stage, though this point has
not been definitely established.

FIG. 321. LONGITUDINAL SEC-
TION THROUGH THE VERTEBRAL
COLUMN OF AN EIGHT WEEKS'
HUMAN EMBRYO IN THE THORACIC
REGION. (From Kolliker.)

v. cartilaginous vertebral body ;
//. intervertebral ligament; ch, noto-
chord.

Mammalia. The early development of the perichordal
cartilaginous tube and rudimentary neural arches is almost the
same in Mammals as in Birds. The differentiation into vertebral
and intervertebral regions is the same in both groups ; but
instead of becoming divided as in Reptilia and Birds into two
segments attached to two adjoining vertebrae, the intervertebral
regions become in Mammals wholly converted into the interverte-
bral ligaments (fig. 322 /*'). There are three centres of ossifica-
tions for each vertebra, two in the arch and one in the centrum.

NOTOCHORD AND VERTEBRAL COLUMN.

559

The fate of the notochord is in important respects different
from that in Birds. It is first constricted in the centre of the
vertebra (figs. 320 E and 321) and disappears there shortly after
the ossification ; while in the intervertebral regions it remains
relatively unconstricted (figs. 320 E, 321 and 322 c] and after

FlG. 3-22. LONGITUDINAL SECTION THROUGH THE INTERVERTEBRAL LIGAMENT
AND ADJACENT PARTS OF TWO VERTEBRA FROM THE THORACIC REGION OF AN
ADVANCED EMBRYO OF A SHEEP. (From Kolliker.)

la. ligamentum longitudinale anterius ; lp. ligamentum long, posterius ; li. liga-
mentum intervertebrale ; k, k'. epiphysis of vertebra ; w. and iv' '. anterior and
posterior vertebrae ; c. intervertebral dilatation of notochord ; c'. and c'. vertebral
dilatation of notochord.

undergoing certain histological changes remains through life as
part of the nucleus pulposus in the axis of the invertebral ligaments1.
There is also a slight swelling of the notochord near the two
extremities of each vertebra (fig. 322 c' and c"}. In the per-
sistent vertebral constriction of the notochord Mammals retain
a more primitive and piscine mode of formation of the vertebral
column than the majority either of the Reptilia or Amphibia.

1 This view was first put forward by Lushka, and his surmises have been con-
firmed by Kolliker and other embryologists. Leboucq (No. 424) however holds that_
the cells of the notochord in the intervertebral regions fuse with those of the
adjoining tissue ; and Dursy and others deny that the nucleus pulposus is derived
from the notochord.

560 BIBLIOGRAPHY.

BIBLIOGRAPHY of Notochord and Vertebral column.

(415) Cartier. "Beitrage zur Entwicklungsgeschichte der Wirbelsaule." Zeit-
schrift furwiss. ZooL, Bd. xxv. Suppl. 1875.

(416) C. Gegenbaur. Untersuchungen zur vergleichenden Anatomic der Wir-
belsaule der Amphibien nnd Reptilien. Leipzig, 1862.

(417) C. Gegenbaur. " Ueber die Entwickelung der Wirbelsaule des Lepi-
dosteus mil vergleichend anatomischen Bemerkungen." Jenaische Zeitschrift, Bd. in.
1863.

(418) C. Gegenbaur. " Ueb. d. Skeletgewebe d. Cyclostomen." Jenaische
Zeitschrift, Vol. v. 1870.

(419) Al. Gotte. "Beitrage zur vergleich. Morphol. des Skeletsystems d.
Wirbelthicre. " II. "Die Wirbelsaule u. ihre Anhange." Archiv f. mikr. Anat., Vol.
xv. 1878 (Cyclostomen, Ganoiden, Plagiostomen, Chimaera), and Vol. xvi. 1879
(Teleostier).

(420) Hasse und Schwarck. "Studien zur vergleichenden Anatomic der
Wirbelsaule u. s. w." Hasse, Anatomische Studien, 1872.

(421) C. Hasse. Das natiirliche System d. Elasmobranchier auf Grundlage d.
Bau. u. d. Entwick. ihrer Wirbelsaule. Jena, 1879.

(422) A. Kolliker. " Ueber die Beziehungen der Chorda dorsalis zur Bildung
der Wirbel der Selachier und einiger anderen Fische." Verhandlungen der physical,
medic in. Gesellschaft in Wiirzburg, Bd. X.

(423) A. Kolliker. " Weitere Beobachtungen iiber die Wirbel der Selachier
insbesondere iiber die Wirbel der Lamnoidei." Abhandlungen der senkenbergischen
naturforschenden Gesellschaft in Frankfurt, Bd. v.

(424) H. Leboucq. " Recherches s. 1. mode de disparition de la corde dorsale
chez les vertebres superieurs." Archives de Biologie, Vol. I. 1 880.

(425) Fr. Leydig. Anatomisch-histologische Untersuchungen iiber Fische nnd
Reptilien. Berlin, 1853.

(426) Aug. Miiller. " Beobachtungen zur vergleichenden Anatomic der Wir-
belsaule." Miiller's Archiv. 1853.

(427) J. Miiller. " Vergleichende Anatomic der Myxinoiden u. der Cyklosto-
men mil durchbohrtem Gaumen, I. Osteologie und Myologie." Abhandlungen der
koniglichen Akademie der Wissenschaften zu Berlin. 1834.

(428) W. Miiller. "Beobachtungen des pathologischen Instituts zu Jena, I.
Ueber den Bau der Chorda dorsalis." Jenaische Zeitschrift, Bd. VI. 1871.

(429) A. Schneider. Beitrage c. vergleich. Anat. u. Entwick. d. Wirbelthicre.
Berlin, 1879.

Ribs and Sternum.

Ribs. Embryological evidence on the development of the
ribs, though somewhat inadequate, indicates that they arise as
cartilaginous bars in the connective tissue of the intermuscular
septa, and that they are placed, in Elasmobranchii and

RIBS. 561

Amphibia, on the level of division between the dorso-lateral and
ventro-lateral divisions of the muscle-plates. This does not
appear to hold true for either Ganoidei or Teleostei. In
Teleostei they are entirely below the muscles along the lines of
the intermuscular septa, and this is partially true for Ganoidei,
though not wholly so in Lepidosteus. They may be attached
either to the haemal (Pisces) or neural (Amphibia and Amniota)
arches. The connective tissue from which they are formed is
continuous with the processes of the vertebrae to which they are
attached ; but the conversion of the tissue into cartilage takes
place more or less independently of that of the arches, although
in many cases the cartilage of the two becomes continuous, the
separation of the ribs being then effected by a subsequent
process of segmentation (Pick, No. 431). It is possible that the
ribs of Pisces may not be homologous with those of Amphibia
and the Amniota, but till the reverse can be proved it is more
convenient to assume that the ribs are homologous structures
throughout the vertebrate series.

In Elasmobranchii the ribs are relatively of less importance in the
adult than in the embryo. By a careful examination of their early develop-
ment, I have satisfied myself that the differentiation of the ribs is indepen-
dent of that of the haemal processes to which they are attached, although
the differentiation proceeds in such a manner that, when both are converted
into cartilage, they are quite continuous. Subsequently the ribs become
segmented off from the haemal processes. At the junction of the tail and
trunk, where the haemal processes commence to be ventrally prolonged,
eventually to unite in the region of the tail below the caudal vein, the ribs
are attached to short processes which spring from the sides of the haemal
arches (fig. 317). The ventral haemal arches of these" fishes are therefore
clearly in no part formed by the ribs.

In Ganoidei and Teleostei there is very great difficulty in determining
the homologies of the ribs.

In the cartilaginous Ganoidei there are well developed rib-like struc-
tures, which might be regarded as homologous with Elasmobranch ribs,
and indeed probably are so ; but at the same time their relations are in
some respects very different from those of Elasmobranch ribs in the caudal
region. In Ganoids the ribs, in approaching the tail, become shorter and then
fuse with the ends of the haemal processes, and finally in the caudal region
form together with the haemal arches a closed haemal canal which super-
ficially resembles that in Elasmobranchii.

In Lepidosteus and Amia, especially the former, the same phenomenon
is still more marked ; and in Lepidosteus it is easy, in passing backwards,

B. III. 36

562 STERNUM.

to trace the ribs bending ventral-wards, and uniting ventrally in the caudal
region to form, with the haemal processes, a complete haemal canal.

It might have been anticipated that the Teleostean Ganoids would
resemble the Teleostei, but, from an examination of adult Teleostei, it
would seem to be clear that the relations of the parts are the same as in
Elasmobranchii, i.e. that the ribs have no share in forming the haemal
canal in the tail. Aug. Miiller and Gotte have however brought embryo-
logical evidence (though not of a conclusive character), to shew that in the
embryo the ribs really fuse with the haemal processes in the tail, and so
assist, as in the Ganoids, in forming the haemal canal. Gotte moreover holds
that the ribs in Elasmobranchii are not homologous with those of Teleostei
and Ganoids ; but that the haemal arches in the tail are homologous in the
three groups.

Without necessarily following Gotte in these views it is worth pointing
out that the undoubtedly close affinity between the bony Ganoids and the
Teleostei is in favour of the view on the haemal arches of Teleostei at
which he has arrived on embryological grounds.

In Amphibia the formation of the ribs from the connective tissue of the
intermuscular septa, their secondary attachment to the transverse processes
of the neural arches, and their subsequent separation was first clearly
established by Pick (No. 431), whose statements have since been confirmed
by Hasse, Born, &c., and in part by Gotte, who holds however that, though
converted into cartilage independently of the transverse processes, they
are formed in membrane as outgrowths of these processes.

In the Amniota the ribs are also independently established (Hasse and
Born), though they subsequently become united to the transverse processes
and to the bodies of the vertebrae, or to the transverse processes only.
This junction is however stated by the majority of authorities, never to
be effected by the fusion of the cartilage of the two parts, but always by
fibrous tissue ; though Hoffmann (No. 435) takes a different view on this
subject, holding that the ribs are at first continuous with the intervertebral
regions of the primitive cartilaginous tube surrounding the notochord.

Sternum. In dealing with the development of the sternum
it will be convenient to leave out of consideration the inter-
clavicle or episternum which is, properly speaking, only part of
the shoulder-girdle and to confine my statements to the sternum
proper.

This structure is found in all the Amniota except the
Ophidia, Chelonia, and some of the Amphisbaenae.

From the older researches of Rathke, and from the newer
ones of Gotte, etc., it appears that the sternum is always formed
from the fusion of the ventral extremities of a certain number of
ribs. The extremities of the ribs unite with each other from

STERNUM. 563

before backwards, and thus give rise to two cartilaginous 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 an unpaired sternum. The Mammalian
presternum (manubrium sterni) and xiphosternum have the
same origin as the main body of the sternum (Ruge, No. 438).

In the Amphibia there is no structure which admits from its
mode of development of a complete comparison with the
sternum of the Amniota ; and it must for this reason be
considered doubtful whether the median structure placed behind
the coracoids in the Anura, which is usually known as the
sternum, is really homologous with the sternum of the
Amniota1.

The remaining Ichthyopsida are undoubtedly not provided
with a sternum.

BIBLIOGRAPHY of Ribs and Sternum.

(430) C. Glaus. " Beitrage z. vergleich. Osteol. d. Vertebraten. I. Rippen u.
unteres Bogensystem. " Sitz. d. kaiserl. Akad. Wiss. Wien, Vol. LXXIV. 1876.

(431) A. E. Fick. " Zur Entwicklungsgeschichte . d. Rippen und Querfort-
satze." Archivf. Anat. und Physiol. 1879.

(432) C. Gegenbaur. "Zur Entwick. d. Wirbelsaule des Lepidosteus mil
vergleich. anat. Bemerk." Jenaische Zeit., Vol. III. 1867.

(433) A. Gotte. " Beitrage z. vergleich. Morphol. d. Skeletsystems d. Wirbel-
thiere Brustbein u. Schultergiirtel." Archivf. mikr. Anat., Vol. xiv. 1877.

(434) C. Hasse u. G. Born. " Bemerkungen lib. d. Morphologic d. Rippen."
Zoologischer A nzeiger, 1879.

(4S5) C. K. Hoffmann. "Beitrage z. vergl. Anat. d. Wirbelthiere." Nieder-
land. Archiv Zool., Vol. IV. 1878.

(436) W. K. Parker. " A monograph on the structure and development of the
shoulder-girdle and sternum." Ray Soc. 1867.

(437) H. Rathke. Ueb. d. Bau n. d. Entrmcklung d. Brustbeins d. Saurier.

i853-

(438) G. Ruge. " Untersuch. lib. Entwick. am Brustbeine d. Menschen.

Morphol. Jahrbuch., Vol. VI. 1880.

1 The so-called sternum of the Amphibia develops in proximity with certain
rudimentary abdominal ribs, and Ruge has with some force urged (against Gotte)
that it may be for this reason a rudimentary structure of the same nature as the
sternum of the higher types.

36—2

CHAPTER XIX.
THE SKULL.

THREE distinct sets of elements may enter into the compo-
sition of the skull. These are (i) the cranium proper, composed
of true endoskeletal elements originally formed in cartilage, to
which are usually added exoskeletal osseous elements, formed
in the manner already described p. 542, and known in the
higher types as membrane bones. (2) The visceral arches formed
primitively as cartilaginous bars, but in the higher types largely
supplemented or even replaced by exoskeletal elements. (3) The
labial cartilages.

These parts present themselves in the most various forms,
and their study constitutes one of the most important depart-
ments of vertebrate morphology, and one which has always
been a favourite subject of study with anatomists. At the end
of the last century and during the first half of the present
century the morphology of the skull was handled from the point
of view of the adult anatomy by Goethe, Oken, Cuvier, Owen,
and many other anatomists, while Duges and, nearer to our own
time, Rathke, laid the foundation of an embryological study of
its morphology. A new era in the study of the skull was
inaugurated by Huxley in his Croonian lecture in 1858, and in
his lectures on Comparative Anatomy subsequently delivered
before the Royal College of Surgeons. In these lectures
Huxley disproved the then widely accepted view that the skull
was composed of four vertebrae ; and laid the foundation of a
more satisfactory method of dealing with the homologies of its
constituent parts. Since then the knowledge of the development
of the skull has made great progress. In this country a number

THE SKULL.

565

of very interesting memoirs have been published on the subject
by Parker, which together constitute a most striking contribution
to our knowledge of the ontogeny of the skull in a series of
types ; and in Germany Gegenbaur's monograph on the cephalic
skeleton of Elasmobranchii has greatly promoted a scientific
appreciation of the nature of the skull.

In the present chapter only the most important features in
the development of the skull will be touched on.

It will be convenient to describe, in the first instance, the
development of the cartilaginous elements of the skull.

The Cranium. The brain is at first enveloped in a contin-
uous layer of mesoblast known as the membranous cranium,
into the base of which the an-
terior part of the notochord is
prolonged for some distance.
The primitive cartilaginous
cranium is formed by a differ-
entiation within the membra-
nous cranium, and is always
composed of the following parts

(fig- 323) :

(1) A pair of cartilaginous
plates on each side of the
cephalic section of the noto-
chord, known as the parachor-
dals (pa. ck}. These plates to-
gether with the notochord (nc)
enclosed between them form a
floor for the hind- and mid-
brain. The continuous plate,
formed by them and the noto-
chord, is known as the basil ar
plate.

(2) A pair of bars forming
the floor for the fore-brain,

iff

Cl?

pa.ch.

CIA

FIG. 323. HEAD OK EMBRYO DOG-
FISH, SECOND STAGE ; BASAL VIEW OF
CRANIUM FROM ABOVE, THE CONTENTS
HAVING BEEN REMOVED. (From

Parker.)

ol. olfactory sacs ; an. auditory capsule;
nc. notochord; py. pituitary body ; pa.ch.
parachordal cartilage ; tr. trabecula ; inf.
infundibulum ; C.ir. cornua trabeculse ;
pn. prenasal element ; sp. spiracular cleft ;
br. external branchiae; Cl. 2, 4. visceral
clefts.

known as the trabeculae (tr).
These bars are continued forward from the parachordals. They
meet behind and embrace the front end of the notochord ; and
after separating for some distance bend in again in such a way

566

THE PARACHORDALS AND NOTOCHORD.

as to enclose a space— the pituitary space. In front of this
space they remain in contact and generally unite. They extend
forwards into the nasal region (pn}.

(3) The cartilaginous capsules of the sense organs. Of these
the auditory (ait) and olfactory capsules (ol} unite more or less
intimately with the cranial walls ; while the optic capsules,
forming the usually cartilaginous sclerotics, remain distinct.

The parachordals and notochord. The first of these sets
of elements, viz. the parachordals and notochord, forming
together the basilar plate, is always an unsegmented continu-
ation 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 (vide p. 314), and its extension is
roughly that of the basioccipital of the adult skull. Its mode of
development is almost identical with that of the vertebral
column, except that the notochord, even in many forms where it
persists in the vertebral column, disappears in the basilar plate ;
though in a certain number of cases remnants of it are found in
the adult state.

It will be convenient to say a few words
notochord in the head. It always extends
along the floor of the mid- and hind-brains,
but ends immediately behind the infundi-
bulum. The limits of its anterior extension
are clearly shewn in fig. 43. The front end
of the notochord often becomes more or
less ventrally flexed in correspondence with
the cranial flexure ; its anterior end being
in some instances (Elasmobranchii) almost
bent backwards (fig. 324).

Kolliker has shewn that in the Rabbit1,
and I believe that 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.

here with reference to the
nib

FIG. 324. LONGITUDINAL
SECTION THROUGH THE BRAIN OF
A YOUNG PRISTIURUS EMBRYO.

cer. commencement of the cere-
bral hemisphere; pn. pineal gland ;
///.infundibulum ; //.ingrowth from
mouth to form the pituitary body ;
nib. mid-brain ; cb. cerebellum ; ch,
notochord; al. alimentary tract;
laa. artery of mandibular arch.

" Embryologische Mittheilungen." Festschrift d. Nattirfor. G^//., Halle, 1879.

THE SKULL.

567

Before the parachordals are formed the anterior end of the notochord has
usually undergone a partial atrophy ; and its front end often becomes
somewhat dorsally flexed. Within the basilar plate it 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
me to be capable of this interpretation.

In Elasmobranchs where, as shewn above, a very primitive
type of development of the vertebral column is retained, we find
that the basilar plate is at first formed of (i) the notochord
invested by its cartilaginous sheath, and (2) of lateral masses of
cartilage, the parachordals, homologous with the arch tissue of the
vertebral column. This development probably indicates that the
basilar plate contains in itself the same elements as
those from which the neural arches and the centra of
the vertebral column are formed ; but that it never passes
beyond the unsegmented stage at first characteristic of
the vertebral column. The hinder end of each parachordal
forms a condyle articulating with the first vertebra ; so that in the
cartilaginous skull there are always two occipital condyles. The
basilar plate always grows up behind (fig. 326, so], and gives rise
to a complete cartilaginous ring enveloping the medulla oblon-
gata, in the same manner that the neural arches envelope the
spinal cord. This ring forms an occipital cartilaginous ring ; in
front of it the basilar plate becomes laterally continuous with the
periotic cartilaginous capsules, and the occipital ring above
usually spreads forward to form a roof for the part of the brain
between these capsules. In the higher Vertebrates the periotic
cartilages may be developed continuously with the basilar plate

The trabeculae. The trabeculae, so far as their mere anato-
mical 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 para-
chordals in one important feature, viz. that, except at their
hinder end (fig. 323), 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 trabeculae would probably seem to indicate, as

568

THE TRABECUL^i.

pointed out by Gegenbaur, that these parts, in spite of their
similarity to the parachordals, have not the same morphological
significance.

C V 1

FlG. 325. VIEW FROM ABOVE OF THE INVESTING MASS AND OF THE TRABECUL/E
OF A CHICK ON THE FOURTH DAY OF INCUBATION. (After 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.

cv i. cerebral vesicle (sliced oft") ; e. eye ; nc. notochord ; iv. investing mass ;
9. foramen for the exit of the ninth nerve ; d. cochlea ; hsc. horizontal semicircular
canal; q. quadrate; 5. notch for the passage of the fifth nerve; Ig. expanded anterior
end of the investing mass ; pts. pituitary space ; tr. trabeculse. The reference line tr.
has been accidentally made to end a little short of the cartilage.

The nature of the trabeculae has been much disputed by morphologists.
The view that they cannot be regarded as the anterior section of the
vertebral axis is supported by the consideration that the forward limit of the
primitive skeletal axis, as marked by the notochord, coincides exactly with
the distinction we have found it necessary to recognise, on entirely indepen-
dent grounds, between the fore-brain, and the remainder of the nervous axis.
But while this distinction between the parachordals and the trabeculas must .
I think be admitted, I see no reason against supposing that the trabecuke
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 equiva-
lent to a complete pair of neural arches enveloping the front end of the brain.
The primitive extension of the base of the fore-brain through the pituitary

THE SKULL.

569

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 trabeculys arise quite
independently of the parachordals, though the two sets of
elements soon unite ; while in Birds (fig. 325) and Mammals the
parachordals and trabeculae are formed as a continuous whole.
The junction between the trabeculae and parachordals becomes
marked by a cartilaginous ridge known as the posterior clinoid.

The trabeculae are usually somewhat lyre-shaped, meeting in
front and behind, and leaving a large pituitary space between
their middle parts (figs. 323 and 325). Into this space there

so

f

bbr

cbr

FJG. 326. SIDE VIEW OF THE CARTILAGINOUS CRANIUM OF A FOWL ON THK

SEVENTH DAY OF INCUBATION. (After Parker.)

pn. prenasal cartilage ; aln. alinasal cartilage ; ale. aliethmoid ; immediately below
this is the aliseptal cartilage, eth. ethmoid ; pp. pars plana ; ps. presphenoid or
interorbital ; pa. palatine ; pg. pterygoid ; z. optic nerve ; as. alisphenoid ; q.
quadrate ; st. stapes ; fr. fenestra rotunda ; hso. horizontal semicircular canal ;
psc. posterior vertical semicircular canal : both the anterior and the posterior semi-
circular canals are seen shining through the cartilage, so. supraoccipital ; eo. exocci-
pital ; oc. occipital condyle ; nc. notochord ; mk. Meckel's cartilage ; ch. cerato-
hyal ; bh. basi-hyal ; cbr. and ebr. cerato-branchial ; bbr. basibranchial.

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 always pass through
it in the embryo ; but in the higher forms it ceases to be
perforated in the adult. The trabeculae soon unite together both
in front and behind and form a complete plate underneath the
fore-brain, and extending into the nasal region1. A special

1 In Man (Kolliker) the trabeculce form from the first a continuous plate in front
of the pituitary space, and the latter very early acquires a cartilaginous floor.

5/0 THE TKAHECUL/E.

vertical growth of this plate in the region of the orbit forms the
interorbital plate of Teleostei, Lacertilia and Aves (fig. 326, 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 in the higher types two regions may be distin-
guished, viz. an alisphenoidal region (fig. 326, as) behind,
growing out from what is known as the basisphenoidal region
of the primitive trabeculae, and an orbitosphenoidal region in
front growing out from the presphenoidal region of the tra-
becula,\ These plates form at first 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 cartilage
forming the anterior boundary of the cranial cavity is known as
the lateral ethmoid region, and it is always perforated for the
passage of the olfactory nerves.

The cartilaginous walls which grow up from the trabecular
floor of the cranium generally extend upwards so as to form a
roof, though almost always an imperfect roof, for the cranial
cavity. In the higher types, in Mammals more especially, this
roof can hardly be said to be formed at all. The region of the
trabeculae in front of the brain is the ethmoid region. The basal
part of this region forms an internasal plate, from which an
internasal septum may grow up (fig. 326). To its sides the
olfactory capsules are attached, and there are usually lateral
outgrowths in front forming the trabecular cornua, while from
the posterior part of the ethmoidal plate, forming the anterior
boundary of the cranial cavity, there often grows out a prefrontal
or lateral ethmoidal process.

These and other processes growing out from the trabeculse have
occasionally been regarded as rudimentary praeoral branchial arches. I
have already stated it as my view that the existence of branchial arches
in this region is highly improbable, and I may add that the development
of these structures as outgrowths of the skull is in itself to my mind a nearly
conclusive argument against their being branchial arches, in that true
branchial arches hardly ever or perhaps never arise in this way.

The sense capsules. The most important of these is the
auditory capsule, which, as we have seen, fuses intimately with

THE SKULL.

the lateral walls of the skull. In front there is usually a cleft
separating it from the alisphenoid region of the skull, through
which the third division of the fifth nerve passes out. This cleft
becomes narrowed to a small foramen (fig. 327, V). The
sclerotic cartilage is always free, but profoundly modifies the
region of the cranium near which it is placed. The nasal invest-
ment forms in Elasmobranchs (fig. 327, No) a capsule open

FIG. 327. SKULL OF ADULT DOGFISH, SIDE VIEW. (From Parker.)
O. C, occipital condyle ; Au. periotic capsule; Pt.O. pterotic ridge ; Sp. 0. sphen-
otic process ; S. Or. supraorbital ridge ; Na. nasal capsule ; P.N. prenasal cartilage;
77. optic foramen ; V. trigeminal foramen ; PL TV., Qu. pterygo-quadrate arcade ;
M.Pt. metapterygoid ligament (including a small cartilage) ; Pl.Tr, ethmo-palatine
or palato-trabecular ligament ; Mck. lower jaw ; Sp. spiracle; H.M. hyomandibular;
C.Hy, ceratohyal ; m.h.l. mandibulo-hyoid ligament; Ph.Br. pharyngobranchial ;
E.Br. epibranchial ; C.br. ceratobranchial ; H.Br. hypobranchial ; B.Br. basi-
branchial ; Ex.Br. extrabranchial ; l\ 2, 3, 4, 5. labial cartilages ; the dotted lines
within Mck. indicate the basihyal.

below, and continuous with the ethmoid region of the trabeculse.
In most types however it becomes more closely united with the
ethmoid region and the accessory parts belonging to it.

The cartilaginous cranium, the development of which has
been thus briefly traced, persists in the adult without even the
addition of membrane bones in the Cyclostomata, Elasmobranchii
(fig. 327) and Holocephali. In the Selachioid 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.

572 THE BRANCHIAL BARS.

Branchial skeleton.

The most primitive type of branchial skeleton in any existing
form would appear to be that of the Petromyzonidae, which is
developed in a superficial subdermal tissue, and consists of a
series of bars united by transverse pieces, so as to form a basket-
work. It is known as an extra-branchial system, and an early
stage of its development in the Lamprey is shewn in fig. 47. In
the higher forms this system is replaced by a series of bars,
known as the branchial bars, so situated as to afford support to
the successive branchial pouches. Outside these bars there may
be present in some primitive forms (Elasmobranchii) cartilaginous
elements, which are supposed to be remnants of the extra-
branchial system (fig. 327, Ex.Br] ; while a series of membrane
bones is also usually added to them, which will be dealt with in a
separate section. The branchial bars are developed as simple
cartilaginous rods in the deeper parts of the mesoblast which
constitutes the primitive branchial arches.

The position of the branchial bars in relation to the somatopleure and
splanchnopleure can be determined from their relation to the so-called head
cavities. These cavities atrophy before the formation of the cartilaginous
branchial bars, but it will be observed (fig. 328), that the artery of each
arch (aa) is placed on the inner side of the head cavity (//). The cartila-
ginous bar arises at a later period on the inner side of the artery, and
therefore on the inner side of the section of the body cavity primitively
present in the arches.

An anterior arch, known as the mandibular arch, placed in
front of the hyo-mandibular cleft, and a second arch, known as
the hyoid arch, placed in front of the hyo-branchial cleft, are
developed in all types. The succeeding arches are known as
the true branchial arches, and are only fully developed in the
Ichthyopsida.

In some Sharks (Notidani) seven branchial arches may be
present (not including the hyoid and mandibular). In other
Ichthyopsida five are usually present, in the embryo at any rate,
while in the Amniota there are usually two or three post-hyoid
membranous arches, in the interior of which a cartilaginous bar
is usually formed. The general form of these bars at an early

THE SKULL.

573

Fir-

HORIZONTAL

stage of development is shewn in the
dog-fish (Scyllium) in fig. 329.

The simple condition of these bars
in the embryo renders it highly pro-
bable that forms existed at one time
with a simple branchial skeleton of
this kind : at the present day however

J SECTION THROUGH THE PEN-

such forms no longer exist. The first ULTIMATE VISCERAL ARCH
arch has in all cases changed its °FRUSAN EMHRYO <
function and has become converted ^ epiblast; vc. pouch of

into a supporting skeleton for the hypoblast which will form the
, , ,11 -1 1 ., i i i. • walls of a visceral cleft ; pp.

mouth ; the hyoid arch, though retain- segment of body-cavity in vis-

ing in Some forms its branchial func- ceral arch ;aa. aortic arch.

tion, has in most acquired additional functions and has under-
gone in consequence various peculiar modifications. The true
branchial arches retain their branchial functions in Pisces and
some Amphibia, but are secondarily modified and largely
aborted in the abranchiate forms. Since the changes undergone

c.a

Bnl

ffm

LrJt

Sn.f

FIG. 329. HEAD OF EMBRYO DOGFISH, n LINES LONG. (From Parker.)
TV. trabecula ; Pl.Pt. pterygo-quadrate ; M.Pt. metapterygoid region; Mn.
mandibular cartilage ; Hy. hyoid arch; Br. i. first branchial arch; Sp. mandilmlo-
hyoid cleft; C/1. hyo-branchial cleft; Lch. groove below the eye; Net. olfactory
rudiment; E. eyeball; An. auditory mass; C i, 2, 3. cerebral vesicles; Hm.
hemispheres; f.n.p. nasofrontal process.

by the true branchial bars are far less complicated than those of
the hyoid and mandibular bars it will be convenient to treat of
them in the first instance.

These bars are, as already mentioned, most numerous in
certain very primitive forms (seven in Notidanus), while as we
ascend the series there is a gradual tendency for the posterior of
them to disappear. This tendency is the result of a gradual
atrophy of the posterior branchial pouches, which commenced at

574

THE BRANCHIAL BARS.

a stage in the evolution of the Chordata long prior to the
appearance of cartilaginous or osseous branchial bars, and
reaches its climax in the Amniota.

In a fully developed branchial bar the primitively simple rod
of cartilage becomes divided into a series of segments, usually
four, articulated so as to be more or less mobile : and either
remaining cartilaginous or becoming partially or wholly ossified.
Each bar (fig. 327) forms a somewhat curved structure, embracing
the pharynx. The dorsal and somewhat horizontally placed
segment is known as the pharyngobranchial (Ph.Br), the next
two as the epibranchial (E.Br) and ceratobranchial (C.Br), and
the ventral segment as the hypobranchial (H.Br). There is
also typically present a basal unpaired segment, uniting the bars
of the two sides, known as the basibranchial (B.Br). The arches
often bear cartilaginous rays which support the gill lamellae.

In Teleostei dental plates are usually developed as an
exoskeletal covering on parts of the branchial arches.

In the Amphibia four or three branchial arches are present in
the embryo. These parts are more or less completely retained
in the Perennibranchiata and Caducibranchiata, but in the
Myctodera and Anura they become largely reduced, and
entirely connected with the hyoid.

In the Anura they never reach any considerable development,
and are soon reduced to a
plate (fig. 330) — the coalesced
basihyal and basibranchial
plate — the posterior processes
of which represent the rem-
nants of the branchial arches.

According to Parker the pos-
terior process of this plate in the
adult is a remnant of the fourth
branchial bar ; the next one is
the third branchial bar, while the
anterior lamina behind the hyoid
is stated by him (though this is
somewhat doubtful) to be a rem-
nant of the first two bars.

In the Amniota, the bran-
chial arches become still more

Pmx

FIG. 330. YOUNG FROG, WITH TAIL
JUST ABSORBED ; SIDE VIEW OF SKULL.
(From Parker.)

An. auditory capsule; in front of it is
the cranial side wall ; A.N. external nostril ;
St. stapes; Mck. Meckelian cartilage; B.Hy.
basihyobranchial plate; St.Hy. stylohyal
or ceratohyal; Br.i. first branchial arch.

Bones: E-0. exoccipital; Pr.O. pro-
otic ; Pa. parietal ; Fr. frontal ; Na. nasal ;
Pmx. premaxillary ; MX. maxillary; Pt.
pterygoid; Sq. squamosal; Qn-J't. quadra -
tojugal; Art. articular; D. dentary.

THE SKULL. 575

degenerated, in correlation with the total disappearance of a
branchial respiration at all periods of life. Their remnants
become more or less important parts of the hyoid bone, and are
solely employed in support of the tongue. Their basal portions
are best preserved, forming parts of the body of the hyoid. The
posterior (thyroid) cornua of the hyoid are remnants of the true
arches. Of these there are two in the Chelonia and Lacertilia,
and one in the Aves and Mammalia. In Aves the cornu formed
from the first branchial arch (fig. 331, cbr) is always larger than
that of the true hyoid arch (cJi).

Mandibular and Hyoid arches. The adaptations of both
the mandibular and hyoid bars, to functions entirely distinct from

FlG. 331. VIEW FROM BELOW OF THE BRANCHIAL SKELETON OF THE SKULL

OF A FOWL ON THE FOURTH DAY OF INCUBATION. (After Parker.)
cv i. cerebral vesicles ; e. eye ; fn. frontonasal process; n. nasal pit; tr. trabeculre ;
pts. pituitary space ; mr. superior maxillary process ; pg. pterygoid ; pa. palatine ;
q. quadrate; mk. Meckel's cartilage; ch. cerato-hyal ; bh. basihyal ; cbr. cerato-
branchial ; ebr. proximal portion of the cartilage in the third visceral (first branchial)
arch; bbr. basibranchial ; i. first visceral cleft; 2. second visceral cleft; 3. third
visceral arch.

those which they primitively served, are most remarkable ; and
the adaptations of the two bars are in many cases so intimately
bound together, that it is not possible to treat them separately.

The most important change of function is undoubtedly that
of the mandibular arch, which becomes entirely converted into a
skeleton for the jaws. It may be noted as a peculiarity of the

576 MAND1BULAR AND HYOID BARS.

mandibular arch that it is never provided with an unpaired basal
element.

The simplest forms of metamorphosis are those undergone
by Elasmobranchii, of which the Dog-fish (Scyllium) and Skate
(Raja) have been studied (Parker, No. 456). In some of these
forms, e.g. the Skate, part of the mandibular bar is still related to
the hyo-mandibular cleft (the spiracle).

Elasmobranchii. In Scyllium the hyoid and mandibular
arches are at first very similar to those which follow. Soon
however each of them sends an anteriorly directed dorsal process
(fig. 329). The regions which may be distinguished owing to the
growth of these processes have received names from ossifications
in them which are found in other types. The anterior process of
the mandibular arch is known as the pterygo-quadrate bar
(Pl.Pt) ; the dorsal end of the primitive bar from which it starts
(M.Pt] is known as the metapterygoid process; while the
ventral end of the bar forms the Meckelian cartilage. The
upper end of the hyoid arch is known as the hyomandi-
bular.

In a somewhat later stage changes take place which cause
these parts practically to assume the adult form (fig. 327). The
mandibular arch becomes segmented at its bend into (i) a
pterygo-quadrate bar (Pl.Pf) which grows forwards in front of
the mouth, and forms an upper jaw, and (2) a Meckelian cartilage
(Mck} which is placed behind the mouth, and forms a lower jaw.
The two jaws are articulated together, and the cartilages of the
two sides composing them meet each other distally.

At the articulation of the Meckelian cartilage with the quad-
rate part of the pterygo-quadrate is situated a ligament (M.Pf),
which takes the place of the metapterygoid process of the
previous stage, and passes up on the anterior side of the spiracle,
to be attached to the cranium in the front part of the auditory
region. This ligament, which is supplemented by a second
ligament, the ethmopalatine ligament, passing from the
pterygo-quadrate bar to the antorbital region of the skull, is not
the most important support of the jaw. The main support is, on
the contrary, given by the hyoid arch ; the hyomandibular
segment of which (H.M) as well as the adjoining segment (cera-
tohyoid C.Hy) are firmly attached by ligament to the mandibular

THE SKULL.

577

arch. The hyomandibular is articulated with the cranium
beneath the pterotic ridge (Pt.O),

In the type just described, the hyoid and mandibular arches
undergo less modification than in almost any other case. The
hyoid arch has altered its form, but retains its respiratory func-
tion. It has however acquired the secondary function of sup-
porting the mandibular arch. The mandibular arch is divided
into two elements, which form respectively the upper and lower
jaws. It is not directly articulated with the skull, and its mode
of support by the hyoid arch has been called by Huxley (No.
445) hyostylic.

The development of the hyoid and mandibular arches in the
Skate is characterised by a few important features (fig. 333). The
anterior element of the hyoid

arch, which forms the hyo- \ ^ Sp

mandibular (H.M], becomes
entirely separate from the
posterior part of the arch, and
only serves to support the
jaws. The posterior part of
the arch (Hy} carries on the
respiratory functions of the
hyoid, and is closely connec-
ted with the first branchial
arch. The upper or meta-
pterygoidelementof the man-
dibular arch (M.Pt} has a
considerable development,

FIG. 333. HEAD OF EMBRYO SKATE, i\
IN. LONG. (From Parker.)

Tr. trabecula ; Pl.Pt. pterygo- quadrate
bar ; Mn. mandibular bar ; M.Pt. meta-
pterygoid cartilage ; H.M. hyomandibu-
lar ; Hy. remainder of hyoid arch ; Br. \.
first branchial arch ; Sp. mandibulo-hyoid
cleft or spiracle ; Pn. pineal gland ; Au. au
ditory vesicle ; C. i, C. 2, and C. 3. vesicles
of the brain.

and, becoming separated from
the remainder of the arch, forms a mass of cartilage with one or
two branchial rays, in the front wall of the spiracle, and con-
stitutes a section of the mandibular arch still retaining traces
of its primitive function in supporting the wall of a branchial
pouch.

Although the development of other Elasmobranch types is
not known, it is necessary to call attention to the mode of
support of the mandibular arch in certain forms, notably Noti-
danus, Hexanchus and Cestracion, where the pterygo-quadrate
region of the mandibular arch is directly articulated to the
B. in. 37

578 MANDIKULAR AND HYOID BARS.

cranium between the optic and trigeminal foramina. In the
two former genera the metapterygoid region of the arch is more-
over continuous with the pterygo-quadrate, and articulates with
the post-orbital process of the auditory region of the skull. In
spite of these attachments the mandibular arch continues to be
partially supported by the hyomandibular. The skulls in which
the mandibular arch has this double form of support have been
called by Huxley amphistylic.

Considering the in many respects primitive characters of the
forms with amphistylic skulls it seems not improbable that they

SOr

l}a.ch.

Gtly

FIG. 334. CRANIAL SKELETON OF A SALMON FRY, SECOND WEEK AFTER
HATCHING; MEMBRANE BONES, EYEBALLS, AND NASAL SACS REMOVED. (From
Parker.)

T.Cr. tegrnen cranii; 'S. Or. supraorbital band; Fo. superior fontanelle; Au.
auditory capsule ; Pa.ch. parachordal cartilage; Ch. notochord; 7>. trabecula; above
the trabecula, the interorbital septum is seen, passing into the cranial wall above and
reaching the supraorbital band; //. optic foramen; V. trigeminal foramen; /', I".
labial cartilages ; PI. Ft. palatopterygoid bar ; M. Pt. metapterygoid tract ; Qu. quad-
rate region; Mck. Meckelian cartilage; H.M. hyomandibular cartilage; Sy.
symplectic tract; I.Hy. interhyal; C.Hy. ceratohyal; II. fly. hypohyal; G.ffy.
glossohyal; Br.\. first branchial arch.

preserve the original mode of support of the mandibular arch ;
from which differentiations in two directions have taken place, viz.
differentiations in the direction of a complete support of the
mandibular arch by the hyoid, which is characteristic of most
Elasmobranchii and, as will be shewn below, of Ganoidei and
Tclcostei ; and differentiations towards a direct articulation or
attachment of the mandibular arch to the cranium, without the

THE SKULL. 579

intervention of the hyoid. The latter mode of attachment is
called by Huxley autostylic. It is found in Holocephala,
Dipnoi, Amphibia and the Amniota.

Teleostei. In addition to that of Elasmobranchii, the skull
of the Salmon is the only hyostylic skull in which, by the admi-
rable investigation of Parker (No. 451), the ontogeny of the hyoid
and mandibular bars has been satisfactorily worked out. Apart
from the presence of a series of membrane bones, the deve-
lopment of these bars agrees on the whole with the types already
described.

The hyoid arch, though largely ossified, undergoes a process
of development very similar to that in Raja. It is formed as a
simple cartilaginous bar, which soon becomes segmented longi-

ft 1.3 Sp.»

FIG. 335. YOUNG SALMON OF THE FIRST SUMMER, AKOUT 2 INCHES LONG;

SIDE VIEW OF SKULL, EXCLUDING BRANCHIAL ARCHES. (From Parker.)

The palato-mandibular and hyoid tracts are detached from their proper situations,

a line indicating the position where the hyomandibular is articulated beneath the

pterotic ridge.

oL olfactory fossa; c.tr. trabecular cornu; «/*. «/''. upper labial cartilages ; p.s.

presphenoid tract ; t.cr. tegmen cranii ; s.o.b. supraorbital band; fo. superior fonta-

nelle; n.c. notochord; b.o. basilar cartilage; //'. trabecula; p.c. condyle for palatine

cartilage; 5. trigeminal foramen ; fa. facial foramen; 8. foramen for glossopharyngeal

and vagus nerves; mk. Meckelian cartilage; op.c. opercular condyle.

Bones: e.o. exoccipital; s.o. supraoccipital; e.p. epiotic; pt.o. pterotic; sp.o.

sphenotic ; op. opisthotic; pro. prootic; I'.s. basisphenoid ; al.s. alisphenoid; o.s.

orbitosphenoid ; I.e. ectethmoid or lateral ethmoid ; pa. palatine ; pg. pterygoid ;

m.pg. mesopterygoid ; mt.pg. metapterygoid ; qu. quadrate; ar. articular; h.m.

hyomandibular; sy. symplectic ; i.h. interhyal ; ep.h. epiceratohyal ; c.h. ceratohyal ;

h.h. hypohyal; g.h. glosso- or basihyal.

37—2

580 MANDIBULAR AND HYOID BARS.

tudinally into an anterior and a posterior part (fig. 334). The
former constitutes the hyomandibular (H.M], while the latter,
becoming more and more separated from the hyomandibular,
constitutes the hyoid arch proper ; owing to the disappearance
of the hyobranchial cleft, it loses its primitive function, and
serves on the one hand to support the operculum covering the
gills, and on the other to support the tongue. It becomes
segmented into a series of parts which are ossified (fig. 335) as
the epiceratohyal (ep./t) above, then a large ceratohyal (c/t),
followed by a hypohyal (JiJi), while the median ventral element
forms the basi- or glossohyal (gJi).

The hyomandibular itself is articulated with the skull below
the pterotic process (fig. 334, H.M}. Its upper element ossifies
as the hyomandibular (fig. 335, fun.}, while its lower part (fig.
334, Sy), which is firmly connected with the mandibular arch,
ossifies as the symplectic (fig. 335, sy). A connecting element
between the two parts of the hyoid bar forms an interhyal (i/i).

There are more important differences in the development of
the mandibular arch in Elasmobranchii and the Salmon than in
that of the hyoid arch, in that, instead of the whole arcade of
the upper jaw being formed from the mandibular arch, a fresh
element, in the form of an independently developed bar of
cartilage, completes the upper arcade in front ; but even with
this bar the two halves of the upper branch of the arch do not
meet anteriorly, but are separated by the ends of the trabeculae.

The anterior bar of the upper arcade is known as the
palatine ; but it appears to me as yet uncertain how far it is to
be regarded as an element, primitively belonging to the upper
arcade of the mandibular arch, which has become secondarily
independent in its development ; or as an entirely distinct
structure which has no counterpart in the Elasmobranch upper
jaw. The latter view is adopted by Parker and Bridge, and a
cartilage attached to the hinder wall of the nasal capsule of
many Elasmobranchii is identified by them with the palatine rod
of the Teleostei.

The arch itself is at first very similar to the succeeding
arches ; its dorsal extremity soon however becomes broadened,
and provided with an anteriorly directed process. This part (fig.
334, M.Pt and Qii] is then segmented from the lower region,

THE SKULL. 581

and forms what may be called the pterygo-quadrate cartilage,
though not completely homologous with the similarly named
cartilage in Elasmobranchs ; while the lower region forms the
Meckelian cartilage (Mck], which has already grown inwards, so
as to meet its fellow ventrally below the mouth. The whole
arch becomes at the same time widely separated from the axial
parts of the skull.

Nearly simultaneously with the first differentiation of the
mandibular arch, a bar of cartilage — the palatine bar already
spoken of — is formed on each side, below the eye, in front of the
mouth. The dilated anterior extremity of this bar soon comes
in contact with an anterior process of the trabeculse, known as
the ethmopalatine process.

In a later stage the pterygoid end of the pterygo-quadrate
cartilage unites with the distal end of the palatine bar (fig. 334,
Pl.Pt], and there is then formed a continuous cartilaginous
arcade for the upper jaw, which is strikingly similar to the
cartilaginous upper jaw of Elasmobranchii.

A large dorsal process of the primitive pterygo-quadrate now
forms a large metapterygoid tract (M.Pt] ; while the whole arch
becomes firmly bound to the hyomandibular (H.M}.

In the later stages the parts formed in cartilage become
ossified (fig. 335). The palatine is first ossified, the pterygoid
region of the pterygo-quadrate is next ossified as a dorsal
mesopterygoid (m.pg] and a ventral pterygoid proper (pg).
The quadrate region, articulating with the Meckelian cartilage,
becomes ossified as a distinct quadrate (qu\ while the dorsal
region becomes also ossified as a metapterygoid (int.pg).

In the Meckelian cartilage a superficial ossification of the
ventral edge and inner surface forms an articulare (ar) ; but the
greater part of the cartilage persists through life.

Some of the above ossifications, at any rate those of the palatine and
pterygoid, seem to be started by dental osseous plates adjoining the carti-
lage. They will be spoken of further in the section dealing with the mem-
brane bones.

Amphibia. The development of the autostylic piscine skulls
has unfortunately not yet been studied ; and the most primitive
autostylic types whose development we are acquainted with are

582 MANDIBULAR AND HYOID BARS.

those of the Amphibia ; on which a large amount of light has
been shed by the researches of Huxley and Parker.

The modifications of the hyoid arch are comparatively simple
and uniform. It forms a rod of cartilage, which soon articulates
in front with the quadrate element of the mandibular arch, and
is subsequently attached by ligaments both to the quadrate and
to the cranium. In those Amphibia in which external gills and
gill clefts are lost, it fuses with the basal element of the hyoid
(fig. 330), which, together with the basal portions of the following
arches, forms a continuous cartilaginous plate. On the com-
pletion of these changes the paired parts of the hyoid arch have
the form of two elongated rods, known as the anterior cornua of
the hyoid, which attach the basihyal plate to the cranium behind
the auditory capsule.

It is still uncertain whether there is any distinct element corresponding
to the hyomandibular of fishes.

Parker holds that the columella auris of the Anura is the homologue
of the hyomandibular. The columella develops comparatively late and
independently of the remainder of the hyoid arch, but the similarity
between its relations to the nerves and those of the hyomandibular is
put forward by Parker as an argument in favour of his view. The early
ligamentous connection between the quadrate and the upper end of the
primitive hyoid is however an argument in favour of regarding the upper
end of the primitive hyoid as the hyomandibular element, not separated
from the remainder of the arch.

The history of the mandibular arch is more complicated than
that of the hyoid. The part of it which corresponds with the
upper jaw of Elasmobranchii exhibits most striking variations in
development ; so striking indeed as to suggest that the secondary
modifications it has undergone are sufficiently considerable to
render great caution necessary in drawing morphological con-
clusions from the processes which are in some instances ob-
servable. A more satisfactory judgment on this point will be .
possible after the publication of a memoir with which Parker is
now engaged on the skulls of the different Anura.

The membrane bones applying themselves to the sides of the
mandibular arch are relatively far more important than in the
lower types. This is especially the case with the upper jaw
where the maxillary and premaxillary bones functionally replace
the primitive cartilaginous jaw ; while membranous pterygoids

THE SKULL.

583

and palatines apply themselves to, and largely take the place of,
the cartilaginous palatine and pterygoid bars.

Two types worked out by Parker, viz. the Axolotl and the
common Frog, may be selected to illustrate the development of
the mandibular arch.

In the Axolotl, which may be taken as the type for the
Urodela, the mandibular arch is constituted at a very early
stage of (i) an enlarged dorsal element, corresponding with the
pterygo-quadrate of the lower types, but usually known as the
quadrate ; and (2) a ventral or Meckelian element. The Mecke-
lian bar very early acquires its investing bones, while the dorsal
part of the quadrate becomes divided into two characteristic

FIG. 336. YOUNG AXOLOTL, i\ INCHES LONG ; UNDER VIEW OF SKULL,

DISSECTED, THE LOWER JAW AND GILL ARCHES HAVING BEEN REMOVED.

(From Parker.)

nc. notochord ; oc.c. occipital condyle; f.o. fenestra ovalis; si. stapes; tr. trabe-
cular cartilage; i.n. internal nares; c.tr. cornu trabeculse; pd. pedicle of quadrate;
(/. quadrate; pg. outline of pterygoid cartilage; 5'. orbito-nasal nerve; 7. facial nerve.

BonCS I pa.s. parasphenoid ; e.o. exoccipital ; v. vomer; px. premaxillary ; mx.
maxillary; pa. palatine; pg. pterygoid.

processes, viz. an anterior dorsal process which grows towards
and soon permanently fuses with the trabecular crest, and a
posterior process known as the otic process, which applies itself
to the outer side of the auditory region. The anterior of these
processes, as pointed out by Huxley, is probably homologous
with the anterior process of the pterygo-quadrate bar in Noti-
danus, which articulates with the trabecular region of the
cranium, while the otic process is homologous with the meta-

584 MANDIBULAR AND HYOID BARS.

pterygoid process. Hardly any trace is present of an anterior
process to form a pterygoid bar, but dentigerous plates forming
a dermal palato-pterygoid bar have already appeared.

At a somewhat later stage a fresh process, called by Huxley
the pedicle, grows out from the quadrate, and articulates with
the ventral side of the auditory region (fig. 336, pd). Shortly
afterwards a rod of cartilage grows forward from the quadrate
under the membranous pterygoid (pg), which corresponds with
the cartilaginous pterygoid bar of other types (fig. 336), and an
independent palatine bar, arising even before the pterygoid
process, is formed immediately dorsal to the dentigerous palatine
plate (pa\ and is attached to the trabecula. These two bars
eventually meet, but never become firmly united to the more
important membrane bones placed superficially to them.

The mandibular arch in the
Frog stands, so far as develop-
ment is concerned, in striking
contrast to the mandibular
arch of the Axolotl, in spite of
the obvious similarity in the
arrangement of the adult parts
in the two types. FlG. 33?. EMBRYO FROG, JUST BE-

In the earliest stage it FORE HATCHING ; SIDE VIEW OF HEAD,

WITH SKIN REMOVED. (From Parker.)
forms a simple bar in the ,, lf , , - . , .. ,

Na. olfactory sack; E. involution for

membranous mandibular arch, eyeball; Ati. auditory sack; 7>. trabe-
11 i , .1 cula; Mn. mandibular : Hy. hyoid ; Br.I.

parallel to and very similar to first branchial arch . 'th/ gili.buds are

are

the hyoid bar behind (figf 337, seen on the first two branchial arches; /.
M \ T u, * u labial cartilages.

Mn). In the next stage ob-

served, that is to say in Tadpoles of four, five, to six lines long,
an astonishing transformation has taken place. The mandibular
arch (fig. 338) is turned directly forwards parallel to the
trabecula, to which it is attached in front (p.pg) and behind
(pd}. The proximal part of the arch thus forms a subocular
bar, and the space between it and the trabecula a subocular
fenestra. In front of the anterior attachment it is continued
forwards for a short distance, and to the free end of this pro-
jecting part is articulated a small Meckelian cartilage directed
upwards (mk}. The Meckelian cartilage is at this stage placed
in front of the nasal sacks, in the lower lip of the suctorial

THE SKULL.

585

mouth. The greater part of the arch, parallel with the trabeculae,
is equivalent to what has been called in the Axolotl the

mJr

FIG. 338. TADPOLE OF COMMON TOAD, ONE-THIRD OF AN INCH LONG ;
CRANIAL AND MANDIBULAR CARTILAGES SEEN FROM ABOVE ; THE PARACHORDAL

CARTILAGES ARE NOT YET DEFINITE. (From Parker.)

nc. notochord; ms. muscular segments; au. auditory capsule; py. region of
pituitary body; tr. trabecula; c.tr, cornu trabeculae ; p-pg. palatopterygoid bar ; pd.
pedicle; q. quadrate condyle; mk. Meckelian piece of mandibular arch; s.o.f.
subocular fenestra ; u.l. upper labial cartilage. The dotted circle within the quadrate
region indicates the position of the internal nostril.

quadrate, while its anterior attachment to the trabeculae is the
rudiment of the palato-pterygoid cartilage. The posterior
attachment is known as the pedicle.

The condition of the mandibular arch during this and the next stage
(fig. 339) is very perplexing. Its structure appears adapted in some way to
support the suctorial mouth of the Tadpole.

Reasons have been offered in a previous part of this volume for sup-
posing that the suctorial mouth of the Tadpole is probably not simply a
structure secondarily acquired by this larva, but is an organ inherited from
an ancestor provided through life with a suctorial mouth.

The question thus arises, is the peculiar modification of the mandibular
arch of the Tadpole an inherited or an acquired feature ?

If the first alternative is accepted we should have to admit that the
mandibular arch became first of all modified in connection with the
suctorial mouth, before it was converted into the jaws of the Gnatho-
stomata ; and that the peculiar history of this arch in the Tadpole is a
more or less true record of its phylogenetic development. In favour of this

586

MANDIBULAR AND HYOID BARS.

view is the striking similarity which Huxley has pointed out between
the oral skeleton of the Lamprey and that of the Tadpole ; and certain
peculiarities of the mandibular arch of Chimaera and the Dipnoi can perhaps
best be explained on the supposition that the oral skeleton of these forms
has arisen in a manner somewhat similar to that in the Frog ; though with
reference to this point further developmental data are much required.

On the other hand the above suppositions would necessitate our
admitting that a great abbreviation has occurred in the development of
the mandibular arch of the otherwise more primitive Urodela ; and that
the simple mode of growth of the jaws in Elasmobranchii, from the
primitive mandibular arch, is phylogenetically a much abbreviated and
modified process, instead of being, as usually supposed, a true record of
ancestral history.

If the view is accepted that the characters of the mandibular arch of
the Tadpole are secondary, it will be necessary to admit that the adaptation
of the mandibular arch to the suctorial mouth took place after the suctorial
mouth had come to be merely a larval organ.

In view of our imperfect knowledge of the development of most Piscine
skulls I would refrain from expressing a decided opinion in favour of
either of these alternatives.

or.p

eth

FIG. 339. TADPOLE WITH TAIL BEGINNING TO SHRINK; SIDE VIEW OF SKULL

WITHOUT THE BRANCHIAL ARCHES. (From Parker.)

n.c. notochord; au. auditory capsule; between it and eth. the low cranial side wall
is seen; eth. ethmoidal region; st. stapes; 5. trigeminal foramen; 2. optic foramen;
ol. olfactory capsules, both seen owing to slight tilting of the skull; c.tr. cornu
trabeculae; «./. upper labial, in outline; su. suspensorium (quadrate); pd. its pedicle;
ot.fr. its otic process; or.p. its orbitar process; t.m. temporal muscle, indicated by
dotted lines passing beneath the orbitar process; pa.pg. palatopterygoid bar; ;;//•.
Meckelian cartilage; /./. lower labial, in outline; c.h. ceratohyal; b.h. basihyal. The
upper outline of the head is shewn by dotted lines.

As the tail of the Tadpole gradually disappears, and the
metamorphosis into the Frog becomes accomplished, the
mandibular arch undergoes important changes (fig. 339): the

THE SKULL.

palato-pterygoid attachment (pa.pg) of the quadrate subocular
bar becomes gradually elongated ; and, as it is so, the front end
of the subocular bar (su) rotates outwards and backwards, and
soon forms a very considerable angle with the trabeculae. The
Meckelian cartilage (ink) at its free end becomes at the same
time considerably elongated. These processes of growth con-
tinue till (fig. 330) the palato-pterygoid bar (Pf) forms a sub-
ocular bar, and is considerably longer than the original sub-
ocular region of the quadrate ; while the Meckelian cartilage
(Mck] has assumed its permanent position on the hinder border
of the no longer suctorial mouth, and has grown forwards so as
nearly to meet its fellow in the median line.

The metapterygoid region of the quadrate gives rise to a
posterior and dorsal process (fig. 339, ot.pr), the end of which is
constricted off as the tympanic annulus (fig. 340, a.f) ; while

pmx

FIG. 340. YOUNG FROG, NEAR END OF FIRST SUMMER ; UPPER VIEW OF
SKULL, WITH LEFT MANDIBLE REMOVED, AND THE RIGHT EXTENDED OUT-
WARDS. (From Parker.)

b.o. basioccipital tract; s.o. supraoccipital tract; fo. frontal fontanelle; e.n,
external nostril; internal to it, internasal plate; a.t. tympanic annulus.

Bones : e.o. exoccipital; pr.o. prootic, partly overlapped by/, parietal; f. frontal ;
eth. rudiment of sphenethmoid ; na. nasal ; pmx. premaxillary ; mx. maxillary; /£-.
pterygoid, partly ensheathing the reduced cartilage; q.j. quadratojugal ; s<j. squa-
mosal; ar. articular; d. dentary; m.mk. mento-Meckelian.

the proximal part of the process remains as the otic (metaptery-
goid) process, articulating with the auditory cartilage.

The pedicle (pd} retains its original attachment to the skull.

588 MAND1BULAR AND IIYOID BARS.

The palato-pterygoid soon becomes segmented into a trans-
versely placed palatine, and a longitudinally placed pterygoid
(fig. 340). With the exception of a few ossifications, which pre-
sent no features of special interest, the parts of the mandibular
arch have now reached their final condition, which is not very
different from that in the Axolotl.

Sauropsida. In the Sauropsida the modifications of the
hyoid and mandibular arches are fairly uniform.

The lower part of the hyoid arch, including the basihyoid,
unites with the remnants of the arches behind to form the hyoid
bone, to which it contributes the anterior cornu and anterior part
of the body.

The columella is believed by Huxley and Parker to represent,
as in the Anura, the independently developed dorsal (hyoman-
dibular) element of the hyoid, together with the stapes with which
it has become united1.

The membranous mandibular arch gives off in the embryos
of all the Sauropsida an obvious bud to form the superior
maxillary process, and the formation of this bud appears to
represent the growth forwards of the pterygoid process in Elas-
mobranchii, which is indeed accompanied by the formation of a
similar bud ; but the skeletal rod, which appears in the axis
of this bud, is as a rule independent of that in the true arch
(fig- SS1./^. PS}- The former is the pterygo-palatine bar; the
latter the Meckelian and quadrate cartilages.

The pterygo-palatine bar is usually if not always ossified
directly, without the intervention of cartilage.

Born has recently shewn that Parker was mistaken in supposing that
the palato-pterygoid bone is cartilaginous in Birds. In the Turtle a short
cartilaginous pterygoid process of the quadrate would seem to be present
(Parker, No. 458).

The quadrate and Meckelian cartilages are either from the
first separate, or very early become so.

1 The strongest evidence in favour of Huxley's and Parker's view of the nature of
the columella is the fusion in the adult Sphenodon of the upper end of the hyoid with
the columella (vide Huxley, No. 445). From an examination of a specimen in the
Cambridge museum I do not feel satisfied that the fusion is not secondary, but have
not been able to examine the junction of the hyoid and columella in section. For a
different view to that of Huxley vide Peters, "Ueb. d. Gehorknochelchen u. ihr
Verhaltniss zu. Zungenbeinbogen b. Sphenodon." Berlin MoHOtsbtnekU, 1874.

THE SKULL.

589

The quadrate cartilage ossifies as the quadrate bone, and
supplies the permanent articulation for the lower jaw. Its upper
end exhibits a tendency to divide into two processes, corres-
ponding with the pedicle and otic processes of the Amphibia.
The Meckelian cartilage becomes soon covered by investing
bones, and its proximal end ossifies as the articulare. The
remainder of the cartilage usually disappears.

Mammalia. The most extraordinary metamorphosis of the
hyoid and mandibular arches occurs in the Mammalia, and has
been in part known since the publication of the memoir of
Reichert (No. 461).

Both the hyoid and mandibular arches develop at first more
completely than in any of the other types above Fishes; and are

pn.ch nc

FIG. 341. EMBRYO PIG, TWO-THIRDS OF AN INCH LONG ; ELEMENTS OF THE

SKULL SEEN SOMEWHAT DIAGRAMMATICALLY FROM BELOW. (From Parker.)
pa.ch. parachordal cartilage; nc. notochorcl; au. auditory capsule; py. pituitary
body; tr. trabeculse; c.lr. trabecular cornu; pn. prenasal cartilage; e.n. external
nasal opening; ol. nasal capsule; p-pg- palatopterygoid tract enclosed in the
maxillopalatine process; mn. mandibular arch ; hy. hyoid arch; th.h. first branchial
arch; ja. facial nerve; 8a. glossopharyngeal ; 86. vagus; 9. hypoglossal.

articulated to each other above, while the pterygo-palatine bar
is quite distinct. The main features of the subsequent develop-
ment are undisputed, with the exception of that of the upper end
of the hyoid, which is still controverted. The following is Parker's
(No. 452) account for the Pig, which confirms in the main the
view originally put forward by Huxley (No. 445).

The mandibular and hyoid arches are at first very similar

5QO MANDIBULAR AND HYOID BARS.

(fig. 341 mn and hy), their dorsal ends being somewhat incurved,
and articulating together.

In a somewhat later stage (fig. 342) the upper end of the
mandibular bar (mb\ without becoming segmented from the
ventral part, becomes distinctly swollen, and clearly corresponds
to the quadrate region of other types. The ventral part of the
bar constitutes the Meckelian cartilage (mk).

The hyoid arch has in the meantime become segmented into
two parts, an upper part (z), which eventually becomes one of

FIG. 342. EMBRYO PIG, AN INCH AND A THIRD LONG; SIDE VIEW OF
MANDIBULAR AND HYOID ARCHES. THE MAIN HYOID ARCH IS SEEN AS DIS-
PLACED BACKWARDS AFTER SEGMENTATION FROM THE INCUS. (From Parker.)

tg. tongue; ink. Meckelian cartilage; ml. body of malleus; mb. manubrium or
handle of the malleus; t.ty. tegmen tympani; i. incus; st. stapes; i.hy, interhyal
ligament; st.h. stylohyal cartilage; h.h. hypohyal ; ^.//.basibranchial; th.h. rudiment
of first branchial arch; -ja. facial nerve.

the small bones of the ear — the incus — and a lower part which
remains permanently as the anterior cornu of the hyoid (st./i).
The two parts continue to be connected by a ligament.

The incus is articulated with the quadrate end of the mandi-
bular arch, and its rounded head comes in contact with the
stapes (fig. 342, st) which is segmented from the fenestra ovalis.
The main arch of the hyoid becomes divided into a hypohyal
(h.h) below and a stylohyal (st. h] above, and also becomes articu-
lated with the basal element of the arch behind (b/i).

In the course of further development the Meckelian part of
the mandibular arch becomes enveloped in a superficial ossifica-
tion forming the dentary. Its upper end, adjoining the quadrate
region, becomes calcified and then absorbed, and its lower, with
the exception of the extreme point, is ossified and subsequently
incorporated in the dentary.

The quadrate region remains relatively stationary in growth

TIIK SKULL. 591

as compared with the adjacent parts of the skull, and finally
ossifies to form the malleus bone of the ear. The processus
gracilis of the malleus is the primitive continuation into Meckel's
cartilage.

The malleus and incus are at first embedded in the connec-
tive tissue adjoining the tympanic cavity (hyomandibular cleft,
vide p. 528) ; and externally to them a bone known as the
tympanic bone becomes developed so that they become placed
between the tympanic bone and the periotic capsule. In late
fcetal life they become transported completely within the tym-
panic cavity, though covered by a reflection of the tympanic
mucous membrane.

The dorsal end of the part of the hyoid separated from the
incus becomes ossified as the tympano-hyal, and is anchylosed
with the adjacent parts of the periotic capsule. The middle part
of the bar just outside the skull forms the stylo-hyal (styloid
process in Man) which is attached by ligament to the anterior
cornu of the hyoid (cerato-hyal).

While the account of the formation of the malleus, incus, and stapes
just given is that usually accepted in this country, a somewhat different
view of the development of these parts has as a rule been adopted in
Germany. Reichert (No. 461) held that both the malleus and the incus
were derived from the mandibular bar ; and this view has been confirmed
by Giinther, Kolliker and other observers, and has recently been adopted
by Salensky (No. 462) after a careful research especially directed towards
this point. Reichert also held that the stapes was derived from the hyoid
bar ; but, though his observations on this point have been very widely
accepted, they have not met with such universal recognition as his views
on the origin of the malleus and incus. Salensky has recently arrived
at a view, which is in accord with that of Parker, in so far as the indepen-
dence of the stapes of both the hyoid and mandibular arches is concerned.
Salensky however holds that it is formed from a mass of mesoblast
surrounding the artery of the mandibular arch, and that the form of the
stapes is due to its perforation by the mandibular artery. A product of
this artery permanently perforates the stapes in a few Mammalia, though
in the majority it atrophies.

In view of the different accounts of the origin of the incus the exact
nature of this bone must still be considered as an open question, but
should Reichert's view be confirmed the identification of the incus with
the columella of the Amphibia and Sauropsida must be abandoned.

592 MEMBRANE BONES.

Membrane bones and ossifications of the cranium.

The membrane bones of the skull may be divided into two
classes, viz. (i) those derived from dermal osseous plates, which
as explained above (p. 542) are primitively formed by the coal-
escence of the osseous plates of scales ; and (2) those formed by
the coalescence of the osseous plates of teeth lining the oral
cavity. Some of the bones sheathing the edge of the mouth
have been formed partly by the one process and partly by the
other.

In the Fishes there are found all grades of transition between
simple dermal scutes, and true subdermal osseous plates forming
an integral part of the internal skeleton. Dermal scutes are best
represented in Acipenser and some Siluroid Fishes.

Where the membrane bones still retain the character of dermal
plates, those on the dorsal surface of the cranium are usually
arranged in a series of longitudinal rows, continuing in the region
of the head the rows of dermal scutes of the trunk ; while the
remaining cranial scutes are connected with the visceral arches.
The dermal bones on the dorsal surface of the head are very
different in number, size, and arrangement in different types of
Fishes ; but owing to their linear disposition it is usually possible to
find a certain number both of the paired and unpaired bones
which have a similar situation in the different forms. These
usually receive the same names, but both from general consider-
ations as to their origin, as well as from a comparison of different
species, it appears to me probable that there is no real homology
between these bones in different species, but only a kind of general
correspondence1.

It is not in fact till we get to the types above the Fishes that
we can find a series of homologous dorsal membrane bones
covering the roof of the skull. In these types three paired sets
of such bones are usually present, viz, from behind forwards the
parietals, frontals and nasals, the latter bounding the posterior
surface of the external nasal opening. Even in the higher

1 For some interesting remarks on the arrangement of these bones in Fishes, vide
Bridge, "On the Osteology of Polyodon folium." Phil. Trans., 1878.

THE SKULL. 593

types these bones are liable to vary very greatly from the usual
arrangement.

Besides these bones there is usually present in the higher
forms a lacrymal bone on the anterior margin of the orbit
derived from one of a series of periorbital membrane bones
frequently found in Fishes. Various supraorbital and postorbital
bones, etc. are also frequently found in Lacertilia, etc. which are
not impossibly phylogenetically independent of the membrane
bones inherited from Fishes; and may have been evolved as
bony scutes in the subdermal tissue of the papillae of the saur-
opsidan scales.

The visceral arches of Fishes, especially of the Teleostei, are
usually provided with a series of membrane bones. In the true
branchial arches these take the form of dentigerous plates ; but
no such plates are found in the Amphibia or Amniota.

The opercular flap attached to the hyoid arch is usually
supported by a series of membrane bones, which attain their
highest development in the Teleostei. One of these bones, the
praeopercular, is very constant and is primitively attached
along the outer edge of the hyomandibular. It seems to be
retained in Amphibia as a membrane bone, overlapping the
attachment of the quadrate and known as the squamosal ;
though it is not impossible that this bone may be derived from a
superficial membrane bone, widely distributed in Teleostei and
Ganoids, which is known as the supra-temporal. In Dipnoi
the bone which appears to be clearly homologous with the
squamosal would seem from its position to belong to the series of
dorsal plates, and therefore to be the supra-temporal ; but it is
regarded by Huxley (No. 446) as the praeopercular1.

In the Amniota the squamosal forms an integral part, of the
osseous roof of the skull ; but in the Sauropsida it continues, as
in Amphibia, to be closely related to the quadrate.

A larger series of persistent membrane bones are related to
the mandibular, and its palato-quadrate process.

Overlying the palato-quadrate process are two rows of bones,

1 It is not impossible that the solution of the difficulty about the praeopercular is
to be found by supposing that the praeopercular as it exists in Teleostei is derived
from a dorsal dermal plate, and that in the Dipnoi this plate retains more nearly than
in Teleostei its primitive position.

B. III. 38

594 MEMBRANE BONES.

one row lying at the edge of the mouth, on the outer side of the
pterygo-palatine process, and the other set on the roof of the
mouth superficial to the pterygo-palatine process.

The outer row is formed of the praemaxilla, maxilla, jugal,
and very often quadrato-jugal. Of these bones the maxilla
and prsemaxilla, as is more especially demonstrated by their
ontogeny in the Urodela, are partly derived from dentigerous
plates and partly from membrane plates outside the mouth;
while the jugal, and quadrato-jugal when present, are entirely
extra-oral. In the Amphibia and Amniota the praemaxillae and
maxillae are the most important bones in the facial region, and
are quite independent of any cartilaginous substratum.

The second row of bones is clearly constituted in the Dipnoi
and Amphibia by the vomer in front, then the palatine, and
finally the pterygoid behind. Of these bones the vomer is
never related to a cartilaginous tract below, while the palatines
and pterygoids usually are so. The position and growth of the
three bones in many Urodela (Axolotl) are especially striking
(Hertwig. No. 442). In the Axolotl they form a continuous
series, the vomer and palatine being covered by teeth, but the
pterygoid being without teeth. The vomer and palatine origi-
nate from the united osseous plates of the bases of the teeth,
while the pterygoid is in the first instance continuous with the
palatine.

In Teleostei, Amia, etc., there are dentigerous plates forming
a palatine and pterygoid, which in position, at any rate, closely
correspond with the similarly named bones in Amphibia ; and
there is also a dentigerous vomer which may fairly be considered
as equivalent to that in Amphibia.

In the Amniota the three bones found in Amphibia are always
present, but with a few exceptions amongst the Lacertilia and
Ophidia, are no longer dentigerous. The cartilaginous bars,
which in the lower types are placed below the palatine and
pterygoid membrane bones, are usually imperfectly or not at all
developed.

On Meckel's cartilage important membrane bones are almost
always grafted. On the outside and distal part of the cartilage a
dentary is usually developed, which may envelope and replace
the cartilage to a larger or smaller extent. Its oral edge

THE SKULL. 595

is usually dentigerous. The splenial membrane bone is the
most important bone on the inner side of Meckel's cartilage, but
other elements known as the coronoid and angular may also be
added. In Mammalia the dentary is the only element present
(vide p. 590).

On the roof of the mouth a median bone, the parasphenoid,
is very widely present in the Amphibia and Fishes, except the
Elasmobranchii and Cyclostomata, and has no doubt the same
phylogenetic origin as the vomer and membranous palatines and
pterygoids.

It is less important in the Sauropsida, and becomes indis-
tinguishably fused with the sphenoid in the adult, while in
Mammalia it is no longer found.

Ossification of the Cartilaginous Cranium. In certain
Fishes the cartilaginous cranium remains quite unossified, while
completely enveloped in dermal bones. Such for instance is its
condition in the Selachioid Ganoids. In most instances, however,
the investment of the cartilaginous cranium by membrane bones
is accompanied by a more or less complete ossification of the
cartilage itself.

In the Dipnoi this occurs to the smallest extent, the only
ossifications occurring in the lateral parts of the occipital region,
and forming the exoccipitals.

In Teleostei and bony Ganoids, a considerably greater number
of ossifications occur in the cartilage.

In the region of the occipital cartilaginous ring there appears
a basioccipital and supraoccipital and two exoccipitals.
The basioccipital is the only bone on the floor of the skull
ossifying that part into which the notochord is primitively con-
tinued1.

In the region of the periotic cartilage a large number of
bones may appear. In front there is the prootic, which often
meets the exoccipital behind ; behind there is above and in close
connection with the supraoccipital the epiotic, and below in
close connection with the exoccipital the opisthotic. On the
dorsal side of the cartilage there is a projecting ridge composed
mainly of a bone known as the pterotic, sometimes erroneously

1 The notochord appears also to enter into the posterior part of the region which
ossifies as the basisphenoid.

38—3

596 OSSIFICATIONS OF THE CARTILAGINOUS CRANIUM.

called the squamosal, and continued in front by the sphenotic.
The pterotic, or the cartilaginous region corresponding to it,
always supplies the articular surface for the hyomandibular.

In the floor of the skull, in the region of the pituitary body,
there is formed a basisphenoid; while in the lateral parts of the
wall of this part of the cranium, there is a bone known as the
alisphenoid.

In front, parts of the lateral walls of the cranium ossify as the
orbitosphenoids.

In view of the very imperfect ossification of the cartilaginous
cranium of the Dipnoi, and of the fact that there is certainly no
direct genetic connection between the Teleostei on the one hand,
and the Amphibia and Amniota on the other, it is very difficult
to believe that most of the ossifications of the cranium in the
Amphibia and Amniota have more than a general correspondence
with those in the Teleostei.

In the Amphibia the ossifications in the cartilage are compa-
ratively few. In the occipital region there is a lateral ossification
on each side of the exoccipital. the basioccipital region being
unossified, and the supraoccipital at the utmost indurated by a
calcareous deposit.

The periotic capsule is ossified by a prootic centre, which
meets the exoccipital behind.

The front part of the cartilaginous cranium is ossified by a
complete ring of bone — the sphenethmoid bone — which embraces
part of the ethmoid region, and of the orbitosphenoid and
presphenoid regions.

In the Amphibia the cartilaginous cranium, with its centres
of ossification, is easily separable from the membranous investing
bones.

In the Amniota the cartilaginous cranium, whose development
in the embryo has already been described, becomes in the adult
much more largely ossified, and the bones which replace the
primitive cartilage unite with the membrane bones to form a
continuous bony cranium.

The centres of ossification become again much more numerous.
In the occipital segment analogous centres to those of Teleostei
are again found ; and it is probable that the exoccipitals are
homologous throughout the series, the supraoccipital and basioc-

THE SKULL. 597

cipital bones of the higher types being merely identical in position
with the similarly named bones in Fishes.

In the periotic there are usually three centres of ossification,
first recognised by Huxley. These are the prootic, the epiotic
and opisthotic, the situations of which have already been defined.
Of these the prootic is the most constant.

In Reptiles, the prootic and opisthotic frequently remain
distinct even in the adult.

In Birds, the epiotic and opisthotic are early united with the
supra- and exoccipital ; and at a later period the prootic is also
indistinguishably fused with the adjacent parts.

In Mammals the three ossifications fuse into a continuous
whole — the periotic bone — which may be partially united with
the adjacent parts.

In the pituitary region of the base of the cranium a pair of
osseous centres or in the higher types a single centre (Parker1)
gives rise to the basisphenoid bone, and in front of this another
basal or pair of basal ossifications forms the presphenoid, while
laterally to these two centres there are formed centres of
ossification in the alisphenoid and orbitosphenoid regions, which
may be extremely reduced in various Sauropsida, leaving the
side walls of the skull almost entirely formed of membrane or
cartilage.

In the ethmoid region there may arise a median ossification
forming the mesethmoid and lateral ossifications forming the
lateral ethmoids or prefrontals ; which may assist in forming the
front wall of the brain-case, or be situated quite externally to the
brain-case and be only related to the olfactory capsules.

The labial cartilages. In most Fishes a series of skeletal structures,
known as the labial cartilages, are developed at the front and sides of the
mouth, and in connection with the olfactory capsules ; and these cartilages
still persist in connection with the olfactory capsules, though in a reduced
form, in the higher types. They are more developed in the Cyclostomata
than in any other Vertebrate type.

The meaning of these cartilages is very obscure ; but, from their being in
part employed to support the lips and horny teeth of the Cyclostomata and
the Tadpole, I should be inclined to regard them as remnants of a primi-
tive skeleton supporting the suctorial mouth, with which, on the grounds
already stated (p. 317), I believe the ancestors of the present Vertebrata
to have been provided.

1 According to Kblliker there are two centres in Man in both the basisphenoid
and presphenoid.

598 BIBLIOGRAPHY.

BIBLIOGRAPHY.

(439) A. Duges. "Recherches sur 1'Osteologie et la myologie des Batraciens a
leur differents ages." Paris, Mem. savans etrang. 1835, and An. Set. A7af. Vol. I.
1834.

(440) C. Gegenbaur. Untersuchwigen z. vergleich. Anat. d. Wirbelthiere, III.
Heft. Das Kopfskelet d. Selachier. Leipzig, 1872.

(441) Giinther. Beob. iib. die Entwick. d. Gehororgans. Leipzig, 1842.

(442) O. Hertwig. " Ueb. d. Zahnsystem d. Amphibien u. seine Bedeutung f.
d. Genese d. Skelets d. Mundhohle. " Archiv f. mikr. Anat., Vol. xi. 1874, suppl.

(443) T.H.Huxley. " On the theory of the vertebrate skull." Proc. Royal
Soc., Vol. ix. 1858.

(444) T. H. Huxley. The Elements of Comparative Anatomy. London, 1869.

(445) T.H.Huxley. "On the Malleus and Incus." Proc. Zool. Soc., 1869.

(446) T.H.Huxley. "On Ceratodus Forsteri." Proc. Zool. Soc., 1876.

(447) T. H. Huxley. " The nature of the craniofacial apparatus of Petromyzon."
Journ. of Anat. and Phys., Vol. X. 1876.

(448) T.H.Huxley. The Anatomy of Vertebrated Animals. London, 1871.

(449) W. K. Parker. "On the structure and development of the skull of the
Common Fowl (Callus Domesticus)." Phil. Trans., 1869.

(450) W. K. Parker. "On the structure and development of the skull of the
Common Frog (Rana temporaria)." Phil. Trans., 1871.

(451) W. K. Parker. "On the structure and development of the skull in the
Salmon (Salmo salar)." Bakerian Lecture, Phil. Trans., 1873.

(452) W. K. Parker. "On the structure and development of the skull in the
Pig (Sus scrofa). " Phil. Trans., 1874.

(453) W. K. Parker. "On the structure and development of the skull in the
Batrachia." Part n. Phil. Trans., 1876.

(454) W. K. Parker. "On the structure and development of the skull in the
Urodelous Amphibia." Part in. Phil. Trans., 1877.

(455) W. K. Parker. "On the structure and development of the skull in the
Common Snake (Tropidonotus natrix)." Phil. Trans., 1878.

(456) W. K. Parker. " On the structure and development of the skull in Sharks
and Skates." Trans. Zoolog. Soc., 1878. Vol. x. pt. iv.

(457) W. K. Parker. "On the structure and development of the skull in the
Lacertilia." Pt. I. Lacerta agilis, L. viridis and Zootoca vivipara. Phil. Trans.,
1879.

(458) W. K. Parker. "The development of the Green Turtle." The Zoology
of the Voyage of H. M.S. Challenger. Vol. I. pt. V.

(459) W. K. Parker. "The structure and development of the skull in the
Batrachia." Pt. in. Phil. Trans., 1880.

(460) W. K. Parker and G. T. Belt any. The Morphology of the Skull.
London, 1877.

(460*) H. Rathke. Entwick. d. Natter. Konigsberg, 1839.

(461) C. B. Reichert. " Ueber die Visceralbogen d. Wirbelthiere." Miiller's
Archiv, 1837.

(462) W. Saleusky. "Beitragez. Entwick. d. knorpeligen Gehorknochelchen."
Morphol. Jahrbuch, Vol. VI. 1880.

Vide also Kolliker (No. 298), especially for the human and mammalian skull;
Gotte (No. 296).

CHAPTER XX.

THE PECTORAL AND PELVIC GIRDLES AND THE
SKELETON OF THE LIMBS.

TJie Pectoral girdle.

Pisces. Amongst Fishes the pectoral girdle presents itself
in its simplest form in Elasmobranchii, where it consists of a
bent band of cartilage on each side of the body, of somewhat
variable form, meeting and generally uniting with its fellow
ventrally. Its anterior border is in close proximity with the
last visceral arch, and a transverse ridge on its outer and
posterior border, forming the articular surface for the skeleton
of the limb, divides it into a dorsal part, which may be called
the scapula, and a ventral part which may be called the
coracoid.

In all the remaining groups of Fishes there is added to the
cartilaginous band, which may wholly or partially ossify, an
osseous support composed of a series of membrane bones.

In the types with such membrane bones the cartilaginous
parts do not continue to meet ventrally, except in the Dipnoi
where there is a ventral piece of cartilage, distinct from that
bearing the articulation of the limb. The cartilage is moreover
produced into two ventral processes, an anterior and a posterior,
below the articulation of the limb ; which may be called, in
accordance with Gegenbaur's nomenclature, the praecoracoid
and coracoid. Of these the praecoracoid is far the most

600 THE PECTORAL GIRDLE.

prominent, and in the majority of cases the coracoid can hardly
be recognised. The coracoid process is however well developed
in the Selachioid Ganoids, and the Siluroid Teleostei. In
Teleostei the scapular region often ossifies in two parts, the
smaller of which is named by Parker praecoracoid, though it is
quite distinct from Gegenbaur's praecoracoid. The membrane
bones, as they present themselves in their most primitive state
in Acipenser and the Siluroids, are dermal scutes embracing the
anterior edge of the cartilaginous girdle. In Acipenser there
are three scutes on each side. A dorsal scute known as the
supra-clavicle, connected above with the skull by the post-
temporal ; a middle piece or clavicle, and a ventral or infra-
clavicle (inter-clavicle), which meets its fellow below.

In most Fishes the primitive dermal scutes have become
subdermal membrane bones, and the infra-clavicle is usually not
distinct, but the two clavicles form the most important part of
the membranous elements of the girdle. Additional mem-
brane bones (post-clavicles) are often present behind the main
row.

The development of these parts in Fishes has been but little
studied.

In Scyllium, amongst the Elasmobranchii, I find that each
half of the pectoral girdle develops as a vertical bar of cartilage
at the front border of the rudimentary fin, and externally to the
muscle-plates.

Before the tissue forming the pectoral girdle has acquired
the character of true cartilage, the bars of the two sides meet
ventrally by a differentiation in situ of the mesoblastic cells, so
that, when the girdle is converted into cartilage, it forms an
undivided arc, girthing the ventral side of the body. There is
developed in continuity with the posterior border of this arc on
the level of the fin a horizontal bar of cartilage, which is
continued backwards along the insertion of the fin, and, as will
be shewn in the sequel, becomes the metapterygium of the adult
(figs. 344, bp and 348, mp). With this bar the remaining skeletal
elements of the fin are also continuous.

The foramina of the pectoral girdle are not in the first
instance formed by absorption, but by the non-development of
the cartilage in the region of pre-existing nerves and vessels.

THE PECTORAL GIRDLE. 6oi

The development of these parts in Teleostei has been recently investigated
by 'Swirski (No. 472) who finds in the Pike (Esox) that the cartilaginous
pectoral girdle is at first continuous with the skeleton of the fin. It forms
a rod with a dorsal scapular and ventral coracoid process. An independent
mass of cartilage gives rise to a prascoracoid, which unites with the main
mass, forming a triradiate bar like that of Acipenser or the Siluroids.
The coracoid process becomes in the course of development gradually
reduced.

'Swirski concludes that the so-called praecoracoid bar is to some extent
a secondary element, and that the coracoid bar corresponds to the whole of
the ventral part of the girdle of Elasmobranchii, but his investigations do
not appear to me to be as complete as is desirable.

Amphibia and Amniota. The pectoral girdle contains a
more or less constant series of elements throughout the
Amphibia and Amniota ; and the differences in structure
between the shoulder girdle of these groups and that of Fishes
are so great that it is only possible to make certain general
statements respecting the homologies of the parts in the two
sets of types.

The generally accepted view, founded on the researches of
Parker, Huxley, and Gegenbaur, is to the effect that there is a
primitively cartilaginous coraco-scapular plate, homologous with
that in Fishes, and that the membrane bones in Fishes are
represented by the clavicle and inter-clavicle in the Sauropsida
and Mammalia, which are however usually admitted to be
absent in Amphibia. These views have recently been challenged
by Gotte (No. 466) and Hoffmann (No. 467), on the ground of
a series of careful embryological observations ; and until the
whole subject has been worked over by other observers it does
not seem possible to decide satisfactorily between the conflicting
views. It is on all hands admitted that the scapulo-coracoid
elements of the shoulder girdle are formed as a pair of carti-
laginous plates, one on each side of the body. The dorsal half
of each plate becomes the scapula, which may subsequently
become divided into a supra-scapula and scapula proper ; while
the ventral half forms the coracoid, which is not always separated
from the scapula, and is usually divided into a coracoid proper,
a praecoracoid, and an epicoracoid. By the conversion of parts
of the primitive cartilaginous plates into membranous tissue
various fenestrae may be formed in the cartilage, and the bars

602 THE NATURE OF THE CLAVICLE.

bounding these fenestrae both in the scapula and coracoid
regions have received special names ; the anterior bar of the
coracoid region, forming the praecoracoid, being especially
important. At the boundary between the scapula and the
coracoid, on the hinder border of the plate, is placed the glenoid
articular cavity to carry the head of the humerus.

The grounds of difference between Gotte and Hoffmann and
other anatomists concern especially the clavicle and inter-clavicle.
The clavicle is usually regarded as a membrane bone which may
become to some extent cartilaginous. By. the above anatomists,
and by Rathke also, it is held to be at first united with the
coraco-scapular plate, of which it forms the anterior limb, free
ventrally, but united dorsally with the main part of the plate ;
and Gotte and Hoffmann hold that it is essentially a cartilage
bone, which however in the majority of the Reptilia ossifies
directly without passing through the condition of cartilage.

The interclavicle (episternum) is held by Gotte to be
developed from a paired formation at the free ventral ends of
the clavicles, but he holds views which are in many respects
original as to its homologies in Mammalia and Amphibia. Even
if Gotte's facts are admitted, it does not appear to me necessarily
to follow that his deductions are correct. The most important
of these is to the effect that the dermal clavicle of Pisces has no
homologue in the higher types. Granting that the clavicle in
these groups is in its first stage continuous with the coraco-
scapular plate, and that it may become in some forms carti-
laginous before ossifying, yet it seems to me all the same quite
possible that it is genetically derived from the clavicle of Pisces,
but that it has to a great extent lost even in development its
primitive characters, though these characters are still partially
indicated in the fact that it usually ossifies very early and
partially at least as a membrane bone1.

In treating the development of the pectoral girdle systematically it will be
convenient to begin with the Amniota, which may be considered to fix the
nomenclature of the elements of the shoulder girdle.

1 The fact of the clavicle going out of its way, so to speak, to become cartilaginous
before being ossified, may perhaps be explained by supposing that its close connection
with the other parts of the shoulder girdle has caused, by a kind of infection, a change
in its histological characters.

II IK PECTORAL GIRDLE.

603

Lacertilia. The shoulder girdle is formed as two membranous plates,
from the dorsal part of the anterior border of each of which a bar projects
(Rathke, Gotte), which is free at its ventral end. This bar, which is usually
(Gegenbaur, Parker) held to be independent of the remaining part of the
shoulder girdle, gives rise to the clavicle and interclavicle. The scapulo-
coracoid plate soon becomes cartilaginous, while at the same time the cla-
vicular bar ossifies directly from the membranous state. The ventral ends
of the two clavicular bars enlarge to form two longitudinally placed plates,
which unite together and ossify as the interclavicle.

Parker gives a very different account of the interclavicle in Anguis. He
states that it is formed of two pairs of bones 'strapped on to the antero-infe-
rior part of the prassternum,' which subsequently unite into one.

Chelonia. The shoulder girdle of the Chelonia is formed (Rathke) of
a triradiate cartilage on each side, with one dorsal and two ventral limbs.
It is admitted on all hands that the dorsal limb is the scapular element,
and the posterior ventral limb the coracoid ; but, while the anterior ventral
limb is usually held to be the praecoracoid, Gotte and Hoffmann maintain
that, in spite of its being formed of cartilage, it is homologous with the
anterior bar of the primitive shoulder-plates of Lacertilia, and therefore the
homologue of the clavicle.

Parker and Huxley (doubtfully) hold that the three anterior elements of
the ventral plastron (entoplastron and epiplastra) are homologous with the
interclavicle and clavicles, but considering that these plates appear to belong
to a secondary system of dermal ossifications peculiar to the Chelonia, this
homology does not appear to me probable.

Aves. There are very great differences of view as to the development
of the pectoral arch of Aves.

About the presence in typical forms of the coraco-scapular plate and two
independent clavicular bars all authors are agreed. With reference to the
clavicle and interclavicle Parker (No. 468) finds that the scapular end of the
clavicle attaches itself to and ossifies a mass of cartilage, which he regards
as the mesoscapula, while the interclavicle is formed of a mass of tissue be-
tween the ends of the clavicles where they meet ventrally, which becomes
the dilated plate at their junction.

Gegenbaur holds that the two primitive clavicular bars are simply clavi-
cles, without any element of the scapula ; and states that the clavicles are
not entirely ossified from membrane, but that a delicate band of cartilage
precedes the osseous bars. He finds no interclavicle.

Gotte and Rathke both state that the clavicle is at first continuous with
the coraco-scapular plate, but becomes early separated, and ossifies entirely
as a membrane bone. Gotte further states that the interclavicles are formed
as outgrowths of the median ends of the clavicles, which extend themselves
at an early period of development along the inner edges of the two halves of
the sternum. They soon separate from the clavicles, which subsequently
meet to form the furculum ; while the interclavicular rudiments give rise, on
the junction of the two halves of the sternum, to its keel, and to the ligament

604 THK PECTORAL GIRDLE.

connecting the furculum with the sternum. The observations of Gotte,
which tend to shew the keel of the sternum is really an interclavicle, appear
to me of great importance.

A prascoracoid, partially separated from the coracoid by a space, is pre-
sent in Struthio. It is formed by a fenestration of a primitively continuous
cartilaginous coracoid plate (Hoffmann). In Dromaeus and Casuarius cla-
vicles are present (fused with the scapula in the adult Dromaeus), though
absent in other Ratitae (Parker, etc.).

Mammalia. The coracoid element of the coraco-scapular plate is
much reduced in Mammalia, forming at most a simple process (except in the
Ornithodelphia) which ossifies however separately1.

With reference to the clavicles the same divergencies of opinion met with
in other types are found here also.

The clavicle is stated by Rathke to be at first continuous with the coraco-
scapular plate. It is however soon separated, and ossifies very early, in the
human embryo before any other bone. Gegenbaur however shewed that
the human clavicle is provided with a central axis of cartilage, and this obser-
vation has been confirmed by Kolliker, and extended to other Mammalia by
Gotte. The mode of ossification is nevertheless in many respects inter-
mediate between that of a true cartilage bone and a membrane bone. The
ends of the clavicles remain for some time, or even permanently, cartila-
ginous, and have been interpreted by Parker, it appears to me on hardly
sufficient grounds, as parts of the mesoscapula and praecoracoid. Parker's
so-called mesoscapula may ossify separately. The homologies of the epister-
num are much disputed. Gotte, who has worked out the development of the
parts more fully than any other anatomist, finds that paired interclavicular
elements grow out backwards from the ventral ends of the clavicles, and
uniting together form a somewhat T-shaped interclavicle overlying the front
end of the sternum. This condition is permanent in the Ornithodelphia,
except that the anterior part of the sternum undergoes atrophy. But in the
higher forms the interclavicle becomes almost at once divided into three
parts, of which the two lateral remain distinct, while the median element
fuses with the subjacent part of the sternum and constitutes with it the pre-
sternum (manubrium sterni). If Gotte' s facts are to be trusted, and they
have been to a large extent confirmed by Hoffmann, his homologies appear to
be satisfactorily established. As mentioned on p. 563 Ruge (No. 438) holds
that Gotte is mistaken as to the origin of the presternum.

Gegenbaur admits the lateral elements as parts of the interclavicle, while
Parker holds that they are not parts of an interclavicle but are homologous
with the omosternum of the Frog, which is however held by Gotte to be a
true interclavicle.

1 This process, known as the coracoid process, is held by Sabatier to be the
pnecoracoid ; while this author also holds that the upper third of the glenoid cavity,
which ossifies by a special nucleus, is the true coracoid. The absence of a prae-
coracoid in the Ornithodelphia is to my mind a serious difficulty in the way of
Sabatier's view.

THE PECTORAL GIRDLE. 605

Amphibia. In Amphibia the two halves of the shoulder girdle are
each formed as a continuous plate, the ventral or coracoid part of which is
forked, and is composed of a larger posterior and a smaller anterior bar-like
process, united dorsally. In the Urodela the two remain permanently free
at their ventral ends, but in the Anura they become united, and the space
between them then forms a fenestra. The anterior process is usually (Gegen-
baur, Parker) regarded as the praecoracoid, but Gotte has pointed out that
in its mode of development it strongly resembles the clavicle of the higher
forms, and behaves quite differently to the so-called praecoracoid of Lizards.
It is however to be noticed that it differs from the clavicle in the fact that it
is never segmented off from the coraco-scapular plate, a condition which has
its only parallel in the equally doubtful case of the Chelonia. Parker holds
that there is no clavicle present in the Amphibia, while Gegenbaur maintains
that an ossification which appears in many of the Anura (though not in the
Urodela) in the perichondrium on the anterior border of the cartilaginous
bar above mentioned is the representative of the clavicle. Gotte's obser-
vations on the ossification of this bone throw doubt upon this view of Gegen-
baur ; while the fact that the cartilaginous bar may be completely enclosed
by the bone in question renders Gegenbaur's view, that there is present both
a clavicle and prsecoracoid, highly improbable.

No interclavicle is present in Urodela, but in this group and in a number
of the Anura, a process grows out from the end of each of the bars (prae-
coracoids) which Gotte holds to be the clavicles. The two processes unite
in the median line, and give rise in front to the anterior unpaired element of
the shoulder girdle (omosternum of Parker). They sometimes overlap the
epicoracoids behind, and fusing with them bind them together in the median
line. Parker who has described the paired origin of the so-called omosternum,
holds that it is not homologous with the interclavicle, but compares it with
his omosternum in Mammals.

BIBLIOGRAPHY.

(463) Bruch. " Ueber die Entwicklung der Clavicula und die Farbe des
Blutes. " Zeit.f. wiss. Zool., \\. 1853.

(464) A. Duges. " Recherches sur 1'osteologie et la myologie des Batraciens a
leurs differens ages." Memoires des savants etrang. Academic royale des sciences de
Finstitut de France^ Vol. vi. 1835.

(465) C. Gegenbaur. Untersuchungen zur vergleichenden Anatomie der Wir-
belthiere, 2 Heft. Schultergiirtel der Wirbelthiere. Bmstflosse der Fische. Leipzig,
1865.

(466) A. Gotte. "Beitrage z. vergleich. Morphol. d. Skeletsystems d. Wirbel-
thiere : Brustbien u. Schultergiirtel." Archivf. mikr, Anat. Vol. xiv. 1877.

(467) C. K. Hoffmann. "Beitrage z. vergleichenden Anatomic d. Wirbel-
thiere." Niederlandisches Archivf. ZooL,Vol.v. 1879.

(468) W. K. Parker. "A Monograph on the Structure and Development of the
Shoulder-girdle and Sternum in the Vertebrata." Ray Society, 1868.

606 PELVIC GIRDLE.

(469) H. Rathke. Ueber die Entwicklung der Schildkrbten. Braunschweig,
1848.

(470) H. Rathke. Ueber den Bau und die Entwicklung des Brustbeins der
Saurier, 1853.

(471) A. Sabatier. Comparaison des ceinfures et des membres antMeurs et pos-
ttrtturs d. la Serie d. Vertttrh. Montpellier, 1880.

(472) Georg 'Swirski. Untersuch. iib. d. Entwick. d. Schultergiirtels n. d.
Skelets d. Brustflosse d. Hechts. Inaug. Diss. Dorpat, 1880.

Pelvic girdle.

Pisces. The pelvic girdle of Fishes is formed of a cartila-
ginous band, to the outer and posterior side of which the basal
element of the pelvic fin is usually articulated. This articulation
divides it into a dorsal iliac, and ventral pubic section. The iliac
section never articulates with the vertebral column.

In Elasmobranchii the two girdles unite ventrally, but the
iliac section is only slightly developed. In Chimaera there is a
well developed iliac process, but the pubic parts of the girdle
are only united by connective tissue.

In the cartilaginous Ganoids the pelvic girdle is hardly to be
separated from the skeleton of the fin. It is not united with its
fellow, and is represented by a plate with slightly developed
pubic and iliac processes.

In the Dipnoi there is a simple median cartilage, articulated
with the limb, but not provided with an iliac process. In bony
Ganoids and Teleostei there is on each side a bone meeting its
fellow in the ventral line, which is usually held to be the rudi-
ment of the pelvic girdle ; while Davidoff attempts to shew that
it is the basal element of the fin, and that, except in Polypterus,
a true pelvic girdle is absent in these types.

From my own observations I find that the mode of develop-
ment of the pelvic girdle in Scyllium is very similar to that of
the pectoral girdle. There is a bar on each side, continuous on
its posterior border with the basal element of the fin (figs. 345
and 347). This bar meets and unites with its fellow ventrally
before becoming converted into true cartilage, and though the
iliac process (il) is never very considerable, yet it is better deve-
loped in the embryo than in the adult, and is at first directed
nearly horizontally forwards.

Amphibia and Amniota. The primitive cartilaginous pelvic

PELVIC GIRDLE. 607

girdle of the higher types exhibits the same division as that of
Pisces into a dorsal and a ventral section, which meet to form
the articular cavity for the femur, known as the acetabulum.
The dorsal section is always single, and is attached by means
of rudimentary ribs to the sacral region of the vertebral column,
and sometimes to vertebrae of the adjoining lumbar or caudal
regions. It always ossifies as the ilium.

The ventral section is usually formed of two more or less
separated parts, an anterior which ossifies as the pubis, and a
posterior which ossifies as the ischium. The space between them
is known as the obturator foramen. In the Amphibia the two
parts are not separated, and resemble in this respect the pelvic
girdle of Fishes. They generally meet the corresponding elements
of the opposite side ventrally, and form a symphysis with them.
The symphysis pubis, and symphysis ischii may be continuous
(Mammalia, Amphibia).

The observations on the development of the pelvic girdle in
the Amphibia and Amniota are nearly as scanty as on those of
Fishes.

Amphibia. In the Amphibia (Bunge, No. 473) the two halves of
the pelvic girdle are formed as independent masses of cartilage, which
subsequently unite in the ventral line.

In the Urodelous Amphibia (Triton) each mass is a simple plate of
cartilage divided into a dorsal and ventral section by the acetabulum.
The ventral parts, which are not divided into two regions, unite in a
symphysis comparatively late.

The dorsal section ossifies as the ilium. The ventral usually contains
a single ossification in its posterior part which forms the ischium ; while
the anterior part, which may be considered as representing the pubis,
usually remains cartilaginous ; though Huxley (No. 475) states that it has
a separate centre of ossification in Salamander, which however does not
appear to be always present (Bunge). There is a small obturator foramen
between the ischium and pubis, which gives passage to the obturator nerve.
It is formed by the part of the tissue where the nerve is placed not be-
coming converted into cartilage.

There is a peculiar cartilage in the ventral median line in front of the
pubis, which is developed independently of and much later than the true
parts of the pelvic girdle. It may be called the praepubic cartilage.

Reptilia. In Lacertilia the pelvic girdle is formed as a somewhat
triradiate mass of cartilage on each side, with a dorsal (iliac) process, and two
ventral (pubic and ischiad) processes. The acetabulum is placed on the
outer side at the junction of the three processes, each of which may be

6o8 PECTORAL AND PELVIC GIRDLES.

considered to have a share in forming it. The distal ends of the pubis
and ischium are close together when first formed, but subsequently separate.
Each of them unites at a late stage with the corresponding process of the
opposite side in a ventral symphysis. A centre of ossification appears in
each of the three processes of the primitive cartilage.

Aves. In Birds the parts of the pelvic girdle no longer develop as a
continuous cartilage (Bunge). Either the pubis may be distinct, or, as in the
Uuck, all the elements. The ilium early exhibits a short anterior process,
but the pubis and ischium are at first placed with their long axes at right
angles to that of the ilium, but gradually become rotated so as to lie paral-
lel with it, their distal ends pointing backwards, and not uniting ventrally
excepting in one or two Struthious forms.

Mammalia. In Mammalia the pelvic girdle is formed in cartilage
as in the lower forms, but in Man at any rate the pubic part of the carti-
lage is formed independently of the remainder (Rosenberg). There are
the usual three centres of ossification, which unite eventually into a single
bone — the innominate bone. The pubis and ischium of each side unite with
each other ventrally, so as completely to enclose the obturator foramen.

Huxley holds that the so-called marsupial bones of Monotremes and
Marsupials, which as shewn by Gegenbaur (No. 474) are performed in carti-
lage, are homologous with the praepubis of the Urodela ; but considering
the great gap between the Urodela and Mammalia this homology can only
be regarded as tentative. He further holds that the anterior prolongations
of the cartilaginous ventral ends of the pubis of Crocodilia are also struc-
tures of the same nature.

BIBLIOGRAPHY.

(473) A. Bunge. Untersuch. z, Entwick. d. Beckengiirtels d. Amphibien,
Reptilien u. Vogel, Inaug. Diss. Dorpat, 1880.

(474) C. Gegenbaur. " Ueber d. Ausschluss des Schambeins von d. Pfanne
d. Hiiftgelenkes." Morph. Jahrbuch, Vol. II. 1876.

(475) Th. H. Huxley. "The characters of the Pelvis in Mammalia, etc."
Proc. of Roy. Soc., Vol. xxvm. 1879.

(476) A. Sabatier. Comparaison des ceintures et des membres anterieurs et
posterieurs dans la Serie d. Vertebrcs. Montpellier, 1880.

Comparison of Pectoral and Pelvic girdles.

Throughout the Vertebrata a more or less complete serial
homology may be observed between the pectoral and pelvic
girdles.

In the cartilaginous Fishes each girdle consists of a continuous
band, a dorsal and ventral part being indicated by the articulation
of the fin ; the former being relatively undeveloped in the pelvic

LIMBS. 609

girdle, while in the pectoral it may articulate with the vertebral
column. In the case of the pectoral girdle secondary membrane
bones become added to the primitive cartilage in most Fishes,
which are not developed in the case of the pelvic girdle.

In the Amphibia and Amniota the ventral section of each
girdle becomes divided into an anterior and a posterior part, the
former constituting the praecoracoid and pubis, and the latter the
coracoid and ischium ; these parts are however very imperfectly
differentiated in the pelvic girdle of the Urodela. The ventral
portions of the pelvic girdle usually unite below in a symphysis.
They also meet each other ventrally in the case of the pectoral
girdle in Amphibia, but in most other types are separated by
the sternum, which has no homologue in the pelvic region, unless
the praepubic cartilage is to be regarded as such. The dorsal or
scapular section of the pectoral girdle remains free ; but that of
the pelvic girdle acquires a firm articulation with the vertebral
column.

If the clavicle of the higher types is derived from the mem-
brane bones of the pectoral girdle of Fishes, it has no homologue
in the pelvic girdle ; but if, as Gotte and Hoffmann suppose, it is
a part of the primitive cartilaginous girdle, the ordinary view as
to the serial homologies of the ventral sections of the two girdles
in the higher types will need to be reconsidered.

Limbs.

It will be convenient to describe in this place not only the
development of the skeleton of the limbs but also that of the
limbs themselves. The limbs of Fishes are moreover so different
from those of the Amphibia and Amniota that the development
of the two types of limb may advantageously be treated separately.

In Fishes the first rudiments of the limbs appear as slight
longitudinal ridge-like thickenings of the epiblast, which closely
resemble the first rudiments of the unpaired fins.

These ridges are two in number on each side, an anterior
immediately behind the last visceral fold, and a posterior on the
level of the cloaca. In most Fishes they are in no way con-
nected, but in some Elasmobranch embryos, more especially in
Torpedo, they are connected together at their first development
B. in. 39

6io

PAIRED FINS OF ELASMOBRANCHII.

by a line of columnar epiblast cells1. This connecting line of
columnar epiblast is a very transitory structure, and after its
disappearance the rudimentary fins become more prominent,
consisting (fig. 343, &) of a projecting ridge both of epiblast and
mesoblast, at the outer edge of which is a fold of epiblast only,
which soon reaches considerable dimensions. At a later stage
the mesoblast penetrates into this fold and the fin becomes a
simple ridge of mesoblast, covered
by epiblast. The pectoral fins
are usually considerably ahead
of the pelvic fins in development.

For the remaining history it
is necessary to confine ourselves
to Scylliurn as the only type
which has been adequately
studied.

The direction of the original
ridge which connects the two fins
of each side is nearly though not
quite longitudinal, sloping some-
what obliquely downwards. It
thus comes about that the attach-
ment of each pair of limbs is
somewhat on a slant, and that
the pelvic pair nearly meet each
other in the median ventral line
a little way behind the anus.

The elongated ridge, forming
the rudiment of each fin, gradually projects more and more, and
so becomes broader in proportion to its length, but at the same
time its actual attachment to the side of the body becomes
shortened from behind forwards, so that what was originally the
attached border becomes in part converted into the posterior
border. This process is much more completely carried out in
the case of the pectoral fins than in that of the pelvic, and the
changes of form undergone by the pectoral fin in its develop-
ment may be gathered from figs. 344 and 348.

FIG. 343. SECTION THROUGH
THE VENTRAL PART OF THE TRUNK
OF A YOUNG EMBRYO OF SCYLLIUM AT
THE LEVEL OF THE UMBILICAL CORD.

b. pectoral fin ; ao. dorsal aorta ;
cav. cardinal vein ; ua. vitelline ar-
tery ; u.v, vitelline vein ; al. duode-
num ; /. liver ; sd. opening of seg-
mented duct into the body cavity ;
mp. muscle plate ; «;«. umbilical
canal.

1 I''. M. I'alfour. Monograph on Elasmobranfh l-'hhes, pp. 101—2.

LIMBS. 6ll

Before proceeding to the development of the skeleton of
the fin it may be pointed out that the connection of the two
rudimentary fins by a continuous epithelial line suggests the
hypothesis that they are the remnants of two continuous lateral
fins1.

Shortly after the view that the paired fins were remnants of
continuous lateral fins had been put forward in my memoir on
Elasmobranch Fishes, two very interesting papers were published
by Thacker (No. 489) and Mivart (No. 484) advocating this
view on the entirely independent grounds of the adult structure
of the skeleton of the paired fins in comparison with that of the
unpaired fins2.

The development of the skeleton has unfortunately not been
as yet very fully studied. I have however made some investi-
gations on this subject on Scyllium, and 'Swirski has also made
some on the Pike.

In Scyllium the development of both the pectoral and pelvic
fins is very similar.

In both fins the skeleton in its earliest stage consists of a bar
springing from the posterior side of the pectoral or pelvic girdle,
and running backwards parallel to the long axis of the body.
The outer side of this bar is continued into a plate which

1 Both Maclise arid Humphry {Journal of Anat. and Pkys., Vol. v.) had
previously suggested that the paired fins were related to the unpaired fins.

2 Davidoff in a Memoir (No. 477) which forms an important contribution to our
knowledge of the structure of the pelvic fins has attempted from his observations to
deduce certain arguments against the lateral fin theory of the limbs. His main
argument is based on the fact that a variable but often considerable number of the
spinal nerves in front of the pelvic fin are united, by a longitudinal commissure, with
the true plexus of the nerves supplying the fin. From this he concludes that the pelvic
fin has shifted its position, and that it may once therefore have been situated close
behind the visceral arches. If this is the strongest argument which can be brought
against the theory advocated in the text, there is I trust a considerable chance of its
being generally accepted. For even granting that Davidoff's deduction from the
character of the pelvic plexus is correct, there is, so far as I see, no reason in the
nature of the lateral fin theory why the pelvic fins should not have shifted, and on the
other hand the longitudinal cord connecting some of the spinal nerves in front of the
pelvic fin may have another explanation. It might for instance be a remnant of the
time when the pelvic fin had a more elongated form than at present, and accordingly
extended further forwards.

In any case our knowledge of the nature and origin of nervous plexuses is far too
imperfect to found upon their character such conclusions as those of Davidoff.

39—2

612

PAIRED FINS OF ELASMOBRANCHII.

extends into the fin, and which becomes very early segmented
into a series of parallel rays at right angles to the longitudinal
bar.

In other words, the primitive skeleton of both the fins
consists of a longitudinal bar running along the base of the fin,

FIG. 344. PECTORAL FIN OF A YOUNG EMBRYO OF SCYLLIUM IN LONGI-
TUDINAL AND HORIZONTAL SECTION.

The skeleton of the fin was still in the condition of embryonic cartilage.
b.p. basipterygium (eventual metapterygium) ; fr. fin rays; p.g. pectoral girdle in
transverse section; /. foramen in pectoral girdle; pc. wall of peritoneal cavity.

and giving off at right angles series of rays which pass into the
fin. The longitudinal bar, which may be called the basiptery-
gium, is moreover continuous in front with the pectoral or
pelvic girdle as the case may be.

The primitive skeleton of the pectoral fin is shewn in
longitudinal section in fig. 344, and that of the pelvic fin at a
slightly later stage in fig. 345.

A transverse section shewing the basipterygium (inpi) of the
pectoral fin, and the plate passing from it into the fin, is shewn
in fig. 346.

Before proceeding to describe the later history of the two
fins it may be well to point out that their embryonic structure
completely supports the view which has been arrived at from
the consideration of the soft parts of the fin.

My observations shew that the embryonic skeleton of the
paired fin consists of a series of parallel rays similar to those
of the unpaired fins. These rays support the soft part of the fin
which has the form of a longitudinal ridge, and are continuous
at their base with a longitudinal bar, which may very probably

LIMBS.

613

be due to secondary development. As pointed out by Mivart, a
longitudinal bar is also occa-
sionally formed to support the
cartilaginous rays of unpaired
fins. The longitudinal bar of
the paired fins is believed by
both Thacker and Mivart to
be due to the coalescence of
the bases of primitively inde-
pendent rays, of which they
believe the fin to have been
originally composed. This
view is probable enough in
itself, but there is no trace

FIG. 345. PELVIC FIN OF A VERY
YOUNG FEMALE EMBRYO OF SCYLLIUM
STELLARE.

bb. basipterygium ; pu. pubic process
of pelvic girdle ; il. iliac process of pelvic
girdle.

in the embryo of the bar in question being formed by the
coalesceace of rays, though the fact of its being perfectly
continuous with the bases of the rays is somewhat in favour
of this view1.

A point may be noticed here which may perhaps appear to be a
difficulty, viz. that to a considerable extent in the pectoral, and to some
extent in the pelvic fin the embryonic cartilage from which the fin-rays
are developed is at first a continuous lamina, which subsequently segments
into rays. I am however inclined to regard this merely as a result of the
mode of conversion of the indifferent mesoblast into cartilage ; and in any
case no conclusion adverse to the above view can be drawn from it, since
I find that the rays of the unpaired fin are similarly segmented from a
continuous lamina. In all cases the segmentation of the rays is to a large
extent completed before the tissue in question is sufficiently differentiated
to be called cartilage by an histologist.

Thacker and Mivart both hold that the pectoral and pelvic
girdles have been evolved by ventral and dorsal growths of the
anterior end of the longitudinal bar supporting the fin-rays.

There is, so far as I see, no theoretical objection to be taken
to this view, and the fact of the pectoral and pelvic girdles
originating continuously, and long remaining united with the

1 Thacker more especially founds his view on the adult form of the pelvic fins in
the cartilaginous Ganoids ; Polyodon, in which the part which constitutes the basal
plate in other forms is divided into separate segments, being mainly relied on. It is
possible that the segmentation of this plate, as maintained by Gegenbaur and Davidoff,
is secondary, but Thacker's view that the segmentation is a primitive character seems
to me, in the absence of definite evidence to the reverse, the more natural one.

614

THE PELVIC FIN.

longitudinal bars of their respective fins is in favour of rather
than against this view. The same may be said of the fact that
the first part of each girdle to be formed is that in the neigh-
bourhood of the longitudinal bar (basipterygium) of the fin, the
dorsal and ventral prolongations being subsequent growths.

The later development of the skeleton of the two fins is more
conveniently treated separately.

The pelvic fin. The changes in the pelvic fin are compara-
tively slight. The fin remains through life as a nearly horizontal
lateral projection of the body, and the longitudinal bar — the

FIG. 346. TRANSVERSE SECTION THROUGH THE PECTORAL FIN OF A YOUNG

EMBRYO OK SCYLLIUM STELLARE.
mpt. basipterygial bar (metapterygium) ; fr. fin ray; m. muscles; hf. horny fibres.

basipterygium — at its base always remains as such. It is for a
considerable period attached to the pelvic girdle, but eventually
becomes segmented from it. Of the fin rays the anterior
remains directly articulated with the pelvic girdle on the sepa-
ration of the basipterygium (fig. 347), and the remaining rays
finally become segmented from the basipterygium, though they
remain articulated with it. They also become to some extent
transversely segmented. The posterior end of the basipterygial
bar also becomes segmented off as the terminal ray.

The pelvic fin thus retains in all essential points its primitive
arrangement.

LIMBS.

6l5

The pectoral fin. The earliest stage of the pectoral fin

There

FIG. 347. PELVIC FIN OF A YOUNG MALE EMBRYO OF SCYLLIUM STELLARE.

bp. basipterygium ; m.o. process of basipterygium continued into clasper; il. iliac
process of pectoral girdle ; pit. pubis.

differs from that of the pelvic fin only in minor points,
is the same longitudinal
or basipterygial bar to
which the fin-rays are
attached, whose position
at the base of the fin is
clearly seen in the trans-
verse section (fig. 346,
mpf). In front the bar is
continuous with the pec-
toral girdle (figs. 344 and

348).

The changes which
take place in the course of
the further development
are however very much
more considerable in the
case of the pectoral than
in that of the pelvic fin. "'«• 3+8. F^OJJL «,,,»« «»v.

By the process spoken mpt metapterygium (basipterygium of earlier

stage); me.p. rudiment of future pro- and meso-
pterygium ; sc. cut surface of scapular process ;
cr. coracoid process;/;', foramen;/, horny fibres.

of above, by which the
attachment of the pec-

6l6 THE PECTORAL FIN.

toral fin to the body wall becomes shortened from behind
forwards, the basipterygial bar is gradually rotated outwards,
its anterior end remaining attached to the pectoral girdle.
In this way this bar comes to form the posterior border of the
skeleton of the fin (figs. 348 and 349, mp], constituting what
Gegenbaur called the metapterygium, and eventually becomes
segmented off from the pectoral girdle, simply articulating
with its hinder edge.

The plate of cartilage, which is continued outwards from the
basipterygium, or as we may now call it, the metapterygium,
into the fin, is not nearly so completely divided up into fin-rays
as in the case of the pelvic fin, and this is especially the case
with the basal part of the plate. This basal part becomes
in fact at first only divided into two parts (fig. 348) a small
anterior part at the front end (me.p), and a larger posterior along
the base of the remainder of the fin. The anterior part directly
joins the pectoral girdle at its base, resembling in this respect
the anterior fin-ray of the pelvic girdle. It constitutes the
rudiment of the mesopterygium and propterygium of Gegenbaur.
It bears four fin-rays at its extremity, the anterior not being
well marked. The remaining fin-rays are borne by the edge of
the plate continuous with the metapterygium.

The further changes in the cartilages of the limb are not
important, and are easily understood by reference to fig. 349
representing the limb of a nearly full-grown embryo. The
front end of the anterior basal cartilage becomes segmented
off as a propterygium, bearing a single fin-ray, leaving the
remainder of the cartilage as a mesopterygium. The remainder
of the now considerably segmented fin-rays are borne by the
metapterygium.

The mode of development of the pectoral fin demonstrates
that, as supposed by Mivart, the metapterygium is the homo-
logue of the basal cartilage of the pelvic fin.

From the mode of development of the fins of Scyllium conclusions
may be drawn adverse to the views recently put forward on the struc-
ture of the fin by Gegenbaur and Huxley, both of whom consider the
primitive type of fin to be most nearly retained in Ceratodus, and to
consist of a central multisegmented axis with numerous rays. Gegenbaur
derives the Elasmobranch pectoral fin from a form which he calls the
archipterygium, nearly like that of Ceratodus, with a median axis and two

LIMBS.

6I7

rows of rays ; but holds that in addition to the rays attached to the median
axis, which are alone found in Ceratodus, there were other rays directly
articulated to the shoulder-girdle. He considers that in the Elasmobranch
fin the majority of the lateral rays on the posterior (median or inner
according to his view of the position of the limb) side have become
aborted, and that the central axis is represented by the metapterygium ;
while the pro- and mesopterygium and their rays are, he believes, derived
from those rays of the archipterygium which originally articulated directly
with the shoulder-girdle.

Gegenbaur's view appears to me to be absolutely negatived by the facts
of development of the pectoral fin in Scyllium ; not so much because the
pectoral fin in this form is necessarily to be regarded as primitive, but
because what Gegenbaur holds to be the primitive axis of the biserial fin
is demonstrated to be really the base, and it is only in the adult that it is
conceivable that a second set of lateral rays could have existed on the
posterior side of the metapterygium. If Gegenbaur's view were correct
we should expect to find in the embryo, if anywhere, traces of the second
set of lateral rays ; but the fact is that, as may easily be seen by an inspec-
tion of figs. 344 and 346, such a second set of lateral rays could not pos-
sibly have existed in a type .
of fin like that found in the
embryo1. With this view of
Gegenbaur's it appears to
me that the theory held by
this anatomist to the effect
that the limbs are modified
gill arches also falls ; in
that his method of deriving
the limbs from gill arches
ceases to be admissible,
while it is not easy to see
how a limb, formed on the
type of the embryonic limb
of Elasmobranchs, could be
derived from a visceral arch
with its branchial rays2.

Gegenbaur's older view

FIG. 349. SKELETON OF THE PECTORAL FIN
AND PART OF PECTORAL GIRDLE OF A NEARLY
RIPE EMBRYO OF SCYLLIUM STELLARE.

m.p. metapterygium ; me.p. mesopterygium ;
//. propterygium ; cr. coracoid process.

1 If, which I very much doubt, Gegenbaur is right in regarding certain rays found
in some Elasmobranch pectoral fins as rudiments of a second set of rays on the
posterior side of the metapterygium, these rays will have to be regarded as structures
in the act of being evolved, and not as persisting traces of a biserial fin.

2 Some arguments in favour of Gegenbaur's theory adduced by Wiedersheim as
a result of his researches on Protopterus are interesting. The attachment which he
describes between the external gills and the pectoral girdle is no doubt remarkable,
but I would suggest that the observations we have on the vascular supply of these
gills demonstrate that this attachment is secondary.

6l8 THE CHEIKOPTERYGIUM.

that the Elasmobranch fin retains a primitive uniserial type appears to me
to be nearer the truth than his more recent view on this subject ; though I
hold that the fundamental point established by the development of these
parts in Scyllium is that the posterior border of the adult Elasmobranch fin
is the primitive base line, i.e. the line of attachment of the fin to the side of
the body.

Huxley holds that the mesopterygium is the proximal piece of the axial
skeleton of the limb of Ceratodus, and derives the Elasmobranch fin from
that of Ceratodus by the shortening of its axis and the coalescence of some
of its elements. The secondary character of the mesopterygium, and its
total absence in the embryo Scyllium, appears to me as conclusive against
Huxley's view, as the character of the embryonic fin is against that of
Gegenbaur ; and I should be much more inclined to hold that the fin of
Ceratodus has been derived from a fin like that of the Elasmobranchii by
a series of steps similar to those which Huxley supposes to have led to the
establishment of the Elasmobranch fin, but in exactly the reverse order.

With reference to the development of the pectoral fin in the Teleostei
there are some observations of 'Swirski (No. 488) which unfortunately do
not throw very much light upon the nature of the limb.

'Swirski finds that in the Pike the skeleton of the limb is formed of a
plate of cartilage, continuous with the pectoral girdle ; which soon becomes
divided into a proximal and a distal portion. The former is subsequently
segmented into five basal rays, and the latter into twelve parts, the number
of which subsequently becomes reduced.

These investigations might be regarded as tending to shew that the
basipterygium of Elasmobranchii is not represented in Teleostei, owing to
the fin rays not having united into a continuous basal bar, but the obser-
vations are not sufficiently complete to admit of this conclusion being
founded upon them with any certainty.

Tlie ckeiropterygium.

Observations on the early development of the pentadactyloid
limbs of the higher Vertebrata are comparatively scanty.

The limbs arise as simple outgrowths of the sides of the
body, formed both of epiblast and mesoblast. In the Amniota,
at all events, they are processes of a special longitudinal ridge
known as the Wolffian ridge. In the Amniota they also bear
at their extremity a thickened cap of epiblast, which may be
compared with the epiblastic fold at the apex of the Elasmo-
branch fin.

Both limbs have at first a precisely similar position, both
being directed backwards and being parallel to the surface of
the body.

I 111: CHEIROPTERYGIUM.

619

In the Urodela (Gotte) the ulnar and fibular sides are
primitively dorsal, and the radial and tibial ventral : in Mam-
malia however Kolliker states that the radial and tibial edges
are from the first anterior.

The exact changes of position undergone by the limbs in the
course of development are not fully understood. To suit a
terrestrial mode of life the flexures of the two limbs become
gradually more and more opposite, till in Mammalia the corre-
sponding joints of the two limbs are turned in completely
opposite directions.

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
digits, become differentiated within this, by the conversion
of definite regions into cartilage, which may either be completely
distinct or be at first united. These cartilaginous elements
subsequently ossify.

The later development of the parts, more especially of the carpus and
tarsus, has been made the subject of considerable study ; and important
results have been thereby obtained as to the homology of the various
carpal and tarsal bones throughout the Vertebrata ; but this subject is too
special to be treated of here. The early development, including the suc-
cession of the growth of the different parts, and the extent of continuity
primitively obtaining between them, has on the other hand been but little
investigated ; recently however the development of the limbs in the Uro-
dela has been worked out in this way by two anatomists, Gotte (No. 482)
and Strasser (No. 487), and their results, though not on all points in com-
plete harmony, are of considerable interest, more especially in their bearing
on the derivation of the pentadactyloid limb from the piscine fin. Till
however further investigations of the same nature have been made upon
other types, the conclusions to be drawn from Gotte and Strasser's observa-
tions must be regarded as somewhat provisional, the actual interpretation
of various ontological processes being very uncertain.

The forms investigated are Triton and Salamandra. We may remind
the reader that the hand of the Urodela has four digits, and the foot five,
the fifth digit being absent in the hand1. In Triton the proximal row of
carpal bones consists (using Gegenbaur's nomenclature) of (i) a radiale, and
(2 and 3) an intermedium and ulnare, partially united. The distal row is
formed of four carpals, of which the first often does not support the first
1 This seems to me clearly to follow from Gotte and Strasser's observations.

620 THE GHE1ROPTERYGIUM.

metacarpal ; while the second articulates with both the first and second
metacarpals. In the foot the proximal row of tarsals consists of a tibiale,
an intermedium and a fibulare. The distal row is formed of four tarsals, the
first, like that in the hand, often not articulating with the first metatarsal,
the second supporting the first and second metatarsals ; and the fourth the
fourth and fifth metatarsals.

The mode of development of the hand and foot is almost the same. The
most remarkable feature of development is the order of succession of the
digits. The two anterior (radial or tibial) are formed in the first instance,
and then the third, fourth and fifth in succession.

As to the actual development of the skeleton Strasser, whose observations
were made by means of sections, has arrived at the following results.

The humerus with the radius and ulna, and the corresponding parts in
the hind limb, are the first parts to be differentiated in the continuous plate
of tissue from which the skeleton of the limb is formed. Somewhat later a
cartilaginous centre appears at the base of the first and second fingers
(which have already appeared as prominences at the end of the limb) in the
situation of the permanent second carpal of the distal row of carpals ; and
the process of chondrification spreads from this centre into the fingers and
into the remainder of the carpus. In this way a continuous carpal plate
of cartilage is established, which is on the one hand continuous with the
cartilage of the two metacarpals, and on the other with the radius and ulna.

In the cartilage of the carpus two special columns may be noticed, the
one on the radial side, most advanced in development, being continuous with
the radius ; the other less developed column on the side of the ulna being
continuous both with the ulna and with the radius. The ulna and radius are
not united with the humerus.

In the further growth the third and fourth digits, and in the foot the fifth
digit also, gradually sprout out in succession from the ulnar side of the
continuous carpal plate. The carpal plate itself becomes segmented from the
radius and ulna, and divided up into the carpal bones.

The original radial column is divided into three elements, a proximal the
radiale, a middle element the first carpal, and a distal the second carpal
already spoken of. The first carpal is thus situated between the basal carti-
lage of the second digit and the radiale, and would therefore appear
to be the representative of a primitive middle row of carpal
bones, of which the centrale is also another representative.

The centrale and intermedium are the middle and proximal products of
the segmentation of the ulnar column of the primitive carpus, the distal
second carpal being common both to this column and to the radial column.

The ulnar or fibular side of the carpus or tarsus becomes divided into a
proximal element — the ulnare or fibulare — the ulnare remaining partially
united with the intermedium. There are also formed from this plate two
carpals to articulate with digits 3 and 4 ; while in the foot the corresponding
elements articulate respectively with the third digit, and with the fourth and
fifth digits.

THE CIIF.IROPTERYGIUM. 621

Gotte, whose observations were made in a somewhat different method to
those of Strasser, is at variance with him on several points. He finds that
the primitive skeleton of the limb consists of a basal portion, the humerus,
continued into a radial and an ulnar ray, which are respectively prolonged
into the two first digits. The two rays next coalesce at the base of the
fingers to form the carpus, and thus the division of the limb into the brachium,
antebrachium and manus is effected.

The ulna, which is primitively prolonged into the second digit, is
subsequently separated from it and is prolonged into the third ; from the side
of the part of the carpus connecting the ulna with the third digit the fourth
digit is eventually budded out, and in the foot the fourth and fifth digits arise
from the corresponding region. Each of the three columns connected
respectively with the first, second, and third digits becomes divided into three
successive carpal bones, so that Gotte holds the skeleton of the hand or foot
to be formed of a proximal, a middle, and a distal row of carpal bones each
containing potentially three elements. The proximal row is formed of the
radiale, intermedium and ulnare ; the middle row of carpal i, the centrale
and carpal 4, and the distal of carpal 2 (consisting according to Gotte of two
coalesced elements) and carpal 3.

The derivation of the cheiropterygium from the ichthyoptcrygium. All
anatomists are agreed that the limbs of the higher Vertebrata are derived
from those of Fishes, but the gulf between the two types of limbs is so great
that there is room for a very great diversity of opinion as to the mode of
evolution of the cheiropterygium. The most important speculations on the
subject are those of Gegenbaur and Huxley.

Gegenbaur holds that the cheiropterygium is derived from a uniserial
piscine limb, and that it consists of a primitive stem, to which a series of
lateral rays are attached on one (the radial) side ; while Huxley holds that the
cheiropterygium is derived from a biserial piscine limb by the "length-
ening of the axial skeleton, accompanied by the removal of its distal
elements further away from the shoulder-girdle and by a diminution in the
number of the rays."

Neither of these theories is founded upon ontology, and the only ontologi-
cal evidence we have which bears on this question is that above recorded
with reference to the development of the Urodele limb.

Without holding that this evidence can be considered as in any way
conclusive, its tendency would appear to me to be in favour of regarding the
cheiropterygium as derived from a uniserial type of fin. The humerus or
femur would appear to be the basipterygial bars (metapterygium), which
have become directed outwards instead of retaining their original position
parallel to the length of the body at the base of the fin. The anterior
(proximal) fin-rays and the pro- and mesopterygium must be supposed to
have become aborted, while the radius or ulna, and tibia or fibula are two
posterior fin-rays (probably each representing several coalesced rays like the
pro- and mesopterygium) which support at their distal extremities more
numerous fin-rays consisting of the rows of carpal and tarsal bones.

622 THE CHEIROPTERYGIUM.

This view of the cheiropterygium corresponds in some respects with that
put forward by Gotte as a result of his investigations on the development of
the Urodele limbs, though in other respects it is very different. A difficulty
of this view is the fact that it involves our supposing that the radial edge of
the limb corresponds with the metapterygial edge of the piscine fin. The
difficulties of this position have been clearly pointed out by Huxley, but the
fact that in the primitive position of the Urodele limbs the radius is ventral
and the ulna dorsal shews that this difficulty is not insuperable, in that it is
easy to conceive the radial border of the fin to have become rotated from its
primitive Elasmobranch position into the vertical position it occupies in the
embryos of the Urodela, and then to have been further rotated from this
position into that which it occupies in the adult Urodela and in all higher
forms.

BIBLIOGRAPHY of the Limbs.

(477) M. v. Davidoff. "Beitrage z. vergleich. Anat. d. hinteren Gliedmaassen
d. Fische I." Morphol. Jahrbuch, Vol. v. 1879.

(478) C. Gegenbaur. Untersuckungen z. vergleich. Anat. d. Wirbelthiere.
Leipzig, 1864 — 5. Erstes Heft. Carpus u. Tarsus. Zweites Heft. Brustflosse d.
Fische.

(479) C. Gegenbaur. "Ueb. d. Skelet d. Gliedmaassen d. Wirbelthiere im
Allgemeinen u. d. Hintergliedmaassen d. Selachier insbesondere." Jenaische Zeit-
sckrift, Vol. V. 1870.

(480) C. Gegenbaur. " Ueb. d. Archipterygium." Jenaische Zeitschrift, Vol.
vii. 1873.

(481) C. Gegenbaur. "Zur Morphologic d. Gliedmaassen d. Wirbelthiere."
Morphologisches Jahrbuch, Vol. II. 1876.

(482) A. Gotte. Ueb. Entivick. u. Regeneration d. Gliedmaassenskelets d. Molche.
Leipzig, 1879.

(483) T. H. Huxley. "On Ceratodus Forsteri, with some observations on the
classification of Fishes." Proc. Zool. Soc. 1876.

(484) St George Mivart. "On the Fins of Elasmobranchii." Zoological
Trans., Vol. x.

(485) A. Rosenberg. "Ueb. d. Entwick. d. Extremitaten-Skelets bei einigen
d. Reduction ihrer Gliedmaassen charakterisirten Wirbelthieren." Zeil.f. iviss. Zool.,
Vol. xxin. 1873.

(486) E. Rosenberg. "Ueb. d. Entwick. d. Wirbelsaule u. d. centrale carpi
d. Menschen. " Morphologisches Jahrbuch, Vol. I. 1875.

(487) H. Strasser. "Z. Entwick. d. Extremitatenknorpel bei Salamandern u.
Tritonen." Morphologisches Jahrbuch, Vol. V. 1879.

(488) G. 'S wirski. Untersitch. iib. d. Entwick. d. Schultergitrtels u. d. Skelcls d.
Brustflosse d. Hechts. Inaug. Diss. Dorpat, 1880.

(489) J. K. Thacker. "Median and paired fins. A contribution to the history
of the Vertebrate limbs." Trans, of the Connecticut Acad., Vol. ill. 1877.

(490) J. K. Thacker. "Ventral fins of Ganoids." Trans, of the Connecticut
Acad., Vol. iv. 1877.

CHAPTER XXI.

THE BODY CAVITY, THE VASCULAR SYSTEM, AND THE
VASCULAR GLANDS.

The Body cavity.

IN the Ccelenterata no body cavity as distinct from the
alimentary cavity is present ; but in the remaining Invertebrata
the body cavity may (i) take the form of a wide space separating
the wall of the gut from the body wall, or (2) may be present in
a more or less reduced form as a number of serous spaces, or
(3) only be represented by irregular channels between the
muscular and connective-tissue cells filling up the interior of the
body. The body cavity, in whatever form it presents itself, is
probably filled with fluid, and the fluid in it may contain special
cellular elements. A well developed body cavity may coexist
with an independent system of serous spaces, as in the Verte-
brata and the Echinodermata ; the perihaemal section of the
body cavity of the latter probably representing the system of
serous spaces.

In several of the types with a well developed body cavity it
has been established that this cavity originates in the embryo
from a pair of alimentary diverticula, and the cavities resulting
from the formation of these diverticula may remain distinct, the
adjacent walls of the two cavities fusing to form a dorsal and a
ventral mesentery.

It is fairly certain that some groups, e.g. the Tracheata, with
imperfectly developed body cavities are descended from ancestors
which were provided with well developed body cavities, but how
far this is universally the case cannot as yet be definitely
decided, and for additional information on this subject the

624 CIIORDATA.

reader is referred to pp. 355 — 360 and to the literature there
referred to.

In the Chaetopoda and the Tracheata the body cavity arises
as a series of paired compartments in the somites of mesoblast
(fig. 350) which have at first a very restricted extension on the
ventral side of the body, but eventually extend dorsalwards and
vcntralwards till each cavity is a half circle investing the
alimentary tract ; on the dorsal side the walls separating the two

FIG. 350. LONGITUDINAL SECTION THROUGH AN EMBRYO OF AGELINA
LABYRINTHICA.

The section is taken slightly to one side of the middle line so as to shew the rela-
tion of the mesoblastic somites to the limbs. In the interior are seen the yolk
segments and their nuclei.

i — 16. the segments ; pr.l. procephalic lobe ; do. dorsal integument.

half cavities usually remain as the dorsal mesentery, while
ventrally they are in most instances absorbed. The transverse
walls, separating the successive compartments of the body
cavity, generally become more or less perforated.

Chordata. In the Chordata the primitive body cavity is
cither directly formed from a pair of alimentary diverticula
(Cephalochorda) (fig. 3) or as a pair of spaces in the mesoblastic
plates of the two sides of the body (fig. 20).

As already explained (pp. 294 — 300) the walls of the dorsal
sections of the primitive body cavity soon become separated
from those of the ventral, and becoming segmented constitute
the muscle plates, while the cavity within them becomes

I

THE BODY CAVITY.

625

the

obliterated : they are dealt with in a separate chapter. The
ventral part of the primitive cavity alone constitutes the
permanent body cavity.

The primitive body cavity in the lower Vertebrata is at first
continued forwards into the region of the head, but on the
formation of the visceral clefts the cephalic section of the body
cavity becomes divided into a series of separate compartments.
Subsequently these sections of the body cavity become oblite-
rated ; and, since their walls give rise to muscles, they may
probably be looked upon as equivalent to the dorsal sections of
the body cavity in the trunk, and will be treated of in connection
with the muscular system.

As a result of its mode of origin the body cavity in
trunk is at first divided into two
lateral halves ; and part of the meso-
blast lining it soon becomes distin-
guished as a special layer of epithe-
lium, known as the peritoneal epithe-
lium, of which the part bounding the
outer wall forms the somatic layer,
and that bounding the inner wall the
splanchnic layer. Between the two
splanchnic layers is placed the gut.
On the ventral side, in the region of
the permanent gut, the two halves
of the body cavity soon coalesce,
the septum between them becoming
absorbed, and the splanchnic layers
of epithelium of the two sides uniting
at the ventral side of the gut, and
the somatic layers at the median
ventral line of the body wall (fig.

In the lower Vertebrata the body
cavity is originally present even in
the post-anal region of the trunk, but
usually atrophies early, frequently
before the two halves coalesce.

On the dorsal side of the gut the
B. III.

FIG. 351. SECTION THROUGH
THE TRUNK OF A SCYLLIUM EM-
BRYO SLIGHTLY YOUNGER THAN

28 F.

sp.c. spinal canal ; W. white
matter of spinal cord ; pr. poste-
rior nerve-roots ; cA. notochord ;
x. sub-notochordal rod ; ao. aorta ;
nip. muscle-plate ; nip1, inner layer
of muscle-plate already converted
into muscles; Vr. rudiment of
vertebral body ; si. segmental
tube ; sd. segmental duct ; sp.v.
spiral valve ; v. subintestinal vein ;
p.o. primitive generative cells.

40

626 ABDOMINAL PORES.

two halves of the body cavity never coalesce, but eventually the
splanchnic layers of epithelium of the two sides, together with a
thin layer of interposed mesoblast, form a delicate membrane,
known as the mesentery, which suspends the gut from the dorsal
wall of the body (figs. 119 and 351). On the dorsal side the
epithelium lining of the body cavity is usually more columnar
than elsewhere (fig. 351), and its cells partly form a covering for
the generative organs, and partly give rise to the primitive
germinal cells. This part of the epithelium is often known as
the germinal epithelium.

Over the greater part of the body cavity the lining epithe-
lium becomes in the adult intimately united with a layer of the
subjacent connective tissue, and constitutes with it a special
lining membrane for the body cavity, known as the peritoneal
membrane.

Abdominal pores. In the Cyclostomata, the majority of the Elas-
mobranchii, the Ganoidei, a few Teleostei, the Dipnoi, and some Sauropsida
(Chelonia and Crocodilia) the body cavity is in communication with the
exterior by a pair of pores, known as abdominal pores, the external
openings of which are usually situated in the cloaca1.

The ontogeny of these pores has as yet been but very slightly investigated.
In the Lamprey they are formed as apertures leading from the body cavity
into the excretory section of the primitive cloaca. This section would
appear from Scott's (No. 87) observations to be derived from part of the
hypoblastic cloacal section of the alimentary tract.

In all other cases they are formed in a region which appears to belong
to the epiblastic region of the cloaca ; and from my observations on Elas-
mobranchs it may be certainly concluded that they are formed there
in this group. They may appear as perforations (i) at the apices of
papilliform prolongations of the body cavity, or (2) at the ends of cloacal
pits directed from the exterior towards the body cavity, or (3) as simple
slit-like openings.

Considering the difference in development between the abdominal pores
of most types, and those of the Cyclostomata, it is open to doubt whether
these two types of pores are strictly homologous.

In the Cyclostomata they serve for the passage outwards of the genera-
tive products, and they also have this function in some of the few Teleostei
in which they are found ; and Gegenbaur and Bridge hold that the primitive
mode of exit of the generative products, prior to the development of the
Miillerian ducts, was probably by means of these pores. I have elsewhere

1 For a full account of these structures the reader is referred to T. W. Bridge,
"Pori Abdominales of Vertebrata. " Journal of Anat. and Physiol. , Vol. XIV., 1879.

THE BODY CAVITY.

627

suggested that the abdominal pores are perhaps remnants of the openings
of segmental tubes ; there does not however appear to be any definite
evidence in favour of this view, and it is more probable that they may have
arisen as simple perforations of the body wall.

Pericardial cavity, pleural cavities, and diaphragm.

In all Vertebrata the heart is at first
placed in the body cavity (fig. 353 A),
but the part of the body cavity contain-
ing it afterwards becomes separated as
a distinct cavity known as the pericar-
dial cavity. In Elasmobranchii, Aci-
penser, etc. a passage is however left
between the pericardial cavity and the
body cavity ; and in the Lamprey a
separation between the two cavities does
not occur during the Ammoccete stage.
In Elasmobranchii the pericardial
cavity becomes established as a distinct
space in front of the body cavity in the
following way. When the two ductus
Cuvieri, leading transversely from the
sinus venosus to the cardinal veins, be-
come developed, a horizontal septum,
shewn on the right side in fig. 352, is
formed to support them, stretching
across from the splanchnic to the so-
matic side of the body cavity, and
dividing the body cavity (fig. 352) in
this part into (i) a dorsal section formed
of a right and left division constituting
the true body cavity (pp), and (2) a
ventral part the pericardial cavity (pc).
The septum is at first of a very small
longitudinal extent, so that both in
front and behind it (fig. 352 on the left
side) the dorsal and ventral sections of the body cavity are in
free communication. The septum soon however becomes pro-
longed, and ceasing to be quite horizontal, is directed obliquely
upwards and forwards till it meets the dorsal wall of the body

40 — 2

-ht

FIG. 352. SECTION
THROUGH THE TRUNK OF A
SCYLLIUM EMBRYO SLIGHT-
LY YOUNGER THAN 28 F.

The figure shews the sepa-
ration of the body cavity from
the pericardial cavity by a
horizontal septum in which
runs the ductus Cuvieri ; on
the left side is seen the narrow
passage which remains con-
necting the two cavities.

sp.c. spinal canal ; w. white
matter of spinal cord ; pr.
commissure connecting the
posterior nerve-roots ; ch. noto-
chord ; x. sub-notochordal
rod ; ao. aorta ; sv. sinus ven-
osus ; cav. cardinal vein ; ht.
heart ; pp. body cavity ; pc.
pericardial cavity ; as. solid
oesophagus ; /. liver ; nip. mus-
cle-plate.

628 THE PERICARDIAL CAVITY.

Anteriorly all communication is thus early shut off between the
body cavity and the pericardial cavity, but the two cavities still
open freely into each other behind.

The front part of the body cavity, lying dorsal to the peri-
cardial cavity, becomes gradually narrowed, and is wholly
obliterated long before the close of embryonic life, so that in
adult Elasmobranch Fishes there is no section of the body cavity
dorsal to the pericardial cavity. The septum dividing the body
cavity from the pericardial cavity is prolonged backwards, till it
meets the ventral wall of the body at the point where the liver
is attached by its ventral mesentery (falciform ligament). In
this way the pericardial cavity becomes completely shut off from
the body cavity, except, it would seem, for the narrow communi-
cations found in the adult. The origin of these communications
has not however been satisfactorily worked out.

The septum between the pericardial cavity and the body
cavity is attached on its dorsal aspect to the liver. It is at first
nearly horizontal, but gradually assumes a more vertical position,
and then, owing to the obliteration of the primitive anterior
part of the body cavity, appears to mark the front boundary of
the body cavity. The above description of the mode of forma-
tion of the pericardial cavity, and the explanation of its relations
to the body cavity, probably holds true for Fishes generally.

In the higher types the earlier changes are precisely the
same as those in Elasmobranch Fishes. The heart is at first
placed within the body cavity attached to the ventral wall of
the gut by a mesocardium (fig. 353 A). A horizontal septum is
then formed, in which the ductus Cuvieri are placed, dividing
the body cavity for a short distance into a dorsal (/./) and
ventral (p.c) section (fig. 353 B). In Birds and Mammals, and
probably also in Reptilia, the ventral and dorsal parts of the
body cavity are at first in free communication both in front of
and behind this septum. This is shewn for the Chick in
fig- 353 A and B, which are sections of the same chick, A being
a little in front of B. The septum is soon continued forwards
so as completely to separate the ventral pericardial and the
dorsal body cavity in front, the pericardial cavity extending at
this period considerably further forwards than the body cavity.

Since the horizontal septum, by its mode of origin, is

THE BODY CAVITY.

629

necessarily attached to the ventral side of the gut, the dorsal
part of the primitive body space is divided into two halves by a
median vertical septum formed of the gut and its mesentery
(fig- 353 B). Posteriorly the horizontal septum grows in a
slightly ventral direction along the under surface of the liver
(fig- 354)j 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 cavity1.

A. B.

FIG. 353. TRANSVERSE SECTIONS THROUGH A CHICK EMBRYO WITH TWENTY-
ONE MESOBLASTIC SOMITES TO SHEW THE FORMATION OF THE PERICARDIAI,
CAVITY, A. BEING THE ANTERIOR SECTION.

p.p. body cavity; p.c. pericardial cavity; al. alimentary cavity ; au. auricle; v. ven-
tricle; s.v. sinus venosus; d.c. ductus Cuvieri ; ao. aorta; nip. muscle-plate; me.
medullary cord.

With the complete separation of the pericardial cavity from
the body cavity, the first period in the development of these
parts is completed, and the relations of the body cavity to the

1 Kolliker's account of this septum, which he calls the mesocardium laterale (No.
298, p. 295), would seem to imply that in Mammals it is completed posteriorly even
before the formation of the liver. I doubt whether this takes place quite so early as
he implies, but have not yet determined its exact period by my own observations.

630

THE PERICARDIAL CAVITY.

pericardial cavity become precisely those found in the embryos
of Elasmobranchii. The later changes are however very differ-
ent. Whereas in Fishes the right and left sections of the body
cavity dorsal to the pericardial cavity soon atrophy, in the
higher types, in correlation with the relatively backward situa-
tion of the heart, they rapidly become larger, and receive the
lungs which soon sprout out from the throat.

The diverticula which form the lungs grow out into the
splanchnic mesoblast, in front of
the body cavity ; but as they
grow, they extend into the two
anterior compartments of the body
cavity, each attached by its me-
sentery to the mesentery of the
gut (fig. 354, lg). They soon more-
over extend beyond the region of
the pericardium into the undivided
body cavity behind. This holds
not only for the embryos of the
Amphibia and Sauropsida, but
also for those of Mammalia.

To understand the further

rrianfrps in rhp nerirardial ravitv FlG> 354- SECTION THROUGH

„ pencaraiai cavity THECARDIACREGION OF AN EMBRYO

it is necessary to bear in mind its OF LACERTA MURALIS OF 9 MM. TO

, ,. ,, ,. . . SHEW THE MODE OF FORMATION OF

relations to the adjoining parts. THE PERICARDIAL CAVITY.

'—-/it

It lies at this period completely
ventral to the two anterior pro-

ht. heart ; pc. pericardial cavity ;
al. alimentary tract; lg. lung; /.
liver ; pp. body cavity ; md. open
longations of the body Cavity COn- end of Mullerian duct ; wd. Wolffian
. . duct; vc. vena cava inferior; ao.

taming the lungs (fig. 354). Its aorta; ch. notochord; me. medullary

dorsal wall is attached to the gut, cord>

and is continuous with the mesentery 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 body cavity which contain the lungs
extending themselves ventrally round the outside of the peri-

THE BODY CAVITY.

631

cardial cavity. This process is illustrated by fig. 355, taken
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 however separated from each
other below by a lamina extending from the ventral wall of the
pericardial cavity 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

SJ3. C.

FIG. 355. SECTION THROUGH AN ADVANCED EMBRYO OF A RABBIT TO SHEW
HOW THE PERICARDIAL CAVITY BECOMES SURROUNDED BY THE PLEURAL
CAVITIES.

ht. heart; pc. pericardial cavity; //./ pleural cavity; Ig. lung; al. alimentary
tract; ao. dorsal aorta; ch. notochord; rp. rib; st. sternum; sp.c. spinal cord.

dorsally to it. These two sections of the body cavity, which in
Amphibia and Sauropsida remain in free communication with
the undivided peritoneal cavity behind, may, from the fact of
their containing the lungs, be called the pleural cavities.

In Mammalia a further change takes place, in that, by the
formation of a vertical partition across the body cavity, known
as the diaphragm, the pleural cavities, containing the lungs,

632 THE VASCULAR SYSTEM.

become isolated from the remainder of the body or peritoneal
cavity. As shewn by their development the so-called pleurae or
pleural sacks are simply the peritoneal linings of the anterior
divisions of the body cavity, shut off from the remainder of
the body cavity by the diaphragm.

The exact mode of formation of the diaphragm is not fully
made out ; the account of it recently given by Cadiat (No. 491)
not being in my opinion completely satisfactory.

BIBLIOGRAPHY.

(491) M. Cadiat. "Du developpement de la partie cephalothoracique de 1'em-
bryon, de la formation du diaphragme, des pleures, du pericarde, du pharynx et de
1'cesophage." Journal de F Anatomic et de la Physiologic, Vol. xiv. 1878.

Vascular System.

The actual observations bearing on the origin of the vascular
system, using the term to include the lymphatic system, are
very scanty. It seems probable, mainly it must be admitted on
d priori grounds, that vascular and lymphatic systems have
originated from the conversion of indefinite spaces, primitively
situated in the general connective tissue, into definite channels.
It is quite certain that vascular systems have arisen indepen-
dently in many types ; a very striking case of the kind being
the development in certain parasitic Copepoda of a closed
system of vessels with a red non-corpusculated blood (E. van
Beneden, Heider), not found in any other Crustacea. Parts of
vascular systems appear to have arisen in some cases by a
canalization of cells.

The blood systems may either be closed or communicate
with the body cavity. In cases where the primitive body cavity
is atrophied or partially broken up into separate compartments
(Insecta, Mollusca, Discophora, etc.) a free communication
between the vascular system and the body cavity is usually
present ; but in these cases the communication is no doubt
secondary. On the whole it would seem probable that the
vascular system has in most instances arisen independently of
the body cavity, at least in types where the body cavity is

THE VASCULAR SYSTEM. 633

present in a well-developed condition. As pointed out by the
Hertwigs, a vascular system is always absent where there is not
a considerable development of connective tissue.

As to the ontogeny of the vascular channels there is still much to be
made out both in Vertebrates and Invertebrates.

The smaller channels often rise by a canalization of cells. This process
has been satisfactorily studied by Lankester in the Leech1, and may easily
be observed in the blastoderm of the Chick or in the epiploon of a newly-
born Rabbit (Schafer, Ranvier). In either case the vessels arise from a net-
work of cells, the superficial protoplasm and part of the nuclei giving rise
to the walls, and the blood-corpuscles being derived either from nucleated
masses set free within the vessels (the Chick) or from blood-corpuscles
directly differentiated in the axes of the cells (Mammals).

Larger vessels would seem to be formed from solid cords of cells, the
central cells becoming converted into the corpuscles, and the peripheral cells
constituting the walls. This mode of formation has been observed by
myself in the case of the Spider's heart, and by other observers in other
Invertebrata. In the Vertebrata a more or less similar mode of formation
appears to hold good for the larger vessels, but further investigations are
still required on this subject. Gotte finds that in the Frog the larger vessels
are formed as longitudinal spaces, and that the walls are derived from the
indifferent cells bounding these spaces, which become flattened and united
into a continuous layer.

The early formation of vessels in the Vertebrata takes place in the
splanchnic mesoblast ; but this appears due to the fact that the circulation
is at first mainly confined to the vitelline region, which is covered by
splanchnic mesoblast.

The Heart.

The heart is essentially formed as a tubular cavity in the
splanchnic mesoblast, on the ventral side of the throat, immedi-
ately behind the region of the visceral clefts. The walls of this
cavity are formed of two layers, an outer thicker layer, which has
at first only the form of a half tube, being incomplete on its
dorsal side; and an inner lamina formed of delicate flattened
cells. The latter is the epithelioid lining of the heart, and the
cavity it contains the true cavity of the heart. The outer layer
gives rise to the muscular wall and peritoneal covering of the
heart. Though at first it has only the form of a half tube (fig.

1 "Connective and vasifactive tissues of the Leech." Quart. J. of Micr. Science,
Vol. XX. 1880.

634

THE HEART.

356), it soon becomes folded in on the dorsal side so as to form
for the heart a complete muscular wall. Its two sides, after thus
meeting to complete the tube of
the heart, remain at first continuous
with the splanchnic mesoblast sur-
rounding the throat, and form a pro-
visional mesentery — the mesocar-
dium — which attaches the heart to
the ventral wall of the throat. The
superficial stratum of the wall of
the heart differentiates itself as the
peritoneal covering. The inner epi-
thelioid tube takes its origin at the
time when the general cavity of the
heart is being formed by the separa-
tion of the splanchnicmesoblastfrom
the hypoblast. During this process
(fig. 357) a layer of mesoblast re-
mains close to the hypoblast, but connected with the main mass

FIG. 356. SECTION THROUGH
THE DEVELOPING HEART OF AN
EMBRYO OF AN ELASMOBRANCH
(Pristiurus).

al. alimentary tract ; sp. splanch-
nic mesoblast ; so. somatic meso-
blast ; ht. heart.

FIG. 357. 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. notochorcl ; x. thickening of
hypoblast (possibly a rudiment of the sub-notochordal rod) ; al. throat; ht. heart;
//. body cavity; so. somatic mesoblast; sf. splanchnic mesoblast; Ay. hypoblast.

THE VASCULAR SYSTEM.

635

of the mesoblast by protoplasmic processes. A second layer
next becomes split from the splanchnic mesoblast, connected
with the first layer by the above-mentioned protoplasmic
processes. These two layers form together the epithelioid lining
of the heart ; between them is the cavity of the heart, which soon
loses the protoplasmic trabeculae 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, and resembles in its
mode of origin that of other large vascular trunks.

The above description applies only to the development of
the heart in those types in which it is formed at a period after
the throat has become a closed tube (Elasmobranchii, Amphibia,
Cyclostomata, Ganoids (?)). In a number of other cases, in
which the heart is formed before the conversion of the throat
into a closed tube, of which the most notable is that of Mammals
(Hensen, Gotte, Kolliker), the heart arises as two independent

A.

B.

mes fir

FIG. 358. TRANSVERSE SECTION THROUGH THE HEAD OF A RABBIT OF THE

SAME AGE AS FIG. 144 B. (From Kolliker.)
B is a more highly magnified representation of part of A.

rf. medullary groove; mp. medullary plate; riv. medullary fold; h. epiblast ;
dd. hypoblast; dd' . notochordal thickening of hypoblast; sp. undivided mesoblast;
^.somatic mesoblast; dfp. splanchnic mesoblast; ph. pericardial section of body
cavity; ahh. muscular wall of heart; ihh. epithelioid layer of heart; vies, lateral
undivided mesoblast ; sw. part of the hypoblast which will form the ventral wall of
the pharynx.

636

THE HEART.

tubes (fig. 358), which eventually coalesce into an unpaired
structure.

In Mammals the two tubes out of which the heart is formed appear at
the sides of the cephalic plates, opposite the region of the mid- and hind-
brain (fig. 358). They arise at a time when the lateral folds which form
the ventral wall of the throat are only just becoming visible. Each half of
the heart originates in the same way as the whole heart in Elasmobranchii,
etc. ; and the layer of the splanchnic mesoblast, which forms the muscular
wall for each part (ahh), has at first the form of a half tube open below to
the hypoblast.

On the formation of the lateral folds of the splanchnic walls, the two
halves of the heart become carried inwards and downwards, and eventually

FlG. 359. TWO DIAGRAMMATIC SECTIONS THROUGH THE REGION OF THE
HIND-BRAIN OF AN EMBRYO CHICK OF ABOUT 36 HOURS ILLUSTRATING THE
FORMATION OF THE HEART.

fib. hind-brain ; nc. notochord ; E. epiblast ; so. somatopleure ; sp. splanchno-
pleure ; d. alimentary tract ; hy. hypoblast ; hs. heart ; of. vitelline veins.

THE VASCULAR SYSTEM.

637

meet on the ventral side of the throat. For a short time they here remain
distinct, but soon coalesce into a single tube.

In Birds, as in Mammals, the heart makes its appearance as two tubes,
but arises at a period when the formation of the throat is very much more
advanced than in the case of Mammals. The heart arises immediately
behind the point up to which the ventral wall of the throat is established
and thus has at first a A -shaped form. At the apex of the A , which forms
the anterior end of the heart, the two halves are in contact (fig. 357),
though they have not coalesced; while behind they diverge to be continued
as the vitelline veins. As the folding in of the throat is continued back-
wards the two limbs of the heart are brought together and soon coalesce
from before backwards into a single structure. Fig. 359 A and B shews the
heart during this process. The two halves have coalesced anteriorly (A)
but are still widely separated behind (B). In Teleostei the heart is formed
as in Birds and Mammals by the coalescence of two tubes, and it arises
before the formation of the throat.

The fact that the heart arises in so many instances as a
double tube might lead to the supposition that the ancestral
Vertebrate had two tubes in the place of the present unpaired
heart.

The following considerations appear to me to prove that this
conclusion cannot be accepted. If the folding in of the splanch-
nopleure to form the throat were deferred relatively to the
formation of the heart, it is clear that a modification in the
development of the heart would occur, in that the two halves of
the heart would necessarily be formed widely apart, and only
eventually united on the folding in of the wall of the throat. It
is therefore possible to explain the double formation of the heart
without having recourse to the above hypothesis of an ancestral
Vertebrate with two hearts. If the explanation just suggested
is the true one the heart should only be formed as two tubes
when it arises prior to the formation of the throat, and as a single
tube when formed after the formation of the throat. Since this
is invariably found to be so, it may be safely concluded that the
formation of the heart as two cavities is a secondary mode of
development, which has been brought about by variations in the
period of the closing in of the wall of the throat.

The heart arises continuously with the sinus venosus, which in
the Amniotic Vertebrata is directly continued into the vitelline
veins. Though at first it ends blindly in front, it is very soon
connected with the foremost aortic arches.

638 THE HEART.

The simple tubular heart, connected as above described, grows
more rapidly than the chamber in which it is contained, and is
soon doubled upon itself, acquiring in this way an S-shaped
curvature, the posterior portion being placed dorsally, and the
anterior ventrally. A constriction soon appears between the
dorsal and ventral portions.

The dorsal section becomes partially divided off behind from
the sinus venosus, and constitutes the relatively thin-walled
auricular section of the heart; while the ventral portion, after
becoming distinct anteriorly from a portion continued forwards
from it to the origin of the branchial arteries, which may be called
the truncus arteriosus, acquires very thick spongy muscular
walls, and becomes the ventricular division of the heart.

The further changes in the heart are but slight in the case of the Pisces.
A pair of simple membranous valves becomes established at the auriculo-
ventricular orifice, and further changes take place in the truncus arteriosus.
This part becomes divided in Elasmobranchii, Ganoidei, and Dipnoi into a
posterior section, called the conus arteriosus, provided with a series of
transverse rows of valves, and an anterior section, called the bulb us
arteriosus, not provided with valves, and leading into the branchial
arteries. In most Teleostei (except Butirinus and a few other forms) the
conus arteriosus is all but obliterated, and the anterior row of its valves
alone preserved ; and the bulbus is very much enlarged1.

In the Dipnoi important changes in the heart are effected, as compared
with other Fishes, by the development of true lungs. Both the auricular
and ventricular chamber may be imperfectly divided into two, and in the
conus a partial longitudinal septum is developed in connection with a
longitudinal row of valves2.

In Amphibia the heart is in many respects similar to that of the Dipnoi.
Its curvature is rather that of a screw than of a simple S. The truncus
arteriosus lies to the left, and is continued into the ventricle which lies
ventrally and more to the right, and this again into the dorsally placed
auricular section.

After the heart has reached the piscine stage, the auricular section
(Bombinator) becomes prolonged into a right and left auricular appendage^
A septum next grows from the roof of the auricular portion of the heart

1 Vide Gegenbaur, "Zur vergleich. Anat. d. Herzens." Jenaische Zeit., Vol. n.
1866, and for recent important observations, J. E. V. Boas, "Ueb. Herz u. Arterien-
bogenbei Ceratodenu. Protopterus," and " Ueber d. Conus arter. b. Butirinus, etc.,"
Morphol. Jahrb., Vol. VI. 1880.

2 Boas holds that the longitudinal septum is formed by the coalescence of a row of
longitudinal valves, but this is opposed to Lankester's statements, "On the hearts of
Ceratodus, Protopterus and Chimaera, etc. Zool. Trans. Vol. x. 1879.

THE VASCULAR SYSTEM. 639

obliquely backwards and towards the left, and divides it in two chambers ;
the right one of which remains continuous with the sinus venosus, while
the left one is completely shut off from the sinus, though it soon enters
into communication with the newly established pulmonary veins. The
truncus arteriosus1 is divided into a posterior conus arteriosus (pylangium)
and an anterior bulbus (synangium). The former is provided with a
proximal row of valves at its ventricular end, and a distal row at its anterior
end near the bulbus. It is also provided with a longitudinal septum, which
is no doubt homologous with the septum in the conus arteriosus of the
Dipnoi. The bulbus is well developed in many Urodela, but hardly exists
in the Anura.

In the Amniota further changes take place in the heart,
resulting in the abortion of the distal rows of valves of the conus
arteriosus2, and in the splitting up of the whole truncus arteriosus
into three vessels in Reptilia, and two in Birds and Mammals,
each opening into the ventricular section of the heart, and
provided with a special set of valves at its commencement. In
Birds and Mammals the ventricle becomes moreover completely
divided into two chambers, each communicating with one of the
divisions of the primitive truncus, known in the higher types
as the systemic and pulmonary aortae. The character of the
development of the heart in the Amniota will be best understood
from a description of what takes place in the Chick.

In Birds the originally straight heart (fig. 109) soon becomes doubled up
upon itself. The ventricular portion becomes placed on the ventral and
right side, while the auricular section is dorsal and to the left. The two
parts are separated from each other by a slight constriction known as the
canalis auricularis. Anteriorly the ventricular cavity is continued into the
truncus, and the venous or auricular portion of the heart is similarly con-
nected behind with the sinus venosus. The auricular appendages grow out
from the auricle at a very early period. The general appearance of the
heart, as seen from the ventral side on the fourth day, is shewn in fig. 360.
Although the external divisions of the heart are well marked even before
this stage, it is not till the end of the third day that the internal partitions
become apparent ; and, contrary to what might have been anticipated from
the evolution of these parts in the lower types, the ventricular septum is the
first to be established.

1 For a good description of the adult heart vide Huxley, Article "Amphibia," in
the Encyclopedia Britannic a.

2 It is just possible that the reverse may be true, vide note on p. 640. If however,
as is most probable, the statement in the text is correct, the valves at the mouth of
the ventricle in Teleostei are not homologous with those of the Amniota ; the former
being the distal rov/ of the valves of the conus, the latter the proximal.

640 THE HEART OF AVES.

It commences on the third day as a crescentic ridge or fold springing
from the convex or ventral side of the rounded ventricular portion of the
heart, and on the fourth day grows rapidly across the ventricular cavity
towards the concave or dorsal side. It thus forms an incomplete longitu-
dinal partition, extending from the canalis auricularis to the commencement
of the truncus 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 commu- A ^) ) CA

nicate with each other, above the free
edge of the partition, along its whole
length.

Externally the ventricular portion
as yet shews no division into two parts.

By the fifth day the venous end of
the heart, though still lying somewhat
to the left and above, is placed as far FIG. 360. HEART OF A CHICK ON

forwards as the arterial end, the whole THE FOURTH DAY OF INCUBATION

VIEWED FROM THE VENTRAL SURFACE.

organ appearing to be drawn together.

The ventricular septum is complete. L?.- left a,uricular appendage; C.A.

„, e.. , . , , canahs auricularis ; v. ventricle ; b. trun-

The apex of the ventricles becomes cus arteriosus.

more and more pointed. In the au-
ricular portion a small longitudinal fold appears as the rudiment of the
auricular septum, while in the canalis auricularis, which is now at its greatest
length, there is also to be seen a commencement of the valvular structures
tending to separate the cavity of the auricles from those of the ventricles.

About the io6th hour, a septum begins to make its appearance in the
truncus arteriosus in the form of a longitudinal fold, which according to
Tonge (No. 495) starts at the end of the truncus furthest removed from the
heart. It takes origin from the wall of the truncus between the fourth and
fifth pairs of arches, and grows downwards in such a manner as to divide the
truncus into two channels, one of which leads from the heart to the third and
fourth pairs of arches, and the other to the fifth pair. Its course downwards
is not straight but spiral, and thus the two channels into which it divides
the truncus arteriosus wind spirally the one round the other.

At the time when the septum is first formed, the opening of the truncus
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, No. 495) are developed
from the wall of that portion of the truncus which lies between the free edge
of the septum and the cavity of the ventricles1.

1 If Tonge is correct in his statement that the semilunar valves develop at some
distance from the mouth of the ventricle, it would seem possible that the portion of
the truncus between them and the ventricle ought to be regarded as the embryonic
conus arteriosus, and that the distal row of valves of the conus (and not the proximal
as suggested above, p. 639) has been preserved in the higher types.

THE VASCULAR SYSTEM.

641

The ventral and the dorsal pairs of valves are the first to appear : the
former as two small solid prominences separated from each other by a
narrow groove ; the latter as a single ridge, in the centre of which is a
prominence indicating the point where the ridge will subsequently become
divided into two. The outer valves appear opposite each other, at a
considerably later period.

As the septum grows downwards towards the heart, it finally reaches
the position of these valves. One of its edges 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 cavities. The free edge of the septum of the truncus now

A. B.

FlG. 361. TWO VIEWS OF THE HEART OF A CHICK UPON THE FIFTH DAY

OF INCUBATION.

A. from the ventral, B. from the dorsal side.

La. left auricular appendage; r.a. right auricular appendage ; r.v. right ventricle;
l.v. left ventricle; b. truncus arteriosus.

fuses with the ventricular septum, and thus the division of the truncus into
two separate channels, each provided with three valves, and each com-
municating with a separate side of the heart, is complete ; the position of
the valves not being very different from that in the adult heart.

That division of the truncus 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.
The former becomes the pulmonary artery, the latter the commencement of
the systemic aorta.

The external constriction actually dividing the truncus 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 H7th to the,i65th hour) in the order of
their appearance.

At the end of the sixth day, and even on the fifth day (figs. 361 and 362),
the appearance of the heart itself, without reference to the vessels which
come from it, is not very dissimilar from that of the adult. The original

B. III.

41

642

THE HEART OF MAMMALIA.

r.a

l.v

FIG. 362. 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 ven-
tricle ; b. truncus arteriosus.

protuberance 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 con-
dition of the truncus arteriosus.

The subsequent changes which the heart
undergoes are concerned more with its in-
ternal 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 conspicuous. It is placed
vertically, and arises from the ventral wall ;
commencing at the canalis auricularis and
proceeding towards the opening into the
sinus venosus.

This latter structure gradually becomes
reduced so as to become a special appendage
of the right auricle. The inferior vena cava

enters the sinus obliquely from the right, so that its blood 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 truncus arteriosus into the
aorta and pulmonary artery becomes visible from the exterior.

By the eleventh to the thirteenth day the right auricle has become as
large as the left, and the auricular septum much more complete, though
there is still a small opening, the foramen ovale, by which the two cavities
communicate with each other.

The most important feature in which the development of the Reptilian
heart differs from that of Birds is the division of the truncus into three
vessels, instead of two. The three vessels remain bound up in a common
sheath, and appear externally as a single trunk. The vessel not represented
in Birds is that which is continued into the left aortic arch.

In Mammals the early stages in the development of the heart present no
important points of difference from those of Aves. The septa in the truncus,
in the ventricular, and in the auricular cavities are formed, so far as
is known, in the same way and at the same relative periods in both groups.
In the embryo Man, the Rabbit, and other Mammals the division of
the ventricles is made apparent externally by a deep cleft, which, though
evanescent in these forms, is permanent in the Dugong.

The attachment of the auriculo-ventricular valves to the wall of the
ventricle, and the similar attachment of the left auriculo-ventricular valves
in Birds, have been especially studied by Gegenbaur and Bernays (No. 492),

ARTERIAL SYSTEM. 643

and deserve to be noticed. In the primitive state the ventricular walls
have throughout a spongy character ; and the auriculo-ventricular valves are
simple membranous projections like the auriculo-ventricular valves of Fishes.
Soon however the spongy muscular tissue of both the ventricular and
auricular walls, which at first pass uninterruptedly the one into the other,
grows into the bases of the valves, which thus become in the main muscular
projections of the walls of the heart. As the wall of the ventricle thickens,
the muscular trabeculas, connected at one end with the valves, remain at the
other end united with the ventricular wall, and form special bands passing
between the two. The valves on the other hand lose their muscular
attachment to the auricular walls. This is the condition permanent in
Ornithorhynchus. In higher Mammalia the ends of the muscular bands
inserted into the valves become fibrous, from the development of inter-
muscular connective tissue, and the atrophy of the muscular elements.
The fibrous parts now form the chordae tendinea?, and the muscular the
musculi papillares.

The sinus venosus in Mammals becomes completely merged into the
right auricle, and the systemic division of the truncus arteriosus is appa-
rently not homologous with that in Birds.

In the embryos of all the Craniata the heart is situated very
far forwards in the region of the head. This position is retained
in Pisces. In Amphibia the heart is moved further back, while
in all the Amniota it gradually shifts its position first of all into
the region of the neck and finally passes completely within the
thoracic cavity. The steps in the change of position may be
gathered from figs. 109, in, and 118.

BIBLIOGRAPHY of the Heart.

(492) A. C. Bernays. " Entwicklungsgeschichte d. Atrioventricularklappen."
Morphol. Jahrbuch,^o\. II. 1876.

(493) E. Gasser. " Ueber d. Entstehung d. Herzens beim Hiihn." Archiv f.
mikr. Anat., Vol. xiv.

(494) A. Thomson. "On the development of the vascular system of the foetus
of Vertebrated Animals." Edinb. New Phil. Journal, Vol. ix. 1830 and 1831.

(495) M. Tonge. "Observations on the development of the semilunar valves
of the aorta and pulmonary artery of the heart of the Chick." Phil. Trans. CLIX.
1869.

Vide also Von Baer (291), Rathke (300), Hensen (182), Kolliker (298), Gotte (296),
and Balfour (292).

Arterial System.

In the embryos of Vertebrata the arterial system consists of
a forward continuation of the truncus arteriosus, on the ventral

41 — 2

644

ARTERIES OF PISCES.

side of the throat (figs. 363, abr, and 364, a), which, with a few
exceptions to be noticed below, divides into as many branches on
each side as there are visceral arches. These branches, after
traversing the visceral arches, unite on the dorsal side of the
throat into a common trunk on each side. This trunk (figs. 363
and 364) after giving off one (or more) vessels to the head (c and
c] turns backv/ards, and bends in towards the middle line, close
to its fellow, immediately below the notochord (figs. 21 and 116)
and runs backwards in this situation towards the end of the tail.
The two parallel trunks below the notochord fuse very early into
a single trunk, the dorsal aorta (figs. 363, ad, and 364, a"}.

ttbr v "a,

FIG. 363. DIAGRAMMATIC VIEW OF THE HEAD OF AN EMBRYO TELEOSTEAN,
WITH THE PRIMITIVE VASCULAR TRUNKS. (From Gegenbaur.)

a. auricle ; v. ventricle ; abr. branchial artery ; c'. carotid ; ad. dorsal aorta ;
s. branchial clefts; sv. sinus venosus; dc. ductus Cuvieri; n. nasal pit

There is given off from each collecting trunk from the visceral
arches, or from the commencement of the dorsal aorta, a subclavian
artery to each of the anterior limbs ; from near the anterior end
of the dorsal aorta a vitelline artery (or before the dorsal aortae
have united a pair of arteries fig. 125, R of A and L of A) to the
yolk-sack, which subsequently becomes the main visceral artery1;
and from the dorsal aorta opposite the hind limbs one (or two)
arteries on each side — the iliac arteries — to the hind limbs ; from
these arteries the allantoic arteries are given off in the higher
types, which remain as the hypogastric arteries after the
disappearance of the allantois.

The primitive arrangement of the arterial trunks is with a
few modifications retained in Fishes. With the development of
the gills the vessels to the arches become divided into two parts
connected by a capillary system in the gill folds, viz. into the

1 In Mammalia the superior inesenteric artery arises from the vitelline artery,
which may probably be regarded as a primitive crclinco-mescnteric artery.

ARTERIAL SYSTEM.

branchial arteries bringing the blood to the gills from the truncus
arteriosus, and the branchial veins transporting it to the dorsal
aorta. The branchial vessels to those arches which do not bear
gills, either wholly or partially atrophy; thus in Elasmobranchii
the mandibular trunk, which is fully developed in the embryo
(fig. 193, \av}, atrophies, except for a small remnant bringing
blood to the rudimentary gill of the spiracle from the branchial
vein of the hyoid arch. In Ganoids the mandibular artery
atrophies, but the hyoid is usually preserved. In Teleostei both
mandibular1 and hyoid arteries are absent in the adult, except
that there is usually left a rudiment of the hyoid, supplying the
pseudobranch, which is similar to the rudiment of the mandibular
artery in Elasmobranchii. In Dipnoi the mandibular artery
atrophies, but the hyoid is sometimes preserved (Protopterus),
and sometimes lost.

In Fishes provided with a well developed air-bladder this
organ receives arteries, which arise sometimes from the dorsal
aorta, sometimes from the caeliac arteries, and sometimes from
the dorsal section of the last (fourth) branchial trunk. The
latter origin is found in Polypterus and Amia, and seems to have
been inherited by the Dipnoi where the air-bladder forms a true
lung.

The pulmonary artery of all the air-breathing Verte-
brata is derived from the pulmonary artery of the
Dipnoi.

In all the types above Fishes considerable changes are
effected in the primitive arrangement of the arteries in the
visceral arches.

In Amphibia the piscine condition is most nearly retained2.
The mandibular artery is never developed, and the hyoid artery
is imperfect, being only connected with the cephalic vessels and
never directly joining the dorsal aorta. It is moreover developed
later than the arteries of the true branchial arches behind. The
subclavian arteries spring from the common trunks which unite
to form the dorsal aorta.

In the Urodela there are developed, in addition to the hyoid,

1 The mandibular artery is stated by Gotte never to be developed in Teleostei, but
is distinctly figured in Lereboullet (No. 71).

2 In my account of the Amphibia, Gotte (No. 296) has been followed.

646 ARTERIES OF THE AMNIOTA.

four branchial arteries. The three foremost of these at first
supply gills, and in the Perennibranchiate forms continue to do
so through life. The fourth does not supply a gill, and very
early gives off, as in the Dipnoi, a pulmonary branch.

The hyoid artery soon sends forward a lingual artery from its
ventral end, and is at first continued to the carotid which grows
forward from the dorsal part of the first branchial vessel.

In the Caducibranchiata, where the gills atrophy, the following
changes take place. The remnant of the hyoid is continued
entirely into the lingual artery. The first branchial is mainly
continued into the carotid and other cephalic branches, but a
narrow remnant of the trunk, which originally connected it with
the dorsal aorta, remains, forming what is known as a ductus
Botalli. A rete mirabile on its course is the remnant of the
original gill.

The second and third branchial arches are continued as
simple trunks into the dorsal aorta, and the blood from the fourth
arch mainly passes to the lungs, but a narrow ductus Botalli still
connects this arch with the dorsal aorta.

In the Anura the same number of arches is present in the
embryo as in the Urodela, all four branchial arteries supplying
branchiae, but the arrangement of the two posterior trunks is
different from that in the Urodela. The third arch becomes at a
very early period continued into a pulmonary vessel, a relatively-
narrow branch connecting it with the second arch. The fourth
arch joins the pulmonary branch of the third. At the metamor-
phosis the hyoid artery loses its connection with the carotid, and
the only part of it which persists is the root of the lingual artery.
The first branchial artery ceases to join the dorsal aorta, and
forms the root of the carotid : the so-called carotid gland placed
on its course is the remnant of the gill supplied by it before the
metamorphosis.

The second artery forms a root of the dorsal aorta. The
third, as in all the Amniota, now supplies the lungs, and also
sends off a cutaneous branch. The fourth disappears. The
connection of the pulmonary artery with both the third and
fourth branchial arches in the embryo appears to me clearly to
indicate that this artery was primitively derived from the fonrtli
arc/i as in the Urodela, and that its permanent connection

ARTERIAL SYSTEM.

647

with the third arch in the Anura and in all the Amniota is
secondary.

In the Amniota the metamorphosis of the arteries is in all
cases very similar. Five arches, viz. the mandibular, hyoid, and
three branchial arches are always developed (fig. 364), but, owing
to the absence of branchiae,
never function as branchial arte-
ries. Of these the main parts of
the first two, connecting the trun-
cus arteriosus with the collecting
trunk into which the arterial
arches fall, always disappear, usu-
ally before the complete develop-
ment of the arteries in the poste-
rior arches.

The anterior part of the col-
lecting trunk into which these
vessels fall is not obliterated
when they disappear, but is on
the contrary continued forwards
as a vessel supplying the brain,
homologous with that found in
Fishes. It constitutes the internal
carotid. Similarly the anterior
part of the trunk from which the mandibular and hyoid arteries
sprang is continued forwards as a small vessel1, which at first
passes to the oral region and constitutes in Reptiles the lingual
artery, homologous with the lingual artery of the Amphibia ; but
in Birds and Mammals becomes more important, and is then
known as the external carotid (fig. 125). By these changes the
roots of the external and internal carotids spring respectively
from the ventral and dorsal ends of the primitive third artery,
i.e. the artery of the first branchial arch (fig. 365, c and c'} ; and
thus this arterial arch persists in all types as the common carotid,

FIG. 364. DIAGRAM OF THE AR-
RANGEMENT OF THE ARTERIAL
ARCHES IN AN EMBRYO OF ONE OF THE

AMNIOTA. (From Gegenbaur ; after
RATHKE.)

a. ventral aorta; a", dorsal aorta;
'» 2> 3> 4> 5- arterial arches ; c. carotid
artery.

1 His (No. 232) describes in Man two ventral continuations of the truncus arte-
riosus, one derived from the mandibular artery, forming the external maxillary artery,
and one from the hyoid artery, forming the lingual artery. The vessel from which
they spring is the external carotid. These observations of His will very probably be
found to hold true for other types.

648

ARTERIAL ARCHES OF THE AMNIOTA.

and the basal part of the internal carotid. The trunk connecting
the third arterial arch with the system of the dorsal aorta persists
in some Reptiles (Lacertilia, fig. 366 A) as a ductus Botalli, but
is lost in the remaining Reptiles and in Birds and Mammals (fig.
366 B, C, D). It disappears earliest in Mammals (fig. 365 C),
later in Birds (fig. 365 B), and still later in the majority of
Reptiles.

The fourth arch always continues to give rise, as in the Anura,
to the system of the dorsal aorta.

In all Reptiles it persists on both sides (fig. 366 A and B),
but with the division of the truncus arteriosus into three vessels

ad

FIG. 365. DEVELOPMENT OF THE GREAT ARTERIAL TRUNKS IN THE EMBRYOS
OF A. A LIZARD ; B. THE COMMON FOWL; C. THE PIG. (From Gegenbaur; after
Rathke.)

The first two arches have disappeared in all three. In A and B the last three are
still complete, but in C the last two are alone complete.

/. pulmonary artery springing from the fifth arch, but still connected with the
system of the dorsal aorta by a ductus Botalli; c. external carotid; <•'. internal
carotid; ad. dorsal aorta; a. auricle; v. ventricle; n. nasal pit; m, rudiment of
fore-limb.

one of these, i.e. that opening furthest to the left side of the
ventricle (e and d), is continuous with the right fourth arch, and
also with the common carotid arteries (c) ; while a second
springing from the right side of the ventricle is continuous with
the left fourth arch (Ji and f). The right and left divisions of the
fourth arch meet however on the dorsal side of the oesophagus to
give origin to the dorsal aorta (g).

In Birds (fig. 366 C) the left fourth arch (h) loses its connec-
tion with the dorsal aorta, though the ventral part remains as

ARTERIAL SYSTEM.

649

the root of the left subclavian. The truncus arteriosus is more-
over only divided into two parts, one of which is continuous
with all the systemic arteries. Thus it comes about that in
Birds the right fourth arch (e) alone gives rise to the dorsal
aorta.

In Mammals (fig. 366 D) the truncus arteriosus is only
divided into two, but the left fourth arch (>), instead of the right,
is that continuous with the dorsal aorta, and the right fourth
arch (/) is only continued into the right vertebral and right
subclavian arteries.

The fifth arch always gives origin to the pulmonary artery
(fig. 365, /) and is continuous with one of the divisions of the
truncus arteriosus. In Lizards (fig. 366 A, i), Chelonians and
Birds (fig. 366 C, i] and probably in Crocodilia, the right and
left pulmonary arteries spring respectively from the right and
left fifth arches, and during the greater part of embryonic life
the parts of the fifth arches between the origins of the pulmonary
arteries and the system of the dorsal aorta are preserved as
ductus Botalli. These ductus Botalli persist for life in the
Chelonia. In Ophidia (fig. 366 B, Ji) and Mammalia (fig.
366 D, m) only one of the fifth arches gives origin to the two
pulmonary arteries, viz. that on the right side in Ophidia, and
the left in Mammalia.

The ductus Botalli of the fifth arch (known in Man as the
ductus arteriosus) of the side on which the pulmonary arteries
are formed, may remain (e.g. in Man) as a solid cord connecting
the common stem of the pulmonary aorta with the systemic
aorta.

The main history of the arterial arches in the Amniota has
been sufficiently dealt with, and the diagram, fig. 366, copied
from Rathke, shews at a glance the character of the metamor-
phosis these arches undergo in the different types. It merely
remains for me to say a few words about the subclavian and
vertebral arteries.

The subclavian arteries in Fishes usually spring from the
trunks connecting the branchial veins with the dorsal aorta.
This origin, which is also found in Amphibia, is typically found
in the embryos of the Amniota. In the Lizards this origin
persists through life, but both subclavians spring from the right

650

ARTERIAL ARCHES OF THE AMNIOTA.

side. In most other types the origin of the subclavians is
carried upwards, so that they usually spring from a trunk
common to them and the carotids (arteria anonyma) (Birds and
some Mammals); or the left one, as in Man and some other
Mammals, arises from the systemic aorta just beyond the
carotids. Various further modifications in the origin of the
subclavians of the same general nature are found in Mammalia,
A 13

FIG. 366. DIAGRAMS ILLUSTRATING THE METAMORPHOSIS OF THE ARTERIAL

ARCHES IN A LlZARD A, A SNAKE B, A BlRD C AND A MAMMAL D. (From
Mivart ; after Rathke.)

A. a. internal carotid; b. external carotid ; c. common carotid; d. ductus Botalli
between the third and fourth arches ; e. right aortic trunk ; /. subclavian ; g. dorsal
aorta; h. left aortic trunk; i. pulmonary artery; k. rudiment of ductus Botalli
between the pulmonary artery and the system of the dorsal aorta.

B. a. internal carotid; b. external carotid; c. common carotid; d. right aortic
trunk; e. vertebral artery;/, left aortic trunk of dorsal aorta; h. pulmonary artery ;
i. ductus Botalli of pulmonary artery.

C. a. internal carotid ; b. external carotid ; c. common carotid ; d. systemic
aorta; e. fourth arch of right side (root of dorsal aorta);/, right subclavian; g. dorsal
aorta; h, left subclavian (fourth arch of left side); i. pulmonary artery; k. and /.
right and left ductus Botalli of pulmonary arteries.

D. a. internal carotid; b. external carotid; c. common carotid; d. systemic aorta;
c. fourth arch of left side (root of dorsal aorta);/ dorsal aorta; g. left vertebral
artery; h. left subclavian artery; i. right subclavian (fourth arch of right side); k.
right vertebral; /. continuation of right subclavian; in. pulmonary artery; n. ductus
Botalli of pulmonary artery.

THE VENOUS SYSTEM.

65I

but they need not be specified in detail. The vertebral arteries
usually arise in close connection with the subclavians, but in
Birds they arise from the common carotids.

BIBLIOGRAPHY of the Arterial System.

(496) H. Rathke. " Ueb. d. Entwick. d. Arterien vv. bei d. Saugethiere von
d. Bogen d. Aorta ausgehen." Miiller's Archiv, 1843.

(-197) H. Rathke. " Untersuchungen lib. d. Aortenwurzeln d. Saurier."
Denkschriften d. k. Akad. Wien, Vol. XIII. 1857.

Vide also His (No. 232) and general works on Vertebrate Embryology.

TJie Venous System,.

The venous system, as it is found in the embryos of Fishes,
consists in its earliest condition of a single large trunk, which
traverses the splanchnic mesoblast investing the part of the
alimentary tract behind the heart. This trunk is directly con-
tinuous in front with the heart, and underlies the alimentary
canal through both its praeanal and postanal sections. It is
shewn in section in fig. 367, v, and may be called the sub-
intestinal vein. This vein has been found in the embryos of
Teleostei, Ganoidei, Elasmobranchii and Cyclostomata, and runs
parallel to the dorsal aorta above, into which it is sometimes
continued behind (Teleostei, Ganoidei, etc.).

In Elasmobranch embryos the subintestinal vein terminates,
as may be gathered from sections (fig. 368, v.cau), shortly before
the end of the tail. The same series of sections also shews that
at the cloaca, where the gut enlarges and comes in contact with
the skin, this vein bifurcates, the two branches uniting into a
single vein both in front of and behind the cloaca.

In most Fishes the anterior part of this vein atrophies, the
caudal section alone remaining, but the anterior section of it
persists in the fold of the intestine in Petromyzon, and also
remains in the spiral valve of some Elasmobranchii. In
Amphioxus, moreover, it forms, as in the embryos of higher
types, the main venous trunk, though even here it is usually
broken up into two or three parallel vessels.

It no doubt represents one of the primitive longitudinal trunks of the
vermiform ancestors of the Chordata. The heart and the branchial artery
constitute a specially modified anterior continuation of this vein. The

652

THE SUBINTESTINAL VEIN.

-p.o

rp.r.

dilated portal sinus of Myxine is probably also part of it ; and if this is
really rhythmically contractile1 the fact would be interesting as shewing that
this quality, which is now localised in the heart, was once probably common
to the subintestinal vessel for its whole length.

On the development of the cardinal veins (to be described
below) considerable changes are
effected in the subintestinal vein.
Its postanal section, which is known
in the adult as the caudal vein,
unites with the cardinal veins. On
this junction being effected retro-
gressive changes take place in the
praeanal section of the original sub-
intestinal vessel. It breaks up in
front into a number of smaller
vessels, the most important of which
is a special vein, which lies in the
fold of the spiral valve, and which is
more conspicuous in some Elasmo-
branchii than in Scyllium, in which
the development of the vessel has
been mainly studied. The lesser of
the two branches connecting it
round the cloaca with the caudal
vein first vanishes, and then the
larger ; and the two posterior car-
dinals are left as the sole forward
continuations of the caudal vein.
The latter then becomes prolonged
forwards, so that the two cardinals
open into it some little distance in
front of the hind end of the kidneys.
By these changes, and by the dis-
appearance of the postanal section of the gut, the caudal vein is
made to appear as a supraintestinal and not, as it really is, a
subintestinal vessel.

From the subintestinal vein there is given off a branch which
supplies the yolk-sack. This leaves the subintestinal vein close

1 J. Miiller holds that this sack is not rhythmically contractile.

FIG. 367. SECTION THROUGH
THE TRUNK OF A SCYLLIUM
EMBRYO SLIGHTLY YOUNGER

THAN 28 F.

sp.c. spinal canal; W. white
matter of spinal cord ; pr. poste-
rior nerve-roots; ch. notochord ;
x. sub-notochordal rod ; ao. aorta ;
mp. muscle plate; ;;//'. inner layer
of muscle-plate already converted
into muscles; Vr. rudiment of
vertebral body; st. segmental
tube ; sd. segmental duct ; sp.v.
spiral valve; v. subintestinal vein ;
p.o. primitive generative cells.

THE VENOUS SYSTEM.

653

to the liver. The liver, on its development, embraces the
subintestinal vein, which then breaks up into a capillary system
in the liver, the main part of its blood coming at this period
from the yolk-sack.

The portal system is thus established from the subintestinal
vein ; but is eventually joined by the various visceral, and some-
times by the genital, veins as they become successively de-
veloped.

The blood from the liver is brought back to the sinus veno-
sus by veins known as the hepatic veins, which, like the hepatic
capillary system, are derivatives of the subintestinal vessel.

There join the portal system in Myxinoids and many
Teleostei a number of veins from the anterior abdominal walls,
representing a commencement of the anterior abdominal or
epigastric vein of higher types1.

In the higher Vertebrates the original subintestinal vessel never attains a
full development, even in the embryo. It is represented by (i) the ductus

FIG. 368. FOUR SECTIONS THROUGH THE POSTANAL PART OF THE TAIL
OF AN EMBRYO OF THE SAME AGE AS FIG. 28 F.

A. is the posterior section.

nc. neural canal; al. post-anal gut; alv. caudal vesicle of post-anal gut; x.
subnotochordal rod; mp. muscle-plate; c/i. notochord; cl.al. cloaca; ao. aorta;
v.cait. caudal vein.

1 Stannius, Vergleich. Anat., p. 251.

654

THE CARDINAL VEINS.

venosus, which, like the true subintestinal vein, gives origin (in the Amniota)
to the vitelline veins to the yolk-sack, and (2) by the caudal vein. Whether
the partial atrophy of the subintestinal vessel was primitively caused by the
development of the cardinal veins, or for some other reason, it is at any rate
a fact that in all existing Fishes the cardinal veins form the main venous
channels of the trunk.

Their later development than the subintestinal vessel as well as their
absence in Amphioxus, probably indicate that they became evolved, at any
rate in their present form, within the Vertebrate phylum.

The embryonic condition of the venous system, with a single
large subintestinal vein is, as has been stated, always modified
by the development of a paired system of vessels, known as the
cardinal veins, which bring to the heart the greater part of the
blood from the trunk.

The cardinal veins appear in Fishes as four paired longi-
tudinal trunks (figs. 363 and 369), two anterior (/) and two
posterior (c). They unite into two transverse trunks on either
side, known as the ductus Cuvieri (dc), which fall into the sinus
venosus, passing from the body wall to the sinus by a lateral
mesentery of the heart already spoken of (p. 627, fig. 352). The
anterior pair, known as the anterior cardinal or jugular veins,
bring to the heart the blood from the head and neck. They
are placed one on each side above the level
of the branchial arches (fig. 299, a.cv). The
posterior cardinal veins lie immediately dor-
sal to the mesonephros (Wolfifian body), and
are mainly supplied by the blood from this
organ and from the walls of the body (fig.
275, c.a.v). In many forms (Cyclostomata,
Elasmobranchii and many Teleostei) they
unite posteriorly with the caudal veins in
the manner already described, and in a large
number of instances the connecting branch
between the two systems, in its passage through
the mesonephros, breaks up into a capillary
network, and so gives rise to a renal portal
system.

The vein from the anterior pair of fins
(subclavian) usually unites with the anterior
jugular vein.

j

FIG. 369. DIA-
GRAM OF THE PAIR-
ED VENOUS SYSTEM

OF A FISH. (From
Gegenbaur. )

j. jugular vein
(anterior cardinal
vein) ; c. posterior
cardinal vein; //. he-
patic veins ; sv. sinus
venosus ; dc. ductus
Cuvieri.

THE VENOUS SYSTEM. 655

The venous system of the Amphibia and Amniota always
differs from that of Fishes in the presence of a new vessel, the
vena cava inferior, which replaces the posterior cardinal veins;
the latter only being present, in their piscine form, during
embryonic life. It further differs from that of all Fishes, except
the Dipnoi, in the presence of pulmonary veins bringing back
the blood directly from the lungs.

In the embryos of all the higher forms the general characters
of the venous system are at first the same as in Fishes, but with
the development of the vena cava inferior the front sections of
the posterior cardinal veins atrophy, and the ductus Cuvieri,
remaining solely connected with the anterior cardinals and their
derivatives, constitute the superior venae cavae. The inferior
cava receives the hepatic veins.

Apart from the non-development of the subintestinal vein
the visceral section of the venous system is very similar to that
in Fishes.

The further changes in the venous system must be dealt
with separately for each group.

Amphibia. In Amphibia (Gotte, No. 296) the anterior and posterior
cardinal veins arise as in Pisces. From the former the internal jugular vein
arises as a branch ; the external jugular constituting the main stem. The
subclavian with its large cutaneous branch also springs from the system of
the anterior cardinal. The common trunk formed by the junction of these
three veins falls into the ductus Cuvieri.

The posterior cardinal veins occupy the same position as in Pisces, and
unite behind with the caudal veins, which Gotte has shewn to be originally
situated below the post-anal gut. The iliac veins unite with the posterior
cardinal veins, where the latter fall into the caudal vein. The original
piscine condition of the veins is not long retained. It is first of all disturbed
by the development of the anterior part of the important unpaired venous
trunk which forms in the adult the vena cava inferior. This is developed
independently, but unites behind with the right posterior cardinal. From
this point backwards the two cardinal veins coalesce for some distance, to
give rise to the posterior section of the vena cava inferior, situated between
the kidneys1. The anterior sections of the cardinal veins subsequently
atrophy. The posterior part of the cardinal veins, from their junction with
the vena cava inferior to the caudal veins, forms a rhomboidal figure. The
iliac vein joins the outer angle of this figure, and is thus in direct communi-
cation with the inferior vena cava, but it is also connected with a longitu-

1 This statement of Gotte's is opposed to that of Rathke for the Amniota, and
cannot be considered as completely established.

656 VEINS OF THE SNAKE.

dinal vessel on the outer border of the kidneys, which receives transverse
vertebral veins and transmits their blood to the kidneys, thus forming a
renal portal system. The anterior limbs of the rhomboid formed by the
cardinal veins soon atrophy, so that the blood from the hind limbs can only
pass to the inferior vena cava through the renal portal system. The
posterior parts of the two cardinal veins (uniting in the Urodela directly
with the unpaired caudal vein) still persist. The iliac veins also become
directly connected with a new vein, the anterior abdominal vein, which
has meanwhile become developed. Thus the iliac veins become united
with the system of the vena cava inferior through the vena renalis advehens
on the outer border of the kidney, and with the anterior abdominal veins by
the epigastric veins.

The visceral venous system begins with the development of two vitelline
veins, which at first join the sinus venosus directly. They soon become
enveloped in the liver, where they break up into a capillary system, which
is also joined by the other veins from the viscera. The hepatic system has
in fact the same relations as in Fishes. Into this system the anterior
abdominal vein also pours itself in the adult. This vein is originally
formed of two vessels, which at first fall directly into the sinus venosus,
uniting close to their opening into the sinus with a vein from the truncus
arteriosus. They become prolonged backwards, and after receiving the
epigastric veins above mentioned from the iliac veins, and also veins from
the allantoic bladder, unite behind into a single vessel. Anteriorly the
right vein atrophies and the left continues forward the unpaired posterior
section.

A secondary connection becomes established between the anterior abdo-
minal vein and the portal system ; so that the blood originally transported
by the former vein to the heart becomes diverted so as to fall into the liver.
A remnant of the primitive connection is still retained in the adult in the
form of a small vein, the so-called vena bulbi posterior, which brings the
blood from the walls of the truncus arteriosus directly into the anterior
abdominal vein.

The pulmonary veins grow directly from the heart to the lungs.

For our knowledge of the development of the venous system of the
Amniota we are mainly indebted to Rathke.

Reptilia. As an example of the Reptilia the Snake may be selected,
its venous system having been fully worked out by Rathke in his important
memoir on its development (No. 300).

The anterior (external jugular) and posterior cardinal veins are formed in
the embryo as in all other types (fig. 370, vj and vc] ; and the anterior
cardinal, after giving rise to the anterior vertebral and to the cephalic veins,
persists with but slight modifications in the adult ; while the two ductus
Cuvieri constitute the superior venos cavas.

The two posterior cardinals unite behind with the caudal veins. They
are placed in the usual situation on the dorsal and outer border of the
kidneys.

THE VENOUS SYSTEM.

657

U-

FIG. 370. ANTERIOR
PORTION OF THE VENOUS
SYSTEM OF AN EMBRYONIC
SNAKE. (From Gegenbaur;
after Rathke.)

vc. posterior cardinal
vein; vj. jugular vein; DC.
ductus Cuvieri ; vu. allan-
toic vein ; v. ventricle ; ba.
truncus arteriosus ; a. vis-
ceral clefts ; /. auditory
vesicle.

With the development of the vena cava inferior, to be described below,
the blood from the kidneys becomes mainly
transported by this vessel to the heart ; and the
section of the posterior cardinals opening into
the ductus Cuvieri gradually atrophies, their
posterior parts remaining however on the outer
border of the kidneys as the vena? renales
advehentes1.

While the front part of the posterior cardinal
veins is undergoing atrophy, the intercostal veins,
which originally poured their blood into the
posterior cardinal veins, become also connected
with two longitudinal veins — the posterior ver-
tebral veins — which are homologous with the
azygos and hemiazygos veins of Man ; and bear
the same relation to the anterior vertebral veins
that the anterior and posterior cardinals do to
each other.

These veins are at first connected by trans
verse anastomoses with the posterior cardinals,
but, on the disappearance of the front part of the
latter, the whole of the blood from the intercostal veins falls into the
posterior vertebral veins. They are united in front with the anterior verte-
bral veins, and the common trunk of the two veins on each side falls into
the jugular vein.

The posterior vertebral veins are at first symmetrical, but after becoming
connected by transverse anastomoses, the right becomes the more important
of the two.

The vena cava inferior, though considerably later in its development
than the cardinals, arises fairly early. It constitutes in front an unpaired
trunk, at first very small, opening into the right allantoic vein, close to the
heart. Posteriorly it is continuous with two veins placed on the inner
border of the kidneys2.

The vena cava inferior passes through the dorsal part of the liver, and in
doing so receives the hepatic veins.

The portal system is at first constituted by the vitelline vein, which is
directly continuous with the venous end of the heart, and at first receives
the two ductus Cuvieri, but at a later period unites with the left ductus.

1 Rathke's account of the vena renalis advehens is thus entirely opposed to that
which Gotte gives for the Frog, but my own observations on the Lizard incline me to
accept Rathke's statements, for the Amniota at any rate.

2 The vena cava inferior does not according to Rathke's account unite behind with
the posterior cardinal veins, as it is stated by Gotte to do in the Anura. Gb'tte
questions the accuracy of Rathke's statements on this head, but my own observations
are entirely in favour of Rathke's observations, and lend no support whatever to
Gotte's views.

B. III.

658 VEINS OF THE CHICK.

It soon receives a mesenteric vein bringing the blood from the viscera,
which is small at first but rapidly increases in importance.

The common trunk of the vitelline and mesenteric veins, which may be
called the portal vein, becomes early enveloped by the liver, and gives off
branches to this organ, the blood from which passes by the hepatic veins
to the vena cava inferior. As the branches in the liver become more
important, less and less blood is directly transported to the heart, and finally
the part of the original vitelline vein in front of the liver is absorbed, and the
whole of the blood from the portal system passes from the liver into the
vena cava inferior.

The last section of the venous system to be dealt with is that of the
anterior abdominal vein. There are originally, as in the Anura, two veins
belonging to this system, which owing to the precocious development of the
bladder to form the allantois, constitute the allantoic veins (fig. 370, vu}.

These veins, running along the anterior abdominal wall, are formed
somewhat later than the vitelline vein, and fall into the two ductus Cuvieri.
They unite with two epigastric veins (homologous with those in the Anura),
which connect them with the system of the posterior cardinal veins. The
left of the two eventually atrophies, so that there is formed an unpaired
allantoic vein. This vein at first receives the vena cava inferior close to the
heart, but eventually the junction of the two takes place in the region of the
liver, and finally the anterior abdominal vein (as it comes to be after the
atrophy of the allantois) joins the portal system and breaks up into capillaries
in the liver1.

In Lizards the iliac veins join the posterior cardinals, and so pour part of
their blood into the kidneys ; they also become connected by the epigastric
veins with the system of the anterior abdominal or allantoic vein. The
subclavian veins join the system of the superior venae cavas.

The venous system of Birds and Mammals differs in two important
points from that of Reptilia and Amphibia. Firstly the anterior abdominal
vein is only a foetal vessel, forming during foetal life the allantoic vein ;
and secondly a direct connection is established between the vena cava
inferior and the veins of the hind limbs and posterior parts of the cardinal
veins, so that there is no renal portal system.

Aves. The Chick may be taken to illustrate the development of the
venous system in Birds.

On the third day, nearly the whole of the venous blood from the body
of the embryo is carried back to the heart by two main venous trunks,
the anterior (fig. 125, S.Ca.V) and posterior (V.Ca) cardinal veins, joining on
each side to form the short transverse ductus Cuvieri (DC), both of which
unite with the sinus venosus close to the heart. As the head and neck
continue to enlarge, and the wings become developed, the single anterior

1 The junction between the portal system and the anterior abdominal vein is
apparently denied by Rathke (No. 300, p. 173), hut this must he an error on
his part.

THE VENOUS SYSTEM.

659

V.C.L

cardinal or jugular vein (fig. 371, /), of each side, is joined by two new
veins : the vertebral vein, bringing back blood from the head and neck, and
the subclavian vein from the wing (W\

On the third day the posterior cardinal veins are the only veins which
return the blood from the hinder part of the body of the embryo.

About the fourth or fifth day, however, the vena cava inferior (fig. 371,
V.C.L) makes its appearance. This, starting
from the sinus venosus not far from the heart,
is on the fifth day a short trunk running back-
ward in the middle line below the aorta, and
speedily losing itself in the tissues of the
Wolffian bodies. When the true kidneys are
formed it also receives blood from them, and
thenceforward enlarging rapidly becomes the
channel by which the greater part of the blood
from the hinder part of the body finds its way
to the heart. In proportion as the vena cava
inferior increases in size, the posterior cardinal
veins diminish.

The blood originally coming to them from
the posterior part of the spinal cord and trunk
is transported into two posterior vertebral veins,
similar to those in Reptilia, which are however
placed dorsally to the heads of the ribs, and
join the anterior vertebral veins. With their
appearance the anterior parts of the posterior
cardinals disappear. The blood from the hind
limbs becomes transported directly through the
kidney into the vena cava inferior, without
forming a renal portal system1.

On the third day the course of the vessels from the yolk-sack is very
simple. The two vitelline veins, of which the right is already the smaller,
form the ductus venosus, from which, as it passes through the liver on its
way to the heart, are given off the two sets of vena advehentes and vena
revehentes (fig. 371).

With the appearance of the allantois on the fourth day, a new feature is
introduced. From the ductus venosus there is given off a vein which
quickly divides into two branches. These, running along the ventral walls
of the body from which they receive some amount of blood, pass to the
allantois. They are the allantoic veins (fig. 371, U] homologous with the
anterior abdominal vein of the lower types. They unite in front to form a
single vein, which becomes, by reason of the rapid growth of the allantois,
very long. The right branch soon diminishes in size and finally disappears.
Meanwhile the left on reaching the allantois bifurcates ; and, its two

FIG. 371. DIAGRAM OF
THE VENOUS CIRCULATION
IN THE CHICK AT THE COM-
MENCEMENT OF THE FIFTH

DAY.

H. heart ; d. c. ductus Cu-
vieri. Into the ductus Cuvieri
of each side fall/, the jugular
vein, W. the vein from the
wing, and c. the inferior car-
dinal vein ; S. V. sinus venosus ;
Of. vitelline vein ; U. allan-
toic vein, which at this stage
gives off branches to the body-
walls ; V.C.l. inferior vena
cava ; /. liver.

The mode in which this is effected requires further investigation.

42 — 2

66o

VEINS OF THE CHICK.

branches becoming large and conspicuous, there still appear to be two
main allantoic veins. 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-sack dwindles, this state of things is reversed, and the less con-
spicuous vitelline appears as a branch of the larger allantoic vein.

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 deve-
loped, it acquires a distinct venous system,
and its blood is returned by veins which
form a trunk, the mesenteric vein (fig. 372,
M") falling into the vitelline vein at its
junction with the allantoic vein.

These three great veins, in fact, form a
large common trunk, which enters at once
into the liver, and which we may now call
the portal vein (fig. 372, P. V}. This, at its
entrance into the liver, partly breaks up
into the vena advehentes, and partly con-
tinues as the ductus venosus (D.V}
straight through the liver, emerging from
which it joins the vena cava inferior. Before
the establishment of the vena cava inferior,
the venas revehentes, carrying back the
blood which circulates through the hepatic
capillaries, join the ductus venosus close to
its exit from the liver. By the time how-
ever 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 ductus venosus making a sepa-
rate junction rather higher up (fig. 372).

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 propor-
tionately diminishes in size and importance, comes to appear as a mere
branch of the portal vein. The ductus venosus becomes obliterated ; and
hence the whole of the blood coming through the portal vein flows into the
substance of the liver, and so by the hepatic veins into the vena cava.

Although the allantoic (anterior abdominal) vein is obliterated in the
adult, there is nevertheless established an anastomosis between the portal
system and the veins bringing the blood from the limbs to the vena cava

FIG. 372. DIAGRAM OF THE
VENOUS CIRCULATION IN THE
CHICK DURING THE LATER DAYS
OF INCUBATION.

H. heart ; V.S.R. right vena
cava superior; V.S.L. left vena cava
superior. The two venas cavrc
superiores are the original 'ductus
Cuvieri,' they open into the sinus
venosus. J. jugular vein; Su.V.
anterior vertebral vein ; In. V. in-
ferior vertebral vein ; W. subcla-
vian; V.C.I, vena cava inferior;
D. V. ductus venosus ; P. V. portal
vein ; M. mesenteric vein bringing
blood from the intestines into the
portal vein ; O.f. vitelline vein ; U.
allantoic vein. The three last men-
tioned veins unite together to form
the portal vein ; /. liver.

THE VENOUS SYSTEM.

66l

inferior, in that the caudal vein and posterior pelvic veins open into a
vessel, known as the coccygeo-mesenteric vein, which joins the portal
vein ; while at the same time the posterior pelvic veins are connected with
the common iliac veins by a vessel which unites with them close to their
junction with the coccygeo-mesenteric vein.

Mammalia. In Mammals the same venous trunks are developed in
the embryo as in other types (fig. 373 A). The anterior cardinals or
external jugulars form the primitive veins of the anterior part of the body,
and the internal jugulars and anterior vertebrals are subsequently formed.
The subclavians (fig. 373 A, j), developed on the formation of the anterior
limbs, also pour their blood into these primitive trunks. In the lower
Mammalia (Monotremata, Marsupialia, Insectivora, some Rodentia, etc.,
the two ductus Cuvieri remain as the two superior venae cavae, but more
usually an anastomosis arises between the right and left innominate veins,
and eventually the whole of the blood of the left superior cava is carried to
the right side, and there is left only a single superior cava (fig. 373 B and C).

FIG- 373- DIAGRAM OF THE DEVELOPMENT OF THE PAIRED VENOUS SYSTEM OF

MAMMALS (MAN). (From Gegenbaur.)

j. jugular vein ; cs. vena cava superior; s. subclavian veins; c. posterior cardinal
vein ; v. vertebral vein ; az. azygos vein ; cor. coronary vein.

A. Stage in which the cardinal veins have already disappeared. Their position
is indicated by dotted lines.

B. Later stage when the blood from the left jugular vein is carried into the right
to form the single vena cava superior ; a remnant of the left superior cava being how-
ever still left.

C. Stage after the left vertebral vein has disappeared; the right vertebral
remaining as the azygos vein. The coronary vein remains as the last remnant of the
left superior vena cava.

A small rudiment of the left superior cava remains however as the sinus
coronartus and receives the coronary vein from the heart (figs. 373 C,
cor and 374, cs).

The posterior cardinal veins form at first the only veins receiving the

662

THE VEINS OF MAMMALIA.

blood from the posterior part of the trunk and kidneys ; and on the
development of the hind limbs receive the blood from them also.

As in the types already described
an unpaired vena cava inferior becomes
eventually developed, and gradually
carries off a larger and larger portion
of the blood originally returned by the
posterior cardinals. It unites with the
common stem of the allantoic and
vitelline veins in front of the liver.

At a later period a pair of trunks
is established bringing the blood from
the posterior part of the cardinal veins
and the crural veins directly into the
vena cava inferior (fig. 374, il}. These
vessels, whose development has not
been adequately investigated, form the
common iliac veins, while the posterior
ends of the cardinal veins which join
them become the hypogastric veins (fig.
374, hy). Owing to the development of
the common iliac veins there is no renal
portal system like that of the Reptilia
and Amphibia.

Posterior vertebral veins, similar to
those of Reptilia and Birds, are estab-
lished in connection with the intercostal
and lumbar veins, and unite anteriorly
with the front part of the posterior

FIG. 374. DIAGRAM OF THE CHIEF

VENOUS TRUNKS OF MAN. (From
Gegenbaur.)

cs. vena cava superior ; s. sub-
clavian vein ; ji. internal jugular ; je.
external jugular ; az. azygos vein ; ha.
hemiazygos vein ; c. clotted line shew-
ing previous position of cardinal veins ;
ci. vena cava inferior ; r. renal veins ;
il. iliac ; hy. hypogastric veins ; h.
hepatic veins.

The dotted lines shew the position
of embryonic vessels aborted in the
adult.

cardinal veins (fig. 373 A)1.

On the formation of the posterior vertebral veins, and as the inferior
vena cava becomes more important, the middle part of the posterior car-
dinals becomes completely aborted (fig. 374, f), the anterior and posterior
parts still persisting, the former as the continuations of the posterior
vertebrals into the anterior vena cava (az\ the latter as the hypogastric veins
(Ay).

Though in a few Mammalia both the posterior vertebrals persist, a
transverse connection is usually established between them, and the one (the
right) becoming the more important constitutes the azygos vein (fig. 374, az),
the persisting part of the left forming the hemiazygos vein (ha}.

The remainder of the venous system is formed in the embryo of the
vitelline and allantoic veins, the former being eventually joined by the
mesenteric vein so as to constitute the portal vein.

1 Rathke, as mentioned above, holds that in the Snake the front part of the
posterior cardinals completely aborts. Further investigations are required to shew
whether there really is a difference between Mammalia and Reptilia in this matter.

THE VENOUS SYSTEM. 663

The vitelline vein is the first part of this system established, and divides
near the heart into two veins bringing back the blood from the yolk-sack
(umbilical vesicle). The right vein soon however aborts.

The allantoic (anterior abdominal) veins are originally paired. They
are developed very early, and at first course along the still widely open
somatic walls of the body, and fall into the single vitelline trunk in front.
The right allantoic vein disappears before long, and the common trunk
formed by the junction of the vitelline and allantoic veins becomes con-
siderably elongated. This trunk is soon enveloped by the liver.

The succeeding changes have been somewhat differently described by
Kolliker and Rathke. According to the former the common trunk of the
allantoic and vitelline veins in its passage through the liver gives off
branches to the liver, and also receives branches from this organ near its
anterior exit. The main trunk is however never completely aborted, as in
the embryos of other types, but remains as the ductus venosus Arantii.

With the development of the placenta the allantoic vein becomes the
main source of the ductus venosus, and the vitelline or portal vein, as it may
perhaps be now conveniently called, ceases to join it directly, but falls into
one of its branches in the liver.

The vena cava inferior joins the continuation of the ductus venosus in
front of the liver, and, as it becomes more important, it receives directly
the hepatic veins which originally brought back blood into the ductus
venosus. The ductus venosus becomes moreover merely a small branch of
the vena cava.

At the close of foetal life the allantoic vein becomes obliterated up to its
place of entrance into the liver ; the ductus venosus becomes a solid cord — •
the so-called round ligament— and the whole of the venous blood is brought
to the liver by the portal vein1.

Owing to the allantoic (anterior abdominal) vein having merely a fcetal
existence an anastomosis between the iliac veins and the portal system by
means of the anterior abdominal vein is not established.

BIBLIOGRAPHY of the Venous System.

(498) J. Marshall. "On the development of the great anterior veins." Phil.
Trans., 1859.

(499) H. Rathke. " Ueb. d. Bildung d. Pfortader u. d. Lebervenen b. Sauge-
thieren." MeckeVs Archiv, 1830.

(500) H. Rathke. "Ueb. d. Bau u. d. Entwick. d. Venensystems d. Wirbel-
thiere." Bericht. Jib. d. natttrh. Seminar, d. Univ. Konigsberg, 1838.

Vide also Von Baer (No. 291), Gotte (No. 296), Kolliker (No. 298), and Rathke
(Nos. 299, 300, and 301).

1 According to Rathke the original trunk connecting the allantoic vein directly
with the heart through the liver is aborted, and the ductus venosus Arantii is a
secondary connection established in the latter part of foetal life.

664 LYMPHATIC SYSTEM.

Lymphatic System.

The lymphatic system arises from spaces in the general parenchyma of
the body, independent in their origin of the true body cavity, though com-
municating both with this cavity and with the vascular system.

In all the true Vertebrata certain parts of the system form definite trunks
communicating with the venous system ; and in the higher types the walls of
the main lymphatic trunks become quite distinct.

But little is known with reference to the ontogeny of the lymphatic vessels,
but they originate late in larval life, and have at first the form of simple
intercellular spaces.

The lymphatic glands appear to originate from lymphatic plexuses, the
cells of which produce lymph corpuscles. It is only in Birds and Mammals,
and especially in the latter, that the lymphatic glands form definite struc-
tures.

The Spleen. The spleen, from its structure, must be classed with the
lymphatic glands, though it has definite relations to the vascular system.
It is developed in the mesoblast of the mesogastrium, usually about the
same time and in close connection with the pancreas.

According to Miiller and Peremeschko 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 cells take place at a
later period to form the so-called Malpighian corpuscles of the spleen.

BIBLIOGRAPHY of Spleen.

(501) W. Miiller. "The Spleen." Strieker's Histology.

(502) Peremeschko. " Ueb. d. Entwick. d. Milz." Sitz. d. Wuti. Akad.
Wiss., Vol. LVI. 1867.

Suprarenal ^bodies.

In Elasmobranch Fishes two distinct sets of structures are found, both of
which have been called suprarenal bodies. As shewn in the sequel both of
these structures probably unite in the higher types to form the suprarenal
bodies.

One of them consists of a series of paired bodies, situated on the
branches of the dorsal aorta, segmentally arranged, and forming a chain
extending from close behind the heart to the hinder end of the body cavity.
Each body is formed of a series of lobes, and exhibits a well-marked
distinction into a cortical layer of columnar cells, and a medullary substance
formed of irregular polygonal cells. As first shewn by Leydig, they are

SUPRARENAL BODIES. 665

closely connected with the sympathetic ganglia, and usually contain numerous
ganglion cells distributed amongst the proper cells of the body.

The second body consists of an unpaired column of cells placed between
the dorsal aorta and unpaired caudal vein, and bounded on each side by the
posterior parts of the kidney. I propose to call it the interrenal body.
In front it overlaps the paired suprarenal bodies, but does not unite with
them. It is formed of a series of well-marked lobules, etc. In the fresh
state Leydig (No. 506) finds that "fat molecules form the chief mass of the
body, and one finds freely imbedded in them clear vesicular nuclei." As
may easily be made out from hardened specimens it is invested by a tunica
propria, which gives off septa dividing it into well-marked areas filled with
polygonal cells. These cells constitute the true parenchyma of the body.
By the ordinary methods of hardening, the oil globules, with which they are
filled in the fresh state, completely disappear.

The paired suprarenal bodies (Balfour, No. 292, pp. 242 — 244) are de-
veloped from the sympathetic ganglia. These ganglia, shewn in an early
stage in fig. 380, sy.g, become gradually divided into a ganglionic part and a
glandular part. The former constitutes the sympathetic ganglia of the adult ;
the latter the true paired suprarenal bodies. The interrenal body is however
developed (Balfour, No. 292, pp. 245 — 247) from indifferent mesoblast cells
between the two kidneys, in the same situation as in the adult.

The development of the suprarenal bodies in the Amniota has been most
fully studied by Braun (No. 503) in the Reptilia.

In Lacertilia they consist of a pair of elongated yellowish bodies, placed
between the vena renalis revehens and the generative glands.

They are formed of two constituents, viz. (i) masses of brown cells placed
on the dorsal side of the organ, which stain deeply with chromic acid, like
certain of the cells of the suprarenals of Mammalia, and (2) irregular cords,
in part provided with a lumen, filled with fat-like globules l, amongst which
are nuclei. On treatment with chromic acid the fat globules disappear, and
the cords break up into bodies resembling columnar cells.

The dorsal masses of brown cells are developed from the sympathetic
ganglia in the same way as the paired suprarenal bodies of the Elasmo-
branchii, while the cords filled with fat-like globules are formed of indifferent
mesoblast cells as a thickening in the lateral walls of the inferior vena cava,
and the cardinal veins continuous with it. The observations of Brunn (No.
504) on the Chick, and Kolliker (No. 298, pp. 953—955) °n the Mammal,
add but little to those of Braun. They shew that the greater part of the
gland (the cortical substance) in these two types is derived from the mesoblast,
and that the glands are closely connected with sympathetic ganglia ; while
Kolliker also states that the posterior part of the organ is unpaired in the
embryo rabbit of 1 6 or 17 days.

The structure and development of what I have called the interrenal body

1 These globules are not formed of a true fatty substance, and this is also probably
true for the similar globules of the interrenal bodies of Elasmobranchii.

666 SUPRARENAL BODIES.

in Elasmobranchii so closely correspond with that of the mesoblastic part of
the suprarenal bodies of the Reptilia, that I have very little hesitation in
regarding them as homologous1; while the paired bodies in Elasmobranchii,
derived from the sympathetic ganglia, clearly correspond with the part of the
suprarenals of Reptilia having a similar origin ; although the anterior parts
of the paired suprarenal bodies of Fishes have clearly become aborted in the
higher types.

In Elasmobranch Fishes we thus have (i) a series of paired
bodies, derived from the sympathetic ganglia, and (2) an un-
paired body of mesoblastic origin. In the Amniota these bodies
unite to form the compound suprarenal bodies, the two consti-
tuents of which remain, however, distinct in their development.
The mesoblastic constituent appears to form the cortical part of
the adult suprarenal body, and the nervous constituent the
medullary part.

BIBLIOGRAPHY of the Suprarenal bodies,

(503) M. Braun. "Bau u. Entwick. d. Nebennieren bei Reptilien. " Arbeit,
a. d. zool.-zoot. Institut Wurzlttrg, Vol. V. 1879.

(504) A. v. Brunn. "Ein Beitrag z. Kenntniss d. feinern Baues u. d. Entwick.
d. Nebennieren." Archiv f. mikr. Anat., Vol. VIII. 1872.

(505) Fr. Leydig. Untersiich. iib. Fische u. fieptilten. Berlin, 1853.

(506) Fr. Leydig. Rochen u. Haie. Leipzig, 1852.

Vide also F. M. Balfour (No. 292), Kolliker (No. 298), Remak (No. 302), etc.

1 The fact of the organ being unpaired in Elasmobranchii and paired in the
Amniota is of no importance, as is shewn by the fact that part of the organ is unpaired
in the Rabbit.

CHAPTER XXII.

THE MUSCULAR SYSTEM.

IN all the Ccelenterata, except the Ctenophora, the contrac-
tile elements of the body wall consist of filiform processes of
ectodermal or entodermal epithelial cells (figs. 375 and 376 B).
The elements provided with these processes, which were first
discovered by Kleinenberg, are known as myo-epithelial
cells. Their contractile parts may either be striated (fig. 376)
or non-striated (fig. 375). In some
instances the epithelial part of the
cell may nearly abort, its nucleus
alone remaining (fig. 376 A) ; and
in this way a layer of muscles lying
completely below the surface may
be established.

There is embryological evidence
of the derivation of the voluntary
muscular system of a large number of types from myo-epithelial
cells of this kind. The more important of these groups are the
Chaetopoda, the Gephyrea, the Chaetognatha, the Nematoda, and
the Vertebrata1.

While there is clear evidence that the muscular system of a
large number of types is composed of cells which had their
origin in myo-epithelial cells, the mode of evolution of the

1 If recent statements of Metschnikoff are to be trusted, the Echinodermata must
be added to these groups. The amoeboid cells stated in the first volume of this
treatise to form the muscles in this group, on the authority of Selenka, give rise,
according to Metschnikoff, only to the cutis, while the same naturalist states the
epithelial cells of the vasoperitoneal vesicles are provided with muscular tails.

FIG. 375. MYO-EPITHELIAL
CELLS OF HYDRA. (From Gegen-
baur ; after Kleinenberg.)

m. contractile fibres.

668 THE MUSCULAR FIBRES.

muscular system of other types is still very obscure. The
muscles may arise in the embryo from amoeboid or indifferent
cells, and the Hertwigs1 hold that in many of these instances the
muscles have also phylogenetically taken their origin from
indifferent connective-tissue cells. The subject is however beset
with very serious difficulties, and to discuss it here would carry
me too far into the region of pure histology.

The voluntary muscular system of the CJiordata.

The muscular fibres. The muscular elements of the
Chordata undoubtedly belong to the myo-epithelial type. The
embryonic muscle-cells are at first simple epithelial cells, but

FIG. 376. MUSCLE-CELLS OF LIZZIA KOLLIKERI. (From Lankester ; after
O. and R. Hertwig.)

A. Muscle-cell from the circular fibres of the subumbrella.

B. Myo-epithelial cells from the base of a tentacle.

soon become spindle-shaped : part of their protoplasm becomes
differentiated into longitudinally placed striated muscular fibrils,
while part, enclosing the nucleus, remains indifferent, and con-
stitutes the epithelial element of the cells. The muscular
fibrils are either placed at one side of the epithelial part of the
cell, or in other instances (the Lamprey, the Newt, the Sturgeon,
the Rabbit) surround it. The latter arrangement is shewn for
the Sturgeon in fig. 57.

The number of the fibrils of each cell gradually increases,
and the protoplasm diminishes, so that eventually only the
nucleus, or nuclei resulting from its division, are left. The
products of each cell probably give rise, in conjunction with a
further division of the nucleus, to a primitive bundle, which,

1 O. and R. Hertwig, Die Calomthcorie. Jena, 1881.

THE MUSCULAR SYSTEM.

669

t>r

except in Amphioxus, Petromyzon, etc., is surrounded by a
special investment of sarcolemma.

The voluntary muscular system. For the purposes of
description the muscular system of the Vertebrata may conve-
niently be divided into two sections, viz. that of the head and
that of the trunk. The main part, if
not the whole, of the muscular system
of the trunk is derived from certain
structures, known as the muscle-plates,
which take their origin from part of
the primitive mesoblastic somites.

It has already been stated (pp.
292 — ^296) that the mesoblastic somites
are derived from the dorsal segmented
part of the primitive mesoblastic plates.
Since the history of these bodies is
presented in its simplest form in Elas-
mobranchii it will be convenient to
commence with this group. Each
somite is composed of two layers — a
somatic and a splanchnic — both formed
of a single row of columnar cells.
Between these two layers is a cavity,
which is at first directly continuous
with the general body cavity, of which
indeed it merely forms a specialised
part (fig. 377). Before long the cavity
becomes however completely constrict-
ed off from the permanent body cavity.

Very early (fig. 377) the inner or splanchnic wall of the
somites loses its simple constitution, owing to the middle part of
it undergoing peculiar changes. The meaning of the changes is
at once shewn by longitudinal horizontal sections, which prove
(%• 378) that the cells in this situation (mp') have become
extended in a longitudinal direction, and, in fact, form typical
spindle-shaped embryonic muscle-cells, each with a large
nucleus. Every muscle-cell extends for the whole length of a
somite. The inner layer of each somite, immediately within
the muscle-band just described, begins to proliferate, and produce

FIG. 377. TRANSVERSE
SECTION THROUGH THETRUNK
OF AN EMBRYO SLIGHTLY
OLDER THAN FIG. 28 E.

nc. neural canal ; pr. pos-
terior root of spinal nerve ; x.
subnotochordal rod ; ao. aorta ;
sc. somatic mesoblast ; sf>.
splanchnic mesoblast ; mp.
muscle-plate ; mp', portion of
muscle-plate converted into
muscle ; Vr. portion of the
vertebral plate which will give
rise to the vertebral bodies ; al.
alimentary tract.

THE MUSCLE-PLATES.

a mass of cells, placed between the muscles and the notochord
( Vr\ These cells form the commencing vertebral bodies, and
have at first (fig. 378) the same segmentation as the somites
from which they sprang.

After the separation of the vertebral bodies from the somites
the remaining parts of the somites may be called muscle-plates ;
since they become directly converted into the whole voluntary
muscular system of the trunk (fig. 379, mp}.

According to the statements of Bambeke and Go'tte, the Amphibians
present some noticeable peculiarities in the development of their muscular
system, in that such distinct muscle-plates as those of other vertebrate types
are not developed. Each side-plate of mesoblast is divided into a somatic
and a splanchnic layer, continuous throughout the vertebral and parietal
portions of the plate. The vertebral portions (somites) of the plates soon
become separated from the parietal, and form independent masses of cells
constituted of two layers, which were originally continuous with the
somatic and splanchnic layers of the parietal plates (fig. 79). The outer or
somatic layer of the vertebral plates is formed of a single row of cells, but
the inner or splanchnic layer is made up of a kernel of cells on the side of
the somatic layer and an inner layer. The kernel of the splanchnic layer
and the outer or somatic layer together correspond to a muscle- plate of other
Vertebrata, and exhibit a similar segmentation.

Osseous Fishes are stated to agree with Amphibians in the development
of their somites and muscular
system1, but further observations
on this point are required.

In Birds the horizontal split-
ting of the mesoblast extends at
first to the dorsal summit of the
mesoblastic plates, but after the
isolation of the somites the split
between the somatic and splanch-
nic layers becomes to a large ex-
tent obliterated, though in the an-
terior somites it appears in part
to persist. The somites on the
second day, as seen in a trans-
verse section (fig. 115, P.?'.), are
somewhat quadrilateral in form
but broader than they are deep.

Each at that time consists of
a somewhat thick cortex of radi-

FlG. 378. HORIZONTALSECTION THROUGH
THE TRUNK OF AN EMBRYO OF SCYLL1UM
CONSIDERABLY YOUNGER THAN 28 F.

The section is taken at the level of the
notochord, and shews the separation of the
cells to form the vertebral bodies from the
muscle-plates.

ch. notochord ; ep. epiblast ; Vr, rudiment
of vertebral body ; mp. muscle- plate ; mp' .
portion of muscle-plate already differentiated
into longitudinal muscles.

1 Ehrlich, " Ueber den peripher. Theil d. Urwirbel." Archiv f. mikr. Anal.,
Vol. XI.

THE MUSCULAR SYSTEM. 671

ating 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 lateral as they are at this period) 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
upper and outer layer of the cortex, together probably with some of the
central cells of the kernel, becomes separated off as a muscle-plate (fig. 1 16).
The muscle-plate when formed (fig. 117) is found to consist of two layers,
an inner and an outer, which enclose between them an almost obliterated
central cavity ; and no sooner is the muscle-plate formed than the middle
portion of the inner layer becomes converted into longitudinal muscles.
The avian muscle-plates have, in fact, precisely the same constitution as
those of Elasmobranchii. The central space is clearly a remnant of the
vertebral portion of the body cavity, which, though it wholly or partially
disappears in a previous stage, reappears again on the formation of the
muscle-plate.

The remainder of the somite, after the formation of the muscle-plate,
is of very considerable bulk ; the cells of the cortex belonging to it lose
their distinctive characters, and the major part of it becomes the vertebral
rudiment.

In Mammalia the history appears to be generally the same as in Elas-
mobranchii. The split which gives rise to the body cavity is continued to
the dorsal summit of the mesoblastic plates, and the dorsal portions of the
plates with their contained cavities become divided into somites, and are
then separated off from the ventral. The later development of the somites
has not been worked out with the requisite care, but it would seem that they
form somewhat cubical bodies in which all trace of the primitive slit is lost.
The further development resembles that in Birds.

The first changes of the mesoblastic somites and the forma-
tion of the muscle-plates do not, according to existing statements,
take place on quite the same type throughout the Vertebrata,
yet the comparison which has been instituted between Elasmo-
branchs and other Vertebrates appears to prove that there are
important common features in their development, which may be
regarded as primitive, and as having been inherited from the
ancestors of Vertebrates. These features are (i) the extension
of the body cavity into the vertebral plates, and subsequent
enclosure of this cavity between the two layers of the muscle-
plates ; (2) the primitive division of the vertebral plate into an
outer (somatic) and an inner (splanchnic) layer, and the formation
of a large part of the voluntary muscular system out of the inner

THE MUSCLE-PLATES.

sp.c

layer, which in all cases is converted into muscles earlier than
the outer layer.

The conversion of the muscle-plates into muscles. It

will be convenient to commence this subject with a description
of the changes which take place in
such a simple type as that of the
Elasmobranchii.

At the time when the muscle-
plates have become independent
structures they form flat two-layered
oblong bodies enclosing a slit-like
central cavity (fig. 379, mp). The
outer or somatic wall is formed of
simple epithelial -like cells. The
inner or splanchnic wall has how-
ever a somewhat complicated struc-
ture. It is composed dorsally and
ventrally of a columnar epithelium,
but in its middle portion of the
muscle-cells previously spoken of.
Between these and the central cavity
of the plates the epithelium forming
the remainder of the layer com-
mences to insert itself; so that be-
tween the first-formed muscle and
the cavity of the muscle-plate there
appears a thin layer of cells, not
however continuous throughout.

When first formed the muscle-
plates, as viewed from the exterior,
have nearly straight edges ; soon
however they become bent in the middle, so that the edges have
an obtusely angular form, the apex of the angle being directed
forwards. They are so arranged that the anterior edge of the
one plate fits into the posterior edge of the one in front. In the
lines of junction between the plates layers of connective-tissue
cells appear, which form the commencements of the intermuscular
septa.

The growth of the plates is very rapid, and their upper ends

FIG. 379. SECTION THROUGH
THE TRUNK OF A SCYLLIUM EM-
BRYO SLIGHTLY YOUNGER THAN

28 F.

sp.c. spinal canal ; W. white
matter of spinal cord ; pr. poste-
rior nerve-roots ; ch. notochord ;
x. sub-notochordal rod ; ao. aorta ;
mp. muscle-plate; mp' . inner layer
of muscle-plate already converted
into muscles ; Vr. rudiment of
vertebral body ; si. segmental
tube ; sd. segmental duct ; sp.v.
spiral valve ; z/. subintestinal vein ;
P.O. primitive generative cells.

THE MUSCULAR SYSTEM. 673

soon extend to the summit of the neural canal, and their lower
ones nearly meet in the median ventral line. The original band
of muscles, whose growth at first is very slow, now increases
with great rapidity, and forms the nucleus of the whole volun-
tary muscular system (fig. 380, mp'). It extends upwards and
downwards by the continuous conversion of fresh cells of the
splanchnic layer into muscle-cells. At the same time it grows
rapidly in thickness by the addition of fresh spindle-shaped
muscle-cells from the somatic layer as well as by the division of
the already existing cells.

Thus both layers of the muscle-plate are concerned in forming
the great longitudinal lateral muscles, though the splanchnic layer
is converted into muscles very much sooner than the somatic1.

Each muscle-plate is at first a continuous structure, extending
from the dorsal to the ventral surface, but after a time it becomes
divided by a layer of connective tissue, which becomes developed
nearly on a level with the lateral line, into a dorso-lateral and
a ventro-lateral section. The ends of the muscle-plates
continue for a long time to be formed of undifferentiated
columnar cells. The complicated outlines of the inter-muscular
septa become gradually established during the later stages of
development, causing the well-known appearances of the muscles
in transverse sections, which require no special notice here.

The muscles of the limbs. The limb muscles are formed
in Elasmobranchii, coincidently with the cartilaginous skeleton,
as two bands of longitudinal fibres on the dorsal and ventral
surfaces of the limbs (fig. 346). The cells, from which these
muscles originate, are derived from the muscle-plates. When
the ends of the muscle-plates reach the level of the limbs they
bend outwards and enter the tissue of the limbs (fig. 380).
Small portions of several muscle-plates (m.pl) come in this way
to be situated within the limbs, and are very soon segmented
off from the remainder of the muscle-plates. The portions of
the muscle-plates thus introduced soon lose their original dis-

1 The brothers Hertwig have recently maintained that only the inner layer of the
muscle-plates is converted into muscles. In the Elasmobranchs it is easy to demon-
strate the incorrectness of this view, and in Acipenser (vide fig. 57, mp) the two layers
of the muscle-plate retain their original relations after the cells of both of them have
become converted into muscles.

B. in. 43

674

THE MUSCLE-PLATES.

3,-n,

FIG. 380. TRANSVERSE SECTION THROUGH THE ANTERIOR PART OF THE TRUNK
OF AN EMBRYO OF SCYLLIUM SLIGHTLY OLDER THAN FIG. 29 B.

The section is diagrammatic in so far that the anterior nerve-roots have been
inserted for the whole length ; whereas they join the spinal cord half-way between
two posterior roots.

sp.c. spinal cord; sp.g. ganglion of posterior root; ar. anterior root; dn. dorsally
directed nerve springing from posterior root; nip. muscle-plate; mp'. part of muscle-
plate already converted into muscles; vi.pl. part of muscle-plate which gives rise to
the muscles of the limbs; «/. nervus lateralis; ao. aorta; ch. notochord; sy.g. sym-
pathetic ganglion; ca.v. cardinal vein; sp.n. spinal nerve; sd. segmental (archinephric)
duct; st. segmental tube; du. duodenum; pan. pancreas; hp.d. point of junction of
hepatic duct with duodenum ; umc. umbilical canal.

THE MUSCULAR SYSTEM. 675

tinctness. There can however be but little doubt that they
supply the tissue for the muscles of the limbs. The muscle-
plates themselves, after giving off buds to the limbs, grow
downwards, and soon cease to shew any trace of having given
off these buds.

In addition to the longitudinal muscles of the trunk just described,
which are generally characteristic of Fishes, there is found in Amphioxus a
peculiar transverse abdominal muscle, extending from the mouth to the
abdominal pore, the origin of which has not been made out.

It has already been shewn that in all the higher Vertebrata
muscle-plates appear, which closely resemble those in Elasmo-
branchii; so that all the higher Vertebrata pass through, with
reference to their muscular system, a fish- like stage. The
middle portion of the inner layers of their muscle-plates be-
comes, as in Elasmobranchii, converted into muscles at a very
early period, and the outer layer for a long time remains formed
of indifferent cells. That these muscle-plates give rise to the
main muscular system of the trunk, at any rate to the episkeletal
muscles of Huxley, is practically certain, but the details of the
process have not been made out.

In the Perennibranchiata the fish-like arrangement of muscles is re-
tained through life in the tail and in the dorso-lateral parts of the trunk.
In the tail of the Amniotic Vertebrata the primitive arrangement is also
more or less retained, and the same holds good for the dorso-lateral trunk
muscles of the Lacertilia. In the other Amniota and the Anura the
dorso-lateral muscles have become divided up into a series of separate
muscles, which are arranged in two main layers. It is probable that the
intercostal muscles belong to the same group as the dorso-lateral muscles.

The abdominal muscles of the trunk, even in the lowest Amphibia,
exhibit a division into several layers. The recti abdominis are the least
altered part of this system, and usually retain indications of the primitive
inter-muscular septa, which in many Amphibia and Lacertilia are also
to some extent preserved in the other abdominal muscles.

In the Amniotic Vertebrates there is formed underneath the vertebral
column and the transverse processes a system of muscles, forming part
of the hyposkeletal system of Huxley, and called by Gegenbaur the sub-
vertebral muscles. The development of this system has not been worked
out, but on the whole I am inclined 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.

43—2

676 THE HEAD-CAVITIES.

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.

The limb-muscles, both extrinsic and intrinsic, as may be concluded
from their development in Elasmobranchii, are derived from the muscle-
plates. Kleinenberg found in Lacertilia a growth of the muscle-plates
into the limbs, and in Amphibia Gotte finds that the outer layer of the
muscle-plates gives rise to the muscles of the limbs.

In the 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.

TJte Somites and muscular system of the head.

The extension of the somites to the anterior end of the body
in Amphioxus clearly proves that somites, similar to those of
the trunk, were originally present in a region, which in the
higher Vertebrata has become differentiated into the head. In
the adult condition no true Vertebrate exhibits indications of
such somites, but in the embryos of several of the lower Verte-
brata structures have been found, which are probably equivalent
to the somites of the trunk : they have been frequently alluded
to in the previous chapters of this volume. These structures
have been most fully worked out in Elasmobranchii.

The mesoblast in Elasmobranch embryos becomes first split
into somatic and splanchnic layers in the region of the head ;
and between these layers there are formed two cavities, one on
each side, which end in front opposite the blind anterior ex-
tremity of the alimentary canal ; and are continuous behind
with the general body-cavity (fig. 20 A, vp}. I propose calling
them the head-cavities. The cavities of the two sides have
no communication with each other.

Coincidently with the formation of an outgrowth from the
throat to form the first visceral cleft, the head-cavity on each
side becomes divided into a section in front of the cleft and a
section behind the cleft ; and at a later period it becomes, owing
to the formation of a second cleft, divided into three sections :

THE MUSCULAR SYSTEM.

677

vn~.

(i) a section in front of the first or hyomandibular cleft; (2) a
section in the hyoid arch between the hyomandibular cleft and
the hyobranchial or first branchial cleft ; (3) a section behind
the first branchial cleft.

The front section of the head-cavity grows forward, and soon
becomes divided, without the intervention of a visceral cleft, into
an anterior and posterior division.
The anterior lies close to the eye,
and in front of the commencing
mouth involution. The posterior
part lies completely within the man-
dibular arch.

As the rudiments of the succes-
sive visceral clefts are formed, the
posterior part of the head-cavity be-
comes divided into successive sec-
tions, there being one section for
each arch. Thus the whole head-
cavity becomes on each side divided
into (i) a premandibular section ; (2)
a mandibular section (vide fig. 29 A,
PP] > (3) a hyoid section ; (4) sections
in each of the branchial arches.

The first of these divisions forms
a space of a considerable size, with
epithelial walls of somewhat short
columnar cells (fig. 381, ipp}. It is
situated close to the eye, and pre-
sents a rounded or sometimes a
triangular figure in section. The
two halves of the cavity are pro-
longed ventralwards, and meet below
the base of the fore-brain. The
connection between them appears to last for a considerable time.
These two cavities are the only parts of the body-cavity within
the head which unite ventrally. The section of the head-cavity
just described is so similar to the remaining sections that it
must be considered as serially homologous with them.

The next division of the head-cavity, which from its position

FIG. 381. TRANSVERSE SEC-
TION THROUGH THE FRONT PART
OF THE HEAD OF A YOUNG PRIS-
TIURUS EMBRYO.

The section, owing to the cra-
nial flexure, cuts both the fore-
and the hind-brain. It shews the
premandibular and mandibular
head-cavities ipp and ipp, etc.
The section is moreover somewhat
oblique from side to side.

fb. fore-brain ; /. lens of eye ;
m. mouth ; pt. upper end of mouth,
forming pituitary involution; lao.
mandibular aortic arch; ipp. and
ipp. first and second head-cavities;
\vc. first visceral cleft; V. fifth
nerve ; aim. auditory nerve ; VII.
seventh nerve ; aa. dorsal aorta ;
acv. anterior cardinal vein ; ch,
notochord.

678 THE HEAD-CAVITIES.

may be called the mandibular cavity, presents a spatulate shape,
being dilated dorsally, and produced ventrally into a long thin
process parallel to the hyomandibular gill-cleft (fig. 20, pp}.
Like the previous space it is lined by a short columnar epi-
thelium.

The mandibular aortic arch is situated close to its inner side
(fig. 381, 2pp). After becoming separated from the lower part
(Marshall), the upper part of the cavity atrophies about the time
of the appearance of the external gills. Its lower part also
becomes much narrowed, but its walls of columnar cells persist.
The outer or somatic wall becomes very thin indeed, the
splanchnic wall, on the other hand, thickens and forms a layer
of several rows of elongated cells. In each of the remaining
arches there is a segment of the original body-cavity fundamen-
tally similar to that in the mandibular arch (fig. 382). A dorsal
dilated portion appears, however, to be present in the third or
hyoid section alone (fig. 20), and even
there disappears very soon, after being
segmented off from the lower part
(Marshall). The cavities in the pos-
terior parts of the head become much
reduced like those in its anterior part,
though at rather a later period. FlG. 382. HORIZONTAL

It has been shewn that the divi- SECTION THROUGH THE PEN-
ULTIMATE VISCERAL ARCH OF

sions of the body-cavity in the head, AN EMBRYO OF PRISTIURUS.
with the exception of the anterior, ep. epiblast; vc. pouch of
early become atrophied, not so how- hypoblast which will form the

walls of a visceral cleit ; //.
CVer their walls. The cells forming segment of body-cavity in vis-

the walls both of the dorsal and ven- ceral arch ; aa' aortic arch'
tral sections of these cavities become elongated, and finally
become converted into muscles. Their exact history has not
been followed in its details, but they almost unquestionably
become the musculus contrictor superficialis and musculus inter-
branchialis1 ; and probably also musculus levator mandibuli and
other muscles of the front part of the head.

The anterior cavity close to the eye remains unaltered much
longer than the remaining cavities.

1 Vide Vetter, " Die Kiemen und Kiefermusculatur d. Fische." Jenaische Zclt-
schrift, Vol. vn.

THE MUSCULAR SYSTEM.

679

Its further history is very interesting. In my original account
of this cavity (No. 292, p. 208) I stated my belief that its walls
gave rise to the eye-muscles, and the history of this process has
been to some extent worked out by Marshall in his important
memoir (No. 509).

Marshall finds that the ventral portion of this cavity, where
its two halves meet, becomes separated from the remainder.
The eventual fate of this part has not however been followed.
Each dorsal section acquires a cup-like form, investing the
posterior and inner surface of the eye. The cells of its outer
wall subsequently give rise to three sets of muscles. The middle
of these, partly also derived from the inner walls of the cup,
becomes the rectus internus of the eye, the dorsal set forms the
rectus superior, and the ventral the rectus inferior. The obliquus
inferior appears also to be in part developed from the walls of
this cavity.

Marshall brings evidence to shew that the rectus externus (as
might be anticipated from its nerve supply) has no connection
with the walls of the premandibular head-cavity, and finds that
it arises close to the position originally occupied by the second
and third cavities. Marshall has not satisfactorily made out the
mode of development of the obliquus superior.

The walls of the cavities, whose history has just been re-
corded, have definite relations with the cranial nerves, an account
of which has already been given at p. 461.

Head-cavities, in the main similar to those of Elasmo-
branchii, have been found in the embryo of Petromyzon (fig. 45,
/ic\ the Newt (Osborn and Scott), and various Reptilia (Parker).

BIBLIOGRAPHY.

(507) G.M.Humphry. " Muscles in Vertebrate Animals." Journ. of Anat.
and Phys., Vol. vi. 1872.

(508) J. Miiller. " Vergleichende Anatomic d. Myxinoiden. Part I. Osteologie
u. Myologie." Akad. Wiss., Berlin, 1834.

(509) A. M. Marshall. "On the head cavities and associated nerves of
Elasmobranchs." Quart. J. of Micr. Science, Vol. xxi. 1881.

(510) A. Schneider. " Anat. u. Entwick. d. Muskelsystems d. Wirbelthierc."
Silz. d. Oberhessischen Gesellschaft, 1873.

(511) A. Schneider. Beitrdge z. vergleich. Anat. ». Entwick. d. Wirbelthiere.
Berlin, 1879.

Vide 2^0 Gotte (No. 296), Kolliker (N o. 298), Balfour (No. 292), Huxley, etc.

CHAPTER XXIII.

EXCRETORY ORGANS.

EXCRETORY organs consist of coiled or branched and often
ciliated tubes, with an excretory pore opening on the outer surface
of the body, and as a rule an internal ciliated orifice placed in the
body-cavity. In forms provided with a true vascular system,
there is a special development of capillaries around the glandular
part of the excretory organs. In many instances the glandular
cells of the organs are filled with concretions of uric acid or some
similar product of nitrogenous waste.

There is a very great morphological and physiological simi-
larity between almost all the forms of excretory organ found in
the animal kingdom, but although there is not a little to be said
for holding all these organs to be derived from some common
prototype, the attempt to establish definite homologies between
them is beset with very great difficulties.

Platyelminthes. Throughout the whole of the Platyel-
minthes these organs are constructed on a well-defined type, and
in the Rotifera excretory organs of a similar form to those of the
Platyelminthes are also present.

These organs (Fraipont, No. 513) are more or less distinctly
paired, and consist of a system of wide canals, often united into a
network, which open on the one hand into a pair of large tubes
leading to the exterior, and on the other into fine canals which
terminate by ciliated openings, either in spaces between the
connective-tissue cells (Platyelminthes), or in the body-cavity
(Rotifera). The fine canals open directly into the larger ones,
without first uniting into canals of an intermediate size.

EXCRETORY ORGANS.

68 1

The two large tubes open to the exterior, either by means of
a median posteriorly placed contractile vesicle, or by a pair of
vesicles, which have a ventral and anterior position. The former
type is characteristic of the majority of the Trematoda, Cestoda.
and Rotifera, and the latter of the Nemertea and some Trematoda.
In the Turbellaria the position of the external openings of the
system is variable, and in a few Cestoda (Wagner) there are
lateral openings on each of the successive proglottides, in addition
to the terminal openings. The mode of development of these
organs is unfortunately not known.

Mollusca. In the Mollusca there are usually present two
independent pairs of excretory organs — one found in a certain
number of forms during early larval life only1, and the other
always present in the adult.

The larval excretory organ has been found in the pulmonate
Gasteropoda (Gegenbaur, Fol2, Rabl), in Teredo (Hatschek), and
possibly also in Paludina. It is placed in the anterior region of
the body, and opens ventrally on each side, a short way behind
the velum. It is purely a larval organ, disappearing before the
close of the veliger stage. In the aquatic Pulmonata, where it is
best developed, it consists on each side of a V-shaped tube, with
a dorsally-placed apex, containing an enlargement of the lumen.
There is a ciliated cephalic limb, lined by cells with concretions,
and terminating by an internal opening near the eye, and a non-
ciliated pedal limb opening to the exterior3.

Two irreconcilable views are held as to the development of
this system. Rabl (Vol. II. No. 268) and Hatschek hold that it
is developed in the mesoblast ; and Rabl states that in Planorbis
it is formed from the anterior mesoblast cells of the mesoblastic
bands. A special mesoblast cell on each side elongates into two
processes, the commencing limbs of the future organ. A lumen
is developed in this cell, which is continued into each limb, while

1 I leave out of consideration an external renal organ found in many marine
Gasteropod larvte, vide Vol. II. p. 280.

2 H. Fol, "Etudes sur le devel. d. Mollusques. " Mem. Hi. Archiv d. Zool.
exfJr. et gener., Vol. VIII.

3 The careful observations of Fol seem to me nearly conclusive in favour of this
limb having an external opening, and the statement to the reverse effect on p. 280 of
Vol. ii. of this treatise, made on the authority of Rabl and Biitschli, must probably be
corrected.

682 POLYZOA.

the continuations of the two limbs are formed by perforated
mesoblast cells.

According to Fol these organs originate in aquatic Pulmonata
as a pair of invaginations of the epiblast, slightly behind the
mouth. Each invagination grows in a dorsal direction, and after
a time suddenly bends on itself, and grows ventralwards and
forwards. It thus acquires its V-shaped form.

In the terrestrial Pulmonata the provisional excretory organs
are, according to Fol, formed as epiblastic invaginations, in the
same way as those in the aquatic Pulmonata, but have the form
of simple non-ciliated sacks, without internal openings.

The permanent renal organ of the Mollusca consists typically
of a pair of tubes, although in the majority of the Gasteropoda
one of the two tubes is not developed. It is placed considerably
behind the provisional renal organ.

Each tube, in its most typical form, opens by a ciliated funnel
into the pericardial cavity, and has its external opening at the
side of the foot. The pericardial funnel leads into a glandular
section of the organ, the lining cells of which are filled with
concretions. This section is followed by a ciliated section, from
which a narrow duct leads to the exterior.

As to the development of this organ the same divergence of
opinion exists as in the case of the provisional renal organ.

Rabl's careful observations on Planorbis (Vol. II. No. 268) tend
to shew that it is developed from a mass of mesoblast cells, near
the end of the intestine. The mass becomes hollow, and,
attaching itself to the epiblast on the left side of the anus,
acquires an opening to the exterior. Its internal opening is not
established till after the formation of the heart. Fol gives an
equally precise account, but states that the first rudiment of the
organ arises as a solid mass of epiblast cells. Lankester finds
that this organ is developed as a paired invagination of the.
epiblast in Pisidium, and Bobretzky also derives it from the
epiblast in marine Prosobranchiata. In Cephalopoda on the
other hand Bobretzky's observations (I conclude this from his
figures) indicate that the excretory sacks of the renal organs are
derived from the mesoblast.

Polyzoa. Simple excretory organs, consisting of a pair of
ciliated canals, opening between the mouth and the anus, have

EXCRETORY ORGAN>.

683

been found by Hatschek and Joliet in the Entoproctous Polyzoa,
and are developed, according to Hatschek, by whom they were
first found in the larva, from the mesoblast

Brachiopoda. One or rarely two (Rhynchonella) pairs of
canals, with both peritoneal and external openings, are found in
the Brachiopoda. They undoubtedly serve as genital ducts, but
from their structure are clearly of the same nature as the
excretory organs of the Chaetopoda described below. Their
development has not been worked out.

Chaetopoda. Two forms of excretory organ have been met
with in the Chaetopoda. The one form is universally or nearly
universally present in the adult, and typically consists of a pair
of coiled tubes repeated in every segment. Each tube has an
internal opening, placed as a rule in the segment in front of that
in which the greater part of the organ and the external opening
are situated.

There are great variations in the structure of these organs,
which cannot be dealt with here. It may be noted however that
the internal opening may be absent, and that there may be
several internal openings for each organ (Polynoe). In the
Capitellidae moreover several pairs of excretory tubes have been
shewn by Eisig (No. 512) to be present in each of the posterior
segments.

The second form of excretory organ has as yet only been
found in the larva of Polygordius, and will be more conveniently
dealt with in connection with the development of the excretory
system of this form.

There is still considerable doubt as to the mode of formation
of the excretory tubes of the Chaetopoda. Kowalevsky (No. 277),
from his observations on the Oligochasta, holds that they develop
as outgrowths of the epithelial layer covering the posterior side
of the dissepiments, and secondarily become connected with the
epidermis.

Hatschek finds that in Criodrilus they arise from a continuous
linear thickening of the somatic mesoblast, immediately beneath
the epidermis, and dorsal to the ventral band of longitudinal
muscles. They break up into S-shaped cords, the anterior end
of each of which is situated in front of a dissepiment, and is
formed at first of a single large cell, while the posterior part is

684 CHvETOPODA.

continued into the segment behind. The cords are covered by
a peritoneal lining, which still envelopes them, when in the
succeeding stage they are carried into the body-cavity. They
subsequently become hollow, and their hinder ends acquire
openings to the exterior. The formation of their internal
openings has not been followed.

Kleinenberg is inclined to believe that the excretory tubes
take their origin from the epiblast, but states that he has not
satisfactorily worked out their development.

The observations of Risig (No. 512) on the Capitellidae
support Kowalevsky's view that the excretory tubes originate
from the lining of the peritoneal cavity.

Hatschek (No. 514) has given a very interesting account of
the development of the excretory system in Polygordius.

The excretory system begins to be formed, while the larva is
still in the trochospere stage (fig. 383, npli), and consists of a
provisional excretory organ, which is placed in front of the future
segmented part of the body, and occupies a position very
similar to that of the provisional excre-
tory organ found in some Molluscan
larvae (vide p. 68 1).

Hatschek, with some shew of rea-
son, holds that the provisional excre-
tory organs of Polygordius are homo-
logous with those of the Mollusca.

In its earliest stage the provisional
excretory organ of Polygordius con-
sists of a pair of simple ciliated tubes, FIG. 383. POLYOORDIUS

, . , • r 11-1 LARVA. (After Hatschek.)

each with an anterior funnel-like open- m_ moulh. ^ supraKBSO.

ing situated in the midst of the meSO- phageal ganglion ; nph. nephri-
11 11 . , dion ; ine.p. mesoblastic band;

blast cells, and a posterior external an_ anus5 oL stomach.
opening. The latter is placed imme-
diately in front of what afterwards becomes the segmented region
of the embryo. While the larva is still unsegmented, a second
internal opening is formed for each tube (fig. 383, np/i) and the
two openings so formed may eventually become divided into
five (fig. 384 A), all communicating by a single pore with the
exterior.

When the posterior region of the embryo becomes segmented,

EXCRETORY ORGANS.

685

paired excretory organs are formed in each of the posterior
segments, but the account of their development, as given by
Hatschek, is so remarkable that I do not think it can be
definitely accepted without further confirmation.

From the point of junction of the two main branches of the
larval kidney there grows backwards (fig. 384 B), to the hind
end of the first segment, a very delicate tube, only indicated by
its ciliated lumen, its walls not being differentiated. Near the
front end of this tube a funnel, leading into the larval body
cavity of the head, is formed, and subsequently the posterior end
of the tube acquires an external opening, and the tube distinct
walls. The communication with the provisional excretory organ
is then lost, and thus the excretory tube of the first segment is
established.

The excretory tubes in the second and succeeding segments
are formed in the same way as in the first, i.e. by the continu-
ation of the lumen of the hind end of the excretory tube from
the preceding segment, and the subsequent separation of this
part as a separate tube.

The tube may be continued with a sinuous course through

A
A

A
+

A.

Y

Y
Y
Y
Y

J)

FIG. 384. DIAGRAM ILLUSTRATING THE DEVELOPMENT OF THE EXCRETORY
SYSTEM OF POLYGORDIUS. (After Hatschek.)

several segments without a distinct wall. The external and
internal openings of the permanent excretory tubes are thus
secondarily acquired. The internal openings communicate with
the permanent body-cavity. The development of the perma-

686 GEPHYREA.

nent excretory tubes is diagrammatically represented in fig.
384 C and D.

The provisional excretory organ atrophies during larval life.

If Hatschek's account of the development of the excretory system of
Polygordius is correct, it is clear that important secondary modifications
must have taken place in it, because his description implies that there sprouts
from the anterior excretory organ, while it has its own external opening, a
posterior duct, which does not communicate either with the exterior or with
the body-cavity! Such a duct could have no function. It is intelligible
either (i) that the anterior excretory organ should lead into a longitudinal
duct, opening posteriorly ; that then a series of secondary openings into the
body-cavity should attach themselves to this, that for each internal opening
an external should subsequently arise, and the whole break up into separate
tubes ; or (2) that behind an anterior provisional excretory organ a series of
secondary independent segmental tubes should be formed. But from Hat-
schek's account neither of these modes of evolution can be deduced.

Gephyrea. The Gephyrea may have three forms of excre-
tory organs, two of which are found in the adult, and one,
similar in position and sometimes also in structure, to the
provisional excretory organ of Polygordius, has so far only been
found in the larvae of Echiurus and Bonellia.

In all the Gephyrea the so-called 'brown tubes' are
apparently homologous with the segmented excretory tubes of
Chaetopods. Their main function appears to be the transport-
ation of the generative products to the exterior. There is but a
single highly modified tube in Bonellia, forming the oviduct and
uterus ; a pair of tubes in the Gephyrea inermia, and two or
three pairs in most Gephyrea armata, except Bonellia. Their
development has not been studied.

In the Gephyrea armata there is always present a pair of
posteriorly placed excretory organs, opening in the adult into
the anal extremity of the alimentary tract, and provided with
numerous ciliated peritoneal funnels. These organs were stated
by Spengel to arise in Bonellia as outgrowths of the gut ; but in
Echinrus Hatschek (No. 515) finds that they are developed from
the somatic mesoblast of the terminal part of the trunk. They
soon become hollow, and after attaching themselves to the
epiblast on each side of the anus, acquire external openings.
They are not at first provided with peritoneal funnels, but these
parts of the organs become developed from a ring of cells at

EXCRETORY ORGANS.

687

their inner extremities ; and there is at first but a single funnel
for each vesicle. The mode of increase of the funnels has not
been observed, nor has it been made out how the organs them-
selves become attached to the hind-gut.

The provisional excretory organ of Echiurus is developed at
an early larval stage, and is functional during the whole of
larval life. It at first forms a ciliated tube on each side, placed
in front of that part of the larva which becomes the trunk of the
adult. It opens to the exterior by a fine pore on the ventral
side, immediately in front of one of the mesoblastic bands, and
appears to be formed of perforated cells. It terminates inter-
nally in a slight swelling, which represents the normal internal
ciliated funnel. The primitively simple excretory organ becomes
eventually highly complex by the formation of numerous
branches, each ending in a slightly swollen extremity. These
branches, in the later larval stages, actually form a network, and
the inner end of each main branch divides into a bunch of fine
tubes. The whole organ resembles in many respects the excre-
tory organ of the Platyelminthes.

In the larva of Bonellia Spengel has described a pair of
provisional excretory tubes, opening near the anterior end of
the body, which are probably homologous with the provisional
excretory organs of Echiurus (vide Vol. II., fig. 162 C, se).

Discophora. As in many of the types already spoken of,
permanent and provisional excretory organs may be present in
the Discophora. The former are usually segmentally arranged,
and resemble in many respects the excretory tubes of the
Chaetopoda. They may either be provided with a peritoneal
funnel (Nephelis, Clepsine) or have no internal opening
(Hirudo).

Bourne1 has shewn that the cells surrounding the main duct
in the medicinal Leech are perforated by a very remarkable
network of ductules, and the structure of these organs in the
Leech is so peculiar that it is permissible to state with due reserve
their homology with the excretory organs of the Chaetopoda.

The excretory tubes of Clepsine are held by Whitman to be
developed in the mesoblast.

1 "On the Structure of the Nephridia of the Medicinal Leech." Quart. J. of
Micr. Science, Vol. XX. 1880.

688 ARTHROPODA.

There are found in the embryos of Nephelis and Hirudo
certain remarkable provisional excretory organs the origin and
history of which are not yet fully made out. In Nephelis they
appear as one (according to Robin), or (according to Biitschli)
as two successive pairs of convoluted tubes on the dorsal side of
the embryo, which are stated by the latter author to develop
from the scattered mesoblast cells underneath the skin. At
their fullest development they extend, according to Robin, from
close to the head to near the ventral sucker. Each of them is
U-shaped, with the open end of the U forwards, each limb of the
U being formed by two tubes united in front. No external
opening has been clearly made out. Fiirbringer is inclined from
his own researches to believe that they open laterally. They
contain a clear fluid.

In Hirudo, Leuckart has described three similar pairs of
organs, the structure of which he has fully elucidated. They
are situated in the posterior part of the body, and each of them
commences with an enlargement, from which a convoluted tube
is continued for some distance backwards; the tube then turns
forwards again, and after bending again upon itself opens to the
exterior. The anterior part is broken up into a kind of
labyrinthic network.

The provisional excretory organs of the Leeches cannot be
identified with the anterior provisional organs of Polygordius
and Echiurus.

Arthropoda. Amongst the Arthropoda Peripatus is the
only form with excretory organs of the type of the segmental
excretory organs of the Chsetopoda1.

These organs are placed at the bases of the feet, in the
lateral divisions of the body-cavity, shut off from the main
median division of the body-cavity by longitudinal septa of
transverse muscles.

Each fully developed organ consists of three parts :

(i) A dilated vesicle opening externally at the base of a
foot. (2) A coiled glandular tube connected with this, and
subdivided again into several minor divisions. (3) A short
terminal portion opening at one extremity into the coiled tube

1 Vide F. M. Balfour, " On some points in the Anatomy of Peripatus Capensis."
Quart. J, of Micr. Science, Vol. XIX. 1879.

EXCRETORY ORGANS. 689

and at the other, as I believe, into the body cavity. This
section becomes very conspicuous, in stained preparations, by
the intensity with which the nuclei of its walls absorb the
colouring matter.

In the majority of the Tracheata the excretory organs have
the form of the so-called Malpighian tubes, which always (vide
Vol. II.) originate as a pair of outgrowths of the epiblastic
proctodaeum. From their mode of development they admit of
comparison with the anal vesicles of the Gephyrea, though in
the present state of our knowledge this comparison must be
regarded as somewhat hypothetical.

The antennary and shell-glands of the Crustacea, and
possibly also the so-called dorsal organ of various Crustacean
larvae appear to be excretory, and the two former have been
regarded by Claus and Grobben as belonging to the same
system as the segmental excretory tubes of the Chaetopoda.

Nematoda. Paired excretory tubes, running for the whole
length of the body in the so-called lateral line, and opening in
front by a common ventral pore, are present in the Nematoda.
They do not appear to communicate with the body cavity, and
their development has not been studied.

Very little is known with reference either to the structure or
development of excretory organs in the Echinodermata and the
other Invertebrate types of which no mention has been so far
made in this Chapter.

Excretory organs and generative ducts of the Craniata.

Although it would be convenient to separate, if possible, the
history of the excretory organs from that of the generative
ducts, yet these parts are so closely related in the Vertebrata, in
some cases the same duct having at once a generative and a
urinary function, that it is not possible to do so.

The excretory organs of the Vertebrata consist of three
distinct glandular bodies and of their ducts. These are (i) a
small glandular body, usually with one or more ciliated funnels
opening into the body cavity, near the opening of which there
projects into the body cavity a vascular glomerulus. It is
situated very far forwards, and is usually known as the head-

44

690 ELASMOBRANCHII.

kidney, though it may perhaps be more suitably called, adopting
Lankester's nomenclature, the pronepliros. Its duct, which forms
the basis for the generative and urinary ducts, will be called the
segmented duct.

(2) The Wolffian body, which may be also called the
mesonepJiros. It consists of a series of, at first, segmentally
(with a few exceptions) arranged glandular canals (segmental
tubes) primitively opening at one extremity by funnel-shaped
apertures into the body cavity, and at the other into the
segmental duct. This duct becomes in many forms divided
longitudinally into two parts, one of which then remains
attached to the segmental tubes and forms the Wolffian or
mesonepJiric duct, while the other is known as the Milllerian
dnct.

(3) The kidney proper or metanephros. This organ is only
found in a completely differentiated form in the amniotic Verte-
brata. Its duct is an outgrowth from the Wolrfian duct.

The above parts do not coexist in full activity in any living
adult member of the Vertebrata, though all of them are found
together in certain embryos. They are so intimately connected
that they cannot be satisfactorily dealt with separately.

Elasmobranchii. The excretory system of the Elasmo-
branchii is by no means the most primitive known, but at the
same time it forms a convenient starting point for studying the
modifications of the system in other groups. The most re-
markable peculiarity it presents is the absence of a pronephros.
The development of the Elasmobranch excretory system has
been mainly studied by Semper and myself.

The first trace of the system makes its appearance as a knob
of mesoblast, springing from the intermediate cell-mass near the
level of the hind end of the heart (fig. 385 K,pd). This knob is
the rudiment of the abdominal opening of the segmental duct,
and from it there grows backwards to the level of the anus a
solid column of cells, which constitutes the rudiment of the
segmental duct itself (fig. 385 B, pd). The knob projects
towards the epiblast, and the column connected with it lies
between the mesoblast and epiblast. The knob and column do
not long remain solid, but the former acquires an opening into
the body cavity (fig. 421, sd) continuous with a lumen, which

EXCRETORY ORGANS.

691

makes its appearance in the column (fig. 386, sd). The knob
forms the only structure which can be regarded as a rudiment of
the pronephros.

spn

spn

FlG. 385. TWO SECTIONS OF A PRISTIURUS EMBRYO WITH THREE VISCERAL

CLEFTS.

The sections illustrate the development of the segmental duct (pd) or primitive
duct of the pronephros. In A (the anterior of the two sections) this appears as a
solid knob (pd) projecting towards the epiblast. In B is seen a section of the column
which has grown backwards from the knob in A.

spn. rudiment of a spinal nerve; me. medullary canal; ch. notochord; X. sub-
notochordal rod; mp. muscle-plate; mp' . specially developed portion of muscle-plate;
ao. dorsal aorta ; pd. segmental duct ; so. somatopleure ; sp. splanchnopleure ; //.
body cavity; ep. epiblast; al. alimentary canal.

While the lumen is gradually being formed, the segmental
tubes of the mesonephros become established. They appear to
arise as differentiations of the parts of the primitive lateral plates
of mesoblast, placed between the dorsal end of the body cavity
and the muscle-plate (fig. 386, st)1, which are usually known as
the intermediate cell-masses.

The lumen of the segmental tubes, though at first very small,
soon becomes of a considerable size. It appears to be established
in the position of the section of the body cavity in the inter-
mediate cell-mass, which at first unites the part of the body
cavity in the muscle-plates with the permanent body cavity.
The lumen of each tube opens at its lower end into the dorsal
part of the body cavity (fig. 386, st}, and each tube curls obliquely

1 In my original account of the development I held these tubes to be invaginations
of the peritoneal epithelium. Sedgwick (No. 549) was led to doubt the accuracy of
my original statement from his investigations on the chick ; and from a re-examina-
tion of my specimens he arrived at the results stated above, and which I am now
myself inclined to adopt.

44—2

692

ELASMOBRANCHII.

sp.c

backwards round the inner and dorsal side of the segmental
duct, near which it at first ends blindly.

One segmental tube makes its
appearance for each somite (fig. 265),
commencing with that immediately
behind the abdominal opening of the
segmental duct, the last tube being
situated a few segments behind the
anus. Soon after their formation
the blind ends of the segmental tubes
come in contact with, and open into
the segmental duct, and each of them
becomes divided into four parts.
These are (i) a section carrying the
peritoneal opening, known as the
peritoneal funnel, (2) a dilated vesicle
into which this opens, (3) a coiled
tubulus proceeding from (2), and
terminating in (4) a wider portion
opening into the segmental duct. At
the same time, or shortly before this,
each segmental duct unites with and
opens into one of the horns of the
cloaca, and also retires from its
primitive position between the epi-
blast and mesoblast, and assumes a
position close to the epithelium lining
the body cavity (fig. 380, sd}. The
general features of the excretory
organs at this period are diagrammatically represented in the
woodcut (fig. 387). In this fig. pd is the segmental duct and
o its abdominal opening; s.t points to the segmental tubes,
the finer details of whose structure are not represented in the
diagram. The mesonephros thus forms at this period an elon-
gated gland composed of a series of isolated coiled tubes, one
extremity of each of which opens into the body cavity, and the
other into the segmental duct, which forms the only duct of the
system, and communicates at its front end with the body cavity,
and behind with the cloaca.

FIG. 386. SECTION THROUGH
THE TRUNK OF A SCYLLIUM EM-
BRYO SLIGHTLY YOUNGER THAN

28 F.

sp.c. spinal canal; W. white
matter of spinal cord ; pr. poste-
rior nerve-roots ; ch. notochord ;
x. sub-notochordal rod ; ao. aorta ;
nip, muscle-plate ; nip', inner layer
of muscle-plate already converted
into muscles ; Vr, rudiment of
vertebral body ; st. segmental
tube; sd. segmental duct; sp.v.
spiral valve ; v. subintestinal vein ;
p.o. primitive generative cells.

EXCRETORY ORGANS. 693

The next important change concerns the segmental duct,
which becomes longitudinally split into two complete ducts in
the female, and one complete duct and parts of a second duct in
the male. The manner in which this takes place is diagram-
matically represented in fig. 387 by the clear line x, and in
transverse section in figs. 388 and 389. The resulting ducts are
(i) the Wolffian duct or mesonephric duct (wd\ dorsally, which
remains continuous with the excretory tubules of the meso-
nephros, and ventrally (2) the oviduct or Miillerian duct in the
female, and the rudiments of this duct in the male. In the

FIG. 387. DIAGRAM OF THE PRIMITIVE CONDITION OF THE KIDNEY IN AN

ELASMOBRANCH EMBRYO.

pd. segmental duct. It opens at o into the body cavity and at its other extremity
into the cloaca; x. line along which the division appears which separates the segmental
duct into the Wolffian duct above and the Miillerian duct below; s.t. segmental
tubes. They open at one end into the body cavity, and at the other into the seg-
mental duct.

female the formation of these ducts takes place (fig. 389) by a
nearly solid rod of cells being gradually split off from the
ventral side of all but the foremost part of the original segmental
duct. This nearly solid cord is the Miillerian duct (pd}. A
very small portion of the lumen of the original segmental duct
is perhaps continued into it, but in any case it very soon acquires
a wide lumen (fig. 389 A). The anterior part of the segmental
duct is not divided, but remains continuous with the Mullerian
duct, of which its anterior pore forms the permanent peritoneal
opening1 (fig. 387). The remainder of the segmental duct (after
the loss of its anterior section, and the part split off from its
ventral side) forms the Wolffian duct. The process of formation
of these ducts in the male differs from that in the female chiefly

1 Five or six segmental tubes belong to the region of the undivided anterior part
of the segmental duct, which forms the front end of the Mullerian duct ; but they ap-
pear to atrophy very early, without acquiring a definite attachment to the segmental
duct.

694

ELASMOBRANCHIL

in the fact of the anterior undivided part of the segmental duct,
which forms the front end of the Miillerian duct, being shorter,

trd/

FIG. 389. FOUR SECTIONS
THROUGH THE ANTERIOR
I'ART OF THE SEGMENTAL
DUCT OF A FEMALE EMBRYO
OF SCYLLIUM CANICULA.

The figure shews how the
segmental duct becomes split
into the Wolffian or meso-
nephric duct above, and Miil-
lerian duct or oviduct below.

wd. Wolffian or meso-
nephric duct; od. Miillerian
duct or oviduct ; sd. segmen-
tal duct.

FIG. 388. DIAGRAMMATIC REPRESEN-
TATION OF A TRANSVERSE SECTION OF A

SCYLLIUM EMBRYO ILLUSTRATING THE
FORMATION OF THE WOLFFIAN AND MlJL-
LERIAN DUCTS BY THE LONGITUDINAL
SPLITTING OF THE SEGMENTAL DUCT.

me. medullary canal; mp. muscle-plate;
ch. notochord; ao. aorta; cav. cardinal
vein; st. segmental tube. On the left side
the section passes through the opening of
a segmental tube into the body cavity. On
the right this opening is represented by
dotted lines, and the opening of the seg-
mental tube into the Wolffian duct has
been cut through; iv.d. Wolffian duct;
m.d. Miillerian duct. The section is taken
through the point where the segmental
duct and Wolffian duct have just become
separate; gr. the germinal ridge with the
thickened germinal epithelium ; /. liver ;
i. intestine with spiral valve.

and in the column of cells with which it is continuous being
from the first incomplete.

The segmental tubes of the mesonephros undergo further
important changes. The vesicle at the termination of each peri-
toneal funnel sends a bud forwards towards the preceding
tubulus, which joins the fourth section of it close to the opening

EXCRETORY ORGANS.

695

into the Wolffian duct (fig. 390, px). The remainder of the
vesicle becomes converted
into a Malpighian body (mg}.

By the first of these changes 10^-4 M @W>f
a tube is established con-
necting each pair of segments
of the mesonephros, and
though this tube is in part
aborted (or only represented
by a fibrous band) in the
anterior part of the excretory
organs in the adult, and most
probably in the hinder part,
yet it seems almost certain
that the secondary and ter-
tiary Malpighian bodies of
the majority of segments are
developed from its persisting
blind end. Each of these

FIG. 390. LONGITUDINAL VERTICAL
SECTION THROUGH PART OF THE MESO-
NEPHROS OF AN EMBRYO OF SCYLLIUM.

The figure contains two examples of the
budding of the vesicle of a segmental tube
(which forms a Malpighian body in its own
segment) to unite with the tubulus in the
preceding segment close to its opening into
the Wolffian (mesonephric) duct.

ge. epithelium of body-cavity; st. peri-
toneal funnel of segmental tube with its
peritoneal opening; mg. Malpighian body;
px. bud from Malphigian body uniting with
preceding segment.

secondary and tertiary Malpighian bodies is connected with a
convoluted tubulus (fig. 391, a.mg), which is also developed from
the tube connecting each pair of segmental tubes, and therefore
falls into the primary tubulus close to its junction with the

st.c

w.d

FIG. 391. THREE SEGMENTS OF THE ANTERIOR PART OF THE MESONEPHROS OF A
NEARLY RIPE EMBRYO OF SCYLLIUM CANICULA AS A TRANSPARENT OBJECT.
The figure shews a fibrous band passing from the primary to the secondary Mal-
pighian bodies in two segments, which is the remains of the outgrowth from the
primary Malpighian body.

sf.o. peritoneal funnel; p. ing. primary Malpighian body; a.mg. accessory Mal-
pighian body; w.d. mesonephric (Wolffian) duct.

696 ELASMOBRANCI1II.

segmental duct. Owing to the formation of the accessory tubuli
the segments of the mesonephros acquire a compound character.

The third section of each tubulus becomes by continuous
growth, especially in the hinder segments, very bulky and
convoluted.

The general character of a slightly developed segment of
the mesonephros at its full growth may be gathered from fig.
391. It commences with (i) a peritoneal opening, somewhat
oval in form (st.d) and leading directly into (2) a narrow tube,
the segmental tube, which takes a more or less oblique course
backwards, and, passing superficially to the Wolffian duct (w.d},
opens into (3) a Malpighian body (p.mg) at the anterior ex-
tremity of an isolated coil of glandular tubuli. This coil forms
the third section of each segment, and starts from the Mal-
pighian body. It consists of a considerable number of rather
definite convolutions, and after uniting with tubuli from one,
two, or more (according to the size of the segment) accessory
Malpighian bodies (a.mg) smaller than the one into which the
segmental tube falls, eventually opens by (4) a narrowish
collecting tube into the Wolffian duct at the posterior end of
the segment. Each segment is probably completely isolated
from the adjoining segments, and never has more than one
peritoneal funnel and one communication with the Wolffian duct.

Up to this time there has been no distinction between the
anterior and posterior tubuli of the mesonephros, which alike
open into the Wolffian duct. The collecting tubes of a con-
siderable number of the hindermost tubuli (ten or eleven in
Scyllium canicula), either in some species elongate, overlap,
while at the same time their openings travel backward so that
they eventually open by apertures (not usually so numerous as
the separate tubes), on nearly the same level, into the hinder-
most section of the Wolffian duct in the female, or into the
urinogenital cloaca, formed by the coalesced terminal parts of
the Wolffian ducts, in the male; or in other species become
modified, by a peculiar process of splitting from the Wolnian
duct, so as to pour their secretion into a single duct on each
side, which opens in a position corresponding with the numerous
ducts of the other species (fig. 392). In both cases the modified
posterior kidney-segments are probably equivalent to the per-

EXCRETORY ORGANS. 697

manent kidney or metanephros of the amniotic Vertebrates, and
for this reason the numerous collecting tubes or single collecting
tube, as the case may be, will be spoken of as ureters. The
anterior tubuli of the primitive excretory organ retain their early
relation to the Wolffian duct, and form the permanent Wolffian
body or mesonephros.

The originally separate terminal extremities of the Wolffian
ducts always coalesce, and form a urinal cloaca, opening by a
single aperture, situated at the extremity of the median papilla
behind the anus. Some of the peritoneal openings of the seg-
mental tubes in Scyllium, or in other cases all the openings,
become obliterated.

In the male the anterior segmental tubes undergo remark-
able modifications, and become connected with the testes.
Branches appear to grow from the first three or four or more of
them (though probably not from their peritoneal openings),
which pass to the base of the testis, and there uniting into a
longitudinal canal, form a network, and receive the secretion of
the testicular ampullae (fig. 393, nf). These ducts, the vasa
efferent ia, carry the semen to the Wolffian body, but before
opening into the tubuli of this body they unite into a canal
known as the longitudinal canal of the Wolffian body (l.c\ from
which pass off ducts equal in number to the vasa efferentia,
each of which normally ends in a Malpighian corpuscle. From
the Malpighian corpuscles so connected there spring the con-
voluted tubuli, forming the generative segments of the Wolffian
body, along which the semen is conveyed to the Wolffian duct
(v.d). The Wolffian duct itself becomes much contorted and
acts as vas deferens.

Figs. 392 and 393 are diagrammatic representations of the
chief constituents of the adult urinogenital organs in the two
sexes. In the adult female (fig. 392), there are present the
following parts :

(1) The oviduct or Mullerian duct (m.d) split off from the
segmental duct of the kidneys. Each oviduct opens at its
anterior extremity into the body cavity, and behind the two
oviducts have independent communications with the general
cloaca.

(2) The mesonephric ducts (w.d), the other product of the

698

ELASMOBRANCHII.

segmental ducts of the kidneys. They end in front by be-
coming continuous with the tubulus of the anterior persisting
segment of the mesonephros on each side, and unite behind to

FIG. 392. DIAGRAM OF THE ARRANGEMENT OF THE URINOGENITAL ORGANS

IN AN ADULT FEMALE ELASMOBRANCH.

m.d. Miillerian duct; w.d. Wolffian duct; s.t. segmental tubes; five of them are
represented with openings into the body cavity, the posterior segmental tubes form
the mesonephros ; ov. ovary.

open by a common papilla into the cloaca. The mesonephric
duct receives the secretion of the anterior tubuli of the primitive
mesonephros.

(3) The ureter which carries off the secretion of the kidney
proper or metanephros. It is represented in my diagram in its
most rare and differentiated condition as a single duct connected
with the posterior segmental tubes.

(4) The segmental tubes (.$-./) some of which retain their

-S.t:

FIG. 393. DIAGRAM OF THE ARRANGEMENT OF THE URINOGENITAL ORGANS

IN AN ADULT MALE ELASMOBRANCH.

m.d. rudiment of Miillerian duct; w.d. Wolffian duct, marked vd in front and
serving as vas deferens; s.t. segmental tubes; two of them are represented with open-
ings into the body cavity; d. ureter; /. testis; nt. canal at the base of the testis;
VE, vasa efferentia; Ic. longitudinal canal of the Wolffian body.

EXCRETORY ORGANS. 699

original openings into the body cavity, and others are without
them. They are divided into two groups, an anterior forming
the mesonephros or Wolffian body, which pours its secretion
into the Wolffian duct ; and a posterior group forming a gland
which is probably equivalent to the kidney proper of amniotic
Craniata, and is connected with the ureter.

In the male the following parts are present (fig. 393):

(1) The Mlillerian duct (m.d], consisting of a small rudi-
ment attached to the liver, representing the foremost end of the
oviduct of the female.

(2) The mesonephric duct (w.d] which precisely corresponds
to the mesonephric duct of the female, but, in addition to
serving as the duct of the Wolffian body, also acts as a vas
deferens (vd}. In the adult male its foremost part has a very
tortuous course.

(3) The ureter (d\ which has the same fundamental con-
stitution as in the female.

(4) The segmental tubes (s.t). The posterior tubes have
the same arrangement in both sexes, but in the male modifica-
tions take place in connection with the anterior tubes to fit them
to act as transporters of the semen.

Connected with the anterior tubes there are present (i) the
vasa efferentia (VE], united on the one hand with (2) the
central canal in the base of the testis («/), and on the other with
the longitudinal canal of the Wolffian body (/<?). From the
latter are seen passing off the successive tubuli of the anterior
segments of the Wolffian body, in connection with which Mal-
pighian bodies are typically present, though not represented in
my diagram.

Apart from the absence of the pronephros the points which
deserve notice in the Elasmobranch excretory system are (i)
The splitting of the segmental duct into Wolffian (mesonephric)
and Mullerian ducts. (2) The connection of the former with
the mesonephros, and of the latter with the abdominal opening
of the segmental duct which represents the pronephros of other
types. (3) The fact that the Mullerian duct serves as oviduct,
and the Wolffian duct as vas deferens. (4) The differentiation
of a posterior section of the mesonephros into a special gland
foreshadowing the metanephros of the Amniota.

/OO CYCLOSTOMATA.

Cyclostomata. The development of the excretory system
amongst the Cyclostomata has only been studied in Petromyzon
(Miiller, Furbringer, and Scott).

The first part of the system developed is the segmental duct.
It appears in the embryo of about 14 days (Scott) as a solid
cord of cells, differentiated from the somatic mesoblast near the
dorsal end of the body cavity. This cord is at first placed
immediately below the epiblast, and grows backwards by a
continuous process of differentiation of fresh mesoblast cells. It
soon acquires a lumen, and joins the cloacal section of the
alimentary tract before the close of foetal life. Before this
communication is established, the front end of the duct sends a
process towards the body cavity, the blind end of which acquires
a ciliated opening into the latter. A series of about four or five
successively formed outgrowths from the duct, one behind the
other, give rise to as many ciliated funnels opening into the body
cavity, and each communicating by a more or less elongated
tube with the segmental duct. These funnels, which have a
metameric arrangement, constitute the pronephros, the whole
of which is situated in the pericardial region of the body
cavity.

On the inner side of the peritoneal openings of each pro-
nephros there is formed a vascular glomerulus, projecting into
the body cavity, and covered by peritoneal epithelium. For a
considerable period the pronephros constitutes the sole func-
tional part of the excretory system.

A mesonephros is formed (Furbringer) relatively late in
larval life, as a segmentally arranged series of solid cords,
derived from the peritoneal epithelium. These cords constitute
the rudiments of the segmental tubes. They are present for a
considerable portion of the body cavity, extending backwards
from a point shortly behind the pronephros. They soon separate
from the peritoneal epithelium, become hollowed out into canals,
and join the segmental duct. At their blind extremity (that
originally connected with the peritoneal epithelium) a Malpighian
body is formed.

The pronephros is only a provisional excretory organ, the
atrophy of which commences during larval life, and is nearly
completed when the Ammoccete has reached 180 mm. in length.

EXCRETORY ORGANS. 70 1

Further changes take place in connection with the excretory
system on the conversion of the Ammoccete into the adult.

The segmental ducts in the adult fall into a common urino-
genital cloaca, which opens on a papilla behind the anus. This
cloaca also communicates by two apertures (abdominal pores)
with the body cavity. The generative products are carried into
the cloaca by these pores ; so that their transportation outwards
is not performed by any part of the primitive urinary system.
The urinogenital cloaca is formed by the separation of the portion
of the primitive cloaca containing the openings of the segmental
ducts from that connected with the alimentary tract.

The mesonephros of the Ammoccete undergoes at the meta-
morphosis complete atrophy, and is physiologically replaced by
a posterior series of segmental tubes, opening into the hinder-
most portion of the segmental duct (Schneider).

In Myxine the excretory system consists (i) of a highly developed pro-
nephros with a bunch of ciliated peritoneal funnels opening into the peri-
cardial section of the body cavity. The coiled and branched tubes of which
the pronephros is composed open on the ventral side of the anterior portion
of the segmental duct, which in old individuals is cut off from the posterior
section of the duct. On the dorsal side of the portion of the segmental duct
belonging to the pronephros there are present a small number of diverticula,
terminating in glomeruli : they are probably to be regarded as anterior
segmental tubes. (2) Of a mesonephros, which commences a considerable
distance behind the pronephros, and is formed of straight extremely simple
segmental tubes opening into the segmental duct (fig. 385).

The excretory system of Myxine clearly retains the characters of the
system as it exists in the larva of Petromyzon.

Teleostei. In most Teleostei the pronephros and mesone-
phros coexist through life, and their products are carried off by
a duct, the nature of which is somewhat doubtful, but which is
probably homologous with the mesonephric duct of other types.

The system commences in the embryo (Rosenberg, Oellacher,
Gotte, Furbringer) with the formation of a groove-like fold of the
somatic layer of peritoneal epithelium, which becomes gradually
constricted into a canal; the process of constriction commencing
in the middle and extending in both directions. The canal does
not however close anteriorly, but remains open to the body
cavity, thus giving rise to a funnel equivalent to the pronephric
funnels of Petromyzon and Myxine. On the inner side of this

702

TELEOSTEI.

funnel there is formed a glomerulus, projecting into the body

cavity ; and at the same time that

this is being formed the anterior end

of the canal becomes elongated and

convoluted. The above structures

constitute a pronephros, while the

posterior part of the primitive canal

forms the segmental duct.

The portion of the body cavity
with the glomerulus and peritoneal
funnel of the pronephros (fig. 395,
po) soon becomes completely iso-
lated from the remainder, so as to
form a closed cavity (gl). The
development of the mesonephros
does not take place till long after
that of the pronephros. The seg-
mental tubes which form it are
stated by Fiirbringer to arise from
solid ingrowths of peritoneal epi-
thelium, developed successively from
before backwards, but Sedgwick
informs me that they arise as dif-
ferentiations of the mesoblastic cells
near the peritoneal epithelium. They
soon become hollow, and unite with
the segmental duct. Malpighian
bodies are developed on their median
portions. They grow very greatly
in length, and become much convoluted, but the details of this
process have not been followed out.

The foremost segmental tubes are situated close behind the
pronephros, while the hindermost are in many cases developed
in the post-anal continuations of the body cavity. The prone-
phros appears to form the swollen cephalic portion of the kidney
of the adult, and the mesonephros the remainder ; the so-called
caudal portion, where present, being derived (?) from the post-
anal segmental tubes.

In some cases the cephalic portion of the kidneys is absent

FIG. 394. PORTIONS OF THE
MESONEPHROS OF MYXINE. (From
Gegenbaur; after J. Miiller.)

a. segmental duct ; b. segmen-
tal tube; c. glomerulus ; d. afferent,
e. efferent artery.

B represents a portion of A
highly magnified.

EXCRETORY ORGANS. 703

in the adult, which probably implies the atrophy of the prone-
phros ; in other instances the cephalic portion of the kidneys is
the only part developed. Its relation to the embryonic pronc-
phros requires however further elucidation.

In the adult the ducts in the lower part of the kidneys lie as
a rule on their outer borders, and almost invariably open into a

pr

FIG. 395. SECTION THROUGH THE PRONEPHROS OF A TROUT AND ADJACENT
PARTS TEN DAYS BEFORE HATCHING.

pr.n. pronephros ; po. opening of pronephros into the isolated portion of the body
cavity containing the glomerulus ; gl. glomerulus ; ao. aorta ; ch. notochord ; x.
subnotochordal rod ; al. alimentary tract.

urinary bladder, which usually opens in its turn on the urino-
genital papilla immediately behind the genital pore, but in a few
instances there is a common urinogenital pore.

In most Osseous Fish there are true generative ducts con-
tinuous with the investment of the generative organs. It
appears to me most probable, from the analogy of Lepidostcus,
to be described in the next section, that these ducts are split off
from the primitive segmental duct, and correspond with the
Miillerian ducts of Elasmobranchii, etc. ; though on this point
we have at present no positive embryological evidence (vide
general considerations at the end of the Chapter). In the
female Salmon and the male and female Eel the generative
products are carried to the exterior by abdominal pores. It is
possible that this may represent a primitive condition, though it

704

GANOIDEI.

is more probably a case of degeneration, as is indicated by the
presence of ducts in the male Salmon and in forms nearly allied
to the Salmonidae.

The coexistence of abdominal pores and generative ducts in
Mormyrus appears to me to demonstrate that the generative
ducts in Teleostei cannot be derived from the coalescence of the
investment of the generative organs with the abdominal pores.

Ganoidei. The true excretory gland of the adult Ganoidei
resembles on the whole that of Teleostei, consisting of an
elongated band on each side — the mesonephros — an anterior
dilatation of which probably represents the pronephros.

There is in both sexes a Mullerian duct, provided, except
in Lepidosteus, with an abdominal funnel, which is however
situated relatively very far back in the abdominal cavity. The
Mullerian ducts appear to serve as generative canals in both sexes.
In Lepidosteus they are continuous with the investment of the
generative glands, and thus a relation between the generative ducts
and glands, very similar to that in Teleostei, is brought about.

Posteriorly the Mullerian ducts and the ducts of the meso-
nephros remain united. The common duct so formed on each
side is clearly the primitive segmental duct. It receives the
secretion of a certain number of the posterior mesonephric
tubules, and usually unites with its fellow to form a kind of
bladder, opening by a single
pore into the cloaca, behind
the anus. The duct which
receives the secretion of the
anterior mesonephric tubules
is the true mesonephric or
Wolffian duct.

The development of the
excretory system, which has
been partially worked out in
Acipenscr and Lepidosteus1,
is on the whole very similar
to that in the Teleostei. The
first portion of the system to

FIG. 396. SECTION THROUGH THE
TRUNK OF A LEPIDOSTEUS EMBRYO ON
THE SIXTH DAY AFTER IMPREGNATION.

me. medullary cord ; ms. mesoblast ; sg.
segmental duct ; ch. notochord ; .r. sub-
notochordal rod; hy. hypoblast.

1 Acipenser has been investigated by Fiirbringer, Salensky, Sedgwick, and also
by myself, and Lepidosteus by W. N. Parker and myself.

EXCRETORY ORGANS.

705

be formed is the segmental duct. In Lepidosteus this duct is
formed as a groove-like invagination of the somatic peritoneal
epithelium, precisely as in Teleostei, and shortly afterwards
forms a duct lying between the mesoblast and the epiblast
(fig. 396, sg}. In Acipenser (Salensky) however it is formed as

FIG. 397. TRANSVERSE SECTION THROUGH THE ANTERIOR PART OF AN ACIPENSER

EMBRYO. (After Salensky.)

Rf. medullary groove ; Alp. medullary plate ; Wg. segmental duct ; Ch. noto-
chord ; En. hypoblast ; Sgp. mesoblastic somite ; Sp. parietal part of mesoblastic
plate.

a solid ridge of the somatic mesoblast, as in Petromyzon and
Elasmobranchii (fig. 397, Wg).

In both forms the ducts unite behind with the cloaca, and a
pronephros of the Teleostean type appears to be developed.
This gland is provided with but one1 peritoneal opening, which
together with the glomerulus belonging to it becomes encapsuled
in a special section of the body cavity. The opening of the
pronephros of Acipenser into this cavity is shewn in fig. ^<^>,pr.n.
At this early stage of Acipenser (larva of 5 mm.) I could find
no glomerulus.

The mesonephros is formed some distance behind, and some
time after the pronephros, both in Acipenser and Lepidosteus,
so that in the larvae of both these genera the pronephros is for
a considerable period the only excretory organ. In Lepidosteus
especially the development of the mesonephros occurs very
late.

The development of the mesonephros has not been worked
out in Lepidosteus, but in Acipenser the anterior segmental
tubes become first established as (I believe) solid cords of cells,
attached at one extremity to the peritoneal epithelium on each

1 I have not fully proved this point, but have never found more than one
opening.

B. III.

45

GANOIDEI.

side of the insertion of the mesentery, and extending upwards
and outwards round the segmental duct1. The posterior seg-
mental tubes arise later than the anterior, and (as far as can be
determined from the sections in my possession) they are formed
independently of the peritoneal epithelium, on the dorsal side
of the segmental duct.

In later stages (larvae of 7 — 10 mm.) the anterior segmental
tubes gradually lose their attachment to the peritoneal epi-
thelium. The extremity near the peritoneal epithelium forms a
Malpighian body, and the other end unites with the segmental
duct. At a still later stage wide peritoneal funnels are es-

sjy.c

mjo

pr.n

FIG. 398. TRANSVERSE SECTION THROUGH THE REGION OF THE STOMACH OF A

LARVA OF ACIPENSER 5 MM. IN LENGTH.

st. epithelium of stomach ; yk. yolk ; ch. notochord, below which is a subnoto-
chordal rod; pr.n. pronephros ; ao. aorta; mf. muscle-plate formed of large cells,
the outer parts of which are differentiated into contractile fibres ; sp.c. spinal cord ;
b.c. body cavity.

tablished, for at any rate a considerable number of the tubes,
leading from the body cavity to the Malpighian bodies. These

1 Whether the segmental tubes are formed as ingrowths of the peritoneal
epithelium, or in situ, could not be determined.

EXCRETORY ORGANS. 707

funnels have been noticed by Furbringer, Salensky and myself,
but their mode of development has not, so far as I know, been
made out. The funnels appear to be no longer present in the
adult. The development of the Mullerian ducts has not been
worked out.

Dipnoi. The excretory system of the Dipnoi is only known in the
adult, but though in some respects intermediate in character between that of
the Ganoidei and Amphibia, it resembles that of the Ganoidei in the
important feature of the Mullerian ducts serving as genital ducts in both
sexes.

Amphibia. In Amphibia (Gotte, Furbringer) the develop-
ment of the excretory system commences, as in Teleostei, by
the formation of the segmental duct from a groove formed by a
fold of the somatic layer of the peritoneal epithelium, near the
dorsal border of the body cavity (fig. 399, u). The anterior end
of the groove is placed immediately behind the branchial
region. Its posterior part soon becomes converted into a canal
by a constriction which commences a short way from the front
end of the groove, and thence extends backwards. This canal
at first ends blindly close to the cloaca, into which however it
soon opens.

The anterior open part of the groove in front of the con-
striction (fig. 399, n] becomes differentiated into a longitudinal
duct, which remains in open communication with the body
cavity by two (many Urodela) three (many Anura) or • four
(Cceciliidae) canals. This constitutes the dorsal part of the
pronephros. The ventral part of the gland is formed from the
section of the duct immediately behind the longitudinal canal.
This part grows in length, and, assuming an S-shaped curvature,
becomes placed on the ventral side of the first formed part of
the pronephros. By continuous growth in a limited space the
convolutions of the canal of the pronephros become more nume-
rous, and the complexity of the gland is further increased by the
outgrowth of blindly ending diverticula.

At the root of the mesentery, opposite the peritoneal openings
of the pronephros, a longitudinal fold, lined by peritoneal epi-
thelium, and attached by a narrow band of tissue, makes its
appearance. It soon becomes highly vascular, and constitutes a
glomerulus homologous with that in Petromyzon and Teleostei.

45—2

AMPHIBIA.

a*'

The section of the body cavity which contains the openings
of the pronephros and the glomerulus,
becomes dilated, and then temporarily
shut off from the remainder. At a
later period it forms a special though
not completely isolated compartment.
For a long time the pronephros and
its duct form the only excretory organs
of larval Amphibia. Eventually how-
ever the formation of the mesonephros
commences, and is followed by the
atrophy of the pronephros. The me-
sonephros is composed, as in other
types, of a series of segmental tubes,
but these, except in Cceciliidae, no
longer correspond in number with the
myotomes, but are in all instances
more numerous. Moreover, in the
posterior part of the mesonephros in
the Urodeles, and through the whole
length of the gland in other types,
secondary and tertiary segmental tubes
are formed in addition to the primary
tubes.

FIG. 399. TRANSVERSE SEC-
TION THROUGH A VERY YOUNG
TADPOLE OF BOMBINATOR AT
THE LEVEL OF THE ANTERIOR
END OF THE YOLK-SACK. (After

Gotte.)

a. fold of epiblast continuous
with the dorsal fin; is", neural
cord; m. lateral muscle; as1.
outer layer of muscle-plate; s.
lateral plate of mesoblast ; b.
mesentery ; u. open end of the
segmental duct, which forms the
pronephros ; f. alimentary tract ;
f. ventral diverticulum which
becomes the liver; e. junction of
yolk cells and hypoblast cells ;
d. yolk cells.

The development of the mesonephros
commences in Salamandra (Fiirbringer) with
the formation of a series of solid cords, which
in the anterior myotomes spring from the
peritoneal epithelium on the inner side of the
segmental duct, but posteriorly arise inde-
pendently of this epithelium in the adjoining
mesoblast. Sedgwick informs me that in the

Frog the segmental tubes are throughout developed in the mesoblast, inde-
pendently of the peritoneal epithelium. These cords next become detached
from the peritoneal epithelium (in so far as they are primitively united to it),
and after first assuming a vesicular form, grow out into coiled tubes, with a
median limb the blind end of which assists in forming a Malpighian body,
and a lateral limb which comes in contact with and opens into the segmental
duct, and an intermediate portion connecting the two. At the junction of
the median with the intermediate portion, and therefore at the neck of the
Malpighian body, a canal grows out in a ventral direction, which meets the

EXCRETORY ORGANS. 709

peritoneal epithelium, and then develops a funnel-shaped opening into the
body cavity, which subsequently becomes ciliated. In this way the peritoneal
funnels which are present in the adult are established.

The median and lateral sections of the segmental tubes become highly
convoluted, and the separate tubes soon come into such close proximity that
their primitive distinctness is lost.

The first fully developed segmental tube is formed in Salamandra macu-
lata in about the sixth myotome behind the pronephros. But in the region
between the two structures rudimentary segmental tubes are developed.

The number of primary segmental tubes in the separate myotomes of
Salamandra is as follows :

In the 6th myotome (i.e. the first with a true

segmental tube) 1—2 segmental tubes

„ „ yth — roth myotome 2—3 „ „

» » IIth » ... 3—4 „ „

» » I2th » 3— 4 or 4— 5 „ „

» » I3th y> 4—5 » »

„ „ 1 3th— i6th „ 5—6 „ „

It thus appears that the segmental tubes are not only more numerous than
the myotomes, but that the number in each myotome increases from before
backwards. In the case of Salamandra there are formed in the region of
the posterior (10— 16) myotomes secondary, tertiary, etc. segmental tubes out
of independent solid cords, which arise in the mesoblast dorsally to the tubes
already established.

The secondary segmental tubes appear to develop out of these cords
exactly in the same way as the primary ones, except that they do not join the
segmental duct directly, but unite with the primary segmental tubes shortly
before the junction of the latter with the segmental duct. In this way com-
pound segmental tubes are established with a common collecting tube, but
with numerous Malpighian bodies and ciliated peritoneal openings. The
difference in the mode of origin of these compound tubes and of those in
Elasmobranchii is very striking.

The later stages in the development of the segmental tubes have not been
studied in the other Amphibian types.

In Cceciliidas the earliest stages are not known, but the tubes present in
the adult (Spengel) a truly segmental arrangement, and in the young each of
them is single, and provided with only a single peritoneal funnel. In the
adult however many of the segmental organs become compound, and may
have as many as twenty funnels, etc. Both simple and compound segmental
tubes occur in all parts of the mesonephros, and are arranged in no definite
order.

In the Anura (Spengel) all the segmental tubes are compound, and an
enormous number of peritoneal funnels are present on the ventral surface,
but it has not yet been definitely determined into what part of the segmental
tubes they open.

710 AMPHIBIA.

Before dealing with the further changes of the Wolffian body
it is necessary to return to the segmental duct, which, at the
time when the pronephros is undergoing atrophy, becomes split
into a dorsal Wolffian and ventral Mullerian duct. The process
in Salamandra (Fiirbringer) has much the same character as in
Elasmobranchii, the Mullerian duct being formed by the gradual
separation, from before backwards, of a solid row of cells from
the ventral side of the segmental duct, the remainder of the duct
constituting the Wolffian duct. During the formation of the
Mullerian duct its anterior part becomes hollow, and attaching
itself in front to the peritoneal epithelium acquires an opening
into the body cavity. The process of hollowing is continued
backwards pari passu with the splitting of the segmental duct.
In the female the process is continued till the Mullerian duct
opens, close to the Wolffian duct, into the cloaca. In the male
the duct usually ends blindly. It is important to notice that
the abdominal opening of the Mullerian duct in the Amphibia
(Salamandra) is a formation independent of the pronephros, and
placed slightly behind it ; and that the undivided anterior part
of the segmental duct (with the pronephros) is not, as in Elasmo-
branchii, united with the Mullerian duct, but remains connected
with the Wolffian duct.

The development of the Mullerian duct has not been satisfactorily
studied in other forms besides Salamandra. In Cceciliidae its abdominal
opening is on a level with the anterior end of the Wolffian body. In other
forms it is usually placed very far forwards, close to the root of the lungs
(except in Proteus and Batrachoseps, where it is placed somewhat further
back), and some distance in front of the Wolffian body.

The Mullerian duct is always well developed in the female, and serves as
oviduct. In the male it does not (except possibly in Alytes) assist in the
transportation of the genital products, and is always more or less rudimen-
tary, and in Anura may be completely absent.

After the formation of the Mullerian duct, the Wolffian duct
remains as the excretory channel for the Wolffian body, and, till
the atrophy of the pronephros, for this gland also. Its anterior
section, in front of the Wolffian body, undergoes a more or less
complete atrophy.

The further changes of the excretory system concern (i) the
junction in the male of the anterior part of the Wolffian body
with the testis ; (2) certain changes in the collecting tubes of the

EXCRETORY ORGANS.

711

posterior part of the mesonephros. The first of these processes
results in the division of the Wolffian body into a sexual and a
non-sexual part, and in Salamandra and other Urodeles the
division corresponds with the distribution of the simple and
compound segmental tubes.

Since the development of the canals connecting the testes with
the sexual part of the Wolffian body has not been in all points
satisfactorily elucidated, it will be convenient to commence with a
description of the adult arrangement of the parts (fig. 400 B). In
most instances a non-segmental system of canals — the vasa effc-
rentia (ve) — coming from the testis, fall into a canal known as the
longitudinal canal of the Wolffian body, from which there pass off
transverse canals, which fall into, and are equal in number to, the
primary Malpighian bodies of the sexual part of the gland. The
spermatozoa, brought to the Malpighian bodies, are thence trans-
ported along the segmental tubes to the Wolffian duct, and so to
the exterior. The system of canals connecting the testis with
the Malpighian bodies is known as the testicular network. The
number of segmental tubes connected with the testis varies
very greatly. In Siredon there are as many as from 30 — 32
(Spengel).

The longitudinal canal of the Wolffian body is in rare instances
(Spelerpes, etc.) absent, where the sexual part of the Wolffian body is
slightly developed. In the Urodela the testes are united with the anterior
part of the Wolffian body. In the Cceciliidas the junction takes place in an
homologous part of the Wolffian body, but, owing to the development of the
anterior segmental tubes, which are rudimentary in the Urodela, it is
situated some way behind the front end. Amongst the Anura the connection
of the testis with the tubules of the Wolffian body is subject to considerable
variations. In Bufo cinereus the normal Urodele type is preserved, and in
Bombinator the same arrangement is found in a rudimentary condition, in
that there are transverse trunks from the longitudinal canal of the Wolffian
body, which end blindly, while the semen is carried into the Wolffian
duct by canals in front of the Wolffian body. In Alytes and Discoglossus
the semen is carried away by a similar direct continuation of the lon-
gitudinal canal in front of the Wolffian body, but there are no rudi-
mentary transverse canals passing into the Wolffian body, as in Bombi-
nator. In Rana the transverse ducts which pass off from the longitudinal
canal of the Wolffian body, after dilating to form (?) rudimentary Malpighian
bodies, enter directly into the collecting tubes near their opening into the
Wolffian duct.

712 AMPHIBIA.

In most Urodeles the peritoneal openings connected with the primary
generative Malpighian bodies atrophy, but in Spelerpes they persist. In
the Cceciliidie they also remain in the adult state.

With reference to the development of these parts little is
known except that the testicular network grows out from the
primary Malpighian bodies, and becomes united with the testis.
Embryological evidence, as well as the fact of the persistence of
the peritoneal funnels of the generative region in the adults
of some forms, proves that the testicular network is not developed
from the peritoneal funnels.

Rudiments of the testicular network are found in the female Cceciliidae
and in the females of many Urodela (Salamandra, Triton). These rudi-
ments may in their fullest development consist of a longitudinal canal and
of transverse canals passing from this to the Malpighian bodies, together
with some branches passing into the mesovarium.

Amongst the Urodela the collecting tubes of the hinder non-sexual part
of the Wolffian body, which probably represents a rudimentary metanephros,
undergo in the male sex a change similar to that which they usually undergo
in Elasmobranchii. Their points of junction with the Wolffian duct are
carried back to the hindermost end of the duct (fig. 400 B), and the collecting
tubes themselves unite together into one or more short ducts (ureters) before
joining the Wolffian duct.

In Batrachoseps only the first collecting tube becomes split off in
this way ; and it forms a single elongated ureter which receives all the
collecting tubes of the posterior segmental tubes. In the female and in
the male of Proteus, Menobranchus, and Siren the collecting tubes retain
their primitive transverse course and open laterally into the Wolffian duct.
In rare cases (Ellipsoglossus, Spengel} the ureters open directly into the
cloaca.

The urinary bladder of the Amphibia is an outgrowth of the
ventral wall of the cloacal section of the alimentary tract, and is
homologous with the allantois of the amniotic Vertebrata.

The subjoined diagram (fig. 400) of the urogenital system of
Triton illustrates the more important points of the preceding
description.

In the female (A) the following parts are present :

(1) The Mullerian duct or oviduct (od) derived from the
splitting of the segmental duct.

(2) The Wolffian duct (sug) constituting the portion of the
segmental duct left after the formation of the Mullerian duct.

(3) The mesonephros (r), divided into an anterior sexual part

EXCRETORY ORGANS.

7'3

connected with a rudimentary testicular network, and a posterior
part. The collecting tubes from both
parts fall transversely into the Wolf-
fian duct.

(4) The ovary (ov).

(5) The rudimentary testicular
network.

In the male (B) the following
parts are present :

(1) The functionless though fairly
developed Miillerian duct (;«).

(2) The Wolffian duct (sug).

(3) The mesonephros (r) divided
into a true sexual part, through the
segmental tubes of which the semen
passes, and a non-sexual part. The
collecting tubes of the latter do not
enter the Wolffian duct directly, but
bend obliquely backwards and only
fall into it close to its cloacal aper-
ture, after uniting to form one or two
primary tubes (ureters).

(4) The testicular network (ve)
consisting of (i) transverse ducts
from the testes, falling into (2) the
longitudinal canal of the Wolffian
body, from which (3) transverse ca-
nals are again given off to the Mal-
pighian bodies.

Amniota. The amniotic Verte-
brata agree, so far as is known, very
closely amongst themselves in the
formation of the urinogenital system.

The most characteristic feature of the system is the full
development of a metanephros, which constitutes the functional
kidney on the atrophy of the mesonephros or Wolffian body,
which is a purely embryonic organ. The first part of the
system to develop is a duct, which is usually spoken of as the
Wolffian duct, but which is really the homologue of the seg-

FIG. 400. DIAGRAM OF THE
URINOGENITAL SYSTEM OF TRI-
TON. (From Gegenbaur ; after
Spengel.)

A. Female. B. Male.
r. mesonephros, on the surface
of which numerous peritoneal fun-
nels are visible ; sug. mesonephric
or Wolffian duct; od. oviduct
(Miillerian duct); in. Miillerian
duct of male ; ve. vasa efferentia of
testis ; t. testis ; ov. ovary ; up.
urinogenital pore.

714 AMNIOTA.

mental duct. It apparently develops in all the Amniota nearly
on the Elasmobranch type, as a solid rod, primarily derived
from the somatic mesoblast of the intermediate cell mass (fig.
401 W.d}\

The first trace of it is visible in an embryo Chick with eight
somites, as a ridge projecting from the intermediate cell mass to-
wards the epiblast in the region of the seventh somite. In the
course of further development it continues to constitute such a
ridge as far as the eleventh somite (Sedgwick), but from this
point it grows backwards in the space between the epiblast and
mesoblast In an embryo with fourteen somites a small lumen
has appeared in its middle part and in front it is connected with
rudimentary Wolffian tubules, which develop in continuity with
it (Sedgwick). In the succeeding stages the lumen of the duct
gradually extends backwards and forwards, and the duct itself
also passes inwards relatively to the epiblast (fig. 402). Its hind-
end elongates till it comes into connection with, and opens into,
the cloacal section of the hind-gut'2.

It might have been anticipated that, as in the lower types,
the anterior end of the segmental duct would either open into
the body cavity, or come into connection with a pronephros.
Neither of these occurrences takes place, though in some types
(the Fowl) a structure, which is probably the rudiment of a
pronephros, is developed ; it does not however appear till a later
stage, and is then unconnected with the segmental duct. The
next part of the system to appear is the mesonephros or
Wolffian body.

This is formed in all Amniota as a series of segmental tubes,
which in Lacertilia (Braun) correspond with the myotomes, but
in Birds and Mammalia are more numerous.

In Reptilia (Braun, No. 542), the mesonephric tubes develop as seg-
mentally-arranged masses on the inner side of the Wolffian duct, and
appear to be at first united with the peritoneal epithelium. Each mass soon
becomes an oval vesicle, probably opening for a very short period into the

1 Dansky and Kostenitsch (No. 543) describe the Wolffian duct in the Chick as
developing from a groove opening to the peritoneal cavity, which subsequently
becomes constricted into a duct. I have never met with specimens such as those
figured by these authors.

2 The foremost extremity of the segmental duct presents, according to Gasser,
curious irregularities and an anterior completely isolated portion is often present.

EXCRETORY ORGANS.

715

peritoneal cavity by a peritoneal funnel. The vesicles become very early
detached from the peritoneal epithelium, and lateral outgrowths from them
give rise to the main parts of the segmental tubes, which soon unite with the
segmental duct.

In Birds the development of the segmental tubes is more complicated1.

The tubules of the Wolffian body are derived from the intermediate cell
mass, shewn in fig. 401, between the upper end of the body cavity and the

g.o.

FIG. 401. TRANSVERSE SECTION THROUGH THE DORSAL REGION OF AN

EMBRYO CHICK OF 45 HOURS.

M.c. medullary canal ; P.v. mesoblastic somite ; W.d. Wolffian duct which is in
contact with the intermediate cell mass ; So. somatopleure ; S.p. splanchnopleure ;
p.p. pleuroperitoneal cavity ; ch. notochord ; op. boundary of area opaca; v. blood-
vessel.

muscle-plate. 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 the intermediate
cell mass becomes detached from the peritoneal epithelium at certain points,
remaining attached to it at other points, there being several such to each
segment. The parts of the intermediate cell mass attached to the peritoneal
epithelium become converted into S-shaped cords (fig. 402, st] which soon
unite with the segmental duct (wd}. Into the commencement of each
of these cords the lumen of the body cavity is for a short distance
prolonged, so that this part constitutes a rudimentary peritoneal funnel.

1 Correct figures of the early stages of these structures were first given by
Kolliker, but the correct interpretation of them and the first satisfactory account of
the development of the excretory organs of Birds was given by Sedgwick (No. 549).

716

AMNIOTA.

In the Duck the attachment of the intermediate cell mass to the peritoneal
epithelium is prolonged further back than in the Chick.

In the foremost segmental tubes, which never reach a very complete
development, the peritoneal funnels widen considerably, while at the same
time they acquire a distinct lumen. The section of the tube adjoining
the wide peritoneal funnel becomes partially invaginated by the formation of
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. 403, gl). There is thus formed a series of free peritoneal
glomeruli belonging to the anterior Wolfnan tubuli 1. These tubuli become
however early aborted.

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 segmental duct. Their blind extremities
constitute the rudiments of Malpighian bodies.

am

FIG. 402. TRANSVERSE SECTION THROUGH THE TRUNK OF A DUCK EMBRYO WITH

ABOUT TWENTY-FOUR MESOBLASTIC SOMITES.

am. amnion ; so. somatopleure ; sp. splanchnopleure ; ivd. Wolffian duct ; st. seg-
mental tube; ca.v. cardinal vein; m.s. muscle-plate; sp.g. spinal ganglion; sp.c.
spinal cord ; ch. notochord ; ao. aorta ; hy. hypoblast.

1 These external glomeruli were originally mistaken by me (No. 539) for the
glomeralus of the pronephros, from their resemblance to the glomerulus of the
Amphibian pronephros. Their true meaning was made out by Sedgwick (No.
550).

EXCRETORY ORGANS.

717

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 considerably later period breaks up into oval vesicles similar to those of
the Reptilia, which form the rudiments of the segmental tubes.

Secondary and tertiary segmental tubules are formed in the Chick, on the
dorsal side of the primary tubules,
as direct differentiations of the meso-
blast. They open independently into
the Wolffian duct.

In Mammalia the segmental tu-
bules (Egli) are formed as solid masses
in the same situation as in Birds and
Reptiles. It is not known whether
they are united with the peritoneal
epithelium. They soon become oval
vesicles, which develop into complete
tubules in the manner already in-
dicated.

After the establishment of
the Wolffian body there is formed
in both sexes in all the Amniota
a duct, which in the female
becomes the oviduct, but which
is functionless and disappears
more or less completely in the
male. This duct, in spite of certain peculiarities in its develop-
ment, is without doubt homologous with the Mullerian duct of

FIG. 403. SECTION THROUGH THE
EXTERNAL GLOMERULUS OF ONE OF
THE ANTERIOR SEGMENTAL TUBES OF
AN EMBRYO CHICK OF ABOUT IOO H.

gl. glomerulus ; ge. peritoneal epi-
thelium ; Wd. Wolffian duct ; ao.
aorta ; me. mesentery. The segmental
tube, and the connection between the
external and internal parts of the glo-
merulus are not shewn in this figure.

FIG. 404. SECTIONS SHEWING TWO OF THE PERITONEAL INVAGINATIONS WHICH
GIVE RISE TO THE ANTERIOR PART OF THE MULLERIAN DUCT (PRONEPHROS).
(After Balfour and Sedgwick. )

A is the nth section of the series.
B „ i 5th

C „ i8th ,, ,,

gri. second groove ; gr$. third groove ; ri. second ridge ; wit. Wolffian duct.

7i8

AMNIOTA.

the Ichthyopsida. In connection with its anterior extremity
certain structures have been found in the Fowl, which are
probably, on grounds to be hereafter stated, homologous with
the pronephros (Balfour and Sedgwick).

The pronephros, as I shall call it, consists of a slightly
convoluted longitudinal canal with three or more peritoneal
openings. In the earliest condition, it consists of three successive
open involutions 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, and is situated some considerable distance behind the
front end of the Wolfifian duct.

In a slightly later stage the ridges connecting the grooves
become partially constricted off from the peritoneal epithelium,

FIG. 405. SECTION OF THE WOLFFIAN BODY DEVELOPING PRONEPHROS AND
GENITAL GLAND OF THE FOURTH DAY. (After Waldeyer.) Magnified 160 times.
m. mesentery; Z. somatopleure ; a', portion of the germinal epithelium from
which the involution (2) to form the pronephros (anterior part of Miillerian duct) takes
place; a. thickened portion of the germinal epithelium in which the primitive
germinal cells C and o are lying ; E. modified mesoblast which will form the stroma
of the ovary ; WK. Wolffian body ; y. Wolffian duct.

EXCRETORY ORGANS. 719

and develop a lumen. The condition of the structure at this
stage is illustrated by fig. 404, representing three transverse
sections through two grooves, and through the ridge connecting
them.

The pronephros may in fact now be described as a slightly
convoluted duct, opening into the body cavity by three groove-
like apertures, and continuous behind with the rudiment of the
true Miillerian duct.

The stage just described is that of the fullest development
of the pronephros. In it, as in all the previous stages, there
appear to be only three main openings into the body cavity ; but
in some sections there are indications of the possible presence of
one or two additional rudimentary grooves.

In an embryo not very much older than the one last
described the pronephros atrophies as such, its two posterior
openings vanishing, and its anterior opening remaining as the
permanent opening of the Miillerian duct.

The pronephros is an extremely transitory structure, and its
development and atrophy are completed between the QOth and
i2Oth hours of incubation.

The position of the pronephros in relation to the Wolffian
body is shewn in fig. 405, which probably passes through a
region between two of the peritoneal openings. As long as the
pronephros persists, the Mullerian duct consists merely of a very

FlG. 406. TWO SECTIONS SHEWING THE JUNCTION OF THE TERMINAL SOLID
PORTION OF THE MtJLLERIAN DUCT WITH THE WOLFFIAN DUCT. (After Balfour

and Sedgwick.)

In A the terminal portion of the duct is quite distinct ; in B it has united with the
walls of the Wolffian duct.

md. Mullerian duct ; Wd. Wolffian duct.

72O AMNIOTA.

small rudiment, continuous with the hindermost of the three
peritoneal openings, and its solid extremity appears to unite
with the walls of the Wolffian duct.

After the atrophy of the pronephros, the Miillerian duct
commences to grow rapidly, and for the first part of its course it
appears to be split off as a solid rod from the outer or ventral
wall of the Wolffian duct (fig. 406). Into this rod the lumen,
present in its front part, subsequently extends. Its mode of
development in front is thus precisely similar to that of the
Miillerian duct in Elasmobranchii and Amphibia.

This mode of development only occurs however in the
anterior part of the duct. In the posterior part of its course its
growing point lies in a bay formed by the outer walls 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 walls of the Wolrfian duct, it may grow back-
wards 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 mode of growth of the Miillerian duct in the posterior part of its
course will best be understood from the following description quoted from
the paper by Sedgwick and myself.

"A few sections before its termination the Miillerian duct appears as a
well-defined oval duct lying in contact with the wall of the Wolffian duct on
the one hand and the germinal epithelium on the other. Gradually, however,
as we pass backwards, the Miillerian duct dilates ; the external wall of the
Wolffian duct adjoining it becomes greatly thickened and pushed in in its
middle part, so as almost to touch the opposite wall of the duct, and so form
a bay in which the Miillerian duct lies. As soon as the Miillerian duct has
come to lie in this bay its walls lose their previous distinctness of outline,
and the cells composing them assume a curious vacuolated appearance. No
well-defined line of separation can any longer be traced between the walls of
the Wolffian duct and those of the Miillerian, but between the two is a
narrow clear space traversed by an irregular network of fibres, in some of
the meshes of which nuclei are present.

The Miillerian duct may be traced in this condition for a considerable
number of sections, the peculiar features above described becoming more
and more marked as its termination is approached. It continues to dilate
and attains a maximum size in the section or so before it disappears. A
lumen may be observed in it up to its very end, but is usually irregular in
outline and frequently traversed by strands of protoplasm. The Miillerian

EXCRETORY ORGANS. 721

duct finally terminates quite suddenly, and in the section immediately
behind its termination the Wolffian duct assumes its normal appearance,
and the part of its outer wall on the level of the Miillerian duct conies into
contact with the germinal epithelium."

Before describing the development of the Mullerian duct in other
Amniotic types it will be well to say a few words as to the identifications
above adopted. The identification of the duct, usually called the Wolffian
duct, with the segmental duct (exclusive of the pronephros) appears to be
morphologically justified for the following reasons : (i) that it gives rise to
part of the Mullerian duct as well as to the duct of the Wolffian body ;
behaving in this respect precisely as does the segmental duct of Elasmo-
branchii and Amphibia. (2) That it serves as the duct for the Wolffian
body, before the Mullerian duct originates from it. (3) That it develops in a
manner strikingly similar to that of the segmental duct of various lower
forms.

With reference to the pronephros it is obvious that the organ identified
as such is in many respects similar to the pronephros of the Amphibia.
Both consist of a somewhat convoluted longitudinal canal, with a certain
number of peritoneal openings ;

The main difficulties in the homology are :

(1) the fact that the pronephros in the Bird is not united with the
segmental duct ;

(2) the fact that it is situated behind the front end of the Wolffian body.
It is to be remembered in connection with the first of these difficulties

that in the formation of the Mullerian duct in Elasmobranchii the anterior
undivided extremity of the primitive segmental duct, with the peritoneal
opening, which probably represents the pronephros, is attached to the
Mullerian duct, and not to the Wolffian duct ; though in Amphibia the
reverse is the case. To explain the discontinuity of the pronephros with the
segmental duct it is only necessary to suppose that the segmental duct and
pronephros, which in the Ichthyopsida develop as a single formation,
develop in the Bird as two independent structures — a far from extravagant
supposition, considering that the pronephros in the Bird is undoubtedly
quite functionless.

With reference to the posterior position of the pronephros it is only
necessary to remark that a change in position might easily take place after
the acquirement of an independent development, and that the shifting is
probably correlated with a shifting of the abdominal opening of the
Mullerian duct.

The pronephros has only been observed in Birds, and is very
possibly not developed in other Amniota. The Mullerian duct
is also usually stated to develop as a groove of the peritoneal
epithelium, shewn in the Lizard in fig. 354, md., which is con-
tinued backward as a primitively solid rod in the space between
B. ill. 46

722

AM N IOTA.

the Wolffian duct and peritoneal epithelium, without becoming
attached to the Wolffian duct.

On the formation of the Miillerian duct, the duct of the
mesonephros becomes the true mesonephric or Wolffian duct.

After these changes have taken place a new organ of great
importance makes its appearance. This organ is the permanent
kidney, or metanephros.

Metanephros. The mode of development of the metane-
phros has as yet only been satisfactorily elucidated in the Chick
(Sedgwick, No. 549). The ureter and the collecting tubes of
the kidney are developed from a dorsal outgrowth of the hinder
part of the Wolffian duct. The 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 meta-
nephric blastema. Sedgwick, of the accuracy of whose
account I have satisfied myself, has shewn that in the Chick it is
derived from the intermediate cell mass of the region of about
the thirty-first to the thirty-fourth somite. It is at first con-
tinuous 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, places itself on
the inner side of the ureter in the position just described.

In the subsequent development of the kidney collecting tubes
grow out from the ureter, and become continuous with masses of
cells of the metanephric blastema, which then differentiate them-
selves into the kidney tubules.

The process just described appears to me to prove that the
kidney of the A mniota is a specially differentiated posterior section
of the primitive mesonephros.

According to the view of Remak and Kolliker the outgrowths from the
ureter give rise to the whole of the tubuli uriniferi and the capsules of the
Malpighian bodies, the mesoblast around them forming blood-vessels, etc.
On the other hand some observers (Kupffer, Bornhaupt, Braun) maintain, in

EXCRETORY ORGANS. 723

accordance with the account given above, that the outgrowths of the ureter
form only the collecting tubes, and that the secreting tubuli, etc. are formed
in situ in the adjacent mesoblast.

Braun (No. 542) has arrived at the conclusion that in the Lacertilia the
tissue, out of which the tubuli of the metanephros are formed, is derived
from irregular solid ingrowths of the peritoneal epithelium, in a region
behind the Wolffian body, but in a position corresponding to that in which
the segmental tubes take their origin. These ingrowths, after separating
from the peritoneal epithelium, unite together to form a cord into which the
ureter sends the lateral outgrowths already described. These outgrowths
unite with secreting tubuli and Malpighian bodies, formed in situ. In
Lacertilia the blastema of the kidney extends into a postanal region.
Braun's account of the origin of the metanephric blastema does not appear
to me to be satisfactorily demonstrated.

The ureter does not long remain attached to the Wolffian
duct, but its opening is gradually carried back, till (in the Chick
between the 6th and 8th day) it opens independently into the
cloaca.

Of the further changes in the excretory system the most im-
portant is the atrophy of the greater part of the Wolffian body,
and the conversion of the Wolffian duct in the male sex into the
vas deferens, as in Amphibia and the Elasmobranchii.

The mode of connection of the testis with the Wolffian duct
is very remarkable, but may be derived from the primitive
arrangement characteristic of Elasmobranchii and Amphibia.

In the structures connecting the testis with the Wolffian body
two parts have to be distinguished, (i) that equivalent to the
testicular network of the lower types, (2) that derived from the
segmental tubes. The former is probably to be found in peculiar
outgrowths from the Malpighian bodies at the base of the testes.

These were first discovered by Braun in Reptilia, and consist
in this group of a series of outgrowths from the primary (?)
Malpighian bodies along the base of the testis : they unite to
form an interrupted cord in the substance of the testis, from
which the testicular tubuli (with the exception of the semi-
niferous cells) are subsequently differentiated. These outgrowths,
with the exception of the first two or three, become detached
from the Malpighian bodies. Outgrowths similar to those in
the male are found in the female, but subsequently atrophy.

Outgrowths homologous with those found by Braun have

46 — 2

724 AMNIOTA.

been detected by myself (No. 555) in Mammals. It is not
certain to what parts of the testicular tubuli they give rise, but
they probably form at any rate the vasa recta and rete vas-
culosum.

In Mammals they also occur in the female, and give rise to
cords of tissue in the ovary, which may persist through life.

The comparison of the tubuli, formed out of these structures,
with the Elasmobranch and Amphibian testicular network is
justified in that both originate as outgrowths from the primary
Malpighian bodies, and thence extend into the testis, and come
into connection with the true seminiferous stroma.

As in the lower types the semen is transported from the
testicular network to the Wolffian duct by parts of the glandular
tubes of the Wolffian body. In the case of Reptilia the anterior
two or three segmental tubes in the region of the testis probably
have this function. In the case of Mammalia the vasa efferentia,
i.e. the coni vasculosi, appear, according to the usually accepted
view, to be of this nature, though Banks and other investigators
believe that they are independently developed structures. Further
investigations on this point are required. In Birds a connection
between the Wolffian body and the testis appears to be estab-
lished as in the other types. The Wolffian duct itself becomes,
in the males of all Amniota, the vas deferens and the convoluted
canal of the epididymis — the latter structure (except the head)
being entirely derived from the Wolffian duct.

In the female the Wolffian duct atrophies more or less
completely.

In Snakes (Braun) the posterior part remains as a functionless canal,
commencing at the ovary, and opening into the cloaca. In the Gecko
(Braun) it remains as a small canal joining the ureter ; in Blindworms a
considerable part of the canal is left, and in Lacerta (Braun) only interrupted
portions.

In Mammalia the middle part of the duct, known as Gaertner's canal,
persists in the females of some monkeys, of the pig and of many ruminants.

The Wolffian body atrophies nearly completely in both
sexes ; though, as described above, part of it opposite the testis
persists as the head of the epididymis. The posterior part of
the gland from the level of the testis may be called the sexual
part of the gland, the anterior part forming the non-sexual part.

EXCRETORY ORGANS. 725

The latter, i.e. the anterior part, is first absorbed ; and in some
Reptilia the posterior part, extending from the region of the genital
glands to the permanent kidney, persists till into the second year.

Various remnants of the Wolffian body are found in the adults of both
sexes in different types. The most constant of them is perhaps the part in
the female equivalent to the head of the epididymis and to parts also of the
coiled tube of the epididymis, which may be called, with Waldeyer, the
epoophoron1. This is found in Reptiles, Birds and Mammals ; though in a
very rudimentary form in the first-named group. Remnants of the anterior
non-sexual part of the Wolffian bodies have been called by Waldeyer
parepididymis in the male, and paroophoron in the female. Such remnants
are not (Braun) found in Reptilia, but are stated to be found in both male
and female Birds, as a small organ consisting of blindly ending tubes with
yellow pigment. In some male Mammals (including Man) a parepididymis
is found on the upper side of the testis. It is usually known as the organ of
Giraldes.

The Mlillerian duct forms, as has been stated, the oviduct in
the female. The two ducts originally open independently into
the cloaca, but in the Mammalia a subsequent modification of
this arrangement occurs, which is dealt with in a separate
section. In Birds the right oviduct atrophies, a vestige being
sometimes left. In the male the Miillerian ducts atrophy more
or less completely.

In most Reptiles and in Birds the atrophy of the Miillerian ducts is
complete in the male, but in Lacerta and Anguis a rudiment of the anterior
part has been detected by Leydig as a convoluted canal. In the Rabbit
(Kolliker)2 and probably other Mammals the whole of the ducts probably
disappears, but in some Mammals, e.g. Man, the lower fused ends of the
Miillerian ducts give rise to a pocket opening into the urethra, known as the
uterus masculinus ; and in other cases, e.g. the Beaver and the Ass, the
rudiments are more considerable, and may be continued into horns homolo-
gous with the horns of the uterus (Weber).

The hydatid of Morgani in the male is supposed (Waldeyer) to represent
the abdominal opening of the Fallopian tube in the female, and therefore to
be a remnant of the Miillerian duct.

Changes in the lower parts of the urinogenital ducts in the Amniota.

The genital cord. In the Monodelphia the lower part of
the Wolffian ducts becomes enveloped in both sexes in a special

1 This is also called parovarium (His), and Rosenmiiller's organ.

2 Weber (No. 553) states that a uterus masculinus is present in the Rabbit, but
his account is by no means satisfactory, and its presence is distinctly denied by
Kolliker.

726

AMNIOTA.

cord of tissue, known as -the genital cord (fig. 407, gc), within the
lower part of which the MUllerian ducts are also enclosed. In
the male the MUllerian ducts in this cord atrophy, except at
their distal end where they unite to form the uterus masculinus.
The Wolffian ducts, after becoming the vasa deferentia, remain
for some time enclosed in the common cord, but afterwards
separate from each other. The seminal vesicles are outgrowths
of the vasa deferentia.

In the female the Wolffian ducts within the genital cord
atrophy, though rudiments of them are for a long time visible or
even permanently persistent. The lower parts of the MUllerian
ducts unite to form the vagina and body of the uterus. The
junction commences in the middle and extends forwards and
backwards ; the stage with a median junction being retained
permanently in Marsupials.

The urinogenital sinus and external generative organs.
In all the Amniota, there open at first into the common cloaca
the alimentary canal dorsally, the allantois ventrally, and the
Wolffian and MUllerian ducts and ureters laterally. In Reptilia
and Aves the embryonic condition is retained. In both groups
the allantois serves as an embryonic urinary bladder, but while
it atrophies in Aves, its stalk dilates to form a permanent
urinary bladder in Reptilia. In Mammalia the dorsal part of
the cloaca with the alimentary tract becomes first of all partially
constricted off from the ventral, which then forms a urinogenital
sinus (fig. 407, ug). In the course of development the urino-
genital sinus becomes, in all Mammalia but the Ornithodelphia,
completely separated from the intestinal cloaca, and the two
parts obtain separate external openings. The ureters (fig. 407,
3) open higher up than the other ducts into the stalk of the
allantois which dilates to form the bladder (4). The stalk
connecting the bladder with the ventral wall of the body con-
stitutes the urachus, and loses its lumen before the close of
embryonic life. The part of the stalk of the allantois below the
openings of the ureters narrows to form the urethra, which opens
together with the Wolffian and MUllerian ducts into the urino-
genital cloaca.

In front of the urinogenital cloaca there is formed a genital
prominence (fig. 407, cp), with a groove continued from the

EXCRETORY ORGANS. 727

urinogenital opening ; and on each side a genital fold (&). In
the male the sides of the groove on the prominence coalesce
together, embracing between them the opening of the urino-
genital cloaca ; and the prominence itself gives rise to the penis,

FIG. 407. DIAGRAM OF THE URINOGENITAL ORGANS OF A MAMMAL AT AN
EARLY STAGE. (After Allen Thomson ; from Quain's Anatomy.)

The parts are seen chiefly in profile, but the Miillerian and Wolffian ducts are
seen from the front.

3. ureter; 4. urinary bladder ; 5. urachus; of. genital ridge (ovary or testis) ; W.
left Wolffian body ; x. part at apex from which coni vasculosi are afterwards
developed ; w. Wolffian duct ; m. Miillerian duct ; gc. genital cord consisting of
Wolffian and Mullerian ducts bound up in a common sheath ; i. rectum ; ug. urino-
genital sinus ; cp. elevation which becomes the clitoris or penis ; Is. ridge from
which the labia majora or scrotum are developed.

along which the common urinogenital passage is continued.
The two genital folds unite from behind forwards to form the
scrotum.

In the female the groove on the genital prominence gradually
disappears, and the prominence remains as the clitoris, which is
therefore the homologue of the penis : the two genital folds form
the labia majora. The urethra and vagina open independently
into the common urinogenital sinus.

728 GENERAL CONCLUSIONS.

General conclusions and Summary.

Pronephros. Sedgwick has pointed out that the pronephros
is always present in types with a larval development, and either
absent or imperfectly developed in those types which undergo
the greater part of their development within the egg. Thus it
is practically absent in the embryos of Elasmobranchii and the
Amniota, but present in the larvae of all other forms.

This coincidence, on the principles already laid down in a
previous chapter on larval forms, affords a strong presumption
that the pronephros is an ancestral organ ; and, coupled with
the fact that it is the first part of the excretory system to be
developed, and often the sole excretory organ for a considerable
period, points to the conclusion that the pronephros and its duct
— the segmental duct — are the most primitive parts of the
Vertebrate excretory system. This conclusion coincides with
that arrived at by Gegenbaur and Fiirbringer.

The duct of the pronephros is always developed prior to the
gland, and there are two types according to which its develop-
ment may take place. It may either be formed by the closing
in of a continuous groove of the somatic peritoneal epithelium
(Amphibia, Teleostei, Lepidosteus), or as a solid knob or rod of
cells derived from the somatic mesoblast, which grows backwards
between the epiblast and the mesoblast (Petromyzon, Elasmo-
branchii, and the Amniota).

It is quite certain that the second of these processes is not a
true record of the evolution of 'the duct, and though it is more
possible that the process observable in Amphibia and the
Teleostei may afford some indications of the manner in which
the duct was established, this cannot be regarded as by any
means certain.

The mode of development of the pronephros itself is ap-
parently partly dependent on that of its duct. In Petromyzon,
where the duct does not at first communicate with the body
cavity, the pronephros is formed as a series of outgrowths from
the duct, which meet the peritoneal epithelium and open into
the body cavity ; but in other instances it is derived from the
anterior open end of the groove which gives rise to the segmental
duct. The open end of this groove may either remain single

EXCRETORY ORGANS. 729

(Teleostci, Ganoidei) or be divided into two, three or more
apertures (Amphibia). The main part of the gland in either
case is formed by convolutions of the tube connected with the
peritoneal funnel or funnels. The peritoneal funnels of the
pronephros appear to be segmentally arranged.

The pronephros is distinguished from the mesonephros by
developmental as well as structural features. The most im-
portant of the former is the fact that the glandular tubules of
which it is formed are always outgrowths of the segmental duct ;
while in the mesonephros they are always or almost always1
formed independently of the duct.

The chief structural peculiarity of the pronephros is the
absence from it of Malpighian bodies with the same relations as
those in the meso- and metanephros; unless the structures found
in Myxine are to be regarded as such. Functionally the place
of such Malpighian bodies is taken by the vascular peritoneal
ridge spoken of in the previous pages as the glomerulus.

That this body is really related functionally to the pronephros appears to
be indicated (i) by its constant occurrence with the pronephros and its
position opposite the peritoneal openings of this body ; (2) by its atrophy at
the same time as the pronephros ; (3) by its enclosure together with the
pronephridian stoma in a special compartment of the body-cavity in
Teleostei and Ganoids, and its partial enclosure in such a compartment in
Amphibia.

The pronephros atrophies more or less completely in most
types, though it probably persists for life in the Teleostei and
Ganoids, and in some members of the former group it perhaps
forms the sole adult organ of excretion.

The cause of its atrophy may perhaps be related to the fact that it is
situated in the pericardial region of the body-cavity, the dorsal part of which
is aborted on the formation of a closed pericardium ; and its preservation in
Teleostei and Ganoids may on this view be due to the fact that in these types
its peritoneal funnel and its glomerulus are early isolated in a special cavity.

Mesonephros. The mesonephros is in all instances com-
posed of a series of tubules (segmental tubes) which are
developed independently of the segmental duct. Each tubule is

1 According t.o Sedgwick some of the anterior segmental tubes of Aves form an
exception to the general rule that there is no outgrowth from the segmental or
metanephric duct to meet the segmental tubes.

730 GENERAL CONCLUSIONS.

typically formed of (i) a peritoneal funnel opening into (2) a
Malpighian body, from which there proceeds (3) a coiled gland-
ular tube, finally opening by (4) a collecting tube into the
segmental duct, which constitutes the primitive duct for the
mesonephros as well as for the pronephros.

The development of the mesonephridian tubules is subject to
considerable variations.

(1) They may be formed as differentiations of the inter-
mediate cell mass, and be from the first provided with a lumen,
opening into the body-cavity, and directly derived from the
section of the body-cavity present in the intermediate cell
mass; the peritoneal funnels often persisting for life (Elasmo-
branchii).

(2) They may be formed as solid cords either attached to
or independent of the peritoneal epithelium, which after first
becoming independent of the peritoneal epithelium subsequently
send downwards a process, which unites with it and forms a
peritoneal funnel, which may or may not persist (Acipenser,
Amphibia).

(3) They may be formed as in the last case, but acquire no
secondary connection with the peritoneal epithelium (Teleostei,
Amniota). In connection with the original attachment to the
peritoneal epithelium, a true peritoneal funnel may however be
developed (Aves, Lacertilia).

Physiological considerations appear to shew that of these
three methods of development the first is the most primitive.
The development of the tubes as solid cords can hardly be
primary.

A question which has to be answered in reference to the segmental tubes
is that of the homology of the secondarily developed peritoneal openings of
Amphibia, with the primary openings of the Elasmobranchii. It is on the
one hand difficult to understand why, if the openings are homologous in the
two types, the original peritoneal attachment should be obliterated in
Amphibia, only to be shortly afterwards reacquired. On the other hand
it is still more difficult to understand what physiological gain there could be,
on the assumption of the non-homology of the openings, in the replacement
of the primary opening by a secondary opening exactly similar to it.
Considering the great variations in development which occur in undoubtedly
homologous parts I incline to the view that the openings in the two types
are homologous.

EXCRETORY ORGANS.

731

In the majority of the lower Vertebrata the mesonephric
tubes have at first a segmental arrangement, and this is no
doubt the primitive condition. The coexistence of two, three, or
more of them in a single segment in Amphibia, Aves and
Mammalia has recently been shewn, by an interesting discovery
of Eisig, to have a parallel amongst Chaetopods, in the co-
existence of several segmental organs in a single segment in
some of the Capitellidae.

In connection with the segmental features of the meso-
nephros it is perhaps worth recalling the fact that in Elasmo-
branchii as well as other types there are traces of segmental
tubes in some of the postanal segments. In the case of all the
segmental tubes a Malpighian body becomes established close
to the extremity of the tube adjoining the peritoneal opening, or
in an homologous position in tubes without such an opening.
The opposite extremity of the tube always becomes attached to
the segmental duct.

In many of the segments of the mesonephros, especially in
the hinder ones, secondary and tertiary tubes become developed
in certain types, which join the collecting canals of the primary
tubes, and are provided, like the primary tubes, with Malpighian
bodies at their blind extremities.

There can it appears to me be little or no doubt that the
secondary tubes in the different types are homodynamous if not
homologous. Under these circumstances it is surprising to find
in what different ways they take their origin. In Elasmo-
branchii a bud sprouts out from the Malpighian body of one
segment, and joins the collecting tube of the preceding segment,
and subsequently, becoming detached from the Malpighian body
from which it sprouted, forms a fresh secondary Malpighian
body at its blind extremity. Thus the secondary tubes of one
segment are formed as buds from the segment behind. In
Amphibia (Salamandra) and Aves the secondary tubes develop
independently in the mesoblast. These great differences in
development are important in reference to the homology of
the metanephros or permanent kidney, which is discussed
below.

Before leaving the mesonephros it may be worth while putting forward
some hypothetical suggestions as to its origin and relation to the pro-

732 GENERAL CONCLUSIONS.

nephros, leaving however the difficult questions as to the homology of the
segmental tubes with the segmental organs of Chastopods for subsequent
discussion.

It is a peculiarity in the development of the segmental tubes that they at
first end blindly, though they subsequently grow till they meet the segmental
duct with which they unite directly, without the latter sending out any
offshoot to meet them1. It is difficult to believe that peritoneal infundibula
ending blindly and unprovided with some external orifice can have had an
excretory function, and we are therefore rather driven to suppose that the
peritoneal infundibula which become the segmental tubes were either from
the first provided each with an orifice opening to the exterior, or were united
with the segmental duct. If they were from the first provided with external
openings we may suppose that they became secondarily attached to the duct
of the pronephros (segmental duct), and then lost their external openings, no
trace of these structures being left, even in the ontogeny of the system.
It would appear to me more probable that the pronephros, with its duct
opening into the cloaca, was the only excretory organ of the unsegmented
ancestors of the Chordata, and that, on the elongation of the trunk and its
subsequent segmentation, a series of metameric segmental tubes became
evolved opening into the segmental duct, each tube being in a sort of way
serially homologous with the primitive pronephros. With the segmentation
of the trunk the latter structure itself may have acquired the more or less
definite metameric arrangement of its parts.

Another possible view is that the segmental tubes may be modified
derivatives of posterior lateral branches of the pronephros, which may at
first have extended for the whole length of the body-cavity. If there is any
truth in this hypothesis it is necessary to suppose that, when the un-
segmented ancestor of the Chordata became segmented, the posterior
branches of the primitive excretory organ became segmentally arranged,
and that, in accordance with the change thus gradually introduced in them,
the time of their development became deferred, so as to accord to a certain
extent with the time of formation of the segments to which they belonged.
The change in their mode of development which would be thereby intro-
duced is certainly not greater than that which has taken place in the case of
segmental tubes, which, having originally developed on the Elasmobranch
type, have come to develop as they do in the posterior part of the mesone-
phros of Salamandra, Birds, etc.

Genital ducts. So far the origin and development of the
excretory organs have been considered without reference to the
modifications introduced by the excretory passages coming to
serve as generative ducts. Such an unmodified state of the

1 As mentioned in the note on p. 729 Sedgwick maintains that the anterior
segmental tubes of the Chick form an exception to this general statement.

EXCRETORY ORGANS. 733

excretory organs is perhaps found permanently in Cyclosto-
mata1 and transitorily in the embryos of most forms.

At first the generative products seem to have been discharged
freely into the body-cavity, and transported to the exterior by
the abdominal pores (vide p. 626).

The secondary relations of the excretory ducts to the
generative organs seem to have been introduced by an opening
connected with the pronephridian extremity of the segmental
duct having acquired the function of admitting the generative
products into it, and of carrying them outwards ; so that
primitively the segmental duct must have served as efferent duct
both for the generative products and the pronepJiric secretion (just
as the Wolffian duct still does for the testicular products and
secretion of the Wolffian body in Elasmobranchii and Am-
phibia).

The opening by which the generative products entered the
segmental duct can hardly have been specially developed for
this purpose, but must almost certainly have been one of the
peritoneal openings of the pronephros. As a consequence (by a
process of natural selection) of the segmental duct having both a
generative and a urinary function, a further differentiation took
place, by which that duct became split into two — a ventral
Mullerian duct and a dorsal Wolffian duct.

The Mullerian duct was probably continuous with one or
more of the abdominal openings of the pronephros which served
as generative pores. At first the segmental duct was probably
split longitudinally into two equal portions, and this mode
of splitting is exceptionally retained in some Elasmobranchii ;
but the generative function of the Mullerian duct gradually
impressed itself more and more upon the embryonic develop-
ment, so that, in the course of time, the Mullerian duct
developed less and less at the expense of the Wolffian duct.
This process appears partly to have taken place in Elasmo-
branchii, and still more in Amphibia, the Amphibia offering in
this respect a less primitive condition than the Elasmobranchii ;
while in Aves it has been carried even further, and it seems
possible that in some Amniota the Mullerian and segmental

1 It is by no means certain that the transportation outwards of the genital products
by the abdominal pores in the Cyclostomata may not be the result of degeneration.

734 GENERAL CONCLUSIONS.

ducts may actually develop independently, as they do exception-
ally in individual specimens of Salamandra (Fiirbringer). The
abdominal opening no doubt also became specialised. At first it
is quite possible that more than one pronephric abdominal
funnel may have served for the entrance of the generative
products ; this function being, no doubt, eventually restricted to
one of them.

Three different types of development of the abdominal
opening of the Mullerian duct have been observed.

In Amphibia (Salamandra) the permanent opening of the
Mullerian duct is formed independently, some way behind the
pronephros.

In Elasmobranchii the original opening of the segmental
duct forms the permanent opening of the Mullerian duct, and no
true pronephros appears to be formed.

In Birds the anterior of the three openings of the rudimentary
pronephros remains as the permanent opening of the Mullerian
duct.

These three modes of development very probably represent
specialisations of the primitive state along three different lines.
In Amphibia the specialisation of the opening appears to have
gone so far that it no longer has any relation to the pronephros.
It was probably originally one of the posterior openings of this
gland.

In Elasmobranchii, on the other hand, the functional opening
is formed at a period when we should expect the pronephros to
develop. This state is very possibly the result of a differenti-
ation by which the pronephros gradually ceased to become
developed, but one of its peritoneal openings remained as the
abdominal aperture of the Mullerian duct. Aves, finally, appear
to have become differentiated along a third line ; since in their
ancestors the anterior (?) pore of the head-kidney appears to
have become specialised as the permanent opening of the
Mullerian duct.

The Mullerian duct is usually formed in a more or less com-
plete manner in both sexes. In Ganoids, where the separation
between it and the Wolffian duct is not completed to the cloaca,
and in the Dipnoi, it probably serves to carry off the generative
products of both sexes. In other cases however only the female

EXCRETORY ORGANS.

735

products pass out by it, and the partial or complete formation
of the Mullerian duct in the male in these cases needs to be
explained. This may be done either by supposing the Ganoid
arrangement to have been the primitive one in the ancestors of
the other forms, or, by supposing characters acquired primitively
by the female to have become inherited by both sexes.

It is a question whether the nature of the generative ducts of
Teleostei can be explained by comparison with those of Ganoids.
The fact that the Mullerian ducts of the Teleostean Ganoid
Lepidosteus attach themselves to the generative organs, and thus
acquire a resemblance to the generative ducts of Teleostei,
affords a powerful argument in favour of the view that the
generative ducts of both sexes in the Teleostei are modified
Mullerian ducts. Embryology can however alone definitely
settle this question.

In the Elasmobranchii, Amphibia, and Amniota the male
products are carried off by the Wolffian duct, and they are
transported to this duct, not by open peritoneal funnels of the
mesonephros, but by a network of ducts which sprout either
from a certain number of the Malpighian bodies opposite the
testis (Amphibia, Amniota), or from the stalks connecting the
Malpighian bodies with the open funnels (Elasmobranchii).
After traversing this network the semen passes (except in
certain Anura) through a variable number of the segmental
tubes directly to the Wolffian duct. The extent of the con-
nection of the testis with the Wolffian body is subject to great
variations, but it is usually more or less in the anterior region.
Rudiments of the testicular network have in many cases become
inherited by the female.

The origin of the connection between the testis and Wolffian body is still
very obscure. It would be easy to understand how the testicular products,
after falling into the body-cavity, might be taken up by the open extremities
of some of the peritoneal funnels, and how such open funnels might have
groove-like prolongations along the mesorchium, which might eventually be
converted into ducts. Ontogeny does not however altogether favour this
view of the origin of the testicular network. It seems to me nevertheless the
most probable view which has yet been put forward.

The mode of transportation of the semen by means of the mesonephric
tubules is so peculiar as to render it highly improbable that it was twice
acquired, it becomes therefore necessary to suppose that the Amphibia and

736 GENERAL CONCLUSIONS.

Amniota inherited this mode of transportation of the semen from the same
ancestors as the Elasmobranchii. It is remarkable therefore that in the
Ganoidei and Dipnoi this arrangement is not found.

Either (i) the arrangement (found in the Ganoidei and Dipnoi) of the
Miillerian duct serving for both sexes is the primitive arrangement, and the
Elasmobranch is secondary, or (2) the Ganoid arrangement is a secondary
condition, which has originated at a stage in the evolution of the Vertebrata
when some of the segmental tubes had begun to serve as the efferent ducts
of the testis, and has resulted in consequence of a degeneration of the latter
structures. Although the second alternative is the more easy to reconcile
with the affinities of the Ganoid and Elasmobranch types, as indicated by
the other features of their organization, I am still inclined to accept the
former ; and consider that the incomplete splitting of the segmental duct in
Ganoidei is a strong argument in favour of this view.

Metanephros. With the employment of the Wolffian duct
to transport the semen there seems to be correlated (i) a
tendency of the posterior segmental tubes to have a duct of
their own, in which the seminal and urinary fluids cannot become
mixed, and (2) a tendency on the part of the anterior segmental
tubes to lose their excretory function. The posterior segmental
tubes, when connected in this way with a more or less specialised
duct, have been regarded in the preceding pages as constituting
a metanephros.

This differentiation is hardly marked in the Anura, but is
well developed in the Urodela and in the Elasmobranchii ; and
in the latter group has become inherited by both sexes. In the
Amniota it culminates, according to the view independently
arrived at by Semper and myself, (i) in the formation of a
completely distinct metanephros in both sexes, formed however,
as shewn by Sedgwick, from the same blastema as the Wolffian
body, and (2) in the atrophy in the adult of the whole Wolffian
body, except the part uniting the testis and the Wolffian duct.

The homology between the posterior metanephridian section of the
Wolffian body, in Elasmobranchii and Urodela, and the kidney of the
Amniota, is only in my opinion a general one, i.e. in both cases a common
cause, viz. the Wolffian duct acting as vas deferens, has resulted in a more
or less similar differentiation of parts.

Fiirbringer has urged against Semper's and my view that no satis-
factory proof of it has yet been offered. This proof has however, since
Fiirbringer wrote his paper, been supplied by Sedgwick's observations.
The development of the kidney in the Amniota is no doubt a direct as
opposed to a phylogenetic development ; and the substitution of a direct for

EXCRETORY ORGANS. 737

a phylogenetic development has most probably been rendered possible by
the fact that the anterior part of the mesonephros continued all the while
to be unaffected and to remain as the main excretory organ during foetal
life.

The most serious difficulty urged by Fiirbringer against the homology is
the fact that the ureter of the metanephros develops on a type of its own,
which is quite distinct from the mode of development of the ureters of the
metanephros of the Ichthyopsidan forms. It is however quite possible, though
far from certain, that the ureter of Amniota may be a special formation
confined to that group, and this fact would in no wise militate against the
homology I have been attempting to establish.

Comparison of the Excretory organs of the Chordata and
Invertebrata.

The structural characters and development of the various forms of
excretory organs described in the preceding pages do not appear to me to
be sufficiently distinctive to render it possible to establish homologies
between these organs on a satisfactory basis, except in closely related
groups.

The excretory organs of the Platyelminthes are in many respects similar
to the provisional excretory organ of the trochosphere of Polygordius
and the Gephyrea on the one hand, and to the Vertebrate pronephros
on the other ; and the Platyelminth excretory organ with an anterior
opening might be regarded as having given origin to the trochosphere organ,
while that with a posterior opening may have done so for the Vertebrate
pronephros1.

Hatschek has compared the provisional trochosphere excretory organ of
Polygordius to the Vertebrate pronephros, and the posterior Chastopod
segmental tubes to the mesonephric tubes ; the latter homology having
been already suggested independently by both Semper and myself. With
reference to the comparison of the pronephros with the provisional excretory
organ of Polygordius there are two serious difficulties :

(1) The pronephric (segmental) duct opens directly into the cloaca,
while the duct of the provisional trochosphere excretory organ opens an-
teriorly, and directly to the exterior.

(2) The pronephros is situated within the segmented region of the
trunk, and has a more or less distinct metameric arrangement of its parts ;
while the provisional trochosphere organ is placed in front of the segmented
region of the trunk, and is in no way segmented.

The comparison of the mesonephric tubules with the segmental excre-
tory organs of the Chaetopoda, though not impossible, cannot be satisfac-
torily admitted till some light has been thrown upon the loss of the supposed
external openings of the tubes, and the origin of their secondary connection
with the segmental duct.

1 This suggestion has I believe been made by Fiirbringer.
B. III. 47

738 BIBLIOGRAPHY.

Confining our attention to the Invertebrata it appears to me fairly clear
that Hatschek is justified in holding the provisional trochosphere excretory
organs of Polygordius, Echiurus and the Mollusca to be homologous. The
atrophy of all these larval organs may perhaps be due to the presence of a
well-developed trunk region in the adult (absent in the larva), in which
excretory organs, probably serially homologous with those present in the
anterior part of the larva, became developed. The excretory organs in the
trunk were probably more conveniently situated than those in the head,
and the atrophy of the latter in the adult state was therefore brought about,
while the trunk organs became sufficiently enlarged to serve as the sole
excretory organs.

BIBLIOGRAPHY OF THE EXCRETORY ORGANS.
Invertebrata.

(512) H. Eisig. " Die Segmentalorgane d. Capitelliden." Mitth. a. d. zool.
Stat. z. Neapel, Vol. I. 1879.

(513) J. Fraipont. " Recherches s. 1'appareil excreteur des Trematodes et d.
Cesto'ides." Archives de Biologic, Vol. I. 1880.

(514) B. Hatschek. "Studien lib. Entwick. d. Anneliden." Arbeit, a. d.
zool. Instit. Wien, Vol. I. 1878.

(515) B. Hatschek. "Ueber Entwick. von Echiurus," etc. Arbeit, a. d.
zool. Instit. Wien, Vol. in. 1880.

EXCRETORY ORGANS OF VERTEBRATA.
General.

(516) F. M. Balfour. "On the origin and history of the urinogenital organs of
Vertebrates." yournal of Anat. and Phys., Vol. X. 1876.

(517) Max. Furbringer1. "Zur vergleichenden Anat. u. Entwick. d. Excre-
tionsorgane d. Vertebraten." Morphol. Jahrbuch, Vol. IV. 1878.

(518) H. Meek el. Zur Morphol. d. Hani- u. Geschlechtnverkz.d. Wirbelthiere,
etc. Halle, 1848.

(519) Joh. Miiller. Bildungsgeschichte d. Genitalien, etc. Diisseldorf, 1830.

(520) H. Rathke. " Beobachtungen u. Betrachtungen u. d. Entwicklung d.
Geschlechtswerkzeuge bei den Wirbelthieren." N. Schriften d. naturf. Gesell. in
Dantzig, Bd. I. 1825.

(521) C. Semper1. "Das Urogenitalsystem d. Plagiostomen u. seine Bedeu-
tung f. d. iibrigen Wirbelthiere." Arb. a. d. zool.-zoot. Instit. Wurzburg, Vol. II.
1875-

(522) W. Waldeyer1. Eierstock u. Ei. Leipzig, 1870.

1 The papers of Furbringer, Semper and Waldeyer contain full references to the
literature of the Vertebrate excretory organs.

BIBLIOGRAPHY. 739

ElasmobrancJdi.

(523) A. Schultz. "Zur Entwick. d. Selachiereies." Archiv f. mikr. Anat.,
Vol. XI. 1875.

Vide also Semper (No. 521) and Balfour (No. 292).

Cyclostomata.

(524) J. Miiller. " Untersuchungen ii. d. Eingeweide d. Fische." Abh. d. k.
Ak. Wiss. Berlin, 1845.

(525) W. Miiller. "Ueber d. Persistenz d. Urniere b. Myxine glutinosa."
Jenaische Zeitschrift, Vol. VII. 1873.

(526) W. Miiller. "Ueber d. Urogenitalsystem d. Amphioxus u. d. Cyclo-
stomen." Jenaische Zeitschri/t, Vol. IX. 1875.

(527) A. Schneider. Beitrdge z. vergleich. Anat. u. Entwick. d. Wirbelthiere.
Berlin, 1879.

(528) W. B. Scott. "Beitrage z. Entwick. d. Petromyzonten." Morphol.
Jahrbuch, Vol. vn. 1881.

Teleostei.

(529) J. Hyrtl. "Das uropoetische System d. Knochenfische." Denkschr. d.
k. k. Akad. Wiss. Wien, Vol. n. 1850.

(530) A. Rosenberg. Untersuchungen iib. die Entivicklung d. Teleostierniere.
Dorpat, 1867.

Vide also Oellacher (No. 72).

Amphibia.

(531) F. H. Bidder. Vergleichend-anatomische u. histologische Untersitchungen
ii. die mdnnlichen Geschleehts- und Harnwerkzeuge d. nackten Amphibien. Dorpat,
1846.

(532) C. L. Duvernoy. "Fragments s. les Organes genito-urinaires des
Reptiles," etc. Mem. Acad. Sciences. Paris. Vol. xi. 1851, pp. 17 — 95.

(533) M. Fiirbringer. Zur Entwicklung d. Amphibienniere. Heidelberg, 1877.

(534) F. Leydig. Anatomie d. Amphibien u. Reptilien. Berlin, 1853.

(535) F. Leydig. Lehrbuch d. Hisiologie. Hamm, 1857.

(536) F. Meyer. "Anat. d. Urogenitalsystems d. Selachier u. Amphibien."
Sitz. d. naturfor. Gesellsch. Leipzig, 1875.

(537) J. W. Spengel. "Das Urogenitalsystem d. Amphibien." Arb. a. d.
zool.- zoot. Instil. Wiirzburg. Vol. III. 1876.

(538) VonWittich. "Harn- u. Geschlechtswerkzeuge d. Amphibien." Zeit.
f. wiss. Zool., Vol. IV.

Vide also Gotte (No. 296).

Amniota.

(539) F. M. Balfour and A. Sedgwick. "On the existence of a head -kidney
in the embryo Chick," etc. Quart. J. of Micr. Science, Vol. xix. 1878.

(540 ) Banks. On the Wolffian bodies of the fatus and their remains in the adult.
Edinburgh, 1864.

47—2

74O BIBLIOGRAPHY.

(541) Th. Bornhaupt. Untersuchungen iib. die Entwicklung d. Urogenital-
systems beim Hiihnchen. Inaug. Diss. Riga, 1867.

(542) Max Braun. "Das Urogenitalsystem d. einheimischen Reptilien."
Arbeiten a. d. zool.-zoot. Instit. Wiirzburg. Vol. iv. 1877.

(543) J. Dansky u. J. Kostenitsch. "Ueb. d. Entwick. d. Keimblatter u. d.
WolfFschen Ganges im Hiihnerei." Mini. Acad. Imp. Petersbourg, vn. Series, Vol.
xxvil. 1880.

(544) Th. Egli. Beitrage zur Anat. und Entwick. d. Geschlechtsorgane. Inaug.
Diss. Zurich, 1876.

(545) E. Gasser. Beitrage zur Entwicklungsgeschichte d. Allantois, der
Milllcr'schen Gange u. des Afters. Frankfurt, 1874.

(546) E. Gasser. "Beob. iib. d. Entstehung d. Wolff schen Ganges bei Em-
bryonen von Hiihnern u. Gansen." Arch, fiir mikr. Anat., Vol. xiv. 1877.

(547) E. Gasser. "Beitrage z. Entwicklung d. Urogenitalsystems d. Hiihner-
embryonen." Sitz. d. GeseU. zur Befdrderung d. gesam. Naturwiss. Marburg, 1879.

(548) C. Kupffer. " Untersuchting iiber die Entwicklung des Harn- und Ge-
schlechtssystems." Archiv fiir mikr. Anat., Vol. II. 1866.

(549) A. Sedgwick. "Development of the kidney in its relation to the
Wolffian body in the Chick." Quart. J. of Micros. Science, Vol. xx. 1880.

(550) A. Sedgwick. "On the development of the structure known as the
glomerulus of the head-kidney in the Chick." Quart. J. of Micros. Science, Vol. xx.
1880.

(551) A. Sedgwick. "Early development of the Wolffian duct and anterior
Wolffian tubules in the Chick ; with some remarks on the vertebrate excretory
system." Quart. J. of Micros. Science, Vol. xxi. 1881.

(552) M. Watson. "The homology of the sexual organs, illustrated by com-
parative anatomy and pathology." Journal of Anat. and Phys., Vol. xiv. 1879.

(553) E. H. Weber. Zusdtze z. Lehre von Baue u. d. Verrichtungen d. Ge-
schlechtsorgane. Leipzig, 1846.

Vide also Remak (No. 302), Foster and Balfour (No. 295), His (No. 297),
Kolliker (No. 298).

CHAPTER XXIV.
GENERATIVE ORGANS AND GENITAL DUCTS.

GENERATIVE ORGANS.

THE structure and growth of the ovum and spermatozoon
were given in the first chapter of this work, but their derivation
from the germinal layers was not touched on, and it is this
subject with which we are here concerned. If there are any
structures whose identity throughout the Metazoa is not open
to doubt these structures are the ovum and spermatozoon ;
and the constancy of their relations to the germinal layers
would seem to be a crucial test as to whether the latter have
the morphological importance usually attributed to them.

The very fragmentary state of our knowledge of the origin of
the generative cells has however prevented this test being so far
very generally applied.

Porifera. In the Porifera the researches of Schulze have
clearly demonstrated that both the ova and the spermatozoa
take their origin from indifferent cells of the general paren-
chyma, which may be called mesoblastic. The primitive germi-
nal cells of the two sexes are not distinguishable ; but a
germinal cell by enlarging and becoming spherical gives rise
to an ovum ; and by subdivision forms a sperm-morula, from
the constituent cells of which the spermatozoa are directly
developed.

Ccelenterata. The greatest confusion prevails as to the
germinal layer from which the male and female products are
derived in the Ccelenterata1.

1 E. van Beneden (No. 556) was the first to discover a different origin for the
generative products of the two sexes in Hydractinia, and his observations have led to
numerous subsequent researches on the subject. For a summary of the observations
on the Hydroids vide Weismann (No. 560).

742 CCELENTERATA.

The following apparent modes of origin of these products
have been observed.

(1) The generative products of both sexes originate in the
ectoderm (epiblast) : Hydra, Cordylophora, Tubularia, all (?) free
Gonophores of Hydromedusae, the Siphonophora, and probably
the Ctenophora.

(2) The generative products of both sexes originate in the
entoderm (hypoblast) : Plumularia and Sertularella, amongst
the Hydroids, and the. whole of the Acraspeda and Actinozoa.

(3) The male cells are formed in the ectoderm, and the
female in the entoderm : Gonothyraea, Campanularia, Hydrac-
tinia, Clava.

In view of the somewhat surprising results to which the
researches on the origin of the genital products amongst the
Ccelenterata have led, it would seem to be necessary either to
hold that there is no definite homology between the germinal
layers in the different forms of Ccelenterata, or to offer some
satisfactory explanation of the behaviour of the genital pro-
ducts, which would not involve the acceptance of the first
alternative.

Though it can hardly be said that such an explanation has
yet been offered, some observations of Kleinenberg (No. 557)
undoubtedly point to such an explanation being possible.

Kleinenberg has shewn that in Eudendrium the ova migrate
freely from the ectoderm into the endoderm, and vice versa ; but
he has given strong grounds for thinking that they originate in
the ectoderm. He has further shewn that the migration in this
type is by no means an isolated phenomenon.

Since it is usually only possible to recognise generative
elements after they have advanced considerably in development,
the mere position of a generative cell, when first observed, can
afford, after what Kleinenberg has shewn, no absolute proof
of its origin. Thus it is quite possible that there is really
only one type of origin for the generative cells in the Ccelen-
terata.

Kleinenberg has given reasons for thinking that the migration of the ova
into the entoderm may have a nutritive object. If this be so, and there are
numerous facts which shew that the position of generative cells is often
largely influenced by their nutritive requirements, it seems not impossible

GENERATIVE ORGANS. 743

that the endodermal position of the generative organs in the Actinozoa and
acraspedote Medusre may have arisen by a continuously earlier migration of
the generative cells from the ectoderm into the endoderm ; and that the
migration may now take place at so early a period of the development, that
we should be justified in formally holding the generative products to be
endodermal in origin.

\Ve might perhaps, on this view, formulate the origin of the generative
products in the Ccelenterata in the following way : —

Both ova and spermatozoa primitively originated in the ectoderm, but in
order to secure a more complete nutrition the cells which give rise to them
exhibit in certain groups a tendency to migrate into the endoderm. This
migration, which may concern the generative cells of one or of both the
sexes, takes place in some cases after the generative cells have become
recognisable as such, and very probably in other cases at so early a period
that it is impossible to distinguish the generative cells from indifferent
embryonic cells.

Very little is known with reference to the origin of the
generative cells in the triploblastic Invertebrata.

Chaetopoda and Gephyrea. In the Chaetopoda and
Gephyrea, the germinal cells are always developed in the adult
from the epithelial lining of the body cavity ; so that their origin
from the mesoblast seems fairly established.

If we are justified in holding the body cavity of these forms
to be a derivative of the primitive archenteron (vide pp. 356 and
357) the generative cells may fairly be held to originate from a
layer which corresponds to the endoderm of the Ccelenterata1.

Chaetognatha. In Sagitta the history of the generative
cells, which was first worked out by Kowalevsky and Biitschli,
has been recently treated with great detail by O. Hertwig2.

The generative cells appear during the gastrula stage, as two
large cells with conspicuous nuclei, which are placed in the
hypoblast lining the archenteron, at the pole opposite the
blastopore. These cells soon divide, and at the same time pass
out of the hypoblast, and enter the archenteric cavity (fig. 408
- A, ge). The division into four cells, which is not satisfactorily
represented ifl my diagram, takes place in such a way that two

1 The Hertwigs (No. 271) state that in their opinion the generative cells arise
from the lining of the body cavity in all the forms whose body cavity is a product of
the archenteron. We do not know anything of the embryonic development of the
generative organs in the Echinodermata, but the adult position of the generative
organs in this group is very unfavourable to the Hertwigs' view.

2 O. Hertwig, Die Chcetognathen. Jena, 1880-

744

CH^ETOGNATHA.

cells are placed nearer the median line, and two externally. The
two inner cells form the eventual testes, and the outer the

FIG. 408. THREK STAGES IN THE DEVELOPMENT OF SAGITTA. (A and C after

Biitschli, and B after Kowalevsky.)
The three embryos are represented in the same positions.

A. Represents the gastrula stage.

B. Represents a succeeding stage, in which the primitive archenteron is com-
mencing to be divided into three.

C. Represents a later stage, in which the mouth involution (in) has become con-
tinuous with the alimentary tract, and the blastopore is closed.

///. mouth ; al. alimentary canal ; ac. archenteron ; bl.p. blastopore ; pv. peri-
visceral cavity ; sp, splanchnic mesoblast ; so. somatic mesoblast ; ge. generative
organs.

ovaries, one half of each primitive cell thus forming an ovary, and
the other a testis.

FIG. 409. Two VIEWS OF A LATE EMBRYO OF SAGITTA. A, from the dorsal

surface. B, from the side. (After Biitschli.)

m. mouth ; al. alimentary canal ; v.g. ventral ganglion (thickening of epiblast) ;
<.'/. epiblast ; c.pv. cephalic section of body cavity ; so. somatopleure ; sp. splanchno-
pleure ; ge. generative organs.

GENERATIVE ORGANS.

745

When the archenteric cavity is divided into a median
alimentary tract, and two lateral sections forming the body
cavity, the generative organs are placed in the common vestibule
into which both the body cavity and alimentary cavity at first
open (fig. 408).

The generative organs long retain their character as simple
cells. Eventually (fig. 409) the two ovaries travel forwards, and
apply themselves to the body walls, while the two testes also
become separated by a backward prolongation of the median
alimentary tract.

On the formation of the transverse septum dividing the tail
from the body, the ovarian cells lie immediately in front of this
septum, and the testicular cells in the region behind it.

Polyzoa. In Pedicellina amongst the entoproctous Polyzoa
Hatschek finds that the generative organs originate from a pair
of specially large mesoblast cells, situated in the space between
the stomach and the floor of the vestibule. The two cells
undergo changes, which have an obvious resemblance to those of
the generative cells of the Chsetognatha. They become sur-
rounded by an investment of mesoblast cells, and divide so as to
form two masses. Each of these masses at a later period
separates into an anterior and a posterior part. The former
becomes the ovary, the latter the testis.

Nematoda. In the Nematoda the generative organs are
derived from the division of a single cell which would appear to
be mesoblastic1.

Insecta. The generative cells have been observed at a very
early embryonic stage in several insect forms (Vol. II. p. 404), but
the observations so far recorded with reference to them do not
enable us to determine with certainty from which of the germinal
layers they are derived.

Crustacea. In Moina, one of the Cladocera, Grobben2 has
shewn that the generative organs are derived from a single cell,
which becomes differentiated during the segmentation. This
cell, which is in close contiguity with the cells from which both
the mesoblast and hypoblast originate, subsequently divides ;

1 Fide Vol. n. p. 374; also Gotte, Zool. Anzeiger, No. 80, p. 189.

2 C. Grobben. "Die Entwick. d. Moina rectirostris." Arbeit, a. d. zool. Instil.
Wien. Vol. II. 1879.

746

CHORDATA.

sp.c

but at the gastrula stage, and after the mesoblast has become
formed, the cells it gives rise to are enclosed in the epiblast, and
do not migrate inwards till a later stage. The products of the
division of the generative cell subsequently divide into two
masses. It is not possible to assign the generative cell of Moina
to a definite germinal layer. Grobben, however, thinks that it
originates from the division of a cell, the remainder of which
gives rise to the hypoblast.

Chordata. In the Vertebrata, the primitive generative cells
(often known as primitive ova) are early distinguishable, being
imbedded amongst the cells of two linear streaks of peritoneal
epithelium, placed on the dorsal side of the body cavity, one on
each side of the mesentery (figs. 405
C and 4io,/0). They appear to be
derived from the epithelial cells
amongst which they lie ; and are
characterized by containing a large
granular nucleus, surrounded by a
considerable body of protoplasm.
The peritoneal epithelium in which
they are placed is known as the
germinal epithelium.

It is at first impossible to distin-
guish the germinal cells which will
become ova from those which will
become spermatozoa.

The former however remain with-
in the peritoneal epithelium (fig. 41 1),
and become converted into ova in a
manner more particularly described
in Vol. II. pp. 54 — 59.

The history of the primitive
germinal cells in the male has not
been so adequately worked out as in
the female.

The fullest history of them is
that given by Semper (No. 559) for
the Elasmobranchii, the general ac-
curacy of which I can fully support ;

FIG. 410. SECTION THROUGH
THE TRUNK OF A SCYLLIUM
EMBRYO SLIGHTLY YOUNGER

THAN 28 F.

sp.c. spinal cord ; W. white
matter of spinal cord ; pr. poste-
rior nerve-roots ; ch. notochord ;
x. sub-notochordal rod ; ao. aorta ;
mp. muscle-plate ; mp'. inner layer
of- muscle-plate already converted
into muscles ; Vr. rudiment of
vertebral body ; st. segmental
tube; sd. segmental duct; sp.v.
spiral valve ; v. subintestinal vein ;
i>.o. primitive generative cells.

GENERATIVE ORGANS.

747

though with reference to certain stages in the history further
researches are still required1.

In Elasmobranchii the male germinal cells, instead of remain-
ing in the germinal epithelium, migrate into the adjacent stroma,
accompanied I believe by some of the indifferent epithelial cells.
Here they increase in number, and give rise to masses of variable
form, composed partly of true germinal cells, and partly of
smaller cells with deeply staining nuclei, which are, I believe,
derived from the germinal epithelium.

FIG. 411. TRANSVERSE SECTION THROUGH THE OVARY OF A YOUNG EMBRYO
OK SCYLLIUM CANICULA, TO SHEW THE PRIMITIVE GERMINAL CELLS (po) LYING
IN THE GERMINAL EPITHELIUM ON THE OUTER SIDE OF THE OVARIAN RIDGE.

These masses next break up into ampullae, mainly formed of
germinal cells, and each provided with a central lumen ; and
these ampullae attach themselves to tubes derived from the
smaller cells, which are in their turn continuous with the
testicular network. The spermatozoa are developed from the
cells forming the walls of the primitive ampulla;; but the
process of their formation does not concern us in this chapter.

In the Reptilia Braun has traced the passage of the primitive
germinal cells into the testicular tubes, and I am able to confirm
his observations on this point : he has not however traced their
further history.

1 Balbiani (No. 554) has also recently dealt with this subject, but I cannot bring
my own observations into accord with his as to the structure of the Elasmobranch
testis.

MODE OF EXIT OF GENITAL PRODUCTS.

In Mammalia the evidence of the origin of the spermato-
spores from the germinal epithelium is not quite complete, but
there can be but little doubt of its occurrence1.

In Amphioxus Langerhans has shewn that the ova and
spermatozoa are derived from similar germinal cells, which may
be compared to the germinal epithelium of the Vertebrata.
These cells are however segmentally arranged as separate
masses (vide Vol. II. p. 54).

BIBLIOGRAPHY.

(554) G. Balbiani. Lemons s. la generation des Vcrlebrcs. Paris, 1879.

(555) F. M. Balfour. "On the structure and development of the Vertebrate
ovary." Quart, J. of Micr. Science, Vol. xvm.

(556) E. van Beneden. "De la distinction originelle dutecticule et clel'ovaire,
etc." Bull. Ac. roy. belgique, Vol. xxxvil. 1874.

(557) N. Kleinenberg. "Ueb. d. Entstehung d. Eier b. Eudendrium." Zcit.
f. -wiss. Zool., Vol. xxxv. 1881.

(558) H. Ludwig. "Ueb. d. Eibildung im Theirreiche." Arbeit, a. d. zool.-
zoot. Inslit. Wilrzburg, Vol. I. 1874.

(559) C. Semper. "Das Urogenilalsystem d. Plagiostomen, etc." Arbeit, a.
d. zooL-zoot. Ins tit. Witrzbiirg, Vol. II. 1875.

(560) A. Weismann. "Zur Frage nach dem Ursprung d. Geschlechtszellen bei
den Hydroiden." Zool. Anzeiger, No. 55, 1880.

Fitffcalso O. and R. Hertwig (No. 271), Kolliker (No. 298), etc.

GENITAL DUCTS.

The development and evolution of the generative ducts is as
yet very incompletely worked out, but even in the light of our
present knowledge a comparative review of this subject brings to
light features of considerable interest, and displays a fruitful
field for future research.

In the Ccelenterata there are no generative ducts.

In the Hydromedusae and Siphonophora the generative
products are liberated by being dehisced directly into the
surrounding medium ; while in the Acraspeda, the Actinozoa
and the Ctenophora, they are dehisced into parts of the gastro-
vascular system, and carried to the exterior through the mouth.

The arrangement in the latter forms indicates the origin of

1 An entirely different view of the origin of the sperm cells has been adopted by
Balbiani, for which the reader is referred to his Memoir (No. 554).

GENITAL DUCTS.

749

the methods of transportation of the genital products to the
exterior in many of the higher types.

It has been already pointed out that the body cavity in a
very large number of forms is probably derived from parts of a
gastrovascular system like that of the Actinozoa.

When the part of the gastrovascular system into which the
generative products were dehisced became, on giving rise to the
body cavity, shut off from the exterior, it would be essential that
some mode of transportation outwards of the generative products
should be constituted.

In some instances simple pores (probably already existing at
the time of the establishment of a closed body cavity) become
the generative ducts. Such seems probably to have been the
case in the Chaetognatha (Sagitta) and in the primitive
Chordata.

In the latter forms the generative products are sometimes dehisced into
the peritoneal cavity, and thence transported by the abdominal pores to the
exterior (Cyclostomata and some Teleostei, vide p. 626). In Amphioxus
they pass by dehiscence into the atrial cavity, and thence through the gill
slits and by the mouth, or by the abdominal pore (?) to the exterior. The
arrangement in Amphioxus and the Teleostei is probably secondary, as
possibly also is that in the Cyclostomata ; so that the primitive mode of
exit of the generative products in the Chordata is still uncertain. It is
highly improbable that the generative ducts of the Tunicata are primitive
structures.

A better established and more frequent mode of exit of the
generative products when dehisced into the body cavity is by
means of the excretory organs. The generative products pass
from the body cavity into the open peritoneal funnels of such
organs, and thence through their ducts to the exterior. This
mode of exit of the generative products is characteristic of the
Chaetopoda, the Gephyrea, the Brachiopoda and the Vertebrata,
and probably also of the Mollusca. It is moreover quite possible
that it occurs in the Polyzoa, some of the Arthropoda, the
Platyelminthes and some other types.

The simple segmental excretory organs of the Polychaeta,
the Gephyrea and the Brachiopoda serve as generative canals,
and in many instances they exhibit no modification, or but a
very slight one, in connection with their secondary generative

750 DERIVATION FROM EXCRETORY ORGANS.

function ; while in other instances, e.g. Bonellia, such modifica-
tion is very considerable.

The generative ducts of the Oligochaeta are probably derived from
excretory organs. In the Terricola ordinary excretory organs are present in
the generative segments in addition to the generative ducts, while in the
Limicola generative ducts alone are present in the adult, but before their
development excretory organs of the usual type are found, which undergo
atrophy on the appearance of the generative ducts (Vedjovsky).

From the analogy of the splitting of the segmental duct of the Vertebrata
into the Miillerian and Wolffian ducts, as a result of a combined generative
and excretory function (vide p. 728), it seems probable that in the genera-
tive segments of the Oligochasta the excretory organs had at first both an
excretory and a generative function, and that, as a secondary result of this
double function, each of them has become split into two parts, a generative
and an excretory. The generative part has undergone in all forms great
modifications. The excretory parts remain unmodified in the Earthworms
(Terricola), but completely abort on the development of the generative ducts
in the Limicola. An explanation may probably be given of the peculiar
arrangements of the generative ducts in Saccocirrus amongst the Poly-
chaeta (vide Marion and Bobretzky), analogous to that just offered for the
Oligochaeta.

The very interesting modifications produced in the excretory
organs of the Vertebrata by their serving as generative ducts
were fully described in the last chapter ; and with reference to
this part of our subject it is only necessary to call attention to
the case of Lepidosteus and the Teleostei.

In Lepidosteus the Mullerian duct appears to have become
attached to the generative organs, so that the generative
products, instead of falling directly into the body cavity and
thence entering the open end of a peritoneal funnel of the
excretory organs, pass directly into the Mullerian duct without
entering the body cavity. In most Teleostei the modification is
more complete, in that the generative ducts in the adult have no
obvious connection with the excretory organs.

The transportation of the male products to the exterior in all
the higher Vertebrata, without passing into the body cavity, is
in principle similar to the arrangement in Lepidosteus.

The above instances of the peritoneal funnels of an excretory
organ becoming continuous with the generative glands, render it
highly probable that there may be similar instances amongst the
In vertebrata.

GENITAL DUCTS.

751

As has been already pointed out by Gegenbaur there are
many features in the structure of the genital ducts in the more
primitive Mollusca, which point to their having been derived
from the excretory organs. In several Lamellibranchiata1
(Spondylus, Lima, Pecten) the generative ducts open into the
excretory organs (organ of Bojanus), so that the generative
products have to pass through the excretory organ on their way
to the exterior. In other Lamellibranchiata the genital and
excretory organs open on a common papilla, and in the remain-
ing types they are placed close together.

In the Cephalopoda again the peculiar relations of the
generative organs to their ducts point to the latter having
primitively had a different, probably an excretory, function.
The glands are not continuous with the ducts, but are placed in
special capsules from which the ducts proceed. The genital
products are dehisced into these capsules and thence pass into
the ducts.

In the Gasteropoda the genital gland is directly continuous
with its duct, and the latter, especially in the Pulmonata and
Opisthobranchiata, assumes such a complicated form that its
origin from the excretory organ would hardly have been
suspected. The fact however that its opening is placed near
that of the excretory organ points to its being homologous with
the generative ducts of the more primitive types.

In the Discophora, where the generative ducts are continuous
with the glands, the structure both of the generative glands and
ducts points to the latter having originated from excretory
organs.

It seems, as already mentioned, very possible that there are
other types in which the generative ducts are derived from the
excretory organs. In the Arthropoda for instance the generative
ducts, where provided with anteriorly placed openings, as in the
Crustacea, Arachnida and the Chilognathous Myriapoda, the
Pcecilopoda, etc., may possibly be of this nature, but the data
for deciding this point are so scanty that it is not at present
possible to do more than frame conjectures.

The ontogeny of the generative ducts of the Nematoda and

1 For a summary of the facts on this subject vide Bronn, Klassen u. Ordnungen d.
Thierreichs, Vol. in. p. 404.

752 DERIVATION FROM EXCRETORY ORGANS.

the Insecta appears to point to their having originated independ-
ently of the excretory organs.

In the Nematoda the generative organs of both sexes
originate from a single cell (Schneider, Vol. I. No. 390).

This cell elongates and its nuclei multiply. After assuming
a somewhat columnar form, it divides into (i) a superficial
investing layer, and (2) an axial portion.

In the female the superficial layer is only developed distinctly
in the median part of the column. In the course of the further
development the two ends of the column become the blind ends
of the ovary, and the axial tissue they contain forms the
germinal tissue of nucleated protoplasm. The superficial layer
gives rise to the epithelium of the uterus and oviduct. The
germinal tissue, which is originally continuous, is interrupted in
the middle part (where the superficial layer gives rise to the
uterus and oviduct), and is confined to the two blind extremities
of the tube.

In the male the superficial layer, which gives rise tc the
epithelium of the vas deferens, is only formed at the hinder ond
of the original column. In other respects the development takes
place as in the female.

In the Insecta again the evidence, though somewhat conflicting,
indicates that the generative ducts arise very much as in Nema-
todes, from the same primitive mass as the generative organs. In
both of these types it would seem probable that the generative
organs were primitively placed in the body cavity, and attached
to the epidermis, through a pore in which their products passed
out ; and that, acquiring a tubular form, the peripheral part of
the gland gave rise to a duct, the remainder constituting the true
generative gland. It is quite possible that the generative ducts
of such forms as the Platyelminthes may have had a similar
origin to those in Insecta and Nematoda, but from the analogy
of the Mollusca there is nearly as much to be said for regarding
them as modified excretory organs.

In the Echinodermata nothing is unfortunately known as to
the ontogeny of the generative organs and ducts. The structure
of these organs in the adult would however seem to indicate that
the most primitive type of echinoderm generative organ consists
of a blind sack, projecting into the body cavity, and opening by

GENITAL DUCTS. 753

a pore to the exterior. The sack is lined by an epithelium,
continuous with the epidermis, the cells of which give rise to the
ova or spermatozoa. The duct of these organs is obviously
hardly differentiated from the gland ; and the whole structure
might easily be derived from the type of generative organ
characteristic of the Hydromedusae, where the generative cells
are developed from special areas of the ectoderm, and, when ripe,
pass directly into the surrounding medium.

If this suggestion is correct we may suppose that the genera-
tive ducts of the Echinodermata have a different origin to those
of the majority of1 the remaining triploblastica.

Their ducts have been evolved in forms in which the
generative products continued to be liberated directly to the
exterior, as in the Hydromedusae ; while those of other types
have been evolved in forms in which the generative products
were first transported, as in the Actinozoa, into the gastrovascular
canals2.

1 It would be interesting to have further information about Balanoglossus.

2 These views fit in very well with those already put forward in Chapter xm. on
the affinities of the Echinodermata.

B. III.

48

CHAPTER XXV.

THE ALIMENTARY CANAL AND ITS APPENDAGES, IN
THE CHORDATA.

THE alimentary canal in the Chordata is always formed of
three sections, analogous to those so universally present in the
Invertebrata. These sections are (i) the mesenteron lined by
hypoblast ; (2) the stomodaeum or mouth lined by epiblast, and
(3) the proctodaeum or anal section lined like the stomodaeum by
epiblast.

Mesenteron.

The early development of the epithelial wall of the mesenteron
has already been described (Chapter XI.). It forms at first a
simple hypoblastic tube extending from near the front end of the
body, where it terminates blindly, to the hinder extremity where
it is united with the neural tube by the neurenteric canal (fig.
420, ne). It often remains for a long time widely open in the
middle towards the yolk-sack.

It has already been shewn that from the dorsal wall of the
mesenteron the notochord is separated off nearly at the same
time as the lateral plates of mesoblast (pp. 292 — 300).

The subnotochordal rod. At a period slightly subsequent
to the formation of the notochord, and before any important
differentiations in the mesenteron have become apparent, a
remarkable rod-like body, which was first discovered by Gotte,
becomes split off from the dorsal wall of the alimentary tract in
all the Ichthyopsida. This body, which has a purely provisional
existence, is known as the subnotochordal rod.

MESENTERON.

755

It develops in Elasmobranch embryos in two sections, one situated in
the head, and the other in the trunk.

The section in the trunk is the first to appear. The wall of the
alimentary canal becomes thickened along the median dorsal line (fig. 412,
r), or else produced into a ridge into which there penetrates a narrow
prolongation of the lumen of the alimentary canal. In either case the cells
at the extreme summit become gradually constricted off as a rod, which lies
immediately dorsal to the alimentary tract, and ventral to the notochord
(fig. 413, *•).

FIG. 412. TRANSVERSE SECTION
THROUGH THE TAIL REGION OF A
PRISTIURUS EMBRYO OF THE SAME
AGE AS FIG. 28 E.

df. dorsal fin ; sp.c. spinal cord ;
//. body cavity ; sp. splanchnic layer
of mesoblast ; so. somatic layer of
mesoblast; mp'. portion of splanchnic
mesoblast commencing to be differen-
tiated into muscles ; ch. notochord ; x.
subnotochordal rod arising as an out-
growth of the dorsal wall of the ali-
mentary tract ; al. alimentary tract.

FIG. 413. TRANSVERSE SEC-
TION THROUGH THE TRUNK OF AN
EMBRYO SLIGHTLY OLDER THAN
FIG. 28 E.

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

In the hindermost part of the body its mode of formation differs some-
what from that above described. In this part the alimentary wall is' very
thick, and undergoes no special growth prior to the formation of the sub-
notochordal rod ; on the contrary, a small linear portion of the wall becomes
scooped out along the median dorsal line, and eventually separates from the
remainder as the rod in question. In the trunk the splitting off of the rod
takes place from before backwards, so that the anterior part of it is formed
before the posterior.

The section of the subnotochordal rod in the head would appear to
develop in the same way as that in the trunk, and the splitting off from the
throat proceeds from before backwards.

48—2

756 MESENTERY.

On the formation of the dorsal aorta, the subnotochordal rod becomes
separated from the wall of the gut and the aorta interposed between the two
(fig. 367, *•).

When the subnotochordal rod attains its fullest development it terminates
anteriorly some way in front of the auditory vesicle, though a little behind
the end of the notochord ; posteriorly it extends very nearly to the extremity
of the tail and is almost co-extensive with the postanal section of the
alimentary tract, though it does not reach quite so far back as the caudal
vesicle (fig. 424, b x). Very shortly after it has attained its maximum size it
begins to atrophy in front. We may therefore conclude that its atrophy,
like its development, takes place from before backwards. During the later
embryonic stages not a trace of it is to be seen. It has also been met with
in Acipenser, Lepidosteus, the Teleostei, Petromyzon, and the Amphibia, in
all of which it appears to develop in fundamentally the same way as in
Elasmobranchii. In Acipenser it appears to persist in the adult as the
subvertebral ligament (Bridge, Salensky). It has not yet been found in a
fully developed form in any amniotic Vertebrate, though a thickening of the
hypoblast, which may perhaps be a rudiment of it, has been found by
Marshall and myself in the Chick (fig. 1 10, x).

Eisig has instituted an interesting comparison between it and an organ
which he has found in a family of Chaetopods, the Capitellidas. In these
forms there is a tube underlying the alimentary tract for nearly its whole
length, and opening into it in front, and probably behind. A remnant of
such a tube might easily form a rudiment like the subnotochordal rod of the
Ichthyopsida, and as Eisig points out the prolongation into the latter during
its formation of the lumen of the alimentary tract distinctly favours such a
view of its original nature. We can however hardly suppose that there is
any direct genetic connection between Eisig's organ in the Capitellidas and
the subnotochordal rod of the Chordata.

Splanchnic mesoblast and mesentery- The mesentcron
consists at first of a simple hypoblastic tube, which however
becomes enveloped by a layer of splanchnic mesoblast. This
layer, which is not at first continued over the dorsal side of the
mesenteron, gradually grows in, and interposes itself between the
hypoblast of the mesenteron, and the organs above. At the same
time it becomes differentiated into two layers, viz. an outer
cpithelioid layer which gives rise to part of the peritoneal
epithelium, and an inner layer of undifferentiated cells which in
time becomes converted into the connective tissue and muscular
walls of the mesenteron. The connective tissue layers become
first formed, while of the muscular layers the circular is the first
to make its appearance.

ALIMENTARY CANAL. 757

Coincidently with their differentiation the connective tissue-
stratum of the peritoneum becomes established.

The Mesentery. Prior to the splanchnic mesoblast growing
round the alimentary tube above, the attachment of the latter
structure to the dorsal wall of the body is very wide. On the
completion of this investment the layer of mesoblast suspending
the alimentary tract becomes thinner, and at the same time the
alimentary canal appears to be drawn downwards and away from
the vertebral column.

In what may be regarded as the thoracic division of the general
pleuroperitoneal space, along that part of the alimentary canal
which will form the oesophagus, this withdrawal is very slight, but
it is very marked in the abdominal region. In the latter the at
first straight digestive canal comes to be suspended from the body
above by a narrow flattened band of mesoblastic tissue. This
flattened band is the mesentery, shewn commencing in fig. 117,
and much more advanced in fig. 1 19, M. It is covered on either
side by a layer of flat cells, which form part of the general
peritoneal epithelioid lining, while its interior is composed of
indifferent tissue.

The primitive simplicity in the arrangement of the mesentery
is usually afterwards replaced by a more complicated disposition,
owing to the subsequent elongation and consequent convolution
of the intestine and stomach.

The layer of peritoneal epithelium on the ventral side of the
stomach is continued over the liver, and after embracing the liver,
becomes attached to the ventral abdominal wall (fig. 380). Thus
in the region of the liver the body cavity is divided into two
halves by a membrane, the two sides of which are covered by the
peritoneal epithelium, and which encloses the stomach dorsally
and the liver ventrally. The part of the membrane between the
stomach and liver is narrow, and constitutes a kind of mesentery
suspending the liver from the stomach : it is known to human
anatomists as the lesser omentum.

The part of the membrane connecting the liver with the
anterior abdominal wall constitutes the fa lei form or suspen-
sory ligament of the liver. It arises by a secondary fusion, and
is not a remnant of a primitive ventral mesentery (vide pp. 624
and 625).

758 MESENTERY.

The mesentery of the stomach, or mesogastrium, enlarges in
Mammalia to form a peculiar sack known as the greater
omentum.

The mesenteron exhibits very early a trifold division. An
anterior portion, extending as far as the stomach, becomes
separated off as the respiratory division. On the formation
of the anal invagination the portion of the mesenteron behind
the anus becomes marked off as the postanal division, and
between the postanal section and the respiratory division is a
middle portion forming an intestinal and cloacal division.

The respiratory division of the mesenteron.

This section of the alimentary canal is distinguished by the
fact that its walls send out a series of paired diverticula, which
meet the skin, and after a perforation has been effected at the
regions of contact, form the branchial or visceral clefts.

In Amphioxus the respiratory region extends close up to the
opening of the hepatic diverticulum, and therefore to a position
corresponding with the commencement of the intestine in higher
types. In the craniate Vertebrata the number of visceral clefts
has become reduced, but from the extension of the visceral clefts
in Amphioxus, combined with the fact that in the higher Verte-
brata the vagus nerve, which is essentially the nerve of the
branchial pouches, supplies in addition the walls of the oesophagus
and stomach, it may reasonably be concluded, as has been pointed
out by Gegenbaur, that the true respiratory region primitively
included the region which in the higher types forms the
oesophagus and stomach.

In Ascidians the respiratory sack is homologous with the
respiratory tract of Amphioxus.

The details of the development of the branchial clefts in the
different groups of Vertebrata have already been described in
the systematic part of this work.

In all the Ichthyopsida the walls of a certain number of
clefts become folded ; and in the mesoblast within these folds a
rich capillary network, receiving its blood from the branchial
arteries, becomes established. These folds constitute the true
internal gills.

ALIMENTARY CANAL.

759

In addition to internal gills external branchial processes covered
by epiblast are placed on certain of the visceral arches in the
larva of Polypterus, Protopterus and many Amphibia. The
external gills have probably no genetic connection with the
internal gills.

The so-called external gills of the embryos of Elasmobranchii
are merely internal gills prolonged outwards through the gill
clefts.

The posterior part of the primitive respiratory division of the
mesenteron becomes, in all the higher Vertebrata, the oesophagus
and stomach. With reference to the development of these parts
the only point worth especially noting is the fact that in
Elasmobranchii and Teleostei their lumen, though present in
very young embryos, becomes at a later stage completely filled
up, and thus the alimentary tract in the regions of the
oesophagus and stomach becomes a solid cord of cells (fig. 23
A, ces)\ as already suggested (p. 61) it seems not impossible that
this feature may be connected with the fact that the cesophageal
region of the throat was at one time perforated by gill clefts.

In addition to the gills two important organs, viz. the
thyroid body and the lungs, take their origin from the respi-
ratory region of the alimentary tract.

Thyroid body. In the Ascidians the origin of a groove-
like diverticulum of the ventral wall of the branchial sack,
bounded by two lateral folds, and known as the endostyle or
hypopharyngeal groove, has already been described (p. 18).
This groove remains permanently open to the pharyngeal sack,

FIG. 414. DIAGRAMMATIC VERTICAL SECTION OF A JUST-HATCHED LARVA

OF PETROMYZON. (From Gegenbaur ; after Calberla.)

o. mouth ; 6. olfactory pit ; v. septum between stomodteum and mesenteron ;
h. thyroid involution ; n. spinal cord ; ch. notochord; c. heart ; a. auditory vesicle.

760

THE THYROID BODY.

and would seem to serve as a glandular organ secreting mucus.
As was first pointed out by W. Miiller there is present in
Amphioxus a very similar and probably homologous organ,
known as the hypopharyngeal groove.

In the higher Vertebrata this organ never retains its primi-
tive condition in the adult state. In the larva of Petromyzon
there is, however, present a ventral groove-like diverticulum of
the throat, extending from about the second to the fourth
visceral cleft. This organ is shewn in longitudinal section in
fig. 414, h, and in transverse section in fig. 415, and has been
identified by W. Muller (Nos. 565 and 566) with the hypo-
pharyngeal groove of Amphi-
oxus and Ascidians. It does
not, however, long retain its
primitive condition, but its open-
ing becomes gradually reduced
to a pore, placed between the
third and fourth of the perma-
nent clefts (fig. 416, tli). This
opening is retained throughout
the Ammoccete condition, but
the organ becomes highly com-
plicated, with paired anterior
and posterior horns and a
median spiral portion. In the adult the connection with the
pharynx is obliterated, and the organ is partly absorbed and
partly divided up into a series of glandular follicles, and event-
ually forms the thyroid body.

From the consideration of the above facts W. Muller was led
to the conclusion tJiat the tJiyroid body of the Craniata was
derived from the endostyle or Jiypopharyngeal groove. In all the
higher Vertebrata the thyroid body arises as a diverticulum of
the ventral wall of the throat in the region either of the mandi-
bular or hyoid arches (fig. 417, Tk}, which after being segmented
off becomes divided up into follicles.

In Elasmobranch embryos it appears fairly early as a diverticulum from
the ventral surface of the throat in the region of the niandibular arc/i,
extending from the border of the mouth to the point where the ventral aorta
divides into the two aortic branches of the mandibular arch (fig. 417, Th}.

FIG. 415. DIAGRAMMATIC TRANS-
VERSE SECTIONS THROUGH THE BRAN-
CHIAL REGION OF YOUNG LARV.K OF
PETROMYZON. (From Gegenbaur ; after
Calberla.)

d. branchial region of throat.

ALIMENTARY CANAL.

761

Somewhat later it becomes in Scyllium and Torpedo solid, though still
retaining its attachment to the wall of the oesophagus. It continues to grow
in length, and becomes divided up into a number of solid branched lobules
separated by connective tissue septa. Eventually its connection with the
throat becomes lost, and the lobules develop a lumen. In Acanthias the
lumen of the gland is retained (W. Miiller) till after its detachment from the

-- "-

Pti

FIG. 416. DIAGRAMMATIC VERTICAL SECTION THROUGH THE HEAD OF A
LARVA OF PETROMYZON.

The larva had been hatched three days, and was 4 '8 mm. in length. The optic
and auditory vesicles are supposed to be seen through the tissues. The letter tv
pointing to the base of the velum is where Scott believes the hyomandibular cleft to
be situated.

c.h. cerebral hemisphere ; th. optic thalamus; in. infundibulum ; pn. pineal gland ;
mb. mid-brain ; cb, cerebellum ; md. medulla oblongata ; au.v. auditory vesicle ; op.
optic vesicle; ol. olfactory pit; m. mouth; br.c. branchial pouches; th. thyroid
involution; v.ao. ventral aorta; ht. ventricle of heart ; ch. notochord.

throat. It preserves its embryonic position through life. In Amphibia it
originates, as in Elasmobranchii, from the region of the mandibular arch ;
but when first visible it forms a double epithelial wall connecting the throat
with the nervous layer of the epidermis. It subsequently becomes detached
from the epidermis, and then has the usual form of a diverticulum from the
throat. In most Amphibians it becomes divided into two lobes, and so
forms a paired body. The peculiar connection between the thyroid diver-
ticulum and the epidermis in Amphibia has been noted by Gotte in
Bombinator, and by Scott and Osborn in Triton. It is not very easy to see
what meaning this connection can have.

In the Fowl (W. Miiller) the thyroid body arises at the end of the second
or beginning of the third day as an outgrowth from the hypoblast of the
throat, opposite the point of origin of the anterior arterial arch. This
outgrowth becomes by the fourth day a solid mass of cells, and by the fifth
ceases to be connected 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

762

THE THYROID BODY.

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 six-
teenth day it is a paired body composed of
a number of hollow branched follicles, each
with a ' membrana propria,' and separated
from each other by septa of connective
tissue. It finally travels back to the point
of origin of the carotids.

Amongst Mammalia the thyroid arises
in the Rabbit (Kolliker) and Man (His) as
a hollow diverticulum of the throat at the
bifurcation of the foremost pair of aortic
arches. It soon however becomes solid,
and is eventually detached from the throat
and comes to lie on the ventral side of the
larynx or windpipe. The changes it under-
goes are in the main similar to those in the
lower Vertebrata. It becomes partially
constricted into two lobes, which remain
however united by an isthmus1. The fact
that the thyroid sometimes arises in the
region of the first and sometimes in that of
the second cleft is probably to be explained

Tli

FIG. 417. SECTION THROUGH
THE HEAD OF AN ELASMOBRANCH
EMBRYO, AT THE LEVEL OF THE
AUDITORY INVOLUTION.

Th. rudiment of thyroid body ;
aup. auditory pit ; aim. ganglion
of auditory nerve ; iv. v. roof of
fourth ventricle ; a.c.v. anterior
cardinal vein ; aa. aorta ; f.aa
aortic trunk of mandibular arch ;
//. head cavity of mandibular
arch ; Ivc. alimentary pouch which
will form the first visceral cleft.

by its rudimentary character.

The Thymus gland. The thymus gland may conveniently be
dealt with here, although its origin is nearly as obscure as its function. It
has usually been held to be connected with the lymphatic system. Kolliker
was the first to shew that this view was probably erroneous, and he
attempted to prove that it was derived in the Rabbit from the walls of one
of the visceral clefts, mainly on the ground of its presenting in the embryo
an epithelial character.

1 Wolfler (No. 571) states that in the Pig and Calf the thyroid body is formed as a
pair of epithelial vesicles, which are developed as outgrowths of the walls of the first
pair of visceral clefts. He attempts to explain the contradictory observations of other
embryologists by supposing that they have mistaken the ventral ends of visceral
pouches for an unpaired outgrowth of the throat. Stieda (No. 569) also states that in
the Pig and Sheep the thyroid arises as a paired body from the epithelium of a pair
of visceral clefts, at a much later period than would appear from the observations of
His and Kolliker. In view of the comparative development of this organ it is
difficult to accept either Wolfler's or Stieda's account. Wolfler's attempt to explain
the supposed errors of his predecessors is certainly not capable of being applied in
the case of Elasmobranch Fishes, or of Petromyzon ; and I am inclined to think that
the method of investigation by transverse sections, which has been usually employed,
is less liable to error than that by longitudinal sections which he has adopted.

ALIMENTARY CANAL. 763

Stieda (No. 569) has recently verified Kolliker's statements. He finds
that in the Pig and the Sheep the thymus arises as a paired outgrowth from
the epithelial remnants of a pair of visceral clefts. Its two lobes may at first
be either hollow (Sheep) or solid (Pig), but eventually become solid, and
unite in the median line. Stieda and His hold that in the adult gland, the
so-called corpuscles of Hassall are the remnants of the embryonic epithelial
part of the gland, and that the lymphatic part of it is of mesoblastic origin ;
but Kolliker believes the lymphatic cells to be direct products of the
embryonic epithelial cells.

The posterior visceral clefts in the course of their atrophy give rise to
various more or less conspicuous bodies of a pseudo-glandular nature, which
have been chiefly studied by Remak1.

Swimming bladder and lungs. A swimming bladder is
present in all Ganoids and in the vast majority of Teleostei.
Its development however is only imperfectly known.

In the Salmon and Carp it arises, as was first shewn by Von
Baer, as an outgrowth of the alimentary tract, shortly in front of
the liver. In these forms it is at first placed on the dorsal side
and slightly to the right, and grows backwards on the dorsal
side of the gut, between the two folds of the mesentery.

The absence of a pneumatic duct in the Physoclisti would
appear to be due to a post-larval atrophy.

In Lepidosteus the air-bladder appears to arise, as in the
Teleostei, as an invagination of the dorsal wall of the oesophagus.

In advanced embryos of Galeus, Mustelus and Acanthias, Miklucho-
Maclay detected a small diverticulum opening on the dorsal side of the
oesophagus, which he regards as a rudiment of a swimming bladder. This
interpretation must however be regarded as somewhat doubtful.

The lungs. The lungs originate in a nearly identical way in
all the Vertebrate forms in which their development has been
observed. They are essentially buds or processes of the ventral
wall of the primitive oesophagus.

At a point immediately behind the region of the visceral
clefts the cavity of the alimentary canal becomes compressed
laterally, and at the same time constricted in the middle, so that
its transverse section (fig. 418 i) is somewhat hourglass-shaped,
and shews an upper or dorsal chamber d, joining on to a lower
or ventral chamber / by a short narrow neck.

1 For details on these organs vide Kolliker, Entwicklungsgeschichte, p. 88 1.

764

THE LUNGS.

The hinder end of the lower tube enlarges (fig. 418 2), and
then becomes partially divided into two lobes (fig. 418 3). All
these parts at first freely com-
municate, but the two lobes,
partly by their own growth,
and partly by a process of con-
striction, soon become isolated
posteriorly; while in front they
open into the lower chamber
of the oesophagus (fig. 422).

By a continuation forwards
of the process of constriction
the lower chamber of the oeso-
phagus, carrying with it the
two lobes above mentioned,
becomes gradually transformed
into an independent tube,
opening in front by a narrow
slit-like aperture into the oeso-
phagus. The single tube in
front is the rudiment of the
trachea and larynx, while the
two diverticula behind become
(fig. 419, Ig) the bronchial tubes
and lungs.

While the above changes
are taking place in the hypo-
blastic walls of the alimentary
tract, the splanchnic 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.

There do not seem to be any essential differences in the mode of
formation of the above structures in the types so far observed, viz. Amphibia,
Aves and Mammalia. Writers differ as to whether the lungs first arise as

FlG. 418. FOUR DIAGRAMS ILLUSTRA-
TING THE FORMATION OF THE LUNGS.

(After Gotte.)

a. mesoblast; b. hypoblast; d. cavity
of digestive canal ; /. cavity of the pul-
monary diverticulum.

In (i) the digestive canal has com-
menced to be constricted into an upper
and lower canal ; the former the true
alimentary canal, the latter the pulmo-
nary tube; the two tubes communicate
with each other in the centre.

In (2) the lower (pulmonary) tube has
become expanded.

In (3) the expanded portion of the
tube has become constricted 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 correspond-
ing to the internal changes which have
been going on.

ALIMENTARY CANAL.

765

re

paired diverticula, or as a single diverticulum ; and as to whether the
rudiments of the lungs are established
before those of the trachea. If the above
account is correct it would appear that
any of these positions might be main-
tained. Phylogenetically interpreted the
ontogeny of the lungs appears however
to imply that this organ was first an
unpaired structure and has become
secondarily paired, and that the trachea
was relatively late in appearing.

The further development of the
lungs is at first, in the higher types
at any rate, essentially similar to
that of a racemose gland. From
each primitive diverticulum nu-
merous branches are given off
In Aves and Mammalia (fig. 355)
they are mainly confined to the
dorsal and lateral parts. These
branches penetrate into the sur-
rounding mesoblast and continue
to give rise to secondary and
tertiary branches. In the meso-

•At

FIG. 419. SECTION THROUGH
THE CARDIAC REGION OF AN EMBRYO
OF LACERTA MURALIS OF 9 MM. TO
SHEW THE MODE OF FORMATION OF
THE PERICARDIAL CAVITY.

ht. heart ; pc . pericardial cavity ;
al. alimentary tract; Ig. lung; /.
liver; pp. body cavity; md. open
end of Mullerian duct; wd. Wolffian
duct ; vc. vena cava inferior ; ao.
aorta; ch. notochord; me, medullary
cord.

blast around them numerous ca-
pillaries make their appearance, and the further growth of the
bronchial tubes is supposed by Boll to be due to the mutual
interaction of the hitherto passive mesoblast and of the hypo-
blast.

The further changes in the lungs vary somewhat in the different forms.

The air sacks are the most characteristic structures of the avian lung.
They are essentially the dilated ends of the primitive diverticula or of their
main branches.

In Mammalia (Kolliker, No. 298) the ends of the bronchial tubes become
dilated into vesicles, which may be called the primary air-cells. At first,
owing to their development at the ends of the bronchial branches, these are
confined to the surface of the lungs. At a later period the primary air-cells
divide each into two or three parts, and give rise to secondary air-cells, while
at the same time the smallest bronchial tubes, which continue all the while
to divide, give rise at all points to fresh air-cells. Finally the bronchial
tubes cease to become more branched, and the air-cells belonging to each
minute lobe come in their further growth to open into a common chamber.

766 THE CLOACA.

Before the lungs assume their function the embryonic air-cells undergo a
considerable dilatation.

The trachea and larynx. The development of the trachea and larynx
does not require any detailed description. The larynx is formed as a simple
dilatation of the trachea. The cartilaginous structures of the larynx are of
the same nature as those of the trachea.

It follows from the above account that the whole pulmonary
structure is 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 epithelium of
the tubes, and the mesoblast providing the elastic, muscular,
cartilaginous, vascular, and other connective tissues of the
tracheal and bronchial walls.

There can be no doubt that the lungs and air-bladder are
homologous structures, and the very interesting memoir of Eisig
on the air-bladder of the Chaetopoda1 shews it to be highly
probable that they are the divergent modifications of a primitive
organ, which served as a reservoir for gas secreted in the
alimentary tract, the gas in question being probably employed
for respiration when, for any reason, ordinary respiration by the
gills was insufficient.

Such an organ might easily become either purely respiratory,
receiving its air from the exterior, and so form a true lung ; or
mainly hydrostatic, forming an air-bladder, as in Ganoidei and
Teleostei.

It is probable that in the Elasmobranchii the air-bladder has
become aborted, and the organ discovered by Micklucho-Maclay
may perhaps be a last remnant of it.

The middle division of the mesenteron. The middle
division of the mesenteron, forming the intestinal and cloacal
region, is primitively a straight tube, the intestinal region of
which in most Vertebrate embryos is open below to the yolk-
sack.

Cloaca. In the Elasmobranchii, the embryos of which
probably retain a very primitive condition of the mesenteron,
this region is not at first sharply separated from the postanal
section behind. Opposite the point where the anus will even-

1 H. Eisig, " Ueb. d. Vorkommen eines schwimmblasenahnlichen Organs bei
Anneliden." Mittheil. a. d. zool. Station z. Neafel, Vol. II. 1881.

ALIMENTARY CANAL.

767

tually appear a dilatation of the mesenteron arises, which comes
in contact with the external skin (fig. 28 E, an}. This dilatation
becomes the hypoblastic section of the cloaca. It communicates
behind with the postanal gut (fig. 424 D), and in front with the
intestine ; and may be defined as the dilated portion of the alimen-
tary tract which receives the genital and urinary ducts and opens
externally by the proctodczum.

In Acipenser and Amphibia the cloacal region is indicated
as a ventral diverticulum of the mesenteron even before the
closure of the blastopore. It is shewn in the Amphibia at an
early stage in fig. 73, and at a later period, when in contact with
the skin at the point where the anal invagination is about to
appear, in fig. 420.

FIG. 420. LONGITUDINAL SECTION THROUGH AN ADVANCED EMBRYO OF

BOMBINATOR. (After Gotte.)

m. mouth ; an. anus ; /. liver ; ne. neurenteric canal ; me. medullary canal ; ch.
notochord ; pn. pineal gland.

In the Sauropsida and Mammalia the cloaca appears as a
dilatation of the mesenteron, which receives the opening of the
allantois almost as soon as the posterior part of the mesenteron
is established.

The eventual changes which it undergoes have been already
dealt with in connection with the urinogenital organs.

Intestine. The region in front of the cloaca forms the
intestine. In certain Vertebrata it nearly retains its primitive
character as a straight tube ; and in these types its anterior
part is characterised by the presence of a peculiar fold, which in
a highly specialised condition is known as the spiral valve.
This structure appears in its simplest form in Ammocoetes. It

768 THE INTESTINE.

there consists of a fold in the wall of the intestine, giving to the
lumen of this canal a semilunar form in section, and taking a
half spiral.

In Elasmobranchii a similar fold to that in Ammoccetes first
makes its appearance in the embryo. This fold is from the
first not quite straight, but winds in a long spiral round the
intestine. In the course of development it becomes converted
into a strong ridge projecting into the lumen of the intestine
(fig. 388, /). The spiral it makes becomes much closer, and it
thus acquires the form of the adult spiral valve. A spiral valve
is also found in Chimaera and Ganoids. No rudiment of such
an organ is found in the Teleostei, the Amphibia, or the higher
Vertebrata.

The presence of this peculiar organ appears to be a very
primitive Vertebrate character. The intestine of Ascidians
exhibits exactly the same peculiarity as that of Ammoccetes,
and we may probably conclude from embryology that the
ancestral Chordata were provided with a straight intestine
having a fold projecting into its lumen, to increase the area of
the intestinal epithelium.

In all forms in which there is not a spiral valve, with the
exception of a few Teleostei, the intestine becomes considerably
longer than the cavity which contains it, and therefore neces-
sarily more or less convoluted.

The posterior part usually becomes considerably enlarged to
form the rectum or in Mammalia the large intestine.

In Elasmobranchii there is a peculiar gland opening into the
dorsal side of the rectum, and in many other forms there is a
caecum at the commencement of the rectum or of the large
intestine.

In Teleostei, the Sturgeon and Lepidosteus there opens into
the front end of the intestine a number of caecal pouches known
as the pancreatic caeca. In the adult Sturgeon these pouches
unite to form a compact gland, but in the embryo they arise as
a series of isolated outgrowths of the duodenum.

Connected with the anterior portion of the middle region of
the alimentary canal, which may be called the duodenum, are
two very important and constant glandular organs, the liver and
the pancreas.

ALIMENTARY CANAL.

769

ITlf

The liver. The liver is the earliest formed and largest
glandular organ in the embryo.

It appears in its simplest
form in Amphioxus as a single
unbranched diverticulum of the
alimentary tract, immediately
behind the respiratory region,
which is directed forwards and
placed on the left side of the
body.

In all true Vertebrata the
gland has a much more compli-
cated structure. It arises as a
ventral outgrowth of the duode-
num (fig. 420, /). This out-
growth may be at first single,
and then grow out into two
lobes, as in Elasmobranchii (fig.
421) and Amphibia, or have from
the first the form of two some-
what unequal diverticula, as in
Birds (fig. 422), or again as in
the Rabbit (Kolliker) one di-
verticulum may be first formed, and a second one appear
somewhat later. The hepatic diverticula, whatever may be
their primitive form, grow into a special thickening of the
splanchnic mesoblast.

From the primitive diverticula there are soon given off a
number of hollow buds (fig. 421) which rapidly increase in
length and number, and form the so-called hepatic cylinders.
They soon anastomose and unite together, and so constitute an
irregular network. Coincidently with the formation of the
hepatic network the united vitelline and visceral vein or veins
(u.v\ in their passage through the liver, give off numerous
branches, and gradually break up into a plexus of channels
which form a secondary network amongst the hepatic cylinders.
In Amphibia these channels are stated by Gotte to be lacunar,
but in Elasmobranchii, and probably Vertebrata generally, they
arc from the first provided with distinct though delicate walls.
B. in. 49

FIG. 421. SECTION THROUGH THE
VENTRAL PART OF THE TRUNK OF A
YOUNG EMBRYO OF SCYLLIUM AT THE
LEVEL OF THE UMBILICAL CORD.

b. pectoral fin ; ao. dorsal aorta ;
cav. cardinal vein; ua. vitelline ar-
tery ; nv. vitelline vein united with
subintestinal vein ; al. duodenum ;
/. liver ; sd. opening of segmental
duct into the body-cavity ; mp. mus-
cle-plate ; urn. umbilical canal.

770

THE LIVER.

It is still doubtful whether the hepatic cylinders are as a rule hollow or
solid. In Elasmobranchii they are at first provided with a large lumen,
which though it becomes gradually smaller never entirely vanishes. The
same seems to hold good for Amphibia and some Mammalia. In Aves
the lumen of the cylinders is even from the first much more difficult
to see, and the cylinders are stated by Remak to be solid, and he has
been followed in this matter by Kolliker. In the Rabbit also Kolliker finds
the cylinders to be solid.

The embryonic hepatic network gives rise to the parenchyma
of the adult liver, with which in
its general arrangement it closely
agrees. The blood-channels are
at first very large, and have a
very irregular arrangement ; and
it is not till comparatively late
that the hepatic lobules with their
characteristic vascular structures
become established.

The biliary ducts are formed
either from some of the primi-
tive hepatic cylinders, or, as
would seem to be the case in
Elasmobranchii and Birds (fig.
422), from the larger diverti-
cula of the two primitive out-
growths.

The gall-bladder is so incon-
stant, and the arrangement of
the ducts opening into the intes-
tine so variable, that no general statements can be made about
them. In Elasmobranchii the primitive median diverticulum
(fig. 421) gives rise to the ductus choledochus. Its anterior end
dilates to form a gall-bladder.

In the Rabbit a ductus choledochus is formed by a diver-
ticulum from the intestine at the point of insertion of the two
primitive lobes. The gall-bladder arises as a diverticulum of
the right primitive lobe.

The liver is relatively very large during embryonic life and
has, no doubt, important functions in connection with the circu-
lation.

r

FIG. 422. DIAGRAM OF THE DIGES-
TIVE TRACT OF A CHICK UPON THE
FOURTH DAY. (After Gotte.)

The black line indicates the hypo-
blast. The shaded part around it is
the splanchnic mesoblast.

Ig. lung ; st. stomach ; p. pancreas ;
/. liver.

ALIMENTARY CANAL.

771

The pancreas. So far as is known the development of the
pancreas takes place on a very constant type throughout the
series of craniate Vertebrata, though absent in some of the
Teleostean fishes and Cyclostomata, and very much reduced in
most Teleostei and in Petromyzon.

It arises nearly at the same time as the liver in the form of a
hollow outgrowth from the dorsal side of the intestine nearly
opposite but slightly behind the hepatic outgrowth (fig. 422, /).
It soon assumes, in Elasmobranchii and Mammalia, somewhat
the form of an inverted funnel, and from the expanded dorsal
part of the funnel there grow out numerous hollow diverticula
into the passive splanchnic mesoblast.

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 funnel-shaped receptacle loses its origi -
nal form, and elongating, assumes the character of a duct.

From the above mode of development it is clear that the
glandular cells of the pancreas are derived from the hypoblast.

Into the origin of the varying arrangements of the pancreatic
ducts it is not possible to enter in detail. In some cases,
e.g. the Rabbit (Kolliker), the two lobes and ducts arise from a
division of the primitive gland and duct. In other cases, e.g. the
Bird, a second diverticulum springs from the alimentary tract.
In a large number of instances the primitive condition with a
single duct is retained.

Postanal section of the mesenteron. In the embryos of
all the Chordata there is a section of the mesenteron placed
behind the anus. This section invariably atrophies at a com-
paratively early period of embryonic life ; but it is much better
developed in the lower forms than in the higher. At its
posterior extremity it is primitively continuous with the neural
tube (fig. 420), as was first shewn by Kowalevsky.

The canal connecting the neural and alimentary canals has
already been described as the neurenteric canal, and represents
the remains of the blastopore.

In the Tunicata the section of the mesenteron, which in all probability
corresponds to the postanal gut of the Vertebrata, is that immediately

49—2

772 POSTANAL SECTION OF THE MESENTERON.

following the dilated portion which gives rise to the branchial cavity

and permanent intestine. It has already

been shewn that from the dorsal and

lateral portions of this section of the

primitive alimentary tract the notochord

and muscles of the Ascidian tadpole are

derived. The remaining part of its walls

forms a solid cord of cells (fig. 423, al'},

which either atrophies, or, according to

Kowalevsky, gives rise to blood-vessels.

In Amphioxus the postanal gut, FIG. 423. TRANSVERSE OPTICAL

.hough distinctly developed, is no, very %
long, and atrophies at a comparatively (After Kowalevsky.)
early period. The section ;s from an embryo of

In Elasmobranchii this section of the the same age as fig. 8 iv.

alimentary tract is very well developed, ch- notochord ; nc neural 1 canal ;

. , , me. mesoblast ; of. hypoblast of
and persists for a considerable period of taji<

embryonic life. The following is a
history of its development in the genus Scyllium.

Shortly after the stage when the anus has become marked out by the
alimentary tract sending down a papilliform process towards the skin, the
postanal gut begins to develop a terminal dilatation or vesicle, connected
with the remainder of the canal by a narrower stalk.

The walls both of the vesicle and stalk are formed of a fairly columnar
epithelium. The vesicle communicates in front by a narrow passage with
the neural canal, and behind is continued into two horns corresponding
with the two caudal swellings previously spoken of (p. 55). Where the
canal is continued into these two horns, its walls lose their distinctness of
outline, and become continuous with the adjacent mesoblast.

In the succeeding stages, as the tail grows longer and longer, the post-
anal section of the alimentary tract grows with it, without however under-
going alteration in any of its essential characters. At the period of the
maximum development, it has a length of about -J of that of the whole
alimentary tract.

Its features at a stage shortly before the external gills have become
prominent are illustrated by a series of transverse sections through the
tail (fig. 424). The four sections have been selected for illustration out of a
fairly-complete series of about one hundred and twenty.

Posteriorly (A) there is present a terminal vesicle (alv) '25 mm. in
diameter, which communicates dorsally by a narrow opening with the
neural canal (nc) ; to this is attached a stalk in the form of a tube, also
lined by columnar epithelium, and extending through about thirty sections
(B al}. Its average diameter is about '084 mm., and its walls are very thick.
Overlying its front end is the subnotochordal rod (x), but this does not
extend as far back as the terminal vesicle.

The thick-walled stalk of the vesicle is connected with the cloacal section

ALIMENTARY CANAL.

773

of the alimentary tract by a very narrow thin-walled tube (C of). This for
the most part has a fairly uniform calibre, and a diameter of not more than
•035 mm. Its walls are formed of flattened epithelial cells. At a point not
far from the cloaca it becomes smaller, and its diameter falls to -03 mm. In

cl.al

FIG. 424. FOUR SECTIONS THROUGH THE POSTANAL PART OF THE TAIL
OF AN EMBRYO OF THE SAME AGE AS FIG. 28 F.

A. is the posterior section.

nc . neural canal ; al. postanal gut ; alv. caudal vesicle of postanal gut ; x.
subnotochordal rod; mp. muscle-plate; ch. notochord; cl.al. cloaca; ao. aorta;
v.cau, caudal vein.

front of this point it rapidly dilates again, and, after becoming fairly wide,
opens on the dorsal side of the cloacal section of the alimentary canal just
behind the anus (D al}.

Very shortly after the stage to which the above figures belong, at a
point a little behind the anus, where the postanal section of the canal
was thinnest in the previous stage, it becomes solid, and a rupture here
occurs in it at a slightly later period.

The atrophy of this part of the alimentary tract having once commenced
proceeds rapidly. The posterior part first becomes reduced to a small
rudiment near the end of the tail. There is no longer a terminal vesicle,
nor a neurenteric canal. The portion of the postanal section of the
alimentary tract, just behind the cloaca, is for a short time represented
by a small rudiment of the dilated part which at an earlier period opened
into the cloaca.

In Teleostei the vesicle at the end of the tail, discovered by Kupffer,

774 THE STOMOD/EUM.

(fig- 34> hyv) is probably the equivalent of the vesicle at the end of the
postanal gut in Elasmobranchii.

In Petromyzon and in Amphibia there is a well-developed postanal
gut connected with a neurenteric canal which gradually atrophies. It is
shewh in the embryo of Bombinator in fig. 420.

Amongst the amniotic Vertebrata the postanal gut is less developed
than in the Ichthyopsida. A neurenteric canal is present for a short period

FIG. 425. DIAGRAMMATIC LONGITUDINAL SECTION THROUGH THE POSTERIOR
END OF AN EMBRYO BlRD AT THE TIME OF THE FORMATION OF THE ALLANTOIS.

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

in various Birds (Gasser, etc.) and in the Lizard, but disappears very early.
There is however, as has been pointed out by Kolliker, a well-marked
postanal gut continued as a narrow tube from behind the cloaca into
the tail both in the Bird (fig. 425, p.a.g.} and Mammals (the Rabbit), but
especially in the latter. It atrophies early as in lower forms.

The morphological significance of the postanal gut and of the neuren-
teric canal has already been spoken of in Chapter xii., p. 323.

The anterior section of the permanent alimentary tract is
formed by an invagination of epiblast, constituting a more or
less considerable pit, with its inner wall in contact with the
blind anterior extremity of the alimentary tract.

In Ascidians this pit is placed on the dorsal surface (fig. 9, o),
and becomes the permanent oral cavity of these forms. In the
larva of Amphioxus it is stated to be formed unsymmetrically

THE STOMOD/EUM.

775

(vide p. 5), but further observations on its development are
required.

In the true Vertebrata it is always formed on the ventral
surface of the head, immediately behind the level of the fore-
brain (fig. 426), and is deeper in Petromyzon (fig. 416, ;«) than
in any other known form.

From the primary buccal cavity or stomodaeum there grows
out the pituitary pit (fig. 426, pt\ the
development of which has already
been described (p. 435).

The wall separating the stomo-
daeum from the mesenteron always
becomes perforated, usually at an
early stage of development, and
though in Petromyzon the boundary
between the two cavities remains
indicated by the velum, yet in the
higher Vertebrata all trace of this
boundary is lost, and the original
limits of the primitive buccal cavity
become obliterated ; while a secon-
dary buccal cavity, partly lined by
hypoblast and partly by epiblast,
becomes established.

This cavity, apart from the organs which belong to it,
presents important variations in structure. In most Pisces it
retains a fairly simple character, but in the Dipnoi its outer
boundary becomes extended so as to enclose the ventral open-
ing of the nasal sack, which thenceforward constitutes the
posterior nares.

In Amphibia and Amniota the posterior nares also open well
within the boundary of the buccal cavity.

In the Amniota further important changes take place.

In the first place a plate grows inwards from each of the
superior maxillary processes (fig. 427, /), and the two plates,
meeting in the middle line, form a horizontal septum dividing
the front part of the primitive buccal cavity into a dorsal
respiratory section («), containing the opening of the posterior
nares, and a ventral cavity, forming the permanent mouth. The

FIG. 426. LONGITUDINAL
SECTION THROUGH THE BRAIN OF
A YOUNG PRISTIURUS EMBRYO.

«r.unpaired rudimentofthecere-
bral hemispheres \pn. pineal gland ;
/w.infundibulum ; //.ingrowth from
mouth to form the pituitary body ;
mb. mid-brain ; cb. cerebellum ; ch.
notochord; al. alimentary tract;
Zaa. artery of mandibular arch.

THE TEETH.

two divisions thus formed open into a common cavity behind.
The horizontal septum, on the development within it of an
osseous plate, constitutes the hard palate.

An internasal septum (fig. 427, e) may more or less com-
pletely divide the dorsal cavity into two canals, continuous
respectively with the two nasal cavities.

In Mammalia a posterior prolongation of the palate, in which
an osseous plate is not formed, constitutes the soft palate.

The second change in the Amniota, which also takes place in
some Amphibia, is caused by the section of the mesenteron into
which the branchial pouches open,
becoming, on the atrophy of these
structures, converted into the pos-
terior part of the buccal cavity.

The organs derived from the
buccal cavity are the tongue, the
various salivary glands, and the
teeth ; but the latter alone will en-
gage our attention here.

The teeth. The teeth are to be
regarded as a special product of the
oral mucous membrane. It has been
shewn by Gegenbaur and Hertwig
that in their mode of development
they essentially resemble the placoid
scales of Elasmobranchii, and that the latter structures extend
in Elasmobranchii for a certain distance into the cavity of the
mouth.

As pointed out by Gegenbaur, the teeth are therefore to be
regarded as more or less specialised placoid scales, whose
presence in the mouth is to be explained by the fact that the
latter structure is lined by an invagination of the epidermis.
The most important developmental point of difference between
teeth and placoid scales consists in the fact, that in the case
of the former there is a special ingrowth of epiblast to
meet a connective tissue papilla which is not found in the
latter.

FIG. 427. DIAGRAM SHEW-
ING THE DIVISION OF THE PRIM-
ITIVE BUCCAL CAVITY INTO THE
RESPIRATORY SECTION ABOVE
AND THE TRUE MOUTH BELOW.
(From Gegenbaur.)

p. palatine plate of superior
maxillary process; m. permanent
mouth ; n. posterior part of nasal
passage; e. internasal septum.

Although the teeth are to be regarded as primitively epiblastic struc-
tures, they are nevertheless found in Teleostei and Ganoidei on the hyoid

THE STOMOD/KUM.

777

and branchial arches ; and very possibly the teeth on some other parts of
the mouth are developed in a true hypoblastic region.

The teeth are formed from two distinct organs, viz. an epithelial cap and
a connective tissue papilla.

The general mode of development, as has been more especially shewn
by the extended researches of Tomes, is practically the same for all Verte-
brata, and it will be convenient to describe it as it takes place in Mam-
malia.

Along the line where the teeth are about to develop, there is formed
an epithelial ridge projecting into the subjacent connective tissue, and
derived from the innermost columnar layer of the oral epithelium. At the
points where a tooth is about to be formed this ridge undergoes special
changes. It becomes in the first place somewhat thickened by the develop-
ment of a number of rounded cells in its interior ; so that it becomes
constituted of (i) an external layer of columnar cells, and (2) a central core
of rounded cells ; both of an epithelial nature. In the second place the
organ gradually assumes a dome-shaped form (fig. 428, e), and covers over a
papilla of the subepithelial connective tissue (p] which has in the meantime
been developed.

From the above epithelial structure, which may be called the enamel
organ, and from the papilla it covers, which
maybe spoken of as the dental papilla,
the whole tooth is developed. After these
parts have become established there is formed
round the rudiment of each tooth a special
connective tissue capsule ; known as the
dental capsule.

Before the dental capsule has become
definitely formed the enamel organ and the
dental papilla undergo important changes.
The rounded epithelial cells forming the core
of the enamel organ undergo a peculiar trans-
formation into a tissue closely resembling
ordinary embryonic connective tissue, while
at the same time the epithelium adjoining
the dental papilla and covering the inner
surface of the enamel organ, acquires a some-
what different structure to the epithelium
on the outer side of the organ. Its cells
become very markedly columnar, and form
a very regular cylindrical epithelium. This
layer alone is concerned in forming the
enamel. The cells of the outer epithelial
layer of the enamel organ become somewhat
flattened, and the surface of the layer is raised into a series of short papilla?
which project into the highly vascular tissue of the dental sheath. Between

FIG. 428. DIAGRAM SHEW-
ING THE DEVELOPMENT OF THE
TEETH. (From Gegenbaur.)

p. dental papilla ; e. enamel
organ.

778 THE PROCTOD/EUM.

the epithelium of the enamel organ and the adjoining connective tissue
there is everywhere present a delicate membrane known as the membrana
praeformativa.

The dental papilla is formed of a highly vascular core and a non-vascular
superficial layer adjoining the inner epithelium of the enamel organ. The
cells of the superficial layer are arranged so as almost to resemble an
epithelium.

The first formation of the hard structures of the tooth commences at
the apex of the dental papilla. A calcification of the outermost layer of
the papilla sets in, and results in the formation of a thin layer of dentine.
Nearly simultaneously a thin layer of enamel is deposited over this,
from the inner epithelial layer of the enamel organ (fig. 428). Both
enamel and dentine continue to be deposited till the crown of the tooth has
reached its final form, and in the course of this process the enamel
organ is reduced to a thin layer, and the whole of the outer layer of the
dental papilla is transformed into dentine — while the inner portion remains
as the pulp.

The root of the tooth is formed later than the crown, but the enamel
organ is not prolonged over this part, so that it is only formed of dentine.

By the formation of the root the crown of the tooth becomes pushed
outwards, and breaking through its sack projects freely on the surface.

The part of the sack which surrounds the root of the tooth gives rise
to the cement, and becomes itself converted into the periosteum of the
dental alveolus.

The general development of the enamel organs and dental papillae is
shewn in the diagram (fig. 428). From the epithelial ridge three enamel
organs are represented as being developed. Such an arrangement may
occur when teeth are successively replaced. The lowest and youngest
enamel organ (e) has assumed a cap-like form enveloping a dental papilla,
but no calcification has yet taken place.

In the next stage a cap of dentine has become formed, while in the
still older tooth this has become covered by a layer of enamel. As may be
gathered from this diagram, the primitive epithelial ridge from which the
enamel organ is formed is not necessarily absorbed on the formation of a
tooth, but is capable of giving rise to fresh enamel organs. When the
enamel organ has reached a certain stage of development, its connection
with the epithelial ridge is ruptured (fig. 428).

The arrangement represented in fig. 428, in which successive enamel
organs are formed from the same epithelial ridge, is found in most Verte-
brata except the Teleostei. In the Teleostei, however (Tomes), a fresh
enamel organ grows inwards from the epithelium for each successively
formed tooth.

The Proctodceuni.

In all Vertebrata the cloacal section of the alimentary tract
which receives the urinogenital ducts is placed in communication

THE PROCTOD/EUM.

779

with the exterior by means of an epiblastic invagination, consti-
tuting a proctodseum.

This invagination is not usually very deep, and in most
instances the boundary wall between it and the hypoblastic
cloaca is not perforated till considerably after the perforation of the
stomodseum ; in Petromyzon, however, its perforation is effected
before the mouth and pharynx are placed in communication.

The mode of formation of the proctodaeum, which is in
general extremely simple, is illustrated by fig. 420 an.

In most forms the original boundary between the cpiblast of
the proctodaeum and the hypoblast of the primitive cloaca
becomes obliterated after the two have become placed in free
communication.

FIG. 429. DIAGRAMMATIC LONGITUDINAL SECTION THROUGH THE POSTERIOR
END OF AN EMBRYO BlRD AT THE TIME OF THE FORMATION OF THE ALLANTOIS.

ep. epiblast ; Sp.c. spinal canal ; ch. notochord ; n.e. neurenteric canal ; hy, hypo-
blast ; 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. peri-
visceral cavity ; am. amnion ; so. somatopleure ; sp. splanchnopleure.

In Birds the formation of the proctodseum is somewhat more compli-
cated than in other types, owing to the outgrowth from it of the bursa
Fabricii.

The proctodseum first appears when the folding off of the tail end of
the embryo commences (fig. 429, an} and is placed near the front (originally
the apparent hind) end of the primitive streak. Its position marks out the
front border of the postanal section of the gut.

The bursa Fabricii first appears on the seventh day (in the chick), as a
dorsal outgrowth of the proctodaeum. The actual perforation of the sep-
tum between the proctodeeum and the cloacal section of the alimentary tract
is not effected till about the fifteenth day of fcetal life, and the approxi-

780 BIBLIOGRAPHY.

mation of the epithelial layers of the two organs, preparatory to their
absorption, is partly effected by the tunneling of the mesoblastic tissue
between them by numerous spaces.

The hypoblastic section of the cloaca of birds, which receives the open-
ings of the urinogenital ducts, is permanently marked off by a fold from
the epiblastic section or true proctodaeum, with which the bursa Fabricii
communicates.

BIBLIOGRAPHY.
Alimentary Canal and its appendages.

(561) B. Afanassiew. "Ueber Bau u. Entwicklung d. Thymus d. Saugeth."
Archivf. mikr. Anat. Bd. xiv. 1877.

(562) Fr. Boll. Das Princip d. Wachsthums. Berlin, 1876.

(563) E. Gasser. "Die Entstehung d. Cloakenoffnung bei Hiihnerembryonen."
Archivf. Anat. u. Physiol., Anat. Abth. 1880.

(564) A. Gotte. Beilrdge zur Entivicklungsgeschichle d. Darmkanah im
Hiihnchen. 1867.

(565) W. Millie r. "Ueber die Entwickelung der Schilddriise." Jenaische
Zeitschrift, Vol. vi. 1871.

(566) W. Miiller. "Die Hypobranchialrinne d. Tunicaten." Jenaische Zeit-
schrift, Vol. VII. 1872.

(567) S. L. Schenk. "Die Bauchspeicheldriise d. Embryo." Anatomisch-
physiologische Untcrsuchungen. 1872.

(568) E. Selenka. " Beitrag zur Entwicklungsgeschichte d. Luftsacke d.
Huhns." Zeit.f. wiss. Zool. 1866.

(569) L. Stieda. Untersuch. iib. d. Entwick. d. Glandula Thymus, Glandula
thyroidea,u. Glandula car otica. Leipzig, 1881.

(570) C. Fr. Wolff. " De formatione intestinorum." Nov. Comment. Akad.
Petrop. 1766.

(571) H. Wolfler. Ueb. d. Entwick. u. d. Bau d. Schilddriise. Berlin, 1880.
Vide also Kolliker (298), Gotte (296), His (232 and 297), Foster and Balfour (295),

Balfour (292), Remak (302), Schenk (303), etc.

Teeth.

(572) T. H. Huxley. "On the enamel and dentine of teeth." Quart. J. of
Micros. Science, Vol. in. 1855.

(573) R. Owen. Odontography . London, 1840 — 1845.

(574) Ch. S. Tomes. Manual of dental anatomy, human and comparative.
London, 1876.

(575) Ch. S. Tomes. " On the development of teeth." Quart. J. of Micros.
Science, Vol. xvi. 1876.

(576) W. Waldeyer. " Structure and development of teeth." Strieker's His-
tology. 1870.

Vide also Kolliker (298), Gegenbaur (294), Hertwig (306), etc.

INDEX TO VOLUME III.

Abdominal muscles, 675

Abdominal pore, 626, 749

Acipenser, development of, 102; affinities
of, 1 1 8 ; comparison of gastrula of, 279 ;
pericardial cavity of, 627

Actinotrocha, 373

Air-bladder of Teleostei, 77; Lepidosteus,
117; blood supply of, 645 ; general ac-
count of, 763 ; homologies of, 766

Alciope, eye of, 480

Alisphenoid region of skull, 569

Alimentary canal and appendages, deve-
lopment of, 754

Alimentary tract ofAscidia, 18; Molgula,
22; Pyrosoma, 24; Salpa, 31 ; Elasmo-
branchii, 52; Teleostei, 75; Petromy-
zon, 93, 97; Acipenser, no; Amphi-
bia, 129, 136; Chick, 167; respiratory
region of, 754; temporary closure of
oesophageal region of, 759

Allantois, development of in Chick, 191,
198; blood-vessels of in Chick, 193;
Lacerta, 205, 209; early development of
in Rabbit, 229, of Guinea-pig, 264;
origin of, 309. See also ' Placenta ' and
'Bladder''

Alternation of generations in Ascidians,
origin of, 35 ; in Botryllus, 35 ; Pyro-
soma, 36; Salpa, 36; Doliolum, 36

Alytes, branchial chamber of, 136; yolk-
sack of, 139; branchiae, 141 ; Miillerian
duct of, 710

Amblystoma, ovum of, 120; larva of, 142,

H3

Amia, ribs of, 561

Ammocoetes, 95; metamorphosis of, 97;

eye of, 498
Amnion, early development of in Chick,

185; later history of in Chick, 196;

Lacerta, 204, 210; Rabbit, 229; origin

of, 3.07. 3°9

Amphibia, development of, 120; vivi-
parous, 121; gastrula of, 277; suctorial
mouth of, 317; cerebellum of, 426; in-
fundibulum of, 431; pineal gland of,
433; cerebrum of, 439; olfactory lobes
of, 444; nares of, 553; notochord and
its sheath, 548; vertebral column of,
554; ribs of, 561 ; branchial arches of,
574; mandibular and hyoid arches of,
582 ; columella of, 582 ; pectoral girdle
of, 605; pelvic girdle of, 607; limbs of,
619; heart of, 638; arterial system of,
f>45 ; venous system of, 655 ; excretory

system of, 707 ; vasa efierentia of, 711;
liver of, 769; postanal gut of, 774;
stomodaeum of, 778

Amphiblastula larva of Porifera, 344

Amphioxus, development of, i ; gastrula
of, 275 ; formation of mesoblast of, 292 ;
development of notochord of, 293; head
of, 314; spinal nerves of, 461; ol-
factory organ of, 462 ; venous system
of, 651; transverse abdominal muscle
°f> 673; generative cells of, 748; liver
of, 769; postanal gut of, 772; stomo-
daeum of, 777

Amphistylic skulls, 578

Angular bone, 594

Anterior abdominal vein, 653

Anura, development of, 121; epiblast of,
125; mesoblast of, 128; notochord of,
128; hypoblast of, 129; general growth
of embryo of, 131; larva of, 134; ver-
tebral column of, 556 ; mandibular arch
of, 584

Anus of Amphioxus, 7 ; Ascidia, 18; Py-
rosoma, 28 ; Salpa, 31 ; Elasmobranchii,
57; Amphibia, 130, 132; Chick, 167;
primitive, 324

Appendicularia, development of, 34

Aqueductus vestibuli, 519

Aqueous humour, 497

Arachnida, nervous system of, 409; eye
of, 481

Area, embryonic, of Rabbit, 218; epiblast

of, 219; origin of embryo from, 228

area opaca of Chick, 150; epiblast,

hypoblast, and mesoblast of, 159
area pellucida of Chick, 150; of La-
certa, 202

area vasculosa of Chick, 194; meso-
blast of, 1 60; of Lizard, 209; Rabbit,
228, 229

Arteria centralis retinas, 503

Arterial system of Petromyzon, 97; con-
stitution of in embryo, 643 ; of Fishes,
644; of Amphibia, 645; of Amniota, 647

Arthropoda, head of, 313 ; nervous system
of, 409 ; eye of, 480 ; excretory organs
of, 688

Articular bone of Teleostei, 581 ; of Sau-
ropsida, 588

Ascidia, development of, 9

Ascidians. See 'Tunicata'

Ascidiozooids, 25

Atrial cavity of Amphioxus, 7; Ascidia,
18; Pyrosoma, 24

782

INDEX.

Atrial pore of Amphioxus, 7; Ascidia, 20;
Pyrosoma, 28 ; Salpa, 32

Auditory capsules, ossifications in, 595,
59.6

Auditory involution of Elasmobranchii,
57; Teleostei, 73; Petromyzon, 89,
92; Acipenser, 106; Lepidosteus, 114;
Amphibia, 127; Chick, 170

Auditory nerve, development of, 459

Auditory organs, of Ascidia, 15; of Salpa,
31; of Ammocoetes, 98; Ganoidei, 108,
114; of Amphibia, 127; of Aves, 170;
general development of, 512; of aquatic
forms, 512; of land forms, 513; of
Ccelenterata, 513; of Mollusca, 515;
of Crustacea, 516; of Vertebrata, 517;
of Cyclostomata, 89, 92, 518; of Te-
leostei, Lepidosteus and Amphibia,
518; of Mammalia, 519; accessory
structures of, 527; ofTunicata, 528

Auriculo-ventricular valves, 642

Autostylic skulls, 579

Aves, development of, 145; cerebellum
of, 426; midbrain of, 427; infundi-
bulum of, 431; pineal gland of, 434;
pituitary body of, 436; cerebrum of,
439 ; olfactory lobes of, 444 ; spinal
nerves of, 449 ; cranial nerves of, 455 ;
vagus of, 458; glossopharyngeal of,
458; vertebral column of, 557; ossifi-
cation of vertebral column of, 558;
branchial arches of, 572, 573; pectoral
girdle of, 603; pelvic girdle of, 608;
heart of, 637 ; arterial system of, 647 ;
venous system of, 658; muscle-plates
of, 670; excretory organs of, 714; me-
sonephros of, 715; pronephros of, 718;
Miillerian duct of, 718, 720; nature of
pronephros of, 721 ; connection of Miil-
lerian duct with Wolffian in, 720 ;
kidney of, 722; lungs of, 764; liver of,
769; postanal gut of, 774

Axolotl, 142, 143; ovum of, 120; mid-
brain of, 427; mandibular arch of, 583

Basilar membrane, 524

Basilar plate, 565

Basipterygium, 612

Basisphenoid region of skull, 569

Bilateral symmetry, origin of, 373-376

Bile duct, 770

Bladder, Amphibia, 131 ; of Amniota, 726

Blastodermic vesicle, of Rabbit, first de-
velopment of, 217; of 7th day, 222;
Guinea-pig, 263; meaning of, 291

Blastoderm of Pyrosoma, 24; Elasmo-
branchii, 41; Chick, 150; Lacerta 202

Blastopore, of Amphioxus, 2; of Ascidia,
II ; Elasmobranchii, 42, 54, 62 ; Petro-
myzon, 87; Acipenser, 104 ; Amphibia,
125, 130; Chick, 153; Rabbit, 216;
true Mammalian, 226; comparative
history of closure of, 284, 288; sum-
mary of fate of, 340; relation of to
primitive anus, 324

Blood-vessels, development of, 633

Body cavity, of Ascidia, 2 1 ; Molgula, 2 1 ;
Salpa, 31; Elasmobranchii, 47 ; of Te-
leostei, 75 ; Petromyzon, 94 ; Chick,
169; development of in Chordata, 325;
views on origin of, 356 — 360, 377; of
Invertebrata, 623; of Chordata, 624;
of head, 676

Bombinator, branchial chamber of, 136;
vertebral column of, 556

Bonellia, excretory organs of, 687

Bones, origin of cartilage bones, 542 ;
origin of membrane bones, 543; de-
velopment of, 543; homologies of mem-
brane bones, 542 ; homologies of carti-
lage bones, 545

Brachiopoda, excretory organs of, 683 ;
generative ducts of, 749

Brain, of Ascidia, IT, 15; Elasmobran-
chii, 56, 59, 60; Teleostei, 77; Petro-
myzon, 89, 92 ; Acipenser, 105 ; Lepid-
osteus, 113; early development of in
Chick, 170; flexure of in Chick, 175;
later development of in Chick, 176;
Rabbit, 229, general account of deve-
lopment of, 419; flexureof, 420; histo-
geny of, 422

Branchial arches, prseoral, 570; disap-
pearance of posterior, 573; dental plates
of in Teleostei, 574; relation of to
head cavities, 571 ; see ' Visceral arches'

Branchial chamber of Amphibia, 136

Branchial clefts, of Amphioxus, 7 ; of
Ascidia, 18, 20; Molgula, 23; Salpa,
32; of Elasmobranchii, 57, 59 — 01;
Teleostei, 77; Petromyzon, 91, 96;
Acipenser, 105; Lepidosteus, 114, 116;
Amphibia, 132, 133; Chick, 178;
Rabbit, 231; praeoral, 312, 318; of
Invertebrata, 326; origin of, 326

Branchial rays, 574

Branchial skeleton, development of, 572,
592; of Petromyzon, 96, 312, 571; of
Ichthyopsida, 572; dental plates of in
Teleostei, 574; relation of to head
cavities, 572

Branchiae, external of Elasmobranchii, 6r,
62; of Teleostei, 77; Acipenser, 107;
Amphibia, 127, 133, 135

Brood-pouch, of Salpa, 29 ; Teleostei, 68,
Amphibia, 12 1

Brown tubes of Gephyrea, 686

Bulbus arteriosus, of Pishes, 638 ; Am-
phibia, 639

Bursa Fabricii, 167, 779

Canalis auricularis, 639
Canalis reuniens, 521
Capitellidre, excretory organs of, 683
Carcharias, placenta of, 66
Cardinal vein, 652
Carnivora, placenta of, 250
Carpus, development of, 620
Cartilage bones of skull, 595 ; homologies
of, 595

INDEX.

783

Cat, placenta of, 250

Caudal swellings of Elasmobranchii, 46,

55; Teleostei, 72; Chick, 162, 170
Cephalic plate of Elasmobranchii, 55
Cephalochorda, development of, i
Cephalopoda, eyes of, 473 — 477
Cerebellum, Petromyzon, 93; Chick, 176;

general account of development of, 424,

425

Cerebrum of Petromyzon, 93, 97; Chick,
175 ; general development of, 429, 438;
transverse fissure of, 443
Cestoda, excretory organs of, 68 1
Cetacea, placenta, 255
Chtetognatha, nervous system of, 349;
eye of, 479 ; generative organs of, 743 ;
generative ducts of, 749
Chcetopoda, head of, 313; eyes of, 479;
excretory organs of, 683; generative
organs of, 743 ; generative ducts of, 749
Charybdnea, eye of, 472
Cheiroptera, placenta of, 244
Cheiropterygium, 618; relation of to ich-

thyopterygium, 621

Chelonia, development of, 210; pectoral
girdle of, 603 ; arterial system of, 649
Chick, development of, 145 ; general
growth of embryo of, 1 70 ; rotation of
embryo of, 173; fcetal membranes of,
185; epiblast of, 150, 166; optic nerve
and choroid fissure of, 500

Chilognatha, eye of, 481

Chilopoda, eye of, 481

Chimasra, lateral line of, 539 ; vertebral
column of, 548; nares of, 533

Chiromantis, oviposition of, 121

Chorda tympani, development of, 460

Chordata, ancestor of, 311; branchial
system of, 312; evidence from Ammo-
cuetes, 312; head of, 312; mouth of,
318; table of phylogeny of, 327

Chorion, 237; villi of, 237, 257

Choroid coat, Ammoccetes, 99; general
account of, 487

Choroid fissure, of Vertebrate eye, 486,
493 ; of Ammocoetes, 498 ; comparative
development of, 500; of Chick, 501;
of Lizards, 501 ; of Elasmobranchii,
502 ; of Teleostei, 503 ; Amphibia, 503 ;
Mammals, 503, 504

Choroid gland, 320

Choroid pigment, 489

Choroid plexus, of fourth ventricle, 425 ;
of third ventricle, 432 ; of lateral ven-
tricle, 442

Ciliated sack of Ascidia, 18; Pyrosoma,
26; Salpa, 31

Ciliary ganglion, 461

Ciliary muscle, 490

Ciliary processes, 488; comparative de-
velopment of, 506

Clavicle, 600

Clitoris, development of, 727

Clinoid ridge, 569

Cloaca, 766

Coccygeo-mesenteric vein, 66 1

Cochlear canal, 519

Coecilia, development of, 143; pronephros
of, 707; mesonephros of, 709; Mill
lerian duct of, 710

Coelenterata, larvae of, 367 ; eyes of, 47 1 ;
auditory organs of, 513; generative
organs of, 741

Columella auris, 529; of Amphibia, 582 ;
of Sauropsida, 588

Commissures, of spinal cord, 417; of
brain, 431, 432, 439, 443

Coni vasculosi, 724

Conus arteriosus, of Fishes, 638; of Am-
phibia, 638

Coracoid bone, 599

Cornea, of Ammocretes, 99 ; general de-
velopment of, 495 ; corpuscles of, 496 ;
comparative development of, 499; of
Mammals, 499

Coronoid bone, 595

Corpora geniculata interna, 428

Corpora quadrigemina, 428

Corpora striata, development of, 437

Corpus callosum, development of, 443

Corti, organ of, 522; structure of, 525;
fibres of, 525 ; development of, 526

Cranial flexure, of Elasmobranchii, 58,
60; of Teleostei, 77; Petromyzon, 93,
94; of Amphibia, 131, 132; Chick,
174; Rabbit, 231; characters of, 321;
significance of, 322

Cranial nerves, development of, 455;
relation of to head cavities, 461 ; an-
terior roots of, 462 — 464; view on
position of roots of, 466

Crocodilia, arterial system of, 649

Crura cerebri, 429

Crustacea, nervous system of, 41 1 ; eye of,
481; auditory organs of, 515; genera-
tive cells of, 745 ; generative ducts of,

75»

Cupola, 524

Cutaneous muscles, 676

Cyathozooid, 25

Cyclostomata, auditory organs of, 517;
olfactory organ of, 532; notochord and
vertebral column of, 546, 549; abdo-
minal pores of, 626 ; segmental duct of,
700 ; pronephros of, 700 ; mesonephros
of, 700 ; generative ducts of, 733, 749 ;
venous system of, 651 ; excretory organs
of, 700

Cystignathus, oviposition of, 122

Dactylethra, branchial chamber of, 136;

branchise of, 136; tadpole of, 140
Decidua reflexa, of Rat, 242 ; of Insecti-

vora, 243; of Man, 245
Deiter's cells, 526
Dental papilla, 777
Dental capsule, 777
Dentary bone, 595
Dentine, 780
Descemet's membrane, 496

784

INDEX.

Diaphragm, 631 ; muscle of, 676

Dipnoi, nares of, 534; vertebral column
of, 548; membrane bones of skull of,
592 ; heart of, 638 ; arterial system of,
645 ; excretory system of, 707 ; stomo-
dseum of, 777

Diptera, eye of, 481

Discophora, excretory organs of, 687

Dog, placenta of, 248

Dohni, on relations of Cyclostomata, 84 ;
on ancestor of Chordata, 311, 319

Doliolum, development of, 28

Ductus arteriosus, 649

Ductus Botalli, 648

Ductus Cuvieri, 654

Ductus venosus Arantii, 663

Dugong, heart of, 642

Dysticus, eye of, 481

Ear, see ' Auditory organ '

Echinodermata, secondary symmetry of
larva of, 380; excretory organs of, 689 ;
generative ducts of, 752

Echinorhinus, lateral line of, 539; verte-
bral column of, 548

Echiurus, excretory organs of, 686

Ectostosis, 543

Edentata, placenta of, 248, 250, 256

Eel, generative ducts of, 703

Egg-shell of Elasmobranchii, 40 ; Chick,
146

Elasmobranchii, development of, 40; vi-
viparous, 40; general features of de-
velopment of, 55 ; gastrulaof, 281 ; de-
velopment of mesoblast of, 294 ; noto-
chord of, 294 ; meaning of formation of
mesoblast of, 295; restiform tracts of,
425 ; optic lobes of, 427 ; cerebellum of,
425 ; pineal gland of, 432 ; pituitary
body of, 435 ; cerebrum of, 438 ; olfac-
tory lobes of, 444 ; spinal nerves, 449 ;
cranial nerves of, 457; sympathetic
nervous system of, 466; nares of, 533;
lateral line of, 539; vertebral column of,
549 ; ribs of, 560 ; parachordals of, 567 ;
mandibular and hyoid arches of, 576 ;
pectoral girdle of, 600 ; pelvic girdle of,
607; limbs of, 609; pericardial cavity
of, 627; arterial system of, 644 ; venous
system of, 65 1 ; muscle-plates of, 668 ;
excretory organs of, 690 ; constitution
of excretory organs in adult of, 697;
spermatozoa of, 747 ; swimming-blad-
der of, 763 ; intestines of, 767 ; liver of,
769; postanal gut of, 772

Elrcoblast of Pyrosoma, 28; Salpa, 30

Elephant, placenta of, 249

Embolic formation of gastrula, 333

Enamel organ, 777

Endolymph of ear, 522

Endostosis, 543

Endostyle of Ascidia, 18, 759; Pyrosoma,
25; Salpa, 32

Epiblast, of Elasmobranchii, 47 ; Tele-
ostei, 71, 75; Petromyzon, 86; Lcpid-

osteus, 112; Amphibia, 122, 125;
Chick, 149, 166; Lacerta, 203; Rabbit,
216, 219; origin of in Rabbit, 221 ;
comparative account of development
of, 300

Epibolic formation of gastrula, 334

Epichordal formation of vertebral column,
556

Epicrium glutinosum, 143

Epidermis, in Ccelenterata, 393; protec-
tive structures of, 394

Epididymis, 724

Epigastric vein, 653

Episkeletal muscles, 676

Episternum, 602

Epoophoron, 725

Ethmoid bone, 597

Ethmoid region of skull, 570

Ethmopalatine ligament of Elasmo-
branchs, 576

Euphausia, eye of, 483

Eustachian tube, of Amphibia, 135;
Chick, 1 80; Rabbit, 232; general
development of, 528

Excretory organs, general constitution of,
680; of Platyelminthes, 680; of Mol-
lusca, 681; of Polyzoa, 682; of Bra-
chiopoda, 683 ; of Choetopoda, 683 ; of
Gephyrea, 686 ; of Discophora, 687 ; of
Arthropoda, 688; of Nematoda, 689;
of Echinodermata, 689 ; constitution of
in Craniata, 689; of Elasmobranchii,
690; constitution of in adult Elasmo-
branch, 697; of Petromyzon, 700; of
Myxine, 701 ; of Teleostei, 701 ; of
Ganoidei, 704; of Dipnoi, 707; of
Amphibia, 707; of Amniota, 713;
comparison of Vertebrate and Inverte-
brate, 737

Excretory system, of Elasmobranchii, 49 ;
Teleostei, 78; Petromyzon, 95, 98;
Acipenser, 99; Amphibia, 133

Exoccipital bone, 595

Exoskeleton, dermal, 393 — 395 ; epider-
mal, 393—396

External generative organs, 726

Extra-branchial skeleton, 572

Eye, of Ascidia, 16; Salpa, 31; Elasmo-
branchii, 56, 57, 58; Teleostei, 73;
Petromyzon, 92, 98; Aves, i/o; Rab-
bit, 229; general development of, 470;
evolution of, 470, 471; simple, 480;
compound, 481 ; aconous, 482; pseudo-
conous, 482 ; of Invertebrata, 471; of
Vertebrata, 483 ; comparative develop-
ment of Vertebrate, 497 ; of Ammo-
ccetes, 497 ; of Tunicata, 507 ; of Chor-
data, general views on, 508 ; accessory
eyes of Fishes, 509; muscles of, 677

Eyelids, development of, 506

Falciform ligament, 757

Falx cerebri, 439

Fasciculi terctes, of Elasmobranchii. 426

Feathers, development of, 396

INDEX.

785

Fenestra rotunda and ovalis, 529

Fertilization, of Amphioxus, 2 ; of Uro-
chorda, 9; Salpa, 29; Elasmobranchii,
46; of Teleostei, 68; Petromyzon, 84 ;
Amphibia, 120; Chick, 145 ; Reptilia,
•202 ; meaning of, 331

Fifth nerve, development of, 460

Fifth ventricle, 443

Fins, of Elasmobranchii, 62 ; Teleostei,
78; Petromyzon, 94, 95; Acipenser,
109; Lepidosteus, 118; relation of
paired to unpaired, 611, 612 ; develop-
ment of pelvic, 614; development of
pectoral, 615; views on nature of paired
fins, 616

Fissures of spinal cord, 417

Foetal development, 360 ; secondary va-
riations in, 361

Foot, 618

Foramen of Munro, 430, 438

Foramen ovale, 642

Forebrain, of Elasmobranchii, 55, 59, 60;
Petromyzon, 93 ; general development
of, 428

Formative cells, of Chick, 154

Fornix, development of, 443

Fornix of Gottsche, 428

Fourth nerve, 464

Frontals, 592

Fronto-nasal process of Chick, 179

Gaertner's canals, 724

Gall-bladder, 770

Ganoidei, development of, 102; relations
of, 118; nares of, 534; notochord of,
546 ; vertebral column of, 546, 553 ;
ribs of, 561 ; pelvic girdle of, 606; arte-
rial system of, 645 ; excretory organs
of, 704; generative ducts of, 734

Gastropoda, eye of, 472

Gastrula, of Amphioxus, 2; of Ascidia, lo;
Elasmobranchii, 43, 44 ; Petromyzon,
86; Acipenser, 103; Amphibia, 123;
comparative development of, in Inver-
tebrata, 275 ; comparison of Mamma-
lian, 291 ; phylogenetic meaning of, 333 ;
ontogeny of (general), 333 ; phylogeny
of, 338 — 343 ; secondary types of, 34!

Geckos, vertebral column of, 557

Generative cells, development of, 74! ;
origin of in Ccelenterata, 741 ; of In-
vertebrata, 743 ; of Vertebrata, 746

Generative ducts, of Teleostei, 704, 735 ;
of Ganoids, 704; of Cyclostomata, 733;
origin of, 733 ; of Lepidosteus, 735,
750 ; development and evolution of,
748 ; of Ccelenterata, 748 ; of Sagitta,
749 ; of Tunicata, 749 ; Cheetopoda,
Gephyrea, etc., 749; of Mollusca, 751;
of Discophora, 751 ; of Echinodermata,

75*

Generative system of Elasmobranchii, 51
Gephyrea, nervous system of, 412; excre-
tory organs of, 686 ; generative cells of,
743 ; generative ducts of, 749

B. III.

Germinal disc, of Elasmobranchii, 40;
Teleostei, 68 ; Chick, 147

Germinal epithelium, 746

Germinal layers, summary of organs <lr-
rived from, in Vertebrata, 304 ; his-
torical account of views of, 332 ; homo-
logies of in the Metazoa, 345

Germinal wall of Chick, 152, 159; struc-
ture and changes of, 160

Geryonia, auditory organ of, 5 r 5

Gill of Salpa, 31

Giraldes, organ of, 725

Glands, epidermic, development of, 397

Glomerulus, external, of Chick, 716

Glossopharyngeal nerve, development of,

456> 457
Grey matter of spinal cord, 417; of brain,

423
Growth in length of Vertebrate embryo,

306
Guinea-pig, primitive streak of, 223;

notochord of, 226 ; placenta of, 242 ;

development of, 262
Gymnophiona, see ' Ccecilia '

Habenula perforata, 525

Hairs, development of, 396

Halichrerus, placenta of, 250

Hand, 619

Head, comparative account of, 313; seg-
mentation of, 314

Head cavities, of Elasmobranchii, 50 ;
Petromyzon, 90, 96; Amphibia, 127;
general development of, 676

Head-fold of Chick, 157, 167

Head kidney, see ' Pronephros '

Heart, of Pyrosoma, 25; Elasmobranchii,
50, 58 ; Petromyzon, 94, 97 ; Acipen-
ser, 106; Chick, 170 ; first appearance
of in Rabbit, 230; general development
of, 633 ; of Fishes, 635, 637 ; of Mam-
malia, 638; of Birds, 637, 639; mean-
ing of development of, 637 ; of Amphi-
bia, 638 ; of Amniota, 639 ; change of
position of, 643

Hind-brain, Elasmobranchii, 55, 59, 60 ;
Petromyzon, 93 ; general account of,
424

Hippocampus major, development of, 442

Hirudo, development of blood-vessels of,
633 ; excretory organs of, 688

Horse, placenta of, 253

Hyaloid membrane, 492

Hylodes, oviposition of, 1 21 ; metamor-
phosis of, -1 37

Hyobranchial cleft, 572

Hyoid arch, of Chick, 179; general ac-
count of, 572, 575 ; modifications of,
e!73> 577 > °f Elasmobranchii, 576; of
Teleostei, 577 ; of Amphibia, 582 ;
of Sauropsida, 588; of Mammalia,

589

Hyomandibular bar of Elasmobranchii,
576, 577 ; of Teleostei, 579 ; of Am-
phibia, 582

50

;86

INDEX.

Hyomandibular cleft, of Fetromyzon, 91 ;
Chick, 179 ; general account of, 572

Hyostylic skulls, 582

Hypoblast of Elasmobranchii, 5! ; Tele-
ostei, 71, 75; Petromyzon, 86; Acipen-
ser, 104; Lepidosteus, 113; Amphibia,
122, 129; Chick, 151, 167 ; Lacerta,
203; Rabbit, 215, 216, 219 ; origin of
in Rabbit, 220

Hyposkeletal muscles, 675

Ilyrax, placenta of, 249

Incus, 529, 590

Infraclavicle, 600

Infundibulum of Petromyzon, 92 ; Chick,
175 ; general development of, 430

Insectivora, placenta of, 243

Insects, nervous system of, 410 ; eye of,
481; generative organs of, 745; gene-
rative ducts of, 751

Intercalated pieces of vertebral column,

551

Interclavicle, homologies of, 602

Intermediate cell-mass of Chick, 183

Intermuscular septa, 672

Interorbital septum, 570

Interrenal bodies, 665

Iris, 489 ; comparative development of,

506

Iris of Ammoccetes, 98
Island of Reil, 444

Jacobson's organ, 537
Jugal bone, 594

Kidney, see ' Metanephros '

Labia majora, development of, 727

Labial cartilages, 597

Labium tympanicum, 525 ; vestibulare,

525

Lacertilia, general development of, 202 ;
nares of, 537 ; pectoral girdle of, 603 ;
pelvic girdle of, 607 ; arterial system
of, 649

Lacrymal bone, 593

Lacrymal duct, 506

Lacrymal glands, 506

Lremargus, vertebral column of, 548

Lagena, 524

Lamina spiralis, 524

Lamina terminalis, 438

Larva of Amphioxus, 2 ; of Ascidia, 1 5 —
it ; Teleostei, 81 ; Petromyzon, 89, 95;
Lepidosteus, 117, 318; Amphibia, 134,
142; types of, in the Invertebrata, 363

Larvre, nature, origin, and affinities of,
360 — 386; secondary variations of less
likely to be retained, 362 ; ancestral
history more fully recorded in, 362 ;
secondary variations in development of,
363 ; ontogenetic record of secondary
variations in, 361; of freshwater and
land animals, 362; types of, 36.2; phos-
phorescence of, 364; of Coelenterata,

367 ; table of, 365 ; of Invertebrata,
367 et seq.

Larynx, 766

Lateral line sense organs, 538 ; compari-
son of, with invertebrate, 538 ; develop-
ment of, in Teleostei, 538 ; develop-
ment of, in Elasmobranchii, 539

Lateral ventricle, 438 ; anterior cornu of,
440 ; descending cornu of, 440 ; choroicl
plexus of, 443

Layers, formation of, in Elasmobrancliii,
41, 56 ; Teleostei, 71 ; Petromyzon,
85 ; Acipenser, 103 ; Lepidosteus, 1 1 1 ;
Amphibia, 121; Chick, 150, 152;
Lacerta, 202; Rabbit, 215 — 227; com-
parison of Mammalia with lower forms,
226, 289; comparison of formation of
in Vertebrata, 275; origin and homolo-
gies of, in the Metazoa, 331

Leech, see ' Hirudo '

Lemuridre, placenta, 256

Lens, of Elasmobranchii, 57, 58 ; Pe-
tromyzon, 94, 99; Acipenser, 106 ;
Lepidosteus, 115 ; Amphibia, 127 ;
Chick, 177 ; of Vertebrate eyes, 485 ;
general account of, 493 ; capsule of, 493 ;
comparative development of, 499 ; of
Amphibia, Teleostei, Lepidosteus, 499

Lepidosteus, development of, 1 1 1 ; larva
of, 117; relations of, 119; spinal nerves
of, 455; ribs of, 561 ; generative ducts
of, 704, 735 ; swimming-bladder of,

763

Ligamentum pectinatum, 490

Ligamentum suspensorium, 557, 558

Ligamentum vesicse medium, 239

Limbs, of Elasmobranchii, 59 ; Teleostei,
80 ; first appearance of in Chick,
184 ; Rabbit, 232 ; muscles of, 673 ; of
Fishes, 609; relation of, to unpaired fins
of Fishes, 611, 612; of Amphibia, 61 8

Liver of Teleostei, 78 ; Petromyzon, 95,
96; Acipenser, no; Amphibia 130;
general account of, 769

Lizard, development of, 202; general
growth of embryo of, 208 ; Mullerian
duct of, 721

Lizzia, eye of, 471

Lobi inferiores, 431

Lungs of Amphibia, 137 ; development
of, 763 ; homology of, 766

Lymphatic system, 664

Malleus, 529, 591 ; views on, 591
Malpighian bodies, development of acces-
sory in Elasmobranchs, 695
Mammalia, development of, 214; com-
parison of gastrula of, 291 ; cerebellum
of, 427 ; infundibulum of, 431 ; pineal
gland of, 434; pituitary body of, 436;
cerebrum of, 439 ; spinal nerves of, 449 ;
sympathetic of, 466; vertebral column
of, 558; branchial arches of, 573, 574;
mandibular and hyoid arches of, 589 ;
pectoral girdle of, 604; pelvic girdle of,

INDEX.

787

608 ; heart of, 636 ; arterial system of,
647; venous system of, 661 ; muscle-
plates of, 671 ; mesonephros of, 714;
testicular network of, 724 ; urinogenital
sinus of, 727 ; spermatozoa of, 747 ;
lungs of, 765 ; intestines of, 768 ; liver
of> 769; postanal gut of, 774; stomo-
dseum of, 775

Mammary gland, development of, 398
Man, placenta of, 244 ; general account of
development of, 265 ; characters of em-
bryo of, 270

Mandibular arch of Elasmobranchii, 62,
576; Petromyzon, 91 ; Acipenser, 106,
116; Chick, 179; general account of,

572, 575; modification of to form jaws,

573, 575; of Teleostei, 580; of Am-
phibia, 582; Sauropsida, 588; Mam-
malia, 589

Mandibular bar, evolution of, 311, 321

Manis, placenta of, 256

Marsupial bones, 608

Marsupialia, foetal membranes of, 240 ; ce-
rebellum of, 426 ; corpus callosum of,
' 443 ; uterus of, 726

Maxilla, 594

Meatus auditorius externus, of Chick, 181;
development of, 527

Meckelian cartilage, of Elasmobranchii,
576; of Teleostei, 581 ; of Amphibia,
584, 585; of Sauropsida, 588 ; of Mam-
malia, 590

Mediastinum anterior and posterior, 630

Medulla oblongata, of Chick, 176 ; gene-
ral development of, 425

Medullary plate of Amphioxus, 4, 5 ; of
Ascidia, n; Elasmobranchii, 44, 47,
55; Teleostei, 72; Petromyzon, 88;
Acipenser, 104; Lepidosteus, 1 1 1 ; Am-
phibia, 126, 127, 131; Chick, 159;
Lacerta, 204; Rabbit, 223, 227, 228;
primitive bilobed character of, 303, 317

Medusae, auditory organs of, 513

Membrana capsulo-pupillaris, 494, 504,

507

Membrana elastica externa, 546

Membrana limitans of retina, 491

Membrana tectoria, 522, 525

Membrane bones, of Amphibia, 582 ; of
Sauropsida, 588; of Mammalia, 590;
of mandibular arch, 593 ; of pectoral
girdle, 599, 602 ; origin of, 592 ; ho-
mologies of, 593

Membranous labyrinth, development of
in Man, 519

Menobranchus, branchial arches of, 142

Mesenteron of Elasmobranchii, 43 ; Tele-
ostei, 75 ; Petromyzon, 85 ; Acipenser,
104; Amphibia, 123, 124, 129; Chick,
167; general account of, 754

Mesentery, 626, 756

Mesoblast, of Amphioxus, 6 ; Ascidia,
17, 20; Pyrosoma, 24; Salpa, 30;
Elasmobranchii, 44, 47; Teleostei, 75;
Petromyzon, 86; Acipenser, 105; Lepi-

dosteus, 113; Amphibia, 125, 128, 129;
of Chick, 154, 167; double origin of in
Chick, 154, 158, 159; origin of from
lips of blastopore in Chick, 158; of
area vasculosa of Chick, iOo; Lacerta,
203; origin of in Rabbit, 218, 223; of
area vasculosa in Rabbit, 227; com-
parative account of formation of, 292 ;
discussion of development of in Verte-
brata, 297 ; meaning of development
of in Amniota, 298; phylogenetic origin
of, 346 ; summary of ontogeny of, 349
— 352 ; views on ontogeny of, 352 —360

Mesoblastic somites, of Amphioxus, 6 ;
Elasmobranchii, 48, 55 ; Petromyzon,
88 ; Acipenser, 105 ; Lepidosteus,
114; Amphibia, 129, 131; Chick,
161, 1 80; Rabbit, 228; development
of in Chordata, 325; meaning of de-
velopment of, 331; of head, 676

Mesogastrium, 758

Mesonephros, of Teleostei, 78, 702; Pe-
tromyzon, 95, 98, 700; Acipenser, 1 10,
705; Amphibia, 134, 708; Chick, 184,
714; general account of, 690 ; develop-
ment of in Elasmobranchs, 691 ; of
Cyclostomata, 700 ; Ganoidei, 705 ;
sexual and non-sexual part of in Am-
phibia, 710; of Amniota, 713, 724;
summary and general conclusions as
to, 729; relation of to pronephros, 731

Mesopterygium, 616

Metagenesis of Ascidians, 34

Metamorphosis of Amphibia, 137, 140

Metanephros, 690; development of in
Elasmobranchii, 697; of Amphibia,
712; of Amniota, 713; of Chick, 722;
of Lacertilia, 723; phylogeny of, 736

Metapterygium, 616

Metapterygoid, of Elasmobranchii, 576;
of Teleostei, 581

Metazoa, evolution of, 339, 342 ; ancestral
form of, 333, 345

Mid-brain, of Elasmobranchii, 55, 58,
59; Petromyzon, 92; general account
of development of, 427

Moina, generative organs of, 745

Molgula, development of, 22

Mollusca, nervous system of, 414 ; eyes of,
472; auditory organs of, 515; excre-
tory organs of, 68 1

Monotremata, foetal membranes of, 240 ;
cerebellum of, 426; corpus callosum
of, 443 ; cerebrum of, 443 ; urinogeni-
tal sinus of, 726

Mormyrus, generative ducts of, 704

Mouth, of Amphioxus, 7; of Ascidia, 18;
Pyrosoma, 27; Salpa, 31; Elasmo-
branchii, 57, 60, 61, 62; Petromyzon,
92, 94, 95, 99; Acipenser, 107; Lepi-
dosteus, 118; Amphibia, 129, 132,
"134; Rabbit, 231 ; origin of, 317

Mouth, suctorial, of Petromyzon, 99;
Acipenser, 107; Lepidosteus, 116, 317;
Amphibia, 133, 141, 317

;88

INDEX.

Mullerian duct, 690; of Elasmobranchs,
693 ; of Ganoids, 704 ; of Amphibia,
710; of Aves, 717,720; opening of in-
to cloaca, 727; origin of, 733; sum-
mary of development of, 733; relation
of to pronephros, 733

Muscle-plates, of Amphioxus, 6; Elas-
mobranchii, 49, 668 ; Teleostei, 670 ;
Petromyzon, 94; Chick, 183, 670; gene-
ral development of, 669 ; of Amphibia,
670; Aves, 670; of Mammalia, 671;
origin of muscles from, 672

Muscles, of Ascidia, II, 17; development
of from muscle-plates, 672; of limbs,
673 ; of head, 676 ; of branchial arches,
678; of eye, 678

Muscular fibres, epithelial origin of, 667

Muscular system, development of, 667;
of Chordata, 668

Mustelus, placenta of, 66

Myoepithelial cells, 667

Mysis, auditory organ of, 517

Myxine, ovum of, loo; olfactory organ
of, 533 ; portal sinus of, 652 ; excretory
system of, 701

Nails, development of, 397

Nares, of Acipenser, 108; of Ichthyop-
sida, 534; development of in Chick,
535; development of in Lacertilia, 537;
development of in Amphibia, 537

Nasal bones, 592

Nasal pits, Acipenser, 108; Chick, 176;
general development of, 531

Nematoda, excretory organs of, 689 ;
generative organs of, 745 ; generative
ducts of, 752

Nemertines, nervous system of, 311 ; ex-
cretory organs of, 68 1

Nerve cord, origin of ventral, 378

Nerves, spinal, 449 ; cranial, 455 — 466

Nervous system, central, general account
of development of in Vertebrata, 415 ;
conclusions as to, 445; sympathetic,
466

Nervous system, of Amphioxus, 4; As-
cidia, 15, 16; Molgula, 22; Pyrosoma,
24, 25; Salpa, 30, 31; Elasmobranchii,
44; Teleostei, 77 ; Petromyzon, 89, 93;
Acipenser, 105; Amphibia, 126; com-
parative account of formation of central,
301; of Sagitta, 349; origin of in
Ccelenterata, 349; of pneoral lobe,
377, 380; evolution of, 400—405; de-
velopment of in Invertebrates, 406;
of Arthropoda, 408; of Gephyrea, 412;
Mollusca, 414

Neural canal, of Ascidia, 10; Teleostei,
72; Petromyzon, 88; Acipenser, 105;
Lepidosteus, 114; Amphibia, 126, 131 ;
Chick, 1 66, 171 ; Lacerta, 208; closure
of in Frog and Amphioxus, 279; closure
of in Elasmobranchii, 284; phylogcuc-
tic origin of, 316

Neural crest, 449, 456, 457

Neurenteric canal, of Amphioxus, 4, 5 ;
Ascidia, lo; Elasmobranchii, 54; Pe-
tromyzon, 88 ; Acipenser, 105 ; Lepi-
dosteus, 113; Aves, 162; Lacerta, 203,
206; general account of, 323; meaning
of, 323

Newt, ovum of, 120; development of,
I255 general growth of, 141

Notidanus, vertebral column of, 548;
branchial arches of, 572

Notochord of Amphioxus, 6; Ascidia,
II, 17; Elasmobranchii, 51; Teleostei,
74; Petromyzon, 86, 94; Acipenser,
104; Lepidosteus, 113; Amphibia, 128,
129; Chick, 157; canal of, in Chick,
163; Lacerta, 204, 205; Guinea-pig,
226; comparative account of formation
of, 292, 325; sheath of, 545; later
histological changes in, 546; cartilagin-
ous sheath of, 547; in head, 566;
absence of in region of trabeculas, 567

Notodelphys, brood-pouch of, 121 ; bran-
chiae of, 140

Nototrema, brood-pouch of, 121

Nucleus pulposus, 559

Oceania, eye of, 471

Occipital bone, 595

CEsophagus, solid, of Elasmobranchii,
61, 759; of Teleostei, 78

Olfactory capsules, 571

Olfactory lobes, development of, 444

Olfactory nerves, Ammoccetes, 99; gene-
ral development of, 464

Olfactory organ, of aquatic forms, 531;
Insects and Crustacea, 531; of Tuni-
cata, 532 ; of Amphioxus, 532 ; of
Vertebrata, 533; Petromyzon, 533;
of Myxine, 533

Olfactory sacks, of Elasmobranchii, 60;
Teleostei, 73; Petromyzon, 92, 97;
Acipenser, 106, 108; Lepidosteus, 116;
Chick, 176

Oligochreta, excretory organs of, 683

Olivary bodies, 426

Omentum, lesser and greater, 757

Onchidium, eye of, 473

Opercular bones, 593

Operculum, of Teleostei, 77; Acipenser,
107; Lepidosteus, 117, 118; Amphibia,

r3.5.

Ophidia, development of, 210; arterial
system of, 649 ; venous system of, 656

Optic chiasma, 430, 493

Optic cup, retinal part of, 488 ; ciliary
portion of, 489

Optic lobes, 428

Optic nerve, development of, 492 ; compa-
rative development of, 500

Optic thalami, development of, 431

Optic vesicle, of Elasmobranchii, 57 — 59;
Teleostei, 74, 499 ; Petromyzon, 89, 92 ;
Acipenser, 106; Lepidosteus, 115;
Chick, 170; Rabbit, 229; general de-
velopment of, 429 ; formation of secon-

INDKX.

7*9

dary, 487 ; obliteration of cavity of, 488 ;
comparative development of, 499; of
Lepidosteus and Teleostei, 499. See
also ' Eye '

Ora serrata, 488

Orbitosphenoid region of skull, 570

Organs, classification of, 391 ; derivation
of from germinal layers, 392

Orycteropus, placenta of, 249

Otic process of Axolotl, 583; of Frog,
585 et seq.

Otoliths, 512

Oviposition, of Amphioxus, i ; Elasmo-
branchii, 40; Teleostei, 68; Petromy-
zon, 84; Amphibia, 121; Reptilia, 202

Ovum, of Amphioxus, i; Pyrosoma, 23;
Elasmobranchii, 40; Teleostei, 68;
Petromyzon, 83 ; Myxine, loo; Acipen-
ser, 102; Lepidosteus, in; Amphibia,
120; Chick, 146; Reptilia, 202 ; Mam-
malia, 214; of Porifera, 741; migra-
tion of in Ccelenterata, 742; Verte-
brata, 746

Palatine bone, of Teleostei, 580; origin
of, 594

Pancreas, Acipenser, no; general de-
velopment of, 770

Pancreatic caeca, of Teleostei, etc. 768

Papillae, oral, of Acipenser, 108; Lepi-
dosteus, n6

Parachordals, 565, 566

Parasphenoid bone, 594

Parepididymis, 725

Parietal bones, 592

Paroophorori, 725

Parovarium, 725

Pectoral girdle, 599 ; of Elasmobranchs,
600; of Teleostei, 600; of Amphibia
and Amniota, 60 1 ; comparison of with
pelvic, 608

Pecten, eye of, 479

Pecten, of Ammoccetes, 498; of Chick,
501 ; Lizard, 501 ; Elasmobranchs, 501

Pedicle, of Axolotl, 484 ; of Frog, 485

Pelobates, branchial apertures of, 136;
vertebral column of, 556

Pelodytes, branchial chamber of, 135

Pelvic girdle, 606; of Fishes, 606; Am-
phibia and Amniota, 607 ; of Lacerti-
lia, 607 ; of Mammalia, 608 ; compari-
son with pectoral, 608

Penis, development of, 727

Peribranchial cavity, of Amphioxus, 7;
of Ascidia, 18; Pyrosoma, 24

Pericardial cavity, of Pyrosoma, 26 ; Elas-
mobranchii, 49 ; Petromyzon, 94; gene-
ral account of, 626; of Fishes, 627 ; of
Amphibia, Sauropsida and Mammalia,
628

Perichordal formation of vertebral column,
5^6

Perilymph of ear, 523
Periotic capsules, ossifications in, 595,
596

Peripatus, nervous system of, 409 ; eye of
480 ; excretory organs of, 688

Peritoneal membrane, 626

Petromyzon, development of, 83; affini-
ties of, 83, 84; general development
of, 87; hatching of, 89; comparison of
gastrula of, 280; branchial skeleton of,
312, 572; cerebellum of, 425; pineal
gland of, 434 ; pituitary body of, 436 ;
cerebrum of, 439; auditory organ of,
517; olfactory organ of, 533; compari-
son of oral skeleton of with Tadpole,
586; pericardial cavity of, 627; abdo-
minal pores of, 626 ; venous system of,
651 ; excretory organs of, 700; segmen-
tal duct of, 700; pronephros of, 700;
mesonephros of, 700 ; thyroid body of,
760; postanalgut of, 774; stomodx-um

of, 775

Phosphorescence of larvae, 364

Phylogeny, of the Chordata, 327; of the
Metazoa, 384

Pig, placenta of, 251; mandibular and
hyoid arches of, 589

Pineal gland, of Petromyzon, 93 ; Chick,
175; general development of, 432;
nature of, 432, 434

Pipa, brood-pouch of, 121 ; metamorpho-
sis of, 139; yolk-sack of, 140; vertebral
column of, 556

Pituitary body, of Rabbit, 231 ; general
development of, 435 ; meaning of, 436 ;
Placenta, of Salpa, 29; Elasmobran-
chii, 66; of Mammalia, 232; villi of,
235 ; deciduate and non-deciduate, 239;
comparative account of, 239 — 259 ; cha-
racters of primitive type of, 240; zo-
nary, 248; non-deciduate, 250; histo-
logy of, 257; evolution of, 259

Placoid scales, 395

Planorbis, excretory organs of, 68 1

Planula, structure of, 367

Pleural cavities, 631

Pleuronectidae, development of, 80

Pneumatoccela, characters of, 327

Polygordius, excretory organs of, 684

Polyophthalmus, eye of, 479

Polypedates, brood-pouch of, 121

Polyzoa, excretory organs of, 682 ; gene-
rative cells of, 745 ; generative ducts

of, 751

Pons Varolii, 426, 427

Pori abdominales, Ammoccetes, 99

Porifera, ancestral form of, 345 ; develop-
ment of generative cells of, 74!

Portal vein, 653

Postanal gut of Elasmobranchii, 58, 59,
60; Teleostei, 75; Chick, 169; gene-
ral account of, 323, 772

Prsemaxilla, 594

Praeopercular bone, 593

Prrcoral lobe, ganglion of, 377, 380

Prefrontals, 597

Presphenoid region of skull, 570

Primitive groove of Chick, 1 55

790

INDEX.

Primitive streak, of Chick, 152, 161;
meaning of, 153; origin of mesoblast
form in Chick, 154; continuity of
hypoblast with epiblast at anterior end
of, in Chick, 156; comparison of with
blastopore, 165 ; fate of, in Chick, 165 ;
of Lacerta, 203; of Rabbit, 221; of
Guinea-pig, 223 ; fusion of layers at, in
Rabbit, 224; comparison of with blas-
topore of lower forms, 226, 287 ; of
Mammalia, 290

Processus falciformis of Ammoccetes, 498 ;
of Elasmobranch, 502 ; of Teleostei , 503
Proctodseum, 778

Pronephros, of Teleostei, 78, 701 ; Pe-
tromyzon, 95, 99, 700; Acipenser, 106,
no; Amphibia, 134, 707; general ac-
count of, 689 ; of Cyclostomata, 700 ;
of Myxine, 701 ; Ganoidei, 705 ; of
Amniota, 714; of Chick, 718; sum-
mary of and general conclusions as to,
728; relation of, to mesonephros, 731 ;
cause of atrophy of, 729
Prootic, 596, 597
Propterygium, 616
Proteus, branchial arches of, 142
Protochordata, characters of, 327
Protoganoidei, characters of, 328
Protognathostomata, characters of, 328
Protopentadactyloidei, characters of, 329
Protovertebrata, characters of, 328
Pseudis, Tadpole of, 139; vertebral

column of, 556

Pseud ophryne, yolk-sack of, 140; Tad-
pole of, 140
Pterygoid bone, of Teleostei, 581; origin

of, 597

Pterygoquadrate bar, of Elasmobranchii,
576; of Teleostei, 581; Axolotl, 584;
Fr°g, 584; ofSauropsida, 588; of Mam-
malia, 589

Pulmonary artery, origin of, 645 ; of
Amphibia, 645 ; of Amniota, 649

Pulmonary vein, 655

Pupil, 489

Pyrosoma, development of, 23

Quadrate bone of Teleostei, 581 ; of
Axolotl, 584; Frog, 585; Sauropsida,
588

Quadratojugal bone, 594

Rabbit, development of, 214; general
growth of embryo of, 227 ; placenta of,
248

Radiate symmetry, passage from to bi-
lateral symmetry, 373 — 376

Raja, caudal vertebras of, 553

Rat, placenta of, 242

Recessus labyrinthi, 519

Reissner's membrane, 524

Reptilia, development of, 202; viviparous,
202; cerebellum of, 426; infundibulum
of, 431; pituitary body of, 436; cere-
brum of, 439; vertebral column of,

556; arterial system of, 648; venous
system of, 656; mesonephros of, 713;
testicular network of, 723; spermatozoa
of, 747

Restiform tracts of Elasmobranchii and
Teleostei, 425

Retina, histogenesis of, 490

Retinulse, 482

Rhabdom, 482

Rhinoderma, brood-pouch of, 121; meta-
morphosis of, 1 39

Ribs, development of, 560

Roseniniiller's organ, 725

Rotifera, excretory organs of, 680

Round ligament of liver, 663

Ruminantia, placenta of, 253

Sacci vasculosi, 437

Sacculus hemisphericus, 519; of Mam-
mals, 519, 520

Sagitta. See ' Chaetognatha'

Salpa, sexual development of, 29; asexual
development of, 33

Salamandra, larva of, 142; vertebral
column of, 553; limbs of, 619; meso-
nephros of, 708; Miillerian duct of,
710

Salmonidse, hypoblast of, 71; generative
ducts of, 704

Sauropsida, gastrula of, 286; meaning of
primitive streak of, 288; blastopore of,
289 ; mandibular and hyoid arches of,
588 ; pectoral girdle of, 60 1

Scala, vestibuli, 522; tympani, 523;
media, 522

Scales, general development of, 396 ; de-
velopment of placoid scales, 395

Scapula, 599

Sclerotic, 488

Scrotum, development of, 727

Scyllium, caudal vertebrse of, 553; man-
dibular and hyoid arches of, 578; pec-
toral girdle of, 600; limbs of, 610; pel-
vic fin of, 614; pectoral fin of, 615

Segmental duct, 690 ; development of in
Elasmobranchs, 690; of Cyclostomata,
700; of Teleostei, 701; of Ganoidei,
704, 705 ; of Amphibia, 707 ; of Am-
niota, 713

Segmental organs, 682

Segmental tubes, 690 ; development of in
Elasmobranchs, 691 ; rudimentary an-
terior in Elasmobranchs, 693 ; develop-
ment of secondary, 731

Segmentation cavity, of Elasmobranchii,
42 — 44; Teleostei, 69, 85, 86; Am-
phibia, 122, 125

Segmentation, meaning of, 331

Segmentation of ovum, in Amphioxus, 2 ;
Ascidia, 9 ; Molgula, 22 ; Pyrosoma,
23; Salpa, 30; Elasmobranchii, 40;
Telostei, 69; Petromyzon, 84; Aci-
penser, IOT, Lcpidosteus, in; Am-
phibia, 122, 124; Newt, 125; Chick,
146; Lizard, 202: Rabbit, 214

INDEX.

791

Semicircular canals, 519

Sense organs, comparative account of
development of, 304

Septum lucidum, 443

Serous membrane, Lacerta, 209; of Rab-
bit, 237

Seventh nerve, development of, 459

Shell-gland of Crustacea, 689

Shield, embryonic, of Chick, 151 ; of
Lacerta, 202

SimiadiK, placenta of, 247

Sinus rhomboidalis, of Chick, 162

Sinus venosus, 637

Sirenia, placenta of, 255

Sixth nerve, 463

Skate, mandibular and hyoid arches of,

577

Skeleton, elements of found in Verte-
brata, 542

Skull, general development of, 564 ; his-
torical account of, 564 ; development of
cartilaginous, 566; cartilaginous walls
of, 570; composition of primitive car-
tilaginous cranium, 565

Somatopleure, of Chick, 170

Spelerpes, branchial arches of, 142

Spermatozoa, of Porifera, 741; of Verte-
brata, 746

Sphenoid bone, 595

Sphenodon, hyoid arch of, 588

Spinal cord, general account of, 415;
white matter of, 415; central canal of,
417, 418; commissures of, 417; grey
matter of, 417; fissures of, 418

Spinal nerves, posterior roots of, 449;
anterior roots of, 453

Spiracle, of Elasmobranchii, 62 ; Acipen-
ser, 105; Amphibia, 136

Spiral valve. See 'Valve'

Spleen, 664

Splenial bone, 595

Squamosal bone, 593

Stapes, 529; of Mammal, 590

Sternum, development of, 562

Stolon of Doliolum, 29 ; Salpa, 33

Stomodaeum, 774

Stria vascularis, 524

Styloid process, 591

Sub-intestinal vein, 65 1 ; meaning of,

651

Syngnathus, brood-pouch of, 68
Subnotochordal rod, of Elasmobranchii,

54; Petromyzon, 94; Acipenser, no;

Lepidosteus, 115; general account of,

754; comparison of with siphon of

Chsetopods, 756

Subzonal membrane, 237; villi of, 236
Sulcus of Munro, 432
Supraclavicle, 600
Suprarenal bodies, 664
Supra-temporal bone, 593
Swimming bladder, see Air bladder
Sylvian aqueduct, 428
Sylvian fissure, 444
Sympathetic ganglia, development of, 467

Tadpole, 134, 139, 140; phylogenetic
meaning of, 137; metamorphosis of,
137; m can ing of suctorial mouth of, 585

Tail of Teleostei, 80; Acipenser, 109;
Lepidosteus, 109; Amphibia, 132

Tarsus, development of, 620

Teeth, horny provisional, of Amphibia,
136; general development of, 776;
origin of, 777

Teleostei, development of, 68; vivipa-
rous, 68; comparison of formation of
layers in, 286; restiform tracts of, 425 ;
mid-brain of, 425 ; infundibulum of,
431 ; cerebrum of, 439; nares of, 534;
lateral line of, 538; notochord and
membrana elastica of, 549 ; vertebral
column of, 553; ribs of, 561; hyoid
and mandibular arches of, 579; pec-
toral girdle of, 601 : pelvic girdle of,
606; limbs of, 618; heart of, 637;
arterial system of, 645; muscle-plates
of, 670; excretory organs of, 701 ; gene-
rative ducts of, 704, 735, 749; swim-
ming bladder of, 763 ; postanal gut of,

Teredo, nervous system of, 414

Test of Ascidia, 14; Salpa, 31

Testicular network, of Elasmobranchs,
697 ; of Amphibia, 712 ; Reptilia, 723 ;
of Mammals, 724

Testis of Vertebrata, 746

Testis, connection of with Wolffian body,
in Elasmobranchii, 697; in Amphibia,
710; in Amniota, 723; origin of, 735

Thalamencephalon of Chick, 175; gene-
ral development of, 430

Third nerve, development of, 461

Thymus gland, 762

Thyroid gland, Petromyzon, 92 ; general
account of, 759; nature of, 760; de-
velopment of in Vertebrata, 761

Tooth. See1 Teeth'

Tori semicirculares, 428

Tornaria, 372

Trabeculas, 565, 567; nature of, 568

Trachea, 766

Trematoda, excretory organs of, 68 1

Triton alpestris, sexual larva of, 143

Triton, development of limbs of, 619}
urinogenital organs of, 712

Truncus arteriosus, 638; of Amphibia,
638; of Birds, 639

Turiicata, development of mesoblast of,
293; test of, 394; eye of, 507; audi-
tory organ of, 530; olfactory organ of,
532; generative duct of, 749 ; intestine
of, 767; postanal gut of, 771; stomo-
dseum of, 775

Turbellaria, excretory organs of, 68 1

Tympanic annulus of *'rog, 587

Tympanic cavity, of Amphibia, 135;
Chick, 1 80; Rabbit, 232; general de-
velopment of, 528; of Mammals, 591

Tympanic membrane, of Chick, 180;
general development of, 528

792

INDEX.

Tympanohyal, 591

Umbilical canal of Elasmobranchii, 54,

57, 58, 59

Umbilical cord, 238; vessels of, 239

Ungulata, placenta of, 250

Urachus, 239, 726

Ureters, of Elasmobranchii, 696; develop-
ment of, 723

Urethra, 727

Urinary bladder of Amphibia, "Jii; of
Amniota, 726

Urinogenital organs, see Excretory or-
gans

Urinogenital sinus of Petromyzon, 700;
of Sauropsida, 726; of Mammalia,
727

Urochorda, development of, 9

Uterus, development of, 726; of Marsu-
pials, 726

Uterus masculinus, 726

Utriculus, 519

Uvea of iris, 489

Vagus nerve, development of, 456, 457;
intestinal branch of, 458; branch of to
lateral line, 459

Valve, spiral, of Petromyzon, 97; Aci-
penser, no; general account of, 767

Valves, semilunar, 641; auriculo-ventri-
cular, 642

Vasa efferentia, of Elasmobranchs, 697 ;
of Amphibia, 711; general origin of,
724

Vascular system, of Amphioxus, 8; Petro-
myzon, 97; Lepidosteus, 116; general
development of, 632

Vas deferens, of Elasmobranchii, 697 ;
of Amniota, 723

Vein, sub-intestinal of Petromyzon, 97 ;
Acipenser, no; Lepidosteus, 116

Velum of Petromyzon, 9 1

Vena cava inferior, development of, 655

Venous system of Petromyzon, 97; gene-
ral development of, 651; of Fishes,
651 ; of Amphibia and Amniota, 655 ;
of Reptilia, 656; of Ophidia, 656; of
Aves, 658; of Mammalia, 661

Ventricle, fourth, of Chick, 176; history
of, 424

Ventricle, lateral, 438, 440; fifth, 443

Ventricle, third, of Chick, 175

Vertebral bodies, of Chick, 183

Vertebral column, development of, 545,
549; epichordal and perichordal de-
velopment of in Amphibia, 556

Vespertilionidse, early development of,
217

Vieussens, valve of, 426

Villi, placental, of zona radiata, 235 ;
subzonal membrane, 235; chorion, 237;

Man, 246; comparative account of,
2575 of young human ovum, 265, 269

Visceral arches, Amphioxus, 7 ; Elasmo-
branchii, 57 — 60; Teleostei, 77; Aci-
penser, 1 06; Lepidosteus, 116; Am-
phibia, 133; Chick, 177; Rabbit,
231; prseoral, 570; relation of to head
cavities, 572; disappearance of pos-
terior, 573; dental plates of in Teleo-
stei, 574

Visual organs, evolution of, 470

Vitelline arteries of Chick, 195

Vitelline veins of Chick, 195

Vitreous humour, of Ammoccetes, 98 ;
general development of, 494; blood*
vessels of in Mammals, 503 ; meso-
blastic ingrowth in Mammals, 503

Vomer, 594

White matter, of spinal cord, 415; of
brain, 423

Wolffian body, see ' Mesonephros '

Wolffian duct, first appearance of in
Chick, 183; general account of, 690;
of Elasmobranchs, 693 ; of Ganoids,
704; of Amphibia, 710; of Amniota,
713; atrophy of in Amniota, 724

Wolffian ridge, 185

Yolk blastopore, of Elasmobranchii, 64

Yolk, folding off of embryo from, in
Elasmobranchii, 55; in Teleostei, 76;
Acipenser, 106; Chick, 168, 170

Yolk nuclei, of Elasmobranchii, 41, 53;
Teleostei, 69, 75

Yolk, of Elasmobranchii, 40; Teleostei,
68; Petromyzon, 96; Acipenser, 109;
Amphibia, 122, 129; Chick, 146; in-
fluence of on formation of layers, 278;
influence of on early development,

341, 342

Yolk-sack, Amphibia, 131, 140, 141; en-
closure of, 123

.Yolk-sack, development of in Rabbit,
227; of Mammalia reduced, 227; cir-
culation of in Rabbit, 233 ; enclosure
of in Sauropsida, 289

Yolk-sack, enclosure of, Petromyzon, 86

Yolk-sack, Lepidosteus, 118

Yolk-sack of Chick, enclosure of, 160;
stalk of, 174; general account of, 193;
circulation of, 195 ; later history of, 198

Yolk-sack of Elasmobranchii, enclosure
of, 62, 283; circulation of, 64

Yolk-sack of Lacerta, 209 ; circulation of,
209

Yolk-sack, Teleostei, 75, 81; enclosure
of, 75 ; circulation of, 81

Zona radiata, villi of, 237
Zonula of Zinn, 495

BIBLIOGRAPHY.

CEPHALOPODA.

(1) A. Kowalevsky. " Entwicklungsgeschichte des Amphioxus lanceolatus."
Mem. Acad. Imper. des Sciences de St Pttersbourg, Series vn. Tom. XI. 1867.

(2) A. Kowalevsky. "Weitere Studien iiber die Entwicklungsgeschichte des
Amphioxus lanceolatus." Archiv f. mikr. Anat., Vol. xui. 1877.

(3) Leuckart u. Pagenstecher. " Untersuchungen tiber niedere Seethiere."
Mutter's Archiv, 1858.

(4) Max Schultze. " Beobachtung junger Exemplare von Amphioxus." Zeit.
f. wiss. Zool., Bd. in. 1851.

(5) A. M. Marshall. "On the mode of Ovi position of Amphioxus." your,
of Anat. and Phys., Vol. x. 1876.

UROCHORDA.

(6) P. J. van Beneden. " Recherches s. 1'Embryogenie, 1'Anat. et la Physiol.
des Ascidies simples." Mem. Acad. Roy. de Belgique, Tom. xx.

(7) W. K. Brooks. "On the development of Salpa." Bull, of the Museum of
Comp. Anat. at Harvard College, Cambridge, Mass.

(8) H. Fol. Eludes surles Appendiculaires du detroit de Mcssine . Geneve et
Bale, 1872.

(9) Ganin. "Neue Thatsachen a. d. Entwicklungsgeschichte d. Ascidien."
Zeit.f. wiss. Zool., Vol. XX. 1870.

(10) C. Gegenbaur. " Ueber den Entwicklungscyclus von Doliolum nebst
Bemerkungen iiber die Larven dieser Thiere." Zeit.f. wiss. Zool., Bd. vu. 1856.

(11) A. Giard. "Etudes critiques des travaux d'embryogenie relatifs a la
parente des Vertebres et des Tuniciers." Archiv Zool. experiment., Vol. I. 1872.

(12) A. Giard. " Recherches sur les Synascidies. " Archiv Zool. exper., Vol. I.
1872.

(13) O. Hertwig. "Untersuchungen lib. d. Bau u. d. Entwicklung des Cellu-
lose-Mantels d. Tunicaten." Jenaische Zeitschrift, Bd. vn. 1873.

(14) Th. H. Huxley. " Remarks upon Appendicularia and Doliolum. " Phil.
Trans., 1851.

(15) Th. H.Huxley. " Observations on the anatomy and physiology of Salpa
and Pyrosoma." Phil. Trans., 1851.

(16) Th. H. Huxley. "Anatomy and development of Pyrosoma." Linnean
Trans., 1860, Vol. XXIII.

(17) Keferstein u. Ehlers. Zoologische Beitrage, 1861. Doliolum.

(18) A. Kowalevsky. "Entwicklungsgeschichte d. einfachen Ascidien." Mem.
Acad. Pctersbourg, VII. serie, T. x. 1866.

(19) A. Kowalevsky. "Beitrag z. Entwick. d. Tunicaten." Nachrichtcn d.
konigl. Gesell.zu Gottingen. 1868.

(20) A. Kowalevsky. "Weitere Studien iib. d. Entwicklung d. einfachen As-
cidien." Archiv f. mikr. Anat., Vol. vn. 1871.

(21) A. Kowalevsky. "Ueber Knospung d. Ascidien." Archiv f. mikr. Anat.,
Vol. X. 1874.

(22) A. Kowalevsky. "Ueber die Entwicklungsgeschichte d. Pyrosoma."
Archiv f. mikr. Anat., Vol. xi. 1875.

(23) A. Krohn. "Ueber die Gattung Doliolum u. ihre Arten." Archiv f.
Natnrgeschichte, Bd. xvm. 1852.

B. Hi. a

BIBLIOGRAPHY.

(24) A. Krohn. "Ueber die Entwicklung d. Ascidien." Mailer's Archiv,
1852.

(25) A. Krohn. "Ueber die Fortpfianzungsverhaltnisse d. Botrylliden. " Archiv
f. Naturgeschichte, Vol. xxxv. 1869.

(26) A. Krohn. "Ueber die fruheste Bildung d. Botryllenstocke." Archiv f.
Naturgeschichte, Vol. xxxv. 1869.

(27) C. Kupffer. " Die Stammverwandschaft zwischen Ascidien u. Wirbelthie-
ren." Archiv f, mikr. Anat., Vol. vi. 1870.

(28) C. Kupffer. "Zur Entwicklung d. einfachen Ascidien." Archiv f. mikr.
Anat., Vol. vm. 187-2.

(29) H. Lacaze Duthiers. "Recherches sur 1'organisation et 1'Embryogenie
des Ascidies (Molgula tubulosa)." Comptes rendus, May 30, 1870, p. 1154.

(30) H. Lacaze Duthiers. "Les Ascidies simples des Cotes de France" (De-
velopment of Molgula). Archiv Zool. exper., Vol. ill. 1874.

(31) R. Leuckart. "Salpa u. Verwandte." Zoologischc Untcrsuchungen,
Heft u.

(32) E. Metschnikoff. " Observations sur le developpement de quelques ani-
maux (Botryllus and Simple Ascidians)." Still, d. fAcad. Petersbottrg, Vol. xm.
1869.

(33) H. Milne-Edwards. "Observations s. 1. Ascidies composees des cotes de
la Manche." Memoir es d. V Instittit, T. xvm. 1842.

(34) W. Salensky. "Ueber d.embryonaleEntwicklungsgeschichtederSalpen."
Zeit.f. wiss. Zool., B. xxvn. 1877.

(35) W. Salensky. "Ueber die Knospung d. Salpen." Morphol. Jahrbuch,
Bd. in. 1877.

(36) W. Salensky. "Ueber die Entwicklung d. Hoden u. iiber den Genera-
tionswechsel d. Salpen." Zeit.f. wiss. Zool., Bd. xxx. Suppl. 1878.

(37) C. Semper. " Ueber die Entstehung d. geschichteten Cellulose-Epidermis
d. Ascidien." Arbeit, a. d. zool.-zoot. Instit. Wiirzburg, Vol. ri. 1875.

(38) Fr. Todaro. Sopra lo sviluppo e F anatomia delle Salpc. Roma, 1875.

(39) Fr. Todaro. "Sui primi fenomeni dello sviluppo delle Salpe." Realc
Accadcmia dci Lincei, Vol. iv. 1880.

ELASMOBRANCHII.

(40) F. M. Balfour. " A preliminary account of the development of the Elasmo-
branch Fishes." Quart. J. of Micr. Science, Vol. xiv. 1876.

(41) F. M. Balfour. "A Monograph on the development of Elasmob ranch
Fishes." London, 1878. Reprinted from the Journal of Anat. and Fhysiol. for 1876,
1877, and 1878.

(42) Z. Gerbe. " Recherches sur la segmentation de la cicatrule et la formation
des prodnits adventifs de Pceuf des Plagiostomes et particulierement des Rates." Vide
also Journal de FAnatomie et de la Physiologic, 1872.

(43) W. His. " Ueb. d. Bildung v. Haifischenembryonen." Zeit. fur Anat. u.
Entwick., Vol. 11. 1877.

(44) A. Kowalevsky. "Development of Acanthias vulgaris and Mustelus
Irevis. " (Russian.) Transactions of the Kiew Society of Naturalists, Vol. I. 1870.

(45) R. Leuckart. "Ueber die allmahlige Bildung d. Korpergestalt bei d.
Rochen." Zeit. f. wiss. Zool., Bd. II., p. 258.

(46) Fr. Ley dig. Rochen u. Hate. Leipzig, 1852.

(47) A. W. Malm. " Bidrag till kannedom om utvecklingen af Rajae." Kongl.
vetenskaps akademiens fo'rhandlingar. Stockholm, 1876.

(48) Joh. M tiller. Clatter Haie des Aristoteles und iiber die Verschiedenheitcn
unler den Haifachen und Rochen in der Entivicklung des Eies. Berlin, 1840.

(49) S. L. Schenk. " Die Eier von Raja quadrimaculata innerhalb der Eileiter."
Sitz. der k. Akad. Wien, Vol. LXXIII. 1873.

(50) Alex. Schultz. " Zur Entwicklungsgeschichte des Selachiereies. " Archiv
fiir micro. Anat., Vol. XI. 1875.

(51) Alex. Schultz. " Beitrag zur Entwicklungsgeschichte d. Knorpelfische. "
Archiv fiir micro. Anat., Vol. xni. 1877.

BIBLIOGRAPHY.

Ill

(52) C. Semper. "Die Stammesverwandschaft d. Wirbelthiere u. Wirlwllo-
sen. Arbeit, a. d. zool.-zoot. Instit. Wiirzburg, Vol. II. 1875.

(53) C. Semper. " Das Urogenitalsystem d. Plagiostomen, etc." Arbeit, a. d.
zool.-zoot. Instit. Wiirzburg, Vol. n. 1875.

(54) Wyman. " Observations on the Development of Raja batis." Memoirs of
the American Academy of Arts and Sciences, Vol. ix. 1864.

TELEOSTEI.

(55) Al. Agassiz. " On the young Stages of some Osseous Fishes. I. Deve-
lopment of the Tail." Proceedings of the American Academy of Arts and Sciences,
Vol. xin. Presented Oct. n, 1877.

(56) Al. Agassiz. "II. Development of the Flounders." Proceedings of the
American Acad. of Arts arid Sciences, Vol. xiv. Presented June, 1878.

(57) K. E. v. Baer. Untersuchungen ilber die Entwicklungsgeschichte der Fische.
Leipzig, 1835.

(58) Ch. van Bambeke. "Premiers effets de la fecondation sur les c«ufs de
Poissons: sur 1'origine et la signification du feuillet muqueux on glandulaire chez les
Poissons Osseux." Comptes Rendus des Seances de f Academic des Sciences, Tome

1872.

Ch. van Bambeke. " Recherches sur 1'Embryologie des Poissons
Mem. couronnes et Mem. de savants etrangers de rAcademie roy. Bclgiqne,

LXXIV.

(59)

Osseux. '
Vol. XL.

(60)

E. v. Beneden. "A contribution to the history of the Embryonic deve-
lopment of the Teleosteans." Quart. J. of Micr. Sci., Vol. xvm. 1878.

(61) E. Calberla. " Zur Entwicklung des Medullarrohres u. d. Chorda
dorsalis d. Teleostier u. d. Petromyzonten." Morphologisches Jahrbuch, Vol. III.
1877.

(62) A. Gotte. "Beitrage zur Entwicklungsgeschichte der Wirbelthiere."
Archiv f. mikr. Anat., Vol. IX. 1873.

(63) A. Gotte. " Ueber d. Entwicklung d. Central-Nervensystems der Teleos-
tier." Archiv f. mikr. Anat., Vol. xv. 1878.

(64) A. Gotte. " Entwick. d. Teleostierkeime." Zoologischer Anzeiger, No. 3.
1878.

(65) W. His. " Untersuchungen Uber die Entwicklung von Knochenfischen, etc."
Zeit.f. Anat. it. Entwicklungsgeschichte, Vol. I. 1876.

(66) W. His. "Untersuchungen Uber die Bildung des Knochenfischembryo
(Salmen)." Archiv f. Anat. u. Physiol., 1878.

(67) E. Klein. "Observations on the early Development of the Common
'Trout." Quart. J. of Micr. Science, Vol. XVI. 1876.

~^* (68) C. Kupffer. " Beobachtungen Uber die Entwicklung der Knochenfische."
Archiv f. mikr. Anat., Bd. iv. 1868.

(69) C. Kupffer. Ueber Laichen u. Entwicklung des Ostsee-Herings. Berlin,
1878.

(70) M. Lereboullet. "Recherches sur le developpement du brochet de la
perche et de 1'ecrevisse." Annales des Sciences Nat., Vol. I., Series iv. 1854.

(71) M. Lereboullet. " Recherches d'Embryologie comparee sur le developpe-
ment de la Truite." An. Sci. Nat., quatrieme serie, Vol. XVI. 1861.

(72) T. Oellacher. " Beitrage zur Entwicklungsgeschichte der Knochenfische
nach Beobachtungen am Bachforellenei." Zeit. f. wiss. Zool., Vol. xxn., 1872, and'
Vol. xxni., 1873.

(72*) H. Rathke. Abh. z. Bildung u. Entwick. d. Menschen u. Thiere. Leipzig,
1832-3. Part n. Blennius.

(73) Reineck. " Ueber die Schichtung des Forellenkeims." Archiv f. mikr.
Anat., Bd. V. 1869.

(74) S. Strieker. "Untersuchungen Uber die Entwicklung der Bachforelle."
Sitzungsberichte der Wiener k. Akad. d. Wiss., 1865. Vol. LI. Abth. 2.

(75) Carl Vogt. " Embryologie des Salmones." Histoire Naturelle des Poissons
de F Europe Centrale. L. Agassiz. 1842.

(76) C.Weil. " Beitrage zur Kenntniss der Knochenfische." Silzungsbcr. do-
Wiener kais. Akad. der Wiss., Bd. i.xvi. 1872.

a 2

BIBLIOGRAPHY.

CYCLOSTOMATA.

(77) E. Calberla. " Der Befruchtungsvorgang beim Petromyzon Planeri."
Zeit.f. iviss. Zool., Vol. xxx. 1877.

(78) E. Calberla. "Ueb. d. Entwicklung d. Medullarrohres u. d. Chorda
clorsalis d. Teleostier u. d. Petromyzonten." Morpholog. Jahrbuch, Vol. in. 1877.

(79) C. Kupffer u. B. Benecke. Der Vorgang d. Befruchtimg am Ei d.
Neunaugen. Konigsberg, 1878.

(80) Aug. Muller. " Ueber die Entwicklung d. Neunaugen." Miiller s
Archiv, 1856.

(81) Aug. Muller. Beobachtungen iib. d. Befruchtungserscheinungen im Ei d.
Neunaugen. Konigsberg, 1864.

(82) W. Muller. "Das Urogenitalsystem d. Amphioxus u. d. Cyclostomen. '
Jcnaische Zeitschrift, Vol. IX. 1875.

(83) Ph. Owsjannikoff. "Die Entwick. von d. Flussneunaugen. " ^ Vorlauf.
Mittheilung. Melanges Biologiqttcs tires du Bulletin de VAcad. Imp. St Pttersbourg,
Vol. vn. 1870.

(84) Ph. Owsjannikoff. On the development of Petromyzon fiuviatihs
(Russian).

(85) Anton Schneider. Beitrdge z. vergleich. Anat. a. Entwick. d. Wirbel-
thiere. Quarto. Berlin, 1879.

(86) M. S. Schultze. "Die Entwickl. v. Petromyzon Planeri." Gekronte
Preisschrift. Haarlem, 1856.

(87) W. B. Scott. " Vorlaufige Mittheilung iib. d. Entwicklungsgeschichte d.
Petromyzonten." Zoologischer Anzeiger, Nos. 63 and 64. ill. Jahrg. 1880.

GANOIDEI.
A cipenseridce.

(88) Knock. "Die Beschr. d. Reise z. Wolga Behufs d. Sterlettbefruchtung. "
Bull. Soc. Nat. Moscow, 1871.

(89) A. Kowalevsky, Ph. Owsjannikoff, and N. Wagner. "Die Entwick.
d. Store." Vorlauf. Mittheilung. Melanges Biologizes tires du Bulletin d. VAcad.
Imp. St Petersbowg, Vol. VII. 1870.

(90) W. Salensky. "Development of the Sterlet (Acipenser ruthenus)." 2 Parts.
Proceedings of the Society of Naturalists in the imperial University of Kasan. 1878 and 9
(Russian). Part I., abstracted in Hoffmann and Schwalbe's Jahresbcricht for 1878.

(91) W. Salensky. " Zur Embryologie d. Ganoiden (Acipenser)." Zoolo-
gischer Anzeiger, Vol. I., Nos. n, 12, 13.

Lepidosteidce.

(92) Al. Agassiz. "The development of Lepidosteus." Proc. Amer. Acad. of
Arts and Sciences, Vol. xm. 1878.

AMPHIBIA.

(93) Ch. van Bambeke. " Recherches sur le developpement du Pelobate
brun." Mc/noires coitronncs, etc. de I1 Acad. roy. de Belgique, 1868.

(94) Ch. van Bambeke. "Recherches sur 1'embryologie des Batraciens."
/!iill,-tin dc V Acad. roy. de Belgique, 1875.

(95) Ch. van Bambeke. " Nouvelles recherches sur 1'embryologie des Batra-
ciens." Archives de Biologic, Vol. I. 1880.

' (9G) K. E. von Baer. " Die Metamorphose des Eies der Batrachier." Miiller's
Archiv, 1834.

(97) B. Benecke. "Ueber die Entwicklung des Erdsalamanders." Zoolo-
.» isch er An zeiger, 1880.

BIBLIOGRAPHY,

(98) S. F. Clarke. "Development of Amblystoma punctatuin," 1'art I. I
lernal. Studies from the Biological Laboratory of the Johns Hopkins University,
No. II. 1880.

(99) H. Cramer. "Bemerkungen iib. d. Zellenleben in d. Entwick. d.
cheies." Mliller's Archiv, 1848.

(100) A. Ecker. Icones Physiolog. 1851 — 1859.

(101) A. Gotte. Die Entivicklungsgeschichte der Unkc. Leipzig, 1875.

(102) C. K. Hoffmann. "Amphibia." Klassen u. Ordnungen d. T/iierrdchs,
1873—1879.

(103) T. H. Huxley. Article "Amphibia in the Encyclopedia Jintannica.

(104) A. Moquin-Tandon. "Developpement des Batraciens anures." Annales
des Sciences Naturelles, III. 1875.

(105) G. Newport. " On the impregnation of the Ovum in Amphibia " (three
memoirs). Phil. Trans. 1851, 1853, and 1854.

(106) W. K. Parker. " On the structure and development of the Skull of the
common Frog." Phil. Trans., CLXI. 1871.

(107) W. K. Parker. " On the structure and development of the Skull of the
Batrachia." Phil. Trans., Vol. CXLVI., Part 2. 1876.

(108) W. C. H. Peters. " Ueber die Entwicklung der Coecilien und besonders
von Coecilia compressicauda." Berlin. Monatsbericht, p. 40, 1874.

(109) W. C. H. Peters. "Ueber die Entwicklung der Coecilien." Berl.
Monalsbericht, p. 483, 1875.

(110) J. L. Prevost and J. B. Dumas. " Deuxieme Mem. s. 1. generation.
Developpement de 1'ceuf d. Batraciens." Ann. Set. Nat. II. 1824.

(111) R. Remak. Untersuchungm iiber die Entwicklung der Wirbelthicn;
1850—1858.

(112) M. Rusconi. Developpement de la grenouille commune depuis le moment de
sa naissance jusqu '« son etat parfait, 1826.

(113) M. Rusconi. Histoire naturelle, developpement et metamorphose de la
Salamandre terrestre, 1854.

(114) W. B. Scott and H. F. Osborn. "On the early development of the
common Newt." Quart. J. of Micr. Science, Vol. xxix. 1879.

(115) S. Strieker. " Entwicklungsgeschichte von Bufo cinereus." Sitzb. der
kaiserl. Acad. zu Wien, 1860.

(116) S. Strieker. " Untersuchungen iiber die ersten Anlagen in Batrachier-
Eiern." Zeitschrift f. wiss. Zoologie, Bd. xi. 1861.

AVES.

(117) K. E. vonBaer. " Ueb. Entwickhmgsgeschichte d. Thiere." Konigsberg,
1828—1837.

(118) F. M. Balfour. "The development and growth of the layers of the
Blastoderm," and "On the disappearance of the Primitive Groove in the Embryo
Chick." Quart. J. of Micros. Science, Vol. xin. 1873.

(119) M. Braun. " Die Entwicklung d. Wellenpapagei's." Parti. Arbeit, d.
zool.-zoot. Instit. Wiirzburg. Vol. V. 1879.

(120) M. Braun. "Aus d. Entwick. d. Papageien; I. Riickenmark ; II.
Entwicklung d. Mesoderms; III. Die Verbindungen zwischen Riickenmark u. Darm
bei Vogeln." Verh. d. phys.-med. Ges. zu Wurzbtirg. N. F. Bd. XIV. and xv. 1879
and 1880.

(121) J. Disse. " Die Entwicklung des mittleren Keimblattes im Htirmerei.
Archiv fur mikr. Anat., Vol. xv. 1878.

(122) J. Disse. "Die Entstehung d. Blutes u. d. ersten Gefasse im Hiihnerei.'
Archiv f. mikr. Anat., Vol. xvi. 1879.

(123) Fr. Durante. "Sulla struttura della macula germinativa delle uova di
Gallina." Ricerche nel Laboratorio di Anatomia della R. Universita di Roma.

(124) E. Dursy. Der Primitivstreif des Hiihnchens. 1867.

(125) M. Duval. "Etude sur la ligne primitive de 1'embryon de Poulet.
Annales des Sciences Naturelles, Vol. vn. 1879.

(126) M. Foster and F. M. Balfour. Elements of Embryology. Part I.
London, 1874.

vi BIBLIOGRAPHY.

(127) Gasser. "Der Primitivstreifen bei Vogelembryonen." Schrifteti d.
Gescll. zur Befbrd. d. gesammten Naturwiss. zu Marburg, Vol. II. Supplement l.
1879.

(128) A. Gotte. " Beitrage zur Entwicklungsgeschichte d. Wirbelthiere. II.
Die Bildung d. Keimblatter u. d. Blutes im Hiihnerei." Archiv fiir mikr. Anat.,
Vol. x. 1874.

(129) V. Hensen. " Embryol. Mitth." Archiv f. mikr. Anat., Vol. in.
1867.

(130) W. His. Untersuch. iib. d. erste Anlage d, Wirbelthierleibes. Leipzig,
1868.

(131) W. His. Unsere Kbrperform tmd das physiol. Problem ihrer Entstehung.
Leipzig, 1875.

(132) W. His. "Der Keimwall des Hiihnereies u. d. Entstehung d. parablas-
tischen Zellen." Zeit.f. Anat.u. Entwicklungsgeschichte. Bd. I. 1876.

(133) W. His. " Neue Untersuchungen iib. die Bildung des Hiihnerembryo I."
Archiv f. Anat. u. Phys. 1877.

(134) E. Klein. "Das mittlere Keimblatt in seiner Bezieh. z. Entwick. d. ers.
Blutgefiisse und Blutkorp. im Hiihnerembryo." Sitzungsber. Wien. Akad., Vol. LXIII.
1871.

(135) A. K6 Hiker. Entwicklungsgeschichte d. Menschen u. d. hbheren 7'hii'rc.
Leipzig, 1879.

(136) C. Kupffer. " Die Entsteh. d. Allantois u. d. Gastrula d. Wirbelth."
Zoolog. Anzeiger, Vol. II. 1879, PP- 52O> 593> 612.

(137) C. Kupffer and B. Benecke. " Photogramme z. Ontogenie d.
Vogel." Nov. Act. d. k. Leop.-Carol.-Deutschen Akad. d. Naturforscher, Vol. XLI.
1879.

(138) J. Oellacher. "Untersuchungen tiber die Furchung u. Blatterbildung
im Hiihnerei." Strieker's Studien. 1870.

(139) C. H. Pander. Beitrage z. Entwick. d. Hiinchens im Eie. Wiirzburg,
1817.

(140) A. Rauber. " Ueber die Etnbryonalanlage des Hiihnchens." Centralblatt
fur d. medic. Wissenschaften. 1874 — 75.

(141) A. Rauber. Ueber die Stellung des Hiihnchens im Entwickhingsplan.
1876.

(142) A. Rauber. " Primitivrinne und Urmund. Beitrage zur Entwicklungs-
geschichte des Hiihnchens." Morphol. Jahrbuch, B. II. 1876.

(143) A. Rauber. Primitivslreifen und Neurula der Wirbelthiere in normalcr
und pathologischer Beziehung. 1877.

(144) R. Remak. Untersuch. iib. d. Entwicklung d. Wirbelthiere. Berlin,
1850—55.

(145) S. L. Schenk. "Beitrage z. Lehre v. d. Organanlage im motorischen
Keimblatt.'' Sitz. Wien. Akad., Vol. LVII. 1860.

(146) S. L. Schenk. " Beitrage z. Lehre v. Amnion." Archiv f. mikr. Anat.,
Vol. vii. 1871.

(147) S. L. Schenk. Lehrbuch d. vergleich. Embryol. d. Wirbelthiere. Wien,
1874.

(148) S. Strieker. " Mittheil. iib. d. selbststiindigen Bewegungen embryonaler
Zellen." Sitz. Wien. Akad., Vol. XLIX. 1864.

(149) S. Strieker. "Beitrage zur Kenntniss des Hiihnereies." Wiener Sit-
zungsber., Vol. LIV. 1866.

(150) H. Virchow. Ueber d. Epithel d. Dottersackes im Hiihnerei. Inaug.
Diss. Berlin, 1875.

(151) W. Waldeyer. "Ueber die Keimblatter und den Primitivstreifen bei
der Entwicklung des Hiihnerembryo." Zeitschrift fiir ratioudle Medicin. 1869.

(152) C. F. Wolff. Theoria generationis. Halse, 1759.

(153) C. F. Wolff. Ueb. d. Bildung d. Darmcanals im bebriitcten Hiinchen.
Halle, 1812.

REPTILIA.

(154) C. Kupffer and Benecke. Die erste Entwicklung am Ei d. Keptilien.
Konigsberg, 1878.

BIBLIOGRAPHY, vii

(155) C. Kupffer. "Die Entstehung d. Allantois u. <1. Gastrula d. Wirbcl-
thiere." Zoologischer Anzeiger, Vol. II. 1879, pp. 520, 593, 612.

Lacertilia.

(156) F. M. Balfour. " On the early Development of the Lacertilia, together
with some observations, etc." Quart. J. of Micr. Science, Vol. xix. 1879.

(157) Emmert u. Hochstetter. " Untersuchung lib. d. Entwick. d. Eidechsen
in ihren Eiern." Reil's Archiv, Vol. X. 1811.

(158) M. Lereboullet. "Developpement de la Truite, du Lc/ard et du
Limnee. II. Embryologie du Lezard." An. Sci. Nat., Ser. iv., Vol. xxvn.
1862.

(159) W. K. Parker. "Structure and Devel. of the Skull in Lacertilia.''
Phil. Trans., Vol. 170, p. 2. 1879.

(160) H. Strahl. " Ueb. d. Canalis myeloentericus d. Eidechse." Schrift. d.
Gesell. z. Be/or, d. gesam. Naturwiss. Marburg. July 23, 1880.

Ophidia.

(161) H. Dutrochet. " Recherches s. 1. en veloppes du foetus." Mem. d. Soc.
Mcd. if Emulation, Paris, Vol. vm. 1.816.

(162) W. K. Parker. "On the skull of the common Snake." Phil. Trans. ,
Vol. 169, Part II. 1878.

(163) H. Rathke. EntTvick. d. Natter. Konigsberg, 1839.

Chelonia.

(164) L. Agassiz. Contributions to the Natural History of the United Slates,
Vol. u. 1857. Embryology of the Turtle.

(165) W. K. Parker. "On the development of the skull and nerves in the
green Turtle." Proc. of the Roy. Soc., Vol. xxvin. 1879. Vide also Nature,
April 14, 1879, and Challenger Reports, Vol. I. 1880.

(166) H. Rathke. Ueb. d. Entwicklung d. Schildkroten. Braunschweig, 1848.

Crocodilia.

(167) H. Rathke. Ueber die Entwicklung d. Krokodile. Braunschweig, 1866.

MAMMALIA.

(168) K. E. von Baer. Ueb. Entwicklungsgcschichte d. Jhiere. Konigsberg,

(169) Barry. "Researches on Embryology." First Series. Philosophical
Transactions, 1838, Part II. Second Series, Ibid. 1839, Part II. Third Series, Ibid.
1840.

(170) Ed. van Beneden. La maturation de Foeuf, la fecondation et les premieres
phases du developpement embryonaire d. Mammiferes. Bruxelles, 1875.

(171) Ed. van Beneden. " Recherches sur 1'embryologie des Mammiferes. •
Archives de Biologic, Vol. I. 1880.

(172) Ed. v. Beneden and Ch. Julin. "Observations sur la maturation etc.
de 1'oeuf chez les Cheiropteres." Archives de Biologie, Vol. I. 1880.

(173) Th. L. W. Bischoff. Entivicklungsgeschichte d. Siiugethiere 11. des
Menschcn. Leipzig, 1842.

(174) Th. L. W. Bischoff. Entivicklungsgeschichte des Kanmcheneies. Braun-
schweig, 1842.

(175) Th. L. W. Bischoff. Entwicklungsgeschuhte des Hundeeies.

schweig, 1845.

(176) Th. L. W. Bischoff. Entivicklungsgesclnchte des Meerschivcinchens.

Giessen. 1852.

viii BIBLIOGRAPHY.

(177) Th. L. W. Bischoff. Entivicklungsgeschichte des Rehcs. Giesscn, 1854.

(178) Th. L. W. Bischoff. " Neue Beobachtungen z. Entwicklungsgesch. des
Meerschweinchens." Abh. d. bayr. Akad., Cl. n. Vol. X. 1866.

(179) Th. L. W. Bischoff. Historisch-kritische B enter kungen z. d. naicstcn
Alittheilungen iil>. d. erste Entwick. d. Siitigethiereier. Miinchen, 1877.

(180) M. Coste. Embryogenie comparee. Paris, 1837.

(181) E. Haeckel. Anthropogenie, Entwicklungsgeschichte des Menschen.
Lci])zig, 1874.

(182) V. Hensen. "Beobachtungen lib. d. Befrucht. u. Entwick. d. Kaninchens
u. Meerschweinchens." Zeit.f. Anat. u. Entwick., Vol. I. 1876.

(183) A. Kolliker. Entivicklungsgeschichte d. Menschen u. d. hb'hcren Thiere.
Leipzig, 1879.

(184) A. Kolliker. "Die Entwick. d. Keimblatter des Kaninchens." Zoolo-
gist her Anseiger, Nos. 61, 62, Vol. in. 1880.

(185) N. Lieberkiihn. Ueber d. Keimblatter d. Siiugethiere. Doctor- Jubelfeier
d. Herrn H. Nasse. Marburg, 1879.

(186) N. Lieberkiihn. "Z. Lehre von d. Keimblattern d. Saugethiere." Sitz.
d. Gesell. z. Beford. d. gesam. Natunviss. Marburg, No. 3. 1880.

(187) Rauber. "Die erste Entwicklung d. Kaninchens." Sitzungsber. d.
naturfor. Gesell. z. Leipzig. 1875.

(188) C. B. Reichert. " Entwicklung des Meerschweinchens." Abh. der.
Berl. Akad. 1862.

(189) E. A. S chafer. " Description of a Mammalian ovum in an early con-
dition of development." Proc. Roy. Soc., No. 168. 1876.

(190) E. A. Schafer. "A contribution to the history of development of the
guinea-pig." Journal of Anal, and Phys. , Vol. x. and xi. 1876 and 1877.

Fcetal Membranes and Placenta of Mammalia.

(191) John Anderson. Anatomical and Zoological Researches in Western
Yunnan. London, 1878.

(192) K. E. von Baer. Untersuchungen iiber die Gef&ssverbindung zwischen
Mutter und Fruc/tf, 1828.

(193) C. G. Cams. Tabulae anatomiam comparali-vam illustrantes. 1831,
1840.

(194) H. C. Chapman. "The placenta and generative apparatus of the
Elephant." Journ. Acad. Nat. Sc., Philadelphia. Vol. viii. 1880.

(195) C. Creighton. " On the formation of the placenta in the guinea-pig."
Journal of Anat. and Phys., Vol. XII. 1878.

(196) Ecker. Icones Physiologicae. 1852-1859.

(197) G. B. Ercolani. 7'he utricular glands of the uterus, etc., translated from
the Italian under the direction of H. O. Marcy. Boston, 1880. Contains translations
of memoirs published in the Mem. deW Accad. d. Scienze d. Bologna, and additional
matter written specially for the translation.

(198) G. B. Ercolani. Nuove ricerche sulla placenta nei pesci cartilaginosi e
nei mammiferi. Bologna, 1 880.

(199) Eschricht. De organis quae respirationi et mttritioni fcetus Mammaliutn
inservinnt. Hafniae, 1837.

(200) A. H. Gar rod and W. Turner. "The gravid uterus and placenta of
Hyomoschus aquaticus." Proc. Zool. Soc., London, 1878.

(201) P. Hart ing. Het ei en de placenta van Halicore Dugong. Inaug. diss.
Utrecht. " On the ovum and placenta of the Dugong." Abstract by Prof. Turner.
Journal of Anat. and Phys., Vol. xin.

(202) Th. H. Huxley. The Elements of Comparative Anatomy. London,
1864.

(203) A. Kolliker. " Ueber die Placenta der Gattung Tragulus." Verh. der
Wiirzb. phys.-med. Gesellschaft, Bd. x.

(204) C. D. Meigs. "On the reproduction of the Opossum (Didelphis Vir-
giniana)." Amer. Phil. Soc. Trans., Vol. x. 1853.

(205) H.Milne-Edwards. " Sur la Classification Naturelle." Ann. Sciences
Nat., Ser. 3, Vol. I. 1844.

BIBLIOGRAPHY.

IX

(206) Alf. Milne-Edwards. "Kecherches sur la famille dcs Chcvrutains.'1
Ann. dcs Sciences Nat., Series V., Vol. II. 1864.

(207) Alf. Milne-Edwards. " Observations sur quelqucs points <le I'Kmbryo-
logie des Lemuriens, etc." Ann. Sci. Nat., Ser. V., Vol. xv. 1872.

(208) Alf. Milne-Edwards. " Sur la conformation du placenta chcz le Ta-
inandua." Ann. des Sci. Nat., xv. 1872.

(209) Alf. Milne-Edwards. " Kecherches s. 1. enveloppes fcetales du Tatou a
neuf bandes." Ann. Sci. Nat., Ser. vi., Vol. vill. 1878.

(210) R. Owen. "On the generation of Marsupial animals, with a description
of the impregnated uterus of the Kangaroo." Phil. Trans., 1834.

(211) R. Owen. "Description of the membranes of the uterine foetus of the
Kangaroo." Mag. Nat. Hist., Vol. I. 1837.

(212) R. Owen. "On the existence of an Allantois in a foetal Kangaroo
(Macropus major)." Zool. Soc. Proc., v. 1837.

(213) R. Owen. "Description of the foetal membranes and placenta of the
Elephant." Phil. Trans., 1857.

(214) R.Owen. On the Anatomy of Vertebrates, Vol. III. London, 1868.

(215) G. Rolleston. " Placental structure of the Tenrec, etc." Transactions
of the Zoological Society, Vol. V. 1866.

(216) W. Turner. "Observations on the structure of the human placenta."
Journal of Anat. and Phys., Vol. vn. 1868.

(217) W. Turner. "On the placentation of the Cetacea." Trans. Roy. Soc.
Edinb,, Vol. xxvi. 1872.

(218) W. Turner. "On the placentation of Sloths (Cholcepus Hoffrnanni)."
Trans, of R. Society of Edinburgh, Vol. xxvn. 1875.

(219) W. Turner. "On the placentation of Seals (Halichcerus gryphus)."
Trans, of R. Society of Edinburgh, Vol. xxvii. 1875.

(220) W.Turner. "On the placentation of the Cape Ant-eater (Orycteropus
capensis)." Journal of Anat. and Phys., Vol. X. 1876.

(221) W. Turner. Lectures on the Anatomy of the Placenta. First Series.
Edinburgh, 1876.

(222) W. Turner. "Some general observations on the placenta, with special
reference to the theory of Evolution." Journal of Anat. and Phys., Vol. XI. 1877.

(223) W.Turner. " On the placentation of the Lemurs." Phil. Trans., Vol.
166, p. 2. 1877.

(224) W.Turner. " On the placentation of Apes." Phil. Trans., 1878.

(225) W. Turner. "The cotyledonary and diffused placenta of the Mexican
deer (Cervus Americanus). " Journal of Anat. and Phys., Vol. xm. 1879.

Human Embryo.

(226) Fried. Ahlfeld. " Beschreibung eines sehr kleinen menschlichen Eies."
Archiv f. Gynaekologie, Bd. xm. 1878.

(227) Herm. Beigel und Ludwig Loewe. "Beschreibung eines menschlichen
Eichens aus der zweiten bis dritten Woche der Schwangerschaft." Archiv f. Gynaeko-
logie, Bd. xn. 1877.

(228) K. Breus. " Ueber ein menschliches Ei aus der zweiten Woche der
Graviditat." Wiener medicinische Wochenschrift, 1877.

(229) M. Coste. Histoire generale et particuliere du developpement des corps or-
ganises, 1847-59.

(230) A. Ecker. Icones Physiologicae. Leipzig, 1851-1859.

(231) V. Hensen. " Beitrag z. Morphologic d. Korperform u. d. Gehirns d.
menschlichen Embryos." Archiv f. Anat. u. Phys., 1877.

(232) W. His. Anatomie menschlicher Etnbryonen, Part I. Embryonen d.
ersten Monats. Leipzig, 1880.

(233) J. Kollmann. " Die menschlichen Eier von 6 MM. Grosse." Archiv f.

Anat. und Phys., 1879.

(234) W. Krause.
Phys., 1875.

(235) W. Krause.
/. wiss. Zool., Vol. xxxv.

Ueber d. Allantois d. Menschen." Archiv f. Anat. und

' Ueber zwei friihzeitige menschliche Embryonen."
1880.

Zeit.

X BIBLIOGRAPHY.

(236) L. Loewe. "Im Sachen cler Eihaute jiingster menschlicher Eicr. "
Archiv fiir Gynaekologie, Bd. xiv. 1879.

(237) C. B. Reichert. " Beschreibung einer friihzeitigen menschlichcn Frucht
im blaschenformigen Bildungszustande (sackformiger Keim von Baer) nebst vergleich-
enden Untersuchungen iiber die blaschenformigen Friichte der Saugethiere und des
Menschen. " Abhandlungcn der konigl. Akad. d, Wiss, zu Berlin, 1873.

(238) Allen Thomson. "Contributions to the history of the structure of the
human ovum and embryo before the third week after conception ; with a description
of some early ova." Edinburgh Med. Siirg.Journal, Vol. LI I. 1839.

COMPARISON OF THE FORMATION OF THE GERMINAL LAYERS
IN THE VERTEBRATA.

(239) F. M. Balfour. "A comparison of the early stages in the development
of Vertebrates." Quart. J. of Micr. Science, Vol. xv. 1875.

(240) F. M. Balfour. "A monograph on the development of Elasmobranch
Fishes." London, 1878.

(241) F. M. Balfour. " On the early development of the Lacertilia together
with some observations, etc." Quart. J. of Micr. Science, Vol. xix. 1879.

(242) A. Gotte. Die Entwicklungsgeschichte d. Unke. Leipzig, 1875.

(243) W. His. "Ueb. d. Bildung d. Haifischembryonen." Zeit. f. Anal. it.
Entwick., Vol. II. 1877. Cf. also His' papers on Teleostei, Nos. 65 and 66.

(244) A. Kowalevsky. " Entwick. d. Amphioxus lanceolatus." Mem. Acad.
des Sciences St Petersbourg, Ser. vii. Tom. XI. 1867.

(245) A. Kowalevsky. " Weitere Studien lib. d. Entwick. d. Amphioxus lan-
ceolatus." Archiv f. mikr. Anal., Vol. XIII. 1877.

(246) C. Kupffer. "Die Entstehung d. Allantois u. d. Gastrula d. Wirbel-
thiere." Zool. Anzeiger, Vol. II. 1879, PP- 52°' 593' 61?.

(247) R. Remak. Untersuchungen iib. d. Entiuicklung d. Wirbelthiere, 1850 —
1858.

(248) A. Rauber. Primitivstreifen ti. Neurula d. Wirbelthiere, Leipzig,
1877.

PHYLOGENY OF THE CHORDATA.

(249) F. M. Balfour. A Monograph on the development of Elasmobranch Fishes,
London, 1878.

(250) A. Dohrn. Der Ursprung d. Wirbelthiere und d. Princip. d. Functions-
wechsel. Leipzig, 1875.

(251) E. Haeckel. Schb'pfungsgeschichte. Leipzig. Vide also Translation.
The History of Creation. King and Co. , London. 1876.

(252) E. Haeckel. Anthropogenie. Leipzig. Vide also Translation. Antliro-
pogeny. Kegan Paul and Co., London, 1878. •

(253) A. Kowalevsky. " Entwicklungsgeschichte d. Amphioxus lanceolatus."
Mem. Acad. d. Scien. St Petersbourg, Ser. VII. Tom. xi. 1867, and Archiv f. ?nikr.
Anat., Vol. XIII. 1877.

(254) A. Kowalevsky. "Weitere Stud. lib. d. Entwick. d. einfachen Ascidien."
Archiv f. mikr. Anat., Vol. VII. 1871.

(255) C. Semper. "Die Stammesverwandschaft d. Wirbelthiere u. Wirbel-
losen." Arbeit, a. d. zool.-zoot. Instit. Wiirzburg, Vol. u. 1875.

(256) C. Semper. "Die Verwandschaftbeziehungen d. gegliederten Thiere."
Arbeit, a. d. zool.-zoot. Instit. Wiirzburg, Vol. in. 1876 — 1877.

GENERAL WORKS ON EMBRYOLOGY.

(257) Allen Thomson. British Association Address, 1877.

(258) A. Agassiz. "Embryology of the Ctenophoroe." Mem. Amcr. Acad. of
Arts and Sciences, Vol. X. 1874.

(259) K. E. von Baer. Ueb. Entivicklnngsgeschichle d. Thiere. Konigsberg,
1828—1837.

BIBLIOGRAPHY.

XI

(260) F. M. Balfour. "A Comparison of the Early Stages in the Development
of Vertebrates." Qttart. Journ. of Micr. Set., Vol. XV. 1875.

(261)
1874.

C. Glaus. Die Typenlehre u. E. HaeckeFs sg. Gastnca-theorie. Wieii,

(262) C. Claus. Grundziige d. Zoologie. Marburg und Leipzig, 1879.

(263) A. Dohrn. Der Ursprung d. Wirbdlhiere u. d. Princip des Functions-
wechsds. Leipzig, 1875.

(264) C. Gegenbaur. Grundriss d. vergleichenden Anatomic. Leipzig, 1878.
Vide also Translation. Elements of Comparative Anatomy. Macmillan £ Co.
1878.

(265) A. Gotte. Ent^vicklungsgeschichte d. Unke. Leipzig, 1874.

(266) E. Haeckel. Studien z. Gastrcca-theorie, Jena, 1877; anc' a'so Jenaische
Zeitschrift, Vols. vm. and IX. 1874-5.

(267) E. Haeckel. Schdpfungsgeschichte. Leipzig. Vide also Translation,
The History of Creation. King & Co., London, 1878.

(268) E. Haeckel. Anthropogenic. Leipzig. Vide also Translation, Atithro-
pogeny. Kegan Paul & Co., London, 1878.

(269) B. Hatschek. "Studien lib. Entwicklungsgeschichte d. Anneliden."
Arbeit, a. d. zool. Instit. d. Univer. Wien. 1878.

(270) O. and R. Hertwig. " Die Actinien." Jenaische Zeitschrift, Vols. xiil.
and XIV. 1879.

(271) O. and R. Hertwig. Die Cctlomtheorie. Jena, 1881.

(272) O. Hertwig. Die Chatognathen. Jena, 1880.

(273) R. Hertwig. Ueb. d. Ban d. Ctenophoren. Jena, 1880.

(274) T. H. Huxley. The Anatomy of Invertebrated Animals. Churchill,
1877.

(274*) T. H. Huxley. "On the Classification of the Animal Kingdom."
Quart. J. of Micr. Science, Vol. XV. 1875.

(275) N. Kleinenberg. Hydra, eine anatomisch-entivicklungsgeschichte Un-
tersnchung. Leipzig, 1872.

(276) A. Kolliker. Entwicklungsgeschichte d. Menschen u. d. hbh. Thiere.
Leipzig, 1879.

(277) A. Kowalevsky. " Embryologische Studien an Wurmern u. Arthropoden."
Mem. Acad. Petersbourg, Series vii. Vol. xvi. 1871.

(278) E. R. Lankester. "On the Germinal Layers of the Embryo as the
Basis of the Genealogical Classification of Animals." Ann. and Mag. of Nat. Hist.

1873-

(279) E. R. Lankester. " Notes on Embryology and Classification." Quart.

Jotirn. of Alter. Set., Vol. xvn. 1877.

(280) E. Metschnikoff. "Zur Entwicklungsgeschichte d. Kalkschwamme."
Zeit. f. wiss. Zool., Vol. xxiv. 1874.

(281) E. Metschnikoff. " Spongiologische Studien." Zeit. f. wiss. Zool.,
Vol. xxxn. 1879.

(282) A. S. P. Packard. Life Histories of Animals, including Man, or Outlines
of Comparative Embryology. Holt and Co., New York, 1876.

(283) C. Rabl. " Ueb. d. Entwick. d. Malermuschel. " Jenaische Zeitsch., Vol.
x. 1876.

(284) C. Rabl. "Ueb. d. Entwicklung. d. Tellerschneke (Planorbis)." Morph.
Jahrbuch, Vol. v. 1879.

(285) H. Rathke. Abhandhmgen z. Bildung und Enlwicklungsgesch.d. Menschen
u. d. Thiere. Leipzig, 1833.

(286) H. Rathke. Ueber die Bildung u. Entwicklungs. d. Flusskrebses. Leipzig,
1829.

(287) R. Remak. Untersuch. ilb. d. Entwick. d. Wirbelthiere. Berlin, 1855.

(288) Salensky. " Bemerkungen lib. Haeckels Gastrsea-theorie." Archiv /.
Naturgeschichte, 1874.

(289) E. Schafer. "Some Teachings of Development." Quart. Jotint. of Micr.
Science, Vol. xx. 1880.

(290) C. Semper. " Die Verwandtschaftbeziehungen d. gegliederten Thiere."
Arbeiten a. d. zool.-zoot. Instit. Wiirzburg, Vol. in. 1876-7.

Xll BIBLIOGRAPHY.

GENERAL WORKS DEALING WITH THE DEVELOPMENT OF
THE ORGANS OF THE CHORDATA.

(291) K. E. von Baer. Ueber Enlwicklungsgeschichte d. Thiere. Konigsberg,
! 828— 1837.

(292) F. M. Balfour. A Monograph on the development of Elasmobranch Fishes.
London, 1878.

(293) Th. C. W. Bischoff. Entwicklungsgesch. d. Siiugdhiere u. d. Menschen.
Leipzig, 1842.

(294) C. Gegenbaur. Grundriss d. vergleichenden Anatomic. Leipzig, 1878.
Vide also English translation, Elements of Comp. Anatomy. London, 1878.

(295) M. Foster and F. M. Balfour. The Elements of Embryology. Part I.
London, 1874.

(296) Alex. Gotte. Entwickhmgsgeschichte d. Unke. Leipzig, 1875.

(297) W. His. Untersuch. ilb. d. erste Anlage d. Wirbelthierleibes. Leipzig,
1868.

(298) A. K 6 Hiker. Entwickhmgsgeschichte d. Menschen u. der hoheren Thiere.
Leipzig, 1879.

(299) H. Rathke. Abhandlungen u. Bildung und Entwickhingsgeschichle d.
Menschen u. d. Thiere. Leipzig, 1838.

(300) H. Rathke. Entwicklungs. d. Natter. Konigsberg, 1839.

(301) H. Rathke. Entwicklungs. d. Wirbelthiere. Leipzig, 1861.

(302) R. Remak. Untersuchungen iib. d. Entwicklung d. Wirbelthiere. Berlin,
1850—1855.

(303) S. L. Schenk. Lehrbuch d. vergleich. Embryologie d. Wirbelthiere.
Wien, 1874.

. EPIDERMIS AND ITS DERIVATIVES.
General.

(304) T. H. Huxley. " Tegumentary organs." Todd's Cyclopedia of Anat.
and Physiol.

(305) P. Z. Unna. "Histol. u. Entwick. d. Oberhaut." Archiv /. mikr. Anat.
Vol. XV. 1876. Pft&also Kolliker (No. 298).

Scales of the Pisces.

(306) O. Hertwig. "Ueber Bau u. Entwicklung d. Placoidschuppen u. d.
Zahne d. Selachier." Jenaische Zeitschrift, Vol. vill. 1874.

(307) O. Hertwig. " Ueber d. Hautskelet d. Fische." Morphol. Jahrbuch,
Vol. u. 1876. (Siluroiden u. Acipenseridae.)

(308) O. Hertwig. "Ueber d. Hautskelet d. Fische (Lepidosteus u. Polypte-
rus)." Morph. Jahrbuch, Vol. v. 1879.

Feathers.

(309) Th. Studer. Die Entwick. d. Federn. Inaug. Diss. Bern, 1873.

(310) Th. Studer. " Beitrage z. Entwick. d. Feder." Zeit.f. wiss. Zool., Vol.
xxx. 1878.

Sweat-glands.

(311) M. S. Ranvier. " Sur la structure des glandes sudoripares." Comptes
Rendus, Dec. 29, 1879.

BIBLIOGRAPHY. xiii

Mammary glands.

(312) C. Creighton. "On the development of the Mamma and the Mammary
function." Jour, of Anat. and Phys. , Vol. xi. 1877.

(313) C. Gegenbaur. " Bemerkungen lib. d. Milchdriisen-Papillen d. Sauge-
thiere." Jenaische Zeit.. Vol. VII. 1873.

(314) M. Huss. " Beitr. z. Entwick. d. Milchdriisen b. Menschen u. b. Wieder-
kauern." Jenaische Zeit., Vol. vil. 1873.

(315) C. Langer. " Ueber d. Bau u. d. Entwicklung d. Milchdriisen." Denk.
d. k. Akad. Wiss. Wien, Vol. in. 1851.

THE NERVOUS SYSTEM.
Evolution of the Nervous System.

(316) F. M. Balfour. " Address to the Department of Anat. and Physiol. of the
British Association." 1880.

(317) C. Claus. "Studien lib. Polypen u. Quallen d. Adria. I. Acalephen,
Discomedusen." Denk. d. math.-natiirwiss. Classe d. k. Akad. Wiss. Wien, Vol.
xxxvin. 1877.

(318) Th. Eimer. Zoologische Studien a. Capri. I. Ueber Beroe ovatus. Ein
Beitrag z. Anat. d. Rippenquallen. Leipzig, 1873.

(319) V. Hensen. " Zur Entwicklung d. Nervensystems. " Virchow's Archiv,
Vol. xxx. 1864.

(320) O. and R. Hertwig. Das Nerven system u. d. Sinnesorgane d. Medusen.
Leipzig, 1878.

(321) O. and R. Hertwig. "Die Actinien anat. u. histol. mit besond. Beriick-
sichtigung d. Nervenmuskelsystem untersucht." Jenaische Zeit., Vol. xiii. 1879.

(322) R. Hertwig. "Ueb. d. Bau d. Ctenophoren." Jenaische Zeitschrift,
Vol. xiv. 1880.

(323) A. W. Hubrecht. "The Peripheral Nervous System in Palseo- and
Schizonemertini, one of the layers of the body-wall." Quart, y. of Micr. Science,
Vol. xx. 1880.

(324) N. Kleinenberg. Hydra, eine anatomisch-entwickhmgsgeschichthche Un-
tersuchung. Leipzig, 1872.

(325) A. Kowalevsky. " Embryologische Studien an Wtirmern u. Arthropo-
den." Mem. Acad. Petersboiirg, Series vil., Vol. XVI. 1871.

(326) E. A. Schafer. "Observations on the nervous system of Aurelia aurita."
Phil. Trans. 1878.

Nervous System of the Invertebrata.

(327) F. M. Balfour. "Notes on the development of the Araneina." Quart.
J. of Micr. Science, Vol. xx. 1880.

(328) B. Hatschek. "Beitr. z. Entwicklung d. Lepidopteren.' Jenaische
Zeitschrift, Vol. XI. 1877.

(329) N. Kleinenberg. "The development of the Earthworm, Lumbncus
Trapezoides." Quart. J. of Micr. Science, Vol. xix. 1879.

(330) A. Kowalevsky. "Embryologische Studien an Wiirmern u. Arthro-
poden." Mem. Acad. Petersbourg, Series vin., Vol. xvi. 1871.

(331) H. Reichenbach. "Die Embryonalanlage u. erste Entwick. d. Fluss-
krebses." Zeit.f. wiss. Zool, Vol. xxix. 1877.

Central Nervous System of the Vertebrata.

(332) C. J. Carus. Versuch einer Darstellung d. Nervensystems, etc. Leipzig,

(333) J. L. Clark. " Researches on the development of the spinal cord in Man,
Mammalia and Birds." Phil. Trans., 1862.

xiv BIBLIOGRAPHY.

(334) E. Dursy. " Beitrage zur Entwicklungsgeschichte des Hirnanhanges. "
Centralblatt f. d. med. \Vissenschaften, 1 868. Nr. 8.

(335) E. Dursy. Zur Entwicklungsgeschichte des Kopfes des Menschen und der
hb'heren Wirbelthiere. Tiibingen, 1869.

(336) A. Ecker. "Zur Entwicklungsgeschichte der Furchen und Windungen
der Grosshirn-Hemispharen im Foetus des Menschen." Archiv f. Anthropologie, v.
Ecker und Lindenschmidt. Vol. ill. 1868.

(337) E. Ehlers. " Die Epiphyse am Gehirn d. Plagiostomen." Zeit.f.wiss.
Zool. Vol. xxx., suppl. 1878.

(338) P. Flechsig. Die Leitungsbahnen im Gehirn und Riickenmark des
Menschen. Auf Grtind entwicklungsgeschichtlicher Untersuchungen. Leipzig, 1876.

(339) V. Hensen. "Zur Entwicklung des Nervensystems." Virchoisfs Archiv,
Bd. xxx. 1864.

(340) L. Lowe. " Beitrage z. Anat. u. z. Entwick. d. Nervensystems d. Sauge-
thiere u. d. Menschen." Berlin, 1880.

(341) L. Lowe. " Beitrage z. vergleich. Morphogenesis d. centralen Nerven-
systems d. Wirbelthiere." Mitthcil. a. d. embryol. Instit. Wien, Vol. u. 1880.

(342) A. M. Marshall. "The Morphology of the Vertebrate Olfactory organ."
Quart. J. of Micr. Science, Vol. xix. 1879.

(343) V. v. Mihalkovics. Entwicklungsgeschichte d. Gehirns. Leipzig, 1877.

(344) W. Miiller. " Ueber Entwicklung und Bau der Hypophysis und des
Processus infundibuli cerebri. " Jenaische Zeitschrift. Bd. vi. 1871.

(345) H. Rahl- Ruck hard. "Die gegenseitigen Verhaltnisse d. Chorda,
Hypophysis etc. bei Haifischembryonen, nebst Bemerkungen lib. d. Deutung d.
einzelnen Theile d. Fischgehirns." Morphol. Jahrbuch, Vol. vi. 1880.

(346) H. Rathke. " Ueber die Entstehung der glandula pituitaria. " Mullens
Archiv f. Anat. und Physiol. , Bd. V. 1838.

(347) C. B. Reich ert. Der Bau des menschlichen Gehirns. Leipzig, 1859 u-
1861.

(348) F. Schmidt. "Beitrage zur Entwicklungsgeschichte des Gehirns."
Zcitschrift f. wiss. Zoologie, 1862. Bd. xi.

(349) G. Schwalbe. "Beitrag z. Entwick. d. Zwischenhirns." Sitz. d.
Jenaischen Gesell.f. Med. u. Natttnviss. Jan. 23, 1880.

(350) Fried. Tiedemann. Anatomie und Bildungsgeschichte des Gehirns im
Foetus des Menschen. Niirnberg, 1816.

Peripheral Nervous System of the Vertebrata.

(351) F. M. Balfour. "On the development of the spinal nerves in Elasmo-
branch Fishes." Philosophical Transactions, Vol. CLXVI. 1876; vide also, A mono-
graph on the development of Elasmobranch Fishes. London, 1878, pp. 191—216.

(352) W. His. " Ueb. d. Anfiinge d. peripherischen Nervensystems." Archiv
f. Anat. u. Physiol., 1879.

(353) A. M. Marshall. " On the early stages of development of the nerves in
Birds." Jottrnal of Anat. and Fkys.,Vo\. XI. 1877.

(354) A. M. Marshall. "The development of the cranial nerves in the Chick."
Quart, y. of Micr. Science, Vol. xvni. 1878.

(355) A. M. Marshall. "The morphology of the vertebrate olfactory organ."
Quart, y. of Micr. Science, Vol. xix. 1879.

(356) A. M. Marshall. " On the head-cavities and associated nerves in Elasmo-
branchs." Quart, y. of Micr. Science, Vol. xxi. 1881.

(357) C. Schwalbe. "Das Ganglion oculomotorii. " Jenaische Zeitschrift,
Vol. xni. 1879.

Sympathetic Nervous System.

(360) F. M. Balfour. Monograph on the development of Elasmobranch Fishes.
London, 1878, p. 173.

(361) S. L. Schenk and W. R. Birdsell. "Ueb. d. Lehre vond. Entwicklung
d. Ganglien d. Sympatheticus." Mittheil. a. d. cmbryologischen Instit. Wien. Heft
III. 1879.

BIBLIOGRAPHY. XV

THE EYE.

Eye of the Mollusca.

(362) N. Bobretzky. " Observations on the development of the Cephalopoda "
(Russian). Nachrichtcn d. kaiserlichen Gesell. d. Frennde der Natuna iss. Anthropolog.
Ethnogr. bei d. Universitdt Moskau.

(363) H. Grenacher. " Zur Entwicklungsgeschichte d. Cephalopoden." Zeit.
f. wiss. Zool., Bd. xxiv. 1874.

(364) V. Hensen. "Ueber d. Auge einiger Cephalopoden." Zeit. f. wiss.
Zool., Vol. xv. 1865.

(365) E. R. Lankester. " Observations on the development of the Cephalo-
poda." Quart. J. of Micr. Science, Vol. xv. 1875.

(366) C. Semper. Ueber Sehorganevon Typus d. Wirbelthicraugen. Wiesbaden,
1877.

Eye of the Arthropoda.

(367) N. Bobretzky. Development of Astacus and Palaemon. Kiew, 1873.

(368) A. Dohrn. " Untersuchungen lib. Bau u. Entwicklung d. Arthropoden.
Palinurus und Scyllarus. " Zeit. f. wiss. Zool., Bd. xx. 1870, p. 264 et seq.

(369) E. Claparede. "Morphologic d. zusammengesetzten Auges bei den Ar-
thropoden." Zeit. f. wiss. Zool., Bd. X. 1860.

(370) H. Grenacher. Untersuchungen iib. d. Sehorgane d. Arthropoden.
Gottingen, 1879.

The Vertebrate Eye.

(371) J.Arnold. Beitrage zur Entwicklungsgeschichle des A uges. Heidelberg,
1874.

(372) Babuchin. "Beitrage zur Entwicklungsgeschichte des Auges." Wiirz-
liurger naturwissenschaftliche Zeitschrift, Bd. 8.

(373) L. Kessler. Zur Ent^vicklung d. Auges d. Wirbclthiere. Leipzig, 1877.

(374) N. Lieberkiihn. Ueber das Auge des Wirbelthierembryo. Cassel, 1872.

(375) N. Lieberkiihn. " Beitrage z. Anat. d. embryonalen Auges." Archiv
f. Anat. und Phys., 1879.

(376) L. Lowe. "Beitrage zur Anatomic des Auges" and "Die Histogenese
der Retina." Archiv f. mikr. Anat., Vol. xv. 1878.

(377) V. Mihalkowics. "Untersuchungen iiber den Kamm des Vogelauges."
Archiv f. mikr. Anat., Vol. IX. 1873.

(378) W. Miiller. " Ueber die Stammesentwickelung des Sehorgans der Wir-
belthiere." Festgabe Carl Ludwig. Leipzig, 1874.

(379) S. L. Schenk. "Zur Entwickelungsgeschichte des Auges der Fische."
Wiener Sitzungsberichte, Bd. LV. 1867.

Accessory organs of the Vertebrate Eye.

(380) G. Born. "Die Nasenhohlen u. d. Thranennasengang d. Amphibien."
Morphologisches Jahrbuch, Bd. II. 1876.

(381) G. Born. " Die Nasenhohlen u. d. Thranennasengang d. amnioten Wir-
belthiere. I. Lacertilia. II. Aves." Morphologisches Jahrbuch, Bd. V. 1879.

Eye of the T2tnicata,

(382) A. Kowalevsky. "Weitere Studien iib. d. Entwicklung d. einfachen
Ascidien." Archiv f. mikr. Anat., Vol. VII. 1871.

(383) C. Kupffer. "Zur Entwicklung d. einfachen Ascidien." Archiv f.
mikr. Anat., Vol. VII. 1872.

xvi BIBLIOGRAPHY.

AUDITORY ORGANS.
Auditory organs of tlie Invertebrata.

(384) V. Hensen. "Studien lib. d. Gehororgan d. Decapoden." Zeil.f. wiss.
Zool., Vol. xui. 1863.

(385) O. and R. Her twig. Das Nervensystem u. d. Sinnesorgane d. Medusen.
Leipzig, 1878.

Auditory organs of the Vertebrata.

(386) A. Boettcher. "Bau u. Entwicklung d. Schnecke." Denkschriften d.
kaiserl. Leap. Carol. Akad. d. Wissenschaft., Vol. xxxv.

(387) C. Hasse. Dievergleich. Morphologieu. Histologied. hciutigen Gehororgane
d. Wirbelthiere. Leipzig, 1873.

(388) V. Hensen. "Zur Morphologie d. Schnecke." Zeit. f, wiss. ZooI.,Vo\.

XIII. 1863.

(389) E. Huschke. "Ueb. d. erste Bildungsgeschichte d. Auges u. Ohres beim
bebrliteten Kiichlein." Isis von Oken, 1831, and Meckel's Archiv, Vol. VI.

(390) Reissner. De Auris internee formatione. Inaug. Diss. Dorpat, 1851.

Accessory parts of Vertebrate Ear.

(391) David Hunt. "A comparative sketch of the development of the ear and
eye in the Pig. " Transactions of the International Otological Congress, 1 876.

(392) W. Moldenhauer. "Zur Entwick. d. mittleren u. ausseren Ohres."
Morphol. Jahrbiich, Vol. ill. 1877.

(393) V. Urbantschitsch. " Ueb. d. erste Anlage d. Mittelohres u. d. Trom-
melfelles." Mittheil. a. d. embryol. Instit. Wien, Heft I. 1877.

OLFACTORY ORGAN.

(394) G. Born. "Die Nasenhohlen u. d. Thranennasengang d. amnioten
Wirbelthiere." Parts I. and II. Morphologisches Jahrbuch, Bd. V., 1879.

(395) A. Kolliker. " Ueber die Jacobson'schen Organe des Menschen."
Festschrift f. Rienecker, 1877.

(396) A. M. Marshall. "Morphology of the Vertebrate Olfactory Organ."
Quart. Journ. of Micr. Science, Vol. xix., 1879.

SENSE-ORGANS OF THE LATERAL LINE.

(397) F. M. Balfour. A Monograph on the development of Elasmobranch Fishes,
pp. 141 — 146. London, 1878.

(398) H. Eisig. "Die Segmentalorgane d. Capitelliden." Mitlhcil. a. d. zool.
Station zu Neapel, Vol. I. 1879.

(399) A. Gotte. Entwicklungsgeschichte d. Unke. Leipzig, 1875.

(400) Fr. Ley dig. Lehrbuch d. Histologie des Menschen u. d. Thiere. Hamm.

T857-

(401) Fr. Ley dig. Nene Beitrdge z. anat. Kenntniss d. Haiitdecke u. IJaut-
sinnesorgane d. Fische. Halle, 1879.

(402) F. E. Schulze. "Ueb. d. Sinnesorgane d. Seitenlinie bei Fischen und
Amphibien." Archiv f. mikr. Anat., Vol. vi. 1870.

(403) C. Semper. "Das Urogenitalsystem d. Selachier." Arbeit, a. d. zool.-
zoot. Instit. Wiirzburg, Vol. II.

(404) B. Solger. "Neue Untersuchungen zur Anat. d. Seitenorgane d. Fische."
Archiv f. mikr. Anat., Vol. xvil. and xvni. 1879 and 1880.

ORIGIN OF THE SKELETON.

(405) C. Gegenbaur. "Ueb. primare u. secundare Knochenliildung mit be-
sonderer Beziehung auf d. Lehre von dem Primordialcranium." Jciiaischc Zeit-
schrifl, Vol. in. 1867.

BIBLIOGRAPHY. xvii

(406) O. Hertwig. "Ueber Bau u. Entwicklung cl. Placoidschuppcn u. d.
Ziihne d. Selachicr." Jetiaische Zeitschrift, Vol. vm. 1874.

(407) O. Hertwig. " Ueb. d. Zahnsystem d. Amphibien u. seine Bcdeutung
f. d. Genese d. Skelets d. Mundhohle." Archiv f. mikr. Anat., Vol. xi. Supple-
mentheft, 1874.

(408) O. Hertwig. " Ueber d. Hautskelet d. Fische." Morphol. Jahrlmch,
Vol. u. 1876. (Siluroiden u. Acipenseriden.)

(409) O. Hertwig. "Ueber d. Hautskelet d. Fische (Lepidosteus u. I'olyp-
terus)." Morph. Jahrbnch, Vol. v. 1879.

(410) A. Kolliker. "AllgemeineBetrachtungenub. die Entstehungd. knoclier-
nen Schadels d. Wirbelthiere. " Berichle v. d. konigl. zoot. Anstalt z. \Viirzlwrg,
1849.

(411) Fr. Leydig. " Histologische Bemerkungen iib. d. Polypterus bichir."
Zeit.f. wiss. Zool., Vol. V. 1858.

(412) H. Muller. "Ueber d. Entwick. d. Knochensubstanz nebst Bemerkun-
gen, etc." Zeit. f. wiss. Zool., Vol. IX. 1859.

(413) Williamson. "On the structure and development of the Scales and
Bones of Fishes." Phil. Trans., 1851.

(414) Vrolik. " Studien iib. d. Verknocherung u. die Knochen d. Schadels d.
Teleostier." Niederldndisches Archiv f. Zoologie, Vol. i.

NOTOCHORD AND VERTEBRAL COLUMN.

(415) Cartier. " Beitrage zur Entwicklungsgeschichte der Wirbelsaule." Zeit-
schrift fur wiss. Zool., Bd. xxv. Suppl. 1875.

(416) C. Gegenbaur. Untersuchungen zur vergleichenden Anatomic der Wir-
belsaule der Amphibien und Reptilien. Leipzig, 1862.

(417) C. Gegenbaur. "Ueber die Entwickelung der Wirbelsaule des Lepi-
dosteus mit vergleichend anatomischen Bemerkungen." Jenaisckc Zeitschrift, Bd. ill.
1863.

(418) C. Gegenbaur. "Ueb. d. Skeletgewebe d. Cyclostomen." Jenaische
Zeitschrift, Vol. v. 1870.

(419) Al. Gotte. "Beitrage zur vergleich. Morphol. des Skeletsystems d.
Wirbelthiere." II. "Die Wirbelsaule u. ihre Anhange." Archiv f. mikr. Anat., Vol.
xv. 1878 (Cyclostomen, Ganoiden, Plagiostomen, Chimaera), and Vol. xvi. 1879
(Teleostier).

(420) Hasse und Schwarck. "Studien zur vergleichenden Anatomic der
Wirbelsaule u. s. w." Hasse, Anatomische Studiett, 1872.

(421) C. Hasse. Das natiirliche System d. Elasmobranchier auf Grundlage d.
Bau. u. d. Entwick. ihrer Wirbelsaule. Jena, 1879.

(422) A. Kolliker. " Ueber die Beziehungen der Chorda dorsalis zur Bildung
der Wirbel der Selachier und einiger anderen Fische." Verhandlungen der physical,
medicin. Gesellschaft in Wiirzburg, Bd. X.

(423) A. Kolliker. " Weitere Beobachtungen iiber die Wirbel der Selachier
insbesondere iiber die Wirbel der Lamnoidei." Abhandhmgen der senkenbergischen
naturforschenden Gesellschaft in Frankfurt, Bd. V.

(424) H. Leboucq. " Recherches s. 1. mode de disparition de la corde dorsale
chez les vertebres superieurs." Archives de Biologie, Vol. I. 1 880.

(425) Fr. Leydig. Anatomisch-histologische Untersuchungen iiber Fische und
Reptilien. Berlin, 1853.

(426) Aug. Muller. "Beobachtungen zur vergleichenden Anatomic der Wir-
belsaule." Miiller's Archiv. 1853.

(427) J. Muller. " Vergleichende Anatomic der Myxinoiden u. der Cyklosto-
men mit durchbohrtem Gaumen, I. Osteologie und Myologie." Abhandlungcn der
koniglichen Akademie der Wissenschaften zu Berlin. 1834.

(428) W. Muller. "Beobachtungen des pathologischen Instituts zu Jena, I.
Ueber den Bau der Chorda dorsalis." Jenaische Zeitschrift, Bd. VI. 1871.

(429) A. Schneider. Beitrage z. vergleich. Anat. u. Entwick. d. Wirbelthiere.
Berlin, 1879.

B. III. *

xviii BIBLIOGRAPHY.

RIBS AND STERNUM.

(430) C. Claus. " Beitrage z. vergleich. Osteol. d. Vertcbraten. I. Rippen u.
unteres Bogensystem." Sitz. d. kaiserl. Akad. Wiss. Wien, Vol. LXXIV. 1876.

(431) A. E. Fick. "Zur Entwicklungsgeschichte d. Rippen und Querfort-
sritze." Archiv f. Anat. und Physiol. 1879.

(432) C. Gegenbaur. "Zur Entwick. d. Wirbelsaule des Lepidosteus mil
vergleich. anat. Bemerk." Jenaische Zeit., Vol. III. 1867.

(433) A. Gotte. "Beitrage z. vergleich. Morphol. d. Skeletsystems d. Wirbel-
thiere Brustbein u. Schultergiirtel." Archiv f. mikr. Anat., Vol. xiv. 1877.

(434) C. Hasse u. G. Born. " Bcmerkungen lib. d. Morphologic d. Rippen."
Zoologischer Anzeiger, 1879.

(435) C.K.Hoffmann. " Beitrage z. vergl. Anat. d. Wirbelthiere." Nieder-
liind. Archiv Zool., Vol. iv. 1878.

(436) W. K. Parker. " A monograph on the structure and development of the
shoulder-girdle and sternum." Ray Soc. 1867.

(437) H. Rathke. Ueb. d. Ban u. d. Enlivicklung d. Brustbeins d. Sanricr.

1853-

(438) G. Ruge. " Untersuch. lib. Entwick. am Brustbeine d. Menschen."
Morphol. Jahrlmch., Vol. VI. 1880.

THE SKULL.

(439) A. Duges. "Recherches sur 1'Osteologie et la myologie des Batraciens a
leur differents ages." Paris, Mem. savans tirang. 1835, and An. Sci. Nat. Vol. I.
1834.

(440) C. Gegenbaur. UntersucJmngen z. vergleich. Anat. d. Wirbelthiere, III.
Heft. Das Kopfskelet d. Selachier. Leipzig, 1872.

(441) Giinther. Beob. iib. die Entwick. d. Gehbrorgans. Leipzig, 1842.

(442) O. Hertwig. " Ueb. d. Zahnsystem d. Amphibien u. seine Bedeutung f.
d. Genese d. Skelets d. Mundhohle. " Archiv f. mikr, Anat., Vol. xi. 1874, suppl.

(443) T. H. Huxley. "On the theory of the vertebrate skull." Proc. Royal
Soc., Vol. ix. 1858.

f444) T.H.Huxley. The Elements of Comparative Anatomy . London, 1869.

(445
(446
(447

T. H. Huxley. "On the Malleus and Incus." Proc. Zool. Soc.,

T. H. Huxley. "On Ceratodus Forsteri." Proc. Zool. Soc., 1876.

T. H. Huxley. " The nature of the craniofacial apparatus of Petromyzon."

Journ. of Anat. and Phys., Vol. X. 1876.

(448) T. H. Huxley. The Anatomy of Vertebrated Animals. London, 1871.

(449) W. K. Parker. "On the structure and development of the skull of the
Common Fowl (Gallus Domesticus). " Phil. Trans., 1869.

(450) W. K. Parker. "On the structure and development of the skull of the
Common Frog (Rana temporaria)." Phil. Trans., 1871.

(451) W. K. Parker. "On the structure and development of the skull in the
Salmon (Salmo salar)." Bakerian Lecture, Phil. Trans., 1873.

(452) W. K. Parker. "On the structure and development of the skull in the
Pig (Susscrofa)." Phil. Trans., 1874.

(453) W. K. Parker. "On the structure and development of the skull in the
Batrachia." Part II. Phil. Trans., 1876.

(454) W. K. Parker. "On the structure and development of the skull in the
Urodelous Amphibia." Part in. Phil. Trans., 1877.

(455) W. K. Parker. "On the structure and development of the skull in the
Common Snake (Tropidonotus natrix)." Phil. Trans. , 1878.

(456) W. K. Parker. "On the structure and development of the skull in Sharks
and Skates." Trans. Zoolog. Soc., 1878. Vol. x. pt. iv.

(1.17) W. K. Parker. "On the structure and development of the skull in the
Lacertilia." Pt. I. Lacerta agilis, L. viridis and Zootoca vivipara. Phil. Trans.,
1879.

BIBLIOGRAPHY,

(458) W. K. Parker. "The development of the Green Turtle." The Zoolo-v
of the Voyage of H.M.S. Challenger. Vol. I. pt. v.

(459) W. K. Parker. "The structure and development of the skull in the
Batrachia." 1't. in. Phil. Trans., 1880.

(460) W. K. Parker and G. T. Bettany. The Morphology of the Skull.
London, 1877.

(460*) H. Rathke. Entwick. d. Natter. Konigsberg, 1830.

(461) C. B. Reichert. " Ueber die Visceralbogen d. Wirbelthiere." Mailer's
Archiv, 1837.

(462) W. Salensky. " Beitrage z. Entwick. d. knorpeligen Gehorknochelchen."
Morphol. Jahrbuch, Vol. VI. 1880.

Vide also Kolliker (No. 298), especially for the human and mammalian skull;
Gotte (No. 296).

THE PECTORAL GIRDLE.

(463) Bruch. "Ueber die Entwicklung der Clavicula und die Farbe des
Blutes." Zeit.f. wiss. Zool., IV. 1853.

(464) A. Duges. " Recherches sur 1'osteologie et la myologie des Batraciens a
leurs differents ages." Memoires des savants etrang. Academic royale des sciences de
Finstitut de France, Vol. VI. 1835.

(465) C. Gegenbaur. Unterstichungen zur vergleichenden Anatomic der Wir-
belthiere, i Heft. Schultergilrtel der Wirbelthiere. Brustflosse der Fische. Leipzig,
1865.

(466) A. Gotte. "Beitrage z. vergleich. Morphol. d. Skeletsystems d. Wirbel-
thiere : Brustbien u. Schultergiirtel. " Archiv f. mikr. Anat. Vol. XIV. 1877.

(467) C. K. Hoffmann. "Beitrage z. vergleichenden Anatomic d. Wirbel-
thiere." Niederldndisches Archiv f. Zool. , Vol. V. 1879.

(468) W. K. Parker. " A Monograph on the Structure and Development of the
Shoulder-girdle and Sternum in the Vertebrata." Ray Society, 1868.

(469) H. Rathke. Ueber die Entwicklung der Schildkroten. Braunschweig,
1848.

(470) H. Rathke. Ueber den Bau und die Entwicklung des Brustbeins der
Satirier, 1853.

(471) A. Sab a tier. Comparaison des ceintures et des menibres anteneurs et pos-
terieurs d. la Serie d. Vertebrcs. Montpellier, 1880.

(472) Georg 'Swirski. Untersuch. lib. d. Entwick. d. Schultergiirtels u. d.
Skelets d. Brustflosse d. Hechts. Inaug. Diss. Dorpat, 1880.

THE PELVIC GIRDLE.

(473) A. Bunge. Untersuch. z. Entwick. d. Beckengilrtels d. Amphibien,
Reptilien u. Vdgel. Inaug. Diss. Dorpat, 1880.

(474) C. Gegenbaur. " Ueber d. Ausschluss des Schambeins von d. Pfanne
d. Hiiftgelenkes." Morph. Jahrbuch, Vol. II. 1876.

(475) Th. H. Huxley. "The characters of the Pelvis in Mammalia, etc."
Proc. of Roy. Soc., Vol. xxvin. 1879.

(476) A. S aba tier. Comparaison des ceintures et des membres anterieurs ct
postb-ieurs dans la Serie d. Vertebres. Montpellier, 1880.

SKELETON OF THE LIMBS.

(477) M. v. Davidoff. "Beitrage z. vergleich. Anat. d. hinteren Gliedmaassen
d. Fische I." Morphol. Jahrbuch, Vol. v. 1879.

(478) C. Gegenbaur. Untersuchungen z. vergleich. Anat. d. Wirbelthiere.
Leipzig, 1864—5. Erstes Heft. Carpus u. Tarsus. Zweites Heft. Brustflosse d.
Fische.

(479) C. Gegenbaur. "Ueb. d. Skelet d. Gliedmaassen d. Wirbelthiere im
Allgemeinen u. d. Hintergliedmaassen d. Selachier insbesondere." Jenaische Zeil-
schrift, Vol. V. 1870.

XX BIBLIOGRAPHY.

(480) C. Gegenbaur. " Ueb. d. Archipterygium." Jenaische Zeitschrift, Vol.
vn. 1873.

(481) C. Gegenbaur. "Zur Morphologic d. Gliedmaassen d. Wirbelthiere."
Morphologisches Jahrbuch, Vol. II. 1876.

(482) A. Gotte. Ueb. Entwick. u. Regeneration d. Gliedmaassenskelets d. Molche.
Leipzig, 1879.

(483) T. H. Huxley. "On Ceratodus Forsteri, with some observations on the
classification of Fishes." Proc. Zool. Soc. 1876.

(484) St George Mivart. "On the Fins of Elasmobranchii." Zoological
Trans., Vol. x.

(485) A. Rosenberg. "Ueb. d. Entwick. d. Extremitaten-Skelets bei einigen
d. Reduction ihrer Gliedmaassen charakterisirten Wirbelthiere." Zeit.f. wiss. Zool.,
Vol. xxin. 1873.

(486) E. Rosenberg. "Ueb. d. Entwick. d. Wirbelsaule u. d. centrale carpi
d. Menschen." Morphologisches Jahrbuch, Vol. I. 1875.

(487) H. Strasser. "Z. Entwick. d. Extremitatenknorpel bei Salamandern u.
Tritonen." Morphologisches Jahrbuch, Vol. V. 1879.

(488) G. 'S wirski. Unterstich. iib. d. Entwick. d. Schnltergiirtels u. d. Skelets d.
Brustflosse d. Hechts. Inaug. Diss. Dorpat, 1880.

(489) J. K. Thacker. "Median and paired fins. A contribution to the history
of the Vertebrate limbs." Trans, oftke Connecticut Acad., Vol. III. 1877.

(490) J. K. Thacker. "Ventral fins of Ganoids." Trans, of the Connecticut
Acad., Vol. IV. 1877.

PLEURAL AND PERICARDIAL CAVITIES.

(491) M. Cadiat. " Du developpement de la partie cephalothoracique de 1'em-
bryon, de la formation du diaphragme, des pleures, du pericarde, du pharynx et de
1'cesophage." Journal de FAnatomie et de la Physiologic, Vol. xiv. 1878.

VASCULAR SYSTEM.
The Heart.

(492) A. C. Bernays. " Entwicklungsgeschichte d. Atrioventricularklappen."
Morphol. Jahrbuch, Vol. 11. 1876.

(493) E. Gasser. " Ueber d. Entstehung d. Herzens beim Hiihn." Archiv f.
mikr. Anat., Vol. xiv.

(494) A. Thomson. "On the development of the vascular system of the foetus
of Vertebrated Animals." Edinb. New Phil. Journal, Vol. ix. 1830 and 1831.

(495) M. Tonge. "Observations on the development of the semilunar valves
of the aorta and pulmonary artery of the heart of the Chick." Phil. Trans. CLIX.
1869.

Vide also Von Baer (291), Rathke (300), Hensen (182), Kolliker (298), Gotte (296),
and Balfour (292).

The Arterial System.

(496) H. Rathke. "Ueb. d. Entwick. d. Arterien w. bei d. Saugethiere von
d. Bogen d. Aorta ausgehen." Miiller's Archiv, 1843.

(41)7) PI. Rathke. " Untersuchungen iib. d. Aortenwurzeln d. Saurier."
Denkschriften d. k. Akad. Wien, Vol. xiil. 1857.

Vide also His (No. 232) and general works on Vertebrate Embryology.

The Venous System.

(498) J.Marshall. "On the development of the great anterior veins." Phil.
Trans., 1859.

BIHLIOGRAI'IIY. XXJ

(499) H. Rathke. " Ueb. d. Bildung d. Pfortader u. d. Lebervenen b. Sauge-

thieren." Meckel 's Archiv, 1830.

(500) H. Rathke. "Ueb. d. Bau u. d. Entwick. d. Venensystems d. Wirbcl-
thiere." Bericht. iib. d. natttrh. Seminar, d. Univ. Konigsberg, 1838.

Vide also Von Baer (No. 291), Gotte (No. 296), Kolliker (No. 298), and Rathke
(Nos. 299, 300, and 301).

THE SPLEEN.

(501) W. Miiller. "The Spleen." Strieker's Histology.

(502) Peremeschko. "Ueb. d. Entwick. d. Milz." Silz. d. Wien. Akad.
Wiss., Vol. LVI. 1867.

THE SUPRARENAL BODIES.

(503) M. Braun. "Bau u. Entwick. d. Nebennieren bei Reptilian." Arbeit,
a. d. zool.-zoot. Institut Wilrzburg, Vol. v. 1879.

(504) A. v. Brunn. "Ein Beitrag z. Kenntniss d. feinern Baues u. d. Entwick.
d. Nebennieren." Archiv f. mikr. Anat., Vol. vni. 1872.

(505) Fr. Leydig. Untersuch. ilb. Fische u. Reptilien. Berlin, 1853.

(506) Fr. Leydig. Rochen u. Haie. Leipzig, 1852.

Vide also F. M. Balfour (No. 292), Kolliker (No. 298), Remak (No. 302), etc.

THE MUSCULAR SYSTEM OF THE VERTEBRATA.

(507) G.M.Humphry. " Muscles in Vertebrate Animals." J our n. of Anat.
and Phys., Vol. vi. 1872.

(508) J. Miiller. "Vergleichende Anatomic d. Myxinoiden. Part I. Osteologie
u. Myologie." Akad. Wiss., Berlin, 1834.

(509) A. M. Marshall. "On the head cavities and associated nerves of
Elasmobranchs." Quart. J. of Micr. Science, Vol. XXI. 1881.

(510) A. Schneider. "Anat. u. Entwick. d. Muskelsystems d. Wirbelthiere."
Sitz. d. Oberhessischen Gesellschaft, 1873.

(511) A. Schneider. Beitrdge z. vergleich. Anat. u. Entwick. d. Wirbelthiere.
Berlin, 1879.

Vide also Gotte (No. 296), Kolliker (No. 298), Balfour (No. 292), Huxley, etc.

EXCRETORY ORGANS.

INVER TEBRA TA .

(512) H. Eisig. " Die Segmentalorgane d. Capitelliden." Mitth. a. d. zool.
Slat. z. Neapel, Vol. I. 1879.

(513) J. Fraipont. " Recherches s. 1'appareil excreteur des Irematc
Cestoides." Archives de Biologie, Vol. I. 1880.

(514) B. Hatschek. "Studien iib. Entwick. d. Annehden. Arbeit, a. d.
zool. Instil. Wien, Vol. I. 1878. .

(515) B. Hatschek. "Ueber Entwick. von Echmrus, etc. Arbeit, a.

zool. Instit. Wien, Vol. ill. 1880.

VERTEBRATA.

General.

(516) F. M. Balfour. "On the origin and history of the urinogenital organs of
Vertebrates." Journal of Anat. and Phys., Vol. X. 1876.

XXJi BIBLIOGRAPHY.

(517) Max. Fiirbringer1. "Zur vergleichenden Anat. u. Entwick. d. Excre-
tionsorgane d. Vertebraten." Morphol. Jahrbuch, Vol. IV. 1878.

(518) H. Meckel. Zur Morphol. d. Harn- u. Geschlechtswerkz.d. Wirbelthiere,
etc. Halle, 1848.

(519) Job. Mtiller. Bildungsgeschichte d. Genitalien, etc. Diisseldorf, 1830.

(520) H. Ratbke. "Beobachtungen u. Betrachtungen ii. d. Entwicklung d.
Geschlechtswerkzeuge bei den Wirbelthieren." N. Schriften d. naturf. Gesell. in
Dantzig, Bd. I. 1825.

(521) C. Semper1. "Das Urogenitalsystem d. Plagiostomen u. seine Bedeu-
tung f. d. ubrigen Wirbelthiere." Arb. a. d. zool.-zoot. Insiit. Wiirzburg, Vol. u.

1875-

(522) W. Waldeyer1. Eierstock u. Ei. Leipzig, 1870.

ElasmobrancJdi.

(523) A. Schultz. "Zur Entwick. d. Selachiereies." Archiv f. mikr. Anal.,
Vol. xi. 1875.

Vide also Semper (No. 521) and Balfour (No. 292).

Cyclostomata.

(524) J. M uller. " Untersuchungen ii. d. Eingeweide d. Fische. " Abh. d. k.
Ak. Wiss. Berlin, 1845.

(525) W. Muller. "Ueber d. Persistenz d. Urniere b. Myxine glutinosa."
Jenaische Zeitschrift, Vol. VII. 1873.

(526) W. Muller. "Ueber d. Urogenitalsystem d. Amphioxus u. d. Cyclo-
stomen." Jenaische Zeitschrift, Vol. ix. 1875.

(527) A. Schneider. Beitrdge z. vergleich. Anat. u. Entwick. d. Wirbelthiere.
Berlin, 1879.

(528) W. B. Scott. "Beitrage z. Entwick. d. Petromyzonten." Morphol.
Jahrbuch, Vol. vn. 1881.

Teleostei.

(529) J. Hyrtl. "Das uropoetische System d. Knochenfische." Denkschr. d.
k. k. Akad. Wiss. Wien, Vol. II. 1850.

(530) A. Rosenberg. Untersuchungen iib. die Enlwicklung d. Teleostierniere.
Dorpat, 1867.

Vide also Oellacher (No. 72).

Amphibia.

(531) F. H. Bidder. Vergleichend-anatomische u. histologisclie Untcrsiiclniii^cn
ii. die mdnnlichcn Geschlec/its- tmd Harmverkzeuge d. nackten Amphibien. Dorpat,
1846.

(532) C. L. Duvernoy. "Fragments s. les Organes genito-urinaires des
Reptiles," etc. Mem. Acad. Sciences. Paris. Vol. xi. 1851, pp. 17 — 95.

(533) M. Fiirbringer. Zur Entwicklung d. Amphibienniere. Heidelberg, 1877.

(534) F. Ley dig. Analomie d. Amphibien u. Keptilien. Berlin, 1853.

(535) F. Leydig. Lehrbuch d. Histologie. Hamm, 1857.

(536) F. Meyer. "Anat. d. Urogenitalsystems d. Selachier u. Amphibien."
Sitz. d. naturfor. Gesellsch. Leipzig, 1875.

(537) J. W. Spengel. "Das Urogenitalsystem d. Amphibien." Arb. a. d.
zool.- zoot. Instil. Wiirzburg. Vol. in. 1876.

(538) Von Wittich. "Harn- u. Geschlechtswerkzeuge d. Amphibien." Zeit.
f. wiss. Zool., Vol. iv.

Vide also Gotte (No. 296).

1 The papers of Fiirbringer, Semper and Waldeyer contain full references to the
literature of the Vertebrate excretory organs.

BIBLIOGRAPHY. xxiii

Amniota.

(539) F. M. Balfour and A. Sedgwick. "On the existence of ahead-kidney
in the embryo Chick," etc. Quart. J. of Micr. Science, Vol. XIX. 1878.

(540) Banks. On the Wolffian bodies of the foetus and their remains in the adult.
Edinburgh, 1864.

(541) Th. Bornhaupt. UntersucJnmgen iib. die Entwicklung d. Urogenital-
systems beim Hiihnchen. Inaug. Diss. Riga, 1867.

(542) Max Braun. "Das Urogenitalsystem d. einheimischen Reptilien."
Arbeiten a. d. zool.-zoot. Instit. Wiirzburg. Vol. IV. 1877.

(543) J. Dansky u. J. Kostenitsch. " Ueb. d. Entwick. d. Keimblatter u. d.
Wolffschen Ganges im Htihnerei." Me"m. Acad. Imp. Petersbourg, vn. Series, Vol.
xxvn. 1880.

(544) Th. Egli. Beitrdge zur Anat. tmd Entiuick. d. Geschlechtsorgane. Inaug.
Diss. Zurich, 1876.

(545) E. Gasser. Beitrdge zur Entwickhmgsgeschichte d. Allantois, der
MiUler' schen Giinge u. des Afters. Frankfurt, 1874.

(546) E. Gasser. " Beob. iib. d. Entstehung d. WolfFschen Ganges bei Em-
bryonen von Hiihnern u. Gansen." Arch, fiir mikr. Anat., Vol. xiv. 1877.

(547) E. Gasser. "Beitrage z. Entwicklung d. Urogenitalsystems d. Htihner-
embryonen." Sitz. d. Cesell. zur Beforderung d. gesam. Naturwiss. Marburg, 1879.

(548) C. Kupffer. " Untersuchung liber die Entwicklung des Harn- und Ge-
schlechtssystems." Archiv fiir mikr. Anat., Vol. II. 1866.

(549) A. Sedgwick. "Development of the kidney in its relation to the
Wolffian body in the Chick." Quart. J. of Micros. Science, Vol. XX. 1880.

(550) A. Sedgwick. "On the development of the structure known as the
glomerulus of the head -kidney in the Chick." Quart. J. of Micros. Science, Vol. XX.
1880.

(551) A. Sedgwick. "Early development of the Wolffian duct and anterior
Wolffian tubules in the Chick ; with some remarks on the vertebrate excretory
system." Quart. J. of Micros. Science, Vol. xxi. 1881.

(552) M. Watson. "The homology of the sexual organs, illustrated by com-
parative anatomy and pathology." Journal of Anat. and Phys., Vol. XIV. 1879.

(553) E. H. Weber. Zusdtze z, Lehre von Bane u. d. Verrichtungen d. Ge-
schlechtsorgane. Leipzig, 1846.

Vide also Remak (No. 302), Foster and Balfour (No. 295), His (No. 297),
Kolliker (No. 298).

GENERATIVE ORGANS.

(554) G. Balbiani. Lemons s. la generation des Vertebres. Paris, 1879.

(555) F. M. Balfour. "On the structure and development of the Vertebrate
ovary." Quart. J. of Micr. Science, Vol. XVIII.

(556) E. van Beneden. "De la distinction originelledutecticuleet del'ovaire,
etc." Bull. Ac. roy. belgique, Vol. xxxvn. 1874.

(557) N. Kleinenberg. "Ueb. d. Entstehung d. Eier b. Eudendrhim." Zeit.
f. wiss. Zool., Vol. xxxv. 1 88 r.

(558) H. Ludwig. "Ueb. d. Eibildung im Theirreiche. " Arbeit, a. d. zool.-
zoot. Instit. Wiirzburg, Vol. I. 1874.

(559) C. Semper. "Das Urogenitalsystem d. Plagiostomen, etc." Arbeit, a.
d. zool.-zoot. Instit. Wiirzburg, Vol. II. 1875.

(560) A. Weismann. "Zur Frage nach clem Ursprung d. Geschlechtszellen bei
den Hydroiden." Zool. Anzeiger, No. 55, 1880.

Vide also O. and R. Hertwig (No. 271), Kolliker (No. 298), etc.

ALIMENTARY CANAL AND ITS APPENDAGES.

(561) B. Afanassiew. " Ueber Bau u. Entwicklung d. Thymus d. Saugeth."
Archiv f. mikr. Anat. Bd. XIV. 1877.

XXIV BIBLIOGRAPHY.

(562) Fr. Boll. Das Princip d. Wachsthums. Berlin, 1876.

(563) E. Gasser. "Die Entstehung d. Cloakenoffhung hei Hiihneremhryonen."
Archiv f. Anat. u. Physiol., Anat. Abth. 1880.

(564) A. Gotte. Beitrage zur Entwicklungsgeschichte 'd. Darmkanah im
Hithnchcn. 1867.

(565) W. Miiller. " Ueber die Entwickelung der Schilddriise." ycnaische
Zeitschrift, Vol. vi. 1871.

(566) W. Miiller. "Die Hypobranchialrinne d. Tunicaten." Jenaischc Zeit-
schrift, Vol. VII. 1872.

(567) S. L. Schenk. "Die Bauchspeicheldriise d. Embryo." Anatomisch-
physiologische UntersucJnmgcn. 1872.

(568) E. Selenka. " Beitrag zur Entwicklungsgeschichte d. Luftsacke d.
Huhns." Zeit.f. wiss. Zool. 1866.

(569) L. Stieda. Untersuch. lib. d. Entivick. d. Glandula Thymus, Glandula
thyroidea, u. Glandula carotica. Leipzig, 1881.

(570) C. Fr. Wolff. " De formatione intestinorum." Nov. Comment. Akad.
Petrop. 1766.

(571) A. Wblfler. Ueb. d. Entwick. it. d. Ban d. Schilddriise. Berlin, 1880.
Vide also Kolliker (298), Qotte (296), His (232 and 297), Foster and Balfour (2!)5),

Balfour (292), Remak (302), Schenk (303), etc.

Teeth.

(572) T. H. Huxley. "On the enamel and dentine of teeth." Quart. J. of
Micros. Science, Vol. III. 1855.

(573) R. Owen. Odontography. London, 1840 — 1845.

(574) Ch. S. Tomes. Manual of dental anatomy, human and comparative.
London, 1876.

(575) Ch. S. Tomes. " On the development of teeth." Quart. J. of Micros.
Science, Vol. xvi. 1876.

(576) W. Waldeyer. " Structure and development of teeth." Strieker 's His-
tology. 1870.

Vide also Kolliker (298), Gegenbaur (294), Hertwig (306), etc.

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