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==Preface to Volume II==
==Preface==


THE present volume completes my treatise on Comparative Embryology. The first eleven chapters deal  
The present volume completes my treatise on Comparative Embryology. The first eleven chapters deal  
with the developmental history of the Chordata. These  
with the developmental history of the Chordata. These  
are followed by three comparative chapters completing  
are followed by three comparative chapters completing  
Line 96: Line 96:
Parker, Turner, and Bridge, I am also greatly indebted  
Parker, Turner, and Bridge, I am also greatly indebted  
for their suggestions with reference to special chapters of  
for their suggestions with reference to special chapters of  
the work.  
the work.
 
 


==Contents Of Volume II==
==Contents Of Volume II==


CHAPTER I. CEPHALOCHORDA. Pp. t 8.
# [[The Works of Francis Balfour 3-1|'''Cephalochorda''']] Segmentation and formation of
 
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. 6873. 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 notochord, pp. 86, 87. General history of the development, pp. 87 97. Metamorphosis, 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. 111119. 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. 125127. Mesoblast and notochord, pp. 128, 129.
Hypoblast, pp. 129131. General groivth of the embryo, pp. 131 143. Anura,
pp. 131 141. Urodela, pp. 141 143. Gymnophiona, p. 143.
 
 
CHAPTER VIII. AVES. Pp. 145201.
 
- Segmentation and formation of the layers, pp. 145166. General history of
the germinal layers, pp. 166169. General development of the embryo, pp. 169
180. Fa-tal membranes, pp. 185199- Amnion, pp. 185191. Allantois,
pp. 191 193. Yolk-sack, pp. 193 199
CHAPTER IX. REPTILIA. Pp. 202213.
 
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. 208210. Ophidia, p. 210. Ckelonia, pp. 210212.
 
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. 239257. Comparative histology of the placenta, pp. 257259. Evolution of the placenta,
pp t 25 g 2 6i. 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 DEVELOPMENT OF VERTEBRATES. Pp. 275 310.
 
Formation of the gastrula, pp. 275292. The formation of the mesoblast
and of the notochord, pp. 292300. The epiblast, pp. 300304. Formation of
the central nervous system, pp. 301304. 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 delamination 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.
 
 
 
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. 400469.
 
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 development 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 Vertebrata, 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 processes, 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- S 1 ^, 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. 538540. 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. Bibliography of the origin of the skeleton, pp. 544, 545. The notochord and its cartilaginous sheath, pp. 545 549. The vertebral arches and the vertebral bodies, pp. 549
559- Cyclostomata, p. 549. Elasmobranchii, pp. 549553. 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. 560562. Sternum, pp. 562, 563.
Bibliography of the ribs and sternum, p. 563.
 
CHAPTER XIX. THE SKULL. Pp. 564598.
 
Preliminary remarks, pp. 564, 565. The cartilaginous cranium, pp.
 
565571. The parachordals and notochord, pp. 566, 567. The trabecula',
pp. 567570. 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. 576579. Teleostei, pp. 579581. Amphibia, pp. 581588. Sauropsida, pp. 588, 589. Mammalia, pp. 589591.
 
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. 599601. 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 cheiropterygium, pp. 618622. Bibliography of limbs, p. 622.
 
CHAPTER XXI. THE BODY CAVITY, THE VASCULAR SYSTEM AND THE
VASCULAR GLANDS. Pp. 623 666.
 
The body cavity, pp. 623632. General, pp. 623, 624. Chordat'a, pp.
624632. Abdominal pores, pp. 626, 627. Pericardial cavities, pleural cavities
and diaphragm, pp. 627 632. Bibliography of body cavity, p. 632.
 
The Vascular System, pp. 632663. General, pp. 632, 633. The heart,
pp. 633643. Bibliography of the heart, p. 643. Arterial system, pp. 643651.
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. 664666.
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 Chordata, 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.
 
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. Excretory organs and generative ducts of the Craniata, pp. 689737.
General, pp. 689, 690. Elasmobranchii, pp. 690 699. Cyclostomata, pp. 700,
701. Teleostei, pp. 701 704. Ganoidei, pp. 704707. Dipnoi, p. 707.
Amphibia, pp. 707 713. Amniota, pp. 713 727. General conclusions
and summary, pp. 728737. Pronephros, pp. 728, 729. Mesonephros, pp.
729732. Genital ducts, pp. 732736. Metanephros, pp. 736, 737. Comparison 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.
 
741753
Generative organs, pp. 741748. 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. 746748. Bibliography of generative organs, p. 748.
 
Genital ducts, pp. 748753.
 
CHAPTER XXV. THE ALIMENTARY CANAL AND ITS APPENDAGES IN
THE CHORDATA. Pp. 754780.
 
Mesenteron, pp. 754774- Subnotochordal rod, pp. 7S475 6 - Splanchnic mesoblast and mesentery, pp. 756758. Respiratory division of the Mesenteron, pp. 758766. Thyroid body, pp. 759762. Thymus gland, pp. 762, 763.
Swimming bladder and lungs, pp. 763766, The middle division of the Mesenteron, pp. 766771. Cloaca, pp. 766, 767. Intestine, pp. 767, 768. Liver,
pp. 769, 770. Pancreas, pp. 770, 771. Posjtanal section of the Mesenteron, pp.
771774.
 
The stomodseum, pp. 774 778. Comparative development of oral cavity,
PP- 774776. Teeth, pp. 776778.
 
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 considered ; 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 Vertebrata 1 . The members of the second and
probably of the first of these groups have undergone degeneration, 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 indications 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 invested 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 hypoblastic.
 
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 ;
so 1 , so 11 , 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 somites 1 . 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 canal 2 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 archenteron 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 continuous 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 understood 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 diverticula 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 continuous 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 mesoblast, and the dorsal to a section of the voluntary muscular
system. The anterior mesoblastic somite long retains its communication 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 backwards 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 mesoblastic 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 afterwards 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 earlier 1 .
 
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.
 
 
 
UROCHORDA 1 .
 
 
 
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 segmentation cavity appears fairly early, and is surrounded, according 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 archenteron, 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 longitudinal 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 MAMMILcanal 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 . notoc b hord ; %.,. neura i
 
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 meantime 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 blastopore 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 a i
 
system in this part also dilates and
forms what may be called the cephalic swelling (fig. 8 IV.), and the
pore at its anterior extremity
gradually narrows and finally disappears. In the region of the tail
we have seen that the dorsal wall of
the archenteron becomes converted
into the notochord, which immediately 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 archenteron (fig. 7, me) in the tail become
converted into elongated cells arranged longitudinally, which form
powerful lateral muscles (fig. 8 IV.
tri). After the formation of the notochord 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 neurenteric 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 MAMMI LLAT A. (After Kowalevsky.)
 
The embryo is of the same age
as fig. 8 ill, but is seen in longitudinal 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. notochord ; 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' . hypoblast 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 head 1 .
 
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 investment 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, developed 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 fluid 2 . After
these papillae have become formed cells first make their appearance 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 considerable 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 condition 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 organ 1 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 superficial 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 developed. 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 peribranchial 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 previous insertion of the
tail in the trunk.
 
The nervous system
also undergoes a very
rapid retrogressive metamorphosis ; 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, including 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
number 1 .
 
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 longitudinal 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 longitudinal 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 completion, the openings of the two atrial sacks gradually approximate 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 involutions 1 , 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 employed 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 segmentation 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 termination 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 ringlike 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 epithelioid 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 epiblastic 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 represented 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 alimentary 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 alimentary cavity becomes, by the
growth of the layer of cells described in the last stage, a complete 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 PYROSOMA SHORTLY BEFORE ITS DIVISION
 
INTO CYATHOZOOID AND ASCIDIOZOOIDS. (After Kowalevsky.)
 
cl. cloacal (atrial) opening; en. endostyle ; 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 Cyathozooid 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 undivided 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 eventually 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 external 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 absorbed ; 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 however 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 together, and united by a vascular tube with the Cyathozooid, and
these connections are not severed till the latter completely atrophies. 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 transparent 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 branchial 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 commences 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 Todaro 1 .
 
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 thickening 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 broodpouch 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 system 1 .
 
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 mesoblast, 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 satisfactory. 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 segmentation 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 unfortunately not satisfactorily described. The folds of the amnion project towards the
placenta, and enclose a cavity which, as the folds never completely meet, is permanently 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 respiratory 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 alternation 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 generations.
 
In the simpler cases, which occur in some Composita
Sedentaria, the process of budding commences with an outgrowth of the body wall into the common test, containing a
prolongation of part of the alimentary tract 1 .
 
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 embryo 1 . The bud eventually 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 commenced by the division of embryos at an early stage of development, and have gradually 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 Pyrosomidse.
 
1 The atrial spaces form somewhat doubtful exceptions to the rule.
 
32
 
 
 
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 alternations 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 rudimentary 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 Ascidiozooids 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 different 1 . 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 independent existence. The solitary asexual Salp clearly corresponds 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 sack 1 .
 
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 openings. 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 following 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 Cellulose-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 Ascidien." 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. Wirbelthieren." 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" (Development 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 animaux (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 Generationswechsel 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, Carcharias, 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, Notidanus, Acanthias, Scymnus, Galeus, Squalus, Mustelus, Carcharias, Sphyrna,
Squatina, Torpedo ; and the following of oviparous genera : Scyllium, Pristiurus, Cestracion, Raja 1 .
 
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 membrane 2 . The greater part of it is formed
of peculiar oval spherules of food-yolk, held together by a
protoplasmic network. The protoplasm is especially concentrated 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 segmentation has the normal meroblastic character (fig. 15) and is
confined to the germinal disc. Before it commences the germinal disc exhibits amoeboid movements. During the segmentation 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 segmentation 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 somewhat 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. hypoblast ; 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 Amphioxus, 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 continuous 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 network 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 disappearance 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 medullary groove ; and its termination at the edge of the blastoderm 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 medullary 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 undifferentiated. A short way from the
edge the lower layer cells
become divided into two distinct layers, a lower one continuous with the hypoblast
in the middle line, and an
upper one between this and
the epiblast (fig. 18 B). The
upper layer is the commencement 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 mesoblast 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 continuous 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 outgrowths of the wall of the alimentary tract ; which differ from the
mesoblastic outgrowths of the wall of the archenteron in Amphioxus 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 continuous 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 distinctly 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.
 
 
 
4 6
 
 
 
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 outlines. 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 represented 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 concentrates 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 differentiation 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
 
 
 
4 8
 
 
 
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, equivalent to the cavity originating as an
outgrowth of the archenteron in Amphioxus. The body cavities of the
two sides are at first quite independent.
 
Coincidentally with the appearance 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 mesoblastic somites. Only the dorsal
parts of the plates become split in
this way, their ventral parts remaining quite intact. As a result of this
each plate becomes divided into a dorsal portion adjoining the
medullary canal, which is divided 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 originally extends uninterruptedly to the summit of the vertebral 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 SECTION THROUGH THE TRUNK OF
AN EMBRYO SLIGHTLY OLDER
THAN FIG. 28 E.
 
nc. neural canal ; pr. posterior
root of spinal nerve ; x. subnotochordal rod ; ao. aorta ; sc. somatic mesoblast ; sp. splanchnic mesoblast ; mp. muscle-plate ; mp' .
portion of muscle -plate converted
into muscle ; Vv. portion of the
vertebral plate which will give
rise to the vertebral bodies ; al.
alimentary tract.
 
 
 
 
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 unsegmented 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 continuous 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 remain 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 primitive 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 mesoblast 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 ?' e P iblast 5 vc - P ouch 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 verymuch 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 backwards. 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
 
42
 
 
 
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 difficult to say at what point
the separation of this rod
from the hypoblast is completed, since all intermediate gradations between
complete separation and
complete attachment are to
be seen.
 
Shortly after the separation 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 notochord.
 
Ch. notochord ; Ch' . developing notochord ; mg.
medullary groove ; Ip. lateral plate of mesoblast ;
ep. epiblast ; Ay. hypoblast.
 
 
 
thick bridge is found connecting the two lateral
halves of the hypoblast,
but this bridge is anteriorly 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 Elasmobranchii tend towards the former view ; but the nearly simultaneous 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 PRISTIURUS 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 blastoderm. The other is a simple growth
of cells independent of any fold.
To the first of these processes the
depth and narrowness of the alimentary cavity is due ; the second
is concerned in forming its ventral
wall. The 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 differentiation 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 alimentary canals.
 
nc. neural canal ; al. alimentary tract ; Ch. notochord ; 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 alimentary 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 immediately below the notochord, and disappears during embryonic life.
 
 
 
 
FIG. 27*. TRANSVERSE
SECTION THROUGH THE
TAIL REGION OF A PRISTIURUS 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 mesoblast ; mp. commencing
differentiation of muscles; ch. notochord; x.
sub-notochordal rod arising as an outgrowth of
the dorsal wall of the
alimentary tract ; al. alimentary 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 blastoderm 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 established, 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 continuous 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 outwards, 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 progressed, 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 spontaneous movements
take place, and consist
in somewhat rapid excursions of the embryo
from side to side, produced 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-brain 1 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 unquestionably 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 movements.
 
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 behind the stalk.
 
The cranial flexure
has greatly increased,
and the angle between
the long axis of the
front part of the head
 
and of the body is less (
 
_, 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 auditory 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 opacity 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 branchial 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 ELASMOHRANCH EMBRYOS AT TWO STAGES AS TRANSPARENT 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 curvature 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 characteristic 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 represented 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 diminution is due to the increase in the size of the cerebral hemispheres,
which grow upwards and forwards, and press the original forebrain 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 anteroposterior 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 attached, forms a bay in
the otherwise regular
outline of the edge of
the blastoderm, and by
the time that about twothirds of the yolk is enclosed 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 embryo at this time is only
connected with the yolksack by a narrow umbilical cord ; but, as shewn
in the figure, is still attached to the edge of the
 
 
 
blastoderm.
 
Shortly subsequent to
this the bay in the blastoderm, at the head of
which the embryo is attached, becomes obliterated by its two sides coming together and coalescing. The embryo then
ceases to be attached at
the edge of the blastoderm. 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 attached at the edge of the blastoderm.
 
B. Older stage with the yolk not quite enclosed 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. portion 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. Throughout all this period the edge of the blastoderm has remained
thickened : a feature which persists till the complete investment
of the yolk, which takes 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 tailswellings, 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 bloodvessels 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 blastoderm 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 connected 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 Elasmobranch 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. Wirbellosen." 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.
 
 
 
52
 
 
 
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 Syngnathus 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. Ostegeniosus 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 segmentation is complete, as well as the solid origin of many of the
organs, receives its most plausible explanation on this hypothesis.
 
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 segmentation 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 granular 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 blastoderm ; 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 doubtful 1 . 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 homologous 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 swelling ; 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 segmentation 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 Elasmobranchii, 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 blastoderm 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 hypoblastic floor and an epiblastic roof several cells deep. It
becomes obliterated shortly after the appearance of the medullary 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 Elasmobranchs. 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 gradually 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 vertebrate 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 unimportant 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 epithelial 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 completed, 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 epiblast at its point of junction with the medullary
keel ; and the optic vesicles spring as solid outgrowths from part of the
keel itself. The olfactory
pits are barely indicated
as thickenings of the nervous layer of the epiblast.
 
 
 
 
FlG. 32. TWO TRANSVERSE SECTIONS OF
 
SYNGNATHUS. (After Calberla. )
 
A. Younger stage before the definite establishment of the notochord.
 
B. Older stage.
 
The epidermic layer of the epiblast is represented in black.
 
ep. epidermic layer of epiblast ; me. neural
cord ; hy. hypoblast ; me. mesoblast ; ch. notochord.
 
 
 
At this early stage all the
organs of special sense are attached 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 yolksack; 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 Elasmobranchs. 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 postanal 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.
 
 
 
7 6
 
 
 
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 somewhat 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 separated 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 remarkable for the small apparent amount of the cranial flexure. This
is probably due to the late development of the cerebral hemispheres. The flexure of the floor
of the brain is however quite as
considerable in the Teleostei as in
other types. The gill clefts develop 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 opercular 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 almost to nothing in the larva.
 
The oesophagus becomes
solid, like that of Elasmobranchs,
and remains so for a considerable 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 Gegenbaur.)
 
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 pronephros 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 fin 1 , 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 simultaneous 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 region where they are present
soon forms a lobe; and an
externally heterocercal tail is
produced (fig. 36 A). The
ventral lobe with its rays continues to grow more prominent and causes the tail fin to
become bilobed (fig. 36 B) ;
there being a dorsal embryonic lobe without fin rays (c),
which contains the notochord,
and a ventral lobe with fin
rays, which will form the permanent 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 gradually 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 DEOF 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 atrophied.
 
c. embryonic caudal fin ; f. permanent
caudal fin ; n. notochord ; it. urostyle.
 
 
 
the body ; and this lobe, together 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 belonging 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-embryonic changes, the most remarkable of which are those undergone by the
Pleuronectidae 1 . 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 bloodvessels.
 
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 subintestinal vessel breaks up into capillaries in the liver, thence passes into the yolksack, 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. Development 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 development 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 Teleostier." 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 developpement 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.
CYCLOSTOMATA 1 .
 
PETROMYZON is the only type of this degenerated but
primitive group of Fishes the development of which has been as
yet studied 2 .
 
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 modification, otherwise only known to occur in Teleostei and
Lepidosteus.
 
Dohrn 3 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.
 
62
 
 
 
8 4
 
 
 
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 gnathostomatous 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 perforated layer, and an outer
structureless layer. There
appears to be a pore perforating the inner layer at
the formative pole, which
may be called a micropyle
(KupfTer and Benecke, No.
79). Enclosing the eggmembranes 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 made 1 .
 
 
 
U
 
 
 
 
FIG. 37. LONGITUDINAL VERTICAL SECTION THROUGH AN EMBRYO OF PETROMYZON
 
PLANERI OF 136 HOURS.
 
me. mesoblast ; yk. yolk-cells ; al. alimentary 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 invagination (fig. 37) which leads to a mode of formation of the hypoblast 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 segmentation 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 prominent 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 invagination. 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. Elasmobranchii, 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 enclosed 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 constituted of a single layer only.
When the mesoblast is fairly 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 between the medullary cord
(nc) (which has by this time
been formed) and a continuous 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 features in the development of
the notochord were first established by Calberla 1 (No.
78).
 
General history of the development. 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. notochord ; yk. yolk-cells ; al. alimentary canal.
 
 
 
m C.
 
 
 
 
FIG. 40. TRANSVERSE SECTION THROUGH
PART OF AN EMBRYO OF PETROMYZON PLANERI 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 contains 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 blastopore 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.
 
 
 
8 9
 
 
 
ventral surface of the yolk (B). With the growth of the embryo 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 eggshell. 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 ;

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Foster M. and Sedgwick A. The Works of Francis Balfour Vol. III. A Treatise on Comparative Embryology 2 (1885) MacMillan and Co., London.

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This historic 1930 book edited by Foster and Sedgwick is the third of Francis Balfour's collected works published in four editions. Francis (Frank) Maitland Balfour, known as F. M. Balfour, (November 10, 1851 - July 19, 1882) was a British biologist who co-authored embryology textbooks.



The Works of Francis Balfour

Foster M. and Sedgwick A. The Works of Francis Balfour Vol. I. Separate Memoirs (1885) MacMillan and Co., London.

Foster M. and Sedgwick A. The Works of Francis Balfour Vol. II. A Treatise on Comparative Embryology 1. (1885) MacMillan and Co., London.

Foster M. and Sedgwick A. The Works of Francis Balfour Vol. III. A Treatise on Comparative Embryology 2 (1885) MacMillan and Co., London.

Foster M. and Sedgwick A. The Works of Francis Balfour Vol. IV. Plates (1885) MacMillan and Co., London.


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The Works of Francis Balfour

Francis Balfour (1851-1882)
Francis Balfour (1851-1882)

Volume III. A Treatise on Comparative Embryology 2

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.


London :

MACMILLAN AND CO. 1885


Preface

The present volume completes my treatise on Comparative 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 Embryology. The remainder of the treatise, from Chapter XIV. onwards, is devoted to Organogeny. For the reasons stated in the introduction to this part the organogeny 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 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

  1. Cephalochorda Segmentation and formation of the layers, Central nervous system, Mesoblast, General history of larva.
  2. Urochorda Solitaria, Development of embryo, Growth and structure of free larva, Retrogressive metamorphosis, Sedentaria, Natantia, Doliolida:, Salpida, Appendicularia, Metagenesis.
  3. Elasmobranchii 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.
  4. Teleostei Segmentation and formation of the layers, General history of the layers, General development of the embryo.
  5. Cyclostomata Segmentation and formation of the layers, Mesoblast and notochord, General history of the development, Metamorphosis, Myxine.
  6. Ganoidei Acipenser, no. Segmentation and formation of the layers, General development of the embryo and larva, Lepidosleus, Segmentation, General development of embryo and larva, General observations on the embryology of Ganoids.
  7. Amphibia. Oviposition and impregnation, Formation of the layers, Epiblast, Mesoblast and notochord, Hypoblast, General growth of the embryo, Anura, Urodela, Gymnophiona.
  8. Aves Segmentation and formation of the layers, General history of the germinal layers, General development of the embryo, Fetal membranes, Amnion, Allantois, Yolk-sack.
  9. Reptilia Lacertilia, Segmentation and formation of the layers, General development of the embryo, Embryonic membranes and yolk-sack, Ophidia, Ckelonia.
  10. Mammalia Segmentation and formation of the layers, General growth of the embryo, Embryonic membranes and yolk-sack, Comparative history of the Mammalian foetal membranes, Comparative histology of the placenta, Evolution of the placenta, Development of the Guinea-pig, The human embryo.
  11. Comparison Of The Formation Of The Germinal Layers And Of The Early Stages In The Development Of Vertebrates Formation of the gastrula, The formation of the mesoblast and of the notochord, The epiblast, Formation of the central nervous system, Formation of the organs of special sense, Summary of organs derived from the three germinal layers, Growth in length of the Vertebrate embryo, The evolution of the allantois and amnion.
  12. Observations On The Ancestral Form Of The Chordata. General considerations, The medullary canal, The origin and nature of the mouth, The cranial flexure, The postanal gut and neurentcric canal, The body-cavity and mesoblastic somites, The notochord, Gill clefts, Phylogeny of the Chordata.
  13. General Conclusions I. Mode of origin and homologies of the germinal layers, Formation of the primary germinal layers, Invagination, Delamination, Phylogenetic significance of delamination and invagination, Homologies of the germinal layers, IT- .545' 346- The origin of the mesoblast, EL Larval forms: their nature, origin, and affinities. Preliminary considerations, pp. 360 362. Types of larva-, pp. 363 384. Phylogenetic conclusions, General conclusions and summary, pp. 385, 386.
  14. The Epidermis And Its Derivatives. Protective epidermic structures, Dermal skeletal structures, Glands.
  15. The Nervous System The origin of the nervous system, Nervous system of the Invertebrata, Central nervous system of the Vertebrata, Spinal chord, General development of the brain, Hind-brain, Mid-brain, General development of fore-brain, Thalamencephalon, Pituitary body, Cerebral Hemispheres, Olfactory lobes, General conclusions as to the central nervous system of the Vertebrata, Development of the cranial and spinal nerves, Spinal nerves, Cranial nerves, Sympathetic nervous system.
  16. Organs Of Vision Coelenterata, Mollusca, Chsetopoda, Chastognatha, Arthropoda, Vertebrata general, Retina, Optic nerve, Choroid fissure, Lens, Vitreous humour, pp. 494, 495. Cornea, Aqueous humour, Comparative development of Vertebrate eye, Ammoccete eye, Optic vesicles, Lens, Cornea, Optic nerve' and choroid fissure, pp. 500 505. Iris and ciliary processes, p. 506. Accessory organs connected with the eye, Eyelids, Lacrymal glands, Lacrymal duct, Eye of the Tunicata, Accessory eyes in the Vertebrata.
  17. Auditory Organ, Olfactory Organ, And Sense Organs Of The Lateral Line Auditory organs, General structure of auditory organs, Auditory organs of the Coelenterata, Auditory organs of the Mollusca, Auditory organs of the Crustacea, p. 516. Auditory organs of the Verlebrata, Auditory vesicle, Organ of Corti, Accessory structures connected with the organ of hearing of terrestrial vertebrata, Auditory organ of the Tunicata, Bibliography of Auditory organs, Olfactory organs, Bibliography of Olfactory organs, Sense organs of the lateral line, Bibliography of sense organs of lateral line.
  18. The Notochord, The Vertebral Column, The Ribs, And The Sternum Introductory remarks on the origin of the skeleton, Bibliography of the origin of the skeleton, The notochord and its cartilaginous sheath, The vertebral arches and the vertebral bodies, Cyclostomata, Elasmobranchii, Ganoidei, Teleostei, Amphibia, Reptilia, Aves, Mammalia, Bibliography of the notochord and vertebral column, Ribs, Sternum, Bibliography of the ribs and sternum.
  19. The Skull Preliminary remarks, The cartilaginous cranium, The parachordals and notochord, The trabecula, The sense capsules, The branchial skeleton, General structure of, Mandibular and hyoid arches, Elasmobranchii, Teleostei, Amphibia, Sauropsida, Mammalia, Membrane bones and ossifications of the cranium, Membrane bones, Ossifications of the cartilaginous cranium, Labial cartilages, Bibliography of the skull.
  20. Pectoral And Pelvic Girdles And The Skeleton Of The Limbs The Pectoral girdle, Pisces, Amphibia and Amniota, Lacertilia, Chelonia, Aves, Mammalia, Amphibia, Bibliography of Pectoral girdle, The Pelvic girdle, Pisces, Amphibia and Amniota, Amphibia, Lacertilia, Mammalia, Bibliography of Pelvic girdle, Comparison of pectoral and -pelvic girdles, Limbs, The piscine fin, The cheiropterygium, Bibliography of limbs.
  21. The Body Cavity, The Vascular System And The Vascular Glands The body cavity, General, Chordata, Abdominal pores, Pericardial cavities, pleural cavities and diaphragm, Bibliography of body cavity, The Vascular System, General, The heart, Bibliography of the heart, Arterial system, Bibliography of the arterial system, Venous system, Bibliography of the venous system, Lymphatic system and spleen, Bibliography of spleen, Suprarenal bodies, Bibliography of suprarenal bodies.
  22. The Muscular System Evolution of muscle-cells, Voluntary muscular system of the Chordata, Muscular fibres, Muscular system of the trunk and limbs, The somites and muscular system of the head, Bibliography of muscular system,
  23. Excretory Organs Platyelminthes, Mollusca, Polyzoa, Branchiopoda, Choctopoda, Gephyrea, Discophora, Arthropocla, Nematoda, Excretory organs and generative ducts of the Craniata, General, Elasmobranchii, Cyclostomata, Teleostei, Ganoidei, Dipnoi, Amphibia, Amniota, General conclusions and summary, Pronephros, Mesonephros, Genital ducts, Metanephros, Comparison of the excretory organs of the Chordata and Invertebrata, Bibliography of Excretory organs.
  24. Generative Organs And Genital Ducts Generative organs, Porifera, Coelenterata, Chtetopoda and Gephyrea, Chastognatha, Polyzoa, Nematoda, Insecta, Crustacea, Chordata, Bibliography of generative organs, Genital ducts.
  25. The Alimentary Canal And Its Appendages In The Chordata Mesenteron, Subnotochordal rod, Splanchnic mesoblast and mesentery, Respiratory division of the Mesenteron, Thyroid body, Thymus gland, Swimming bladder and lungs, The middle division of the Mesenteron, Cloaca, Intestine, Liver, Pancreas, Posjtanal section of the Mesenteron, The stomodseum, Comparative development of oral cavity, Teeth, The proctodseum, Bibliography of alimentary canal.
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