Fig. 423. Transverse optical section of the tail of an embryo of
Phallusia mammillata. (After Kowalevsky.)
The section is from an embryo of the same age as fig. 8 IV.
ch. notochord; n.c. neural canal; me. mesoblast; al´.
hypoblast of tail.
In Amphioxus the postanal gut, though distinctly developed, is not very long, and atrophies at a comparatively early period.
In Elasmobranchii this section of the alimentary tract is very well developed, and persists for a considerable period of embryonic life. The following is a history of its development in the genus Scyllium.
Shortly after the stage when the anus has become marked out by the alimentary tract sending down a papilliform process towards the skin, the postanal gut begins to develop a terminal dilatation or vesicle, connected with the remainder of the canal by a narrower stalk.
The walls both of the vesicle and stalk are formed of a fairly columnar epithelium. The vesicle communicates in front by a narrow passage with the neural canal, and behind is continued into two horns corresponding with the two caudal swellings previously spoken of (p. 55). Where the canal is continued into these two horns, its walls lose their distinctness of outline, and become continuous with the adjacent mesoblast.
In the succeeding stages, as the tail grows longer and longer, the postanal section of the alimentary tract grows with it, without however undergoing alteration in any of its essential characters. At the period of the maximum development, it has a length of about 1⁄3 of that of the whole alimentary tract.
Its features at a stage shortly before the external gills have become prominent are illustrated by a series of transverse sections through the tail (fig. 424). The four sections have been selected for illustration out of a fairly-complete series of about one hundred and twenty.
Posteriorly (A) there is present a terminal vesicle (alv) .25 mm. in diameter, which communicates dorsally by a narrow opening with the neural canal (nc); to this is attached a stalk in the form of a tube, also lined by columnar epithelium, and extending through about thirty sections (B al). Its average diameter is about .084 mm., and its walls are very thick. Overlying its front end is the subnotochordal rod (x), but this does not extend as far back as the terminal vesicle.
The thick-walled stalk of the vesicle is connected with the cloacal section of the alimentary tract by a very narrow thin-walled tube (C al). This for the most part has a fairly uniform calibre, and a diameter of not more than .035 mm. Its walls are formed of flattened epithelial cells. At a point not far from the cloaca it becomes smaller, and its diameter falls to .03 mm. In front of this point it rapidly dilates again, and, after becoming fairly wide, opens on the dorsal side of the cloacal section of the alimentary canal just behind the anus (D al).
Fig. 424. Four sections through the postanal part of the tail
of an embryo of the same age as fig. 28 F.
A. is the posterior section.
nc. neural canal; al. postanal gut; alv. caudal vesicle of
postanal gut; x. subnotochordal rod; mp. muscle-plate; ch.
notochord; cl.al. cloaca; ao. aorta; v.cau. caudal vein.
Very shortly after the stage to which the above figures belong, at a point a little behind the anus, where the postanal section of the canal was thinnest in the previous stage, it becomes solid, and a rupture here occurs in it at a slightly later period.
The atrophy of this part of the alimentary tract having once commenced proceeds rapidly. The posterior part first becomes reduced to a small rudiment near the end of the tail. There is no longer a terminal vesicle, nor a neurenteric canal. The portion of the postanal section of the alimentary tract, just behind the cloaca, is for a short time represented by a small rudiment of the dilated part which at an earlier period opened into the cloaca.
In Teleostei the vesicle at the end of the tail, discovered by Kupffer, (fig. 34, hyv) is probably the equivalent of the vesicle at the end of the postanal gut in Elasmobranchii.
In Petromyzon and in Amphibia there is a well-developed postanal gut connected with a neurenteric canal which gradually atrophies. It is shewn in the embryo of Bombinator in fig. 420.
Fig. 425. Diagrammatic longitudinal section through the posterior
end of an embryo Bird at the time of the formation of the Allantois.
ep. epiblast; Sp.c. spinal canal; ch. notochord; n.e.
neurenteric canal; hy. hypoblast; p.a.g. postanal gut; pr.
remains of primitive streak folded in on the ventral side; al.
allantois; me. splanchnic mesoblast; an. point where anus will
be formed; p.c. perivisceral cavity; am. amnion; so.
somatopleure; sp. splanchnopleure.
Amongst the amniotic Vertebrata the postanal gut is less developed than in the Ichthyopsida. A neurenteric canal is present for a short period in various Birds (Gasser, etc.) and in the Lizard, but disappears very early. There is however, as has been pointed out by Kölliker, a well-marked postanal gut continued as a narrow tube from behind the cloaca into the tail both in the Bird (fig. 425, p.a.g.) and Mammals (the Rabbit), but especially in the latter. It atrophies early as in lower forms.
The morphological significance of the postanal gut and of the neurenteric canal has already been spoken of in Chapter XII., p. 323.
The Stomodæum.
The anterior section of the permanent alimentary tract is formed by an invagination of epiblast, constituting a more or less considerable pit, with its inner wall in contact with the blind anterior extremity of the alimentary tract.
In Ascidians this pit is placed on the dorsal surface (fig. 9, o), and becomes the permanent oral cavity of these forms. In the larva of Amphioxus it is stated to be formed unsymmetrically (vide p. 5), but further observations on its development are required.
Fig. 426. Longitudinal section through the brain of a young
Pristiurus embryo.
cer. unpaired rudiment of the cerebral hemispheres; pn. pineal
gland; In. infundibulum; pt. ingrowth from mouth to form the
pituitary body; mb. mid-brain; cb. cerebellum; ch. notochord;
al. alimentary tract; Iaa. artery of mandibular arch.
In the true Vertebrata it is always formed on the ventral surface of the head, immediately behind the level of the fore-brain (fig. 426), and is deeper in Petromyzon (fig. 416, m) than in any other known form.
From the primary buccal cavity or stomodæum there grows out the pituitary pit (fig. 426, pt), the development of which has already been described (p. 435).
The wall separating the stomodæum from the mesenteron always becomes perforated, usually at an early stage of development, and though in Petromyzon the boundary between the two cavities remains indicated by the velum, yet in the higher Vertebrata all trace of this boundary is lost, and the original limits of the primitive buccal cavity become obliterated; while a secondary buccal cavity, partly lined by hypoblast and partly by epiblast, becomes established.
This cavity, apart from the organs which belong to it, presents important variations in structure. In most Pisces it retains a fairly simple character, but in the Dipnoi its outer boundary becomes extended so as to enclose the ventral opening of the nasal sack, which thenceforward constitutes the posterior nares.
In Amphibia and Amniota the posterior nares also open well within the boundary of the buccal cavity.
In the Amniota further important changes take place.
In the first place a plate grows inwards from each of the superior maxillary processes (fig. 427, p), and the two plates, meeting in the middle line, form a horizontal septum dividing the front part of the primitive buccal cavity into a dorsal respiratory section (n), containing the opening of the posterior nares, and a ventral cavity, forming the permanent mouth. The two divisions thus formed open into a common cavity behind. The horizontal septum, on the development within it of an osseous plate, constitutes the hard palate.
Fig. 427. Diagram shewing the division of the primitive buccal
cavity into the respiratory section above and the true mouth
below. (From Gegenbaur.)
p. palatine plate of superior maxillary process; m. permanent
mouth; n. posterior part of nasal passage; e. internasal
septum.
An internasal septum (fig. 427, e) may more or less completely divide the dorsal cavity into two canals, continuous respectively with the two nasal cavities.
In Mammalia a posterior prolongation of the palate, in which an osseous plate is not formed, constitutes the soft palate.
The second change in the Amniota, which also takes place in some Amphibia, is caused by the section of the mesenteron into which the branchial pouches open, becoming, on the atrophy of these structures, converted into the posterior part of the buccal cavity.
The organs derived from the buccal cavity are the tongue, the various salivary glands, and the teeth; but the latter alone will engage our attention here.
The teeth. The teeth are to be regarded as a special product of the oral mucous membrane. It has been shewn by Gegenbaur and Hertwig that in their mode of development they essentially resemble the placoid scales of Elasmobranchii, and that the latter structures extend in Elasmobranchii for a certain distance into the cavity of the mouth.
As pointed out by Gegenbaur, the teeth are therefore to be regarded as more or less specialised placoid scales, whose presence in the mouth is to be explained by the fact that the latter structure is lined by an invagination of the epidermis; The most important developmental point of difference between teeth and placoid scales consists in the fact, that in the case of the former there is a special ingrowth of epiblast to meet a connective tissue papilla which is not found in the latter.
Although the teeth are to be regarded as primitively epiblastic structures, they are nevertheless found in Teleostei and Ganoidei on the hyoid and branchial arches; and very possibly the teeth on some other parts of the mouth are developed in a true hypoblastic region.
The teeth are formed from two distinct organs, viz. an epithelial cap and a connective tissue papilla.
The general mode of development, as has been more especially shewn by the extended researches of Tomes, is practically the same for all Vertebrata, and it will be convenient to describe it as it takes place in Mammalia.
Along the line where the teeth are about to develop, there is formed an epithelial ridge projecting into the subjacent connective tissue, and derived from the innermost columnar layer of the oral epithelium. At the points where a tooth is about to be formed this ridge undergoes special changes. It becomes in the first place somewhat thickened by the development of a number of rounded cells in its interior; so that it becomes constituted of (1) an external layer of columnar cells, and (2) a central core of rounded cells; both of an epithelial nature. In the second place the organ gradually assumes a dome-shaped form (fig. 428, e), and covers over a papilla of the subepithelial connective tissue (p) which has in the meantime been developed.
Fig. 428. Diagram shewing the development of the teeth. (From
Gegenbaur.)
p. dental papilla; e. enamel organ.
From the above epithelial structure, which may be called the enamel organ, and from the papilla it covers, which may be spoken of as the dental papilla, the whole tooth is developed. After these parts have become established there is formed round the rudiment of each tooth a special connective tissue capsule; known as the dental capsule.
Before the dental capsule has become definitely formed the enamel organ and the dental papilla undergo important changes. The rounded epithelial cells forming the core of the enamel organ undergo a peculiar transformation into a tissue closely resembling ordinary embryonic connective tissue, while at the same time the epithelium adjoining the dental papilla and covering the inner surface of the enamel organ, acquires a somewhat different structure to the epithelium on the outer side of the organ. Its cells become very markedly columnar, and form a very regular cylindrical epithelium. This layer alone is concerned in forming the enamel. The cells of the outer epithelial layer of the enamel organ become somewhat flattened, and the surface of the layer is raised into a series of short papillæ which project into the highly vascular tissue of the dental sheath. Between the epithelium of the enamel organ and the adjoining connective tissue there is everywhere present a delicate membrane known as the membrana præformativa.
The dental papilla is formed of a highly vascular core and a non-vascular superficial layer adjoining the inner epithelium of the enamel organ. The cells of the superficial layer are arranged so as almost to resemble an epithelium.
The first formation of the hard structures of the tooth commences at the apex of the dental papilla. A calcification of the outermost layer of the papilla sets in, and results in the formation of a thin layer of dentine. Nearly simultaneously a thin layer of enamel is deposited over this, from the inner epithelial layer of the enamel organ (fig. 428). Both enamel and dentine continue to be deposited till the crown of the tooth has reached its final form, and in the course of this process the enamel organ is reduced to a thin layer, and the whole of the outer layer of the dental papilla is transformed into dentine—while the inner portion remains as the pulp.
The root of the tooth is formed later than the crown, but the enamel organ is not prolonged over this part, so that it is only formed of dentine.
By the formation of the root the crown of the tooth becomes pushed outwards, and breaking through its sack projects freely on the surface.
The part of the sack which surrounds the root of the tooth gives rise to the cement, and becomes itself converted into the periosteum of the dental alveolus.
The general development of the enamel organs and dental papillæ is shewn in the diagram (fig. 428). From the epithelial ridge three enamel organs are represented as being developed. Such an arrangement may occur when teeth are successively replaced. The lowest and youngest enamel organ (e) has assumed a cap-like form enveloping a dental papilla, but no calcification has yet taken place.
In the next stage a cap of dentine has become formed, while in the still older tooth this has become covered by a layer of enamel. As may be gathered from this diagram, the primitive epithelial ridge from which the enamel organ is formed is not necessarily absorbed on the formation of a tooth, but is capable of giving rise to fresh enamel organs. When the enamel organ has reached a certain stage of development, its connection with the epithelial ridge is ruptured (fig. 428).
The arrangement represented in fig. 428, in which successive enamel organs are formed from the same epithelial ridge, is found in most Vertebrata except the Teleostei. In the Teleostei, however (Tomes), a fresh enamel organ grows inwards from the epithelium for each successively formed tooth.
The Proctodæum.
In all Vertebrata the cloacal section of the alimentary tract which receives the urinogenital ducts is placed in communication with the exterior by means of an epiblastic invagination, constituting a proctodæum.
This invagination is not usually very deep, and in most instances the boundary wall between it and the hypoblastic cloaca is not perforated till considerably after the perforation of the stomodæum; in Petromyzon, however, its perforation is effected before the mouth and pharynx are placed in communication.
The mode of formation of the proctodæum, which is in general extremely simple, is illustrated by fig. 420 an.
In most forms the original boundary between the epiblast of the proctodæum and the hypoblast of the primitive cloaca becomes obliterated after the two have become placed in free communication.
Fig. 429. Diagrammatic longitudinal section through the posterior
end of an embryo Bird at the time of the formation of the Allantois.
ep. epiblast; Sp.c. spinal canal; ch. notochord; n.e.
neurenteric canal; hy. hypoblast; p.a.g. postanal gut; pr.
remains of primitive streak folded in on the ventral side; al.
allantois; me. mesoblast; an. point where anus will be formed;
p.c. perivisceral cavity; am. amnion; so. somatopleure; sp.
splanchnopleure.
In Birds the formation of the proctodæum is somewhat more complicated than in other types, owing to the outgrowth from it of the bursa Fabricii.
The proctodæum first appears when the folding off of the tail end of the embryo commences (fig. 429, an) and is placed near the front (originally the apparent hind) end of the primitive streak. Its position marks out the front border of the postanal section of the gut.
The bursa Fabricii first appears on the seventh day (in the chick), as a dorsal outgrowth of the proctodæum. The actual perforation of the septum between the proctodæum and the cloacal section of the alimentary tract is not effected till about the fifteenth day of fœtal life, and the approximation of the epithelial layers of the two organs, preparatory to their absorption, is partly effected by the tunneling of the mesoblastic tissue between them by numerous spaces.
The hypoblastic section of the cloaca of birds, which receives the openings of the urinogenital ducts, is permanently marked off by a fold from the epiblastic section or true proctodæum, with which the bursa Fabricii communicates.
Bibliography.
Alimentary Canal and its appendages.
(561) B. Afanassiew. “Ueber Bau u. Entwicklung d. Thymus d. Säugeth.”
Archiv f. mikr. Anat. Bd. XIV. 1877.
(562) Fr. Boll. Das Princip d. Wachsthums. Berlin, 1876.
(563) E. Gasser. “Die Entstehung d. Cloakenöffnung bei
Hühnerembryonen.” Archiv f. Anat. u. Physiol., Anat. Abth. 1880.
(564) A. Götte. Beiträge zur Entwicklungsgeschichte d. Darmkanals im
Hühnchen. 1867.
(565) W. Müller. “Ueber die Entwickelung der Schilddrüse.” Jenaische
Zeitschrift, Vol. VI. 1871.
(566) W. Müller. “Die Hypobranchialrinne d. Tunicaten.” Jenaische
Zeitschrift, Vol. VII. 1872.
(567) S. L. Schenk. “Die Bauchspeicheldrüse d. Embryo.”
Anatomischphysiologische Untersuchungen. 1872.
(568) E. Selenka. “Beitrag zur Entwicklungsgeschichte d. Luftsäcke d.
Huhns.” Zeit. f. wiss. Zool. 1866.
(569) L. Stieda. Untersuch. üb. d. Entwick. d. Glandula Thymus,
Glandula thyroidea, u. Glandula carotica. Leipzig, 1881.
(570) C. Fr. Wolff. “De formatione intestinorum.” Nov. Comment. Akad.
Petrop. 1766.
(571) H. Wölfler. Ueb. d. Entwick. u. d. Bau d. Schilddrüse. Berlin,
1880.
Vide also Kölliker (298), Götte (296), His (232 and 297), Foster and
Balfour (295), Balfour (292), Remak (302), Schenk (303), etc.
Teeth.
(572) T. H. Huxley. “On the enamel and dentine of teeth.” Quart. J.
of Micros. Science, Vol. III. 1855.
(573) R. Owen. Odontography. London, 1840-1845.
(574) Ch. S. Tomes. Manual of dental anatomy, human and comparative.
London, 1876.
(575) Ch. S. Tomes. “On the development of teeth.” Quart. J. of
Micros. Science, Vol. XVI. 1876.
(576) W. Waldeyer. “Structure and development of teeth.” Stricker’s
Histology. 1870.
Vide also Kölliker (298), Gegenbaur (294), Hertwig (306), etc.
[278] Wölfler (No. 571) states that in the Pig and Calf the thyroid body is formed as a pair of epithelial vesicles, which are developed as outgrowths of the walls of the first pair of visceral clefts. He attempts to explain the contradictory observations of other embryologists by supposing that they have mistaken the ventral ends of visceral pouches for an unpaired outgrowth of the throat. Stieda (No. 569) also states that in the Pig and Sheep the thyroid arises as a paired body from the epithelium of a pair of visceral clefts, at a much later period than would appear from the observations of His and Kölliker. In view of the comparative development of this organ it is difficult to accept either Wölfler’s or Stieda’s account. Wölfler’s attempt to explain the supposed errors of his predecessors is certainly not capable of being applied in the case of Elasmobranch Fishes, or of Petromyzon; and I am inclined to think that the method of investigation by transverse sections, which has been usually employed, is less liable to error than that by longitudinal sections which he has adopted.
[279] For details on these organs vide Kölliker, Entwicklungsgeschichte, p. 881.
[280] H. Eisig, “Ueb. d. Vorkommen eines schwimmblasenähnlichen Organs bei Anneliden.” Mittheil. a. d. zool. Station z. Neapel, Vol. II. 1881.
INDEX TO VOLUME III.
| A | B | C | D | E | F | G | H | I | J | K | L | M |
| N | O | P | Q | R | S | T | U | V | W | Y | Z |
- Abdominal muscles, 675
- Abdominal pore, 626, 749
- Acipenser, development of, 102;
- Actinotrocha, 373
- Air-bladder of Teleostei, 77;
- Alciope, eye of, 480
- Alisphenoid region of skull, 569
- Alimentary canal and appendages, development of, 754
- Alimentary tract of Ascidia, 18;
- Allantois, development of in Chick, 191, 198;
- Alternation of generations in Ascidians, origin of, 35;
- Alytes, branchial chamber of, 136;
- Amblystoma, ovum of, 120;
- Amia, ribs of, 561
- Ammocoetes, 95;
- Amnion, early development of in Chick, 185;
- Amphibia, development of, 120;
- viviparous, 121;
- gastrula of, 277;
- suctorial mouth of, 317;
- cerebellum of, 426;
- infundibulum of, 431;
- pineal gland of, 433;
- cerebrum of, 439;
- olfactory lobes of, 444;
- nares of, 553;
- notochord and its sheath, 548;
- vertebral column of, 554;
- ribs of, 561;
- branchial arches of, 574;
- mandibular and hyoid arches of, 582;
- columella of, 582;
- pectoral girdle of, 605;
- pelvic girdle of, 607;
- limbs of, 619;
- heart of, 638;
- arterial system of, 645;
- venous system of, 655;
- excretory system of, 707;
- vasa efferentia of, 711;
- liver of, 769;
- postanal gut of, 774;
- stomodæum of, 778;
- Amphiblastula larva of Porifera, 344
- Amphioxus, development of, 1;
- Amphistylic skulls, 578
- Angular bone, 594
- Anterior abdominal vein, 653
- Anura, development of, 121;
- Anus of Amphioxus, 7;
- Appendicularia, development of, 34
- Aqueductus vestibuli, 519
- Aqueous humour, 497
- Arachnida, nervous system of, 409;
- eye of, 481;
- Area, embryonic, of Rabbit, 218;
- Arteria centralis retinæ, 503
- Arterial system of Petromyzon, 97;
- Arthropoda, head of, 313;
- Articular bone of Teleostei, 581;
- of Sauropsida, 588;
- Ascidia, development of, 9
- Ascidians. _See_ ‘Tunicata’
- Ascidiozooids, 25
- Atrial cavity of Amphioxus, 7;
- Atrial pore of Amphioxus,7;
- Auditory capsules, ossifications in, 595, 596
- Auditory involution of Elasmobranchii, 57;
- Auditory nerve, development of, 459
- Auditory organs, of Ascidia, 15;
- of Salpa, 31;
- of Ammocoetes, 98;
- Ganoidei, 108, 114;
- of Amphibia, 127;
- of Aves, 170;
- general development of, 512;
- of aquatic forms, 512;
- of land forms, 513;
- of Coelenterata, 513;
- of Mollusca, 515;
- of Crustacea, 516;
- of Vertebrata, 517;
- of Cyclostomata, 89, 92, 518;
- of Teleostei, Lepidosteus and Amphibia, 518;
- of Mammalia, 519;
- accessory structures of, 527;
- of Tunicata, 528;
- Auriculo-ventricular valves, 642
- Autostylic skulls, 579
- Aves, development of, 145;
- cerebellum of, 426;
- mid-brain of, 427;
- infundibulum of, 431;
- pineal gland of, 434;
- pituitary body of, 436;
- cerebrum of, 439;
- olfactory lobes of, 444;
- spinal nerves of, 449;
- cranial nerves of, 455;
- vagus of, 458;
- glossopharyngeal of, 458;
- vertebral column of, 557;
- ossification of vertebral column of, 558;
- branchial arches of, 572, 573;
- pectoral girdle of, 603;
- pelvic girdle of, 608;
- heart of, 637;
- arterial system of, 647;
- venous system of, 658;
- muscle-plates of, 670;
- excretory organs of, 714;
- mesonephros of, 715;
- pronephros of, 718;
- Müllerian duct of, 718, 720;
- nature of pronephros of, 721;
- connection of Müllerian duct with Wolffian in, 720;
- kidney of, 722;
- lungs of, 764;
- liver of, 769;
- postanal gut of, 774;
- Axolotl, 142, 143;
- Basilar membrane, 524
- Basilar plate, 565
- Basipterygium, 612
- Basisphenoid region of skull, 569
- Bilateral symmetry, origin of, 373-376
- Bile duct, 770
- Bladder, Amphibia, 131;
- of Amniota, 726;
- Blastodermic vesicle, of Rabbit, first development of, 217;
- Blastoderm of Pyrosoma, 24;
- Blastopore, of Amphioxus, 2;
- Blood-vessels, development of, 633
- Body cavity, of Ascidia, 21;
- Bombinator, branchial chamber of, 136;
- vertebral column of, 556;
- Bonellia, excretory organs of, 687
- Bones, origin of cartilage bones, 542;
- Brachiopoda, excretory organs of, 683;
- generative ducts of, 749;
- Brain, of Ascidia, 11, 15;
- Branchial arches, præoral, 570;
- Branchial chamber of Amphibia, 136
- Branchial clefts, of Amphioxus, 7;
- Branchial rays, 574
- Branchial skeleton, development of, 572, 592;
- Branchiæ, external of Elasmobranchii, 61, 62;
- Brood-pouch, of Salpa, 29;
- Brown tubes of Gephyrea, 686
- Bulbus arteriosus, of Fishes, 638;
- Amphibia, 639;
- Bursa Fabricii, 167, 779
- Canalis auricularis, 639
- Canalis reuniens, 521
- Capitellidæ, excretory organs of, 683
- Carcharias, placenta of, 66
- Cardinal vein, 652
- Carnivora, placenta of, 250
- Carpus, development of, 620
- Cartilage bones of skull, 595;
- homologies of, 595;
- Cat, placenta of, 250
- Caudal swellings of Elasmobranchii, 46, 55;
- Cephalic plate of Elasmobranchii, 55
- Cephalochorda, development of, 1
- Cephalopoda, eyes of, 473-477
- Cerebellum, Petromyzon, 93;
- Cerebrum of Petromyzon, 93, 97;
- Cestoda, excretory organs of, 681
- Cetacea, placenta, 255
- Chætognatha, nervous system of, 349;
- Chætopoda, head of, 313;
- Charybdæa, eye of, 472
- Cheiroptera, placenta of, 244
- Cheiropterygium, 618;
- relation of to ichthyopterygium, 621;
- Chelonia, development of, 210;
- Chick, development of, 145;
- Chilognatha, eye of, 481
- Chilopoda, eye of, 481
- Chimæra, lateral line of, 539;
- Chiromantis, oviposition of, 121
- Chorda tympani, development of, 460
- Chordata, ancestor of, 311;
- Chorion, 237;
- Choroid coat, Ammocoetes, 99;
- general account of, 487;
- Choroid fissure, of Vertebrate eye, 486, 493;
- Choroid gland, 320
- Choroid pigment, 489
- Choroid plexus, of fourth ventricle, 425;
- Ciliated sack of Ascidia, 18;
- Ciliary ganglion, 461
- Ciliary muscle, 490
- Ciliary processes, 488;
- comparative development of, 506;
- Clavicle, 600
- Clitoris, development of, 727
- Clinoid ridge, 569
- Cloaca, 766
- Coccygeo-mesenteric vein, 661
- Cochlear canal, 519
- Coecilia, development of, 143;
- Coelenterata, larvæ of, 367;
- Columella auris, 529;
- Commissures, of spinal cord, 417;
- Coni vasculosi, 724
- Conus arteriosus, of Fishes, 638;
- of Amphibia, 638;
- Coracoid bone, 599
- Cornea, of Ammocoetes, 99;
- Coronoid bone, 595
- Corpora geniculata interna, 428
- Corpora quadrigemina, 428
- Corpora striata, development of, 437
- Corpus callosum, development of, 443
- Corti, organ of, 522;
- Cranial flexure, of Elasmobranchii, 58, 60;
- Cranial nerves, development of, 455;
- Crocodilia, arterial system of, 649
- Crura cerebri, 429
- Crustacea, nervous system of, 411;
- Cupola, 524
- Cutaneous muscles, 676
- Cyathozooid, 25
- Cyclostomata, auditory organs of, 517;
- Cystignathus, oviposition of, 122
- Dactylethra, branchial chamber of, 136;
- Decidua reflexa, of Rat, 242;
- Deiter’s cells, 526
- Dental papilla, 777
- Dental capsule, 777
- Dentary bone, 595
- Dentine, 780
- Descemet’s membrane, 496
- Diaphragm, 631;
- muscle of, 676;
- Dipnoi, nares of, 534;
- Diptera, eye of, 481
- Discophora, excretory organs of, 687
- Dog, placenta of, 248
- Dohrn, on relations of Cyclostomata, 84;
- Doliolum, development of, 28
- Ductus arteriosus, 649
- Ductus Botalli, 648
- Ductus Cuvieri, 654
- Ductus venosus Arantii, 663
- Dugong, heart of, 642
- Dysticus, eye of, 481
- Ear, _see_ ‘Auditory organ’
- Echinodermata, secondary symmetry of larva of, 380;
- Echinorhinus, lateral line of, 539;
- vertebral column of, 548;
- Echiurus, excretory organs of, 686
- Ectostosis, 543
- Edentata, placenta of, 248, 250, 256
- Eel, generative ducts of, 703
- Egg-shell of Elasmobranchii, 40;
- Chick, 146;
- Elasmobranchii, development of, 40;
- viviparous, 40;
- general features of development of, 55;
- gastrula of, 281;
- development of mesoblast of, 294;
- notochord of, 294;
- meaning of formation of mesoblast of, 295;
- restiform tracts of, 425;
- optic lobes of, 427;
- cerebellum of, 425;
- pineal gland of, 432;
- pituitary body of, 435;
- cerebrum of, 438;
- olfactory lobes of, 444;
- spinal nerves, 449;
- cranial nerves of, 457;
- sympathetic nervous system of, 466;
- nares of, 533;
- lateral line of, 539;
- vertebral column of, 549;
- ribs of, 560;
- parachordals of, 567;
- mandibular and hyoid arches of, 576;
- pectoral girdle of, 600;
- pelvic girdle of, 607;
- limbs of, 609;
- pericardial cavity of, 627;
- arterial system of, 644;
- venous system of, 651;
- muscle-plates of, 668;
- excretory organs of, 690;
- constitution of excretory organs in adult of, 697;
- spermatozoa of, 747;
- swimming-bladder of, 763;
- intestines of, 767;
- liver of, 769;
- postanal gut of, 772;
- Elæoblast of Pyrosoma, 28;
- Salpa, 30;
- Elephant, placenta of, 249
- Embolic formation of gastrula, 333
- Enamel organ, 777
- Endolymph of ear, 522
- Endostosis, 543
- Endostyle of Ascidia, 18, 759;
- Epiblast, of Elasmobranchii, 47;
- Epibolic formation of gastrula, 334
- Epichordal formation of vertebral column, 556
- Epicrium glutinosum, 143
- Epidermis, in Coelenterata, 393;
- protective structures of, 394;
- Epididymis, 724
- Epigastric vein, 653
- Episkeletal muscles, 676
- Episternum, 602
- Epoophoron, 725
- Ethmoid bone, 597
- Ethmoid region of skull, 570
- ethmo-palatine ligament of Elasmobranchs, 576
- Euphausia, eye of, 483
- Eustachian tube, of Amphibia, 135;
- Excretory organs, general constitution of, 680;
- of Platyelminthes, 680;
- of Mollusca, 681;
- of Polyzoa, 682;
- of Brachiopoda, 683;
- of Chætopoda, 683;
- of Gephyrea, 686;
- of Discophora, 687;
- of Arthropoda, 688;
- of Nematoda, 689;
- of Echinodermata, 689;
- constitution of in Craniata, 689;
- of Elasmobranchii, 690;
- constitution of in adult Elasmobranch, 697;
- of Petromyzon, 700;
- of Myxine, 701;
- of Teleostei,701;
- of Ganoidei, 704;
- of Dipnoi, 707;
- of Amphibia, 707;
- of Amniota, 713;
- comparison of Vertebrate and Invertebrate, 737;
- Excretory system, of Elasmobranchii, 49;
- Exoccipital bone, 595
- Exoskeleton, dermal, 393-395;
- epidermal, 393-396;
- External generative organs, 726
- Extra-branchial skeleton, 572
- Eye, of Ascidia, 16;
- Salpa, 31;
- Elasmobranchii, 56, 57, 58;
- Teleostei, 73;
- Petromyzon, 92, 98;
- Aves, 170;
- Rabbit, 229;
- general development of, 470;
- evolution of, 470, 471;
- simple, 480;
- compound, 481;
- aconous, 482;
- pseudoconous, 482;
- of Invertebrata, 471;
- of Vertebrata, 483;
- comparative development of Vertebrate, 497;
- of Ammocoetes, 497;
- of Tunicata, 507;
- of Chordata, general views on, 508;
- accessory eyes of Fishes, 509;
- muscles of, 677;
- Eyelids, development of, 506
- Falciform ligament, 757
- Falx cerebri, 439
- Fasciculi teretes, of Elasmobranchii, 426
- Feathers, development of, 396
- Fenestra rotunda and ovalis, 529
- Fertilization, of Amphioxus, 2;
- Fifth nerve, development of, 460
- Fifth ventricle, 443
- Fins, of Elasmobranchii, 62;
- Fissures of spinal cord, 417
- Foetal development, 360;
- secondary variations in, 361;
- Foot, 618
- Foramen of Munro, 430, 438
- Foramen ovale, 642
- Fore-brain, of Elasmobranchii, 55, 59, 60;
- Formative cells, of Chick, 154
- Fornix, development of, 443
- Fornix of Gottsche, 428
- Fourth nerve, 464
- Frontals, 592
- Frontonasal process of Chick, 179
- Gaertner’s canals, 724
- Gall-bladder, 770
- Ganoidei, development of, 102;
- Gastropoda, eye of, 472
- Gastrula, of Amphioxus, 2;
- Geckos, vertebral column of, 557
- Generative cells, development of, 741;
- Generative ducts, of Teleostei, 704, 735;
- Generative system of Elasmobranchii, 51
- Gephyrea, nervous system of, 412;
- Germinal disc, of Elasmobranchii, 40;
- Germinal epithelium, 746
- Germinal layers, summary of organs derived from, in Vertebrata, 304;
- Germinal wall of Chick, 152, 159;
- structure and changes of, 160;
- Geryonia, auditory organ of, 515
- Gill of Salpa, 31
- Giraldes, organ of, 725
- Glands, epidermic, development of, 397
- Glomerulus, external, of Chick, 716
- Glossopharyngeal nerve, development of, 456, 457
- Grey matter of spinal cord, 417;
- of brain, 423;
- Growth in length of Vertebrate embryo, 306
- Guinea-pig, primitive streak of, 223;
- Gymnophiona, _see_ ‘Coecilia’
- Habenula perforata, 525
- Hairs, development of, 396
- Halichærus, placenta of, 250
- Hand, 619
- Head, comparative account of, 313;
- segmentation of, 314;
- Head cavities, of Elasmobranchii, 50;
- Head-fold of Chick, 157, 167
- Head kidney, _see_ ‘Pronephros’
- Heart, of Pyrosoma, 25;
- Hind-brain, Elasmobranchii, 55, 59, 60;
- Hippocampus major, development of, 442
- Hirudo, development of blood-vessels of, 633;
- excretory organs of, 688;
- Horse, placenta of, 253
- Hyaloid membrane, 492
- Hylodes, oviposition of, 121;
- metamorphosis of, 137;
- Hyobranchial cleft, 572
- Hyoid arch, of Chick, 179;
- Hyomandibular bar of Elasmobranchii, 576, 577;786.png
- Hyomandibular cleft, of Petromyzon, 91;
- Hyostylic skulls, 582
- Hypoblast of Elasmobranchii, 51;
- Hyposkeletal muscles, 675
- Hyrax, placenta of, 249
- Incus, 529, 590
- Infraclavicle, 600
- Infundibulum of Petromyzon, 92;
- Insectivora, placenta of, 243
- Insects, nervous system of, 410;
- Intercalated pieces of vertebral column, 551
- Interclavicle, homologies of, 602
- Intermediate cell-mass of Chick, 183
- Intermuscular septa, 672
- Interorbital septum, 570
- Interrenal bodies, 665
- Iris, 489;
- comparative development of, 506;
- Iris of Ammocoetes, 98
- Island of Reil, 444
- Labia majora, development of, 727
- Labial cartilages, 597
- Labium tympanicum, 525;
- vestibulare, 525;
- Lacertilia, general development of, 202;
- Lacrymal bone, 593
- Lacrymal duct, 506
- Lacrymal glands, 506
- Læmargus, vertebral column of, 548
- Lagena, 524
- Lamina spiralis, 524
- Lamina terminalis, 438
- Larva of Amphioxus, 2;
- Larvæ, nature, origin, and affinities of, 360-386;
- secondary variations of less likely to be retained, 362;
- ancestral history more fully recorded in, 362;
- secondary variations in development of, 363;
- ontogenetic record of secondary variations in, 361;
- of freshwater and land animals, 362;
- types of, 362;
- phosphorescence of, 364;
- of Coelenterata, 367;
- table of, 365;
- of Invertebrata, 367 et seq.
- Larynx, 766
- Lateral line sense organs, 538;
- Lateral ventricle, 438;
- Layers, formation of, in Elasmobranchii, 41, 56;
- Leech, _see_ ‘Hirudo’
- Lemuridæ, placenta, 256
- Lens, of Elasmobranchii, 57, 58;
- Lepidosteus, development of, 111;
- Ligamentum pectinatum, 490
- Ligamentum suspensorium, 557, 558
- Ligamentum vesicæ medium, 239
- Limbs, of Elasmobranchii, 59;
- Liver of Teleostei, 78;
- Lizard, development of, 202;
- Lizzia, eye of, 471
- Lobi inferiores, 431
- Lungs of Amphibia, 137;
- Lymphatic system, 664
- Malleus, 529, 591;
- views on, 591;
- Malpighian bodies, development of accessory in Elasmobranchs, 695
- Mammalia, development of, 214;
- comparison of gastrula of, 291;
- cerebellum of, 427;
- infundibulum of, 431;
- pineal gland of, 434;
- pituitary body of, 436;
- cerebrum of, 439;
- spinal nerves of, 449;
- sympathetic of, 466;
- vertebral column of, 558;
- branchial arches of, 573, 574;
- mandibular and hyoid arches of, 589;
- pectoral girdle of, 604;
- pelvic girdle of, 608;
- heart of, 636;
- arterial system of, 647;
- venous system of, 661;
- muscle-plates of, 671;
- mesonephros of, 714;
- testicular network of, 724;
- urinogenital sinus of, 727;
- spermatozoa of, 747;
- lungs of, 765;
- intestines of, 768;
- liver of, 769;
- postanal gut of, 774;
- stomodæum of, 775;
- Mammary gland, development of, 398
- Man, placenta of, 244;
- Mandibular arch of Elasmobranchii, 62, 576;
- Mandibular bar, evolution of, 311, 321
- Manis, placenta of, 256
- Marsupial bones, 608
- Marsupialia, foetal membranes of, 240;
- Maxilla, 594
- Meatus auditorius externus, of Chick, 181;
- development of, 527;
- Meckelian cartilage, of Elasmobranchii, 576;
- Mediastinum anterior and posterior, 630
- Medulla oblongata, of Chick, 176;
- general development of, 425;
- Medullary plate of Amphioxus, 4, 5;
- Medusæ, auditory organs of, 513
- Membrana capsulo-pupillaris, 494, 504, 507
- Membrana elastica externa, 546
- Membrana limitans of retina, 491
- Membrana tectoria, 522, 525
- Membrane bones, of Amphibia, 582;
- Membranous labyrinth, development of in Man, 519
- Menobranchus, branchial arches of, 142
- Mesenteron of Elasmobranchii, 43;
- Mesentery, 626, 756
- Mesoblast, of Amphioxus, 6;
- Ascidia, 17, 20;
- Pyrosoma, 24;
- Salpa, 30;
- Elasmobranchii, 44, 47;
- Teleostei, 75;
- Petromyzon, 86;
- Acipenser, 105;
- Lepidosteus, 113;
- Amphibia, 125, 128, 129;
- of Chick, 154, 167;
- double origin of in Chick, 154, 158, 159;
- origin of from lips of blastopore in Chick, 158;
- of area vasculosa of Chick, 160;
- Lacerta, 203;
- origin of in Rabbit, 218, 223;
- of area vasculosa in Rabbit, 227;
- comparative account of formation of, 292;
- discussion of development of in Vertebrata, 297;
- meaning of development of in Amniota, 298;
- phylogenetic origin of, 346;
- summary of ontogeny of, 349-352;
- views on ontogeny of, 352-360;
- Mesoblastic somites, of Amphioxus, 6;
- Mesogastrium, 758
- Mesonephros, of Teleostei, 78, 702;
- Petromyzon, 95, 98, 700;
- Acipenser, 110, 705;
- Amphibia, 134, 708;
- Chick, 184, 714;
- general account of, 690;
- development of in Elasmobranchs 691;
- of Cyclostomata, 700;
- Ganoidei, 705;
- sexual and non-sexual part of in Amphibia, 710;
- of Amniota, 713, 724;
- summary and general conclusions as to, 729;
- relation of to pronephros, 731;
- Mesopterygium, 616
- Metagenesis of Ascidians, 34
- Metamorphosis of Amphibia, 137, 140
- Metanephros, 690;
- Metapterygium, 616
- Metapterygoid, of Elasmobranchii, 576;
- of Teleostei, 581;
- Metazoa, evolution of, 339, 342;
- Mid-brain, of Elasmobranchii, 55, 58, 59;
- Moina, generative organs of, 745
- Molgula, development of, 22
- Mollusca, nervous system of, 414;
- Monotremata, foetal membranes of, 240;
- Mormyrus, generative ducts of, 704
- Mouth, of Amphioxus, 7;
- Mouth, suctorial, of Petromyzon, 99;
- Müllerian duct, 690;
- Muscle-plates, of Amphioxus, 6;
- Muscles, of Ascidia, 11, 17;
- Muscular fibres, epithelial origin of, 667
- Muscular system, development of, 667;
- of Chordata, 668;
- Mustelus, placenta of, 66
- Myoepithelial cells, 667
- Mysis, auditory organ of, 517
- Myxine, ovum of, 100;
- Nails, development of, 397
- Nares, of Acipenser, 108;
- Nasal bones, 592
- Nasal pits, Acipenser, 108;
- Nematoda, excretory organs of, 689;
- Nemertines, nervous system of, 311;
- excretory organs of, 681;
- Nerve cord, origin of ventral, 378
- Nerves, spinal, 449;
- cranial, 455-466;
- Nervous system, central,
- Nervous system, of Amphioxus, 4;
- Ascidia, 15, 16;
- Molgula, 22;
- Pyrosoma, 24, 25;
- Salpa, 30, 31;
- Elasmobranchii, 44;
- Teleostei, 77;
- Petromyzon, 89, 93;
- Acipenser, 105;
- Amphibia, 126;
- comparative account of formation of central, 301;
- of Sagitta, 349;
- origin of in Coelenterata, 349;
- of præoral lobe, 377, 380;
- evolution of, 400-405;
- development of in Invertebrates, 406;
- of Arthropoda, 408;
- of Gephyrea, 412;
- Mollusca, 414;
- Neural canal, of Ascidia, 10;
- Neural crest, 449, 456, 457
- Neurenteric canal, of Amphioxus, 4, 5;
- Newt, ovum of, 120;
- Notidanus, vertebral column of, 548;
- branchial arches of, 572;
- Notochord of Amphioxus, 6;
- Ascidia, 11, 17;
- Elasmobranchii, 51;
- Teleostei, 74;
- Petromyzon 86, 94;
- Acipenser, 104;
- Lepidosteus, 113;
- Amphibia, 128, 129;
- Chick, 157;
- canal of, in Chick, 163;
- Lacerta, 204, 205;
- Guinea-pig, 226;
- comparative account of formation of, 292, 325;
- sheath of, 545;
- later histological changes in, 546;
- cartilaginous sheath of, 547;
- in head, 566;
- absence of in region of trabeculæ, 567;
- Notodelphys, brood-pouch of, 121;
- branchiæ of, 140;
- Nototrema, brood-pouch of, 121
- Nucleus pulposus, 559
- Oceania, eye of, 471
- Occipital bone, 595
- OEsophagus, solid, of Elasmobranchii, 61, 759;
- of Teleostei, 78;
- Olfactory capsules, 571
- Olfactory lobes, development of, 444
- Olfactory nerves, Ammocoetes, 99;
- general development of, 464;
- Olfactory organ, of aquatic forms, 531;
- Olfactory sacks, of Elasmobranchii, 60;
- Oligochæta, excretory organs of, 683
- Olivary bodies, 426
- Omentum, lesser and greater, 757
- Onchidium, eye of, 478
- Opercular bones, 593
- Operculum, of Teleostei, 77;
- Ophidia, development of, 210;
- Optic chiasma, 430, 493
- Optic cup, retinal part of, 488;
- ciliary portion of, 489;
- Optic lobes, 428
- Optic nerve, development of, 492;
- comparative development of, 500;
- Optic thalami, development of, 431
- Optic vesicle, of Elasmobranchii, 57-59;
- Ora serrata, 488
- Orbitosphenoid region of skull, 570
- Organs, classification of, 391;
- derivation of from germinal layers, 392;
- Orycteropus, placenta of, 249
- Otic process of Axolotl, 583;
- of Frog, 585 et seq.
- Otoliths, 512
- Oviposition, of Amphioxus, 1;
- Ovum, of Amphioxus, 1;
- Palatine bone, of Teleostei, 580;
- origin of, 594;
- Pancreas, Acipenser, 110;
- general development of, 770;
- Pancreatic cæca, of Teleostei, etc. 768
- Papillæ, oral, of Acipenser, 108;
- Lepidosteus, 116;
- Parachordals, 565, 566
- Parasphenoid bone, 594
- Parepididymis, 725
- Parietal bones, 592
- Paroophoron, 725
- Parovarium, 725
- Pectoral girdle, 599;
- Pecten, eye of, 479
- Pecten, of Ammocoetes, 498;
- Pedicle, of Axolotl, 484;
- of Frog, 485;
- Pelobates, branchial apertures of, 136;
- vertebral column of, 556;
- Pelodytes, branchial chamber of, 135
- Pelvic girdle, 606;
- Penis, development of, 727
- Peribranchial cavity, of Amphioxus, 7;
- Pericardial cavity, of Pyrosoma, 26;
- Perichordal formation of vertebral column, 556
- Perilymph of ear, 523
- Periotic capsules, ossifications in, 595, 596
- Peripatus, nervous system of, 409;
- eye of 480;
- excretory organs of, 688;
- Peritoneal membrane, 626
- Petromyzon, development of, 83;
- affinities of, 83, 84;
- general development of, 87;
- hatching of, 89;
- comparison of gastrula of, 280;
- branchial skeleton of, 312, 572;
- cerebellum of, 425;
- pineal gland of, 434;
- pituitary body of, 436;
- cerebrum of, 439;
- auditory organ of, 517;
- olfactory organ of, 533;
- comparison of oral skeleton of with Tadpole, 586;
- pericardial cavity of, 627;
- abdominal pores of, 626;
- venous system of, 651;
- excretory organs of, 700;
- segmental duct of, 700;
- pronephros of, 700;
- mesonephros of, 700;
- thyroid body of, 760;
- postanal gut of, 774;
- stomodæum of, 775;
- Phosphorescence of larvæ, 364
- Phylogeny, of the Chordata, 327;
- of the Metazoa, 384
- Pig, placenta of, 251;
- mandibular and hyoid arches of, 589;
- Pineal gland, of Petromyzon, 93;
- Pipa, brood-pouch of, 121;
- Pituitary body, of Rabbit, 231;
- Placoid scales, 395
- Planorbis, excretory organs of, 681
- Planula, structure of, 367
- Pleural cavities, 631
- Pleuronectidæ, development of, 80
- Pneumatocoela, characters of, 327
- Polygordius, excretory organs of, 684
- Polyophthalmus, eye of, 479
- Polypedates, brood-pouch of, 121
- Polyzoa, excretory organs of, 682;
- Pons Varolii, 426, 427
- Pori abdominales, Ammocoetes, 99
- Porifera, ancestral form of, 345;
- development of generative cells of, 741;
- Portal vein, 653
- Postanal gut of Elasmobranchii, 58, 59, 60;
- Præmaxilla, 594
- Præopercular bone, 593
- Præoral lobe, ganglion of, 377, 380
- Prefrontals, 597
- Presphenoid region of skull, 570
- Primitive groove of Chick, 155
- Primitive streak, of Chick, 152, 161;
- meaning of, 153;
- origin of mesoblast form in Chick, 154;
- continuity of hypoblast with epiblast at anterior end of, in Chick, 156;
- comparison of with blastopore, 165;
- fate of, in Chick, 165;
- of Lacerta, 203;
- of Rabbit, 221;
- of Guinea-pig, 223;
- fusion of layers at, in Rabbit, 224;
- comparison of with blastopore of lower forms, 226, 287;
- of Mammalia, 290;
- Processus falciformis of Ammocoetes, 498;
- Proctodæum, 778
- Pronephros, of Teleostei, 78, 701;
- Prootic, 596, 597
- Propterygium, 616
- Proteus, branchial arches of, 142
- Protochordata, characters of, 327
- Proto-ganoidei, characters of, 328
- Proto-gnathostomata, characters of, 328
- Proto-pentadactyloidei, characters of, 329
- Protovertebrata, characters of, 328
- Pseudis, Tadpole of, 139;
- vertebral column of, 556;
- Pseudophryne, yolk-sack of, 140;
- Tadpole of, 140;
- Pterygoid bone, of Teleostei, 581;
- origin of, 597;
- Pterygoquadrate bar, of Elasmobranchii, 576;
- Pulmonary artery, origin of, 645;
- Pulmonary vein, 655
- Pupil, 489
- Pyrosoma, development of, 23
- Rabbit, development of, 214;
- Radiate symmetry, passage from to bilateral symmetry, 373-376
- Raja, caudal vertebræ of, 553
- Rat, placenta of, 242
- Recessus labyrinthi, 519
- Reissner’s membrane, 524
- Reptilia, development of, 202;
- Restiform tracts of Elasmobranchii and Teleostei, 425
- Retina, histogenesis of, 490
- Retinulæ, 482
- Rhabdom, 482
- Rhinoderma, brood-pouch of, 121;
- metamorphosis of, 139;
- Ribs, development of, 560
- Rosenmüller’s organ, 725
- Rotifera, excretory organs of, 680
- Round ligament of liver, 663
- Ruminantia, placenta of, 253
- Sacci vasculosi, 437
- Sacculus hemisphericus, 519;
- Sagitta. _See_ ‘Chætognatha’
- Salpa, sexual development of, 29;
- asexual development of, 33;
- Salamandra, larva of, 142;
- Salmonidæ, hypoblast of, 71;
- generative ducts of, 704;
- Sauropsida, gastrula of, 286;
- Scala, vestibuli, 522;
- Scales, general development of, 396;
- development of placoid scales, 395;
- Scapula, 599
- Sclerotic, 488
- Scrotum, development of, 727
- Scyllium, caudal vertebræ of, 553;
- Segmental duct, 690;
- Segmental organs, 682
- Segmental tubes, 690;
- Segmentation cavity, of Elasmobranchii, 42-44;
- Segmentation, meaning of, 331
- Segmentation of ovum, in Amphioxus, 2;
- Semicircular canals, 519
- Sense organs, comparative account of development of, 304
- Septum lucidum, 443
- Serous membrane, Lacerta, 209;
- of Rabbit, 237;
- Seventh nerve, development of, 459
- Shell-gland of Crustacea, 689
- Shield, embryonic, of Chick, 151;
- of Lacerta, 202;
- Simiadæ, placenta of, 247
- Sinus rhomboidalis, of Chick, 162
- Sinus venosus, 637
- Sirenia, placenta of, 255
- Sixth nerve, 463
- Skate, mandibular and hyoid arches of, 577
- Skeleton, elements of found in Vertebrata, 542
- Skull, general development of, 564;
- Somatopleure, of Chick, 170
- Spelerpes, branchial arches of, 142
- Spermatozoa, of Porifera, 741;
- of Vertebrata, 746;
- Sphenoid bone, 595
- Sphenodon, hyoid arch of, 588
- Spinal cord, general account of, 415;
- Spinal nerves, posterior roots of, 449;
- anterior roots of, 453;
- Spiracle, of Elasmobranchii, 62;
- Spiral valve. _See_ ‘Valve’
- Spleen, 664
- Splenial bone, 595
- Squamosal bone, 593
- Stapes, 529;
- of Mammal, 590;
- Sternum, development of, 562
- Stolon of Doliolum, 29;
- Salpa, 33;
- Stomodæum, 774
- Stria vascularis, 524
- Styloid process, 591
- Subintestinal vein, 651;
- meaning of, 651;
- Syngnathus, brood-pouch of, 68
- Subnotochordal rod, of Elasmobranchii, 54;
- Subzonal membrane, 237;
- villi of, 236;
- Sulcus of Munro, 432
- Supraclavicle, 600
- Suprarenal bodies, 664
- Supra-temporal bone, 593
- Swimming bladder, _see_ Air bladder
- Sylvian aqueduct, 428
- Sylvian fissure, 444
- Sympathetic ganglia, development of, 467
- Tadpole, 134, 139, 140;
- Tail of Teleostei, 80;
- Tarsus, development of, 620
- Teeth, horny provisional, of Amphibia, 136;
- Teleostei, development of, 68;
- viviparous, 68;
- comparison of formation of layers in, 286;
- restiform tracts of, 425;
- mid-brain of, 425;
- infundibulum of, 431;
- cerebrum of, 439;
- nares of, 534;
- lateral line of, 538;
- notochord and membrana elastica of, 549;
- vertebral column of, 553;
- ribs of, 561;
- hyoid and mandibular arches of, 579;
- pectoral girdle of, 601;
- pelvic girdle of, 606;
- limbs of, 618;
- heart of, 637;
- arterial system of, 645;
- muscle-plates of, 670;
- excretory organs of, 701;
- generative ducts of, 704, 735, 749;
- swimming bladder of, 763;
- postanal gut of, 774;
- Teredo, nervous system of, 414
- Test of Ascidia, 14;
- Salpa, 31;
- Testicular network, of Elasmobranchs, 697;
- Testis of Vertebrata, 746
- Testis, connection of with Wolffian body, in Elasmobranchii, 697;
- Thalamencephalon of Chick, 175;
- general development of, 430;
- Third nerve, development of, 461
- Thymus gland, 762
- Thyroid gland, Petromyzon, 92;
- Tooth. _See_ ‘Teeth’
- Tori semicirculares, 428
- Tornaria, 372
- Trabeculæ, 565, 567;
- nature of, 568;
- Trachea, 766
- Trematoda, excretory organs of, 681
- Triton alpestris, sexual larva of, 143
- Triton, development of limbs of, 619;
- urinogenital organs of, 712;
- Truncus arteriosus, 638;
- Tunicata, development of mesoblast of, 293;
- Turbellaria, excretory organs of, 681
- Tympanic annulus of Frog, 587
- Tympanic cavity, of Amphibia, 135;
- Tympanic membrane, of Chick, 180;
- general development of, 528;
- Tympanohyal, 591
- Umbilical canal of Elasmobranchii, 54, 57, 58, 59
- Umbilical cord, 238;
- vessels of, 239;
- Ungulata, placenta of, 250
- Urachus, 239, 726
- Ureters, of Elasmobranchii, 696;
- development of, 723;
- Urethra, 727
- Urinary bladder of Amphibia, 712;
- of Amniota, 726;
- Urinogenital organs, _see_ Excretory organs
- Urinogenital sinus of Petromyzon, 700;
- Urochorda, development of, 9
- Uterus, development of, 726;
- of Marsupials, 726;
- Uterus masculinus, 726
- Utriculus, 519
- Uvea of iris, 489
- Vagus nerve, development of, 456, 457;
- Valve, spiral, of Petromyzon, 97;
- Valves, semilunar, 641;
- auriculo-ventricular, 642;
- Vasa efferentia, of Elasmobranchs, 697;
- Vascular system, of Amphioxus, 8;
- Vas deferens, of Elasmobranchii, 697;
- of Amniota, 723;
- Vein, subintestinal of Petromyzon, 97;
- Velum of Petromyzon, 91
- Vena cava inferior, development of, 655
- Venous system of Petromyzon, 97;
- Ventricle, fourth, of Chick, 176;
- history of, 424;
- Ventricle, lateral, 438, 440;
- fifth, 443;
- Ventricle, third, of Chick, 175
- Vertebral bodies, of Chick, 183
- Vertebral column, development of, 545, 549;
- epichordal and perichordal development of in Amphibia, 556;
- Vespertilionidæ, early development of, 217
- Vieussens, valve of, 426
- Villi, placental, of zona radiata, 235;
- Visceral arches, Amphioxus, 7;
- Visual organs, evolution of, 470
- Vitelline arteries of Chick, 195
- Vitelline veins of Chick, 195
- Vitreous humour, of Ammocoetes, 98;
- Vomer, 594
- White matter, of spinal cord, 415;
- of brain, 423;
- Wolffian body, _see_ ‘Mesonephros’
- Wolffian duct, first appearance of in Chick, 183;
- Wolffian ridge, 185
- Yolk blastopore, of Elasmobranchii, 64
- Yolk, folding off of embryo from, in Elasmobranchii, 55;
- Yolk nuclei, of Elasmobranchii, 41, 53;
- Yolk, of Elasmobranchii, 40;
- Yolk-sack, Amphibia, 131, 140, 141;
- enclosure of, 123;
- Yolk-sack, development of in Rabbit, 227;
- Yolk-sack, enclosure of, Petromyzon, 86
- Yolk-sack, Lepidosteus, 118
- Yolk-sack of Chick, enclosure of, 160;
- Yolk-sack of Elasmobranchii, enclosure of, 62, 283;
- circulation of, 64;
- Yolk-sack of Lacerta, 209;
- circulation of, 209;
- Yolk-sack, Teleostei, 75, 81;