FORNIA LIBRARY OF THE UNIVERSITY OF CALIFORNIA LIBRARY OF THE UNIVERSITY 0 ';^ o\s ' THE UNIVERSITY OF CALIFORNIA LIBRARY OF THE UNIVERSITY OF CALIFORNIA LIBRAR !2J=5 F THE UNIVERSITY OF C CALIFORNIA LIBRA1 iF THE UNIVERSITY OF CAl THE LIBRARY OF THE UNIVERSITY OF CALIFORNIA PRESENTED BY PROF. CHARLES A. KOFOID AND MRS. PRUDENCE W. KOFOID LIBRA ®tittion THE WORKS OF FRANCIS MAITLAND .BALFOUR VOL. III. A TREATISE ON COMPARATIVE EMBRYOLOGY. Vol. II. Vertebrata. MACMILLAN AND CO. 1885 THE WORKS OF FRANCIS MAITLAND BALFOUR. VOL. III. Cambridge : PRINTED BY C. J. CLAY, M.A. AND SON, AT THE UNIVERSITY PRESS. jHetnorfal <£toftfon. THE WORKS OF FRANCIS MAITLAND BALFOUR, M.A., LL.D., F.R.S., FELLOW OF TRINITY COLLEGE, AND PROFESSOR OF ANIMAL MORPHOLOGY IN THE UNIVERSITY OF CAMBRIDGE. EDITED BY M. FOSTER, F.R.S., PROFESSOR OF PHYSIOLOGY IN THE UNIVERSITY OF CAMBRIDGE; AND ADAM SEDGWICK, M.A., FELLOW AND LECTURER OF TRINITY COLLEGE, CAMBRIDGE. VOL. III. A TREATISE ON COMPARATIVE EMBRYOLOGY. Vol. II. Vertebrata. Uonfcon : MACMILLAN AND CO. 1885 [The Right of Translation is reserved.} PREFACE TO VOLUME II. THE present volume completes my treatise on Com- parative Embryology. The first eleven chapters deal with the developmental history of the Chordata. These are followed by three comparative chapters completing the section of the work devoted to Systematic Embry- ology. The remainder of the treatise, from Chapter XIV. onwards, is devoted to Organogeny. For the reasons stated in the introduction to this part the or- ganogeny of the Chordata has been treated with much greater fulness than that of the other groups of Metazoa. My own investigations have covered the ground of the present volume much more completely than they did that of the first volume ; a not inconsiderable proportion of the facts recorded having been directly verified by me. The very great labour of completing this volume has been much lightened by the assistance I have received B. III. B M34558O vi PREFACE. from my friends and pupils. Had it not been for their co-operation a large number of the disputed points, which I have been able to investigate during the preparation of the work, must have been left untouched. My special thanks are due to Mr Sedgwick, who has not only devoted a very large amount of time and labour to correcting the proofs, but has made for me an index of this volume, and has assisted me in many other ways. Dr Allen Thomson and Professor Kleinenberg of Messina have undertaken the ungrateful task of looking through my proof-sheets, and have made suggestions which have proved most valuable. To Professors Parker, Turner, and Bridge, I am also greatly indebted for their suggestions with reference to special chapters of the work. CONTENTS OF VOLUME II. CHAPTER I. CEPHALOCHORDA. Pp. i — 8. Segmentation and formation of the layers, pp. 1—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. Doliolidce, pp. 28, 29. Salpidce, pp. 29 — 34. Appendicularia, p. 34. Metagenesis, pp. 34 — 38. CHAPTER III. ELASMOBRANCHII. Pp. 40 — 67. Segmentation and formation of the layers, pp. 40 — 47. Epiblast, p. 47. Mesoblast, pp. 47 — 51. Hypoblast and notochord, pp. 51 — 54. General features of the embryo at successive stages, pp. 55 — 62. The yolk-sack, pp. 62-66. CHAPTER IV. TELEOSTEI. Pp. 68 — 82. Segmentation and formation of the layers, pp. 68—73. General history of the layers, pp. 73 — 75. General development of the embryo, pp. 76 — 8l. CHAPTER V. CYCLOSTOMATA. Pp. 83 — 101. Segmentation and formation of the layers, pp. 83 — 86. Mesoblast and noto- chord, pp. 86, 87. General history of the development, pp. 87—97. Metamor- phosis, pp. 97 — 100. Myxine, p. 100. CHAPTER VI. GANOIDEI. Pp. 102 — 119. Acipensi'r, pp. 102 — no. Segmentation and formation of the layers, pp. 102 — 104. General development of the embryo and larva, pp. 104 — 1 10. Lepidosteus, pp. 111—119. Segmentation, pp. m, 112. General development of embryo and larva, pp. 1 12 — 1 19. General observations on the embryology of Ganoids, p. 1 19. CHAPTER VII. AMPHIBIA. Pp. 120 — 144. Oviposition and impregnation, pp. 120, 121. Formation of the layers, pp. 121 — 124. Epiblast, pp. 125—127. Mesoblast and notochord, pp. 128, 129. Hypoblast, pp. 129 — 131. General growth of the embryo, pp. 131 — 143. Anura, pp. 131 — 141. Urodela, pp. 141 — 143. Gymnophiona, p. 143. viii CONTENTS OF VOLUME II. CHAPTER VIII. AVES. Pp. 145—201. Segmentation and formation of the layers, pp. 145—166. General history of the germinal layers, pp. 166 — 169. General development of the embryo, pp. 169 — 180. Fatal membranes, pp. 185 — 199. Amnion, pp. 185 — 191. Allantois, pp. 191 — 193. Yolk-sack, pp. 193— 199. CHAPTER IX. REPTILIA. Pp. 202 — 213. Lacertilia, pp. 202 —209. Segmentation and formation of the layers, pp. 202 — 207. General development of the embryo, p. 208. Embryonic membranes and yolk-sack, pp. 208 — 210. Ophidia, p. 210. Ckelonia, pp. 210 — 212. CHAPTER X. MAMMALIA. Pp. 214 — 274. Segmentation and formation of the layers, pp. 214 — 227. General growth of the embryo, pp. 227 — 232. Embryonic membranes and yolk-sack, pp. 232 — 239. Comparative history of the Mammalian foetal membranes, pp. 239 — 257. Com- parative histology of the placenta, pp. 257 — 259. Evolution of the placenta, pp. 259 — 261. Development of the Guinea-pig, pp. 262 — 265. The human embryo, pp. 265 — 270. CHAPTER XI. COMPARISON OF THE FORMATION OF THE GERMINAL LAYERS AND OF THE EARLY STAGES IN THE DE- VELOPMENT OF VERTEBRATES. Pp. 275 — 310. Formation of the gastrula, pp. 275 — 292. The formation of the mesoblast and of the notochord, pp. 292 — 300. The epiblast, pp. 300 — 304. Formation of the central nervous system, pp. 301 — 304. Formation of the organs of special sense, p. 304. Summary of organs derived from the three germinal layers, pp. 304 — 306. Growth in length of the Vertebrate embryo, pp. 306 — 309. The evolution of the allantois and amnion, pp. 309, 310. CHAPTER XII. OBSERVATIONS ON THE ANCESTRAL FORM OF THE CHORDATA. Pp. 311 — 330. General considerations, pp. 311 — 316. The medullary canal, pp. 316, 317. The origin and nature of the mouth, pp. 317 — 321. The cranial flexure, pp. 321, 322. The postanal gut and neurenteric 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. 331 — 360. Formation of the primary germinal layers, pp. 332, 333. Imagination, pp. 333 — 335. Delamination, pp. 335 — 338. Phylogenetic significance of delami- nation and invagination, pp. 338 — 345. Homologies of the germinal layers, pp. 345, 346. The origin of the mesoblast, pp. 346 — 360. II. Larval forms: their nature, origin, and affinities. Preliminary considerations, pp. 360 — 362. Types of larvse, pp. 363 — 384. Phylogenetic conclusions, pp. 384, 385. General conclusions and summary, pp. 385, 386. CONTENTS OF VOLUME II. IX PART II. ORGANOGENY; INTRODUCTION. Pp. 391, 392. CHAPTER XIV. THE EPIDERMIS AND ITS DERIVATIVES. Pp. 393—399. Protective epidermic structures, pp. 393 — 397. Dermal skeletal structures, p. 397. Glands, pp. 397, 398. CHAPTER XV. THE NERVOUS SYSTEM. Pp. 400—469. The origin of the nervous system, pp. 400 — 405. Nervous system of the Invertebrata, pp. 405 — 414. Central nervous system of the Vertebrata, pp. 415 — 447. Spinal chord, pp. 415 — 419. General development of the brain, pp. 419 — 423. Hind-brain, pp. 424 — 427. Mid-brain, pp. 427, 428. General develop- ment of fore-brain, pp. 428 — 430. Thalamencephalon, pp. 430 — 435. Pituitary body, pp. 435 — 437. Cerebral Hemispheres, pp. 437 — 444. Olfactory lobes, pp. 444, 445. General conclusions as to the central nervous system of the Verte- brata, pp. 445 — 447. Development oj '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. Coelenterata, pp. 471, 472. Mollusca, pp. 472 — 479. Chastopoda, p. 479. Chretognatha, 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. Ammocoete eye, pp. 498, 499. Optic vesicles, p. 499. Lens, p. 499. Cornea, p. 500. Optic nerve and choroid fissure, pp. 500 — 505. Iris and ciliary pro- cesses, p. 506. Accessory organs connected with the eye, p. 506. Eyelids, p. 506. Lacrymal glands, p. 506. Lacrymal duct, pp. 506, 507. Eye of the Tunicata, pp. 507 — 509. Accessory eyes in the Vertebrata, pp. 509, 510. CHAPTER XVII. AUDITORY ORGAN, OLFACTORY ORGAN, AND SENSE ORGANS OF THE LATERAL LINE. Pp. 5 12 — 541. Auditory organs, pp. 512 — 531. General structure of auditory organs, PP- 512, 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 Vertebrata, pp. 516 — 530. Auditory vesicle, pp. 517 — 524. Organ of Corti, pp. 524 — 527. Accessory structures connected with the organ of hearing of terrestrial vertebrata, pp. 527 — 530. Auditory organ of the Tunicata, pp. 530, 531. Bibliography of Auditory organs, p. 531. Olfactory organs, pp. 531 — 538. Bibliography of Olfactory organs, p. 538. Sense organs of the lateral line, pp. 538—540. Bibliography of sense organs of lateral line, pp. 540, 541. CHAPTER XVIII. THE NOTOCHORD, THE VERTEBRAL COLUMN, THE RIBS, AND THE STERNUM. Pp. 542 — 563. Introductory remarks on the origin of the skeleton, pp. 542 — 544. Biblio- graphy of the origin of the skeleton, pp. 544, 545. The notochord and its cartilngi- CONTENTS OF VOLUME II. nous sheath, pp. 545 — 549. The vertebral arches and the vertebral bodies •, pp. 549 — 559. Cyclostomata, p. 549. Elasmobranchii, pp. 549 — 553. Ganoidei, p. 553. Teleostei, p. 553. Amphibia, pp. 553—556- Reptilia, pp. 556, 557. Aves, PP- 557> 558. Mammalia, pp. 558, 559. Bibliography of the notochord and vertebral column, p. 560. l?tl>s, pp. 560—562. Sternum, pp. 562, 563. Bibliography of the ribs and sternum, p. 563. CHAPTER XIX. THE SKULL. Pp. 564—598. Preliminary remarks, pp. 564, 565. The cartilaginous cranium, pp. 565 — 571. The parachordals and notochord, pp. 566, 567. The trabeculse, pp. 567 — 570. The sense capsules, pp. 570, 571. The branchial skeleton, pp. 572 — 591. General structure of, pp. 572 — 575. Mandibular and hyoid arches, pp. 575 — 591. Elasmobranchii, pp. 576 — 579. Teleostei, pp. 579 — 581. Am- phibia, pp. 581—588. Sauropsida, pp. 588, 589. Mammalia, pp. 589—591. Membrane bones and ossifications of the cranium, pp. 592—597. Membrane bones, pp. 592 — 595. Ossifications of the cartilaginous cranium, pp. 595 — 597. Labial cartilages, p. 597. Bibliography of the skull, p. 598. CHAPTER XX. PECTORAL AND PELVIC GIRDLES AND THE SKELETON OF THE LIMBS. Pp. 599 — 622. The Pectoral girdle, pp. 599—606. Pisces, pp. 599—601. Amphibia and Amniola, 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 ami Amniota, pp. 606, 607. Amphibia, p. 607. Lacertilia, p. 607. Mammalia, p. 608. Bibliography of Pelvic girdle, p. 608. Comparison of pectoral and pelvic girdles, pp. 608, 609. Limbs, pp. 609--622. The piscine fin, pp. 609 — 618. The cheiroptery- gium, pp. 618 — 622. Bibliography of limbs, p. 622. CHAPTER XXI. THE BODY CAVITY, THE VASCULAR SYSTEM AND THE VASCULAR GLANDS. Pp. 623 — 666. The body Cavity, pp. 623 — 632. General, pp. 623, 624. Chordala, pp. 624 — 632. Abdominal pores, pp. 626, 627. Pericardial cavities, pleural cavities and diaphragm, pp. 627 — 632. Bibliography of body cavity, p. 632. The vascular system, pp. 632—663. General, pp. 632, 633. The heart, pp. 633—643. Bibliography of the heart, p. 643. Arterial system, pp. 643—651. Bibliography of the arterial system, p. 651. Venous system, pp. 651—663. Bibliography of the venous system, p. 663. Lymphatic system and spleen, p. 664. Bibliography of spleen, p. 664. Suprarenal bodies, pp. 664 — 666. Bibliography of suprarenal bodies, p. 666. CHAPTER XXII. THE MUSCULAR SYSTEM. Pp. 667 — 679. Evolution of muscle-cells, pp. 667, 668. Voluntary muscular system of the Chor- data, pp. 668 — 679. Muscular fibres, pp. 668, 669. Muscular system of the trunk and limbs, pp. 673 — 676. The somites and muscular system of the head, pp. 676 — 679. Bibliography of muscular system, p. 679. CONTENTS OF VOLUME II. xi CHAPTER XXIII. EXCRETORY ORGANS. Pp. 680 — 740. Platyelminthes, pp. 680, 681. Mollusca, pp. 681, 682. Polyzoa, pp. 682, 683. Branchiopoda, p. 683. Choctopoda, pp. 683 — 686. Gephyrea, pp. 686, 687. Discophora, pp. 687, 688. Arthropoda, pp. 688, 689. Nematoda, p. 689.- Excre- tory organs and generative ducts of the Craniata, pp. 689—737. General, pp. 689, 690. Elasmobranchii, pp. 690 — 699. Cyclostomata, pp. 700, 701. Teleostei, pp. 701 — 704. Ganoidei, pp. 704—707. Dipnoi, p. 707. Amphibia, pp. 707 — 713. Amniota, pp. 713 — 727. General conclusions and summary, pp. 728 — 737. Pronephros, pp. 728, 729. Mesonephros, pp. 729 — 732. Genital ducts, pp. 732 — 736. Metanephros, pp. 736, 737. Com- parison of the excretory organs of the Chordata and Invertebrata, pp. 737, 738. Bibliography of Excretory organs, pp. 738 — 740. CHAPTER XXIV. GENERATIVE ORGANS AND GENITAL DUCTS. Pp. 741—753- Generative organs, pp. 741—748. Porifera, p. 741. Ccelenterata,- pp. 741 — 743. Chaetopoda and Gephyrea, p. 743. Choetognatha, pp. 743 — 745. Polyzoa, p. 745. Nematoda, p. 745. Insecta, p. 745. Crustacea, pp. 745, 746. Chordata, pp. 746 — 748. Bibliography of generative organs, p. 748. Genital ducts, pp. 748—753. CHAPTER XXV. THE ALIMENTARY CANAL AND ITS APPENDAGES IN THE CHORDATA. Pp. 754 — 780. Mesenteron, pp. 754 — 774. Subnotochordal rod, pp. 754 — 756. Splanch- nic mesoblast and mesentery, pp. 756 — 758. Respiratory division of the Mesen- teron, pp. 758 — 766. Thyroid body, pp. 759—762. Thymus gland, pp. 762, 763. Swimming bladder and lungs, pp. 763 — 766, The middle division of the Mesen- teron, pp. 766 — 771. Cloaca, pp. 766, 767. Intestine, pp. 767, 768. Liver, pp. 769, 770. Pancreas, pp. 770, 771. Postanal section of the Mesenteron, pp. 771—774. The stomodaeum, pp. 774 — 778. Comparative development of oral cavity, pp. 774—7/6- Teeth, pp. 776— 778. The proctodaeum, pp. 778 — 780. Bibliography of alimentary canal, p. 780. EMBRYOLOGY. CHAPTER I. CEPHALOCHORDA. THE developmental history of the Chordata has been studied far more completely than that of any of the groups so far con- sidered ; and the results which have been arrived at are of striking interest and importance. Three main subdivisions of this group can be recognized : (i) the Cephalochorda containing the single genus Amphioxus ; (2) the Urochorda or Tunicata ; and (3) the Vertebrata1. The members of the second and probably of the first of these groups have undergone degenera- tion, but at the same time the members of the first group especially undergo a less modified development than that of other Chordata. CEPHALOCHORDA. Our knowledge of the development of Amphioxus is mainly due to Kowalevsky (Nos. 1 and 2). The ripe eggs appear to be dehisced into the branchial or atrial cavity, and to be transported thence through the branchial clefts into the pharynx, and so through the mouth to the exterior. (Kowalevsky, No. 1, and Marshall, No. 5.) 1 The term Vertebrata is often used to include the Cephalochorda. It is in many ways convenient to restrict its use to the forms which have at any rate some indica- tions of vertebne ; 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. 13. III. I FORMATION OF THE LAYERS. When laid the egg is about 0*105 mm. in diameter. It is in- vested by a delicate membrane, and is somewhat opaque owing to the presence of yolk granules, which are however uniformly distributed through it, and proportionately less numerous than in the ova of most Chordata. Impregnation is external and the segmentation is nearly regular (fig. i). A small segmentation FIG. i. THE SEGMENTATION OF AMPHIOXUS. A. Stage with two equal segments. B. Stage with four equal segments. (Copied from Kowalevsky. ) C. Stage after the four segments have become divided by an equatorial furrow into eight equal segments. D. Stage in which a single layer of cells encloses a central segmentation cavity. E. Somewhat older stage in optical section. SS- segmentation cavity. cavity is visible at the stage with four segments, and increases during the remainder of the segmentation ; till at the close (fig. i E) the embryo consists of a blastosphere formed of a single layer of cells enclosing a large segmentation cavity. One side of the blastosphere next becomes invaginated, and during the process the embryo becomes ciliated, and commences to rotate. The cells forming the invaginated layer become gradually more columnar than the remaining cells, and constitute the hypoblast; and a structural distinction between the epiblast and hypoblast is thus established. In the course of the invagination the seg- CEPHALOCHORDA. mentation cavity becomes gradually obliterated, and the embryo first assumes a cup-shaped form with a wide blastopore, but soon becomes elongated, while the communication of the archenteron, or cavity of invagination, with the exterior is reduced to a: small blastopore (fig. 2 A), placed at the pole of the long axis which the subsequent development shews to be the hinder end oj the FIG. i. EMBRYOS OF AMPHIOXUS. (After Kowalevsky.) The parts in black with white lines are epiblastic; the shaded parts are hypo- blastic. A. Gastrula stage in optical section. B. Slightly later stage after the neural plate np has become differentiated, seen as a transparent object from the dorsal side. C. Lateral view of a slightly older larva in optical section. D. Dorsal view of an older larva with the neural canal completely closed except for a small pore (no) in front. E. Older larva seen as a transparent object from the side. bl. blastopore (which becomes in D the neurenteric canal) ; ne. neurenteric canal ; np. neural or medullary plate ; no, anterior opening of neural canal ; ch. notochord ; so1, so", first and second mesoblastic somites. embryo. The blastopore is often known in other Chordata as the anus of Rusconi. Before the invagination is completed the larva throws off the egg-membrane, and commences to lead a free existence. Up to this stage the larva, although it has acquired a cylindrical elongated form, has only the structure of a simple two-layered gastrula ; but the changes which next take place I — 2 MEDULLARY CANAL. give rise on the one hand to the formation of the central nervous system, and on the other to the formation of the notochord and mesoblastic somites1. The former structure is developed from the epiblast and the two latter from the hypoblast. The formation of the central nervous system commences with the flattening of the dorsal surface of the embryo. The flattened area forms a plate (fig. 2 B and fig. 3 A, np\ extending backwards to the blastopore, which has in the meantime passed round to the dorsal surface. The sides of the plate become raised as two folds, which are most prominent posteriorly, and meet behind the blastopore, but shade off in front. The two folds next unite dorsally, so as to convert the previous groove into a canal2 — the neural or medullary canal. They unite first of all over the blastopore, and their line of junction extends from this point forwards (fig. 2 C, D, E). There is in this way formed a tube on the floor of which the blastopore opens behind, and which is itself open in front. Finally the medullary canal is formed for the whole length of the embryo. The anterior opening persists however for some time. The communication between the neural and alimentary tracts becomes interrupted when the caudal fin appears and the anus is formed. The neural canal then extends round the end of the notochord to the ventral side, but subsequently retreats to the dorsal side and terminates in a slight dilatation. In the formation of the medullary canal there are two points deserving notice — viz. (i) the connection with the blastopore; (2) the relation of the walls of the canal to the adjoining epiblast. With reference to the first of these points it is clear that the fact of the blastopore opening on the floor of the neural canal causes a free communication to exist between the archen- teron or gastrula cavity and the neural canal ; and that, so long as the anterior pore of the neural canal remains open, the archenteron communicates indirectly with the exterior (vide fig. 2 E). It must not however be supposed (as has been done by some embryologists) that the pore at the front end of the neural canal represents the blastopore carried forwards. It is 1 The protovertebrse 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 Kovvalevsky describes as the carrying of the blastopore to the dorsal side is really the commencement of the formation of the neural canal, the walls of which are con- tinuous with the lips of the blastopore. This interpretation receives support from the fact that at a later stage, when the neural and alimentary canals become separated, the neural canal extends round the posterior end of the notochord to the ventral side. The embryonic communication between the neural and alimentary canals is common to most Chordata ; and the tube connecting them will be called the neurenteric canal. It is always formed in fundamentally the same manner as in Amphioxus. With reference to the second point it is to be noted that Amphioxus is exceptional amongst the Chordata in the fact that, before the closure of the neural groove, the layer of cells which will form the neural tube becomes completely separated from the adjoining epiblast (fig. 3 A), and forms a FIG. 3. SECTIONS OF AN AMPHIOXUS EMBRYO AT THREE STAGES. (After Kowalevsky.) A. Section at gastrula stage. B. Section of an embryo slightly younger than that represented in fig. 2 D. C. Section through the anterior part of an embryo at the stage represented in fig. 2 E. np. neural plate ; nc . neural canal ; mes. archenteron in A and B, and mesenteron in C ; ch. notochord ; so. mesoblastic somite. structure which may be spoken of as the medullary plate ; and that in the closure of the neural canal the lateral epiblast forms a complete layer above this plate before the plate itself is folded over into a closed canal. This peculiarity will be easily under- stood from an examination of fig. 3 A, B and C. The formation of the mesoblastic somites commences, at about the same time as that of the neural canal, as a pair of hollow outgrowths of the walls of the archenteron. These MESOBLASTIC SOMITES. outgrowths, which are shewn in surface view in fig. 2 B and D, so, and in section in fig. 3 B and C, so, arise near the front end of the body and gradually extend backwards as wing-like diver- ticula of the archenteric cavity. As they grow backwards their dorsal part becomes divided by transverse constrictions into cubical bodies (fig. 2 D and E), which, with the exception of the foremost, soon cease to open into what may now be called the mesenteron, and form the mesoblastic somites. Each mesoblastic somite, after its separation from the mesenteron, is constituted of two layers, an inner one — the splanchnic — and an outer — the somatic, and a cavity between the two which was originally con- tinuous with the cavity of the mesenteron. Eventually the dorsal parts of the outgrowths become separated from the ventral, and form the muscle-plates, while their cavities atrophy. The cavity of the ventral part, which is not divided into separate sections by the above described constrictions, remains as the true body cavity. The ventral part of the inner layer of the mesoblastic outgrowths gives rise to the muscular and connective tissue layers of the alimentary tract, and the dorsal part to a section of the voluntary muscular system. The ventral part of the outer layer gives rise to the somatic meso- blast, and the dorsal to a section of the voluntary muscular system. The anterior mesoblastic somite long retains its com- munication with the mesenteron, and was described by Max Schultze, and also at first by Kowalevsky, as a glandular organ. While the mesoblastic somites are becoming formed the dorsal wall of the mesenteron develops a median longitudinal fold (fig. 3 B, c/i), which is gradually separated off from before back- wards as a rod (fig. 3 C, cJt), underlying the central nervous system. This rod is the notochord. After the separation of those parts the remainder of the hypoblast forms the wall of the mesenteron. With the formation of the central nervous system, the meso- blastic somites, the notochord, and the alimentary tract the main systems of organs are established, and it merely remains briefly to describe the general changes of form which accompany the growth of the larva into the adult. By the time the larva is but twenty-four hours old there are formed about seventeen mesoblastic somites. The body, during the period in which CEPHALOCHORDA. these are being formed, remains cylindrical, but shortly after- wards it becomes pointed at both ends, and the caudal fin appears. The fine cilia covering the larva also become replaced by long cilia, one to each cell. The mesenteron is still completely closed, but on the right side of the body, at the level of the front end of the mesenteron, the hypoblast and epiblast now grow together, and a perforation becomes formed through their point /v VVJl 'IL Jl 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. mes. mesenteron ; br.c. branchial cavity; //. body cavity. of contact, which becomes the mouth. The anus is probably formed about the same time if not somewhat earlier1. Of the subsequent changes the two most important are (i) the formation of the gill slits or clefts ; (2) the formation of the peribranchial or atrial cavity. The formation of the gill slits is, according to Kowalevsky's description, so peculiar that one is almost tempted to suppose that his observations were made on pathological specimens. The following is his account of the process. Shortly after the formation of the mouth there appears on the ventral line a coalescence between the epiblast and hypoblast. Here an opening is formed, and a visceral cleft is thus established, which passes to the left side, viz. the side opposite the mouth. A second and apparently a third slit are formed in the same way. The stages immediately following were not observed, but in the next stage twelve slits were present, no longer however on the left side, but in the median ventral line. There now appears on the side opposite the mouth, and the same therefore as that originally occupied by the first three clefts, a series of fresh clefts, which in their 1 The lateral position of the mouth in the embryo Amphioxus has been regarded as proving that the mouth represents a branchial cleft, but the general asymmetry of the organs is such that no great stress can, I think, be laid on the position of the mouth. 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." Mem. Acad. Imper. des Sciences de St Petersbourg, Series VII. Tom. XI. 1867. (2) A. Kowalevsky. "Weitere Studien iiber die Entwicklungsgeschichte des Amphioxus lanceolatus." Archiv f. mikr. Anat., Vol. xm. 1877. (3) Leuckart u. Pagenstecher. " Untersuchungen iiber niedere Seethiere." Mutter's Archiv, 1858. (4) Max Schultze. " Beobachtung junger Exemplare von Amphioxus." Zeit. f. wiss. Zool., Bd. in. 1851. (5) A. M.Marshall. " On the mode of Oviposition of Amphioxus." Jour, of Anat. and Phys., Vol. x. 1876. CHAPTER II. UROCHORDA1. IN the Solitaria, except Cynthia, the eggs are generally laid, and impregnation is effected sometimes before and sometimes after the eggs have left the atrial cavity. In Cynthia and most Caducichordata development takes place within the body of the parent, and in the Salpidae a vascular connection is established between the parent and the single foetus, forming a structure physiologically comparable with the Mammalian placenta. Solitaria. The development of the Solitary Ascidians has been more fully studied than that of the other groups, and appears moreover to be the least modified. It has been to a great extent elucidated by the splendid researches of Kowalevsky (Nos. 18 and 20), whose statements have been in the main followed in the account below. Their truth seems to me to be established, in spite of the scepticism they have met with in some quarters, by the closeness of their correspondence with the developmental phenomena in Amphioxus. 1 The following classification of the Urochorda is adopted in the present chapter. I- Caducichordata. ( Solitaria ex. Ascidia. A. SIMPLICIA \ ( Sociaha ex. Llavelltna. B. COMPOSITA Sedentaria ex. Sotryllus. ( Natantia ex. Pyrosoma. C. CONSERTA ( Dohohdae. II. Perennichordata. Ex. Appetuiicularia. 10 MEDULLARY GROOVE. The type most fully investigated by Kowalevsky is Ascidia (Phallusia) mammillata ; and the following description must be taken as more especially applying to this type. The segmentation is complete and regular. A small seg- mentation cavity appears fairly early, and is surrounded, ac- cording to Kowalevsky, by a single layer of cells, though on this point Kupffer (No. 27) and Giard (No. 11) are at variance with him. The segmentation is followed by an invagination of nearly the same character as in Amphioxus. The blastosphere resulting from the segmentation first becomes flattened on one side, and the cells on the flatter side become more columnar (fig. 8 I.). Very shortly a cup-shaped form is assumed, the concavity of which is lined by the more columnar cells. The mouth of the cup or blastopore next becomes narrowed ; while at the same time the embryo becomes oval. The blastopore is situated not quite at a pole of the oval but in a position which subsequent development shews to be on the dorsal side close to the posterior end of the embryo. The long axis of the oval corresponds with the long axis of the embryo. At this stage the embryo consists of two layers ; a columnar hypoblast lining the central cavity or archen- teron, and a thinner epiblastic layer. The dorsal side of the embryo next becomes flattened (fig. 8 II.), and the epiblast cover- ., . . ,. r , , FIG. 5. TRANSVERSE SECTION ing it is shortly afterwards marked THROUGH THE FRONT KND OF AN EM- by an axial groove continued for- .BfJ° °J PHALLUSIA MAMMILLATA. 0 (After Kowalevsky.) wards from the blastopore to near The embryo is slightly younger the front end of the body (fig. 5, than that represented in fig. 8 in. mg}. 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. floor of the groove, and, on the conversion of the groove into a canal, the blastopore connects the canal with the archenteric cavity, and forms a short neurenteric canal. The closure of the medullary canal commences at the blastopore and is thence continued forwards, the anterior end of the canal remaining open. The above me- processes are represented in longitu- dinal section in fig. 8 III, n. When the neural canal is completed for its whole length, it still communicates by a terminal pore with the exterior. FIG. 6. TRANSVERSE OPTICAL T , , . c ,. ... SECTION OF THE TAIL OF AN EM- In the relation of the medullary BRYO OF PHALLUSIA MAMMIL- canal to the blastopore, as well as LATA- FIG. 21. TRANSVERSE SEC- TION THROUGH THE TRUNK OF AN EMBRYO SLIGHTLY OLDER THAN FIG. 28 E. nc. neural canal ; pr. posterior root of spinal nerve ; x. subnoto- chordal rod ; ao. aorta ; sc. soma- tic mesoblast ; sp. splanchnic me- soblast ; mp. muscle-plate ; mp' . portion of muscle-plate converted into muscle ; Vv. portion of the vertebral plate which will give rise to the vertebral bodies ; al. alimentary tract. FIG. 22. HORIZONTAL SECTION THROUGH THE TRUNK OF AN EMBRYO OF ScYLLIUM CONSIDERABLY YOUNGER THAN 28 F. The section is taken at the level of the notochord, and shews the separation of the cells to form the vertebral bodies from the muscle-plates. ch. notochord ; ep. epiblast ; Vr. rudiment of vertebral body; mp. muscle-plate; mp' . portion of muscle-plate already differentiated into longitudinal muscles. ELASMOBRANCHII. 49 portion ; thereby the upper segmented part of the body cavity becomes isolated, and separated from the lower and unseg- mented part. As a consequence of this change the vertebral plate comes to consist of a series of rectangular bodies, the mesoblastic somites, each composed of two layers, a somatic and a splanchnic, between which is the cavity originally continu- ous with the body cavity (fig. 23, mp). The splanchnic layer of the plates buds off cells to form the rudiments of the vertebral bodies which are at first segmented in the same planes as the mesoblastic somites (fig. 22, Vr}. The plates themselves re- main as the muscle-plates (mp], and give rise to the whole of the voluntary muscular system of the body. Between the vertebral and lateral plates there is left a connecting isthmus, with a narrow prolongation of the body cavity (fig. 23 B, st], which gives rise (as described in a special chapter) to the segmental tubes and to other parts of the excretory system. In the meantime the lateral plates of the two sides unite ventrally throughout the intestinal and cardiac regions of the body, and the two primitively isolated cavities contained in them coalesce. In the tail however the plates do not unite ventrally till somewhat later, and their contained cavities remain distinct till eventually obliterated. At first the pericardial cavity is quite continuous with the body cavity ; but it eventually becomes separated from the body cavity by the attachment of the liver to the abdominal wall, and by a horizontal septum in which run the two ductus Cuvieri (fig. 23 A, sv}. Two perforations in this septum (fig. 23 A) leave the cavities in permanent communication. The parts derived from the two layers of the mesoblast (not including special organs or the vascular system) are as follows : — From the somatic layer are formed (1) A considerable part of the voluntary muscular system of the body. (2) The dermis. (3) A large part of the inter-muscular connective tissue. (4) Part of the peritoneal epithelium. From the splanchnic layer are formed (i) A great part of the voluntary muscular system. B. III. 4 THE MESOBLAST. (2) Part of the inter-muscular connective tissue. (3) The axial skeleton and surrounding connective tissue. (4) The muscular and connective-tissue wall of the alimentary tract. (5) Part of the peritoneal epithelium. In the region of the head the mesoblast does not at first become divided into somites ; but on the formation of the gill A. B. sp.c lit FIG. 23. SECTIONS THROUGH THE TRUNK OF A SCYLLIUM EMBRYO SLIGHTLY YOUNGER THAN 28 F. Figure A shews the separation of the body cavity from the pericardial cavity by a horizontal septum in which runs the ductus Cuvieri ; on the left side is seen the narrow passage which remains connecting the two cavities. Fig. B through a posterior part of the trunk shews the origin of the segmental tubes and of the primi- tive ova. sp.c. spinal canal ; W. white matter of spinal cord ; pr. commissure connecting the posterior nerve-roots ; ch. notochord ; x. sub-notochordal rod ; ao. aorta ; sv. sinus venosus ; cav. cardinal vein ; ht. heart ; pp. body cavity ; pc. pericardial cavity ; ces. solid oesophagus; /. liver; mp. muscle- plate ; nip', inner layer of muscle-plate; Vr. rudiment of vertebral body ; st. segmental tube ; sd. segmental duct ; sp.v. spiral valve ; v. subintestinal vein. clefts a division takes place, which is apparently equivalent to the segmentation of the mesoblast in the trunk. This division causes the body cavity of the head to be divided up into a series ELASMOBRANCHII. 51 of separate segments, one of which is shewn in fig. 24, pp. The walls of the segments eventually give rise to the main muscles of the branchial clefts, and probably also to the muscles of the mandibular arch, of the eye, and of other parts. The cephalic sections of the body cavity will be spoken of as head cavities. In addition to the parts already mentioned the meso- blast gives rise to the whole of the vascular system, and to the generative system. The heart is formed from part of the splanchnic meso- FlG ^ HORIZONTAL SECTION THROUGH blast, and the generative THE LAST VISCERAL ARCH BUT ONE OF AN , c c . , EMBRYO OF PRISTIURUS. system from a portion of the mesoblast of the dorsal nart #• ePiblast; vc> Pouch of hypoblast which will form the walls of a visceral cleft ; of the body Cavity. //• segment of body-cavity in visceral arch ; m, ,, 17 aa. aortic arch. The hypoblast. Very shortly after the formation of the mesoblastic plates as lateral differentiations of the lower layer cells, an axial differentiation of the hypoblast appears, which gives rise to the notochord very much in the same way as in Amphioxus. At first the hypoblast along the axial line forms a single layer in contact with the epiblast. Along this line a rod-like thickening of the hypoblast very soon appears (fig. 25, B and C, Ch'} at the head end of the embryo, and gradually extends back- wards. This is the rudiment of the notochord ; it remains attached for some time to the hypoblast, and becomes separated from it first at the head end of the embryo (fig. 25 A, ch} : the separation is then carried backwards. A series of sections taken through an embryo shortly after the first differentiation of the notochord presents the following characters. In the hindermost sections the hypoblast retains a perfectly normal structure and uniform thickness throughout. In the next few sections (fig. 25 C, Ch} a slight thickening is to be observed in it, immediately below the medullary groove. The layer, which elsewhere is composed of a single row of cells, here becomes two cells deep, but no sign of a division into two layers is exhibited. In the next few sections the thickening of the hypoblast becomes much more pronounced ; we have, in fact, a ridge projecting from the hypoblast towards the epiblast (fig. 25 B, Ch'). This ridge is pressed firmly against 4—2 THE HYPOBLAST. the epiblast, and causes in it a slight indentation. The hypoblast in the region of the ridge is formed of two layers of cells, the ridge being entirely due to the uppermost of the two. In sections in front of this a cylindrical rod, which can at once be recognized as the notochord, and is continuous with the ridge just described, begins to be split off from the hypoblast (fig. 25 A, Ch}. It is diffi- cult to say at what point the separation of this rod from the hypoblast is com- pleted, since all intermedi- ate gradations between complete separation and complete attachment are to be seen. Shortly after the separ- ation takes place, a fairly thick bridge is found con- necting the two lateral halves of the hypoblast, but this bridge is anterior- ly excessively delicate and thin, and in some cases is FIG. 25. THREE SECTIONS OF A PRISTIURUS EMBRYO SLIGHTLY OLDER THAN FIG. 28 B. The sections shew the development of the noto- chord. Ch. notochord ; Ch'. developing notochord ; mg. medullary groove ; Ip. lateral plate of mesoblast ; ep. epiblast ; hy, hypoblast. barely visible except with high powers. In some sections I have observed possible indications of the process like that described by Calberla for Petronyzon, by which the lateral parts of the hypoblast grow in underneath the axial part, and so isolate it bodily as the notochord. It is not absolutely clear whether the notochord is to be regarded as an axial differentiation of the hypoblast, or as an axial differentiation of the lower layer cells. The facts of development both in Amphioxus and Elasmo- branchii tend towards the former view ; but the nearly simul- taneous differentiation of the notochord and the mesoblastic plates lends some support to the supposition that the notochord may be merely a median plate of mesoblast developed slightly later than the two lateral plates. The alimentary canal or mesenteron was left as a space between the hypoblast and the yolk, ending blindly in front, but ELASMOBRANCIIII. 53 FIG. 26 SECTION THROUGH THE ANTERIOR PART OF A PRIS- TIURUS EMBRYO TO SHEW THE FORMATION OF THE ALIMENTARY TRACT. Ch. notochord ; hy. hypoblast; al, alimentary tract ; na. cells passing in from the yolk to form the ventral wall of the alimentary tract. opening behind by a widish aperture, the blastopore or anus of Rusconi (vide fig. 19 B). The conversion of this irregular cavity into a closed canal commences first of all at the anterior extremity. In this conversion two distinct processes are concerned. One of these is a process of folding off of the embryo from the blasto- derm. The other is a simple growth of cells independent of any fold. To the first of these processes the depth and narrowness of the ali- mentary cavity is due ; the second is concerned in forming its ventral wall. The process of the folding off of the embryo from the blastoderm resembles exactly the similar process in the embryo bird. The fold is a perfectly continuous one round the front end of the embryo, but may be conveniently spoken of as composed of a head-fold and two lateral folds. Of far greater interest than the nature of these folds is the formation of the ventral wall of the alimentary canal. This originates in a growth of cells from the two sides to the middle line (fig. 26). The cells for it are not however mainly derived from pre-existing hypoblast cells, but are formed de novo around the nuclei of the yolk which have already been spoken of (fig. 26, no). The ventral wall of the mesenteron is in fact, to a large extent at any rate, formed as a dif- ferentiation of the primitive yolk floor. The folding off and closing of the alimentary canal in the anterior part of the body proceeds rapidly, and not only is a considerable tract of the alimentary canal formed, but a great part of the head is completely folded off from the yolk before the medullary groove is closed. FIG. 27. LONGITUDINAL VERTICAL SECTION OF AN EMBRYO SLIGHTLY YOUNGER THAN THAT IN FIG. 26 D. The section shews the communication which exists between the neural and ali- mentary canals. nc. neural canal ; al. ali- mentary tract ; Ch. noto- chord; 7s. tail swelling. 54 THE HYPOBLAST. The posterior part of the alimentary canal retains for a longer time its primitive condition. Finally however it also becomes closed in, by the lips of the blastopore at the hind end of the embryo meeting and uniting. The peculiarity of the closing in of the posterior part of the alimentary canal consists in the fact that a similar continuity to that in Amphioxus obtains between the neural and alimentary canals. This is due to the medullary folds being continuous at the end of the tail with the lips of the blastopore, which close in the hind end of the alimentary canal ; so that, when the medullary folds unite to form a canal, this canal becomes continuous with the ali- mentary canal, which is closed in at the same time. In other words, the medullary folds assist in enveloping the blastopore which does not therefore become absolutely closed, but opens into the floor of the neural canal. It will afterwards be shewn that it is only the posterior part of the blastopore that becomes closed during the above process, and that the anterior and ventral part long remains open. The general arrangement of the parts, at the time when the hind end of the mesenteron is first closed, is shewn in fig. 27. The same points may be seen in the diagrammatic longitudinal section fig. 19 C. The middle portion of the alimentary tract is the last to be closed in since it remains till late in embryonic life as the umbilical or vitelline canal, connecting the yolk-sack with the alimentary cavity. The umbilical canal falls into the alimentary tract immediately behind the entrance of the hepatic duct. At a fairly early stage of development a rod is constricted off from the dorsal wall of the alimentary canal (figs. 27* and 23 x), which is known as the subnotochordal rod. It is placed immedi- ately below the notochord, and disappears during embryonic life. FIG. 27*. TRANSVERSE SECTION THROUGH THE TAIL REGION OF A PRIS- TIURUS EMBRYO OF THE SAME AGE AS FIG. 28 E. df, dorsal fin ; sp.c. spinal cord ; pp. body cavity ; sp. splanchnic layer of mesoblast ; so. somatic layer of meso- blast ; mp. commencing differentiation of mus- cles ; ch. notochord ; jc. sub-notochordal rod aris- ing as an outgrowth of the dorsal wall of the alimentary tract ; al. ali- mentary tract. ELASMOBRANCHII. 55 General features of tlic ElasmobrancJi 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 (h) — while the tail end at the edge of the blasto- derm is also enlarged, being formed of a pair of swellings — the tail swellings (ts) — derived from the lateral parts of the original embryonic rim. By this stage a certain number of mesoblastic somites have become formed but are not shewn in my figure. They are the foremost somites of the trunk, and those behind them continue to be added, like the segments in Chaetopods, between the last formed somite and the end of the body. The increase in length of the body mainly takes place by growth in the region between the last mesoblastic somite and the end of the tail. The anterior part of the body is now completely folded off from the blastoderm, and the medullary groove of the earlier stage has become converted into a closed canal. By the next stage (fig. 28 D) the embryo has become so much folded off from the yolk both in front and behind that the separate parts of it begin to be easily recognizable. The embryo is attached to the yolk by a distinct stalk or cord, which in the succeeding stages gradually narrows and elongates, and is known as the umbilical cord (so. s.). The medullary canal has now become completely closed. The anterior region constitutes the brain ; and in this part slight constrictions, not perceptible in views of the embryo as a transparent object, mark off three vesicles. These vesicles are known as the fore, mid, and hind brain. From the fore-brain there is an outgrowth on each side, the first rudiment of the optic vesicles (op). The tail swellings are still conspicuous. GENERAL GROWTH OF THE EMBRYO. The tissues of the body have now become fairly transparent, and there may be seen at the sides of the body seventeen mesoblastic somites. The notochord, which was formed long 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 ; es. embryonic swelling. B. A somewhat older blastoderm in which the medullary groove has been es- tablished, mg. medullary groove. C. An embryo from the dorsal surface, as an opaque object, after the medullary groove has become posteriorly converted into a tube. mg. medullary groove : the reference line points very nearly to the junction between the open medullary groove with the medullary tube ; h. cephalic plate ; ts. tail swelling. D. Side view of a somewhat older embryo as a transparent object, ch. notochord ; op. optic vesicle ; I.v.c. ist visceral cleft; al. alimentary tract ; so.s. stalk connecting the yolk-sack with the embryo. E. Side view of an older embryo as a transparent object, mp. muscle-plates ; au.v. auditory vesicle ; vc. visceral cleft ; ht. heart ; ;;/. mouth invagination ; an. anal diverticulum ; al.v. posterior vesicle of post-anal gut. F. G. H. Older embryos as opaque objects. ELASMOBRANCHII. 57 before the stage represented in figure 28 D, is now also distinctly visible. It extends from almost the extreme posterior to the anterior end of the embryo, and lies between the ventral wall of the spinal canal and the dorsal wall of the intestine. Round its posterior end the neural and alimentary tracts become continu- ous with each other. Anteriorly the termination of the notochord cannot be seen, it can only be traced into a mass of mesoblast at the base of the brain, which there separates the epiblast from the hypoblast. The alimentary canal (ai) is completely closed anteriorly and posteriorly, though still widely open to the yolk-sack in the middle part of its course. In the region of the head it exhibits on each side a slight bulging out- wards, the rudiment of the first visceral cleft. This is represented in the figure by two lines (I. v.c.). The embryo represented in fig. 28 E is far larger than the one just described, but it has not been convenient to represent this increase of size in the figure. Accompanying this increase in size, the folding off from the yolk has considerably pro- gressed, and the stalk which unites the embryo with the yolk is proportionately narrower and longer than before. The brain is now very distinctly divided into the three lobes, the rudiments of which appeared during the last stage. From the foremost of these the optic vesicles now present themselves as well-marked lateral outgrowths, towards which there has appeared an involution from the external skin (op) to form the lens. A fresh organ of sense, the auditory sack, now for the first time becomes visible as a shallow pit in the external skin on each side of the hind-brain (au.v). The epiblast which is involuted to form this pit becomes much thickened, and thereby the opacity, indicated in the figure, is produced. The mesoblastic somites have greatly increased in number by the formation of fresh somites in the tail. Thirty-eight of them were present in the embryo figured. The mesoblast at the base of the brain is more bulky, and there is still a mass of unsegmented mesoblast which forms the tail swellings. The first rudiment of the heart (///) becomes visible during this stage as a cavity between the mesoblast of the splanchnopleure and the hypoblast. GENERAL GROWTH 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 (at.v), becomes eventually completely atrophied. It is known as the post-anal gut. In the region of the throat the rudiment of a second visceral cleft has appeared behind the first ; neither of them is as yet open to the exterior. In a somewhat older embryo the first spon- taneous movements take place, and consist in somewhat rapid ex- cursions of the embryo from side to side, pro- duced by a serpentine motion of the body. A ventral flexure of the prae-oral part of the head, known as the cranial flexure, which commenced in earlier stages (fig. 28 D and E), has now become very evident, and the mid-brain1 begins to project in the same manner as in the embryo fowl on the 1 The part of the brain which I have here called mid-brain, and which unquestion- ably corresponds to the part called mid-brain in the embryos of higher vertebrates, becomes in the adult what Miklucho-Maclay and Gegenbaur called the vesicle of the third ventricle or thalamencephalon. al FIG. 28*. FOUR SECTIONS THROUGH THE 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 ; mp. muscle-plate; eh. notochord; cl.al. cloaca; ao. aorta ; v.cau. caudal vein. ELASMOBRANCHII. 59 third day, and will soon form the anterior termination of the long axis of the embryo. The fore-brain has increased in size and distinctness, and the anterior part of it may now be looked on as the unpaired rudiment of the cerebral hemispheres. Further changes have taken place in the organs of sense, especially in the eye, in which the involution for the lens has made considerable progress. The number of the muscle-plates has again increased, but there is still a region of unsegmented mesoblast in the tail. The thickened portions of mesoblast, which caused the tail swellings, are still to be seen, and would seem to act as the reserve from which is drawn the matter for the rapid growth of the tail, which occurs soon after this. The mass of the mesoblast at the base of the brain has again increased. No fresh features of interest are to be seen in the notochord. The heart is very much more conspicuous than before, and its commencing flexure is very apparent. It now beats actively. The post-anal gut is much longer than during the last stage ; and the point where the anus will appear is very easily detected by a bulging out of the gut towards the external skin. The alimentary vesicle at the end of the post-anal gut, first observable during the last stage, is now a more conspicuous organ. There are three visceral clefts, none of which are as yet open to the exterior. Figure 28 F represents a considerably older embryo viewed as an opaque object, and fig. 29 A is a view of the head as a transparent object. The stalk connecting it with the yolk is now, comparatively speaking, quite narrow, and is of sufficient length to permit the embryo to execute considerable move- ments. The tail has grown immensely, but is still dilated terminally. The terminal dilatation is mainly due to the alimentary vesicle (fig. 28* ah], 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 OF THE EMBRYO. ol posterior pair, which is the later developed and less conspicuous of the two, is situated ^ ^ mr some little distance be- hind the stalk. The cranial flexure has greatly increased, and the angle between the long axis of the front part of the head and of the body is less than a right angle. The conspicuous mid-brain (29 A, mb) forms the anterior termination of the long axis of the body. The thin roof of the fourth ventricle (hb} may be noticed in the figure behind the mid-brain. The audi- tory sack (au. V} is nearly closed, and its opening is not shewn in the figure. In the eye (op} the lens is completely formed. The olfactory pit (ol} is seen a little in front of the eye. Owing to the opa- city of the embryo, the muscle-plates are only indistinctly indicated in fig. 28 F, and no other features of the mesoblast are to be seen. The mouth is now a deep pit, the hind borders of which are almost completely formed by a thickening in front of the first branchial or visceral cleft, which may be called the first branch- ial arch or mandibular arch. Four branchial clefts are now visible, all of which are open to the exterior, but in the embryo, viewed as a transparent FlG. 29. VIEWS OF THE HEAD OF ELASMO- BRANCH EMBRYOS AT TWO STAGES AS TRANS- PARENT OBJECTS. A. Pristiurus embryo of the same stage as fig. 28 F. B. Somewhat older Scyllium embryo. ///. third nerve ; V. fifth nerve ; VII. seventh nerve; au.n. auditory nerve; gl. glossopharyngeal nerve ; Vg. vagus nerve ; fb. fore-brain ; pn. pineal gland ; mb. mid-brain ; hb. hind-brain ; iv.v. fourth ventricle ; cb. cerebellum ; ol. olfactory pit ; op. eye ; au. V. auditory vesicle ; m. mesoblast at base of brain ; ch. notochord ; ht. heart ; Vc. visceral clefts ; eg. external gills ; pp. sections of body cavity in the head. ELASMOBRANCHII. 6 1 object, two more, not open to the exterior, are visible behind the last of these. Between each of these and behind the last one there is a thickening of the mesoblast which gives rise to a branchial arch. The arch between the first and second cleft is known as the hyoid arch. Fig. 29 B is a representation of the head of a slightly older embryo in which papillae may be seen in the front wall of the second, third, and fourth branchial clefts : these papillae are the commencements of filiform processes which grow out from the gill-clefts and form external gills. The peculiar ventral curva- ture of the anterior end of the notochord (c/i) both in this and in the preceding figure deserves notice. A peculiar feature in the anatomy makes its appearance at this period, viz. the replacement of the original hollow oesophagus by a solid cord of cells (fig. 23 A, ces) in which a lumen does not reappear till very much later. I have found that in some Teleostei (the Salmon) long after they are hatched a similar solidity in the oesophagus is present. It appears not impossible that this feature in the oesophagus may be connected with the fact that in the ancestors of the present types the oesophagus was perforated by gill slits ; and that in the process of embryonic abbreviation the stage with the perforated oesophagus became replaced by a stage with a cord of indifferent cells (the oesophagus being in the embryo quite functionless) out of which the non-perforated oesophagus was directly formed. In the higher types the process of development appears to have become quite direct. By this stage all the parts of the embryo have become established, and in the succeeding stages the features character- istic of the genus and species are gradually acquired. Two embryos of Scyllium are represented in fig. 28 G and H, the head and anterior part of the trunk being repre- sented in fig. G, and the whole embryo at a much later stage in fig. H. In both of these, and especially in the second, an apparent diminution of the cranial flexure is very marked. This diminu- tion is due to the increase in the size of the cerebral hemispheres, which grow upwards and forwards, and press the original fore- brain against the mid-brain behind. In fig. G the rudiments of the nasal sacks arc clearly visible as small open pits. 62 FORMATION OF THE YOLK-SACK. The first cleft is no longer similar to the rest, but by the closure of the lower part has commenced to be metamorphosed into the spiracle. Accompanying the change in position of the first cleft, the mandibular arch has begun to bend round so as to enclose the front as well as the sides of the mouth. By this change in the mandibular arch the mouth becomes narrowed in an antero- posterior direction. In fig. H are seen the long filiform external gills which now project out from all the visceral clefts, including the spiracle. They are attached to the front wall of the spiracle, to both walls of the next four clefts, and to the front wall of the last cleft. They have very possibly become specially developed to facilitate respiration within the egg ; and they disappear before the close of larval life. When the young of Scyllium and other Sharks are hatched they have all the external characters of the adult. In Raja and Torpedo the early stages, up to the acquirement of a shark-like form, are similar to those in the Selachoidei, but during the later embryonic stages the body gradually flattens out, and assumes the adult form, which is thus clearly shewn to be a secondary acquirement. An embryonic gill cleft behind the last present in the adult is found (Wyman, No. 54) in the embryo of Raja batis. The unpaired fins are developed in Elasmobranchs as a fold of skin on the dorsal side, which is continued round the end of the tail along the ventral side to the anus. Local developments of this give rise to the dorsal and anal fins. The caudal fin is at first symmetrical, but a special lower lobe grows out and gives to it a heterocercal character. Enclosure of the yolk-sack and its relation to the embryo. The blastoderm at the stage represented in fig. 28 A and B forms a small and nearly circular patch on the surface of the yolk, composed of epiblast and lower layer cells. While the body of the embryo is gradually being moulded this patch grows till it envelopes the yolk ; the growth is not uniform, but ELASMOBRANCHII. is less rapid in the immediate neighbourhood of the embryonic part of the blastoderm than elsewhere. As a consequence of this, that part of the edge, to which the embryo is at- tached, forms a bay in the otherwise regular outline of the edge of the blastoderm, and by the time that about two- thirds of the yolk is en- closed this bay is very conspicuous. It is shewn in fig. 30 A, where bl points to the blastoderm, and yk to the part of the yolk not yet covered by the blastoderm. The em- bryo at this time is only connected with the yolk- sack by a narrow umbili- cal cord ; but, as shewn in the figure, is still at- tached to the edge of the blastoderm. Shortly subsequent to this the bay in the blas- toderm, at the head of which the embryo is at- tached, becomes oblitera- ted by its two sides com- ing together and coales- cing. The embryo then ceases to be attached at the edge of the blasto- derm. But a linear streak formed by the coalesced FIG. 30. THREE VIEWS OF THE VITELLUS OF AN ELASMOBRANCH, SHEWING THE EMBRYO, THE BLASTODERM, AND THE VESSELS OF THE YOLK-SACK. The shaded part (bl) is the blastoderm; the white part the uncovered yolk. A. Young stage with the embryo still at- tached at the edge of the blastoderm. B. Older stage with the yolk not quite en- closed by the blastoderm. C. Stage after the complete enclosure of the yolk. yk. yolk ; bl. blastoderm ; v. venous trunks of yolk-sack ; a. arterial trunks of yolk-sack ; y. point of closure of the yolk blastopore ; x. por- tion of the blastoderm outside the arterial sinus terminalis. edges of the blastoderm is left connecting the embryo with the 64 FORMATION OF THE YOLK-SACK. edge of the blastoderm. This streak is probably analogous to (though not genetically related with) the primitive streak in the Amniota. This stage is represented in fig. 30 B. In this figure there is only a small patch of yolk (yK) not yet enclosed, which is situated at some little distance behind the embryo. Through- out all this period the edge of the blastoderm has remained thickened : a feature which persists till the complete investment of the yolk, which takes place shortly after the stage last described. In this thickened edge a circular vein arises which brings back the blood from the yolk-sack to the embryo. The opening in the blastoderm, exposing the portion of the yolk not yet covered, may be conveniently called the yolk blastopore. It is interesting to notice that, owing to the large size of the yolk in Elasmobranchs, the posterior part of the primitive blastopore becomes encircled by the medullary folds and tail- swellings, and is so closed long before the anterior and more ventral part, which is represented by the uncovered portion of the yolk. It is also worth remarking that, owing to the embryo becoming removed from the edge of the blastoderm, the final closure of the yolk blastopore takes place at some little distance from the embryo. The blastoderm enclosing the yolk is formed of an external layer of epiblast, a layer of mesoblast below in which the blood- vessels are developed, and within this a layer of hypoblast, which is especially well marked and ciliated (Leydig, No. 46) in the umbilical stalk, where it lines the canal leading from the yolk-sack to the intestine. In the region of the yolk-sack proper the blastoderm is so thin that it is not easy to be quite sure that a layer of hypoblast is throughout distinct. Both the hypoblast and mesoblast of the yolk-sack are formed by a differentiation of the primitive lower layer cells. Nutriment from the yolk-sack is brought to the embryo partly through the umbilical canal and so into the intestine, and partly by means of blood-vessels in the mesoblast of the sack. The blood-vessels arise before the blastoderm has completely covered the yolk. Fig. 30 A represents the earliest stage of the circulation of the yolk-sack. At this stage there is visible a single arterial ELASMOBRANCHII. 65 trunk (a) passing forwards from the embryo and dividing into two branches. No venous trunk could be detected with the simple microscope, but probably venous channels were present in the thickened edge of the blastoderm. In fig. 30 B the circulation is greatly advanced. The blasto- derm has now nearly completely enveloped the yolk, and there remains only a small circular space (yk} not enclosed by it. The arterial trunk is present as before, and divides in front of the embryo into two branches which turn backwards and form a nearly complete ring round the embryo. In general appearance this ring resembles the sinus terminalis of the area vasculosa of the Bird, but in reality bears quite a different relation to the circulation. It gives off branches on its inner side only. A venous system of returning vessels is now fully developed, and its relations are very remarkable. There is a main venous ring in the thickened edge of the blastoderm, which is con- nected with the embryo by a single stem running along the seam where the edges of the blastoderm have coalesced. Since the venous trunks are only developed behind the embryo, it is only the posterior part of the arterial ring that gives off branches. The succeeding stage (fig. 30 C) is also one of considerable interest. The arterial ring has greatly extended, and now embraces nearly half the yolk, and sends off trunks on its inner side along its whole circumference. More important changes have taken place in the venous system. The blastoderm has now completely enveloped the yolk, and the venous ring is therefore reduced to a point. The small veins which originally started from it may be observed diverging in a brush-like fashion from the termination of the unpaired trunk, which originally connected the venous ring with the heart. At a still later stage the arterial ring embraces the whole yolk, and, as a result of this, vanishes in its turn, as did the venous ring before it. There is then present a single arterial and a single venous trunk. The arterial trunk is a branch of the dorsal aorta, and the venous trunk originally falls into the heart together with the subintestinal or splanchnic vein. On the formation of the liver the proximal end of the subintestinal vein becomes the portal vein, and it is joined just as it enters B. in. 5 66 BIBLIOGRAPHY. the liver by the venous trunk from the yolk-sack. The venous trunk leaves the body on the right side, and the arterial on the left. The yolk-sack persists during the whole of embryonic life, and in the majority of Elasmobranch embryos there arises within the body walls an outgrowth from the umbilical canal into which a large amount 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. Balfour. " A preliminary account of the development of the Elasmo- branch Fishes." Quart, jf. of Micr. Science, Vol. xiv. 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 des produits adventifs de Foeuf des Plagiostomes et particulierement des Raies" Vide also Journal de rAnatomie et de la Physiologic, 1872. (43) W. His. " Ueb. d. Bildung v. Haifischenembryonen." Zeit. fiir Anat. u. Entwick., Vol. n. 1877. (44) A. Kowalevsky. "Development of Acanthias vulgaris and Mustelus laevis." (Russian.) Transactions of the Kiew Society of Naturalists, Vol. I. 1870. (45) R. Leuckart. " Ueber die allmahlige Bildung d. Korpergestalt bei d. Rochen." Zeit. f. wiss. Zool., Bd. n., p. 258. (46) Fr. Leydig. Rochen u. Haie. Leipzig, 1852. (47) A. W. Malm. " Bidrag till kannedom om utvecklingen af Rajse." Kongl. vetenskaps akademiens for hand lingar. Stockholm, 1876. (48) Joh. Miiller. Clatter Haie des Aristoteles und ilber die Verschiedenheiten unter den Haifischen und Rochen in der Entwick lung des Eies. Berlin, 1840. (49) S. L. Schenk. " Die Eier von Raja quadrimaculata innerhalb der Eileiter." Site, der k. Akad, Wien, Vol. Lxxm. 1873. BIBLIOGRAPHY. 67 (50) Alex. Schultz. " Zur Entwicklungsgeschichte des Selachiereies. " Archiv fur micro. Anat., Vol. XI. 1875. (51) Alex. Schultz. " Beitrag zur Entwicklungsgeschichte d. Knorpelfische. " Archiv fur micro. Anat.> Vol. xiu. 1877. (52) C. Semper. "Die Stammesverwandschaft d. Wirbelthiere u. Wfrbdlo- sen." Arbeit, a. d. zool.-zoot. Instit. Wurzbtirg, Vol. II. 1875. (53) C. Semper. " Das Urogenitalsystem d. Plagiostomen, etc." Arbeit, a. d. zooL-zoot. Instit. Wurzburg, Vol. II. 1875. (54) Wyman. " Observations on the Development of Raja batis." Memoirs of the American Academy of Arts and Sciences, Vol. IX. 1864. 5—2 CHAPTER IV. TELEOSTEI. THE majority of the Teleostei deposit their eggs before impregnation, but some forms are viviparous, e.g. Blennius viviparus. Not a few carry their eggs about ; but this operation is with a few exceptions performed by the male. In Syngna- thus the eggs are carried in a brood-pouch of the male situated behind the anus. Amongst the Siluroids the male sometimes carries the eggs in the throat above the gill clefts. Ostegenio- sus militaris, Arius falcarius, and Arius fissus have this peculiar habit. The ovum when laid is usually invested in the zona radiata only, though a vitelline membrane is sometimes present in addition, e.g. in the Herring. It is in most cases formed of a central yolk mass, which may either be composed of a single large vitelline sphere, or of distinct yolk spherules. The yolk mass is usually invested by a granular protoplasmic layer, which is especially thickened at one pole to form the germinal disc. In the Herring's ovum the germinal disc is formed, as in many Crustacea, at impregnation; the protoplasm which was previously diffused through the egg becoming aggregated at the germinal pole and round the periphery. Impregnation is external, and on its occurrence a contraction of the vitellus takes place, so that a space is formed between the vitellus and the zona radiata, which becomes filled with fluid. The peculiarities in the development of the Teleostean ovum can best be understood by regarding it as an Elasmobranch TELEOSTEI. 69 ovum very much reduced in size. It seems in fact very probable that the Teleostei are in reality derived from a type of Fish with a much larger ovum. The occurrence of a meroblastic segmentation, in spite of the ovum being usually smaller-than that of Amphibia and Acipenser, etc., in which the segment- ation is complete, as well as the solid origin of many of the organs, receives its most plausible explanation on this hypo- thesis. The proportion of the germinal disc to the whole ovum varies considerably. In very small eggs, such as those of the Herring, the disc may form as much as a fifth of the whole. The segmentation, which is preceded by active movements of the germinal disc, is meroblastic. There is nothing very special to note with reference to its general features, but while in large ova like those of the Salmon the first furrows only penetrate for a certain depth through the germinal disc, in small ova like those of the Herring they extend through the whole thickness of the disc. During the segmentation a great increase in the bulk of the blastoderm takes place. In hardened specimens a small cavity amongst the segment- ation spheres may be present at any early stage ; but it is probably an artificial product, and in any case has nothing to do with the true segmentation cavity, which does not appear till near the close of segmentation. The peripheral layer of granu- lar matter, continuous with the germinal disc, does not undergo division, but it becomes during the segmentation specially thickened and then spreads itself under the edge of the blasto- derm ; and, while remaining thicker in this region, gradually grows inwards so as to form a continuous sub-blastodermic layer. In this layer nuclei appear, which are equivalent to those in the Elasmobranch ovum. A considerable number of these nuclei often become visible simultaneously (van Beneden, No. 60) and they are usually believed to arise spontaneously, though this is still doubtful1. Around these nuclei portions of protoplasm are segmented off, and cells are thus formed, which enter the blastoderm, and have nearly the same destination as the homo- logous cells of the Elasmobranch ovum. 1 Vide Vol. II. p. 108. /O SEGMENTATION. During the later stages of segmentation one end of the blastoderm becomes thickened and forms the embryonic swell- ing ; and a cavity appears between the blastoderm and the yolk which is excentrically situated near the non-embryonic part of the blastoderm. This cavity is the true segmentation cavity. Both the cavity and the embryonic swelling are seen in section in fig. 31 A and B. In Leuciscus rutilus Bambeke describes a cavity as appearing in the middle of the blastoderm during the later stages of segmentation. From his figures it might be supposed that this cavity was equivalent to the segment- ation cavity of Elasmobranchs in its earliest condition, but Bambeke states that it disappears and that it has no connection with the true segmentation cavity. Bambeke and other investigators have failed to recognize the homology of the segmentation cavity in Teleostei with that in Elasmo- branchii, Amphibia, etc. With the appearance of the segmentation cavity the portion of the blastoderm which forms its roof becomes thinned out, so that the whole blastoderm consists of (i) a thickened edge especially prominent at one point where it forms the embryonic swelling, and (2) a thinner central portion. The changes which now take place result in the differentiation of the embryonic layers, and in the -rapid extension of the blastoderm round the yolk, accompanied by a diminution in its thickness. A FIG. 31. LONGITUDINAL SECTIONS THROUGH THE BLASTODERM OF THE TROUT AT AN EARLY STAGE OF DEVELOPMENT. A. at the close of the segmentation; B. after the differentiation of the germinal layers. ep' '. epidermic layer of the epiblast; sc. segmentation cavity. The first differentiation of the layers consists in a single row of cells on the surface of the blastoderm becoming distinctly TELEOSTEI. 7 1 marked off as a special layer (fig. 31 A); which however does not constitute the whole epiblast but only a small part of it, which will be spoken of as the epidermic layer. The complete differentiation of the epiblast is effected by the cells of the thickened edge of the blastoderm becoming divided into two strata (fig. 31 B). The upper stratum constitutes the epiblast. It is divided into two layers, viz., the external epidermic layer already mentioned, and an internal layer known as the nervous layer, formed of several rows of vertically arranged cells. According to the unanimous testimony of investigators the roof of the segmentation cavity is formed of epiblast cells only. The lower stratum in the thickened rim of the blastoderm is several rows of cells deep, and corresponds with the lower layer cells or primitive hypoblast in Elasmobranchii. It is continuous at the edge of the blastoderm with the nervous layer of the epiblast. In smaller Teleostean eggs there is formed, before the blasto- derm becomes differentiated into epiblast and lower layer cells, a complete stratum of cells around the nuclei in the granular layer underneath the blastoderm. This layer is the hypoblast ; and in these forms the lower layer cells of the blastoderm are stated to become converted into mesoblast only. In the larger Teleostean eggs, such as those of the Salmonidae, the hypoblast, as in Elasmobranchs, appears to be only partially formed from the nuclei of the granular layer. In these forms however, as in the smaller Teleostean ova and in Elasmobranchii, the cells derived from the granular stratum give rise to a more or less complete cellular floor for the segmentation cavity. The segmentation cavity thus becomes enclosed between an hypo- blastic floor and an epiblastic roof several cells deep. It becomes obliterated shortly after the appearance of the medul- lary plate. At about the time when the three layers become established the embryonic swelling takes a somewhat shield-like form (fig. 33 A). Posteriorly it terminates in a caudal prominence (ts) homologous with the pair of caudal swellings in Elasmo- branchs. The homologue of the medullary groove very soon appears as a shallow groove along the axial line of the shield. After these changes there takes place in the embryonic layers a series of differentiations leading to the establishment of the 72 FORMATION OF THE LAYERS. definite organs. These changes are much more difficult to follow in the Teleostei than in the Elasmobranchii, owing partly to the similarity of the cells of the various layers, and partly to the primitive solidity of all the organs. The first changes in the epiblast give rise to the central nervous system. The epiblast, consisting of the nervous and epidermic strata already indicated, becomes thickened along the axis of the embryo and forms a keel projecting towards the yolk below : so great is the size of this keel in the front part of the embryo that it influences the form of the whole body and causes the outline of the surface adjoining the yolk to form a strong ridge moulded on the keel of the epiblast (fig. 32 A and B). Along the dorsal line of the epiblast keel is placed the shallow medullary groove ; and according to Calberla (No. 61) the keel is formed by the folding together of the two sides of the primitively uniform epiblastic layer. The keel becomes gradu- ally constricted off from the external epiblast and then forms a solid cord below it. Subsequently there appears in this cord a median slit-like canal, which forms the permanent central canal of the cerebrospinal cord. The peculiarity in the formation of the central nervous system of Teleostei consists in the fact that it is not formed by the folding over of the sides of the medullary groove into a canal, but by the separation, below the medullary groove, of a solid cord of epiblast in which the central canal is subsequently formed. Various views have been put forward to explain the apparently startling difference between Teleostei, with which Lepidosteus and Petromyzon agree, and other verte- brate forms. The explanations of Gotte and Calberla appear to me to contain between them the truth in this matter. The groove above in part represents the medullary groove ; but the closure of the groove is represented by the folding together of the lateral parts of the epiblast plate to form the medullary keel. According to Gotte this is the whole explanation, but Calberla states for Syngnathus and Salmo that the epidermic layer of the epiblast is carried down into the keel as a double layer just as if it had been really folded in. This ingrowth of the epidermic layer is shewn in fig. 32 A where it is just commencing to pass into the keel ; and at a later stage in fig. 32 B where the keel has reached its greatest depth. TELEOSTEI. 73 Gotte maintains that Calberla's statements are not to be trusted, and I have myself been unable to confirm them for Teleostei or Lepidosteus ; but if they could be accepted the difference in the formation of the medullary canal in Teleostei and in other Vertebrata would become altogether unimpor- tant and consist simply in the fact that the ordinary open medullary~groove is in Teleostei obliterated in its inner part by the two sides of the groove coming together. Both layers of epiblast would thus have a share in the formation of the central nervous system ; the epidermic layer giving rise to the lining epithe- lial cells of the central canal, and the nervous layer to the true nervous tissue. Jtlt The separation of the solid nervous system from the epiblast takes place relatively very late ; and, before it has been com- pleted, the first traces of the auditory pits, of the optic vesicles, and of the olfactory pits are -visible. The auditory pit arises as a solid thickening of the nervous layer of the epi- blast at its point of junc- tion with the medullary keel ; and the optic vesi- cles spring as solid out- growths from part of the keel itself. The olfactory pits are barely indicated as thickenings of the ner- vous layer of the epiblast. FlG. 32. TWO TRANSVERSE SECTIONS OF SYNGNATHUS. (After Calberla. ) A. Younger stage before the definite es- tablishment of the notochord. B. Older stage. The epidermic layer of the epiblast is repre- sented in black. ep. epidermic layer of epiblast ; me. neural cord ; hy. hypoblast ; me. mesoblast ; ch, noto- chord. At this early stage all the organs of special sense are at- tached to a layer continuous with or forming part of the central nervous system ; and this fact has led Gotte (No. 63) to speak of a special- sense plate, belonging to the central nervous system and not to the skin, from which 74 FORMATION OF THE LAYERS. all the organs of special sense are developed ; and to conclude that a serial homology exists between these organs in their development. A comparison between Teleostei and other forms shews that this view cannot be upheld ; even in Teleostei the auditory and olfactory rudiments arise rather from the epiblast at the sides of the brain than from the brain itself, while the optic vesicles spring from the first directly from the medullary keel, and are therefore connected with the central nervous system rather than with the external epiblast. In a slightly later stage the different connections of the two sets of sense organs is conclusively shewn by the fact that, on the separation of the central nervous system from the epiblast, the optic vesicles remain attached to the former, while the auditory and olfactory vesicles are continuous with the latter. After its separation from the central nervous system the remainder of the epiblast gives rise to the skin, etc., and most probably the epidermic stratum develops into the outer layer of the epidermis and the nervous stratum into the mucous layer. The parts of the organs of special sense, which arise from the epiblast, are developed from the nervous layer. In the Trout (Oellacher, No. 72) both layers are continued over the yolk- sack; but in Clupeus and Gasterosteus only the epidermic has this extension. According to Gotte the distinction between the two layers becomes lost in the later embryonic stages. Although it is thoroughly established that the mesoblast originates from the lower of the two layers of the thickened embryonic rim, it is nevertheless not quite certain whether it is a continuous layer between the epiblast and hypoblast, or whether it forms two lateral masses as in Elasmobranchs. The majority of observers take the former view, while Calberla is inclined to adopt the latter. In the median line of the embryo underneath the medullary groove there are undoubtedly from the first certain cells which eventually give rise to the notochord ; and it is these cells the nature of which is in doubt. They are certainly at first very indistinctly separated from the mesoblast on the two sides, and Calberla also finds that there is no sharp line separating them from the secondary hypoblast (fig. 32 A). Whatever may be the origin of the notochord the mesoblast very soon forms two lateral plates, one on each side of the body, and between them is placed the notochord (fig. 32 B). The general fate of the two mesoblast plates is the same as in Elas- mobranchs. They are at first quite solid and exhibit relatively TELEOSTEI. 75 late a division into splanchnic and somatic layers, between which is placed the primitive body cavity. The dorsal part of the plates becomes transversely segmented in the region of the trunk ; and thus gives rise to the mesoblastic somites, from which the muscle plates and the perichordal parts of the vertebral column are developed. The ventral or outer part remains unsegmented. The cavity of the ventral section becomes the permanent body cavity. It is continued forward into the head (Oellacher), and part of it becomes separated off from the remainder as the pericardial cavity. The hypoblast forms a continuous layer below the mesoblast, and, in harmony with the generally confined character of the development of the organs in Teleostei, there is no space left between it and the yolk to represent the primitive alimentary cavity. The details of the formation of the true alimentary tube have not been made out ; it is not however formed by a folding in of the lateral parts of the hypoblast, but arises as a solid or nearly solid cord in the axial line, between the notochord and the yolk, in which a lumen is gradually established. In the just hatched larva of an undetermined fresh-water fish with a very small yolk-sack I found that the yolk extended along the ventral side of the embryo from almost the mouth to the end of the gut. The gut had, except in the hinder part, the form of a solid cord resting in a concavity of the yolk. In the hinder part of the gut a lumen was present, and below this part the amount of yolk was small and the yolk nuclei numerous. Near the limit of its posterior extension the yolk broke up into a mass of cells, and the gut ended behind by falling into this mass. These incomplete observations appear to shew that the solid gut owes its origin in a large measure to nuclei derived from the yolk. When the yolk has become completely enveloped a postanal section of gut undoubtedly becomes formed ; and although, owing to the solid condition of the central nervous system, a communication between the neural and alimentary canals cannot at first take place, yet the terminal vesicle of the post- anal gut of Elasmobranchii is represented by a large vesicle, originally discovered by Kupffer (No. 68), which can easily be seen in the embryos of most Teleostei, but the relations of which have not been satisfactorily worked out (vide fig. 34, Jiyv), As the tail end of the embryo becomes separated off from the yolk the postanal vesicle atrophies. 76 GENERAL GROWTH OF THE EMBRYO. General development of the Embryo. Attention has already been called to the fact that the embryo first appears as a thickening of the edge of the blastoderm which soon assumes a somewhat shield-like form (fig. 33 A). The hinder end of the embryo, which is placed at the edge of the blastoderm, is some- what prominent, and forms the caudal swelling (ts). The axis of the embryo is marked by a shallow groove. The body now rapidly elongates, and at the same time or. md cb FIG. 33. THREE STAGES IN THE DEVELOPMENT OF THE SALMON. (After His.) ts. tail-swelling; ait.v. auditory vesicle; oc. optic vesicle; ce. cerebral rudiment; m.b. mid-brain; ^.cerebellum; md. medulla oblongata ; M.SO. mesoblastic somite. becomes considerably narrower, while the groove along the axis becomes shallower and disappears. The anterior, and at first proportionately a very large part, soon becomes distinguished as the cephalic region (fig. 33 B). The medullary cord in this region becomes very early divided into three indistinctly sepa- rated lobes, representing the fore, the mid, and the hind brains : of these the anterior is the smallest. With it are connected the optic vesicles (oc) — solid at first — which are pushed back into the region of the mid-brain. The trunk grows in the usual way by the addition of fresh somites behind. After the yolk has become completely enveloped by the blastoderm the tail becomes folded off, and the same process takes place at the front end of the embryo. The free tail end of TELEOSTET. 77 the embryo continues to grow, remaining however closely applied to the yolk-sack, round which it curls itself to an extent varying with the species (vide fig. 34). The general growth of the embryo during the later stages presents a few special features of interest. The head is remark- able for the small apparent amount of the cranial flexure. This is probably due to the late deve- lopment of the cerebral hemi- spheres. The flexure of the floor of the brain is however quite as considerable in the Teleostei as in other types. The gill clefts deve- lop from before backwards. The first cleft is the hyomandibular, and behind this there are the hyobranchial and four branchial clefts. Simultaneously with the clefts there are developed the branchial arches. The postoral arches formed are the mandibular, hyoid and five branchial arches. In the case of the Salmon all of these appear before hatching. The first cleft closes up very early (about the time of hatching in the Salmon) ; and about the same time there springs a membranous fold from the hyoid arch, which gradually grows backwards over the arches following, and gives rise to the operculum. There appear in the Salmon shortly before hatching double rows of papillae on the four anterior arches behind the hyoid. They are the rudiments of the branchiae. They reach a considerable length before they are covered in by the opercu- lar membrane. In Cobitis (Gb'tte, No. 64) they appear in young larvae as filiform processes equivalent to the external gills of Elasmobranchs. The extremities of these processes atrophy; while the basal portions become the permanent gill lamellae. The general relation of the clefts, after the closure of the hyomandibular, is shewn in fig. 35. The air-bladder is formed as a dorsal outgrowth of the alimentary tract very slightly in front of the liver. It grows in between the two limbs of the mesentery, in which it extends itself backwards. It appears in the Salmon, FlG. 34. VIEW OF AN ADVANCED EMBRYO OF A HERRING IN THE EGG. (After Kupffer.) oc. eye ; ht. heart ; hyv. post-anal vesicle ; ch. notochord. FORMATION OF THE TAIL. Carp, and other types to originate rather on the right side of the median dorsal line, but whether this fact has any special significance is rather doubtful. In the Salmon and Trout it is formed considerably later than the liver, but the two are stated by Von Baer to arise in the Carp nearly at the same time. The absence of a pneumatic duct in the Physoclisti is due to a post-larval atrophy. The region of the stomach is reduced al- most to nothing in the larva. The cesophagus becomes solid, like that of Elasmobranchs, and remains so for a consider- able period after hatching. The liver, in the earliest stage in which I have met with it in the Trout (27 days after impregnation), is a solid ventral diverticulum of the intestine, which in the region of the liver is itself without a lumen. The excretory system com- FIG. 35. DIAGRAMMATIC VIEW OF THE HEAD OF AN EMBRYO TELEOSTEAN, WITH THE PRIMITIVE VASCULAR TRUNKS. (From Gegen- baur.) a. auricle ; v. ventricle ; abr. branchial artery ; the sides of which form the optic lobes, and behind this again the hind-brain (md} ; the front border of the roof of which is thickened to form the cere- bellum (cb}. The medulla passes without any marked line of demarca- tion into the spinal cord. The histological differentiation of the brain has already proceeded to some extent ; and it has in the main the same character as the spinal cord. Before the larva has been hatched very long a lateral investment of white matter is present through- out. The notochord (ck) is continued forwards in the head to the hinder border of the infundibulum. It is slightly flexed anteriorly. From the hinder border of the auditory region to the end of the branchial region the mesoblast is dorsally divided into 1 Max Schultze's statements as to the structure and histology of the brain are very inadequate in the present state of our knowledge. —4. FIG. 45. HORIZONTAL SEC- TION THROUGH THE HEAD OF A JUST-HATCHED LARVA OF PETRO- MYZON SHEWING THE DEVELOP- MENT OF THE LENS OF THE EYE. th.c. thalamencephalon ; op.v. optic vesicle ; /. lens of eye ; A.c. head cavity. 94 GENERAL DEVELOPMENT. myotomes, which nearly, though not quite, correspond in number with the branchial pouches. The growth of the myotomes would seem, as might be anticipated from their indepen- dent innervation, not to be related to that of the branchial pouches, so that there is a want of correspondence between these parts, the extent of which varies at different periods of life. The relation between the two in an old larva is shewn in fig. 47. The head of the larva of Petromyzon FlG. ^ EYE OF A LARVA differs very strikingly in general ap- OF PETROMYZON NINE DAYS .. . AFTER HATCHING. pearance from that of the normal /. lens • r. retina. Vertebrata. This is at once shewn The section passes through by a comparison of fig. 43 with fig. 29. one side of the lens* The most important difference between the two is due to the absence of a pronounced cranial flexure in Petromyzon ; an absence which is in its turn probably caused by the small development of the fore-brain. The stomodaeum of Petromyzon is surprisingly large, and its size and structure in this type militate against the view of some embryologists that the stomodaeum originated from the coa- lescence of a pair of branchial pouches. In the region of the trunk there is present an uninterrupted dorsal fin continuous with a ventral fin round the end of the tail. There is a well-developed body cavity, which is especially dilated in front, in the part which afterwards becomes the pericardium. In this region is placed the nearly straight heart, divided into an auricle and ventricle (figs. 42 and 43), the latter continued forwards into a bulbus arteriosus. The myotomes are now very numerous (about 57, including those of the head, in a three days' larva). They are separated by septa, but do not fill up the whole space between the septa, and have a peculiar wavy outline. The notochord is provided with a distinct sheath, and below it is placed a subnotochordal rod. The alimentary canal consists of a narrow anterior section free from yolk, and a posterior region, the walls of which are CYCLOSTOMATA. 95 largely swollen with yolk. The anterior section corresponds to the region of the cesophagus and stomach, but exhibits no dis- tinction of parts. Immediately behind this point the alimentary canal dilates considerably, and on the ventral side is placed the opening of a single large sack, which forms the commencement of the liver. The walls of the hepatic sack are posteriorly united to the yolk-cells. At the region where the hepatic sack opens into the alimentary tract the latter dilates considerably. The posterior part of the alimentary tract still constitutes a kind of yolk-sack, the ventral wall being enormously thick and formed of several layers of yolk-cells. The dorsal wall is very thin. The excretory system is composed of two segmental ducts, each connected in front with a well-developed pronephros (head- kidney), with about five ciliated funnels opening into the peri- cardial region of the body cavity. The segmental ducts in the larvae open behind into the cloacal section of the alimentary tract. The development of the larva takes place with considerable rapidity. The yolk becomes absorbed and the larva becomes accordingly more transparent. It generally lies upon its side, and resembles in general appearance and habit a minute Am- l 567 FIG. 47. HEAD OF A LARVA OF PETROMYZON six WEEKS OLD. (Altered from Max Schultze.) au.v. auditory vesicle; op. optic vesicle; ol. olfactory pit; ul. upper lip; //. lower lip ; or.p. papillce at side of mouth ; v. velum ; br.s. extra branchial skeleton ; i — 7. branchial clefts. phioxus. It is soon able to swim with vigour, but usually, unless disturbed, is during the day quite quiescent, and chooses by 96 GENERAL DEVELOPMENT. preference the darkest situations. It soon straightens out, and, with the disappearance of the yolk, the tail becomes narrower than the head. A large caudal fin becomes developed. When the larva is about twenty days old, it bears in most anatomical features a close resemblance to an Ammoccetes ; though the histological differences between my oldest larva (29 days) and even very young Ammoccetes are considerable. The mouth undergoes important changes. The upper lip becomes much more prominent, forming of itself the anterior end of the body (fig. 47, «/). The opening of the nasal pit is in this way relatively thrown back, and at the same time is caused to assume a dorsal position. This will be at once understood by a comparison of fig. 43 with fig. 47. On the inner side of the oral cavity a ring of papillae is formed (fig. 47, or.p}. Dorsally these papillae are continued forward as a linear streak on the under side of the upper lip. A communication between the oral cavity and the branchial sack is very soon established. The gill pouches gradually become enlarged ; but it is some time before their small external openings are established. Their walls, which are entirely lined by hypoblast, become raised in folds, forming the branchial lamellae. The walls of the head cavities between them become resolved into the contractors and dilators of the branchial sacks. The extra-branchial basketwork becomes established very early (it is present in the larva of 6 millimetres, about 9 days after hatching) and is shewn in an older larva in fig. 47, br.s. It is not so complicated in these young larvae as in the Ammoccetes, but in Max Schultze's figure, which I have reproduced, the dorsal elements of the system are omitted. On the dorsal wall of the branchial region a ciliated ridge is formed, which may be homologous with the ridge on the dorsal wall of the branchial sack of Ascidians. It has been described by Schneider in Ammoccetes. With reference to the remainder of the alimentary canal there is but little to notice. The primitive hepatic diverticulum rapidly sprouts out and forms a tubular gland. The opening into the duodenum changes from a ventral to a lateral or even dorsal position. The duct leads into a gall- bladder imbedded in the substance of the liver. Ventrally the liver is united with the abdominal wall, but laterally passages are left by which the pericardial and body cavities continue to communicate. The greater part of the yolk becomes employed in the formation of the intestinal wall. This part of the intestine in a nine days' larva (67 mm.) has the form of a cylindrical tube with very thick columnar cells entirely filled with yolk particles. The dorsal wall is no longer appreciably thinner than the ventral. In the later stages the cells of this part of the intestine become gradually less columnar as the yolk is absorbed. The fate of the yolk-cells in the Lamprey is different from that in most other Vertebrata with an equally large amount of yolk. They no doubt CYCLOSTOMATA. 97 supply nutriment for the growth of the embryo, and although in the anterior part of the intestine they become to some extent enclosed in the alimentary tract and break up, yet in the posterior part they become wholly transformed into the regular epithelium of the intestine. On the ninth day a slight fold filled with mesoblastic tissue is visible on the dorsal wall of the intestine. This fold appears to travel towards the ventral side ; at any rate a similar but better-marked fold is visible in a ventro-lateral position at a slightly later period. This fold is the com- mencement of the fold which in the adult makes a half spiral, and is no doubt equivalent to the spiral valve of Elasmobranchs and Ganoids. It contains a prolongation of the coeliac artery, which constitutes at first the vitelline artery. The nervous system does not undergo during the early larval period changes which require a description. The opening of the olfactory sack becomes narrowed and ciliated (fig. 47, TADPOLE OF BOMBINATOR FROM THE VENTRAL SIDE, WITH THE ABDOMINAL WALL REMOVED. other gills but these