In conclusion, my best thanks are due to Drs Dohrn and Eisig for the uniformly kind manner in which they have forwarded my researches both at the Zoological Station in Naples, and after my return to England; and also to Mr Henry Lee and to the Manager and Directors of the Brighton Aquarium, who have always been ready to respond to my numerous demands on their liberality.

To my friend and former teacher Dr Michael Foster I tender my sincerest thanks for the never-failing advice and assistance which he has given throughout the whole course of the work.

[66] Zeitschrift f. Anat. u. Entwicklungsgeschichte, Bd. II.

[67] Archiv f. Micr. Anat. Bd. XIII.

[68] Erste Anlage des Wirbelthierleibes.

[69] Entwicklungsgeschichte der Unke.


TABLE OF CONTENTS.
CHAPTER I.
THE RIPE OVARIAN OVUM, pp. [213-221].
Structure of ripe ovum. Atrophy of germinal vesicle. The extrusion of its membrane and absorption of its contents. Oellacher's observations on the germinal vesicle. Götte's observations. Kleinenberg's observations. General conclusions on the fate of the germinal vesicle. Germinal disc.
CHAPTER II.
THE SEGMENTATION, pp. [222-245].
Appearance of impregnated germinal disc. Stage with two furrows. Stage with twenty-one segments. Structure of the sides of the furrows. Later stages of segmentation. Spindle-shaped nuclei. Their presence outside the blastoderm. Knobbed nuclei. Division of nuclei. Conclusion of segmentation. Nuclei of the yolk. Asymmetry of the segmented blastoderm. Comparison of Elasmobranch segmentation with that of other meroblastic ova. Literature of Elasmobranch segmentation.
CHAPTER III.
FORMATION OF THE LAYERS, pp. [246-285].
Division of blastoderm into two layers. Formation of segmentation cavity. Disappearance of cells from floor of segmentation cavity. Nuclei of yolk and of blastoderm. Formation of embryonic rim. Appearance of a layer of cells on the floor of the segmentation cavity. Formation of mesoblast. Formation of medullary groove. Disappearance of segmentation cavity. Comparison of segmentation cavity of Elasmobranchii with that of other types. Alimentary cavity. Formation of mesoblast in two lateral plates. Protoplasmic network of yolk. Summary. Nature of meroblastic ova. Comparison of Elasmobranch development with that of other types. Its relation to the Gastrula. Haeckel's views on vertebrate Gastrula. Their untenable nature. Comparison of primitive streak with blastopore. Literature.
CHAPTER IV.
GENERAL FEATURES OF THE ELASMOBRANCH EMBRYO AT SUCCESSIVE STAGES, pp. [286-297].
Description of Stages A-Q. Enclosure of yolk by blastoderm. Relation of the anus of Rusconi to the blastopore.
CHAPTER V.
STAGES B-G, pp. [298-314].
General features of the epiblast.—Original uniform constitution. Separation into lateral and central portions. The medullary groove.—Its conversion into the medullary canal. The mesoblast.—Its division into somatic and splanchnic layers. Formation of protovertebræ. The lateral plates. The caudal swellings. The formation of the body-cavity in the head. The alimentary canal.—Its primitive constitution. The anus of Rusconi. Floor formed by yolk. Formation of cellular floor from cells formed around nuclei of the yolk. Communication behind of neural and alimentary canals. Its discovery by Kowalevsky. Its occurrence in other instances. General features of the hypoblast. The notochord.—Its formation as a median thickening of the hypoblast. Possible interpretations to be put on this. Its occurrence in other instances.
CHAPTER VI.
DEVELOPMENT OF THE TRUNK DURING STAGES G TO K, pp. [315-360].
Order of treatment. External epiblast.—Characters of epiblast. Its late division into horny and epidermic layers. Comparison of with Amphibian epiblast. The unpaired fins. The paired fins.—Their formation as lateral ridges of epiblast. Hypothesis that the limbs are remnants of continuous lateral fins. Mesoblast.—Constitution of lateral plates of mesoblast. Their splanchnic and somatic layers. Body-cavity constituting space between them. Their division into lateral and vertebral plates. Continuation of body-cavity into vertebral plates. Protovertebræ. Division into muscle-plates and vertebral bodies. Development of muscle-plates. Disappearance of segmentation in tissue to form vertebral bodies. Body-cavity and parietal plates. Primitive independent halves of body-cavity. Their ventral fusion. Separation of anterior part of body-cavity as pericardial cavity. Communication of pericardial and peritoneal cavities. Somatopleure and splanchnopleure. Résumé. General considerations on development of mesoblast. Probability of lateral plates of mesoblast in Elasmobranchii representing alimentary diverticula. Meaning of secondary segmentation of vertebral column. The urinogenital system.—Development of segmental duct and segmental tubes as solid bodies. Formation of a lumen in them, and their opening into body-cavity. Comparison of segmental duct and segmental tubes. Primitive ova. Their position. Their structure. The notochord.—The formation of its sheath. The changes in its cells.
CHAPTER VII.
GENERAL DEVELOPMENT OF THE TRUNK FROM STAGE K TO THE CLOSE OF EMBRYONIC LIFE, pp. [361-377].
External epiblast.—Division into separate layers. Placoid scales. Formation of their enamel. Lateral line.—Previous investigations. Distinctness of lateral line and lateral nerve. Lateral nerve a branch of vagus. Lateral line a thickening of epiblast. Its greater width behind. Its conversion into a canal by its cells assuming a tubular arrangement. The formation of its segmental apertures. Mucous canals of the head. Their nerve-supply. Reasons for dissenting from Semper's and Götte's view of lateral nerve. Muscle-plates.—Their growth. Conversion of both layers into muscles. Division into dorso-lateral and ventro-lateral sections. Derivation of limb-muscles from muscle-plates. Vertebral column and notochord.—Previous investigations. Formation of arches. Formation of cartilaginous sheath of notochord and membrana elastica externa. Differentiation of neural arches. Differentiation of hæmal arches. Segmentation of cartilaginous sheath of notochord. Vertebral and intervertebral regions. Notochord.
CHAPTER VIII.
DEVELOPMENT OF THE SPINAL NERVES AND OF THE SYMPATHETIC NERVOUS SYSTEM, pp. [378-396].
The spinal nerves.—Formation of posterior roots. Later formation of anterior roots. Development of commissure uniting posterior roots. Subsequent development of posterior roots. Their change in position. Development of ganglion. Further changes in anterior roots. Junction of anterior and posterior roots. Summary. General considerations.—Origin of nerves. Hypothesis explaining peripheral growth. Hensen's views. Later investigations. Götte. Calberla. Relations between Annelidan and Vertebrate nervous systems. Spinal canal. Dr Dohrn's views. Their difficulties. Hypothesis of dorsal coalescence of lateral nerve cords. Sympathetic nervous system.—Development of sympathetic ganglia on branches of spinal nerves. Formation of sympathetic commissure.
CHAPTER IX.
DEVELOPMENT OF THE ORGANS IN THE HEAD, pp. [397-445].
Development of the Brain, pp. [397-407]. General history. Fore-brain.—Optic vesicles. Infundibulum. Pineal gland. Olfactory lobes. Lateral ventricles. Mid-brain. Hind-brain.—Cerebellum. Medulla.—Previous investigations. Huxley. Miklucho-Maclay. Wilder. Organs of sense, pp. [407-412]. Olfactory organ.—Olfactory pit. Schneiderian folds. Eye. General development. Hyaloid membrane. Lens capsule. Processus falciformis. Auditory organs.—Auditory pit. Semicircular canals. Mouth involution and Pituitary body, pp. [412-414]. Outgrowth of pituitary involution. Separation of pituitary sack. Junction with infundibulum. Development of cranial nerves, pp. [414-428]. Early development of 5th, 7th, 8th, 9th and 10th cranial nerves. Distribution of the nerves in the adult. The fifth nerve.—Its division into ophthalmic and mandibular branches. Later formation of superior maxillary branch. Seventh and auditory nerves.—Separation of single rudiment into seventh and auditory. Forking of seventh nerve over hyomandibular cleft. Formation of anterior branch to form ramus ophthalmicus[TN8] superficialis of adult. General view of morphology of branches of seventh nerve. Glossopharyngeal and vagus nerves.—General distribution at stage L. Their connection by a commissure. Junction of the commissure with commissure connecting posterior roots of spinal nerves. Absence of anterior roots. Hypoglossal nerve. Mesoblast of Head, pp. [429-432]. Body-cavity and myotomes of head.—Continuation of body-cavity into head. Its division into segments. Development of muscles from their walls. General mesoblast of head. Notochord in Head, P. [433]. Hypoblast of the Head, pp. [433-434]. The formation of the gill-slits. Layer from which gills are derived. Segmentation of the Head, pp. [434-440]. Indication of segmentation afforded by (1) cranial nerves, (2) visceral clefts, (3) head-cavities. Comparison of results obtained.
CHAPTER X.
THE ALIMENTARY CANAL, pp. [446-459].
The solid œsophagus.—Œsophagus originally hollow. Becomes solid during Stage K. The postanal section of the alimentary tract.—Continuity of neural and alimentary canals. Its discovery by Kowalevsky. The postanal section of gut. Its history in Scyllium. Its disappearance. The cloaca and anus.—The formation of the cloaca. Its junction with segmental ducts. Abdominal pockets. Anus. The thyroid body.—Its formation in region of mandibular arch. It becomes solid. Previous investigations. The pancreas.—Arises as diverticulum from dorsal side of duodenum. Its further growth. Formation of duct. The liver.—Arises as ventral diverticulum of duodenum. Hepatic cylinders. Comparison with other types. The subnotochordal rod.—Its separation from dorsal wall of alimentary canal. The section of it in the trunk. In the head. Its disappearance. Views as to its meaning.
CHAPTER XI.
THE VASCULAR SYSTEM AND VASCULAR GLANDS, pp. [460-478].
The heart.—Its development. Comparison with other types. Meaning of double formation of heart. The general circulation. The venous system. The primitive condition of. Comparison of, with Amphioxus and Annelids. The cardinal veins. Relations of caudal vein. The circulation of the yolk-sack.—Previous observations. Various stages. Difference of type in amniotic Vertebrates. The vascular glands.—Suprarenal and interrenal bodies. Previous investigations. The suprarenal bodies.—Their structure in the adult. Their development from the sympathetic ganglia. The interrenal body.—Its structure in the adult. Its independence of suprarenal bodies. Its development.
CHAPTER XII.
THE ORGANS OF EXCRETION, pp. [479-520].
Previous investigations. Excretory organs and genital ducts in adult. In male.—Kidney and Wolffian body. Wolffian duct. Ureters. Cloaca. Seminal bladders. Rudimentary oviduct. In female.—Wolffian duct. Ureters. Cloaca.—Segmental openings. Glandular tubuli of kidney. Malpighian bodies. Accessory Malpighian bodies. Relations of to segmental tubes. Vasa efferentia. Comparison of Scyllium with other Elasmobranchii. Development of segmental tubes. Their junction with segmental duct. Their division into four segments. Formation of Malpighian bodies. Connection between successive segments. Morphological interest of. Development of Müllerian and Wolffian ducts. In female—General account. Formation of oviduct as nearly solid cord. Hymen. In male—Rudimentary Müllerian duct.—Comparison of development of Müllerian duct in Birds and Elasmobranchii. Own researches. Urinal cloaca. Formation of Wolffian body and kidney proper.—General account. Details of formation of ureters. Vasa efferentia.—Views of Semper and Spengel. Difficulties of Semper's views. Unsatisfactory result of own researches. General homologies. Résumé. Postscript.

CHAPTER I.