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. 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.