TABLE OF CONTENTS. 



CHAPTER I. 



THE RIPE OVARIAN OVUM, pp. 1 9. 



Structure of ripe ovum. Atrophy of germinal vesicle. The extrusion of its 

 membrane and absorption of its contents, Oellaeher's observations on the 

 germinal vesicle. Gotte's observations. Kleineuberg's observations. General 

 conclusions on the fate of the germinal vesicle. Germinal disc. 



CHAPTER IL 



THE SEGMENTATION, pp. 10—32. 



Appearance of impregnated germinal disc. Stage with two furrows. Stage 

 with twenty-one segments. Structure of the sides of the fun-ows. 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 Elasmo- 

 branch segmentation with that of other meroblastic ova. Literature of 

 Elasmobranch segmentation. 



CHAPTER III. 



FORMATION OF THE LAYERS, pp. 33 — 70. 



Division of blastoderm into two layers. Formation of segmentation cavity. 

 Disappearance of cells from floor of se.^mentation 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 Elasmobranchs 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 Elasmobianch 

 development with that of other types. Its relation to the Gastrula. Haeckel's 

 views on vertebrate Gastrula. Their untenable natiu'e. Comparison of primitive 

 streak with blastopore. Literature. 



