viii TABLE OF CONTENTS. 



CHAPTER IV. 



GENERAL FEATURES OF THE ELASMOBRANCH EMBRYO AT SUCCESSIVE 

 STAGES, pp. 71 — 81. 



Description of Stages A-Q. Enclosure of yolk by blastoderm. Relation of 

 the anus of Eusconi to the blastopore. 



CHAPTER Y. 



STAGES B — G, pp. 82 — 96. 



General features of the eplblast. — Original uniform constitution. Separation 

 into lateral and central portions. The medullary groove. — Its conversion into 

 the medullary canal. The mesohlast. — Its division into somatic and splanchnic 

 layers. Formation of protovertebroe. The lateral plates. The caudal swellings. 

 The formation of the body-cavity in the head. The alimentary canal. — Its 

 primitive constitution. The anus of Eusconi. Floor formed by yolk. Forma- 

 tion of cellular floor from cells formed around nuclei of the yolk. Communica- 

 tion behind of neural and ahmentary 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 interpreta- 

 tions to be put on this. Its occurrence in other instances. 



CHAPTER VI. 



DEVELOPMENT OF THE TRUNK DURING STAGES G TO K, pp. 97 — 139. 



Order of treatment. External eplblast. — 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 contiuuous lateral fins. 

 Mesoblast. — Constitution of lateral plates of mesoblast. Their splanchnic and 

 Bomatic layers. Body-cavity constituting space between them. Their division 

 into lateral and vertebral plates. Continuation of body-cavity into vertebral 

 plates. Protovertfcbrffi. 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, Sepai'ation of anterior part of body- 

 ca\dty as pericardial cavity. Communication of pericardial and peritoneal 

 ca\'ities. Somatopleure and splanchnopleure. Resume. General considerations 

 on development of mesoblast. Probability of lateral plates of mesoblast in 

 Elasmobranchs representing alimentary diverticula. Meaning of secondary 

 segmentation of vertebral column. The urinoycnital system. — Development of 

 segmental duct and segmental tubes as sohd 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. 



