CHAPTER V. 

 Stacres B to G. 



"O- 



The present cha23ter deals with tlie history of the development 

 of the Elasmobranch embryo from the period when the medul- 

 lary groove first arises till that in which it becomes completely 

 closed, and converted into the medullary canal. The majority 

 of the observations recorded were made on Pristiurus embryos, 

 a few on embryos of Torpedo. Where nothing is said to the 

 contrary the statements made apply to the embryos of Pristiurus 

 only. 



The general external features for this period have already 

 been given in sufficient detail in the last chapter ; and I 

 proceed at once to describe consecutively the history of the 

 three layers. 



General features of the Epihlast. 



At the commencement of this period, during the stage inter- 

 mediate between B and C, the epiblast is composed of a single 

 layer of cells. (PI. ix. fig. 1.) 



These are very much elongated in the region of the embryo, 

 but flattened in other parts of the blastoderm. Throughout 

 they contain numerous yolk spherules. 



In a Torpedo embryo of this age (as determined by the con- 

 dition of the notochord) the epiblast presents a very different 

 structure. It is composed of small spindle-shaped cells several 

 rows deep. The nuclei of these are very large in proportion to 

 the cells containing them, and the yolk spherules are far less 

 numerous than in the cells of corresponding Pristiurus embryos. 



During stage C the condition of the epiblast does not under- 

 go any important change, with the exception of the layer be- 

 coming much thickened, and its cells two or three deep in the 

 anterior parts of the embryo, (PI, ix. fig. 2.) 



