CHAPTER V. 

 STAGES B TO G. 



THE present chapter deals with the 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 Epiblast. 



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. 10, fig. i.) 



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 un- 

 dergo any important change, with the exception of the layer 



