DEVELOPMENT OF ELASMOBRA^X'H FISHES. 139 



The diameter of a notochord in the anterior part of the 

 body of a Pristiurus embryo of this stage is about 021 Mm. 

 Round the exterior of the notochord the mesoblast cells are 

 commencing to arrange themselves as a special sheath. 



In Torpedo the notochord at first presents the same 

 structure as in Pristiurus, i. e. it forms a cylindrical rod of 

 flattened cells. 



The vacuolation of these cells does not however commence 

 till a relatively very much later period than in Pristiurus, and 

 also presents a very different character (PI. x. fig. 7). 



The vacuoles are smaller, more numerous, and more rounded 

 than in the other genera, and there can be no question that in 

 many cases there is more than one vacuole in a cell. The 

 most striking point in which the notochord of Torpedo differs 

 from that of Pristiurus consists in the fact that in Torpedo 

 there is never any aggregation of the nuclei at the centre of the 

 cord, but the nuclei are always distributed uniformly through 

 it. As the vacuolation proceeds the differences between Tor- 

 pedo and the other genera become less and less marked. The 

 vacuoles become angular in form, and the cells of the cord 

 cease to be flattened, and become polygonal. 



At my final stage for Torpedo (slightly younger than K) 

 the only important feature distinguishing the notochord from 

 that of Pristiurus, is the absence of any signs of nuclei 

 or protoplasm passing to the periphery. Around the exterior 

 of the cord there is early found in Torpedo a special invest- 

 ment of mesoblastic cells. 



10—2 



