THE NOTOCHORD. 357 



The whole notochord becomes very delicate, and can with 

 great difficulty be conserved whole in transverse sections. 



The layer of protoplasm which appeared during the last 

 stage on the inner side of the cuticular membrane of the noto- 

 chord becomes during the present stage a far thicker and more 

 definite structure. It forms a continuous layer with irregular 

 prominences on its inner surface ; and contains numerous nuclei. 

 The layer sometimes presents in transverse sections hardly any 

 indication of a division into a number of separate cells, but in 

 longitudinal sections this is generally very obvious. The cells 

 are directed very obliquely forwards, and consist of an oblong 

 nucleus invested by protoplasm. The layer formed by them 

 is very delicate and very easily destroyed. In one example its 

 thickness varied from '004 to '006 Mm., in another it reached 

 012 Mm. The thickness of the cuticular membrane is about 

 'OO2 Mm. or rather less. 



The diameter of a notochord in the anterior part of the 

 body of a Pristiurus embryo of this stage is about O'2i 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 struc- 

 ture 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. 11, 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 Torpedo 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 



