354 DEVELOPMENT OF ELASMOBRANCH FISHES. 



gradually lose them in the course of development. Against this 

 interpretation is the fact that the primitive ova in the younger 

 embryo first described are completely without these bodies; this 

 embryo however unquestionably presented an abnormally early 

 development of the ova; and I am satisfied that embryos present 

 considerable variations in this respect. 



If the primitive ova are in reality in the first instance filled 

 with yolk-spherules, the question arises as to whether, consider- 

 ing that they are the only mesoblast cells filled at this period 

 with yolk-spherules, we must not suppose that they have 

 migrated from some peripheral part of the blastoderm into their 

 present position. To this question I can give no satisfactory 

 answer. Against a view which would regard the spherules in 

 the protoplasm as bodies which appear subsequently to the first 

 formation of the ova, is the fact that hitherto no instances in 

 which these spherules were present have been met with in the 

 late stages of development; and they seem therefore to be 

 confined to the first stages. 



Notochord. 



The changes -undergone by the notochord during this period 

 present considerable differences according to the genus examined. 

 One type of development is characteristic of Scyllium and 

 Pristiurus ; a second type, of Torpedo. 



My observations being far more complete for Scyllium and 

 Pristiurus than for Torpedo, it is to the two former genera only 

 that the following account applies, unless the contrary is ex- 

 pressly stated. Only the development of the parts of the noto- 

 chord in the trunk are here dealt with ; the cephalic section of 

 the notochord is treated of in a subsequent section. 



During stage G the notochord is composed of flattened cells 

 arranged vertically, rendering the histological characters of the 

 notochord difficult to determine in transverse sections. In longi- 

 tudinal sections, however, the form and arrangement of the cells 

 can be recognised with great ease. At the beginning of stage 

 G each cell is composed of a nucleus invested by granular pro- 

 toplasm frequently vacuolated and containing in suspension 

 numerous yolk-spherules. It is difficult to determine whether 



