DEVELOPMENT OF ELASMOBRANCH FISHES. 135 



wliose primitive ova were described at tlie commencement of 

 this section and the embr)^o last described, the primitive ova 

 presented some peculiarities, about the meaning of which I 

 am not quite clear, but which may perhaps throw some light 

 on the origin of these bodies. 



Instead of the protoplasm around the nucleus being clear or 

 slightly granular, as in the cases just described, it was filled in 

 the most typical instances with numerous highly refracting 

 bodies resembling yolk-spherules. In osmic acid specimens (PL 

 XI. fig. 15) these stain very darkly, and it is then as a rule very 

 difficult to see the nucleus ; in specimens hardened in picric 

 acid and stained with hsematoxylin these bodies are stained of 

 a deep purple colour, but the nucleus can in most cases be dis- 

 tinctly seen. In addition to the instances in which the proto- 

 plasm of the ova is quite filled with these bodies, there are 

 others in which they only occupy a small area adjoining the 

 nucleus (PI. XI. fig. 15 a), and finally some in which only one or 

 two of these bodies are present. The protoplasm of the 

 primitive ova appears in fact to present a series of gradations 

 between a state in which it is completely filled with highly 

 refracting spherules and one in which these are completely 

 absent. 



This state of things naturally leads to the view that the 

 primitive ova, when they are first formed, are filled with these 

 spherules, which are probably yolk- spherules, but that they 

 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, 

 considering 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 



