134 PRIMITIVE OVA. 



morphosis of adjoining cells, or may not be introduced from else- 

 where. In some of the lower animals, e. g. Hydrozoa, there is no 

 question that the ova are derived from the epiblast ; we might 

 therefore expect to find that they had the same origin in Verte- 

 brates. Further than this, ova are frequently capable in a 

 young state of executing amoeboid movements, and accordingly 

 of migrating from one layer to another. In the Elasmobranchs 

 the primitive ova exhibit in a hardened state an irregular form 

 which might appear to indicate that they possess a power of 

 altering their shape, a view which is further supported by some 

 of them being at the present stage situated in a position very 

 different from that which they eventually occupy, and which 

 they can only reach by migration. If it could be shewn that 

 there were no intermediate stages between the primitive ova 

 and the adjoining cells (their migratory powers being admitted) 

 a strong presumption would be offered in favour of their having 

 migrated from elsewhere to their present position. In view 

 of this possibility I have made some special investigations, 

 which have however led to no very satisfactory results. There 

 are to be seen in the stages immediately preceding the present 

 one, numerous cells in a corresponding position to that of the 

 primitive ova, v/hich might very well be intermediate between 

 the primitive ova and ordinary cells, but which offer no suffi- 

 ciently well marked features for a certain determination of 

 their true nature. 



In the particular embryo whose primitive ova have been 

 described these bodies were more conspicuous than in the 

 majority of cases, but at the same time they presented no 

 special or peculiar characters. • 



In a somewhat older embryo of Scyllium the cells amongst 

 which the primitive ova lay had become very distinctly dif- 

 ferentiated as an epithelium (the germinal epithelium of 

 Waldeyer) well separated by what might almost be called a 

 basement membrane from the adjoining connective-tissue cells. 

 Hardly any indication of a germinal ridge had appeared, but 

 the ova were more definitely confined than in previous embryos 

 to the restricted area which eventually forms this. The ova 

 on the average were somewhat smaller than in the previous 

 cases. 



In several embryos intermediate in age between the embryo 



