584 THE STRUCTURE AND DEVELOPMENT 



is thick. Peculiar highly refracting nucleoli, usually enclosing a 

 large vacuole, are present in considerable numbers, and are either 

 arranged in a circle round the periphery, or sometimes aggre- 

 gated towards one side of the vesicle ; and in addition, numerous 

 deeply staining smaller granular aggregations, probably belong- 

 ing to the same category as the nucleoli (from which in the 

 living ovum they can only be distinguished by their size), are 

 scattered close to the inner side of the membrane over the whole 

 or only a part of the surface of the germinal vesicle. In a fair 

 number of instances bodies like that figured on PL 25, fig. 27, 

 are to be found in the germinal vesicle. They appear to be 

 nucleoli in which a number of smaller nucleoli are originating by 

 a process of endogenous growth, analogous perhaps to endogenous 

 cell-formation. The nucleoli thus formed are, no doubt, destined 

 to become free. The above mode of increase for the nucleoli 

 appears to be exceptional. The ordinary mode is, no doubt, 

 that by simple division into two, as was long ago shewn by 

 Auerbach. 



Of the later stages of the germinal vesicle and its final fate, I 

 can give no account beyond the very fragmentary statements 

 which have already appeared in my monograph on Elasmobranch 

 Fishes. 



Formation of fresh ova and ovarian nests in the post-embryonic 

 stages. Ludwig 1 was the first to describe the formation of ova in 

 the post-embryonic periods. His views will be best explained 

 by quoting the following passage : 



" The follicle of Skates and Dog fish, with the ovum it con- 

 tains, is to be considered as an aggregation of the cells of the 

 single-layered ovarian epithelium which have grown into the 

 stroma, and of which one cell has become the ovum and the 

 others the follicular epithelium. The follicle, however, draws in 

 with it into the stroma a number of additional epithelial cells 

 in the form of a stalk connecting the follicle with the superficial 

 epithelium. At a later period the lower part of the stalk at 

 its junction with the follicle becomes continuously narrowed, 

 and at the same time a rupture takes place in the cells which 

 form it. In this manner the follicle becomes at last constricted 



1 Lot. fif. 



