5/4 TFiE STRUCTURE AND DEVELOPMENT 



and columnar and the nucleus oval (PI. 25, fig. 28). In this 

 condition the thickness of the epithelium is about 0^025 mm. 

 The epithelium does not, however, become uniformly thick over 

 the whole ovum, but in the neighbourhood of the germinal 

 vesicle it is very flat and formed of granular cells with indistinct 

 outlines, rather like the hypodermis cells of many Annelida. 

 Coincidently with this change in the follicular epithelium the 

 commencement of the atrophy of the membranes of the ovum, 

 described in the last section, becomes apparent. 



The original membrana propria folliculi is still present round 

 the follicular epithelium, but is closely associated with a fibrous 

 layer with elongated nuclei. Outside this there is now a layer 

 of cells, very much like an ordinary epithelial layer, which may 

 possibly be formed of cells of the true germinal epithelium (fig. 

 28, ft''). This layer, which will be spoken of as the secondary 

 follicle layer, might easily be mistaken for the follicular epithe- 

 lium, and it is possible that it has actually been so mistaken by 

 Eimer, Clark, and Klebs, in Reptilia, and that the true follicular 

 epithelium (in a flattened condition) has been then spoken of as 

 the Binnenepithel. 



In slightly older eggs the epithelial cells are no longer uni- 

 form or arranged as a single layer. The general arrangement of 

 these cells is shewn in PI. 25, fig. 29. A considerable number of 

 them are more or less flask-shaped, with bulky protoplasm pro- 

 longed into a thin stem directed towards the vitelline membrane, 

 with which, in many instances if not all, it comes in contact. 

 These larger cells are arranged in several tiers. Intercalated 

 between them are a number of elongated small cells with scanty 

 protoplasm and a deeply staining nucleus, not very dissimilar 

 to, though somewhat smaller than, the columnar cells of the 

 previous stage. There is present a complete -series of cells 

 intermediate between the larger cells and those with a deeply 

 stained nucleus, and were it not for the condition of the epithe- 

 lium in Raja, to be spoken of directly, I should not sharply 

 divide the cells into two categories. In surface views of the 

 epithelium the division into two kinds of cells would not be 

 suspected. There can, it appears to me, be no question that 

 both varieties of cell are derived from the primitive uniform 

 follicle cells. 



