STRUCTURE AND DEVELOPMENT OF VERTEBRATE OVARY. 405 



The first important change is the assumption of a palisade- 

 like character by the follicle cells, each cell becoming very narrow 

 and columnar and the nucleus oval (PL XIX^ fig. 28). In this 

 condition the thickness of the epithelium is about 0£5 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, bat 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, fe'). This layer, which will be spoken of as the 

 secondary follicle layer, might easily be mistaken for the folli- 

 cular epithelium, 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 Binnenejnthel. 



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

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

 these cells is shown in (PL XIX, fig. 29.) A considerable 

 number of them are more or less flask-shaped, with bulky proto- 

 plasm prolonged 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 epithelium in Eaja, 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. 



The fibrous layer bounding the membrana propria folliculi is 

 thicker than in the last stage, and the epithelial-like layer {fe'), 

 which bounds it externally is more conspicuous than before. 

 Immediately adjoining it are vascular and lymph sinuses. The 

 thickness of the follicular epithelium at this stage may reach as 



