222 THE FOURTH DAY. [CHAP. 



stages it is impossible to detect the one sex from the 

 other. 



The ovary. In the female the primordial ova en- 

 large and become more numerous, the whole epithelium 

 growing thicker and more prominent, and the spindle- 

 shaped cells of the underlying mesoblast also increase 

 rapidly and thus form the stroma of the ovary. The 

 primordial ova after undergoing some further changes, 

 into which it is not within the scope of this work to 

 enter, become surrounded by a number of the ordinary 

 epithelium cells. These form a distinct layer, the folli- 

 cular epithelium, round the ovum. After a time there 

 appear numerous vascular ingrowths from the stroma, 

 which penetrate through all parts of the germinal epi- 

 thelium and break it up into a sponge-like structure 

 formed of trabeculse of germinal epithelium interpene- 

 trated by vascular strands of stroma. The trabeculse 

 of the germinal epithelium form the egg-tubes of 

 Pflliger. 



In this way each ovum becomes invested by a cap- 

 sule of vascular connective tissue lined internally by 

 a layer of epithelium ; the whole constituting a Graafian 

 follicle. 



The large nucleus of the primordial ovum becomes 

 the germinal vesicle, while the ovum itself remains as 

 the true ovum ; this subsequently becomes enlarged by 

 the addition of a quantity of yolk derived from a differ- 

 entiation of its protoplasm. 



The testis. The first traces of the testes are found 

 in the dorsal and inner side of the intermediate cell- 

 mass, and appear about the sixth day. From the first 

 they differ from the rudimentary ovaries, by coming into 



