240 Vertebrate Embryology 



same in all embryos, whether male or female, 

 and it is not at first possible to say whether 

 the embryo will develop into a male or into a 

 female. 



In the female the epithelium increases 

 enormously in thickness, and the cells of the 

 thickened patch of mesoblast under it increase 

 in numbers to form the stroma of the ovary. 

 The primitive ova increase in size, and the 

 smaller cells of the epithelium arrange them- 

 selves around each ovum as a sort of capsule, 

 the follicular epithelium. Some of the ova 

 sink down into the underlying mesoblast, which 

 also sends processes up into the epithelium, 

 and each ovum becomes surrounded by a vas- 

 cular sheath of connective tissue ; the ovum 

 with its follicular epithelium and vascular 

 sheath now constitutes a Graafian follicle. 



It is only on the left side that the above- 

 described changes take place : the ovary on 

 the right side of the chick remains in a rudi- 

 mentary condition throughout life, or may 

 disappear entirely. 



The development of the testis is not so 

 easily determined as that of the ovary. From 

 the first, it is more closely associated with the 

 Wolffian body. As the epithelium thickens, 



