166 THE FOURTH DAY. [CHAP. 



abdominal cavity, appears in direct light as a fusiform white 

 patch or streak, in its early stages extending along the whole 

 length of the Wolffian body and genital ridge, but sub- 

 sequently restricted to its anterior portion. Its appearance 

 under these circumstances has been well described by Von Baer. 

 This 'sexual eminence' is present in the early stages of 

 both sexes. In both the epithelium consists of several layers 

 of short cylindrical cells, a few of which are conspicuous on 

 account of their size and their possessing a highly refractive 

 oval nucleus of considerable bulk ; in both, the underlying 

 thickened mesoblast consists — as indeed at this epoch it does 

 generally in all parts of the body — of spindle-shaped cells. 



The larger conspicuous cells of the epithelium, which 

 appear to have quite a common origin with their fellow cells, 

 and to rise from them by direct differentiation, and which 

 are seen at the first in male as well as female embryos, are 

 the primordial ova (Fig. 51, o). Thus in quite early stages 

 it is impossible to detect the one sex from the other. At 

 about the 80th to the 100th hour, however, a distinction 

 becomes apparent. 



In the males, the epithelium with its underlying meso- 

 blast ceases to develope ; the primordial ova neither increase 

 nor multiply. On the contrary, they disappear, and the 

 whole sexual eminence fades away. 



In females, on the other hand, the primordial ova enlarge 

 and become more numerous, the whole epithelium growing 

 thicker and more prominent. The spindle-shaped cells of 

 the underlying mesoblast also increase rapidly, and thus 

 form the stroma of the ovary. The growth of this stroma 

 bears subsequently such a relation to that of the epithelium, 

 that the primordial ova appear to sink into the stroma, and 

 each ovum, as it descends, to carry with it a number of the 

 ordinary epithelium-cells, which arrange themselves round it 

 in a distinct layer. In this way each ovum becomes invested 

 by a capsule of vascular connective tissue, lined internally by 

 a layer of epithelium ; the whole constituting a Graffian 

 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 ad- 

 dition of a quantity of yolk derived from the epithelial lining 

 of the follicle. 



