THE URIXOGEXITAL SYSTEM 395 



right one; (4) behavior of the stroma, particularly the albuginea. 

 According to Semon the nature of the gonad may be detected on 

 the fifth, or, at the latest, on the sixth day, by the fact that the 

 right ovary is already much smaller than the left, owing to the 

 more rapid growth of the latter. Although the right testis 

 frequently develops more slowly than the left, the difference is 

 not so great as in the case of the ovary. In Grallatores and Nata- 

 tores, according to Hoffmann, retrogression of the right ovary 

 does not begin until shortly before hatching. 



Histological differentiation manifests itself first in the ger- 

 minal epithelium and sexual cords. In the males the germinal 

 epithelium never attains as great thickness as in the females, 

 and the sexual cords are much better developed and the stroma 

 therefore less abundant than in the females. It is impossible 

 to tell from the literature just how early these differentiating 

 characters become decisive; but it is between the sixth and 

 eighth days. 



Development of the Testis. We have seen that, during the 

 indifferent period, the primitive ova multiply in the germinal 

 epithelium; small groups may thus be formed, and such groups, 

 or single primitive ova, soon appear in the stroma and in the 

 sexual cords (Fig. 227). Their appearance in these situations 

 is attributed to migration, and not neo-formation in situ for 

 the following reasons: (1) The primitive ova are found in the 

 germinal epithelium before they appear either in the stroma or 

 sexual cords; (2) the boundary between the germinal epithelium 

 and the stroma is not sharp, and both ordinary epithelial cells 

 and primitive ova are found in intermediate positions before 

 they appear in the stroma and sexual cords; (3) the primitive 

 ova in the stroma and in the sexual cords are precisely like those 

 originally found in the germinal epithelium; (4) the sexual cords 

 have no basement membrane in early stages, and primitive ova 

 may be found in the margin of the cords. 



By this process of migration, then, the primitive ova leave 

 the germinal epithelium and pass either directly or through the 

 stroma into the sexual cords, which thus come to be composed 

 of two kinds of cells, viz., the epithelial cells and the primitive 

 ova (Fig. 227). This process appears to go on until about the 

 end of the second week of incubation. The sexual cords increase 

 in number very rapidly and become closely pressed together so as 



