THE URINOGENITAL SYSTEM 3995 
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 
