THE DEVELOPMENT OF THE UROGENITAL SYSTEM. 407 



large vesicular nuclei (Fig. 364) . The latter are the sex cells; and the whole 

 epithelial (mesothelial) band is known as the germinal epithelium. The sex 

 cells are destined to give rise to the sexual elements in the female to' the ova, 

 in the male to the spermatozoa. In the earlier stages, however, it is impossible 

 to determine whether the sex cells will give rise to male or female elements. 

 The differentiation of sex and the corresponding histological differentiation of 

 the sex cells occur at a later period. 



In his recent work on the ovary and testis in Mammals, Allen has ob- 

 served in very early stages (pig embryos of 6 mm., rabbit embryos of 13 days) 

 certain large cells, with large clear nuclei, in the mesenchymal tissue of the 

 mesentery, outside of the genital ridge. These, from their resemblance to the 

 sex cells within the genital ridge, should probably also be classed as sex cells. 

 Their origin in these animals, however, is not known with certainty; but 

 the fact that in turtle embryos Allen has found cells of a similar character 

 apparently migrating from the entoderm through the mesoderm to the site of 

 the genital glands suggests the possibility that they are entodermal derivatives. 

 It is doubtful whether these aberrant sex cells take part in the development of 

 the mature sexual elements, the latter in all probability being derived from 

 the sex cells of the mesothelium of the genital ridge. 



Beard, Eigenmann, Rabl, Woods, and others, have described sex cells, undoubtedly 

 homologous with the aberrant sex cells mentioned above, as occurring in various regions of 

 the embryos of certain Fishes. These investigators also assert that the sex cells become 

 specialized and, so to speak, segregated at a very early period of development, even at the 

 stage of blastomere formation. Beard contends that the early differentiated sex (or germ) 

 cells are significant in the origin of certain teratomata (see Chapter on Teratogenesis). 



The cells of the germinal epithelium increase in number by mitotic division 

 and, for some time at least, the sex cells continue to increase in number by 

 differentiation from the small cuboidal (indifferent) cells, as indicated by the 

 presence of intermediate stages between the two types. The germinal epi- 

 thelium soon becomes separated into two layers (i) a superficial layer which 

 retains its epithelial character and contains the sex cells, and (2) a deeper layer 

 composed of smaller cells which resemble those of the mesenchyme and which 

 give rise to a part, at least, of the stroma of the genital glands. The elevation 

 formed by these two layers projects into the body cavity from the medial side 

 of the mesonephros and constitutes the genital ridge (Fig. 346). From the 

 superficial epithelial layer, columns or cords of cells, containing some of the 

 sex cells, grow into the underlying tissue. This ingrowth, however, does not 

 occur equally in all parts of the genital ridge, for three fairly distinct regions 

 can be recognized. In the cephalic end comparatively few columns appear, 

 but these few grow far down into the underlying tissue and constitute the rete 

 fords. In the middle region a greater number of columns grow into the 



