374 TEXT-BOOK OF EMBRYOLOGY. 



cytoplasm and large vesicular nuclei (Fig. 326). The latter are the sex cells; 

 and the whole epithelial (mesothelial) band is known as the germinal epi- 

 thelium. 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 earlier 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 oj the genital ridge. In his investigation of the chick, 

 Swift has discerned the sex cells at the time when the primitive streak and 

 primitive axis are being formed. They are located in the entoderm and in 

 the space between entoderm and ectoderm in the anterior part of the germ 

 wall. When the mesoderm appears in this region the sex cells enter this 

 layer, then enter the blood vessels. They are apparently amoeboid. By the 

 blood stream they are carried to all parts of the blastoderm and embryo. 

 Later the cells accumulate in the vicinity of the ccelomic angle and finally 

 enter the thickened mesothelium (germinal epithelium) of the genital ridge. 



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

 homologous with the early 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 the sex cells continue to increase in number by proliferation of their own 

 members since there are no intermediate stages between the two types. 

 The germinal epithelium 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. 308). 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 cords. In the middle region a greater 



