188 



GENITAL GLANDS 



p. 180). These are the primitive sex-cells, and, as far as any evidence yet 

 available permits conclusions, they arise from the germinal epithelium in situ. 

 It is, notwithstanding, a question of great theoretical importance, which must 

 be left to the future to decide, whether the sex-cells actually do so arise in situ, 

 or whether they are only segregated here, having been set apart from the somatic 

 cells at the earliest stages of development. Large sex- cells are already present in 

 or under the peritoneum at the root of the mesentery in the region of the first 

 five trunk segments in a human embryo of 4'9 mm. (Ingalls, 1907). 



The proliferating epithelium now grows inwards to form strands of cells known 

 as the genital cords. At first there is a very small amount of connective tissue 

 between them, but this soon increases in amount, and the cords become 



86. 



FIG. 237. SECTION OP THE GERMINAL EPITHELIUM AND ADJACENT STBOMA IN A MALE 

 CHICK-EMBRYO. (Semon.) 



g.ep^ germinal epithelium forming a thickened ridge-like projection ; pr.ov., primitive ova of various 

 sizes, some in the germinal epithelium and others somewhat beyond the limit of this epithelium ; 

 st., strands of cells which have grown from the germinal epithelium, and one of which appears connected 

 with an enlarged primitive ovum. 



more clearly defined. The cords consist of epithelial cells, among which are seen 

 primitive sex-cells, and, it is said, elements which have characters which are 

 transitional between the two. 



According to the researches of Coert and Allen on embryos of the pig and rabbit, the epithelium 

 over the cranial end of the genital ridge is lower than the true germinal epithelium, but here 

 again strands are produced by proliferation which are easily distinguishable from the genital 

 strands by their smaller cells and darker-staining nuclei. These extend inwards and also back- 

 wards between the gland-rudiment and the Wolffian body, and when later they acquire a lumen 

 become the rete tubules (see below). 



After the gland has reached this stage specialisation begins in the hitherto 

 indifferent rudiment, and it acquires the distinctive characters of ovary or testis. 



