DEVELOPMENT OF REPRODUCTIVE SYSTEM 791 



1. Early Developmental Features; the Indifferent Gonad 



The gonads or reproductive glands are associated intimately with the devel- 

 oping mesonephric kidneys. The typical site of origin is the area between 

 the dorsal mesentery and the anterior portion of the mesonephric kidney 

 (figs. 345F, G; 350C). As development progresses, it tends to move laterad 

 and in doing so becomes located along the mesial aspect of the developing 

 mesonephric ridge (figs. 3 A; 345G). 



The reproductive gland arises as an elongated fold, the genital ridge or 

 genital fold. The extent of this fold, in general, is longer than the actual site 

 from which the rudimentary gonad or reproductive gland arises, and it may 

 extend for a considerable distance along the mesonephric kidney. Felix ('06) 

 designates three general areas of the primitive genital ridge: 



( 1 ) a gonal portion, from which the sex gland arises, 



(2) a progonal area in front of the gonal area, which gives origin to the 

 anterior suspensory ligament of the gonad, and 



(3) an epigonal area behind, which continues caudally as a peritoneal sup- 

 port along the mesonephric kidney (fig. 3A). 



The rudimentary structural parts of the early genital ridge in the gonal 

 area, viewed in transverse section, consist of the following (fig. 350A-C): 



(1) primitive germ cells (origin of the germ cells discussed in Chapter 3, 

 see figure 60), 



(2) the germinal (coelomic) epithelium and the primitive sex cords and 

 cells proliferated therefrom, and 



(3) contributions from mesonephric tissue, forming in most vertebrates 

 the rete tissue of the urogenital union together with the primitive 

 mesenchyme of the gonad. 



The first stages in the development of the gonad consist of a thickening 

 of the germinal (coelomic) epithelium and of a rapid and copious prolifera- 

 tion of cells from its inner surface. The primitive (primordial) germ cells 

 become associated with the thickened germinal epithelium and its proliferated 

 cells, and migrate inward into the substance of the gonad with the cells of 

 the germinal epithelium (fig. 350B). 



As a result of the activities of the germinal epithelium, a mass of cells, the 



Fig. 350 — (Continued) 



1912, reference same as in C, above.) (G) Differentiating testis in the wood frog, Rana 

 sylvatica. (Redrawn from Witschi, 1931, Sex and Internal Secretions, edited by Allen 

 et al., Williams and Wili<ins, Baltimore.) (H) Ingrowth of sex cords from germinal 

 epithelium of ovary of 6 weeks old rabbit. (Redrawn from Brambell, 1930, The Devel- 

 opment of Sex in Vertebrates, Macmillan, N. Y.) (I) Section through differentiating 

 ovary in the opossum, 63 mm. pouch young. (J) Differentiating ovary in the wood 

 frog, Rana sylvatica. (Redrawn from Witschi, 1931, reference same as G, above.) 



