THE LATER DEVELOPMENT OF THE FROG 



205 





definite median strand of cells between the posterior cardinal 

 veins (8-9 mm.) (Allen). 



This germinal strand now divides longitudinally and the 

 halves move more laterally, projecting slightly into the body 

 cavity as the genital ridges, near the attachment of the mesen- 

 tery and just beneath the cardinal veins. The genital ridges 

 now become more conspicuous through the proliferation of the 

 primitive germ cells and the peritoneal cells covering them, to 

 which are added mesenc^yme cells 

 from the body wall. In this cell mass 

 the mesenchyme elements are con- 

 cerned in the formation of the stroma 

 of the ridge. The peritoneum continues 

 to form a thin superficial covering and 

 later forms the suspensory folds (mes- 

 orchia, mesovaria) of. the gonads, as the 

 ridges may be called when they pro- 

 ject freely into the body cavity. As 

 the primitive germ cells begin to 

 multiply, they form the nests of cells 

 described in the preceding chapter, 

 the further development of which 

 need not be repeated here. 



Before metamorphosis begins the ., FlG - 7 *'~ Sect i n throu ^ h 



the gonad of a 35 mm. tad- 



anterior third or half of each genital pole of R. temporaria. After 

 ridge commences to degenerate and 



becomes converted into the fat body ter y>. " "nests" formed by 



multiplication of the primitive 

 (See above). The posterior portion germ cells; s, genital strand 



has previously acquired secondary ( 



connections with the mesonephric duct in the 'folio wing man- 

 ner. From the stalks of a few (7-8, Nussbaum) of the Malpi- 

 ghian bodies of the posterior part of the mesonephros, there 

 grow out solid strands of cells known as the sexual cords. 

 These become tubular and gradually extend downward into 

 the substance of the gonad, either forming or connecting 

 with spaces within this organ (Fig. 77). From this point 

 onward their history is different in the two sexes. In the 



