30 THE EGGS OF MAMMALS 



chorio-allantoic grafts of the gonad-forming areas of chick 

 gonads that germ cells remaining outside the germinal ridge 

 area do not differentiate into oogonia or spermatogonia, 

 whereas those that become situated under the germinal 

 epithelium develop as typical sex cells. On the basis of 

 this and other evidence he agrees with Witschi (1929) that 

 the cortex {e.g., the cortical sex cords) of the gonad acts 

 upon the germ cells as a specific organizer of female sex 

 cells, and the medulla as organizer of spermatogenetic tissue. 

 In the free-martin of cattle, which is a female twin develop- 

 ing in utero under the influence of the hormones of its male 

 partner, a sterile testis-like organ develops. It is notable 

 that while typical male sex cords are present, germ cells 

 are absent (Chapin, 1917; Willier, 1921). Perhaps in the 

 case of the free-martin (as in the frogs with delayed ferti- 

 lization) a spermatogenetic tissue is not formed because 

 primordial germ cells do not reach the gonad. 



If these data are generally applicable to manamals it would 

 seem that although ovogenesis takes place from the germinal 

 epithelium the formation of a functional ovary is dependent 

 upon the primordial germ cells. We have seen, in the case 

 of x-rayed ovaries, that an ovary with morphologically 

 normal germinal epithelium may be incapable of forming 

 ova. A necessary mechanism is lacking. It may be that the 

 primordial germ cells are the precursors to this mechanism 

 in normally developing ovaries. 



The evidence from the free-martin and recent data on the 

 transplantation of embryonic gonad rudiments indicates 

 that, as in amphibia and birds, the development of an ovary 

 in embryogeny is dependent upon the formation of a cortex 

 in the developing gonad. Normally in ontogeny the gonads 

 of both sexes are morphologically indistinguishable for some 

 time. The genital ridge, as already noted, consists of ger- 

 minal epithelium overlying primordial germ cells. At about 

 the 10 mm. stage in both the pig (Allen, 1904) and cat 

 (Sainmont, 1905) and at the 12th day post coitum in the 

 mouse (Brambell, 1930) the germinal epithelium begins to 



