THE ORIGIN OF THE DEFINITIVE OVA 31 



proliferate the primary sex cords from its inner surface. 

 During the formation of these cords (or nest of medullary 

 cells as in man [Felix, 1912]) the gonad is still morphologi- 

 cally indifferent. Morphological differentiation may be con- 

 sidered as initiated when these primary cords become iso- 

 lated in the medulla by the formation of the primitive tunica 

 albuginea under the germinal epithelium in the male gonad 

 and the proliferation of a second set of cortical sex cords 

 from the germinal epithelium in the female gonad. In the 

 embryonic ovary the medullary cords persist for some time 

 but are rarely found after birth; the cortical cords break up 

 to form primitive follicle cells surrounding the primordial 

 ova. 



Buyse (1935) has transplanted rat gonads in the morpho- 

 logically indifferent stage onto the kidney of adult rats of 

 both sexes. Over 60 per cent of the transplants developed 

 as testes, 16 per cent as ovaries and the remainder were 

 bisexual gonads or gonads of undetermined sex. A small 

 percentage of the gonads classified as rudimentary testes 

 seemed to be transformed ovaries. It will be seen that if 

 these are included in the group of gonads other than testes 

 the normal sex ratio is approximated. Since the type of 

 gonad developed was not correlated with the sex of the 

 host Buyse concludes that adult sex hormones do not affect 

 sex differentiation. The differentiation was then dependent 

 on the history of the sex cords in the transplanted tissue. 

 Presumably the clear cut segregation of testes was due to 

 the presence of formed primary sex cords, e.g., the testis 

 organizers, whereas various types of zygotic ovaries were 

 obtained dependent on the probability of formation or 

 partial formation of the cortical sex cords. 



