226 AN INTRODUCTION TO MODERN GENETICS 



ments, a considerable amount is known about the developmental 

 mechanisms by which the sex factors produce their effects. As was 

 pointed out earlier, the actual germ-cells develop into eggs or sperm 

 according to the type of gonad in which they find themselves, and the 

 determination of their development can be regarded as an embryonic 

 induction. The somatic constituents of the gonad consist mainly of the 

 outer layer or cortex and the central medulla, both of which are present 

 in the larval organ. In the female, the medulla largely disappears and 

 it is the cortex which is the inductor of egg-development, while in the 

 male things are reversed, the cortex vanishes and the sperms are induced 



racdulUrln 



genetical male genetical female 



Fig. 104. Differentiation of the Gonads in Salamanders. — In the male the 

 central medulla becomes fully developed and produces "medullarin" (the male 

 substance), while in the female it is the cortex which becomes highly developed. 

 The arrows indicate the suppressing action of the sex substances in heterosexual 

 parabiotic pairs; the action of medullarin in suppressing cortical development 

 is the stronger of the two. 



(From Witschi.) 



by the medulla. As well as the balance of the M and F factors, certain 

 environmental conditions influence the relative development of these 

 two parts; for instance, in frogs kept at high temperatures the cortex is 

 inhibited and the medulla may become more highly developed and 

 females converted into males. Cold has the opposite effect. In the toad 

 no medulla is ever formed in the anterior part of the gonad and in the 

 male this part cannot develop into a proper testis but remains a 

 purely cortical organ, more or less like an ovary, known as Bidder's 

 organ. 



The cortex and medulla perform their inductions by substances 

 which are capable of some diffusion. If a male and female are grafted 

 together, there is antagonism between the cortical and medullary sub- 

 stances. In toads there is Uttle effect, probably because the substances 

 are not easily diffusible. In frogs grafted side by side the male medulla 

 will suppress the cortex of the ovary nearest to it, but the diffusion is 

 not sufficient to have much effect on the further ovary. In urodeles the 



