258 THE MOSAIC STAGE OF DIFFERENTIATION 



female-differentiation combined with a certain degree of encourage- 

 ment of male-differentiation, resulting in progressive transforma- 

 tion away from the somatic female towards the somatic male-type 

 of gonad. 



The most unexpected result is that the effect always manifests 

 itself first in the " inside " ovary, i.e. that nearer to the male partner, 

 and always on the inside margin of this ovary ; from here it gradually 

 spreads, but with diminishing intensity, to the more distant 

 regions. It appears clear that the medulla of the male produces a 

 substance which not only promotes masculine sex-differentiation of 

 germ-cells, but is also antagonistic to cortical development: further, 

 that this substance is not strictly localised but can diffuse outwards 

 in diminishing concentration and with diminishing effectiveness. 

 In chain-pairs, the gonads of the female partner lie beyond the 

 limit of effectiveness; in parallel pairs, they lie across a rapidly 

 decreasing concentration-gradient (fig. 123). 



Comparative studies on other forms provide further striking 

 results. In toads, no effect is ever observed on the gonads of either 

 partner, whether in parallel or chain-pairs. In the Urodele Triturus, 

 however, the effect is equally marked in both kinds of combination. 

 (The details here are slightly different: there is a long period of 

 mutual inhibition, in which both male and female gonads are 

 delayed and rendered nearly sterile. Later the male recovers, and 

 reduces the female gonads still further to small rudiments almost 

 free from germ-cells. There is no male transformation of the 

 genetic females. In other Urodeles the effect is similar in affecting 

 the female partner equally in chains and in parallel pairs, but 

 neither the mutual antagonism nor the final inhibition of female- 

 differentiation are so extreme.) 



It would thus appear that the morphogenetic (inductive) sub- 

 stances produced by cortex and medulla are in toads strictly 

 localised within the regions where they are produced, and wholly or 

 almost incapable of diffusion. This is borne out by the existence in 

 toads of Bidder's organ, an anterior section of the gonad of ovarian 

 character, which develops from a portion of the gonad-rudiment 

 consisting wholly of cortex.^ This could not well develop, as it does, 

 in males if the medullary substance could diffuse even a short dis- 



1 Witschi, 1933 B. 



