257 



Fig. 123 

 Antagonistic sex-differentiation in Amphibia. Diagram of parabiotic twin pairs 

 of unlike sex in toads (left), frogs (centre) and Urodeles (right). Above, parallel 

 pairs; below, chain pairs. In the gonad: male differentiation, black; female 

 differentiation, white. In toads, there is no effect of one twin upon the other 

 (note Bidder's organ at the anterior end of the gonad in both sexes). In frogs, 

 there is no effect in chain pairs ; but in parallel pairs, the male gonad affects the 

 gonads of the female partner, the effect diminishing with distance (degree of 

 shading of circles). In Urodeles (Trm/m^), the male gonad completely inhibits the 

 female gonad in both types of twin pairs. (Based upon Witschi, Chap, v in 

 E. Allen, Sex and Internal Secretions, London and Baltimore, 1932; modified.) 



HEE 17 



