26o THE MOSAIC STAGE OF DIFFERENTIATION 



potency. The progressive restriction of potencies during ontogeny, 

 and its variation between types of tissue and types of organism, 

 remains a central problem for developmental physiology. 



Two further points may be mentioned. Occasionally in Anuran 

 parabiotic twins, the female partner may obtain an unusually early 

 start. In this case, the male-differentiation of the male partner is 

 inhibited and a female phase may be passed through. This however 

 is only to be seen in the parts of gonads nearest to the female 

 partner, and is transitory, normal male-differentiation eventually 

 gaining the upper hand again. 



Secondly, in heteroplastic parabiosis between two species of 

 frogs, a curious effect is visible in pairs in which both partners are 

 genetic females, and in which one member 

 belongs to the species Rana sylvatica. The 

 sylvatica ovaries hypertrophy, those of the co- 

 twin become reduced and degenerate. This 

 can be explained on the assumption that some 

 substance necessary for ovarian growth is 

 present in limited quantity in the embryo, ^ 

 and that the faster-growing sylvatica ovaries 

 obtain a disproportionate share of it.^ In rare C- 

 cases, the reduced ovary of the co-twin may 

 even show some changes in the direction of 

 male transformation. This may be explained Fig. 124 



on the hypothesis of antagonism between Diagram of the two 

 cortex and medulla; when the cortex is in- Sramprbkn^gonad- 

 hibited by being starved of the substance rudiment. M, medulla, 

 necessary for its growth, the medulla is re- ^^2^°"''^^^- ^^^ J^^^^" 



•' .,.,., differentiation ; C, cor- 



leased from mhlbltion. tex, responsible for 



Thus we come to the general conclusion female -differentiation. 



that sex-differentiation in Amphibia is under \^ in^E.^ Allen, S^x 



the control of substances provided by the a7id Internal Secretions, 



gonad cortex and medulla respectively: that Baltimore and London, 



^ • -r: 1 1932- Modified.) 



these substances are not speci6s-specinc : that 



they are mutually antagonistic : that they are capable of various 



^ The same hypothesis will account for the fact that removal of the gonad 

 proper in either sex in toads leads to the hypertrophy of Bidder's organ to form 

 a functional ovary. The substance in question is very possibly a hormone pro- 

 duced by the pituitary. See Witschi, 1933 b. 



