Huxley: Glands and Development 



351 



LARVAL FORM OF WHITE AXOLOTL 



Figure 8. The axolotl is found in Mexico, where it has been used as an article of 

 food for centuries. The "white" axolotls are a pale flesh color, with beautiful red gills. 

 Normally axolotls do not metamorphose, but retain their larval form throughout life. 

 Iodine or thyroid treatment produces metamorphosis, and this brings up the interesting 

 question of why this change does not normally occur, as the axolotls have functional thy- 

 roids which produce metamorphosis when grafted into anuran larvae. 



this will only happen in the second 

 generation from a cross. 



There would seem to be no reason- 

 able doubt, after Goldsmith's work, 

 that some such quantitative change in 

 the "potency" of male and female-de- 

 termining factors does occur, and that 

 this change in potency implies a quan- 

 titative change in the amount of some 

 substance produced during the course 

 of development, which in its turn af- 

 fects the development of sexual char- 

 acters. 



As to the precise way in which 

 these sex-determining substances act, 

 however, we are very much in the 

 dark. In any event, we shall not be 

 far wrong if we try to think in terms 

 of metabolism. There is, at least, no 

 doubt that the males and females of 

 insects, (as well as of other organisms), 

 possess different types of metabolism. 

 The evidence for this is reviewed in 

 Goldschmidt's book.* We may conceive 

 of this in two main ways. Either the 

 sex-determining substances are markedly 



specific in their action on sexual charac- 

 ters, i. e., act locally on particular organs, 

 as does the hormone secretin upon the 

 pancreas ; this would appear to be the 

 case with the sex-hormones of mam- 

 mals and birds. ^^' ^^ The sexual dif- 

 ferences in metabolism would then be 

 secondary results of the different de- 

 velopment of the gonads. Or else 

 these substances act primarily upon 

 metabolism, turning on one of two 

 alternative switches, so to speak, which 

 lead to two different types of meta- 

 bolism, which result in different 

 internal environments. In one environ- 

 ment one chain of relations would 

 occur and would lead to male develop- 

 ment. The primary effect of the 

 sex-factor might concern some general 

 fundamental function, such as oxida- 

 tion, and not be specific or local in 

 its action on sexual characters at all ; 

 in this case the primary effect would 

 be on general metabolism, which would 

 lead, as a secondary result, to sex- 

 chvergence. There remains a third 



