Huxley : Glands and Development 



353 



PARTLY METAMORPHOSED 



Figure 9. Iodine treatment is having its effect most markedly in the reduced size of 

 the gills. The dorsal fin is also smaller, but there is little change in the size or shape 

 of the head. Metamorphosis appears to be caused by a change in the internal environment, 

 resulting in a higher rate of metabolism. This is favorable to specifically adult tissues, 

 but larval organs and tissues cannot "keep the pace" and break down. The sipecimen 

 shown here is a black axolotl, while the one shown in Figure 8 is white. 



viously the simplest way to envisage 

 the problem here presented to us is 

 to suppose, first, that a complete set 

 of chromosomes is necessary for de- 

 velopment ; second, that a definite 

 quantitative preponderance of certain 

 factors contained in the autosomes re- 

 sults in a type of metabolism leading to 

 an internal environment in which male 

 characters are expressed ; and third, 

 that a preponderance of certain factors 

 contained in the X-chromosome pro- 

 duces another type of internal envir- 

 onment in which female characters can 

 develop. 



It further follows that, in such a 

 case, it is not necessary to postulate 

 the existence of specific sex-determin- 

 ing factors at all, but merely of fac- 

 tors which influence metabolism in a 

 way favorable to the expression of 

 male or female characters, respective- 

 ly. The difference between this and 

 the state of affairs in plants like 



Polygonum amphibiiini is that in our 

 cases the genetically-determined sex- 

 chromosome mechanism normally pro- 

 vides the two proper quantitative 

 relations of the two sets of factors, 

 whereas in Polygonum there is no 

 genetic basis for the regulation of the 

 dimorphism. Haldane" has recently ex- 

 pressed the view not only that the 

 assumption of specific sex-factors is 

 unnecessary, but that it should not be 

 made until the more economical alter- 

 native has been tested. Such an idea is 

 borne out by the recent remarkable 

 work of Blakeslee and his co-workers, 

 on the effect produced by the presence 

 of abnormal numbers of certain chro- 

 mosomes in Datura.^ 



In any event, what is iinportant for 

 our present purpose is definitely es- 

 tablished : ( I ) that the dimorphism of 

 sex is brought about in higher ani- 

 mals by a zygotic difference in the 

 relative amount of certain factors in 



