356 



The Journal of Heredity 



may be direct and not. as was previous- 

 ly supposed, exerted solely indirectly 

 by stimulating the thyroid.'" 



The fact that not only are limbs 

 fully formed before metamorphosis in 

 Urodeles, but that they do not appear 

 to be affected in their growth by iodine 

 treatment,"' whereas they may be 

 formed quite precociously as the re- 

 sult of that treatment in Anura, shows 

 that the same organ responds in a 

 totally different way in the two groups. 

 The resorption of the tail in Anura, 

 and its non-resorption in Urodela, is 

 a fact of the same order. Our next 

 assumption, therefore, is that meta- 

 morphosis is not. in the main, a specific 

 effect of thyroid secretion in the sense 

 that thyroid secretion has a special 

 growth-promoting effect on certain 

 types of organs, but that it is what 

 we have called a general effect upon 

 metabolism, inducing a different in- 

 ternal environment. The larval organ- 

 ization is adapted to one environment, 

 the adult to the other ; as the first 

 environment changes into the second, 

 the larval organs can no longer main- 

 tain themselves, and break down, just 

 as do the female structures, such as 

 ovary and oviduct, in a female frog 

 which is becoming transformed into 

 a male, in which, therefore, the in- 

 ternal environment favors the growth 

 of male and discourages that of fe- 

 male characters. 



It is further clear that, if this be so, 

 we need no longer go to the trouble 

 of postulating a change of function in 

 the thyroid, for we can think of the 

 change as occuring ivhen a certain 

 relative concentration of thyroid se- 

 cretion exists in the organism. In 

 other words, metamorphosis, like sex, 

 is an affair of balance. It is not the 

 absolute arriount of thyroid secretion 

 which is at the root of the matter, but 

 the amount of thyroid secretion con- 

 sidered in relation to the amount of 

 certain other substances, or the in- 



tensity of other processes. There is 

 therefore a normal threshold value 

 for the amount of thyroid secretion 

 necessary to produce metamorphosis 

 in a given species, but this need not 

 be the same, even when calculated 

 relative to body weight, for different 

 species; and it may be possible to 

 alter this value artificially in any one 

 species. It need not be the same, for 

 it will be exerting its effects relatively 

 to other metabolic processes, and these 

 may be different in different species.* 



That the thyroid exerts its effects 

 by a special effect on cell metabolism 

 is made probable by the important 

 discovery of Hogben, that injections 

 of anterior lobe of the pituitary will 

 produce metamorphosis even in thy- 

 roidless Axolotls.^^ That pituitary as 

 well as thyroid is concerned with in- 

 creasing basal metabolism, is rendered 

 highly probable by the work of Hill" 

 and Benedict and Romans'. 



Let us consider a few facts in the 

 light of these considerations. Uhleri- 

 huth, in a very interesting paper,^ 

 has shown that in certain Urodeles, 

 which in this regard are probably sim- 

 ilar to Anura, metamorphosis is co- 

 related not with growth-rate, but with 

 the attainment of a certain size. At 

 a given temperature this size will be 

 constant, whether the animal is well 

 fed and grows rapidly, or half starved 

 and grows very slowly. We may take 

 this to mean that differentiation and 

 growth of thyroid and of the rest of 

 the body are, as one would expect, 

 bound up together, one affecting the 

 other. He also finds that the "me- 

 tamorphic size" changes with the tem- 

 perature ; at low temperatures the size 

 is greater (by about 10% in bady 

 length). This would imply that the 

 temperature-coefficients for thyroid 

 and general body differentiation are 

 slightly different. It is worth noting 

 that Goldschmidt has found similar 

 temperature effects in his intersexual 



* This threshold value refers to the concentration just prior to and at the time of 

 metamorphosis. There is some evidence to show that in adult Anura, the concentration of 

 thyroid hormone decreases again ; but, since metamorphosis is irreversible, no morphogenetic 

 processes ensue. 



