542 METAMORPHOSIS IN AMPHIBIANS 



certain substance with progressing age, the temperature coefficient 

 of that change being lower than that of the processes which produce 

 the substance in question. 



It is possible that mechanisms similar to the mechanism involved 

 in the development of the skin color are responsible also for the de- 

 velopment of the tongue and palatal teeth, both phenomena being 

 checked definitely by low temperature, as pointed out by Cope^^ and 

 Powers.^^ 



DISCUSSION. 



In previous experiments^^ it has been shown, by grafting organs 

 from one larva to a larva of different age, that the development of 

 certain stages of the eye and of the skin pigmentation is caused, like 

 the processes of metamorphosis, by substances which are not pro- 

 duced in the organs themselves but reach them through the blood 

 circulation. In this paper further proof of the existence of such 

 substances has been found in experiments made by a different method. 

 But at the same time it has been shown here that the substances which 

 are causing development of these organs are not identical with the 

 agents causing metamorphosis and consequently the development of 

 these organs cannot be included under metamorphosis. It is certain 

 that in the salamanders examined at least five groups of organs, the 

 structures leading to pigmentation of the skin, sex organs, legs, tongue, 

 and palatal teeth, are independent of each other and of metamorphosis. 

 It is probable that at least six different chemical mechanisms exist 

 in the amphibian organism each of them for another group of organs. 

 Consequently it should be possible to change the succession in time 

 of these developmental phenomena by feeding directly substances 

 which contain or lack the agent for only one of them; this can be 

 actually accomplished in the case of metamorphosis and development 

 of the skin color by iodine treatment or thymus feeding. But the 

 same result should be accomplished by keeping the larvae in different 

 temperatures, since the temperature coefficients are different for the 

 chemical reactions constituting each of these mechanisms. This has 



^^Uhlenhuth, E., Arch. Entwcklngsmechn. Organ., 1913, xxxvi, 211; Arch, 

 vergl. Ophthalm., 1913, iii, 343; Proc. Soc. Exp. Biol, and Med., 1917, xiv, 88; 

 J. Exp. ZooL, 1917-18, xxiv, 237. 



