540 METAMORPHOSIS IN AMPHIBIANS 



is more retarded than metamorphosis; second, the development of 

 yellow bands is inhibited permanently in low temperature. 



The first phenomenon can be explained as follows: In a previous 

 paper it was shown that one of the substances causing metamor- 

 phosis, the excretor substance, is evolved during chemical processes, 

 the rate of which depends on the rate of the processes leading to 

 growth. Since the temperature coefficient of the excretor-forming 

 processes is greater than that of the growth processes, the formation of 

 excretor substance is decreased more than the rate of growth if the 

 larvae are kept at low temperature, and consequently the relation be- 

 tween size and metamorphosis is changed, the larvae being larger at 

 the time of metamorphosis when kept at low temperature than when 

 kept at high temperature. Since the temperature coefficients of 

 different physiological processes are frequently different, we might 

 expect that the temperature coefficients of the processes causing meta- 

 morphosis are different from those of the processes causing develop- 

 ment of the structures which give rise to the skin coloration of the 

 metamorphosed animal. If this were so, we should expect that the 

 time relation between metamorphosis and the development of the 

 skin color should be changed not only by feeding or omitting sub- 

 stances which cause precocious metamorphosis but also by keeping 

 the animals at low temperatures. 



This is actually the case with the development of the yellowish 

 spots in Amhystoma tigrinum. The retardation of the appearance 

 of the yellowish spots as compared with metamorphosis can be ex- 

 plained if we assume that the chemical processes, causing the de- 

 velopment of the spots have a higher temperature coefficient than 

 the chemical processes causing metamorphosis. 



In order to understand the second phenomenon, the inhibition 

 of the development of the yellowish bands, we shall again refer to 

 metamorphosis. 



One of the cases of metamorphosis most difficult to explain is the 

 occurrence of a perennibranchiate form in the species Amhystoma 

 tigrinum in certain localities. On the basis of the results so far ob- 

 tained in our experiments the perennibranchiate form of the Amhy- 

 stoma tigrinum can be explained tentatively as follows. 



