No. 63S] INTERNAL SECRETIONS OF AMPHIBIANS 207 



facts, is also capable of explaining the problem of neoteny 

 of the so-called axolotl. In the course of experiments 

 carried on during several years in the laboratory, and by 

 inspection of the conditions prevailing rn the Rocky 

 Mountain lakes, the natural habitat of the American 

 axolotl, I have become convinced that the neoteny of this 

 species is due to the effect of low temperatures. We have 

 in the amphibians an experimental material in wliich the 

 relation between the development of the body and certain 

 endocrine glands can be changed by the iiiihience of tem- 

 perature, owing to the differences of the temperature 

 coefficients of the processes governing the development 

 of different glands. 



Although my experiments are not yet finished, they 

 seem to permit the following conclusions in connection 

 with my field observations : 



1. The thyroid gland of salamanders undergoes a de- 

 velopmental change consisting of two periods, one of 

 early development, lasting at least 63 weeks, in the course 

 of which the thyroid becomes more and more sensitive to 

 the action of a releasing factor (called excretor substance 

 ill my earlier work) and one of aging in the course of 

 whieli the thyroid loses gradually its sensibility to the 



■2. In order to release the hormone of the thyroid ghuul, 

 a i)artieidar releasing factor is required (the nature of 

 whieh is entiivly unknown); the quantity of tliis factor 

 ]iecessary to release the thyroid hormone dei.en<ls on the 

 sensitivity of tlie thyroid gland. :\retamorpliosis can 



;;. The tem])eratnre coellieient for the elaboration of 

 the i-eleasiiiu- factor is highei' than the temperature coeffi- 

 cients for growth and the thyroid change. 



The following facts seem to warrant these assnnq^tions : 

 1. Salamander larva', ker-t at an identical temi.eratinv. 

 are nearly all of the same >i/e when they metainoi-phoe. 

 LarvcT ke])t at low tenq)eratures grow consideral)ly larger 



