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THE AMEBIC AN NATURALIST [Vol. LV 



not due to differences existing- between the endocrine 

 system of the numerous species inhal)iting- tlie United 

 States, but is explained by the fact that A. tigrinum, 

 among the closely related species which I had an oppor- 

 tunity to test, is the only species that can stand tempera- 

 tures low enough to bring about the necessary differ- 

 ence between the rate of the thyroid development and 

 that of the elaboration of the releasing factor. 



8, The fact that many individuals among the offspring 

 of female specimens of the Mexican axolotl do not meta- 

 morphose even if they are brought, immediately after 

 hatching, into conditions permitting normal metamor- 

 phosis of other salamander species, is not necessarily 

 related to the factors discussed above, but may be due 

 to the development of congenital thyroid disturbance in 

 the young born by an athyroidous female. 



It is, of course, well known that many structural 

 changes, only a few of which have been studied, are re- 

 cjuired to make, out of the aquatic larvap, the terrestrial 

 amphibian. This is true for the anurans as well as for 

 the urodelans. Since we know that the complex phe- 

 nomenon of metamorphosis is initiated by the thyroid 

 effect, the question arises now which of the component 

 changes are directly caused ])y the action of the thyroid 

 hoi'iiu.nc. Th<. fact thnt certain .h'v.'Iopin.'Htal i)rocesses 

 frtM|iit'iitIy take place upon thyroid admiiiist I'ation and 

 therefore are a \'ery convenient iiuiicator in studying 

 quantitatively the effect of thyroid substance, of iodine 

 or of any other metamorphosis-causing agent, does 

 not mean, in itself, that these developmental processes 

 are caused directly by the action of the thyroid; it is pos- 

 sible and indeed supported by many facts, that certain 

 of these changes will follow antoniaticallx', aftei' the 

 initial changes have been effected 1)\ the th\ loid action. 

 Thus, while under normal conditions, the pigmentary 

 pattern, the legs, the tongue, the pahatal teeth and the 

 sex organs mature in sahimanders (hiring metamor- 

 phosis, they can be shown to be highly in(U'pendent of 



