198 



THE AMEBIC AN NATURALIST [Vol. LV 



thyroid substance can cause such an increase of metab- 

 olism as to bring about metamorphosis. Several experi- 

 ments have been carried out to answer this question. But 

 until thyroidectomized tadpoles have been employed in 

 these experiments, no definite conclusions are possible; 

 in larvae possessing a normal thyroid gland it can not be 

 decided whether the experimental conditions employed 

 have caused metamorphosis by raising the metabolism 

 directly or merely through the intermediation of the thy- 

 roid by precociously releasing the thyroid hormone. 

 Powers as well as Barfurth has shown that a sudden 

 cessation of food supply results in precocious metamor- 

 phosis. Although this is certainly true, it does not decide 

 the point in question, but may mean that sudden starva- 

 tion may precociously release the thyroid honnone. At 

 any rate, starvation in itself does not cause metamorpho- 

 sis, but is effective only if well-fed larvjE which are ap- 

 proaching metamorphosis and possess a thyroid capable 

 already of functioning are suddenly starved. The same 

 criticism applies to Kaufman's (8) recent experiments, 

 in which an advanced neotenous larva (axolotl) of Am- 

 hystoma tigrinum was given salicylic acid, whereupon it 

 metamorphosed promptly. This result is extremely in- 

 teresting as it raises most urgently the question whether 

 the action of iodothyrine is specific and whether the 

 changes of metabolism resulting from thyroid adminis- 

 tration are merely quantitative or also qualitative. As 

 pointed out, Kaufman's experiment, however, does not 

 answer any of these questions. 



In accord with the highly increased catabolism as 

 caused by the action of the tliyi-oid hormone is the fact 

 that metamorphosis, in lis initinl >tages, appears to be 

 more a process of ])r()l'(>uii(l atr<>|)h\- than one of construc- 

 tive .k'velopmeiit, althnu.oh phnmniena of the latter 

 kind rnM,„..ntlv ncroinpaiiv ill.. (h-,.H.M<itiv.. i.nKvss.'S. 

 Aniono- th.. ino.t conspicuous pro.-..s>r. nf d.'slnu-tion 



cular apparatus whicli serves the gill circulation, a con- 



