ENDOCRINE SYSTEM OF TYPHLOMOLGE RATHBUNI. 323 



have been small, attain a size and structure capable of producing 

 metamorphosis. But as shown in my iodine experiments, the 

 feeding of iodine would greatly accelerate the elaboration of the 

 hormone and, if the releasing mechanism is set active (which it 

 was in Swingle's specimens, according to his own statements), 

 metamorphosis may occur months before it takes place in the 

 untreated controls. Swingle has observed his animals apparently 

 only for 6 months; it would be important to know whether the 

 untreated " thyroidectomized " animals did not finally meta- 

 morphose. 



Swingle 13 - 21 mentions also that 3-5 di-brom-tyrosine, when 

 fed to thyroidectomized axolotl larvae, is incapable of enforcing 

 metamorphosis and thinks that this result is contrary to my own 

 views on the role of iodine in the amphibian metamorphosis and 

 in the thyroid hormone. Apparently he did not see the following 

 statement, in which my views were summarized 10 (p. 1 14) : 

 ' The views elaborated above are in no way contradictory to the 

 fact that nevertheless, in a biological sense, iodine is an important 

 and essential part of the thyroid hormone; if it were possible to 

 substitute the iodine by any other substance without changing 

 the reactivity of the hormone, biologically this would not make 

 iodine less important, for it is the only substance which, by the 

 mechanism actually available to the organisms, can be used in the 

 manufacture of the thyroid hormone. Although chemically 

 bromine or any other halogen may be able to substitute iodine 

 without changing the chemical or even the physiological reac- 

 tivity of the thyroid hormone, the organism is unable to use 

 bromine, as shown by Swingle, and presumably the other halo- 

 gens to make thyroid hormone." I have never claimed that the 

 thyroid or any other organ can manufacture the thyroid hormone 

 from bromine. Swingle has not touched, by his experiments, the 

 real problem. This centers around the question whether the 

 finished thyroid hormone could enforce metamorphosis if it con- 

 tained bromine instead of iodine; Swingle did not employ such a 

 product in his experiments. 



As Swingle correctly states, the crux of the problem of thyroid 

 function is now to find the organ or tissue or substance which 

 plays the role of a releasing mechanism to the thyroid gland. I 



have intentionally refrained, in my previous papers, from forming 

 22 



