152 E. UHLENHUTH. 



in the specimens of thyroid gland which were fed to the tadpoles 

 corresponded to an increased amount of thyroid hormone ( A-iodine, 

 Kendall) ; in this case Lenhart's experiments would mean that 

 metamorphosis is the more accelerated, the more thyroid hormone 

 there is supplied to the tadpole, a result which corresponds with 

 quantitative experiments on the action of iodothyrin in sala- 

 manders 5 and does not throw any light on the role of iodine in the 

 thyroid hormone. 



Swingle 12 in support of his viewpoint that inorganic iodine as 

 such causes the amphibian metamorphosis, has emphasized the 

 specificity of iodine, the administration of inorganic bromine be 

 ing incapable of causing metamorphosis. It is not impossible, how- 

 ever, that this fact merely means that the organism does not pos- 

 sess a mechanism by means of which bromine can be employed in 

 the manufacture of thyroid hormone; if it were possible to make 

 synthetically, outside the organism, a substance identical with 

 Kendall's thyroxin in every respect, but possessing a bromine 

 atom in place of every iodine atom, this substance may be capable 

 of producing metamorphosis. 



SUMMARY. 



1. Iodothyrin and inorganic iodine, in known quantities, were 

 administered to so-called axolotl larvae of the salamander Atnby- 

 stoma tigrinum. 



2. A dosis of iodothyrin containing only 0.03 mgm. iodine per 

 1,000 c.c. of water caused metamorphosis 13 days after its first 

 administration. 



3. A dosis of inorganic iodine 33 to 86 times larger and feeding 

 still larger doses directly per mouth did not cause metamorphosis. 



4. The amphibian metamorphosis is truly the expression of the 

 thyroid activity and not the result of the effect of inorganic iodine. 



5. Inorganic iodine as such is not the active principle of the 

 thyroid hormone. 



12 Swingle, W. W., J. Gen. Physiol., 1919, i, 593-606. 



