W. W. SWINGLE 599 



received (m/1, 100,000 solution) soon showed the effect of its admin- 

 istration. It is safe to assume that an absolutely iodine-free diet 

 fed to frog larvae reared in iodine-free water would inhibit metamor- 

 phosis indefinitely. The writer is conducting experiments along this 

 line, but the data are not yet ready for publication. As stated else- 

 where, the function of the thyroid appears to be that of collecting, 

 storing, and passing into the circulation at the proper time such 

 iodine as finds its way into the organism by means of food and water. 

 Probably most if not all organic matter contains very minute traces 

 of iodine in some form; most surface water contains traces of it, 

 though iodine is said not to occur in deep fresh water springs. Lack 

 of iodine in the environment may possibly explain why Amhly stoma 

 trigrinum fails to undergo metamorphosis and permanently retains 

 its larval form in certain locaHties such as Lakes Xochimilco and 

 Chalco 8 miles southeast of the City of Mexico.* This phenomenon 

 is not unknown elsewhere; for instance, in certain pools and dis- 

 tricts in Lombardy the larvae of Triton are said to fail to metamorphose 

 and may grow to an immense size. It seems reasonable to suppose, 

 in the light of these iodine studies, that the failure of the Axolotl and 

 Triton larvae to metamorphose is due to lack of iodine in the environ- 

 ment or else the thyroids of these animals are defective and unable to 

 collect sufficient iodine for the organism's needs. Just as there are, 

 certain districts where cretinism and myxedema are endemic, so 

 there are certain districts where amphibians fail to undergo meta- 

 morphosis. In both instances lack of sufficient iodine in the food and 

 water is probably the causal factor. 



Moreover, it seems likely that in the experiments recorded in the 

 literature where frog larvae restricted to certain diets, for example 

 thymus gland tissue, apparently fail to undergo metamorphosis at the 

 proper time, or not at all, the cause of the delayed metamorphosis 

 is lack of sufficient iodine. Thymus tissue is said not to contain 

 iodine, but in the mammalia it is not at all uncommon to find thyroid 

 tissue embedded in the thymus. The discordant results obtained by 

 investigators who have fed thymus tissue to frog larvae may be ex- 

 plained on this hypothesis, although the writer believes that certain 



* Gadow, H., Through Southern Mexico, London, 1910. 



