EFFECT OF IODINE ON LARVAE OF SALAMANDERS. 145 



of the tadpoles or certain parts of it do not possess the ability of 

 storing the thyroid hormone after its elaboration. The tadpole 

 experiments in themselves, however, do not decide anything as to 

 the role of the inorganic iodine in the action of the thyroid hor- 

 mone. When viewed in the light of the experiments on salaman- 

 ders they support the conception that inorganic iodine as such does 

 not play an essential role in the effects characteristic for the thy- 

 roid hormone, and that the amphibian metamorphosis can be ef- 

 fected only by the thyroid hormone. 



I have shown 5 that inorganic iodine, when administered to such 

 amphibians in which the control of the excretion of the thyroid 

 hormone is similar to that in mammalians i.e., in which no hor- 

 mone can escape from the thyroid in the absence of the action of 

 a particular releasing mechanism is completely unable to produce 

 the amphibian metamorphosis. In the experiments to be reported 

 below I have used so-called axolotl larvae. The axolotls, which 

 are the larvae of the species Ambystoma tigrinum, are characterized 

 by an inhibition of metamorphosis resulting in a considerable pro- 

 longation of the larval period. It is possible, therefore, to extend, 

 in this species, the experiments over long periods and to administer 

 excessively large quantities of iodine. The ineffectiveness of such 

 large quantities of inorganic iodine as compared to the prompt 

 effect of amounts of iodothyrin containing only minute quantities 

 of iodine disproves, in a quantitative manner, that inorganic iodine 

 is the active principle of the thyroid hormone in the amphibian 

 metamorphosis. 



EXPERIMENTS. 



From about 100 larvae of the salamander Ambystoma tigrinum. 

 which had developed into so-called " axolotl larvae," 7 9 larvae were 

 selected for the experiment to be reported here. They were kept 

 in ordinary tap water and fed earthworms in the beginning, beef 

 liver later on. 



Beginning of Experiment (December 9, 1920}. 



No. i measured 189.0 mm.; No. 2, 160.6 mm. ; and No. 3, 194.4 

 mm. All three animals were completely larval (large gills, large 



7 The axolotl larvx were collected in the Colorado Rocky Mountains in 

 August and September, 1920. 



