IODINE AND AMPHIBIAN METAMORPHOSIS. 251 



3. Larval Spelerpes bilineatus readily metamorphoses in strong 

 solutions of 3-5 diiodotyrosine, but do not transform in equivalent 

 concentrations of tyrosine and 3-5 dibromtyrosine even when 

 kept in such solutions over comparatively long periods. 



4. Rana clamitans tadpoles with from six to eight months of 

 larval life remaining (i.e., passing the winter as tadpoles) were 

 metamorphosed within twenty days by rearing the animals in 

 strong concentrations of 3-5 diiodotyrosine and feeding them 

 with this substance. Control larvae of similar age and develop- 

 mental stage reared in equivalent solutions of 3-5 dibromtyrosine 

 and fed quantities of the compound failed to transform. 



5. The experiments are clean cut and admit of but one inter- 

 pretation ; it is the iodine in the amino acid and protein molecule 

 that is responsible for amphibian metamorphosis. Why the 

 linking of iodine to the third and fifth carbon atoms of the benzene 

 ring of the tyrosine molecule should so greatly increase the reac- 

 tive powers of the iodine is unknown. 



6. Iodine other than thyroid iodine is effective in inducing the 

 metamorphosis of thyroidless urodele and anuran larvae. There 

 is no evidence that any other tissue of the vertebrate organism 

 has the power to function vicariously for the thyroid in the latter's 

 absence. 



7. Bromine has little or no influence upon amphibian meta- 

 morphosis and can not be substituted for iodine. The substitution 

 of two bromine atoms for two hydrogen atoms (the third and fifth) 

 of the tyrosine molecule fails to change this substance into a meta- 

 morphosis-inducing agent. 



8. The physiologic responses (metabolic changes) of mammals 

 to thyroid administration are due to the CO-NH group within 

 the thyroxin molecule; amphibian metamorphosis to the iodine 

 in the molecule. Consequently thyroidless animals of these two 

 vertebrate groups are hardly comparable in regard to their 

 response to iodine. Only thyroid iodine containing the CO-NH 

 group is effective in thyroidless mammals whereas iodine other 

 than thyroid iodine will metamorphose thyroidless amphibian 

 larvae. 



9. Data are presented showing why experiments on amphibian 

 larvae devised to test out the effect upon metamorphosis by 

 feeding or injecting various substances are unsound unless per- 



