248 W. W. SWINGLE. 



bolic rate is due to the specific chemical structure of the thyroxin 

 molecule particularly the CO-NH group, whereas metamorphic 

 response of amphibians is dependent upon a peculiar property of 

 iodine in certain types of combination, although not necessarily 

 that characteristic of thyroid iodine, e.g., iodized amino acids and 

 proteins. Further evidence of the difference as to cause between 

 metabolic changes in mammals and amphibian metamorphosis 

 is furnished by the acetonitrile test where iodized substances 

 shown to be specific in accelerating metamorphosis completely 

 fail to simulate the thyroid function in protecting mice against 

 the lethal effects of actonitrile. 



Hunt and Seidell ('09) made the interesting observation that 

 feeding thyroid tissue to white mice greatly increases the resist- 

 ance of these animals to lethal doses of acetonitrile, and that the 

 efficiency of the gland seems dependent upon its iodine content. 

 They concluded that the increased resistance of the mice to the 

 poison was due to the changed metabolism of the animals follow- 

 ing thyroid feeding, the metabolic change preventing the acetoni- 

 trile from breaking down into its posionous product hydrocyanic 

 acid. This assumption was based upon the fact that thyroid 

 feeding does not raise the resistance of mice to lethal doses of 

 hydrocyanic acid itself. 



Koch ('13) and Miura ('22) found that iodized amino acids 

 such as diiodotyrosine iodotryptophan and tetra-iodohistidine 

 when administered to mice fails to increase their resistance to 

 acetonitrile. 



Strouse and Voegtlin ('09-' 10) failed to observe any thyroid- 

 like effect on the nitrogen metabolism or on the blood pressure 

 of normal dogs, nor was there any favorable effect on the condi- 

 tion of myxedematous and cretinous mammals following admin- 

 istration of iodized amino acid. Other investigators have tried 

 in vain to obtain thyroid effects on mammals (metabolic changes) 

 by the use of various iodized substances; tri-iodo-imidazol and, 

 iodophenylalanie have also proven ineffective. 



If, however, we bear in mind the results of Kendall's experi- 

 ment, it becomes clear why iodized amino acids, e.g., iodotyrosine, 

 give negative results when administered to myxydematous and 

 cretinous mammals, and positive results when fed to thyroidless 

 amphibian larvae. In the latter group the metamorphic response 



