IODINE AND AMPHIBIAN METAMORPHOSIS. 247 



imino group in the thyroxin is replaced with acetyl, the substance 

 loses its physiologic activity and there follows no demonstrable 

 effect upon the metabolic rate. This emphasized the importance 

 of the imino group in thyroxin (in so far as the metabolic effect 

 upon mammals is concerned) and minimizes the importance of 

 the iodine in the molecule. Bearing in mind the effect of iodine 

 administration upon tadpole metamorphosis, Kendall was led 

 to try the acetyl derivative of thyroxin on tadpoles, for if meta- 

 morphosis depends only upon the increase in the basal metabolic 

 rate of the larvae then thyroxin should increase the rate of meta- 

 morphosis but the acetyl derivative involving the imino group 

 should not. If, however, iodine alone is concerned in accelerating 

 metamorphosis, then both thyroxin and the derivative should 

 affect the transformation. Kendall found that both thyroxin 

 and the acetyl derivative would induce a rapid metamorphosis 

 of the bull frog tadpole. Kendall's conclusion was that thyroxin 

 appears to have two separate and distinct functions: the effect 

 upon the metabolic rate which is brought about by the CO-NH 

 groups within the molecule; and the physiological changes in- 

 volved in the metamorphosis of the tadpole due to the iodine 

 contained in the molecule. Our own experiments have demon- 

 strated that this action of iodine is not specific to thyroxin, but 

 can be obtained in thyroidless amphibians (though the effects 

 are not so rapid, and the amounts administered must be larger) 

 by a large number of other iodine compounds and by adminis- 

 tration of elemental iodine itself. 



Kendall's experiment sheds considerable light on the reason 

 for the conflicting results obtained by investigators working with 

 iodine, iodized proteins and amino acids on mammals, and the 

 students of amphibian metamorphosis. In mammals the criterion 

 employed for testing the phsiologic action of iodine and thyroid 

 upon the organism is the effect upon metabolism as indicated by 

 changes in the nitrogen excretion, CO 2 elimination and oxygen 

 consumption ; in amphibians the criterion has been the rate and 

 degree of the degenerative and regenerative processes incident 

 to metamorphosis. However, it is becoming clear that the two 

 types of physiologic response are not in the same class and hence 

 not to be compared because they owe their origin to different 

 causes. The unique effects of thyroid or thyroxin upon the meta- 



