RELATION OF lODIN TO THYROID 413 



portion is dissolved. It was soon found that both normal and 

 thyroidless animals are quickly killed by very weak dilutions. 

 Two cc. of such a solution of iodin in 500 cc. of water suffices to 

 kill both kinds of animals in a few hours. No conclusions could 

 be drawn from the results, save, perhaps, that flour when mixed 

 with iodin must in some way protect the tissues of the larvae 

 from the latter. 



Discussion of effects of feeding iodin to ihyroidectomized larvae. 

 The results of iodin feeding just recorded should in some measure 

 serve to clarify the conflicting views regarding the relation of 

 iodin to the physiological activity of the thyroid and inciden- 

 tally throw some light upon the causes underlying amphibian 

 metamorphosis. 



The various views regarding the relation of iodin to the thyroid 

 held by investigators to-day may be briefly summarized under 

 three heads: 1. Some are of the opinion that the activity of the 

 thyroid depends upon its iodin content and that thyroid free of 

 iodin has no physiological activity. 2. Another group of 

 writers take the view that there is no relation whatever between 

 the physiological activity of the thyroid and its iodin content; 

 i.e., that the iodin usually present has no importance in the 

 economy of the organism. 3. Still other investigators admit the 

 parallelism between physiological activity and iodin content, but 

 deny that iodin is the causal agent, believing rather it is simply 

 associated accidentally with the active principle of the gland. 



The effects of iodin feeding to normal tadpoles give additional 

 confirmation to the first of these views. But especially interesting 

 in this connection ai'e the results with feeding iodin to thyroid- 

 less larvae. If animals without the vestige of a thyroid gland 

 are stimulated to complete metamorphosis in an abnormally 

 short time by iodin, it would appear that iodin functions within 

 the organism as a hormone itself and that the gland functions 

 chiefly for storage purposes. The evidence from the thyroidee- 

 tomized larvae indicates that the animal body is capable of 

 utilizing iodin directly without the intermediation of the gland. 



The fact that the thyroid gland of man and animals does not 

 invariably contain iodin (shown by Miwa and Stoeltzner, Roos 



THE JOURNAL OP EXPERIMENTAL ZOOLOGY, VOL. 27, NO. 3 



