IODINE AND THE THYROID. 



IV. Quantitative Experiments on Iodine Feeding and 

 Metamorphosis. 



By W. W. swingle. 



(From the Department of Biology, Princeton University, Princeton.) 

 (Received for publication, September 15, 1919.) 



This paper deals with experiments undertaken with the object of 

 determining, by quantitative feeding, the approximate amount of ele- 

 mental iodine necessary to induce complete metamorphosis in normal 

 and thyroidless toad larvae, reared under identical environmental 

 conditions. 



In previous studies on the relation of iodine to the thyroid,^ as de- 

 termined by feeding this halogen and its compounds to tadpoles, the 

 experimental evidence led to the suggestion that probably the chief 

 function of the thyroid apparatus, at any rate in the Anura,.is the ex- 

 traction from the blood, and storage of the extremely minute quanti- 

 ties of iodine taken into the body by means of food and water, and the 

 subsequent release of this substance perhaps in a modified form, as 

 the thyroid hormone. The purpose of this experiment was to test 

 the validity of this conclusion, for it is evident, that if the thyroid ap- 

 paratus in Anura serves the purpose of collecting, storing, and trans- 

 forming the incoming iodine into a more active hormonic agent, then 

 tadpoles possessing normal thyroid glands should react more promptly 

 by metamorphosis to iodine feeding than tadpoles of similar age and 

 parentage, whose thyroids had been removed before the period of 

 their first functioning, and hence were devoid of the requisite mechan- 

 ism for collecting, storing, and transforming iodine. Moreover, it 

 was hoped that such an experiment would show whether or not there 

 is an irreducible minimum quantity of iodine required by anurans 

 for metamorphosis. 



1 Swingle, W. W., /. Exp. Zool., 1918-19, xxvii, 397, 417; J. Gen. Physiol, 

 1918-19, i, 593; Endocrinology, 1918, ii, 283. 



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