IODINE AND THE THYROID. 



III. The Specific Action of Iodine in Accelerating Amphibian 



Metamorphosis. 



By W. W. swingle. 

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



(Received for publication, May 13, 1919.) 



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

 termined by the effects produced by feeding this substance and its 

 compounds to larval Anurans,^ the following conclusions were either 

 stated or impHed, all of which have a direct bearing on the present 

 experiments. (1) Inorganic iodine and its compounds, iodoform and 

 potassium iodide, greatly accelerate metamorphosis of tadpoles. 

 (2) Animals from which the thyroid gland had been removed at its 

 inception {i.e. 6 mm. larvae), and which under normal conditions 

 never undergo metamorphosis but grow to an abnormal size, quickly 

 transform into frogs when fed iodine. (3) The follicles of the thy- 

 roids of tadpoles on an iodine diet show a greater colloid content 

 than do the glands of normally fed animals. These facts led to the 

 conclusion that iodine is essential for amphibian metamorphosis, 

 that it is the active constituent of the thyroid glands of these ani- 

 mals, and, judging by its action on thyroidless tadpoles, that it 

 exerts its action directly upon the cells and tissues of the organism 

 without the necessity of undergoing transformation in the gland 

 tissue; i.e., that iodine is capable of functioning as the thyroid hor- 

 mone itself within the body, or else is transformed into this hor- 

 mone through the activity of tissue other than that of the thyroid. 

 (4) The chief function of the thyroid is the extraction from the 

 blood and storage of the minute quantities of iodine taken into the 

 organism in food and water, and the subsequent release of this sub- 



^ Swingle, W. W., Studies on the relation of iodin to the thyroid, J. Exp. Zool., 

 1918-19, xxvii, 397, 417. 



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