• W. W. SWINGLE 169 



There can be little doubt that the iodine of the thyroid is obtained 

 by the organism from the food and water. Marine and Rogoff ^ have 

 clearly shown that the affinity of thyroid tissue for iodine is so great 

 that practically all iodine injected into the organism by experimental 

 methods is "fixed" in the thyroid within a few minutes. Their 

 experiments explain why it is that considerably more iodine is re- 

 quired to induce metamorphosis in thyroidless frog larvse than in 

 animals possessing glands. 



The importance of iodine in the economy of the organism is at 

 present little understood and much underestimated. The writer 

 holds the view that this halogen will eventually take rank along with 

 chlorine, phosphorous, and other elements essential to the mainte- 

 nance of normal metabolism, growth, and development. The almost 

 universal occurrence of a thyroid mechanism among the vertebrates 

 for the assimilation and utilization of iodine in minute quantities, 

 points to the conclusion that there is a definite iodine metabolism in 

 these forms, and that it is necessary for the normal functioning of 

 vital processes. 



That elemental iodine functions within the thyroidless organism as 

 such, is highly improbable; it is far more likely that it is combined 

 with other substances in the blood and tissues, though what these 

 substances are is unknown, just as it is unknown how the presence 

 of the thyroid affects the iodine. In either case, however, the result 

 is the same in frog larvae; i.e., normal metamorphosis. The fact that 

 more iodine is required to bring about metamorphosis in thyroidless 

 animals than in normal ones may indicate either that the thyroid 

 gland tissue has added something to the iodine, thereby increasing its 

 physiological activity, or that the gland has functioned as a storage 

 organ and fixed the iodine in greater quantities, thus preventing its 

 passage through the organism unused. It may well be that both 

 these possibilities are realized in normal animals with intact glands. 

 It is to this latter view that the writer adheres. 



A recent writer^ has stated that iodine has nothing to do with limb 

 development in the larvee of Urodela, and doubt is expressed that con- 



^ Marine, D., and Rogoff, J. M., /. Pharm. and Exp. Therap., 1916, viii, 439. 

 8 Uhlenhuth, E., /. Gen. Physiol., 1918-19, i, 534. 



