The Effect of Iodine and Iodothyrin on the Larvæ of Salamanders

In previous papers I have shown that feeding thymus gland to the larvae of salamanders retards or completely inhibits metamorphosis 1'~ as well as growth. 3 I t was also demonstrated ~ that the inhibition of both growth and metamorphosis is not caused by specific growthinhibiting substances contained in the thymus, but by a deficiency of the thymus in certain substances which are necessary for growth and metamorphosis. Later I suggested that these substances might be identical with iodine. 4 Recently, however, I found 5 that inorganic iodine even when administered in excessive quantifies does not accelerate either growth or metamorphosis of salamander larvae. Although this result does not necessarily mean that salamander larvae can grow and metamorphose in the complete absence of iodine, the correctness of my assumption that inhibition of growth and metamorphosis of thymus-fed salamander larvae may be caused by a deficiency of the thymus in iodine, became doubtful, and special experiments were carried out to test this assumption. In the light of these experiments which will be briefly reported below, it appears that the substances which are deficient in the food of thymus-fed salamander larvae are not identical with iodine.