Relation between Thyroid Gland, Metamor-

I t has been demonstrated by a number of workers that the active principle of the thyroid gland in the causation of amphibian metamorphosis is iodine. Recently Swingle 1 has obtained definite proof that inorganic iodine when fed to larvm of Salientia induces metamorphosis in a short time after the be#nning of the feeding. On the other hand, it has been shown by the writer 2 that the retarding influence of the feeding of thymus upon amphibian metamorphosis is due to the absence from the thymus gland of a substance required for metamorphosis, and it is possible that this lacking substance is iodine. Although minute amounts of iodothyrin have been found in some thymus glands, it is likely that the amounts found are insufficient to produce metamorphosis and that some thymus glands may not contain the iodine at all. This would account for the variability of the results obtained with thymus feeding. Swingle 1 also subjected tadpoles deprived of their thyroid to an iodine diet; although such larvm when kept on a normal diet never metamorphose, they very soon metamorphosed when fed on iodine crystals. Hence it is manifest that iodine is one of the substances involved in the causation of amphibian metamorphosis. The quantities contained in normal food are, however, so small as to have no immediate effect upon the organism. If the larvm have no thyroid glands, the small quantities of iodine taken up with the food apparently cannot be retained by the organism and the iodine leaves the body without