IODINE AND AMPHIBIAN METAMORPHOSIS. 235 



impossible to make any definite statements on the point until 

 more is known about the relation of size to age in the larvae of 

 this urodele. 



The fact is well known that the thyroid gland of vertebrates 

 exhibits a remarkable selective action in regard to iodine absorp- 

 tion, taking this element from the blood and synthesizing it into 

 the thyroid hormone by the addition of other substances. In 

 view of this property of thyroid tissue, it is possible, though 

 rather improbable, that in the experiment just cited, the thyroid 

 apparatus of the iodotyrosine-fed animals took up the iodine 

 pouring into the organism and elaborated excessive quantities of 

 the hormone, thus inducing metamorphosis. However, the 

 rapid transformation of both thyroidless frog and salamander 

 larvae when fed or injected with iodized proteins and amino acids 

 renders such an assumption doubtful. 



It is interesting to note that Huxley and Hogben meta- 

 morphosed Salamandra and Triton larvae by rearing them in dilute 

 solutions of inorganic iodine. But in these experiments the thyroid 

 glands of the animals were intact, hence it is impossible to know 

 whether the action of the iodine was direct or through the 

 mediation of the thyroid. 



III. EXPERIMENTS ON Amblystoma punctatum. 



It was considered desirable to test the effects of iodo- and 

 bromtyrosine upon the metamorphosis of Amblystoma since it 

 was owing to negative results obtained by administration of 

 inorganic iodine to animals of this group that led Uhlenhuth to 

 assert that iodine has no effect upon salamander transformation. 



Eighty young larvae of Amblystoma punctatum, averaging 30 

 mm. total length were divided into four groups of twenty animals 

 each. The larva? of three groups were isolated in finger bowls 

 containing 50 c.c of w r ater, one animal to each container. The 

 twenty larvae remaining were reared in large aquaria and fed 

 quantities of tubifex. The animals in the finger bowls received 

 food only at definite intervals and in very small amounts so that 

 they were in a state of semi-starvation during the course of the 

 experiment. 



Twenty larvae received small amounts of diodotyrosine crystals 

 dissolved in the 50 cc. of water in the finger bowls. Twenty 



