EDUARD UHLENHUTH 477 



Metamorphosis and Rate of Growth in Thymus-Fed LarvB. 



In a recent publication Janney^ has shown that while a certain 

 minimum amount of the thyroid hormone results in an increased pro- 

 tein breakdown, it finally leads to a positive nitrogen balance and an 

 increase in weight and size, since it not only accelerates the protein 

 breakdown but at the same time facilitates the assimilation of the 

 nitrogen into the proteins of the body tissues. This is shown also in 

 the metamorphosis of the amphibians where the initial breakdown of 

 the tissues, as demonstrated, for instance, by Morse^ on tadpoles, 

 and the decrease in size, resulting probably, as suggested above, from 

 the excretion of an overdose in the beginning of the functional period 

 of the thyroid, are followed soon by an increase in size and weight. 



On the other hand, Janney's experiments have shown that in cer- 

 tain diseases, such as exophthalmic goiter, the normal synthesis by 

 the thyroid of the thyroid hormone from iodine and certain organic 

 substances is disturbed, and on the basis of Swingle's experiments we 

 may assume that in these diseases the thyroid is unable to retain the 

 iodine consumed by the organism with the food. This assumption is 

 supported by the fact, as mentioned by Janney, that in exophthalmic 

 goiter the thyroid frequently is found very poor in iodine. But 

 since in this case the excretory function of the thyroid remains undis- 

 turbed, the thyroid is excreting, instead of the hormone, certain sub- 

 stances, probably the indole-containing amino-acid tryptophane 

 which normally is used to build up the hormone, but which in itself 

 is toxic causing a permanent protein breakdown without facilitating 

 assimilation of the food nitrogen; consequently a negative nitrogen 

 balance is brought about and a permanent loss of weight. 



If we apply this hypothesis to amphibian metamorphosis, we 

 should expect that in such larvae, which have not been able to take up 

 an amount of iodine sufficient for metamorphosis and whose thyroid 

 consequently was unable to develop the normal hormone, the action 

 of the excretor substance would lead to a prolonged and increased loss 

 in size of the larvae, without resulting metamorphosis, since in these 

 animals excretion by the thyroid would commence as soon as the 

 excretor substance reached the amount required, but the excreted 



5 Morse, W., Biol. Bull., 1918, xxxiv, 149. 



