EDUARD UHLENHUTH 311 



DISCUSSION. 



From the above experiments it is evident that an exclusive thymus 

 diet sometimes can retard or even prevent metamorphosis. But 

 while the abihty of the thyroid to enforce metamorphosis is due to 

 the presence in the thyroid of a special substance, the inhibitory 

 action of the thymus is due to the absence of a substance without 

 which metamorphosis is impossible. 



In order to appreciate this fact fully we must remember the ex- 

 periments performed by Allen^ and Hoskins,^ which have demonstrated 

 that tadpoles whose thyroid glands have been extirpated are unable to 

 metamorphose. This means that under normal conditions at the 

 time of metamorphosis the thyroid begins to excrete the metamor- 

 phosis-producing substance which under experimental conditions is 

 introduced into the organism by feeding thyroid to the larvae or keep- 

 ing them in a solution of thyroid substance. Since under normal 

 conditions no thyroid is fed to the animals and since it is the thyroid 

 of the animal itself which excretes the substance in question, we 

 must assume that the normal food of the larvae contains a substance 

 which is necessary to develop the thyroid of the larva to a state 

 in which it can excrete the metamorphosis-producing substance. 

 Whether or not the substance necessary to develop the thyroid and 

 furnished in normal conditions by the normal food of the larvae is 

 identical with the metamorphosis-producing substance excreted later 

 on by the thyroid, cannot be decided at present; but in this respect 

 the- attempts made by Allen^ to enforce metamorphosis of thyroid- 

 less larvae by feeding them on thyroid are important. If it is possible 

 to enforce metamorphosis in thyroidless larvae by feeding them on thy- 

 roid, but impossible by feeding them normal food, the substances 

 contained in the normal food are able to develop the thyroid to the 

 excreting stage, but they are unable to evoke metamorphosis in the 

 absence of the thyroid, and, therefore, they are not identical with the 

 thyroid substance. The results so far obtained by Allen point in the 

 latter direction. It is this substance, necessary to develop the secre- 

 tory stage in the thyroid, which is missing in the thymus. 



« Allen, B. M., Science, 1916, xliv, 755; /. Exp. ZooL, 1917-18, xxiv, 499. 

 ' Hoskins, E. R., and Hoskins, M. M., Froc. Soc. Exp. Biol, and Med., 1917-18, 

 XV, 102. 



