EDUARD XJHLENHUTH 307 



larvae of the foregoing series, the larvae were fed on thymus and with 

 the largest possible amount of this diet ; otherwise they were kept like 

 the worm-fed controls. The average time of metamorphosis in this 

 series was 17 weeks and 5 days; i.e., less than in the worm-fed series. 

 In this number, however, two larv£e are not included, both of which 

 died before metamorphosis; one of them reached the age of 31 weeks 

 and 2 days, the other 34 weeks and 1 day without having metamor- 

 phosed. Thus in this series again a considerable delay (if not an in- 

 hibition) of metamorphosis was produced by the th3rmus diet in two 

 animals. 



A third species, Amby stoma tigrinum, was experimented on. This 

 species, as mentioned in a recent publication^ shows the least effects 

 when fed on thymus. Among six animals kept at approximately 

 25°C. and fed on thymus, the larvae metamorphosed simultaneously 

 with the controls. Among six other animals kept at approximately 

 15°C. and fed on thymus, only five larvae metamorphosed simultane- 

 ously with the controls (between 22 and 26 weeks after hatching), 

 while one individual though now over 74 weeks old is still in larval 

 condition. Thus also in this species thymus feeding apparently 

 resulted in a considerable retardation of metamorphosis though only 

 in one individual. 



From these experiments on urodelan larvae it is again evident that 

 the effect of the thymus is extremely variable. But it is also a fact 

 that in some of the larvae metamorphosis was retarded by the thymus 

 diet and in two probably completely prevented. Further experi- 

 ments, therefore, were carried out to determine this point. 



Inhibitive EJ'ect of Thymus upon Amphibian Metamorphosis Is a 

 Deficiency Phenomenon. 



It is clear that the considerable variability of the action of the 

 thymus cannot be explained on the assumption that the inhibitive 

 effect of that gland is due to the presence of a specific inhibiting sub- 

 stance in the thymus. But it can be explained if this effect is due to 

 the absence from certain parts of the thymus of a substance necessary 

 to produce metamorphosis and which is contained, in minute quan- 



^Uhlenhuth, E., /. Gen. Physiol., 1918, i, 23. 



