■ EDUARD UHLENHUTH 35 



mals suffering from tetany was smaller on a mixed diet than on pure 

 thymus diet. This is due to several factors. 



First, it was pointed out in a previous article^ that the salamander 

 larvae, though they have no parathyroids, must possess some other 

 mechanism capable of doing away with a certain limited amount of 

 the tetany-producing poison; otherwise even normal larvae not fed on 

 thymus should shov; tetany as soon as their own thymus glands are 

 sufficiently developed. In one species {Amhy stoma tigrinum) this 

 mechanism is strong enough to prevent tetany even if by means of 

 feeding thymus exclusively the body is flooded with a large amount of 

 the tetany toxin. Hence, it is not surprising that if a smaller dose of 

 the toxin is introduced into the body, the larvae should be capable of 

 removing — or antagonizing — so much of the toxin that the remaining 

 part may be less effective than a maximum dose would be. Hence 

 the fact that tetanic attacks occurred less frequently in each single 

 individual on a mixed diet than on an exclusive thymus diet is due 

 to the difference in the amount of thymus substance. 



A second factor is still more important as regards the decrease of 

 tetany in the mixed food series than the first factor mentioned. It 

 has been pointed out in another communication^ that larvae of the 

 species of Amby stoma maculatum and Amby stoma opacum cease to show 

 tetanic convulsions after metamorphosis, probably because of the 

 action of the parathyroids developing during metamorphosis. Meta- 

 morphosis started in the mixed food series 7 weeks earlier than in the 

 pure thymus series, and was finished in the former series 19 weeks 

 earlier than in the latter series. 



We have alluded to the fact that some authors believe they have 

 demonstrated that thymus feeding retarded the metamorphosis of 

 the tadpoles of frogs and that this effect was specific. This conclusion 

 is not correct. It became clear in the course of our study of amphibian 

 metamorphosis^ that the retardation of metamorphosis upon exclusive 

 thymus feeding is probably not due to a specific effect of the thymus, 

 but to a deficiency in the diet. If retardation of metamorphosis upon 

 thymus feeding were due only to the absence of certain substances 

 from the thymus and tetany were due to the presence of certain toxic 



^ Uhlenhuth, E., J. Exp. ZooL, 1918, xxv, 135. 



