310 INFLUENCE OF THYMUS 



the larval organism in such a large amount that metamorphosis oc- 

 curred in this series simultaneously with the controls. The ability 

 of the normal food to counteract the inhibitive effect of the thymus 

 upon metamorphosis was the more conspicuous as the same food 

 was quite ineffective in preventing the tetanic convulsions produced 

 by the tetany toxin present in the thymus; this may be seen from 

 the curves which indicate the percentage of tetanic animals found 

 at the time of observation among the thymus-fed and mixed food 

 series.^ 



That the action of the thymus is merely due to the absence of the 

 substance necessary to produce metamorphosis, is also demonstrated 

 in experiments in which thymus-fed larvae are placed in a solution of 

 thyroid substance. If the thymus contained a specific metamorphosis- 

 preventing substance, one would expect an antagonistic neutralization 

 of the thyroid substance by the thymus substance. But instead the 

 thyroid substance even when present in minute quantities induces 

 prompt metamorphosis in the thymus-fed animals. For the sake of 

 illustration one experiment may be reported here. A set of six 

 thymus-fed larvas of Ambystoma opacum was fed on thymus. At an 

 age of 6 weeks the larvae were placed in a 0.02 per cent solution of 

 iodothyrin which after about 20 hours was replaced by a 0.006 per 

 cent solution. 8 days after the beginning of the thyroid treat- 

 ment all the larvae were metamorphosed although fed on thymus, 

 while the controls not treated with iodothyrin needed from 6 to 7 

 weeks more to metamorphose. In this series again we observe that 

 the effects of the thymus, which actually are due to the presence of a 

 speciaP substance in the thymus, are not stopped by the thyroid 

 treatment. The larvae exhibited severe tetanic convulsions caused 

 by the tetany toxin of the thymus; these convulsions occurred in 

 spite of the thyroid treatment with undiminished strength. 



^ For detailed discussion see Uhlenhuth, /. Gen. Physiol. , 1918, i, 33. 



^ The term specific in connection with the active principle of the thyroid 

 gland has been avoided here, for it does not seem to be proved definitely that the 

 effects exerted by the iodothyrin cannot be brought about by any other sub- 

 stance or any other factor. Since the term specific refers not only to the origin 

 of the inner secretory substances but also to their effects, it is misleading in 

 connection with the thyroid substance. 



