FEEDING EXPERIMENTS ON TADPOLES 

 TABLE 5 



447 



ings. The figures in the other columns give the number of days 

 required by each set in developing hind and fore legs. 



The similarity of the results in the two series is striking. There 

 is a gradual decrease in the rapidity of action of the thyroid 

 influence up to the age of about 5 weeks followed by a steady 

 increase after this time. Whether or not this is a general rule 

 must be decided by further experiments. It is also difficult to 

 give a satisfactory explanation for this phenomenon, yet the fol- 

 lowing suggestion may be advanced. The young tadpoles were 

 very quickly affected by the thyroid feedings because the pre- 

 vious thymus diet had not acted long enough to delay or counter- 

 balance the thyroid stimulus. Older animals had enough thymus 

 agens accumulated to retard the thyroid action. Still older tad- 

 poles were more nearly approaching the normal time of meta- 

 morphosis and may thus have been ripe to respond to an acceler- 

 ating stimulus. The last argument, however, seems especially 

 weak, since it introduces a new factor in the action of thymus 

 diet; namely, that if it is prolonged, it loses the refarding influ- 

 ence and thus is less able to counteract the thyroid. This factor 

 has not yet been demonstrated ; thus no conclusions as to the 

 rapidity of the thyroid influence on different age tadpoles can be 

 based upon it. 



THE AMEMCAN JOrRNAL OF ANATOMY, VOL. 15, NO. 4 



