FEEDING EXPERIMENTS ON TADPOLES 469 



kind of food that had been given before the thyroid diet was 

 started. There is also a difference in response between previously 

 starved and fed tadpoles. The experiments with mixed diets 

 showed that the accelerating influence of the thyroid could be 

 checked to some extent and the rapid differentiation more or less 

 retarded. However, there is no complete counteraction against 

 the thyroid stimulus, so that sooner or later any tadpole, receiv- 

 ing thyroid after other or mixed with other diets, must respond to 

 its influence. 



Experiments were carried out to determine the least amount of 

 thyroid food necessary to produce the typical reactions, and also 

 to determine whether or not the tadpoles could recover from the 

 thyroid shock if afterwards put on other food. A feeding of only 

 24 hours — that is to say, the thyroid was kept in the dish for 

 about 24 hours, though the animals did not feed on it continuously 

 — sufficed to cause a hastened differentiation. A feeding for three 

 days was enough to give the fastest rate of differentiation, a rate 

 that could not be increased by longer feedings. 



When thyroid is applied too rapidly, the animals usually die 

 very soon after the appearance of their fore limbs and the simul- 

 taneous reduction of their tails. By careful feedings at rather 

 long intervals and in all not more than four times, the animals maj' 

 be kept alive for several weeks. They will not undergo, however, 

 any further changes, except perhaps a continued reduction of 

 their tails, nor will they ever feed again. In 1911 I succeeded m 

 bringing some tadpoles to a complete absorption of then* tails and 

 these thyroid frogs were kept alive on wet sand for from 2 to 



4 days. In 1912 some Bufo tadpoles almost completely ab- 

 sorbed their tails under the thyroid treatment (fig. 9 e, 9 n), but 

 could not be kept alive for more than 24 hours. 



A recovery from the thyroid influence is extremely rare. Only 



5 individuals out of 400 in one experiment and 6 in another 

 were able to survive, and, although, they were never seen to feed, 

 began slowly to grow again after a standstill of several weeks. 



