468 



J. F. GUDERNATSCH 



TABLE 15 



influence of different diets on the regeneration of the tail in 

 Rana esculenta tadpoles. 



The experiments of 1911 and 1912 leave no doubt that the 

 mammalian thyroid gland contains an agens which, when the 

 gland is given as food, calls forth a rapid differentiation in a 

 developing vertebrate organism. This differentiation may be 

 brought about at any stage of development, before maturity is 

 reached. The differentiation may therefore be highly precocious 

 in cases where the treatment is begun on extremely young ani- 

 mals. Experiment I showed that tadpoles could be brought to 

 the point of metamorphosis within 18 days after hatching, while 

 normally they would require 10 to 12 weeks to reach such a stage. 

 The results of this premature metamorphosis are perfect frogs 

 of minute size, pygmy frogs, as the figures show (fig. 4 o, 4 p and 

 others). The thyroid influence is very decided and there is no 

 escape from it for any tadpole given a thyroid diet. All of the 

 individuals, even if thousands be employed as was the case in 

 several experiments, will react almost immediately to the th}^- 

 roid stimulus, so that certain changes in their structure may be 

 observed after 24 hours, when only one application of thyroid 

 has been made. They will all react simultaneously so that any 

 individual differences in development will become unobservable. 



The experiments further bring out the fact that the time of re- 

 action to the thyroid stimulus varies to a certain degree with the 



