470 J. F. GUDERNATSCH 



Not one of the many thousand tadpoles fed on thymus in the 

 sprmg of 1912 could be brought to metamorphosis during the 15 

 weeks in which the animals were under observation, while tad- 

 poles of the same set, though fed on other substances, metamor- 

 phosed before that time (fig. 4 o, p, fiver). In some experiments 

 the thymus fed tadpoles never succeeded in growing their hind 

 extremities before the controls completely metamorphosed. Yet 

 the thymus tadpoles would grow very rapidly, especially during 

 the first weeks of the experiments. 



The tadpoles feed on spleen behaved in very much the same 

 way as the thymus-fed ones, though they always were somewhat 

 ahead of the latter and did not counteract the thyroid feedings 

 so strongly. Of the great number of spleen fed tadpoles also not 

 one could be caused to metamorphose. 



"We must therefore conclude that the thymus and to some ex- 

 tent the spleen also, and probably the lymphatic organs, when 

 given as food, will cause a rapid growth followed by a rather late 

 differentiation or none at all.. 



These experiments with various foods, except thyroid, thymus 

 and spleen, and the investigations of other workers show that the 

 animals must reach a certain constant minimum size before the 

 final metamorphosis can begin. There is on the other hand a 

 constant assimilation of food and a gradual increase in body size 

 up to a constant maximum, beyond which a normal animal may 

 not pass without the beginning of the final differentiation and 

 metamorphosis. 



In the thyroid-fed tadpoles there is differentiation without 

 growth, in the thymus- (and spleen- ) fed tadpoles growth with- 

 out differentiation. The experiments, therefore, emphasize the 

 fact that in development we deal with two entirely separate factors, 

 the factor of growth and the factor of differentiation. 



The two naturally work simultaneously; but differentiation is 

 not the result of growth, otherwise there could be no thyroid differ- 



