FEEDING EXPERIMENTS ON TADPOLES 471 



entiation without growth. Nor must growth necessarily be fol- 

 lowed by differentiation, as seen in the growing th^oiius tadpoles. 



One might say that ordinary foods which are being assimilated 

 bring with them the two factors of growth and differentiation. 

 Then the assmnption is necessary that the thyroid food lacks the 

 power of causing growth, while the thymus and spleen lack the 

 power of causing differentiation. But we can hardly assume that 

 all the various kinds of foods an animal may take in, with the only 

 exceptions of thyroid and thymus, contain these two factors in the 

 proper proportions. 



The factors for growth and differentiation can only be located 

 within the organism itself. 



We face the following propositions: 



1. The thyroid has the power to excite differentiation, but it 

 lacks the power to cause growth. 



The thyroid calls forth differentiation, whether the animals 

 be small or large, and without regard to the standard minimum 

 size, necessary for the final change. It must possess an agens for 

 stimulating differentiation which other foods do not possess. 

 That the thyroid also possesses a power which prevents growth is 

 not evident. The suppression of growth may merely be inci- 

 dental, for rapid differentiation does not allow growth. 



2. The thymus has the power to stimulate growth, but lacks 

 the power to excite differentiation. 



It has been stated above that tadpoles feeding on any food 

 (except thyroid) reach a maximum size, and when this maximum 

 is reached, differentiation begins independently. The th;>Tnus-f ed 

 tadpoles reach this maximum size and differentiation should set in 

 of itself, even if the thymus lacked the necessary stimulus. But 

 differentiation does not begin; therefore the thymus (and spleen) 

 must exert an influence not possessed by the other foods which 

 suppresses differentiation. That the thymus possesses the power 

 to stimulate growth, is again not so evident, since the th3anus 

 growth may be merely the normal result of the intake of food. 

 The thymus growth is rather rapid, but this may be attributed to 

 the better nutritive qualities of the thymus tissue. The thymus- 

 fed tadpoles may also grow bej'ond the normal as brought out in 



