472 J. F. GUDERNATSCH 



1911, yet this again may be merely incidental. There is no be- 

 ginning of differentiation in thymus-fed tadpoles which would 

 check a further growth. 



We can only say with reasonable safety: 



1. The thyroid possesses a quality that stimulates differentia- 

 tion, not contained in any other food used.- 



2. The thymus (and spleen) possess a quality that suppresses 

 differentiation not contained in any other food used. 



Thus the thyroid and thymus must produce, or at least con- 

 tain, agents which, when passed into developing organisms, in the 

 one case stimulate, in the other suppress, differentiation. The 

 production of such substances to be thrown into the circulation 

 characterises the thyroid and thymus as glands with a positive 

 internal secretion. That these two types of tissues may also be 

 capable of performing the reverse action, viz. the elimination of 

 certain substances from the circulation, as has been assumed 

 especially for the thyroid, is neither demonstrated nor denied by 

 these experiments. 



The other glands used in the studies may or may not contain 

 either the accelerating or the depressing power. However, the 

 macroscopic differences in the rate of growth and differentiation 

 of tadpoles fed on such glands and of the control animals are only 

 slight and might easily be attributed to differences in the nutri- 

 tive values of the various foods. The feeding experiments on tad- 

 poles are, therefore, not likely to reveal these factors, if at all 

 present in other glands, in a striking degree. Their study must 

 be left to experiments of a different kind. 



2 By 'differentiation' is meant merely the macroscopic changes, hind, fore- 

 limbs, metamorphosis. The microscopic differentiation will be discussed in a 

 later paper. 



