Significance of the Internal Secretory System 143 



so many proofs for their existence have been obtained that 

 the idea is no longer questioned. Such substances are known 

 now under the name of 'internal secretions' or 'hormones'; 

 their connection with the theory of Sachs was forgotten with 

 the introduction of the new nomenclature." l 



A case to which Loeb makes special reference as proof 

 that internal secretions are the same as Sachs' organ-form- 

 ing substances is that of the effect of thyroid on the meta- 

 morphosis of the tadpoles of frogs and toads, demonstrated 

 by Gudernatsch. The author's mode of using this case in 

 illustration of his contention is highly instructive. He re- 

 fers to the legless condition of the tadpoles and calls atten- 

 tion to so much of Gudernatsch's results as pertain to these 

 members. Gudernatsch found, Loeb points out, that where- 

 as in the usual course of things the tadpoles live from four 

 months to a year before the legs grow out, by feeding them 

 on thyroid gland these members can be induced to appear 

 at any time. "We must, therefore, draw the conclusion," 

 Loeb says, "that the normal outgrowth of legs in a tadpole 

 is due to the presence in the body of substances similar to 

 the thyroid in their action (it may possibly be thyroid sub- 

 stance) which are either formed in the body or taken up in 

 the food." When the case is presented in this way and 

 nothing more said about it, it certainly looks considerably 

 as if the thyroid substance or something like it is leg-form- 

 ing substance; and such an interpretation would be enticing 

 were it really true, as Loeb says, that no other substance 

 seems to have such an effect. 



When, however, one consults the account given by Guder- 

 natsch himself as to what his experiments were and what 

 they established, the whole matter stands in a quite different 

 light. First of all, the fact that in these experiments mam- 

 malian thyroid was largely used as food for the tadpoles, thus 

 bringing it to pass that if thyroid substance was specific 

 organ-forming substance, then frogs' legs were produced by 



