ENDOCRINE SYSTEM OF TYPHLOMOLGE RATHBUNI. 317 



of the introduction to this article and of the facts described above ; 

 (3) the lack of an effective releasing mechanism may have been 

 the primary cause of the neoteny of Typhlomolge^ but so far 

 nothing as to this effect can be quoted ; (4) the question of whether 

 or not Typhlomolge and phsyiologically similar species, such as 

 Proteus and Necturus, possess, in their present state, the ability to 

 metamorphose upon thyroid administration is a problem entirely 

 aside from the role of the releasing mechanism and has been con- 

 sidered so in the writer's previous papers. (It is possible that 

 permanent suppression of the thyroid function over long periods 

 may cause complete loss of the reactivity of the organism. The 

 demonstration of such complete loss, however, does not decide 

 the question as to whether primarily the thyroid ceased to func- 

 tion, in Typhlomolge, on account of a defective releasing mecha- 

 nism) ; (5) it has not been proved as yet that Typhlomolge, Proteus, 

 and Necturus have completely lost their ability to react to the 

 thyroid hormone. It was merely this point which gave occasion 

 for the criticism of Jensen's work. Jensen 17 exposed only the 

 adult specimens of Proteus and Necturus to the action of the 

 thyroid hormone. That he did not succeed in enforcing meta- 

 morphosis does not necessarily mean that the responsiveness of 

 these animals has been completely lost. In order to show that 

 Necturus, Proteus, and Typhlomolge could not metamorphose even 

 if they were in the possession of a complete and normal thyroid 

 mechanism, the young larvae or even the parents at the time of 

 development or ripening of the ova and spermatozoa may have to 

 be subjected to thyroid administration. Swingle has merely 

 repeated Jensen's experiments on Necturus, without modifying 

 Jensen's technique. Like Jensen, he did not use the young 

 larvae, but the adult animals. Nowhere in Swingle's paper, how- 

 ever, can there be found any reference to Jensen's experiments on 

 Necturus. 



The same attitude is met with in Swingle's paper regarding the 

 releasing mechanism. Although no progress beyond the present 

 state of the problem has been accomplished, there is, in Swingle's 

 article, no mention made anywhere of previous work on the same 

 problem. 



That the thyroid mechanism of salamanders consists of two 

 physiologically distinct parts was found by the writer of this 



