316 EDUARD UHLENHUTH. 



Throughout his paper Swingle attempts to create in the reader's 

 mind the impression that in my previous work I have laid too 

 much one-sided emphasis on the thyroid gland as the only organ 

 potent in the amphibian metamorphosis. This attitude is per- 

 plexing in view of the circumstance that I have been able to 

 disclose facts demonstrating the existence of a releasing mecha- 

 nism outside of the thyroid and in view of the other circumstance 

 that Swingle received the discovery of this "releasing mecha- 

 nism" when communicated by the writer in 19 19 u in the following 

 way 15 (p. 600): "Uhlenhuth, while accepting the conclusions 

 stated regarding the relation of iodine to amphibian metamor- 

 phosis, thinks that still another substance is needed to cause the 

 thyroid gland to excrete the iodine necessary for metamorphosis. 

 This hypothetical factor he terms excretor substance and thinks 

 that it is evolved during the growth processes of the organism. 

 The assumption of an excretor substance obscures rather than 

 clarifies the already sufficiently complicated problem of amphi- 

 bian metamorphosis." 



As to Swingle's criticism regarding the omission, on my part, 

 of the possible defectiveness of the releasing mechanism in the 

 explanation of the neoteny of Typhlomolge, it should be pointed 

 out that the writer of this article has, reluctantly, refrained from 

 suggesting this possibility, because no experiments suggesting it 

 have been or are today available. As to the actual interpre- 

 tation of the neoteny of Typhlomolge and as to my attitude 16 to- 

 wards Jensen's 17 experiments which showed that adult Proteus 

 and Necturus do not metamorphose upon thyroid administration, 

 the following statements may be made: (i) the interpretation 

 of this phenomeqon as given in my previous papers 16 does not 

 form an integral part of the theory of a releasing mechanism; 

 (2) the most pertinent problem in regard to the neoteny of Typhlo- 

 molge was the question whether or not this animal possesses a 

 thyroid gland proper, a question to the solution of which Swingle 

 has contributed nothing, as will become evident from a perusal 



"'Uhlenhuth, E., Jour. Gen. Phys.> 1919, I., 473. 



15 Swingle, W. W., Jour. Gen. Phys., 1919, I., 593. 



16 Uhlenhuth, E., American Naturalist, 1921, LV., 193. 



17 Jensen, C. O. Oversigt klg. Danske Vidensk. Forhandl., Copenhagen, 1916, No. 

 3. 251. 



