32O EDUARD UHLENHUTH. 



As to the first point I should like to refer the reader to the fol- 

 lowing statement, 10 (p. 114) into which my results on the role of 

 iodine were summarized : ' That iodine if supplied in excess does 

 not produce metamorphosis of salamander larvae does not mean, 

 according to what has been said above, that it is not necessary 

 in the metamorphosis of salamanders. Very likely if larvae of 

 salamanders would be raised on an iodine-free diet and kept in 

 iodine-free water, metamorphosis could not take place." 



Regarding the second point Swingle quotes against me his 

 own experiments 13 > 21 in which he thinks he has shown that 3-5 

 di-iodo-tyrosine can enforce metamorphosis of thyroidectomized 

 axolotls, and Jensen's experiments (22) with iodized proteins. 

 Neither Swingle's own experiments nor Jensen's experiments 

 referred to have proved that inorganic iodine can be utilized 

 directly by the axolotl tissues to elaborate the thyroid hormone. 

 The facts regarding the influence of inorganic iodine on the 

 axolotl metamorphosis are, however, widely different from what 

 Swingle would like them to be. 



In the first place, Jensen has not only not shown that inor- 

 ganic iodine does enforce metamorphosis of the axolotl, but on 

 the contrary has shown that inorganic iodine as such is ineffective 

 in the axolotl metamorphosis. In one of his papers, Jensen 23 

 points out that the effectiveness of thyroid preparations in en- 

 forcing the axolotl metamorphosis does not correspond to the 

 iodine-content of these preparations. In a personal conversation, 

 Professor C. O. Jensen told me that he had tested the action of 

 inorganic iodine, but found it ineffective in enforcing the axolotl 

 metamorphosis. Jensen's experiments are therefore entirely 

 in accord with my own experiments. Moreover, Professor 

 Jensen's experiences which are well in accord with my own ob- 

 servations may serve as a warning against the reliability of those 

 experiments which resulted in "enforced metamorphosis" of the 

 axolotl. Among Professor Jensen's strains of the European race 

 of the axolotl there were, in the beginning, animals which gave 

 rise to offspring 50 per cent, of which would metamorphose 



21 Swingle, W. W., Science, 1922, X. S., LVI., 720. ' 



22 Jensen, C. O., Compl. rend. Soc. Biol., 1920, LXXXIII., 315; 1921, LXXXIV. 

 423; 1921, LXXXV., 391. 



23 Jensen, C. O., Hospilalstidende, 1920, LXIII., 505. 



