ENDOCRINE SYSTEM OF TYPHLOMOLGE RATHBUXI. 319 



has been found in the iodine experiments. 10 - 1S - 19 - 20 Adminis- 

 tration of an excess of inorganic iodine does not enforce the meta- 

 morphosis of salamander larvae, 10 - 20 yet the elaboration of the 

 colloid is accelerated by iodine feeding. This result was to be 

 expected if the release of the hormone does not depend on the 

 quantity of hormone developed in the follicles of the thyroid but 

 is controlled by a particular releasing mechanism. 



The results outlined above were checked also by histological 

 sections of large numbers of thyroids of normal and experimental 

 animals. Although the publication in full of this work has been 

 postponed in order to assure greater completeness, single results 

 have been referred to in various papers and have been demon- 

 strated to colleagues and before meetings. In every case it was 

 found that the elaboraion of the colloid and the excretion of it 

 are two distinct and independent processes, physiologically as 

 well as structurally. Elaboration of normal colloid is frequently 

 met with in cases of inhibition of metamorphosis and in normal 

 larvae long before metamorphosis, and, in this case, is combined 

 with complete absence of the structures characteristic of the 

 excreting stage of the thyroid. This relation has been interpreted 

 as further testimony in favor of the existence of a releasing 

 mechanism. 



I must also refer here to Swingle's criticism of my iodine ex- 

 periments, since, if correct, it would question the value of these 

 experiments as supporting the theory of the releasing mechanism. 

 My experiments 10 - 20 showed that, contrary to anuran larvae, in 

 the larvae of salamanders metamorphosis cannot be enforced by 

 the administration of inorganic iodine. The bearing of this fact 

 upon a general theory of the role of iodine in the specific effect 

 of the thyroid hormone has been outlined in detail in two previous 

 papers. 10 - 20 Swingle's general attitude in his paper tends to 

 create the impression (i) that I have claimed "iodine has nothing 

 to do with the axolotl metamorphosis" 13 (p. 417) and (2} that 

 somewhere in my papers are to be found statements to the effect 

 that organic iodine compounds cannot enforce the metamorpho- 

 sis of the axolotl and other salamanders. 



18 Uhlenhuth, E., Jour. Gen. Phys., 1922, IV., 319. 



19 Uhlenhuth, E., BIOL. BULL., 1921, XLI., 307. 



20 Uhlenhuth, E., BIOL. BULL., 1922, XLII., 143. 



