322 EDUARD UHLENHUTH. 



It is evident that none of the observations according to which 

 inorganic iodine does enforce the metamorphosis of salamanders 

 can be accepted as correct at the present time. 



That organic iodine compounds may enforce the metamorphosis 

 of neotenous forms of salamanders has been claimed repeatedly 

 and may be true, although the axolotl used generally in these 

 experiments appears, for reasons stated above, to be an unrelia- 

 ble material. Jensen was the first one who studied, in an exten- 

 sive manner, the influence of organic iodine-compounds upon the 

 metamorphosis of axolotls. Where he left the problem it is still 

 at the present time. In particular, Jensen deserves the credit for 

 having recognized that the experiments with iodine could not 

 advance the problem unless thyroidectomized larvae are used. 

 He was the first one who administered organic iodine compounds 

 to thyroidectomized axolotls and stated 26 that thyroxine can be 

 used directly by the organism without the intermediation of the 

 thyroid. Swingle repeated these experiments 13 - 21 using 3-5 

 di-iodo-tyrosine, a substance which Jensen 27 had found ineffec- 

 tive in the normal axolotl. Swingle reports that 3-5 di-iodo- 

 tyrosine does enforce the metamorphosis of thyroidectomized 

 axolotls. Both Jensen's and Swingle's experiments, however, 

 should be taken with caution as far as the successful thyroidec- 

 tomy is concerned. I am not certain at all that Swingle realizes 

 that an axolotl possesses 4 thyroid glands, two main portions and 

 two accessory ones. He mentions it nowhere and it is likely that 

 only the main portions were extirpated. The accessory thyroid 

 glands of A. tigrinum have a tendency to become very large and, 

 after removal of the main portions, may enlarge considerably, so 

 as to cause finally metamorphosis, as I observed in many larvae 

 of A. tigrinum. It is likely that Swingle's "thyroidless" axolotls 

 were in the possession of two developing accessory glands; that 

 an axolotl does not possess accessory glands I would be willing 

 to believe only if sections through the entire region of the lower 

 jaws, throat, and heart could be presented, since dissection, 

 because of the hidden position of these accessory glands, may 

 fail to demonstrate them. If the main thyroids are removed, it 

 takes a long time before the accessories, in the event that they 



26 Jensen, C. O., Compt. rend. Soc. BioL, 1921, LXXXV., 391. 



27 Jensen, C. O., Compt. rend. Soc. BioL, 1920, LXXXIII., 315. 



