244 w - w - SWINGLE. 



because the metamorphosis may have been due to increased 

 activity of the thyroid due to increased iodine supply. On the 

 other hand the absorbed iodine may have combined with other 

 than thyroid proteins thus inducing metamorphosis. Either or 

 both of these possibilities may have been realized in Hirschler's 

 experiment, consequently work involving iodine administration 

 should be performed only on thyroidectomized larvse since only 

 this type of experiment will shed any light upon the role of iodine 

 in amphibian metamorphosis. This is especially true of neoten- 

 ous forms like axolotl; no work on the role of iodine in the meta- 

 morphosis of this form should be regarded as conclusive unless 

 performed upon animals from which all trace of thryoid tissue 

 has been removed. 



Thyroid conditions in axolotl are very peculiar (Swingle, '22) 

 and vitiate the results of experiments done on animals with 

 intact glands. The New Mexican strain of axolotl has perfectly 

 developed thyroids, the vesicles filled to capacity with the phy- 

 siologically active hormone yet the secretion is apparently unable 

 to escape into the blood stream in sufficient quantities to trans- 

 form the animal. This is demonstrated by heteroplastic thyroid 

 transplants. An axolotl thyroid is sufficient to metamorphose 

 thirteen normal, thyroidless and pituitaryless anuran tadpoles 

 w r hen grafted, but left intact within the a.xolot's body is incapable 

 of initiating metamorphosis. : It is obvious that in this form we 

 are dealing with a thyroid mechanism which selects, stores and 

 transforms the iodine of the animal's food and water into the 

 thyroid hormone but fails to release the elaborated product. 

 Consequently, if axolotls are fed elemental iodine they do not 

 metamorphose, and for exactly the same reason, they fail to 

 transform under the ordinary dietary regime the iodine is 

 picked up by the gland and synthesized but not released. It 

 is probable that feeding organic iodine preparations would give 

 similar results. On the other hand it is erroneous to conclude 

 from such an experiment that iodine has no influence on axolotl 

 metamorphosis. Ou'r experiments on both thyroidectomized 

 and partially thyroidectomized axolotls show quite clearly that 

 iodized amino acids and proteins promptly metamorphose these 



1 This experiment was performed by Mr. Karl Mason, of this laboratory, and 

 reported at the Boston meeting of the American Society of Zoologists, 1922. 



