IODINE AND AMPHIBIAN METAMORPHOSIS. 245 



animals, and that the amino-acid and protein without the iodine 

 in the molecule are inert. 



Hirschler ('22) metamorphosed axolotls (the European strain 

 which rarely spontaneously transforms) by implanting iodoform 

 paste within the body cavity. However, in this experiment the 

 glands were intact so it may be that the iodoform merely served 

 to stimulate the thyroid mechanism to release its stored hormone 

 thus inducing metamorphosis. It will be recalled that Kaufman 

 ('18) metamorphosed axolotl by injections of salicylic acid. So 

 far as is known salicylic acid has no influence on metamorphosis, 

 the effect of injecting the substance into axolotl was to stimulate 

 the secretory activity of the thyroid apparatus in some way. 

 Anyone not familiar with thyroid conditions in axolotl might 

 conclude that salicylic acid per se was the metamorphosis-induc- 

 ing agent, whereas probably nothing could be farther from the 

 real facts of the case. 



The New Mexican strain of axolotl if removed from its native 

 habitat to New Haven soon undergoes spontaneous metamor- 

 phosis. Why? Certainly not because the railroad journey exerts 

 any mysterious metamorphosis-inducing power, but probably 

 because the changed food, water, the jolting and confinement 

 incident to the trip acted as a stimulating agent thus releasing the 

 thyroid hormone from the gland vesicles thereby causing trans- 

 formation. 



Where the thyroid apparatus is left intact the fate of the sub- 

 stances fed or injected into an amphibian larva is problematical. 

 If an effect upon metamorphosis is produced it is impossible to 

 determine whether the effect is due directly to the substance 

 itself, or indirectly through intermediation of the thyroid unless 

 the work is checked by repeating it upon thyroidless forms. An 

 excellent illustration of this statement has been furnished by 

 Allen ('20). Allen observed that transplantation of the anterior 

 lobe of the pituitary into hypophysectomized tadpoles (but pos- 

 sessing a rudimentary and functionless thyroid apparatus) 

 metamorphosed the animals, whereas transplantation of the 

 pituitary gland into thyroidectomized larvae had no such effect. 

 The pituitary secretion had no direct influence upon meta- 

 morphosis but acted indirectly by stimulating the rudimen- 

 tary thyroid into functional activity. Any one working with 

 17 



