No. 647] METAMORPHOSIS OF AMERICAN AXOLOTL 565 



scended from these animals. Gadow also states that the 

 axolotls of Lake Xochimilco have never been known to 

 metamorphose in their native habitat. However, several 

 of the descendants of the animals taken to Europe did 

 metamorphose, so that spontaneous transformation in 

 the Mexican strain does sometimes occur, though rarclx . 



In an earlier paper ('22) the writer showed that the 

 thyroid mechanism of axolotls is filled with physiologi- 

 cally active hormone capable of inducing metamorphosis 

 but that the secretion is apparently not liberated into the 

 blood stream, hence the retention of the larval characters 

 despite the possession of a large well-formed gland. The 

 thyroid of a fourteen-inch axolotl several years of age 

 was extirpated and cut into small pieces, each piece then 

 transplanted into an immature Anuran larva. The single 

 axolotl thyroid promptly metamorphosed five such tad- 

 poles within fourteen days, whereas left intact within the 

 axolotl's body it was quite incapable of inducing trans- 

 formation. 



This same experiment was repeated upon thyroidecto- 

 mized and hypophysectomized Rana sylvatica tadpoles 

 with similar results. Small pieces of axolotl thyroid 

 when engrafted into thyroidless and pituitaryless larviv 

 promptly induce metamorphosis within ten or twi'hc 

 days. 



It is quite clear from these experiments that axolotl 

 neoteny is due to retention of the thyroid hormone within 

 the gland vesicles. Under normal conditions and in its 

 native habitat, the releasing mechanism apparently fails 

 to act, but when the animals are shipped from one place 

 to another and subjected to new environmental conditions 

 metamorphosis promptly ensues. In the New ^lexican 

 strain slight stimulation is sufficient to initiate meta- 

 morphosis, but in tlic Kni'()j)t"aji and Mexican forms very 

 powerful stimulation is nei^dcd to overcome the thyroi<l 

 inhibition and release tlic sccrtnion. Tn llir Kurojioan 

 strain the followinir ayvnts liavp h,4.n uhmI <u,vcs<t'ul]y 

 for inducing metainoi-i>liosis : thxi'oid iV'cdinn- (Laui])er- 



