METAMORPHOSIS OF NEOTENOUS AMPHIBIANS 409 



system. On theoretical grounds, the writer beheves that elec- 

 trical stimulation, thyroid puncture, and extirpation of the thyroid 

 and reimplanting it into the same individual will metamorphose 

 axolotl, but to date has been unable to obtain sufficient animals 

 for experimentation along the lines indicated. 



If such procedure should cause metamorphosis, then it is clear 

 that the physiologically active hormone is not released from the 

 thjToid in sufficient quantity to induce transformation. From 

 the e\'idence at hand it seems to the writer that such is probably 

 the case. Axolotls readily respond to thyroid feeding or to in- 

 jections of iodothyrine by transforming, and the amount of 

 thyroid substance required is not excessive. If the assumption 

 were correct that the blood and tissues of this neotenic form con- 

 tained substances which neutralized or rendered impotent the 

 thyroid hormone, thus preventing metamorphosis, why should 

 small amounts of thyroid substance, when fed, be able to pro- 

 duce an effect? The evidence obtained from thyroid transplanta- 

 tion experiments with neotenous anurans is interesting in this 

 connection. 



EXPERIMENTS WITH THE THYROIDS OF NEOTENOUS ANURANS 



The larvae of the green frog, Rana clamata, have a larval 

 period of approximately one year: i.e., 370 to 400 days from the 

 date of egg deposition. The animals attain a length of about 

 65 mm. at metamorphosis, which occurs in late July and early 

 August. However, it has been repeatedly observed by the 

 writer that many larvae fail to transform at the usual time and 

 remain an extra year as tadpoles. Such animals are typically 

 neotenous forms and, as they continue growing throughout the 

 larval period, they ultimately reach a size considerably in excess 

 of that generally exhibited by the species at metamorphosis. 

 Larvae measuring 75 to 90 mm. total length, wdth differentiated 

 hind legs varying from 4 to 20 mm., have been captured in the 

 months of November, December, and January from various 

 pools in the vicinity of New Haven. The tendency of the species 

 is to prolong rather than curtail the span of larval existence. 

 Because of this fact, a series of thyroid-transplantation experi- 



