41G W. W. SWINGLE 



5. A giant axolotl, 14.25 inches long, several years of age, was 

 found to have a highly active metamorphosis-inducing thyroid 

 apparatus. The thyroid of a single specimen cut into six pieces 

 and transplanted metamorphosed five immature anuran tadpoles 

 within two weeks. The sixth animal died following the operation. 



6. The axolotl's thyroid is normal in appearance and of large 

 size, consisting of numerous large vesicles filled with colloid. 

 The gland is surrounded by a rich network of capillaries. 



7. The failure of the axolotl to metamorphose appears to be 

 due to the inhibition or the defective development of some un- 

 known factor which normally serves to release the fully formed 

 hormone from the thyroid into the blood stream. It is suggested 

 that defective nervous stimulation or perhaps inhibition is the 

 immediate cause of retention of the secretion within the thyroid 

 vesicles, but that in the last analysis some defect of interrelation 

 of the various components of the endocrine system is probably 

 responsible for the nervous inhibition or lack of normal 

 stimulation. 



8. Experiments on large neotenous anuran tadpoles indicate 

 that the failure of these animals to metamorphose at the proper 

 time probably is due to the same causes responsible for axolotl 

 neoteny: i.e., the thyroid glands apparently do not secrete their 

 fully formed hormone into the blood stream because of some 

 unknown inhibiting influence. The thyroid inhibition seems to 

 be less marked in anurans than in the axolotl, since neotenous 

 tadpoles eventually metamorphose if given sufficient time. 



9. The next step in the analysis of amphibian neoteny is to 

 determine the nature of the factor responsible for the failure of 

 the thyroid to release its hormone (or at any rate to render it 

 impotent in so far as metamorphosis is concerned). Is this 

 unknown factor hormonal, or nervous, or both? 



10. In the older work on amphibian neoteny too much stress 

 was laid upon the exogenous factors as causative agents, and too 

 little, if any at all, upon endogenous factors, and heredity. 



