404 W. W. SWINGLE 



These experiments, however, do not rule out the possibility 

 of inducing the metamorphosis of perennibranchiates by other 

 means than that of thyroid feeding or transplantation. The 

 cause of the non-metamorphosis of these forms may be pluri- 

 glandular in origin, and a result of defective interrelation of various 

 endocrine glands. It should be added that the writer fed one 

 Necturus small quantities of desiccated ovarian, testicular, ad- 

 renal, and anterior pituitary lobe tissues, along with large quanti- 

 ties of thyroid extract, but without avail. It is probable that 

 transplants of these various glands simultaneously would have 

 had more effect than feeding the desiccated substances, in case 

 the animal possessed the capacity to transform. Administration 

 of endocrine secretions, no matter in what quantity given, can 

 give positive results only when acting upon an appropriate hered- 

 itary substratum. The indications are that the hereditary factors 

 concerned in the metamorphosis of Necturus have become so 

 modified that the appropriate substratum is lacking, thus render- 

 ing the thyroid hormone powerless. It is obvious that hereditary 

 conditions in the perennibranchiates are quite different from those 

 in axolotl, in regard to metamorphosis, since the latter readily 

 respond to thyroid feeding and the former do not. 



EXPERIMENTS WITH AXOLOTL THYROIDS 



Through the courtesy of Prof. Henry Laurens, of the Depart- 

 ment of Physiology, the writer obtained a very large specimen of 

 Axolotl mexicanum (neotenic larva of Amblystoma tigrinum) 

 for thyroid transplantation work. The animal was a very large 

 one, measuring 14.25 inches from snout to tail-tip. The exact 

 age of the specimen is unknown, as it was obtained, along with 

 several others, from Albuquerque, New Mexico. When first 

 brought to the laboratory the animal was about 8 inches long, 

 and hence presumably about two years of age at the time; it 

 was kept under laboratory conditions for two more years, thus 

 making four years the animal's approximate age when used by 

 me. This specimen was the only one of the lot that failed to 

 metamorphose within a few months following removal from its 

 native habitat to New Haven. 



