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. 
