METAMORPHOSIS OF NEOTENOUS AMPHIBIANS 407 



thej^ originally formed a part. The metamorphosis of an anuran 

 larva involves much more fundamental transformation and 

 reorganization of tissues and organs than the same process in 

 axolotl. 



Our experiment suggests four possible factors to account for 

 this anomalous situation: 1) Possibly the blood supply taking 

 the hormone away from the gland is defective and the thyroid 

 consequently unable to release its secretion. 2) The thyroid is 

 able to collect, store, and transform the incoming iodine taken 

 from the food and water into the physiologically active hormone, 

 but owing to defective nervous stimulation the gland is unable 

 to release the secretion into the blood stream. 3) The secretion, 

 though perfectly formed, is unable to escape from the gland, 

 owing to some defective interrelation between the pituitary and 

 thyroid, or possibly some other endocrine gland which supplies 

 the necessary stimulus to the thyroid, thereby acting as the releas- 

 ing factor. 4) The blood and tissues of neotenous forms may 

 contain substances that neutralize or render impotent the meta- 

 morphosis-inducing agent of the thyroid hormone. 



If it is true, however, that the thyroid hormone is unable to 

 escape because of defective outlet through the blood stream, 

 or because of defective interrelationship between various com- 

 ponents of the endocrine system, how is it that axolotls usually 

 promptly metamorphose when taken from their native habitat 

 or when subjected to sudden environmental changes, such as a 

 change from New Mexico to New Haven?^ Furthermore, aside 

 from the nervous system, there are few anatomical or physio- 

 logical mechanisms which hold such power over other structures 

 as to permit a gland like the thyroid to manufacture and store 

 in large quantities a highly complex substance, but apparently 

 prohibits its release. When we consider the thyroid glands of 

 an axolotl filled to capacity with highly active secretion, as our 

 experiments clearly show, yet apparently unable to release the 



2 Of the 109 axolotls received from Albuquerque, only those that were thyroid- 

 ectomized failed to metamorphose spontaneously a few weeks after removal from 

 their native habitat. Left unmolested in the New Mexican environment, the 

 animals may remain permanent larvae and grow to a length of 14 inches. 



