On Amphibian Metamorphosis and Internal Secretions. 37 



First, however, a record of one of our own experiments will be given. 

 Two black Axolotls, one male, the other female, and both sexually mature 

 (though not quite full-grown), were kept in a large tank, together with two 

 fine full-grown white Axolotls as controls. The controls were fed on worms, 

 while the others were given pieces of ox-thyroid. In every case the feeding- 

 was controlled, the worm or piece of thyroid being kept near the animal's 

 mouth until swallowed ; if persistently refused, it was removed. The thyroid 

 feeding began on November 30, 1919. On December 29, the thyroid-fed 

 animals were past the critical stage of metamorphosis (see Boulenger, 1913); 

 on January 13 no trace of gills was left; on January 19 the animals left the 

 water, and the metamorphosis could, therefore, be considered as complete. The 

 temperature was at first that of the room, varying from 8° to 15° C. ; from 

 January 2, when the water was warmed by an electric-light bulb, it rose to 

 between 14° and 16° C. The length of the specimens at completion of meta- 

 morphosis was 17"4 cm. ( $ ) and 14'6 cm. ( ¥ ) ; the male had the typical sign 

 of maturity — enlargement of the lips of the cloaca — well-marked. The 

 controls during the same period showed no changes. It is an interesting fact 

 that the metamorphosed animals, although tending to come more frequently 

 to the surface about the time that the critical stage was reached, showed no 

 inclination to leave the water until after the morphological alterations had 

 run their course. It is only when a few days have elapsed after the com- 

 pletion of the visible external changes that the animals emerge from the 

 water. 



(b) Jensens Work. 



Curiously enough, Jensen also believed that he was the first to obtain the 

 metamorphosis of Axolotls by thyroid, and it was only after his paper was 

 completed that he found that he too had been anticipated — byLaufberger (1913). 

 Jensen's experiments, however, are much fuller. He used calf-thyroid as 

 diet. Two-week old larvae on this diet all died within a few days, without 

 change of form. Four immature specimens (10 to 16 cm. in length) meta- 

 morphosed in 17 to 25 days without exception. They were fed only three to 

 eight times, with 1 to 3 grm. thyroid in all. An intra-abdominal injection of 

 iodothyrin caused metamorphosis in the same length of time (10 specimens). 

 The amount of iodothyrin injected had no influence upon the rate of meta- 

 morphosis within the limits used (1 to 10*5 mgrm.). One specimen fed upon 

 iodothyrin started to metamorphose, but after 21 days of very slow change 

 began to reverse the process, and became larval again. Four mature 

 specimens, 22 to 24 cm. long, and two of them known to be over 6 years old, 

 were also successfully metamorphosed. The time required was here greater, 



