Thymus Gland. 



39 



pictures) and metamorphosis started earlier in the thymus-fed 

 sets than in the worm-fed series. Yet as seen from the curves, 

 the thymus-fed animals also grew more rapidly than the worm- 

 fed animals. Still when they metamorphosed, they were smaller 

 than the worm-fed animals at the time of metamorphosis. It is 

 only recently that I have felt the necessity of knowing more about 

 the relation between metamorphosis and growth in amphibians, 

 and only when our experiments on this point are completed will it 

 be known definitely whether or not the thymus feeding actually 

 does accelerate metamorphosis in salamanders. 



The experiments just mentioned are also confusing on account 

 of the fact that only part of the thymus-fed animals actually 

 metamorphosed earlier than the worm-fed animals. In each series 

 of the Opacum group there were several individuals which, after 

 they had arrived at the stage preceding metamorphosis, suddenly 

 stopped their development and either died or metamorphosed 

 only after a certain period had elapsed. 



In the A. tigrinum set there is one thymus-fed animal, which 

 did not metamorphose when the others did. It is still a larva — 

 io weeks after the rest of the series metamorphosed. 



In a thymus-fed series of A. punctatum, two individuals re- 

 mained as larvae for almost 15 months; and died upon commencing 

 to metamorphose. 



It is important to note that "long time" larvae occurred mostly 

 in thymus-fed series which were kept in low temperatures. 



One cannot, of course, but recognize the important bearing 

 on the problem in question, of these "long-time" larvae, and an 

 examination of them will be necessary. Yet it does not seem 

 that the phenomenon has anything to do with the thymus; for 

 entirely similar cases are encountered in normally kept animals. 

 We had, for instance, one worm-fed animal among a series of A. 

 punctatum kept in a cool temperature, which was still a larva 8 

 months after hatching and then unfortunately was killed by an 

 unsuccessful operation. In another series of only 4 A. punctatum 

 larvae, kept in high temperature, there was one individual which 

 grew normally but did not show any signs of development on its 

 skin throughout its life; it remained larval when the other animals 

 metamorphosed and died as a larva one month later. 



