On Amphibian Metamorphosis and Internal Secretions. 39 



(q) External Agencies, Age, and Rate of Metamorpliosis. 



As will be seen later, our young Axolotl larvae fed on thyroid, all 

 metamorphosed in from 24 to 32 days. We are thus in a position to 

 compare the time necessary for metamorphosis induced by thyroid-feeding 

 with that induced by air-breathing. Marie von Chauvin's classical experi- 

 ments were done with great precautions, and the time taken was very large. 

 Those of Boulenger (1913) will serve as our example, for it is not probable 

 that the time for metamorphosis by this means can be much reduced beyond 

 what was found by him. His results show that the time required for 

 metamorphosis by air-breathing is a function of the temperature. Those 

 kept at 75°. to 80° F. metamorphosed in 89 to 109 days, while those kept at 

 55° to 60° F. took 116 to 120 days. The times required by animals fed on 

 thyroid were always much smaller, varying from 17 to 50 days. The times 

 recorded by Jensen are uniformly less than those found by us. This very 

 probably is connected with a difference of temperature, since our experi- 

 ments were all carried out in the depth of winter, at temperatures, be it 

 noted, considerably below the lowest used in Boulenger's experiments. 

 Mature animals always require a longer time than do immature. Jensen's 

 results show that the minimum time for metamophosis was 10 days longer in 

 adults than in immature specimens, the maximum 7 days longer ; the 

 •minimum time for adults was thus greater than the maximum for immature 

 specimens. The difference in our own experiments was still greater. For 

 immature specimens, it does not appear to make any difference whether 

 they are quite young (under 3 inches in length) or half-grown ; the time 

 taken is approximately the same. This appears to indicate a real difference 

 between the metabolism of adult and immature specimens, and that the 

 time-difference observed is not due merely to differences of size. We may 

 sum up by saying that thyroid-feeding causes metamorphosis much more 

 rapidly than does enforced air-breathing ; and that the change to sexual 

 maturity in Axolotls increases the time necessary for metamorphosis by 

 25 to 50 per cent. 



Some Amblystoma used in our experiments were bred in the laboratory in 

 the spring of 1920. The eggs were laid in the second half of April. Young- 

 specimens from these eggs were used for experiment later in the same 

 year. The first experiments were made in the first week of July, the 

 larvae then being not quite 3 months old, and measuring almost precisely 

 50 mm. in length. Four of these were fed upon thyroid, and showed all the 

 usual metamorphie changes ; the gills were reduced to mere stumps without 

 filaments, the fin along the back was resorbed, and the eyes became 



