2Ua THE AME1UCAX XATl'EALIST [Vol. LV 



than those kept at hiuli tcinpt'iatiire, before they can 

 metamorphose. This is shown in Table III (2). The 

 temperature coefficient for the releasing factor is higher 



that for growth. 



Temperature a 



TABLE III 



2. In very low temperatures (6° C. to 10° C.) growth 

 is greatly slowed down and consequently the elaboration 

 of the releasing factor must be still more retarded; yet 

 larvae kept at 6" C. grow less and less large before meta- 

 morphosis, when they are traiisl'erred, at increasing 

 ages, to IS"" C, as shown by an exjjerinu'iit lasting (58 

 weeks thus far. Apparently the thyroid has gone on to 

 mature at a relatively high rate and at 63 weeks is highly 

 sensitive and responds to smaller quantities of the releas- 

 ing factor. The temperature coethcient for the thyroid 

 change is considerably lower than those for growth and 

 for the elaboration of the releasing factor. 



3. If the thyroid can continue to develop in the absence 

 of growth, it probably can also commence to age. Should 

 this assumption be correct, the larvae kept at 6° C. should 

 finally become unable to metamorphose, if the time dur- 

 ing which they are kept in fi (\ is sufficiently long. At 

 present this assumption would explain wli\- many speci- 

 mens of the Colorado axolotl x ield only slowly, ii' at all, 

 to tlie influence of high tenipci'ainre, and the Mexican- 

 axolotl rre(|nent!v loses eomph'telv its ahilitv to meta- 



