Morphological Variation and Its Causes in A. tigrinum I'j 



growth at the tip, as evidenced by its extreme thinness and lack 

 of pigment ; spots of color which showed in the immature animal 

 at or near the tip being, after the elongation, removed by the same 

 amount from its extremity. So thin are these tail tips during the 

 first season of their development that the extension of the spinal 

 column shows on either side as a strong convex ridge along its 

 middle. Only very gradually does this secondary growth take 

 on the character of the original portion, and it always remains 

 relatively thin. 



A further question occurs, viz., whether this caudal elongation 

 which accompanies the first advent of sexual maturity is followed 

 by subsequent elongation w'ith each recurring cycle of sexual de- 

 velopment. Cope thought that animals with very long tails were 

 very old ; and if, with each yearly period of sexual development, 

 the tail increased in length disproportionately to the body, his 

 assumption would be borne out. As a general rule, however, I 

 do not find this to be the case. Besides, I find that very slowly 

 grown larvae may produce males that equal, in their first season'"s 

 sexual maturity. Cope's extremest .measurements, i. e. forms in 

 which the tail equals or even exceeds the length of head and body. 

 Nevertheless, I do find that this sexual character is capable of 

 repeated expansion, in case the animals are very highly fed dur- 

 ing the summer. The tail does not grow disproportionately dur- 

 ing the period of feeding. Rather the reverse seems to be the 

 case. But when the animals are taken from the earth the follow- 

 ing spring another increment of the tail is evident, rendering this 

 member still more disproportionate. Even several old males, after 

 excessive feeding had been induced, showed this power of renewed 

 tail-growth. I hope thus, by repeated feeding, to produce exces- 

 sively long-tailed animals, suggestive of some related types. In 

 •nature, however, this repeated extension of the tail certainly 

 takes place but rarely. The average Amblystoma does not get 

 sufficient food, it would seem, after metamorphosis, for even 

 moderate growth. 



By careful study a number of means may be discovered of 

 judging or estimating the age of animals taken in nature : year- 

 lings can be certainly told by the form of the tail, and two-year- 



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