38 /, H. Powers 



mer, the time when metamorphosis is most easy. Neotenic larvae 

 metamorphose usually without developing it. Yet a number of 

 large, old larvae, metamorphosing, with resistance, so to speak, 

 in the fall, have shown projecting lower jaws, quite equal to 

 Cope's A. xiphias. Out of about a dozen ordinary larvae which I 

 was wintering, and finally induced to metamorphose in midwin- 

 ter, several developed and permanently retained this same feature 

 despite their later growth to. twice the size. - 



As to the permanence of special types of head in the adult 

 during its life subsequent to the time of metamorphosis some- 

 thing must also be said. For if the various types of head in this 

 species should turn out to be due to nothing more than the vary- 

 ing conditions of the individual as such, fatness, leanness, etc., 

 their importance as variations is much reduced. At first I thought 

 this would prove to be largely the case, that the amblystoma with 

 prodigiously bulging parotids was simply an individual very 

 highly fed, whether during larval or adult life. This supposition 

 was, however, borne out only in part by the facts. I have kept 

 many adults, young and old, for three and even four years, and 

 have subjected them to very varying conditions of nutrition, tem- 

 perature, etc., and by means of photographs I have compared the 

 appearance of many during successive seasons. Starvation does, 

 of course, produce a marked effect on many organs and upon 

 the animal's whole appearance, save color. More important is 

 the fact that young adults, particularly males, often undergo con- 

 siderable changes in the size and shape of the head during their 

 first winter, especially if they have developed from very full fed 

 larvae. Heads of good sized specimens may, in extreme cases, 

 lose two to three mm. in width, and this makes a great difference 

 in the proportions of the animal. Probably the change would 

 never be so great as this if the animals were not wintered at too 

 high a temperature. All in all, however, it turns out that sexually 

 mature adults show a nearly constant form of head, so long as 

 they are kept under approximately natural conditions. And even 

 very old adults, although they become much worn and wrinkled 

 about the head, yet show no marked deviation from younger ani- 

 mals of similar general types. 



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