Metamorphosis of Axolotls 
47 
reduced gills. Such reductions are a reaction of the organism 
against the influence of surroundings^ they do not induce profound 
changes in the organism and thus are reversible. Metamorphosis, 
on the contrary, is caused by internal factors modifying the 
structure and function .of the organism in such a degree that it is 
no longer able to assume the larval form. 
Petrik succeeded in retarding for half a year the metamor¬ 
phosis of the salamander by feeding it on thymus. This gland 
therefore contains a factor whose influence is contrary to that of 
the thyroid gland. In Petri k’s experiments the animals, in which 
metamorphotic processes had already progressed far, did not return 
to larval form. In these experiments we are confronted with 
a retardation but not with a reversion of evolutive pro¬ 
cesses. 
The appearance of the skin of specimens fed on thyroidine 
changes gradually. Bright spots which in black larval individuals 
formed indistinct marblings now become brighter and of a greenish 
yellow hue. At the same time, the fundamental dark olive colour 
becomes almost completely black; this produces an impression as 
if the pigment, formerly spread all over the body, had left the 
brighter parts. All that happens gradually; the skin becomes dist¬ 
inctly spotted only after several moults. (See phot. 1, representing 
a black terrestrial Amblystoma). The colour of the skin of alb- 
inotic axolotls becomes darker before each moult, for the epithe¬ 
lium which is cast off contains much pigment. After the moult, 
the skin again assumes a pink colour which differs from the larval 
white. This complection is due to blood-vessels which are more 
visible here than in the thick larval skin. To this question I shall 
return in the histological part of the present paper. 
The transformed animal differs considerably in appearance from 
the larva. Its head becomes smaller, on the back we see a groo¬ 
ve in the place of the larval crest, the tail is narrow and flat as 
in newts, the fingers long and deprived of webs. Photograph 1, 
table .2, represents a black, photograph 2 a white, totally trans¬ 
formed Amblystoma. A striking character of these terrestrial animals 
are the prominent eyes, different from the eyes of aquatic specimens. 
Such eyes characterise all terrestrial forms of amphibians; on the 
other hand, a similar prominence of the eyes („exophthalmus“) is 
a known pathological phenomenon caused by supernormal or ab- 
