Metamorphosis of Axolotls 
m 
phenomena, analogous to those which are characteristic of the pro¬ 
cesses naturally occurring. 
Since all the phenomena of natural and artificial metamorphosis 
elicited by thyroidine feeding are identical, their causes might prob¬ 
ably be also identical. In my experiments, the transformation of 
axolotls has been caused by increased metabolism produced by 
thyroidine food. What is the cause accelerating disassimilatory pro¬ 
cesses at a given stage of the larval life of amphibians under norm¬ 
al conditions? Bata il Ion’s theory of asphyxia (which explains 
changes in an organism undergoing metamorphosis by anatomical 
changes occurring during evolution and causing decrease of tension 
in the larval respiratory organs) does not appear to take into con¬ 
sideration that histolytical processes occur not only in Anura , but 
also in Urodela in which anatomical changes (the formation of 
the „spiracula complémentaires“), which are believed to cause hist¬ 
olysis, are absent. The factor producing histolysis acts like ex¬ 
tracts of the thyroid gland given as food. I therefore suppose that ? 
in a certain evolutionary stage, the product of the thyroidea is 
abundantly secreted; this would cause disturbances analogous to 
those of feeding with thyroidine. That neotenia and metamorphosis 
are due to internal secretion, is admitted by Bâbâk and by Lauf- 
berger. Indirect signs of a dependence of the transformation of 
the amphibians on the function of the thyroid gland may be found 
in Adler’s paper on the removal of the hypophysis of tadpoles, 
on the influence of hypermaturity of eggs and the influence of 
temperature changes on metamorphosis. 
If we assume that metamorphosis depends on an internal factor 
(the function of the thyroid gland), we have to bring into harmony 
with that view the well-known results of previous authors which 
ascertained the influence of external factors on the duration of the 
larval stage of amphibians. 
The influence of food on metamorphosis, as I endeavoured to 
show, depends on the fact that starvation intensifies the function of 
the thyroid gland, when it acts simultaneously with it. 
The well known observation that the course of metamorphosis 
depends upon temperature, is explained by Adler’s results. 
He found a close connection between temperature and the structure 
of the thyroid gland. Temperature therefore influences metamor¬ 
phosis indirectly, regulating the secretion of the thyroidea. 
