152 EDUARD UHLENHUTH 
individuals are obviously the worm-fed animals of the third 
series, which are allowed to have as much food as they wish; in 
the experimental group with A. opacum (O 1916) it is the thymus 
animals which take in a greater quantity of nutritive material 
through eating thymus. The experiments furthermore show 
that qualitative influences exerted on the rate of growth would 
have to be very considerable in order that they can be experi- 
mentally tested in the case of amphibia, for in these animals the 
slightest quantitative differences, such as can hardly be controlled, 
would bring about very misleading differences in growth. 
With respect to the ultimate size attained by the animals, 
Salamander larvae resemble tadpoles in the fact that under 
certain conditions the later they metamorphose the greater is 
their final size; this is not only true for thymus-fed animals in 
comparison to worm-fed animals, but also for worm-fed animals 
kept in high temperature in comparison to worm-fed animals 
kept in low temperature. 
The action of thymus on development and metamorphosis 
may be summarized in the following way: 
In animals fed on thymus the development presumably of the 
‘organism as a whole but certainly of the legs, gills, shape of the 
head and color of the skin, is greatly accelerated during the 
larval period. The thymus-fed animals, therefore, reach the 
stage at which worm-fed animals are ready for metamorphosis, 
much more quickly than worm-fed animals. As development at 
least to some degree may be dependent on growth, on the rate of 
growth and on size, it is impossible to examine the specific 
influence upon development of any substance without keeping 
alike the conditions of growth in both the experimental and 
control series; such was attempted by admitting an equal amount 
of food to both series. 
When the thymus animals have: reached the stage at which 
worm-fed animals go into metamorphosis, the development of 
most organs seems to stop, while certain characteristics of the 
skin continue to develop; the skin of such animals then behaves 
very similarly to the sex-organs of neotenic larvae, since the skin — 
at least with regard to the structures determining pigmentation, 
