154 EDUARD UHLENHUTH 
to the fact that in the thymus-fed Salamander larvae develop- 
ment and differentiation and consequently metamorphosis also 
depend on the general conditions of growth; the experiments on 
Salamander larvae reported suggest that rate of growth and size 
play an important role in metamorphosis. The difference noted 
between Anura and Urodela when fed on thymus can be ex- 
plained only by assuming a fundamental difference between 
the organization of these two groups of animals. It will be 
pointed out in another article that such a difference, namely 
the absence in the Salamander larvae and the presence in the 
anuran larvae of the parathyroids, seems to explain why thymus- 
feeding should develop tetany in Salamander larvae and should 
not in anuran larvae. It suggests itself that. metamorphosis 
in part must depend on a factor similarly being present in one 
group but absent in the other group. The development of that 
factor may be induced primarily by processes occurring in a 
certain stage of differentiation, but also may be influenced and 
inhibited or disturbed by thymus diet. The action upon this 
factor of the thymus may be widely different from that upon 
developmental processes preceding its development; this is 
indicated by the fact that development while accelerated during 
the larval period is on the contrary retarded from the time at 
which metamorphosis should occur. It is this phenomenon 
which emphasizes the fact that metamorphosis to some degree 
must occupy a particular place among the processes of develop- 
ment. In this connection, finally, frequent reports may be 
remembered according to which thymus causes disturbances of 
the blood circulation; in metamorphosis of the Amphibians the 
blood circulation undergoes a fundamental change in the course 
of which the gills are absorbed, and in Salamanders, the absorp- 
tion of the gills according to Maurer, is a prerequisite for the 
formation of the parathyroids. It may be worth while to keep 
these facts in mind during further studies of the influence exerted 
upon metamorphosis by the thymus. 
Though the effect of thymus feeding on development and meta- 
morphosis is very evident, it appears to the writer that similar 
effects may be produced by other and purely quantitative exter- 
