136 EDUARD UHLENHUTH 
is produced. The last mentioned phenomenon will be discussed 
in another article; the effects of thymus upon development, 
metamorphosis and growth will be outlined briefly in the follow- 
ing pages. : 
In the experiments to be reported, only larvae of Urodela were 
studied (Amblystoma punctatum and A. opacum). The ad- 
vantage of using Salamander larvae is that the quantity of food 
given to them can be controlled and measured with exactness, 
up to a certain degree, which cannot be done if tadpoles are used; 
and it should be emphasized that in order to avoid errors the 
possibility of measuring the quantity of food is very desirable 
and even should be demanded in experiments in which it is 
suspected that qualitative relations are involved. 
1. DEVELOPMENT AND METAMORPHOSIS 
Gudernatsch found that the development of tadpoles was 
delayed if the animals were fed on thymus. Similar results 
were obtained by Romeis in his first experiments. This led to 
the inference that thymus contains a substance whose specific 
property is a retarding effect upon development. Only recently 
Gudernatsch again published a paper based on this hypothesis. 
However, in his work published in 1915 relative to the influence 
of the glands of internal secretion on Anura larvae, Romeis 
reports upon two series of experiments, which cannot be ex- 
plained from the above-mentioned standpoint, and which caused 
the author himself to doubt whether inhibition of development 
were indeed a specific function of the thymus. ‘The first series 
consisted of fairly old larvae of the species Rana esculenta, 
the individuals of which developed in a perfectly normal manner, 
in spite of being fed with thymus; but in this case it might have 
been supposed that the thymus feeding had been started too 
late. In the second series, however, in which larvae of Rana 
temporaria were employed, the thymus feeding was commenced 
at a very early stage, in spite of which fact the development of the 
larvae was not delayed. On the contrary, they underwent 
metamorphosis at an earlier stage than did the larvae fed with 
