INFLUENCE OF THYMUS FEEDING UPON DEVELOPMENT 151 
dimensions than do those which first underwent metamorphosis. 
It can be seen that this is not connected with a specific influence 
of thymus feeding, from the fact that exactly the same relation 
exists between the warm and the cold worm series, the warm 
worm-fed series which first underwent metamorphosis meta- 
morphosing while smaller in size than the cold worm series, and 
the latter continuing to grow after the former has metamorphosed. 
While yet in the larval stage the cold worm-fed animals attain a 
size which when the largest animals of both series are used for 
comparison, already exceeds that of the largest warm worm-fed 
larva by 12.5 mm.’ From another point of view the salamander 
larvae of those species so far examined show the very opposite 
characteristics from those possessed by the anuran larvae; for 
it is not the worm-fed salamander larvae which first undergo 
metamorphosis but the thymus-fed individuals. Thus, the 
point to be primarily emphasized is not the greater size ulti- 
mately attained by the worm-fed salamanders and thymus-fed 
tadpoles, for we have seen that this does not depend upon the 
specific qualities of the thymus, but that it is a general phe- 
nomenon peculiar to amphibia and one dependent upon the 
time at which the animals undergo metamorphosis. The point 
of importance in both cases—the larvae of Anura as well as of 
A. opacum and A. punctatum is the circumstance that thymus- 
feeding produces, metamorphosis in the Anura only when con- 
siderable size has been attained, whereas in the Urodela, on the 
other hand, this occurs while the animal is but small in size. 
To summarize, we may make the following statement: The 
differences in the rate of growth to be noted before metamorphosis 
are not the result of a specific growth-promoting influence of the 
thymus; they are based on the circumstance that animals which 
are better fed grow more quickly. In the experimental group of 
A. punctatum (P 1917) discussed in the preceding section, these 
‘Although the cold worm larvae are at the time of writing larger than the 
largest metamorphosed warm worm animals, we do not here intend to take up 
the question of this relation; moreover, a comparison of the experiments 
hitherto conducted in connection with the Anura shows this not to be possible, 
as the respective authors never observed their experimental animals beyond 
the period of metamorphosis. 
