INFLUENCE OF THYMUS FEEDING UPON DEVELOPMENT 141 
velopment of network, the first thymus animal attained the 
network stage when only 41.3 mm. in length. 
We thus see that the time at which the various phases of 
development are attained varies according to the quantity of 
food, with the result that sometimes the thymus animals, at 
other times the worm-fed animals appear to lead. But the 
constant factor is the size at which the various stages are at- 
tained; that is, constant to the extent that the thymus animals 
always develop more quickly than do the worm animals, if the 
various stages are referred to the size of the animals. This 
relationship is directly opposed to that of the Anura larvae, for 
in these animals the thymus-fed individuals must usually attain 
a considerably greater size than the worm-fed animals in order 
to arrive at the same degree of development. 
Identical relationships as occur in the development are also 
found in the metamorphosis; but in this case one or more addi- 
tional factors seem to play a réle to complicate considerably the 
phenomena, as we shall see. 
Here again we must differentiate between the experiments in 
which the food was quantitatively equal and those in which each 
animal was allowed to eat to the point of satiety. But it should 
be emphasized that only the first method permits of a correct 
comparison. For when the worm animals feed at will they eat 
approximately 10 to 20 times the quantity of food that is con- 
sumed by the thymus animals when the latter begin to suffer 
from tetany; as the worm animals also grow much more rapidly 
as aresult, it would not be surprising that they also metamorphose 
earlier, since we might expect that if a definite size of the animal 
is indispensable to metamorphosis, metamorphosis will be ac- 
celerated if we accelerate growth by some external conditions. 
We will now turn our attention again to the group O 1916 of 
A. opacum in which each series was given approximately the 
same quantity of food. In this group the warm thymus animals 
were the first to undergo metamorphosis; thus in the warm thy- 
mus series (22.6°C.) the first animal underwent metamorphosis 
in the 13th week, in the warm worm series only in the 24th week; 
in the cold thymus series (14.8°C.) the first animal left the water 
