Abstract
1. When thymus is fed to salamander larvæ, this gland does not exert any specific growth-promoting influence.
This is best shown in the curves obtained from the average sizes of four series of A. tigrinum. Two of the series were kept in high temperature and two in low temperature and in each temperature one series was fed on thymus and one on normal food (mainly earthworms). The thymus diet did not accelerate growth. (The same fact is shown with the aid of pictures.)
Why other writers have reported faster growth in thymus-fed amphibians will be demonstrated by curves obtained from the average sizes of four series of A. opacum (A, B, C, D, 1916). In all four series, the food was given in pieces of approximately the same size and exactly the same quantity. Since a piece of earthworm contains a great deal of indigestible matter, i. e., soil, cuticule, etc., the worm-fed animals actually received less food than the thymus-fed animals. Therefore, the thymus-fed series grew more rapidly than the worm-fed series. Had we not recorded the number of pieces which were fed to each series, we would have been led to the erroneous conclusion that thymus promotes growth in a specific way.
That thymus does not promote growth in a specific way is also seen from the two following curves obtained from two series (A, B, 1917) of A. opacum. Again in both series the food was given in pieces on a forceps; but to each animal food was given every day, until it would take no more food. Here the wormfed animals grew even more rapidly than the thymus-fed series. When the differences in the quantity of food become still greater, the animals show differences in size which are far greater than any reported due to thymus feeding.
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