Abstract
In the larvæ of amphibians when the thyroid glands begin to excrete the thyroid hormone, metamorphosis of the larvæ into adult animals takes place. Since larvæ with fully developed thyroid glands frequently do not metamorphose, and since in the thyroid gland of the normal larvæ, large quantities of “colloid” are present in the follicles for a considerable time before metamorphosis, it is very probable that the thyroid gland cannot begin to excrete its hormone unless a second factor is present in the larvæ. It seems that this factor is elaborated during the process of growth and must be present in a definite quantity in order that the thyroid function may begin. This is shown in two tables in which for several series of the marbled and the tiger salamanders, the age at which metamorphosis took place and the rate of growth were recorded. The greater the rate of growth the shorter the length of the larval period. As a consequence the product of the rate of growth into the duration of the larval period gives a fairly constant value K. The maximum deviations observed can be traced back to certain causes which will be discussed immediately.
Hence it is evident that metamorphosis not only depends upon the thyroid hormone but also on a second substance, the quantity of which increases in the same ratio as growth. This second substance must be present in a certain quantity in order that metamorphosis can take place. The rate at which it is formed from the same kind of food and for the same rate of growth is distinctly influenced by two factors; by the quantity of food available to the larvae, and by the temperature.
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