The well-known effects upon amphibian metamorphosis produced by feeding thyroid preparations and by transplantation of living glandular substance have been clearly established and yet there is need of quantitative studies on this score in order to solve interesting questions, as for instance:
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The minimum amount of thyroid hormone to produce evidence of metamorphosis.
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The degree of response to different quantities of thyroid hormone.
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The different thresholds of stimulation and rate of reaction of different organs.
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The capacity of the organism to react to different amounts of thyroid stimulus at different stages of development.
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The age at which the organism shows a capacity to respond to the hormone.
Certain of these questions have been investigated already by Romeis, '23; Kosmin and Resnitschenko, '27; and by Blacher, '28. Romeis, '23, carried on a series of experiments with tadpoles, his method consisting chiefly in immersing them in the test solution for definite periods and at definite This work was supported by grants from the National Research Council and the University of California.
21t is also a pleasure t o express my deep appreciation for the pleasant laboratory quarters placed at rnr disposal through the kindness of the Del Amo Foundation.