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A U T H O~S ABSTRACTThis paper presents a study of the growth and histoIogica1 development of the thyroid in three species of frogs, the small Pseudacris triseriata, the large Rana catesbeians, and the intermediate Rana palustris. It was found that the history of the gland in rdation to metamorphosis is essentially the same in all. Previous to metamorphosis the gland is small, and the cells very poor in cytoplasm. At the beginning of metamorphosis (i.e., rapid hind leg growth) the thyroid shows a marked acceleration of growth and activity which continues and reaches a oeak a t the time of the emergence of the front lers. hate in metamorphosis a regression occurs.The conclusion is drawn that there is a rising concentration of thyroid hormone during the course of metamorphosis. The author has previously shown that such a rising concentration will induce the normal pattern a8 to sequence and time spacing of metamorphic events.The beginning of metamorphosis is conceived as dependent upon the time of stimulation of the thyroid and the pattern of its subsequent continuance a s dependent upon the pattern of response of this gland.--

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