MECHANISMS OF AMPHIBIAN METAMORPHOSIS: HORMONES

Abstract

The study of amphibian metamorphosis might well be begun with a consideration of the more recent reviews of this topic, particu? larly those by Allen (1938), Needham (1942), Lynn and Wachowski (1951), and Etkin (1955). Metamorphosis can be thought of in a variety of ways. It can be considered in the restricted role of converting for terrestrial life those larval parts which are initially adapted for aquatic life, or are as yet but rudimentary. It can be considered, more broadly, as the sum of all changes which the postembryonic amphibian undergoes in the transformation from larval to juvenile form. In perhaps a more restricted sense, but one which emphasizes causal mechanisms, am? phibian metamorphosis can be thought of as the sum of all transformations which depend, either directly or indirectly, upon circulating thyroid hormone. Through the pioneer feeding studies of Gudernatsch (1912), the thyroid gland was shown to contain metamorphosis-stimulating material, and later Allen (1916) demon? strated that metamorphosis was inhibited in thyroidless tadpoles. It was already known, from the work of Adler (1914), that the pituitary gland was implicated, since tadpoles whose pituitary glands had been cauterized remained larval in appearance and developed atrophic thyroid glands. Special attention has been given to the size and to the histo? logical and cytological appearance of the thyroid gland throughout larval life. The correlation between gland structure and detectable metamorphic change is clear. De? tailed studies by Uhlenhuth (1927) in salamanders record the growth of primary follicles, their filling with colloid, and their fusion with one another. The climax phase of metamorphosis is characterized by the discharge of colloid from the follicles. In