Hormones in Metamorphosis of Amphibia
In amphibians, the changes that take place during metamorphosis are brought about by hormonal secretions of the thyroid gland. The first sign of this was obtained in 1912 when Gudematsch reported that frog tadpoles while fed dried and powdered sheep thyroid underwent precocious or early metamorphosis. Identical results however were not observed when tadpoles were fed along with preparation of other glands. These experiments made it feasible to postulate that thyroid hormones bring about metamorphosis.
In 1918 B.M. Allen observed that elimination of thyroid rudiment from early frog tadpoles, prevented them from going through metamorphosis, and caused them to become giant tadpoles instead. Conversely, if they were fed with thyroid or immersed in water containing soluble extracts from thyroid glands they then proceeded immediately to metamorphosis. Identical experiments in urodele amphibians, in particular in axolotl, Ambystoma mexicanum, also indicated the significance of the thyroid gland in urodele metamorphosis.