Date of Award
Life & Environmental Sciences
Rev. Joseph Harrington
Knowledge of the thyroid gland as an organ was known as far back as the third and second centuries B.C. when many of the Greek and Roman physicians recognized that goiter was a pathologic enlargement of the thyroid. It received its name "glandula thyroidea" from Thomas Wharton in 1656 in the first thorough account of its anatomy.However, it wasn't until the latter part of the nineteenth century that the significance of the thyroid gland as an endocrine organ regulating metabolic activity was recognized.
After this, much interest was turned toward the importance and function of the thyroid gland, but all attempts to determine its affect by thyroidectomy proved to be futile as it only resulted in the death of the animal. "It was not until Gudernatsch’s discovery (1912) of the role of the thyroid in control of amphibian metamorphosis that extensive experimental study of thyroid function in cold-blooded vertebrates was undertaken."2 Of all the mysteries of life, one of the most intriguing is animal metamorphosis. Metamorphosis of amphibians has been especially studied because they undergo extensive morphologic changes, and they also proved to be relatively stable experimental animals in that they could adequately withstand the necessary experimental treatments with a relatively low mortality rate. Metamorphosis of tadpoles will be specifically dealt with in this paper. It is found that metamorphosis in tadpoles involves two general stages~a prometamorphic stage, and a metamorphic climax. The tadpole goes through this initial prometamorphic stage in which its initial streamlined, fishlike form is somewhat altered by the development of hind legs. This stage varies in length from two weeks to a year and a half, depending upon the particular species. The stage comes to an end when the length of the hind legs eventually develops to a length equal to that of the torso of the animal. With this the tadpole enters into its metamorphic climax. Metamorphic climax entails a number of macroscopic morphological changes. First, forelegs suddenly erupt through small openings close to the gills, followed by a widening and strengthening of the jaw? the skin and lungs are transformed? and finally the gills and tail are resorbed. One of the most remarkable phenomena about this transformation is the exquisite timing and perfectly coordinated pattern involved. The tail is not resorbed until the legs have nearly completed growth? the nostrils and mechanism for respiration by means of the lungs appear just before the gills are resorbed.
Dernback, Timothy, "The Thyrotropic Effect On The Metamorphic Development Of Rana catesbiana" (1968). Life and Environmental Sciences Undergraduate Theses. 536.