Abstract
The removal of the thyroid gland anlage and the buccal anlage of the hypophysis from amphibian larvae affords animals in which these glands have never functioned. In the first series Rana aurora tadpoles kept for over 4 months after the operation were placed in thyroxin solutions of 1-500,000; 1-1,000,000; and 1-2,000,000 concentration in tap water. In all cases there was a very rapid tendency to metamorphosis that proceeded with equal speed and to an equal degree in each of the groups, resulting in the appearance of all 4 limbs except in a few cases in which the right fore-limb failed to pierce the skin. The tail rapidly shrivelled and typical changes took place in the position of the eyes, the character of the mouth, the retrogression of the gills, and, all of the other features characteristic of thyroid induced metamorphosis. The rapidity of the changes of metamorphosis was to a slight degree inversely proportional to the concentration of the thyroxin solution, as we should expect; but some diversity in size of specimens precludes our attaching too great importance to this point.
Controls maintained during this time under identical conditions failed to show any tendency to metamorphosis. The largest had only the hind-limbs developed to the point of showing the knee-joints, and the fore-limbs had not appeared in any, nor was there any shortening of the tail or other evidence of metamorphosis. Because of these facts we may conclude that the thyroid gland and anterior lobe of the hypophysis in the normal specimens placed in thyroxin were not factors to be considered; probably because they had not yet become active to any significant degree.
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