1. Coincident progressive and regressive changes are induced in the tail of the frog larva by simultaneous thyroid administration and removal of a portion of the tail. The rate of growth of the regenerating tail is greatest in animals not treated with thyroid, next greatest in animals treated with thyroid 2 days after tail removal, somewhat less in animals treated with thyroid simultaneously with tail removal, and least in those treated with thyroid 2 days before removal of the tail.
2. The leucocytes chiefly concerned are neutrophilic granulocytes, lymphocytes, hemoblasts, and lymphoid phagocytes (monocytes, macrophages). The neutrophils further tissue lysis, the lymphoid phagocytes remove tissue débris. The lymphocytes are apparently not numerous enough to exert any important growth-promoting influence. The hemoblasts represent merely a part of the general reaction to thyroid treatment. Eosinophils are absent except for a few that appear in the late stages. Basophils of mesenchymal origin are present in the later stages of thyroid-treated animals. These appear to differentiate further into eosinophils.
3. The rate and amount of growth are correlated directly with the degree of crowding of cells, and inversely with the degree of vascularity. The greatest crowding and least vascularity occur in animals not treated with thyroid, somewhat less crowding and greater vascularity in those treated with thyroid 2 days after tail removal, still less crowding and somewhat greater vascularity in those treated with thyroid at the time of tail removal, and finally, least crowding and greatest vascularity occur in animals treated with thyroid 2 days before tail removal. These results support the views of Burrows (4) upon the mechanism of growth stimulation of cells.