The Behavior of the Leucocytes during Coinci- Dent Regeneration and Thyroid-induced Metamorphosis in the Frog Larva, with a Consideration of Growth Factors. By

Various aspects of the problem of growth stimulation of tissues have been emphasized recently. From experiments with tissue cultures Carrel (1) has obtained results showing that leucocytes have a growth-promoting function, that they give off substances which stimulate the growth of adjacent cells. These growth-promoting substances he has named trephones (2). On the basis of the specific differential distribution of leucocytes in thyroid-treated tadpoles, the writers (3) have concluded that, if leucocytes have a growthpromoting function, this function is probably to be ascribed to the lymphocytes; that the granulocytes, on the contrary, probably give off substances that aid in the breaking down of tissues.