The lymphocyte-like cell of rat marrow is capable of performing as a multipotential cell. It resembles a small lymphocyte, and reacts to the effects of cortisone and thio-TEPA, as do small lymphocytes in other organs, thus should be called a small lymphocyte.
Its sudden appearance in large numbers to occupy almost 50 per cent of the marrow space in suckling rats fourteen days of age can best be explained on the basis of migration of these cells to the marrow from other organs via the blood stream. The rate at which lymphocytosis of the bone marrow occurs in rats is influenced by the size of the suckling litters, and may be related to a non-caloric factor in mother's milk. The small lymphocyte probably represents the inactive phase of a multipotential cell, a dormant cell which can give rise to other cell forms when appropriately stimulated.