The dominant function of the lymphoid tissues is to arrest foreign substances brought to them in suspension and to manufacture phagocytic cells for the blood and tissue spaces. In certain situations, notably the gastro-intestinal tract, the geographical intimacy between the lymphoid tissues and an absorptive surface of enormous dimensions would seem to indicate that the lymphoid cells were placed there primarily for purposes of filtration, restraining such foreign bodies as bacteria, and filtering their toxins. That the lymphoid depots in question are a source of supply of wandering phagocytes cannot, of course, be denied, but that this function is subsidiary appears to be indicated by the fact that, in lymphatic leukemia, which is essentially a disease of the lymphoid hemopoietic system, the gastro-intestinal lymphoid follicles escape altogether, or exhibit a negligible degree of hyperplasia. The regional lymph nodes, on the contrary, such as those of the neck, axilla and groin,