The presence in the peripheral blood of a relatively large number of granular polymorphonuclear leukocytes and of lymphocytes has greatly facilitated the investigations by means of which these white blood cells have been traced to the bone marrow and to the lymphoid tissue, respectively. The determination of the origin of the mononuclear phagocytes which are much less numerous in the normal blood has been attended with greater difficulty. Ehrlich,1 by the use of differential staining and especially by the method employing the triacid stain, recognized in the blood a large mononuclear leukocyte which he thought came from the spleen and the bone marrow and which was in the circulating blood transformed into the polymorphonuclear leukocyte. He observed further that some mononuclear leukocytes had indentations in the nuclei together with a few fine granules in the cytoplasm, and these he regarded as the transitional forms between the mononuclear leukocyte and the