Not a little light has been shed on certain of the phagocytic cells found in areas of aseptic inflammation by the use, in histophysiologic investigations, of the so-called "vital-dyes," and particularly by the employment of the azo dyes of the benzidin series. Tschaschin1produced lesions of lymph glands, liver and spleen by burning with a red-hot needle, and introduced trypan blue, and other similar substances, into the circulating fluids of the animals during the period of repair. He showed that the mononuclear phagocytes, which throng the inflamed tissues around these lesions, rapidly became filled with large quantities of the dyestuff, held in the form of numberless minute granules or droplets. Other workers have noted similar phenomena, and it is generally recognized now that these mononuclear phagocytic cells, during a certain period of their existence at least, take up such dyes from the surrounding fluids with avidity, and store them