The great divergence of opinion as to the origin of the clasmatocytes, epithelioid cells, and giant cells of the Langhans type can be ascribed either to inadequate technic or to the possibility of a varied origin. The simple technic applied first by Mrs. Lewis to mammalian (man, dog, cat, mouse, guinea-pig) and avian (chicken) blood and by both of us to the blood of elasmobranchs, bony fishes, amphibia and reptiles enables one to prove beyond question that the white blood cells, in all the various forms studied, can give rise to clasmatocytes, epithelioid cells, and giant cells of the Langhans type. The method is so simple that any competent investigator can repeat our observations. Small drops of whole blood, taken under aseptic conditions from the heart or periphery, made into ordinary hanging drops and kept at suitable temperatures, contain, after a few days (from two to twenty-one), varying numbers of