;2 PA THOGENIC BA CTERIA. movement of the phagocytic cells of the human body, and observation of these phenomena is not difficult. If a small capillary tube be filled with sweet oil and placed beneath the skin, only a short time need pass before it will be found full of leucocytes—positive chemotaxis. If, instead of sweet oil, oil of turpentine be used, not a leucocyte will be found—negative chemotaxis. Phagocytosis is almost universal in the inicro-or- ganismal diseases at some stage or another. If the blood of a patient suffering from relapsing fever be studied beneath the microscope, it will be found to contain numerous active mobile spirilla, all free in the liquid portion of the blood. As soon as the apyretic stage comes on not a single free spirillum can be found. Every one is seen to be enclosed in the leucocytes. At the edge of an erysipelatous patch a most active warfare is waged between the streptococci and the cells. Xear the centre of the patch there are many free strep- tococci and a few cells. At the margin there are free streptococci, and also a great many streptococci en- closed in cells (leucocytes) which are, for the most part, dead. In the newly-invaded tissue we find hosts of active living cells engaged in eating up the enemies as fast as they can. The phagocytologists tell us that at the centre the bacteria are fortified, actively growing, and virulent ; in the next zone the leucocytes which have feasted upon the bacteria are poisoned by them ; outside, the cells, which are more powerful and which are con- stantly being reinforced, are waging successful warfare against the streptococci. In this manner the battle con- tinues, the cells now being obliged to yield to the bacteria and the patch spreading, while the cells subsequently re- inforce and destroy the bacteria, so that the disease comes to a termination. Metschnikoff introduced fragments of tissue from ani- mals dead of anthrax under the skin of the back of a froo- and found it surrounded and penetrated by leucocytes con- taming many of the bacilli.