nic bacteria. But, as already stated, in the light of recent experiments, this theory now appears to us to be untenable as a general explanation of acquired immunity.
The Theory of Phagocytosis.—The fact that in certain infectious diseases due to bacteria the parasitic invaders, at the point of inoculation or in the general blood-current, are picked up by the leucocytes, and in properly stained preparations may be seen in their interior, has been known for some years. Now, the theory of phagocytosis assumes that the bacilli are picked up by the leucocytes and destroyed in their interior, and that immunity depends largely upon the power of these "phagocytes" to capture and destroy living pathogenic bacilli.
The writer suggested this as a hypothesis as long ago as 1881, in a paper read August 18, 1881, before the American Association for the Advancement of Science, in the following language: "It has occurred to me that possibly the white corpuscles may have the office of picking up and digesting bacterial organisms which by any means find their way into the blood. The propensity exhibited by the leucocytes for picking up inorganic granules is well known, and that they may be able not only to pick up but to assimilate, and so dispose of, the bacteria which come in their way, does not seem to me very improbable, in view of the fact that amoebae, which resemble them so closely, feed upon bacteria and similar organisms."
At a later date (1884) Metschnikoff offered experimental evidence in favor of this view, and the explanation suggested in the above quotation is commonly spoken of as the Metschnikoff theory. The observations which first led Metschnikoff to adopt this view were made upon a species of daphnia which is subject to fatal infection by a torula resembling the yeast fungus. Entering with the food, this fungus penetrates the walls of the intestine and invades the tissues. In certain cases the infection does not prove fatal, owing, as Metschnikoff asserts, to the fact that the fungus cells are seized upon by the leucocytes, which appear to accumulate around the invading parasite (chemiotaxis) for this special purpose. If they are successful in overpowering and destroying the parasite, the animal recovers; if not, it succumbs to the general infection which results. In a similar manner, Metschnikoff supposes, pathogenic bacteria are destroyed when introduced into the body of an immune animal. The colorless blood-corpuscles, which he designates phagocytes, accumulate at the point of invasion and pick up the living bacteria, as they are known to pick up inorganic particles injected into the circulation. So far there can be no doubt that Metschnikoff is right. The presence of bacteria in the leucocytes in considerable numbers, both at the point of inoculation and in the general circulation, has been repeatedly