These are facts, perfectly well proved, and confirmed by numberless observations made upon both the leucocytes of higher vertebrates and the amœboid cells of lower organisms. In fact, the whole first part of Metchnikoff's Leçons sur l'lnflammation is given to the description of like observations upon the ingestion and digestion of bacteria and other micro-organisms, and these observations are so conclusive that we already see growing a new science—comparative pathology—which will have to study the diseases and the means of defense against disease in all classes of animals. More than that. Not only those leucocytes which happen to be near to a microbe introduced within the body, do swallow it. It is now certain that as soon as microbes, or even some foreign substance like a splinter or coloring matter, is introduced into the body, the wandering white corpuscles of the body immediately move toward the foreign matter or organism, as if they were endowed with a certain irritability or sensibility, which directs their movements. This fact is so usual that Metchnikoff is even brought to advocate the idea that the distinctive feature of every inflammation is such a gathering of leucocytes around the infected spot, in order to destroy, if possible, the cause of infection. The defense of the living body by means of its phagocytes would thus be a fundamental character of all organisms, high and low, acquired and perfected during their evolution under the necessities of struggle for life.
However, not all bacteria are ingested by leucocytes. Thus, the leucocytes of mice (which so easily succumb to anthrax) do not swallow the anthrax bacilli; and those of pigeons and rabbits (who succumb to chicken-cholera) do not swallow the bacilli of that special disease. This fact has, however, nothing very astonishing in it, as it has its analogy in the life of the lowest organisms. Thus it has been proved that the Plasmodium of the slime-fungi, or Mycetozoa (it occurs as a gelatinous mass on the surface of trees), which consists of numberless nucleated amœbulæ, and creeps by itself over the bark of the trees, most distinctly displays a certain option in choosing the direction of its movements. If cauterized at some spot of the part which moves foremost, it changes the direction of its motion, and leaves the cauterized spot behind. A decoction of dead leaves attracts it, while a solution of sugar or salt repels it. The same is known of isolated amœbæ. So also the leucocytes immediately attack and ingest some microbes, living or dead, but avoid some others, and various kinds of leucocytes behave in various ways. The mono-nuclear leucocytes of man seem loath to attack the bacilli of erysipelas, while the many-nuclear ones display no such re-
- Metchnikoff's Leçons sur l'lnflammation, pp. 38 et seq.