pre-eminent importance. It is not the bacteria alone, and the so-called toxins, that have to be considered in the whole question of immunity reactions, but most probably the intermediate products of their metabolism, as well as certain decomposites which are, at any rate partly, formed quite outside the cells in question. And, above all, ve have to consider the structure of the organism. The host directs its struggle, not only against the living micro-organism, but also against the particles which appear with the decay of the dead organism, and more particularly against the intermediate products which originate during the preparation of the nutritive medium. The organism attacks every point with its ferments, and tries to decompose or reconstruct anything that is disharmonious with its structure or configuration, or even its physical properties. The more it succeeds in this respect, the more does it deprive the micro-organisms of the conditions required for their existence, and protect its own cells against the injurious action of these substances.
We come, then, to the conclusion, that at least one part of the means of defence, possessed by an organism against infections of any kind, depends on its power of liberating ferments, which attack the disharmonious substances—be they by-products or end-products of metabolism, or products of cellular disintegration—and deprive them as quickly as
