the cells of the host catch the secreted ferments of the micro-organism, or render them inactive in some other way, and in this manner either increase the difficulties of existence for the invaders, or else completely destroy them.
How sensitive individual organisms are, in regard to these nutritive substrates, is shown, by numerous laboratory observations on the cultivation of the most varied micro-organisms. We know that many of them only thrive, when very definite substrates are offered to them. The fact, that an alteration of the nutritive medium deprives certain organisms of their means of existence, is shown in the clearest manner by the observation that an infection with trichophyton fungi cures itself at the time of puberty. Evidently the cells of the skin become so modified, at the onset of sexual maturity, that the substrate of the host—the components of the skin—can no longer act as a means of subsistence for the fungus. From this point of view we may well imagine that medicines and other therapeutical means effect a curative action, without directly attacking particular kinds of cells which may be living as parasites in the animal organism, for they need only to destroy the conditions of existence required by the organism in question by means of a modification of its nutritive substrate. It is imaginable that certain means do modify certain cells to such an extent, that their com-
