[Illustration: FIG. 46.—_a_, _b_, _c_, _d_. The cholera spirillum, or comma-bacillus of Koch; _a_, spirillum stage of growth, with vibrating flagellum, by which it is driven along with screw-like movement; _b_, the spirillum has lost its flagellum, and is motionless: it is marked off into separate segments; _c_, the segments have separated from one another as comma-shaped pieces, hence the name “comma-bacillus” given to it by Koch; _d_, a number of comma-bacilli of cholera which have developed tails of vibratile protoplasm (like a single cilium), and are swimming about, being driven by the lashing of these tails; _e_, a cubical packet of sarcina; _f_, a double row of the spherical units (cocci or micrococci), which form a sarcina-packet; _g_, similar cocci separated.] The circumstances which determine whether the cholera-bacillus, when it gets into the human intestine, will develop and cause an attack of cholera, or will simply be digested or will remain alive, but inactive, for a time, have yet to be exactly determined. Obviously a knowledge of them must be of immense importance. Certain experiments show that other minute parasitic organisms—especially those called _Sarcina_ (Fig. 46, _e_), which often, but by no means always, are abundant in the human intestine—favour the growth of the cholera-bacillus—in fact, prepare the ground or soil, as we may call it, for that deadly organism. This has been shown experimentally by sowing cholera-bacillus on plates of slightly acid gelatine, or jelly. It will not grow on this, but if at certain points on the surface of the jelly the _Sarcina_ organism is planted, then it is found that all around the points where the _Sarcina_ is growing the cholera-bacillus also flourishes and multiplies. And it seems probable that, just as there are microbes which are adjuvant or helpful to the cholera microbe, so there are others which are repressive or destructive of it. We know that this is the case with regard to some other microbes—namely, that a microbe which will flourish abundantly on a prepared jelly if it is alone, is entirely repressed and arrested in its growth by the presence of one other ascertained kind. It is, in fact, thus that some of the commoner putrefactive kinds of microbes occurring in river water are repressive of the typhoid-bacillus, which, if it should get there, flourishes best in the purest water or in water containing no other microbe. There is some ground for thinking that in certain districts there may be microbes present which make their way into the human intestine, and then actually repress the cholera-bacillus, should it subsequently be taken in with food or water. It would, of course, be of immense importance to discover such a microbe, if it exist, and the inquiry is at the present moment proceeding in Paris.