When the blood of one mammal is injected into the body of another, the latter shows remarkable modifications. When there is added to a serum, prepared from the blood of a rabbit and consisting of a colourless transparent liquid, a few drops of blood drawn from another rodent (for instance a guinea-pig), nothing unusual happens. The blood of the guinea-pig preserves its normal colour, and its corpuscles remain practically unaltered. If, instead of adding guinea-pig’s blood to the serum of rabbit’s blood, we add a serum drawn from the blood of the guinea-pig, still no special change occurs. [Illustration: FIG. 10.—Fœtus of gorilla (after Deniker). ] If, however, a serum be prepared from the blood of a rabbit into which there had first been injected the blood of a guinea-pig, the serum shows new and striking qualities. The addition to it of some drops of guinea-pig’s blood brings about, in a very short time, a changed appearance. The red liquid, at first opaque, becomes transparent. The mixture of the prepared serum of the rabbit with the blood of the guinea-pig will assume the colour of claret mixed with water. The change is due to solution of the red corpuscles of the guinea-pig in the blood serum of the rabbit. [Illustration: FIG. 11.—Human fœtus of about five months. ] This serum has still another property not less worthy of attention. If there is added to it not pure blood but only blood serum of the guinea-pig, a disturbance in the mixture occurs almost at once, and leads to the forming of a precipitate more or less abundant. The injection of the blood of the guinea-pig into a rabbit has therefore changed the serum of the latter by introducing new properties: that of dissolving the red corpuscles of the guinea-pig and of giving a precipitate with the blood serum of the same animal. Frequently the blood serum of animals prepared by previous injections of the blood of other species of animals is strictly specific. In such cases the serum only gives a precipitate with the serum of the species which has furnished the blood for the injections, and only dissolves the red corpuscles of this same species. But there are some instances in which a serum of a prepared animal dissolves, not only the red corpuscles of the species which has furnished the injected blood, but those of allied species. Thus the blood serum of the rabbit, after some injections of blood of the chicken, becomes capable of dissolving not only the red corpuscles of the chicken but also those of the pigeon, although in a less degree.