The work of M. Paul Bert, at the close of 1876, was surrounded with circumstances no less complex. To thoroughly understand them we must call to mind Pasteur's discovery as to the mode of reproducing the anaerobic germs of putrefaction. These vibrios reproduce themselves by spores. In the vibrio of acute septicæmia this is the mode of generation. Short or long jointed filaments show themselves studded with brilliant points, which are precisely the spores of which we speak. Experience proves that these spores resist perfectly the poisonous action of compressed oxygen. Inoculating an animal with blood which is at the same time septic and splenic, after the blood has been compressed, the septic germs, remaining alive, produce death, although neither bacteria nor filaments may be perceptible in its blood at the moment of death. It was likewise from Chartres that M. Paul Bert obtained his supply of splenic fever blood. The blood he had received was without doubt not only splenic but also septic. The filaments of bacteria and the filaments of septic vibrios had perished under the influence of the compressed oxygen; but the spores were there, and the great pressure of oxygen gas had not affected them. The new contagium which had appeared, and which had killed the inoculated animals, was due to these spores. As regards the proof that this virulence in the blood of the body of an animal which has died of splenic fever is really the effect of the septic vibrio, Pasteur, assisted by Joubert and a new assistant, M. Chamberland, has given that proof, as he did in the case of the bacterium of splenic fever, by resorting to the method of successive cultivations in an artificial medium. These cultivations, however, of the septic vibrio require very special precautions and conditions. They should be carried on in as perfect a vacuum as it is possible to obtain, or in contact with carbonic acid gas without the presence of air. In contact with air the cultivations of septic vibrios would prove sterile, because the vibrio is exclusively anaerobic and air kills it. If a spore of this organism could germinate in contact with the air, the product of the germination would be at once arrested and would perish by the action of the oxygen. It is exactly the contrary with the bacilli of splenic fever, which prove sterile in a vacuum or in presence of carbonic acid gas. If one of the spores of the splenic fever bacillus (for it also produces spores) could germinate, the product of the germination, deprived of free oxygen, would at once perish. And, to mention in passing a very ingenious experiment of Pasteur's, we thus obtain a means of separating by culture the bacillus of splenic fever from the septic vibrio when they are temporarily associated together. If this mixture of pathogenic organisms is cultivated in contact with the air, the bacilli of splenic fever alone will be developed. If this same mixture is cultivated without air,