The inoculation of anthropoid apes with syphilis was successful. The chimpanzee was found to be most sensitive to the disease; it manifests primary and secondary symptoms identical with those of man. Lower monkeys, though less sensitive, also contract syphilis but generally only show primary characteristic manifestations. The possibility of rapidly provoking in apes, even of the inferior kinds, syphilitic lesions similar to those of man has a very great importance, for it provides a sure means of diagnosis in doubtful human cases. Owing to the liability of apes to contract syphilis, experimental vaccination and serotherapy could be attempted on them; but, though these experiments were sometimes encouraging, the results obtained were not constant enough to justify their application to man. Thus, it was found possible to attenuate the virus by successive passages in certain lower apes, and yet, though attenuated for the chimpanzee, it did not confer upon him immunity against the active virus. In 1905, Schaudinn discovered the syphilitic treponema in man. By using this discoverer's method, the same microbe was found in apes inoculated with human virus, which confirmed the specific character of the treponema. An observation was then made which was of great importance on account of its consequences: it was ascertained that the syphilitic microbe was absorbed by the less mobile mononuclear phagocytes and remained localised near the entrance point long enough to allow of a local treatment which might succeed in being curative as it had time to act before the microbes had passed into the general circulation of the organism. This supposition was proved to be correct by a series of experiments on monkeys, and, in 1906, a young doctor, M. Maisonneuve, inoculated himself with syphilis and applied the treatment with a perfectly satisfactory result. It might have been thought that this simple, safe, and innocuous method would at once come into practice, but it was not so. Between opposition on the one hand, and carelessness of the subjects themselves on the other, this useful discovery remained for a long time without being utilised. All the above results were obtained through experiments on anthropoid apes, and the study of syphilis, until then purely clinical, entered at last into the field of experimental science. * * * * * Researches upon syphilis were but an interlude; Metchnikoff, returning to his principal work, resumed the study of senility and of the intestinal flora. During many years he applied himself to researches concerning the part played by the latter within the organism. He was able to confirm the deductions expounded in his _Études sur la nature humaine_, and in 1907 he published a new work, _Essais optimistes_, in which he developed the same ideas, amplified by the results of his new researches, and answering the criticisms excited by his first book.