suggested by the familiar
observation, made by Pasteur and numerous other workers, that animals
of different species differ widely in their susceptibility to various
maladies, and that the virus of a given disease may become more and more
virulent when passed through the systems of successive individuals
of one species, and, contrariwise, less and less virulent when passed
through the systems of successive individuals of another species. These
facts suggested the theory that the blood of resistant animals might
contain something directly antagonistic to the virus, and the hope that
this something might be transferred with curative effect to the blood
of an infected susceptible animal. Numerous experimenters all over the
world made investigations along the line of this alluring possibility,
the leaders perhaps being Drs. Behring and Kitasato, closely followed by
Dr. Roux and his associates of the Pasteur Institute of Paris. Definite
results were announced by Behring in 1892 regarding two important
diseases--tetanus and diphtheria--but the method did not come into
general notice until 1894, when Dr. Roux read an epoch-making paper on
the subject at the Congress of Hygiene at Buda-Pesth.

In this paper Dr. Roux, after adverting to the labors of Behring,
Ehrlich, Boer, Kossel, and Wasserman, described in detail the methods
that had been developed at the Pasteur Institute for the development of
the curative serum, to which Behring had given the since-familiar name
antitoxine. The method consists, first, of the cultivation, for some
months, of the diphtheria bacillus (called the Klebs-Loeffler bacillus,
in honor of its discoverers) in an artificial bouillon, for the
development of a powerful toxine capable of giving the disease in a
virulent form.

This toxine, after certain details of mechanical treatment, is injected
in small but increasing doses into the system of an animal, care being
taken to graduate the amount so that the animal does not succumb to the
disease. After a certain course of this treatment it is found that a
portion of blood serum of the animal so treated will act in a curative
way if injected into the blood of another animal, or a human patient,
suffering with diphtheria. In other words, according to theory, an
antitoxine has been developed in the system of the animal subjected to
the progressive inoculations of the diphtheria toxine. In Dr. Roux's
experience the animal best suited for the purpose is the