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Annual Address by the President. 15 
bacillus, which was not virulent enough to cause death, was able to im- 
part to an inoculated animal powers of resisting the invasion of a 
virulent culture; and that by repeated inoculations of the milder virus, 
the degree of resistance to the strong virus co.uld be raised to a very 
high limit. 
However, although this method of direct inoculation with the organ- 
isms of the virus was successfully carried out, it was found to be rather 
dangerous; and, as a substitute, it was found that inoculation with the 
toxin of the germs would produce similar results. By this method the 
organisms were filtered away and the filtrate, containing the poisons 
produced by the growth of the organisms, was employed. (This was 
the basis of the first tuberculin made by Koch that caused such con- 
sternation in 1890 and that proved such .a lamentable failure.) 
Further, it was found that if toxin free from organisms was repeatedly 
injected into an animal it could be rendered immune to the action of 
the bacteria forming the toxin; and if the serum of this animal so im- 
munized was injected into man, he in turn became resistive and at 
times immune to the action of organisms. 
The methods by which an animal is successfully immunized against 
a given micro-organism are two in number. We can employ either the 
micro-organisms or the culture fluid from which the organisms have 
been carefully filtered. But a difference results in the serum if only 
one method is employed. If we use the organisms only, the serum gains 
marked bacteriolytic powers, but seems to have very little power of 
neutralizing the toxin that the bacteria produce. If, however, the fil- 
tered culture is used, the blood serum possesses both bacteriolytic and 
antitoxic properties. Therefore, as both these properties are essential, 
it is the rule to immunize an animal by injecting both bacteria (living 
or dead) and the filtered culture also. 
From the above it will be gathered that whatever the nature of the 
immune substance may be, it can not be single, but must consist of two 
substances at least, possessing, respectively, bacteriolytic and antitoxic 
properties. 
Keturning to the second method by which bacteria can be destroyed, 
we are led to a discussion of the role played by the living cells of the 
tissues in resisting the attacks of micro-organisms. The phenomena 
observed were first carefully described by Metschnikoff, whose earnest 
investigations laid the foundation of the phagocytosis theory. This 
observer, who has worked, out the part played by the cells of the body 
in resisting the attacks of organisms, believes that the white cells of the 
blood and the fixed cells of connective tissue actually attack the organ- 
isms, ingest them, and destroy them. In short, he believes they are the 
main agents by w'hich they are destroyed and gave to them the name 
