thogenic
organism was known, the manner and site of invasion was clear, it was
also evident that if the multiplication of the parasite was unchecked
the animal died, but if the parasite was opposed by the body cells and
destroyed the animal recovered. The studies were carried further into
the diseases of the higher animals, and it was found the leucocytes in
these played the same part as did the cells in the body cavity of the
daphnea. The introduction of bacteria into certain animals was
followed by their destruction within cells and no disease resulted; if
this did not take place, the bacteria multiplied and produced disease.
Support also was given the theory by the demonstration at about the
same time that in most of the infectious diseases the leucocytes of
the blood became increased in number,--that in pneumonia, for
instance, instead of the usual number of eight thousand in a cubic
millimeter of blood, there were often thirty thousand or even fifty
thousand. At about the same time also chemotaxis, or the action of
chemical substances in attracting or repelling organisms, excited
attention, and all these facts together became woven into the theory.
It was soon seen, however, that this theory, based as it was on
observation and supported by the facts observed, was not, at least in
its first crude form, capable of general application. Many animals
have natural immunity to certain diseases; they do not have the
disease under natural conditions, nor do they acquire the disease when
the organisms causing it are artificially introduced into their
tissues by inoculation. Such natural immunity seemed to be unconnected
with defence by phagocytosis, for the leucocytes of the animal might
or might not have phagocytic reaction to the particular organisms to
which the animal was immune. It was also seen that recovery from
infection in certain diseases was unconnected with phagocytosis. It
had also been demonstrated, by German observers chiefly, that the
serum of the blood, the colorless fluid in which the corpuscles float,
was itself destructive, and that in an animal rendered immune to a
special bacterium the destructive action of the serum on that organism
was greatly increased. In this hostile serum the bacteria often became
clumped together in masses, the bodies became swollen, broken up, and
finally disintegrated. This property of the serum was described as due
to a substance in the serum called _alexine_, which in the immu