athogenetic bacteria are in some cases so great as to completely
overwhelm the differences; again the same bacterium may under varying
conditions assume appearances so different from those regarded as
typical or normal as to throw doubt on its identity. In each case a
simple inoculation experiment may decide the point at once. As a
concrete example may be instanced an autopsy on an animal dead from an
unknown infection. Cultivations from the heart blood gave a pure growth
of a typical (capsulated) pneumococcus. Cultivations from the liver gave
a pure growth of what appeared to be a typical (non-capsulated)
Streptococcus pyogenes longus. The latter inoculated into a rabbit
caused the death of the animal from pneumococcic septicaemia, and
cultures from the rabbit's blood gave a pure growth of a typical
(capsulated) pneumococcus.

~D. Study of the Problems of Immunity.~

It is only by a careful and elaborate study of the behaviour of the
animal cell and the body fluids vis-a-vis with the infecting bacterium
that it becomes possible to throw light upon the complex problem whereby
the cell opposes successful resistance to the diffusion of the invading
microbe, or succeeds in driving out the microbe subsequently to the
occurrence of that diffusion.

At the moment, however, our attention is directed to the first of these
broad headings, for it is by the application of the knowledge acquired
in its pursuit that we are able to deal with problems arising under any
of the remainder.

For whatever purpose the inoculation is performed, it is essential that
the experiment should be planned to secure the maximum amount of
information and the minimum of discomfort to the animal used. Every care
therefore must be taken to ensure that the virus is introduced into the
exact tissue or organ selected; and the operation itself must be carried
out with skill and expedition, and under strictly aseptic conditions.

In the course of inoculation studies many instances of natural immunity,
both racial and individual, will be met with; but it must be recollected
that natural immunity is relative only and never absolute, and care be
taken not to label an organism as _non-pathogenic_ until many different
methods of inoculation have been performed upon different species of
animals, combined when necessary with various procedures calculated to
overcome any apparent immunity, and have invariably given negative
results.

In some countries experime