n
the opsonic index (negative phase) which, however, is followed later by a
rise to a higher level than before. If the amounts of vaccine used and the
times of the injection are suitably chosen, there may thus be produced by a
series of steps a rise of the opsonic index to a high level. One of the
chief objects in registering the opsonic power in such cases is to avoid
the introduction of additional vaccine when the opsonic index is low, that
is, during the negative phase, as if this were done a further diminution of
the opsonic action might result. The principle in such treatment by means
of vaccines is to stimulate the general production of anti-substances
throughout the body, so that these may be carried to the sites of bacterial
growth, and aid the destruction of the organisms by means of the cells of
the tissues. A large number of favourable results obtained by such
treatment controlled by the observation of the opsonic index have already
been published, but it would be unwise at present to offer a decided
opinion as to the ultimate value of the method.

Active immunity has thus been shown to be associated with the presence of
certain anti-substances in the serum. After these substances have
disappeared, however, as they always do in the course of time, the animal
still possesses immunity for a varying period. This apparently depends upon
some alteration in the cells of the body, but its exact nature is not
known.

[Sidenote: Phagocytosis.]

The destruction of bacteria by direct cellular agency both in natural and
acquired immunity must not be overlooked. The behaviour of certain cells,
especially leucocytes, in infective conditions led Metchnikoff to place
great importance on phagocytosis. In this process there are two factors
concerned, viz. the ingestion of bacteria by the cells, and the subsequent
intracellular digestion. If either of these is wanting or interfered with,
phagocytosis will necessarily fail as a means of defence. As regards the
former, leucocytes are guided chiefly by chemiotaxis, _i.e._ by
sensitiveness to chemical substances in their surroundings--a property
which is not peculiar to them but is possessed by various unicellular
organisms, including motile bacteria. When the cell moves from a less to a
greater degree of concentration, _i.e._ towards the focus of production,
the chemiotaxis is termed positive; when the converse obtains, negative.
This apparently purposive movement has been point