way or another, but some of them cause certain of the diseases that
we will have to discuss later so attention may be called here to a few
of the important facts in regard to their organization and life-history
in order that we may better understand how they may be so easily
transferred from one host to another.

Although these bacilli are so extremely minute (Fig. 7), some of them so
small that they cannot be seen with the most powerful microscopes, they
differ in size, shape, methods of division and spore-formation. Each
species makes a characteristic growth on gelatin, agar or other media
upon which it may be cultivated. In this way as well as by the
inoculation of animals the presence of the ultramicroscopic kinds may be
demonstrated.

The method of reproduction is very simple. They increase to a certain
point in size, then divide. This growth and division takes place very
rapidly. Twenty to thirty minutes is sufficient time in some cases for
a just-divided cell to attain full size and divide again. Thus in a few
days time the number of bacteria resulting from a single individual
would be inconceivable if they should all develop.

Fortunately for us, however, they do not all multiply so rapidly as this
and besides there are natural checks, not the least of which are the
substances given off by the bacteria themselves in their growth and
development. Such excretions often serve to inhibit further
multiplication. Sometimes, though not often, they form spores which not
only provide for a more rapid multiplication, but enable the organism to
live under conditions that would otherwise prove fatal to it.

Bacteria may be conveniently grouped under two heads: those that live
upon dead organic matter, known as the saprophytic forms, and those that
are found in living plants or animals, the true parasites. Such a
grouping is not always entirely satisfactory, for many of the kinds that
live saprophytically under normal conditions may become parasitic if
opportunity offers, and also many of those that are usually regarded as
parasitic may be grown in cultures of agar or other media, under which
conditions they may be regarded as living saprophytically.

It is this power of easily adapting themselves to different conditions
that makes many of the kinds dangerous. The bacillus which causes
tetanus or lockjaw will illustrate this. It is a rather common bacillus
in soil in many localities. As long as it remains there it is of