differences in size being noticed. This method of spore formation
occurs only in a few special forms of bacteria.

The matter of spore formation serves as one of the points for
distinguishing species. Some species do not form spores, at least
under any of the conditions in which they have been studied.
Others form them readily in almost any condition, and others again
only under special conditions which are adverse to their life. The
method of spore formation is always uniform for any single
species. Whatever be the method of the formation of the spore, its
purpose in the life of the bacterium is always the same. It serves
as a means of keeping the species alive under conditions of
adversity. Its power of resisting heat or drying enables it to
live where the ordinary active forms would be speedily killed.
Some of these spores are capable of resisting a heat of 180
degrees C. (360 degrees F.) for a short time, and boiling water
they can resist for a long time. Such spores when subsequently
placed under favourable conditions will germinate and start
bacterial activity anew.

MOTION.

Some species of bacteria have the power of active motion, and may
be seen darting rapidly to and fro in the liquid in which they are
growing. This motion is produced by flagella which protrude from
the body. These flagella (Fig. 15) arise from a membrane
surrounding the bacterium, but have an intimate connection with
the protoplasmic content. Their distribution is different in
different species of bacteria. Some species have a single
flagellum at one end (Fig. 15 a). Others have one at each end