cleavage at right angles to the long axis forming two individuals. In
some of the spherical forms division takes place alternately in two
planes, and not infrequently the single individuals adhere, forming
figures of long threads or chains or double forms. The rate of growth
varies with the species and with the environment, and under the best
conditions may be very rapid. A generation, that is, the interval
between divisions, has been seen to take place in twenty minutes. At
this rate of growth from a single cholera bacillus sixteen quadrillion
might arise in a single day. Such a rate of growth is extremely
improbable under either natural or artificial conditions, both from
lack of food and from the accumulation in the fluid of waste products
which check growth. Many species of bacteria in addition to this
simple mode of multiplication form spores which are in a way analogous
to the seeds of higher plants and are much more resistant than the
simple or vegetative forms; they endure boiling water and even higher
degrees of dry heat for a considerable time before they are destroyed.
When these spores are placed in conditions favorable for bacterial
life, the bacterial cells grow out from them and the usual mode of
multiplication continues. This capacity for spore formation is of
great importance, and until it was discovered by Cohn in 1876, many of
the conditions of disease and putrefaction could not be explained.
Spores, as the seeds of plants, often seem to be produced when the
conditions are unfavorable; the bacterium then changes into this form,
which under natural conditions is almost indestructible and awaits
better days.

The bacteria are divided into species, the classification being based
on their forms, on the mode of growth, the various substances which
they produce and their capacity for producing disease. The
differentiation of species in bacteria is based chiefly upon their