ng the
free-living non-pathogenic forms much larger examples are found. In
shape bacteria are round, or rod-shaped, or spiral; the round forms
are called micrococci, the rod-shaped bacilli and the spiral forms are
called spirilli. A clearer idea of the size is possibly given by the
calculation that a drop of water would contain one billion micrococci
of the usual size. Their structure in a general way conforms with that
of other cells. On the outside is a cell membrane which encloses
cytoplasm and nucleus; the latter, however, is not in a single mass,
but the nuclear material is distributed through the cell. Many of the
bacteria have the power of motion, this being effected by small
hair-like appendages or flagellae which may be numerous, projecting
from all parts of the organisms or from one or both ends, the movement
being produced by rapid lashing of these hairs. A bacterium grows
until it attains the size of the species, when it divides by simple
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 t