difficult to conceive of the minuteness of the bacteria;
the following may give some idea of their size. In a drop of cream
ready for churning may be found as many as 10,000,000 and in a piece
of fresh cheese as large as a cherry there may be as many living
bacteria as there are people on our earth. While the bacteria are
very minute, the effect which they exert in milk and other dairy
products is great on account of their enormous numbers.

=Manner of growth.= The cells of which all plants and animals consist
increase in numbers by the division of each cell into two cells
through the formation of a division wall across the cell. The new
cells divide and the plant or animal continues to grow. The same
cell division occurs in the bacteria but since the bacteria are
single celled, division of the cells means an increase in numbers
rather than growth as in the higher forms of life.

[Illustration: Fig. 2.--Division of Bacteria.

The bacteria increase in numbers by the division of each cell into
two cells. (After Novy.)]

In the case of those bacteria that have a greater length than
diameter, the new wall is formed at right angles to the long axis of
the cell. As soon as the division is complete each cell is a
complete individual, capable of carrying on all of its life
processes. The cells may, however, cohere and thus form distinctive
groupings that may serve to identify certain types. Some of the
cocci form long chains and the term _streptococcus_ is applied to
such. Other groupings may be similar to a bale of twine or they may
be massed in clusters with no regularity distinguishable.

=Spores.= Just as ordinary plants form resistant structures, known as
seeds, capable of retaining vitality under conditions unfavorable
for growth thereby perpetuating the species, so with certain of the
bacteria, definite structures, known as _spores_, that are analogous
in some respects to the seeds of the higher plants, are produced
within the mother cell. The spores are exceedingly resistant to the
influence of an unfavorable environment, such as heat, cold, drying,
and even chemical agents. It is this property of the spores which
makes it so difficult to destroy the bacterial life in the process
of sterilizing milk. The property of spore-formation is fortunately
confined to a comparatively small number of different species of
bacilli.

=Movement.= Many of the bacteria are provided with vibratory organs of
locomotion, known as _