fferent forms of bacilli, _g_ shows
internal endospores within cell; _h_ and _i_, bacilli with motile organs
(cilia).]

In size, the bacteria are the smallest organisms that are known to
exist. Relatively there is considerable difference in size between the
different species, yet in absolute amount this is so slight as to
require the highest powers of the microscope to detect it. As an average
diameter, one thirty-thousandth of an inch may be taken. It is difficult
to comprehend such minute measurements, but if a hundred individual
germs could be placed side by side, their total thickness would not
equal that of a single sheet of paper upon which this page is printed.

~Manner of Growth.~ As the cell increases in size as a result of growth,
it elongates in one direction, and finally a new cell wall is formed,
dividing the so-called mother-cell into two, equal-sized daughter-cells.
This process of cell division, known as _fission_, is continued until
growth ceases and is especially characteristic of bacteria.

~Cell Arrangement.~ If fission goes on in the same plane continually, it
results in the formation of a cell-row. A coccus forming such a chain of
cells is called _strepto-coccus_ (chain-coccus). If only two cells
cohere, it is called a _diplo-coccus_ (twin-coccus). If the second cell
division plane is formed at right angles to the first, a _cell surface_
or _tetrad_ is formed. If growth takes place in three dimensions of
space, a _cell mass_ or _sarcina_ is produced. Frequently, these cell
aggregates cohere so tenaciously that this arrangement is of value in
distinguishing different species.

~Spores.~ Some bacteria possess the property of forming _spores_ within
the mother cell (called _endospores_, fig. 1g) that are analogous in
function to the seeds of higher plants and spores of fungi. By means of
these structures which are endowed with greater powers of resistance
than the vegetating cell, the organism is able to protect itself from
the effect of an unfavorable environment. Many of the bacilli form
endospores but the cocci do not. It is these spore forms that make it
so difficult to thoroughly sterilize milk.

~Movement.~ Many bacteria are unable to move from place to place. They
have, however, a vibrating movement known as the _Brownian_ motion that
is purely physical. Many other kinds are endowed with powers of
locomotion. Motion is produced by means of fine thread-like processes of
protoplasm known as