know first how they look and where they are found. A short
consideration of certain morphological facts will therefore be
necessary at the start.

FORM OF BACTERIA.

In shape bacteria are the simplest conceivable structures.
Although there are hundreds of different species, they have only
three general forms, which have been aptly compared to billiard
balls, lead pencils, and corkscrews. Spheres, rods, and spirals
represent all shapes. The spheres may be large or small, and may
group themselves in various ways; the rods may be long or short,
thick or slender; the spirals may be loosely or tightly coiled,
and may have only one or two or may have many coils, and they may
be flexible or stiff; but still rods, spheres, and spirals
comprise all types.

In size there is some variation, though not very great. All are
extremely minute, and never visible to the naked eye. The spheres
vary from 0.25 u to 1.5 u (0.000012 to 0.00006 inches). The rods
may be no more than 0.3 u in diameter, or may be as wide as 1.5 u
to 2.5 u, and in length vary all the way from a length scarcely
longer than their diameter to long threads. About the same may be
said of the spiral forms. They are decidedly the smallest living
organisms which our microscopes have revealed.

In their method of growth we find one of the most characteristic
features. They universally have the power of multiplication by
simple division or fission. Each individual elongates and then
divides in the middle into two similar halves, each of which then