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.