[Illustration: FIG. 17.--VARIOUS FORMS OF BACTERIA, _a_, _b_, _c_, _d_, Round bacteria or cocci: (_a_) Staphylococci, organisms which occur in groups and a common cause of boils; (_b_) streptococci, organisms which occur in chains and produce erysipelas and more severe forms of inflammation; (_c_) diplococci, or paired organisms with a capsule, which cause acute pneumonia; (_d_) gonococci, with the opposed surfaces flattened, which cause gonorrhoea. _e_, _f_, _g_, _h_, Rod-shaped bacteria or bacilli: (_e_) diphtheria bacilli; (_f_) tubercle bacilli; (_g_) anthrax bacilli; (_h_) the same bacilli in cultures and producing spores; a small group of spores is shown. (_i_) Cholera spirillae. (_j_) Typhoid bacilli. (_k_) Tetanus bacillus; _i_, _j_, _k_ are actively motile, motion being effected by the small attached threads. (_l_) The screw-shaped spirochite which is the cause of syphilis.] The bacteria (Fig. 17) are unicellular organisms and vary greatly in size, shape and capacity of growth. The smallest of the pathogenic or disease-producing bacteria is the influenza bacillus, 1/51000 of an inch in length and 1/102000 of an inch in thickness; and among the largest is a bacillus causing an animal disease which is 1/2000 of an inch in length and 1/25000 of an inch in diameter. Among 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