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 lack of food and from the accumulation in the fluid of waste products which check growth.  Many species of bacteria in addition to this simple mode of multiplication form spores which are in a way analogous to the seeds of higher plants and are much more resistant than the simple or vegetative forms; they endure boiling water and even higher degrees of dry heat for a considerable time before they are destroyed.  When these spores are placed in conditions favorable for bacterial life, the bacterial cells grow out from them and the usual mode of multiplication continues.  This capacity for spore formation is of great importance, and until it was discovered by Cohn in 1876, many of the conditions of disease and putrefaction could not be explained.  Spores, as the seeds of plants, often seem to be produced when the conditions are unfavorable; the bacterium then changes into this form, which under natural conditions is almost indestructible and awaits better days.