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—After bacteria have fully developed, each cell divides into two equal parts; the process of division is called fission. Each of these two parts rapidly grows into a full-sized organism. Then fission again takes place, so that four bacteria replace the original one. In each of the four, fission occurs again, and so the process of multiplication continues. As bacteria develop they group themselves in characteristic ways. Some, like the streptococci, arrange themselves in chains; the diplococci, in pairs; the tetrads, in groups of four; others in packets called sarcinæ, and still others, the staphylococci, form masses supposed to resemble bunches of grapes.

Fig. 3.—Spirilla of Various Forms. (Williams.)

Fig. 4.—Bacteria showing Flagella. (Williams.)

Under favorable conditions fission occurs rapidly; in some types a new generation may appear as often as every 15 minutes. Enormous multiplication would result if nothing occurred to check the process. But in nature such increase never continues unhindered, and bacteria, acting upon their food substances, produce acids and other materials injurious to themselves. Furthermore, lack of proper food, moisture, or favorable temperature, and competition with other organisms tend to prevent their unrestricted growth and multiplication.

Fig. 5.—Bacteria with Spores. (Williams.)