~Form and size.~ When a plant is composed of a single cell but little difference in form is to be expected. While there are intermediate stages that grade insensibly into each other, the bacteria may be grouped into three main types, so far as form is concerned. These are spherical, elongated, and spiral, and to these different types are given the names, respectively, _coccus_, _bacillus_ and _spirillum_ (plural, _cocci_, _bacilli_, _spirilla_) (fig. 1). A ball, a short rod, and a corkscrew serve as convenient models to illustrate these different forms. [Illustration: FIG. 1. Different forms of bacteria. _a_, _b_, _c_, represent different types as to form: _a_, coccus, _b_, bacillus, _c_, spirillum; _d_, diplococcus or twin coccus; _e_, staphylococcus or cluster coccus; _f_ and _g_, different forms of bacilli, _g_ shows internal endospores within cell; _h_ and _i_, bacilli with motile organs (cilia).] In size, the bacteria are the smallest organisms that are known to exist. Relatively there is considerable difference in size between the different species, yet in absolute amount this is so slight as to require the highest powers of the microscope to detect it. As an average diameter, one thirty-thousandth of an inch may be taken. It is difficult to comprehend such minute measurements, but if a hundred individual germs could be placed side by side, their total thickness would not equal that of a single sheet of paper upon which this page is printed. ~Manner of Growth.~ As the cell increases in size as a result of growth, it elongates in one direction, and finally a new cell wall is formed, dividing the so-called mother-cell into two, equal-sized daughter-cells. This process of cell division, known as _fission_, is continued until growth ceases and is especially characteristic of bacteria. ~Cell Arrangement.~ If fission goes on in the same plane continually, it results in the formation of a cell-row. A coccus forming such a chain of cells is called _strepto-coccus_ (chain-coccus). If only two cells cohere, it is called a _diplo-coccus_ (twin-coccus). If the second cell division plane is formed at right angles to the first, a _cell surface_ or _tetrad_ is formed. If growth takes place in three dimensions of space, a _cell mass_ or _sarcina_ is produced. Frequently, these cell aggregates cohere so tenaciously that this arrangement is of value in distinguishing different species. ~Spores.~ Some bacteria possess the property of forming _spores_ within the mother cell (called _endospores_, fig. 1g) that are analogous in function to the seeds of higher plants and spores of fungi. By means of these structures which are endowed with greater powers of resistance than the vegetating cell, the organism is able to protect itself from the effect of an unfavorable environment. Many of the bacilli form endospores but the cocci do not. It is these spore forms that make it so difficult to thoroughly sterilize milk.