10 THE GENERAL CHARACTERS OF MICROORGANISMS. 
for a long time prevented the development of the science. Bacterio- 
logical study to-day commonly begins with cultivating the bacteria, 
i.e., allowing them to grow in some medium adapted to them until 
they become abundant enough to be handled in bulk. To prevent 
the mixing of the different kinds that may be in the material we are 
studying, they are usually grown in a solid or jelly-like medium, 
which holds the individuals fast in one spot. As the individuals 
multiply in this solid medium, they are unable to separate from each 
other; so they remain in little clusters which in time become large 
enough to be seen without a microscope. 
Such clusters are called colonies, and 
figures of some of them are shown in Fig. 6. 
The shape and appearance of the colonies 
produced by different kinds of bacteria are 
often very different, showing, indeed, greater 
FIG. 6 Colonies of bacteria, varieties than can be seen in the bacteria 
themselves with a microscope. As a result the shape and appear- 
ance of these colonies are often used to separate the numerous 
bacteria from each other and to classify them. A colony, when it 
comes from the multiplication of a single individual bacterium, is 
made of one kind of bacteria only. This colony may easily be 
picked out with a sterile needle, and when properly placed in 
another culture medium it becomes a pure culture. The starting- 
point in practical bacteriological study is thus the colony rather 
than the individual bacterium (see Laboratory Work). 
Form of Bacteria. Bacteria are of three quite different shapes, 
but are all very simple, i. Simple spheres (see Fig. 7, a). Such 
spherical forms are called Cocci. In common microscopical 
preparations no internal structure can be seen, the bacteria appearing 
as deeply stained balls. The Cocci, however, differ somewhat 
in their method of growth, thus enabling the microscopist to distin- 
guish different kinds, as will be mentioned presently. 2. The rod- 
formed bacteria (Fig. 7, b). These organisms are longer than they 
are broad, sometimes only slightly so, but at other times very much 
longer, forming, indeed, long, slender threads. 3. The spiral- 
formed bacteria. These are either long, coiled spirals, or very 
