rature, tiny spots will begin to appear on
the surface, or in the depth of the culture-medium. These spots are
called _colonies_, and are composed of an almost infinite number of
individual cells, the result of the continued growth of a single
organism that was in the drop of milk and which was firmly held in
place when the gelatin solidified. The number of these colonies
represents approximately the number of living bacteria that were
present in the amount of milk added to the tube of gelatin. If the
plate is not too thickly sown with the bacteria, the colonies will
continue to grow and increase in size, and as they do, minute
differences will begin to appear. These differences may be in the
color, the contour, and the texture of the colony, or the manner in
which it acts toward gelatin.

[Illustration: Fig. 4.--Plate Culture.

Each of the dots is a colony that has been formed by the growth of
an organism embedded in the solid culture-medium. By counting the
colonies, the number of living bacteria in the amount of milk added
to the culture is determined.]

In order to make sure that the number of colonies is not so numerous
as to prevent counting and further study of their characteristics, a
series of plate cultures is usually made in which varying amounts of
milk are added to the tubes of gelatine. This is attained by adding
a definite amount of the milk or other substance to be examined to a
measured amount of sterile water, e.g., one cubic centimeter of milk
to ninety-nine cubic centimeters of water. One cubic centimeter of
this mixture may be used for the inoculation of the plate culture.
This dilution may be carried on to any desired extent; in the
examination of many dairy products, it is necessary to use very
minute quantities of material, often only one one-millionth of a
cubic centimeter.

To study further the peculiarities of the different bacteria, small
portions of the individual colonies are transferred to tubes of
sterile culture-media. In order to do this the colony is touched
with a piece of platinum wire; the minute amount of growth that
adheres to the wire is sufficient to seed the tube of fresh
culture-medium. The inoculating needle must always be sterilized
before use by passing it through a gas flame.

A culture thus obtained is called a _pure culture_ since it contains
but a single kind of an organism, as the colony is the result of the
growth of a single cell. These cultures then serve as