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56 THE MICBO-ORGAMSMS OF THE HUMAN MOUTH.

colonies on the different plates will of course depend upon
the number in the original material. In these colonies we
have pure cultures or growths of all the different organisms
present in the material used, as far as thej^ are cultivable in
gelatine. At this time, but much better a day or two later, it
may be seen by the unaided eye (better, naturally, under a low
power of the microscope) that the colonies do not all present
the same appearance. In color they may be white, j^ellowish,
dirty white, gray, brownish gray, brown, orange, red, etc., or
they may impart color to the gelatine; they maybe perfectly
round or irregular, thick or thin, transparent or opaque, moist
or dry, homogeneous or knott}', soft or cartilaginous, etc. Some
liquefy the gelatine, others do not. Under the microscope, other
differences of structure not discernible to the naked eye may be
made out. (See Fig. 17.) By transferring one or more colonies
of each kind to tubes of gelatine or agar-agar, as described
below, we obtain larger, pure growths, in a form in which they
may be kept a greater length of time without becoming impure.
Test-tube Cultures.
When it is desired to obtain pure growths in larger masses,
and in a form in which they may be kept for a longer time, we
make use of the so-called tube cultures. These are obtained by
inoculating tubes of nutrient gelatine or agar-agar with sepa-
rate colonies, either from line or dilution cultures. Placing the
plate under a low power of the microscope (50 diameters), we
adjust an isolated colony, which we pick up with the bent point
of a platinum needle directly under the microscope (an opera-
tion which requires some practice), and transfer at once to the
tube, puncturing the gelatine with the needle to a depth of
about one and a half inches, the tube being held in the left hand,
with the mouth downward, to prevent the falling in of germs
from the air. Larger colonies may be picked up without the
aid of the microscope, provided only we have satisfied ourselves
beforehand by the microscopic examination that the colony is
really isolated, and that no small colony of some other species,
invisible to the naked eye, is lying near it. These test-tube
iirrowths also o-eiierallv furnish a ready means of distinguishing