CULTURES, AND THEIR STUDY.

149

Certain bacteria, as the tubercle bacillus, will not grow
at all upon gelatin. Some forms which are rigidly ae-
robic will only grow upon or near the surface ; others,
anaerobic, only in the deeper parts. The majority, how-
ever, grow both upon the surface and in the puncture
made by the wire. Sometimes the consistence of the
gelatin is unaltered ; sometimes it is liquefied throughout,
sometimes only at the surface. Sometimes offshoots ex-
tend from the colonies into the gelatin, giving the culture

FIG. 33.—Various forms of gelatin puncture-cultures: a, Bacillus typhi ab-
dominalis; If, B. anthracis; c, B. mycoides; d, B. mesentericus vulgatus;
e, B. of malignant edema • f, B. radiatis.

a bristling appearance. Figure 33 will serve to illustrate
different varieties of gelatin growth.

The growth in gelatin is generally so far removed from
the walls of the tube (a central puncture nearly always
being made in the culture-medium, in order that the
growth be symmetrical) that it is next to impossible to
make a microscopical examination of it with any power
beyond that given by a hand-lens.

Much attention has been given of late to the preparation
of microtome sections of the gelatin growth. To accom-
plish this the glass is warmed sufficiently to allow the
gelatin to be removed and placed in Miiller's fluid (bi-