466

PATHOGENIC BACTERIA.

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place slowly unless such tubes a-re placed in a Buchner's
jar. The deeper colonies are the
largest. Sometimes the growth only
takes place within 10-12 mm. of the
surface, at others within 3-4 cm. of
it. After repeated cultivation the
organism seems to become somewhat
accustomed to the presence of oxy-
gen, and will grow higher up in the
tube than when freshly secured from
animal tissue (see Fig. 133).

The colonies seen in the culture-
media are grayish-white or brownish-
white by transmitted light, and some-
times exhibit a central dark dot. At
the end of twenty-four hours the larger
colonies do not exceed 0.5-1.0 mm.
in diameter, though they may subse-
quently attain a diameter of 2-3 mm.
or more. Their first appearance is
as little spheres or ovals, more or less
flattened, with rather irregular con-
tours, due to the presence of small
projecting prongs, which are quite
distinct under a lens. The colonies
may appear as little irregular masses
with projections.

After several days or weeks, single,
well-separated colonies may attain a
large size and be surrounded by pro-
jections, either in the form of little
knobs or spikes or of fine branchings
—hair-like or feathery. Their ap-
pearance has been compared to
thistle-balls or powder-puffs and to
thorn-apples. When the growth
takes place in the puncture the
feathery projections are continuous. Bubbles of gas

FIG.

aerogenes capsulatus,
with gas-production (from
photograph by Prof. Si-
mon Flexner1!.