466 PATHOGENIC BACTERIA. \-r 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!.