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324 terminal flagellum. A capsule, according to the same observer, or the appearance of a capsule, can generally be made out, especially in those bacilli which are present in the blood. The bacillus is readily stained by aniline dyes, the extremities taking on a deeper colour than the interpolar part. It is usually decolorized by Gram.

Culture characters.—When sown on blood serum and kept at body temperature, in from twenty-four to forty-eight hours an abundant moist, yellowish-grey growth is formed without liquefaction of the culture medium. On agar, but better on

Fig. 66.—Bacillus of plague in chains showing polar staining. From a young culture in bouillon, x 1,000. (Muir and Ritchie.)

glycerine agar, the growths have a greyish-white appearance. In agar plate cultures they show a bluish translucence, the individual colonies being circular, with slightly irregular contours and a moist surface; on mannite neutral-red-bile-salt agar the colonies are bright red, but are colourless on a similar medium in which lactose is substituted. Litmus-milk and glucose-broth are rendered slightly acid, lactose-broth is unchanged. Young colonies are glass-like, but older colonies are thick at the centre and more opaque; they are singularly coherent and may be removed en bloc with a platinum needle. Stab cultures show after one or two days a fine dust-like line of growth. According to Yersin, when sown on gelatin the bacillus gives rise to white transparent colonies which, when examined in reflected light, present iridescent borders.