438 PLAGUE. 



find evidence of true motility. They stain readily with the 

 basic aniline stains, but are decolorised by Gram's method. 



Cultivation. From the affected glands, etc., the bacillus 

 can readily be cultivated on the ordinary media. It grows best 

 at the temperature of the body, though growth occurs as low as 

 1 8 C. On agar and on blood serum the colonies are circular 

 discs of somewhat transparent appearance and smooth, shining 

 surface. When examined with a lens, their borders appear 

 slightly wavy. In stroke-cultures on agar there forms a con- 

 tinuous line of growth with the same appearance, showing partly 

 separated colonies at its margins. When agar cultures are kept 

 at the room temperature, some of the colonies show a more 

 luxuriant growth with more opaque appearance than the rest of 

 the growth, the appearance in fact being often such as to sug- 

 gest the presence of impurities in the cultures. In stab-cultures 

 in peptone gelatin, growth takes place along the needle track as 

 a white line, composed of small spherical colonies. On the sur- 

 face of the gelatin a thin, semi-transparent layer may be formed, 

 which is usually restricted to the region of puncture, though 

 sometimes it may spread to the wall of the tube ; sometimes, 

 however, there is practically no surface growth. There is no 

 liquefaction of the medium. In gelatin plates the superficial 

 colonies develop first and form slightly raised semi-transparent 

 discs with somewhat crenated margins; the deeper colonies are 

 smaller and of spherical shape with smooth outline. In bouillon 

 the growth usually forms a slightly granular or powdery deposit 

 at the foot and sides of the flask, somewhat resembling that of 

 a streptococcus. If oil or melted butter is added to the bouillon 

 so that drops float on the surface, then a striking mode of growth 

 may result, to which the term "stalactite" has been applied. 

 This consists in the growth starting from the under surface 

 of the fat globules and extending downwards in the form of 

 pendulous, string-like masses. These masses are exceedingly 

 delicate, and readily break off on the slightest shaking of the 

 flask ; accordingly during their formation the culture must be 

 kept absolutely at rest. This manner of growth constitutes 

 an important but not absolutely specific character of the 

 organism; unfortunately it is not supplied by all races of 

 the organism, and varies from time to time with the same 

 race. The organism flourishes best in an abundant supply of 



