438 PLAGUE. 



find evidence of true motility. They stain readily with the 

 basic aniline stains, but are decoloris.ed 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 



