BACTERIUM PESTIS. 215 



ible involution forms are produced, the like of which are 



dd to occur in no other variety. The cell bodies are 



swollen in the center, similar to yeast cells, or they become 



rounded, like spherical forms. Very often there appear 



jells many times larger than normal cells. The staining 



>roperties in these forms are lessened (13, viii). Accord- 



ig to the statement of the German Pest Commission, 



ipon Hankin's 3% chlorid of sodium-agar involution 



forms are produced almost exclusively (C. B. xxn, 438). 



Gelatin Plate. — (a) Natural size: Small, crumbly, 



gray, transparent colonies, which are directly elevated 



above the surface. After a longer time, they spread out 



but little (13, v 6). 



(b) Magnified sixty times: Answering to the elevation 

 which occurs, there is observed a marked reflection from 

 the surface. The colonies are roundish, smooth-bordered 

 to lobulated, sharply outlined, with a yellowish to a green- 

 ish shimmer. Very often the superficial colony is sur- 

 rounded by a very delicate, transparent lobulated zone, 

 which occurs also in somewhat altered form upon other 

 nutrient media. The structure varies from homogeneous 

 to faintly granular. The deep-lying colonies are similar, 

 but never present this delicate zone (13, vi). 



Gelatin Stab. — In stab-canal, a faint, homogeneous, 

 whitish, thread-like growth. On the surface the growth is 

 like the colony in the gelatin plate. 



Agar Plate. — The description refers to cultures which 

 have been cultivated a long time. Cultures recently ob- 

 tained from pest cadavers behave somewhat differently. 

 (See under agar streak. ) 



(a) Natural size : After forty-eight hours the colonies 

 are plainly visible macroscopically, have wavy, smooth 

 borders, are slightly elevated, and cannot be differentiated 

 from those of the colon bacterium. They are gray to 

 grayish-white, and have an oily or moist luster (13, v a). 



(b) Magnified sixty times : Colonies roundish, transparent 

 at the periphery, in the interior yellowish to yellowish- 

 gray. They are universally very crumbly. Sometimes 

 one is reminded of a very granular colony of diphtheria 

 or of a delicate sarcina colony (13, vn a). The better the 

 nutrient medium, the more luxuriant the growth. Thus 



