BUBONIC PLAGUE. 555 



pin's head. The gelatin is not liquefied. The borders 

 of the colonies are, upon microscopic examination, found 

 to be sharply defined and to become more granular as 

 their age increases. The superficial colonies occasionally 

 are surrounded by a fine, semi-transparent zone. 



In gelatin puncture-cultures the development is scant. 

 The medium is not liquefied ; the growth takes place in 

 the form of a fine duct, little points being seen on the 

 surface and in the line of puncture. Sometimes fine 

 filaments project into the gelatin from the central 

 puncture. 



Upon agar-agar the bacilli grow freely but slowly, the 

 colonies being whitish in color, with a bluish tint by re- 

 flected light, and first appearing to the naked eye when 

 cultivated from the blood of an infected animal after 

 about thirty-six hours' incubation at 37 C. Under the 

 microscope they appear moist, with rounded, uneven 

 edges. The small colonies are said to resemble little 

 tufts of glass-wool; the larger ones have large round 

 centres. Microscopic examination of the bacilli grown 

 upon agar-agar reveals the presence of long chains resem- 

 bling streptococci. 



Upon glycerin agar the development of the colonies is 

 slower, though in the end the colonies attain a larger 

 size than those grown upon plain agar. 



Klein x says that the colonies develop quite readily 

 upon gelatin made from beef-bouillon (not infusion), 

 appearing in twenty-four hours, at 20 C, as small, gray, 

 irregularly rounded dots. Magnification shows the col- 

 onies to be serrated at the edges and made up of short, 

 oval, sometimes double bacilli. Some colonies contrast 

 markedly with their neighbors in that they are large, 

 round, or oval, and consist of longer or shorter, straight 

 or looped threads of bacilli. The appearance was much 

 like that of the young colonies of the Proteus vulgaris. 

 At first these were regarded as contaminations, but later 

 he was led to believe that their occurrence was character- 



1 Centralbl. f. Bakt. und Parasitenk., July 10, 1897, xxi., Nos. 24 and 25. 



