THE PLAGUE BACILLUS 469 



blood in the septicsemic variety, but only in the earlier 

 stages of the disease. Later the typical forms tend to 

 disappear, their place being taken by a few large, rounded, 

 ovoid, or pear-shaped involution forms. Under cultiva- 

 tion the bacilli in young cultures (twenty-four to forty - 

 eight hours) are so short as to be almost coccoid or slightly 

 ovoid ; on agar their size is about the same as that in the 

 animal body, on gelatin they are somewhat smaller, but 

 a few well-marked rods and even threads are always pre- 

 sent. In older cultures, rod, thread and involution forms 

 occur more numerously ; on agar containing 2-3 per cent, 

 of salt the latter are swollen and yeast-like. 



In broth chains of slightly ovoid organisms occur, 

 resembling streptococci (Plate XV, b). 



The organism is non-sporing and non-motile, although 

 Gordon described the presence of one or two fine spiral 

 terminal flagella (others have not found flagella). 



Sometimes in hanging-drop cultivations a capsule is 

 apparently present, but the author has failed to verify this 

 by staining methods. 



The B. pestis stains well with Loffler's blue and anilin- 

 gentian violet, polar-staining being a marked feature, 

 especially in smear preparations. It does not stain by 

 Gram's method. With old laboratory strains polar stain- 

 ing may be completely absent, but in such cases may 

 sometimes be obtained by first treating the preparations 

 with alcohol or by the Gram method, and subsequently 

 re-staining with Loffler's blue or weak gentian violet. Sec- 

 tions are best stained with carbol methylene or thionine 

 blue. 



Cultural characters. The B. pestis is aerobic and facul- 

 tatively anaerobic. On blood-serum it forms moist, 

 smooth, shining, cream-coloured colonies or growths, 

 slightly raised above the surrounding medium. The 

 blood-serum is not liquefied. 



