476 



PLAGUE 



for the most part lying singly, though pairs are also seen. On 

 the other hand, in cultures in fluids, e.g. bouillon, they grow 

 chiefly in chains, sometimes of considerable length, the form 

 known as a streptobacillus resulting (Fig. 143). In young agar 

 cultures the bacilli show greater variation in size, and polar 

 staining is less marked than in the tissues : sometimes forms 

 of considerable length are present. After a time involution 



^ 



FIG. 141. Film preparation from a plague bubo showing enormous 

 numbers of bacilli, most of which show well-marked bipolar staining. 

 Stained with weak gentian-violet, x 1000. 



forms appear, especially when the surface of the agar is dry; 

 but the formation of these is much more rapid and more marked 

 when 2 to 5 per cent, of sodium chloride is added to the medium, 

 constituting the so-called " salt agar " (Hankin and Leumann). 

 On this medium, especially with the higher percentage, the in- 

 volution forms assume a great size and a striking variety of 

 shapes, large globular, oval, or pyriform bodies resulting (Fig. 

 144) ; with about 2 per cent, sodium chloride, after twenty- 

 four hours' incubation, the most striking feature is -a general 



