BACILLUS OF PLAGUE 



427 



often left uncoloured, giving the so-called " polar staining." In 



films from the tissues they 



are found scattered 



amongst the cells, for the 



most part lying singly, 



though pairs are also seen. 



On the other hand, in cul- 

 tures in fluids, e.g. bouillon, 



they grow chiefly in chains, 



sometimes of considerable 



length, the form known 



as a streptobacillus result- 

 ing (Fig. 145). In young 



agar cultures the bacilli 



show greater variation in 



size, and polar staining is 



less marked than in the 



tissues: sometimes forms Fia 14 4. -Bacillus of plague from a young 

 ,. . j 111 i. culture on agar. 



ot considerable length are staine(T with weak ca rbol-fuchsin. x 1000. 



present. After a time in- 

 volution forms appear, especially when the surface of the agar 



is dry ; but the formation 

 of these is much more 

 rapid and more marked 

 when 2-5 per cent of 

 sodium chloride is added 

 to the medium, constitut- 

 ing the so-called "salt- 

 agar " (Hankin and Leu- 

 mann). On this medium, 

 especially with the higher 

 percentage, the involution 

 forms assume a great size 

 and a striking variety of 

 shapes, large globular, 

 oval, or pyriform bodies 

 resulting (Fig. 146) ; with 



FIG. 145. Bacillus of plague iii chains show- about 2 per cent sodium 

 ing polar staining. From a young culture chloride, aftertwenty-f our 

 in bouillon. , '. , , . ,1 



Stained with thionin-blue. x 1000. hours incubation, the 



most striking feature is a 



general enlargement of all the bacilli. Sometimes in the tissues 



they are seen to be surrounded by an unstained capsule, though 



this appearance is by no means common. They do not form 



