BACILLUS PESTIS. 437 



tissues they are found scattered amongst the cells, 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 consider- \ \ 



able length, the form known ^ x 



as a streptobacillus resulting \ / 



(Fig. 149). In young agar cul- ^ N f*t \ 



tures the bacilli show greater 



variation in size, and polar % 



staining is less marked than in \ t / 



**** *^ / 



the tissues : sometimes forms *X 



of considerable length are pres- ^s \ / 



ent. After a time involution 

 forms appear, especially when 



r r T 49' Bacillus of plague in chains, 



the Surface Of the agar IS dry; showing, polar staining. From a young cul- 



but the formation of these is ^ in bouillon. 



Stained with thionm-blue. x 1000. 



much more rapid and more 



marked when 2-5 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 per- 



centage, the involution forms 



^* "' ;*-" C . assume a great size and a strik- 



ing variety of shapes, large 



0+ ^# ***** globular, oval, or pyriform 



*^.T bodies resulting (Fig. 150); 



with about 2 per cent sodium 

 chloride,, after twenty-four 

 hours' incubation, the most 

 * 4 I v striking feature is a general 



* enlargement of all the bacilli. 



Sometimes in the tissues they 

 * * * * / J 



are seen to be surrounded by 

 an unstained capsule, though 



FIG. 150. Culture of the bacillus of , , . -, 



plague on 4 per cent salt agar, showing in- thls appearance is by no means 



volution forms of great variety of size and common. They do not form 



stained with carboi-thionin-biue. xiooo. spores. Gordon, who has found 



that they possess flagella which, 



however, stain with difficulty, states that they are motile. Most 

 observers, however, and with these we agree, have failed to 





