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 ifuids, e.g. 



bouillon, they grow chiefly m 



chains, sometimes of consider- \ \ 



able length, the form known "-y •' . 



as a streptobacillus resulting \ .^ 



(Fig. 149). In young agar cul- ^»^ /''/ \ 



tures the bacilli show greater 



variation in size, and polar 



staining is less marked than in \ * / 



the tissues: sometimes forms >' ***» 



of considerable length are pres- \^ ^^' , ^ 



ent. After a time involution 



forms appear, especially when _ „-,,., , ■ 



^ ^ ^ ■' ^ Fig. 149. — Bacillus ot plague in chains, 



the surface of the agar is dry; showing polar staining. From a young cul- 



but the formation of these is '""'"''T"".?'..- ■ k, 



Stained with thionin-blue. X looo. 



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 

 ^ ' . * ' »._^ assume a great size and a strik- 



• •♦ -i**^ ing variety of shapes, large 



^^ ^^, -V» ^ , globular, oval, or pyriform 



'*-y 



bodies resulting (Fig. 150); 



^ j-^ •'^ with about 2 per cent sodium 



L • i .. • ''^»^1 * ■■ " chloride, after twenty-four 



'j • \ 'j^^ hours' incubation, the most 



) striking feature is a general 





enlargement of all the bacilli. 

 Sometimes in the tissues they 

 ■ ' V ' are seen to be surrounded bv 



^ an unstained capsule, though 



Fig. ic;o. — Culture of the bacillus of , i ■ • ■, 



plague on 4 per cent salt agar, showing in- this appearance IS by no means 



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



' "suinedwithcarboi-thionin-biue. X 1000. 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 



