488 



PLAGUE 



stained preparations a portion in the middle of the bacillus is 

 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 cultures in fluids, e.g., bouillon, they grow 

 chiefly in chains, sometimes of considerable length, the form 

 known as a streptobacillus resulting (Fig. 147). In young agar 





'' -'4.,. '•••»•' *.W~ ; - 



'*• 



Pig. 145. — 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. , 



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 

 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 involution forms assume a great size and a 



