DIPHTHERIA 267 



and by subculturing from these pure cultures may be 

 obtained. 



Characters of the Diphtheria Bacillus 



Morphology. The B. diphtheria is a small, delicate 

 bacillus, with rounded ends, measuring 3 JUL or 4 //. in 

 length. It is non-motile and does not form spores. 

 The size varies somewhat even on the same medium, 

 and three varieties of the bacillus have been described, 

 viz. long, medium, and short, according to the length. 

 These varieties tend to be constant and to breed true. 

 Some of the rods both in cultures and in the mem- 

 brane have a swollen end, the so-called clubbing, 

 and parallel grouping, both in the membrane and in 

 cultures, is almost universal, the bacilli lying parallel 

 side by side (Plate VI. a). This parallel arrangement 

 arises from the peculiar mode of division of the bacillus. 

 If a cell be observed upon a warm stage it first elongates, 

 then becomes constricted at about its middle, and then 

 suddenly one side of the cell-membrane seems to rupture 

 and one half of the cell bends over to the other, so that 

 the two halves form a V . This mode of division, occurring 

 in contiguous cells and being repeated, and the cells thus 

 becoming more and more crowded together, leads to the 

 arrangement in parallel series. The bacilli are generally 

 joined end to end in pairs, and distinct thread and branch- 

 ing forms, though of rare occurrence, may be met with. 

 On different media the same strain exhibits considerable 

 variation in size. On blood-serum and on gelatin the 

 bacilli are of medium length and on the whole fairly regular 

 in shape ; in broth they tend to be short and stunted ; 

 while on agar, especially glycerin agar, they are much 

 larger than on the former media, and long club-shaped, 

 spindle-shaped and barred or segmented involution forms 



