322 A MANUAL OF BACTERIOLOGY 



of the rods both in cultures and in the membrane 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 branching 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 

 are abundant ; on blood-serum club-shaped involution 

 forms also occur, but sparsely in a young, eighteen to 

 twenty hours' culture, in a forty-eight hours' culture 

 more numerously. 



Staining reactions. The B. diphtheria stains well with 

 the ordinary anilin dyes and is Gram-positive. With 

 Lomer's methylene blue the coloration is usually some- 

 what irregular, more deeply stained portions alternating 

 with paler intervals, the so-called segmentation, and 

 especially marked with agar cultures. The ends of the 

 organisms are also frequently deeply stained, the so-called 

 polar staining, while the phenomenon known as " meta- 



