MORPHOLOGY. 23 



example. There are also considerable differences in the transverse 

 diameter of bacilli belonging to the same species when cultivated in 

 different media, or even in the same medium, although, as a rule, 

 the transverse diameter is tolerably uniform in pure cultures. 



Again, the form of the extremities of the rods is to be observed 

 (Fig. 7). This may be square, or the corners may be slightly 

 rounded, or the extremities may be quite round or lance-oval, or 

 the outlines of the rod may be spindle-shaped from the formation of 



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a large central spore "clostridium" or one end may be dilated 

 from the formation of a large terminal spore. 



In old cultures we frequently find irregular forms due to swellings 

 and constrictions, which probably occur in bacilli which have but 

 little vitality or are already dead. These are spoken of as involution 

 forms (Fig. 8). 



The bacilli multiply by binary division in a direction transverse 

 to the longitudinal axis, and, as a result of such binary division, long 



Fio. 9. 



chains in which the elements remain associated may be formed 

 (Fig. 9) ; or the rods may be for the most part solitary or united in 

 pairs. Like the micrococci, the bacilli are sometimes surrounded by 

 a gelatinous envelope or capsule. They may also be united by a 

 glutinous material into zoogloea masses. 



Bacilli which under certain conditions are seen as short rods 

 may, under other circumstances, grow out into long filaments, and 

 these may be associated in bundles or in tangled masses. 



The spirilla differ from the bacilli in the form of the rods and fila- 



