38 BACTERIOLOGY. 



Those which divide in three directions and cling to- 

 gether form packets in cubes (sarcinse). Those which 

 apparently divide irregularly in any axis form irregu- 

 larly shaped, grape-like bunches (staphylococci). 



There are a considerable number of bacteria which 

 appear to frequently assume spherical forms, or at least 

 forms so like spheres that they cannot be differentiated 

 from them, and yet under other conditions they generate 

 rod-like forms. These apparently spherical bacteria 

 we can properly regard as short forms of bacilli, which, 

 owing to the rapidity of division, are for the time being 

 of the same size in both diameters. Under suitable 

 conditions, however, the true rod- shape is always de- 

 veloped. 



2. Rod Form, or Bacillus. The type of this group is 

 the cylinder. The length of the fully developed cell 

 is always longer than its breadth. The size of the 

 cells of different varieties varies enormously, from a 

 length of 30// and a breadth of 4// to a length of 0.2// 

 and a breadth of 0.1 //. The largest bacilli met with 

 in disease do not, however, average over 3/*. In des- 

 cribing their forms bacilli are roughly classed as slender 

 when the ratio of the long to the transverse diameter is 

 from 1 : 4 to 1 : 10, and as thick when the proportions 

 of the long to the short diameter is approximately 1 : 2. 



The characteristic form of the bacillus is one with a 

 straight axis, uniform thickness throughout, and flat 

 ends (Fig. 13, page 47); but there are many exceptions 

 to this typical form. Thus frequently the motile bac- 

 teria have rounded ends (Fig. 10, page 43); many of 

 the more slender forms have the long axis bent; some 

 few species, such as the diphtheria bacilli (Fig. 5), in- 

 variably produce many cells whose thickness is very 



