38 BACTERIOLOGY. 
Those which divide in three directions and cling to- 
gether form packets in cubes (sarcinz). 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 0.24 
and a breadth of 0.14. The largest bacilli met with 
in disease do not, however, average over 34. 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 
