446 BACTERIOLOGY. 
Growth on Gelatin. In gelatin plates colonies are 
developed in from twenty-four to forty-eight hours, 
which vary considerably in their appearance according 
to their age, and in different cultures in the same 
medium. They resemble greatly the colonies of the 
typhoid bacillus, except that they are somewhat larger 
for the same period of growth. When located in the 
depths of the gelatin and examined by a low-power 
lens they are at first seen to be finely granular, almost 
homogeneous, in structure round, and of a pale yellow- 
ish to brownish color; later they become larger, denser, 
darker, and more coarsely granular. In shape they 
may be round, oval, or ‘‘ whetstone-like.’? The super- 
ficial colonies appear as small, dry, irregular, flat, 
blue-white points that are commonly somewhat dentated 
at the margin. 
In stab cultures on gelatin the growth usually takes 
the form of a nail with a flattened head, the surface 
extension generally reaching out rapidly to the sides 
of the tube. 
On Nutrient Agar and Blood-serum. On nutrient 
agar and blood-serum an abundant, soft, white layer is 
quickly developed in the incubator, but the growth 
is not characteristic. 
In Bouillon. In bouillon the bacillus coli produces 
diffuse clouding with sedimentation; in some cultures 
a tendency to pellicle formation on the surface is seen 
occasionally. In old cultures, in the absence of sugar, 
the reaction becomes alkaline, and a decided fecal odor 
may be noticed. 
The colon bacillus produces indol in bouillon and in 
peptone solutions, this reaction being most pronounced 
after a week’s development in the incubator. It pos- 
