48 
Growth of Bacilli 
In both these groups it is evident from the mode of post-fission 
movement that widely separated bacilli tend to produce separate, more 
or less rounded colonies, and that more closely aggregated bacilli tend 
to produce separate colonies whose peripheral portions fuse with each 
other, resulting in a slightly uneven continuous film. Such fusion 
only occurs on moist agar which allows of growth over its surface for a 
considerable time. 
In the “snapping” or diphtheroid group continuous growth was 
never obtained unless the cultures had been very copiously sown, and 
in any case numerous elevations in such a growth marked the situations 
of the originally discrete colonies which had partially coalesced to form 
this film. 
In the “ slipping ” group, however, the structure and appearance of 
the colony is very markedly influenced by the condition of the medium. 
The variations are so great that the colonies in different parts of the 
culture look as if they had been produced by different species of 
organisms. To illustrate the range of variation in a single species a 
member of the subtilis group has been chosen. 
Plate VIII, Fig. 9 is a photograph of a plate sown shortly after 
solidification in two parallel streaks with an emulsion of this organism, 
and incubated for 18 hours. The thicker portion is at the lower end. 
Over the lower portion a uniform film is seen. (If further incubated 
folds are often produced on the film, which are due to small areas of the 
film being thrust away from the surface of the agar by the increasing- 
lateral pressure. Such folds are very commonly present in the lower 
two-thirds of agar slant cultures.) Towards the middle of the plate 
the film becomes less dense, and at the upper part discontinuous. 
Here branched lines of growth, each terminating in a slightly thickened 
expansion, are seen radiating from the needle tracks. Smaller radiating 
processes are seen all round the film, being least marked at its thicker 
portions. 
Fig. 8 shows a similar culture on agar which has been dried for 
a short time after solidification. Here there is practically no film, 
but in the thicker (lower) parts radiating growths are seen round 
denser centres. In the thinner (upper) parts the colonies are mostly 
dense and round. 
Though the exact causes of these variations have not been determined, 
numerous cultivations have shown that they constantly occur under 
given conditions, and the mode of formation of each variety has been 
ascertained. 
