360 PA THOGENIC BA CTERIA.
In gelatin puncture-cultures the growth is even more
characteristic than are the colonies. The bacilli begin
to grow along the entire track of the wire, most luxuri-
antly at the surface, where oxygen is plentiful. As the
growth progresses fine filaments like bristles, extend
from the puncture into the neighboring gelatin giving
the growth somewhat the appearance of an evergreen
tree inverted (Fig. 104).
FIG. 104.—Bacillus anthracis : gelatin puncture-culture seven days old
The more superficial of these threads reach about half-
way to the sides of the tube, while the deeper ones are
shorter and shorter, until near the apex branches cease.
When the projections are pretty well developed a distinct
surface-growth will be discerned, and if the tube be tilted,
one can observe that the gelatin beneath it has liquefied.
As the growth becomes older the liquefaction increases,
until-ultimately the entire gelatin is fluid and the growth
Upon agar-agar the characteristics are few. The
growth takes place all along the line of inoculation as
a slightly translucent, slightly wrinkled layer with irreg-
ular edges, from which sufficient bacillary threads pro-
ject to give it a ciliated appearance to the naked eye.
When the culture is old the agar-agar turns a distinct
brown. Spore-formation is luxuriant upon agar-agar.