vision._--Whatever the shape and size of the individual cell,
cell-filament or cell-colony, the immediate visible results of active
nutrition are elongation of the cell and its division into two equal
halves, across the long axis, by the formation of a septum, which either
splits at once or remains intact for a shorter or longer time. This process
is then repeated and so on. In the first case the separated cells assume
the character of the parent-cell whose division gave rise to them; in the
second case they form filaments, or, if the further elongation and
divisions of the cells proceed in different directions, plates or
spheroidal or other shaped colonies. It not unfrequently happens, however,
that groups of cells break away from their former connexion as longer or
shorter straight or curved filaments, or as solid masses. In some
filamentous forms this "fragmentation" into multicellular pieces of equal
length or nearly so is a normal phenomenon, each partial filament repeating
the growth, division and fragmentation as before (cf. figs. 2 and 6). By
rapid division hundreds of thousands of cells may be produced in a few
hours,[3] and, according to the species and the conditions (the medium,
temperature, &c.), enormous collections of isolated cells may cloud the
fluid in which they are cultivated, or form deposits below or films on its
surface; valuable characters are sometimes obtained from these appearances.
When these dense "swarms" of vegetative cells become fixed in a matrix of
their own swollen contiguous cell-walls, they pass over into a sort of
resting state as a so-called zoogloea (fig. 3).

[Illustration: FIG. 6.--_Bacillus megaterium._ (After de Bary.)

a, a chain of motile rodlets still growing and dividing (_bacilli_).

b, a pair of bacilli actively growing and dividing.

p, a rodlet in this condition (but divided into four segments) after
treatment with alcoholic iodine solution.

c, d, e, f, successive stages in the development of the spores.

r, a rodlet segmented in four, each segment containing one ripe spore.

g1, g2, g3, early stages in the germination of the spores (after being
dried several days);

h1, h2, k, l and m, successive stages in the germination of the spore.]

[Illustration: FIG. 7.--_Bacillus anthracis_. (After Koch.)

A. _Bacilli_ mingled with blood-corpuscles from the blood of a guinea-pig;
some of the _bacilli_ dividing.

B. The rodlets after three hours' culture in a drop of