aggregated at opposite
poles.

2. A circular constriction of the organism takes place midway between
these aggregations, and a septum is formed in the interior of the cell
at right angles to its length.

3. The division deepens, the septum divides into two lamellae, and
finally two cells are formed.

[Illustration: FIG. 88.--Fission of cocci.]

[Illustration: FIG. 89.--Fission of bacteria.]

4. The daughter cells may remain united by the gelatinous envelope for a
variable time. Eventually they separate and themselves subdivide.

Cultures on artificial media, after growing in the same medium for some
time--i. e., when the pabulum is exhausted--show "involution forms"
(Fig. 90), well exemplified in cultures of B. pestis on agar two days
old, B. diphtheriae on potato four to six days old.

[Illustration: FIG. 90.--Involution forms.]

They are of two classes, viz.:

(a) Involution forms characterised by alterations of shape (Fig. 90).
(Not necessarily dead.)

(b) Involution forms characterised by loss of staining power. (Always
dead.)

_Resting Stage._--Spore Formation.--Conditions influencing spore
formation: In an old culture nothing may be left but spores. It used to
be supposed that spores were _always_ formed, so that the species might
not become extinct, when

(a) The supply of nutrient was exhausted.

(b) The medium became toxic from the accumulation of metabolic
products.

(c) The environment became unfavourable; e. g., change of
temperature.

This is not altogether correct; e. g., the temperature at which spores
are best formed is constant for each bacterium, but varies with
different species; again, aerobes require oxygen for sporulation, but
anaerobes will not spore in its presence.

(A) Arthrogenous: Noted only in the micrococci. One complete element
resulting from ordinary fission becomes differentiated for the purpose,
enlarges, and develops a dense cell wall. One or more of the cells in a
series may undergo this alteration.

This process is probably not real spore formation, but merely relative
increase of resistance. These so-called arthrospores have never been
observed to "germinate," nor is their resistance very marked, as they
fail to initiate new cultures, after having been exposed to a
temperature of 80 deg. C. for ten minutes.

(B) Endogenous: The cell protoplasm becomes differentiated and condensed
into a spherical or oval mass (very rarely cylindrical). After further
contra