32 THE MORPHOLOGY OF BACTERIA 
nucleus makes bacterial reproduction structurally a simple process, 
and doubtless the rapid sequence of generations observed in various 
bacteria depends in part upon this simplicity of structure. 
Reproduction takes place in the following manner: A bacterial 
cell placed in a favorable environment increases in size until it reaches 
a maximum which is relatively constant for each variety; then a slight 
equatorial constriction occurs, which deepens until a distinct septum 
is produced by invagination, which divides the original cell into two 
morphologically complete, fully mature individuals of approximately 
equal size. It is obvious that this septum consists ordinarily of at 
least two layers, since one layer is required to complete each of the 
dividing individuals. Successive generations may be produced at 
intervals which may be as frequent as every fifteen minutes in the more 
rapidly growing types. Septation usually takes place deliberately; 
that is to say, the septum forms relatively slowly. Diphtheria bacilli 
and possibly related bacteria divide somewhat differently; the parental 
cell appears to be under tension when the septum becomes visible, and 
the daughter cells spring apart suddenly when septation is completed. 
So forcible is this separation that the daughter cells lie at an angle with 
each other: Xakanishi^ has observed that the septum in this group of 
organisms frequently forms at a metachromatic granule. Septation 
in the Bacillacese and Spirillacese normally takes place at right angles to 
the long axis of the organism, and mid-way between the ends, thus 
effecting the separation into two individuals with the minimal expendi- 
ture of material; in the Coccacese, which are usually isodiametric, no 
economy of material in septation is apparent, and no known force 
determines the initial plane of septation: subsequent fission may be 
definitely related to the initial plane. 
F. CELL GROUPING. 
In Bacilli and Spirilla, where septation typically occurs at right 
angles to the long axis of the organism, it is obvious that no geometrical 
arrangement of cells is possible other than the formation of chains of 
rods or of spirals if the individual organisms remain adherent. The 
cocci, on the other hand, are spherical and have no longer or shorter 
axis, consequently a definite sequence of septation in one, two or three 
planes of space can give rise to: (1) Chains of cocci, if the plane of 
septation is always in one plane of space; (2) groups of four cocci, if 
septation takes place alternately at right angles in two planes of space; 
or (3) in packets of cocci, if septation is alternate and at right angles in 
three planes of space. Many cocci do not exhibit a definite sequence 
of planes of septation. 
1 Centralbl. f. Bakteriol., 1900, 27, 641. 
