REPRODUCTION AND CELL DIVISION 31 



E. REPRODUCTION AND CELL DIVISION. 



Bacteria are structurally the simplest known organisms which main- 

 tain an independent existence: all their vital functions are exhibited 

 in a single asexual cell devoid of a morphologically definable nucleus. 

 The absence of sexual characters and of a morphologic nucleus makes 

 bacterial reproduction mechanically 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. Diptheria 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 com- 

 pleted. So forcible is this separation that the daughter cells lie at an 

 angle with each other: Nakanishi 1 has observed that the septum in 

 this group of organisms frequently forms at a met achromatic granule. 

 Septation in the Bacillacese and Spirillacese normally takes place 

 at right angles to the long axis of the organism, and midway between 

 the ends, thus effecting the separation into two individuals with the 

 minimal expenditure 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. Noguchi 2 has 

 brought forward striking evidence and photographic illustrations in 

 favor of the view that the Spirocheta (Treponemata) may reproduce 

 by longitudinal fission rather than by transverse fission. If this view 

 be generally adopted, it would contradict the "minimal requirement 



1 Centralbl. f. Bakteriol., 1900, xxvii, 641. 



2 Jour. Exper. Med., 1912, xv, 201. 



