THE LIFE HISTORY OF BACTERIA. 43 



stained preparations this dividing line remains colorless, it 

 is evident that it largely consists of the same material as 

 the outer layer of the cell membrane, namely, the softened, 

 gelatinized mantle which, unlike the membrane proper, 

 does not stain with anilin dyes. Obviously the entire cell- 

 wall, inner as well as outer membrane, is depressed at the 

 zone of constriction (Fig. 8 a). When the ingrowing wall 

 reaches the center the inner membrane coalesces and 

 divides, leaving the space between the two new end walls 

 filled with the gelatinized outer membrane. Continuous 

 filaments showing no apparent division into cells are some- 

 times met with. In such cases division may have occurred, 

 but is not visible, owing to the absence of the outer mem- 

 brane. 



/ 



FIG. 9. Division forms of bacilli, a single; b in pairs; c in threads. 



As soon as the cell has completely divided, the two 

 new cells may at once tear apart and lead a separate exist- 

 ence. Many bacilli are therefore characterized as growing 

 singly. In some species there is a tendency for the two 

 cells to remain attached owing to the firm union of the two 

 cells, which are held together by the gelatinous connecting 

 zone. The bacillus is then spoken of as growing in pairs 

 diplo-bacillus. In other species the cells are likely to remain 

 attached even after repeated division. The individual rods 

 remain attached, end to end, by the undivided outer layer 

 of the cell-wall, and thus give rise to long filaments which 

 are commonly designated as threads. A thread may be long 

 or short; that is, it may consist of four or five, or fifty to one 



