WHAT ARE BACTERIA ? 31 



division, complete or partial, of this lengthened rod into 

 two shorter daughter-cells. Under suitable conditions these 

 two again increase in length, each again is divided into two, 

 and so regular chains or swarms of bacteria are developed. 

 The first sign of division is a delicate transverse mark or 

 line across the middle of the bacterium in a plane at right 

 angles to the longitudinal axis of the cell. 



By staining, as De Bary recommends, with an ajcoholic 

 solution of iodine and causing the young protoplasm to 

 retract, it may be made out thaf this line is due to the 

 ingrowth of the cell membrane from the periphery towards 

 the centre so as to form a septum, more or less com- 

 plete, between the little rods of protoplasm, which are 

 thus gradually cut off from one another by the constricting, 

 and growing in, cell membrane. 



In the rod-shaped bacteria this division takes place in one 

 plane only — at right angles to the longitudinal axis of the 

 rod — and when it is imperfect or incomplete it gives rise to 

 chain-bacteria or Strepto-bacteria.' 



The individual bacteria of which the chain consists are 

 held together in series by the constricted but incompletely 

 divided membranous portion. 



Similar divisions take place in the rounded bacteria or 

 micrococci, and we have then chain-cocci or strepto-cocci 

 formed, but instead of going on to form or remaining in 

 long chains, they may be arranged in pairs, and are then 

 spoken of as diplococci. There is sometimes division taking 

 place in two dimensions of space, the one at right angles to 

 the other, a good example of which is seen in the Bacterium 

 Merismopcedioides described by Zopf, in which the lines of 

 divisions are placed at right angles to one another, but in 

 one surface plane, /'.«., at the surface of a fluid. In other 

 cases there appear to be division, multijjlication, and 

 growth in three directions. To obtain an idea of what 

 occurs, suppose that a two-inch cube is divided into single 

 cubes, each one inch in diameter : and that these single-inch 

 cubes then grow until each reaches the size of the original 

 two-inch cube, when it, in turn, is again divided into one- 



' A long spiral may divide into short curved rods, as in the case of the 

 Cholera bacillus. In Cladothrix, where there is a false branching, the 

 terminal cell before branching, instead of dividing transversely divides 

 vertically, and then the division again goes on transversely. 



