22 OUTLINES OF BACTERIOLOGY 



the bacteria are alive. What usually happens in a tube ciiilture is 

 that the excretion products of the bacteria ultimately effect their 

 destruction, and naturally the motility is lost when the protoplasm 

 is destroyed. In the case of those species which form spores, the 

 formation of these structures involves the abstraction of protoplasm 

 from the remainder of the cell. Consequently the cilia, being detached 

 from their base, fall off, and movement ceases. It is by no means 

 uncommon, however, to see motile individuals possessing spores. This 

 results from the fact that all the protoplasm has not been used up in 

 spore-formation, a notable example being the case of Sarcina ureae. 



The cilia are very delicatfe structures, and unfavourable external 

 circumstances often cause them to fall off, although the individuals 

 are not dead. In other cases, without falling oif, they may become 

 paralysed, thus causing a cessation of motility. We know this because 

 we can sometimes demonstrate the presence of cilia in organisms which 

 are motionless, but which have been previously in active motion. 



§ 2. CELL-DIVISION. 



Multiplication among bacteria is effected almost entirely by division of 

 cells, whereby two individuals are formed out of one. After division, 

 the two daughter-cells separate, grow into mature cells, and very soon 

 repeat the process of division, so that, in a comparatively short time, if 

 conditions are favourable, many millions may be formed from a single 

 individual. The process of division can easily be followed by 

 comparing individuals in an actively growing culture, when, if the 

 preparation has been properly stained, the various stages are exhibited. 



1. Division in the Bacteriaceae. This can be followed by comparison 

 of the individuals shown in Fig. 36. As seen in h, the first sign of 

 division, after slight elongation, is the formation of a wall at right 

 angles to the long axis of the cell. This wall is very thin at first, and 

 does not show the double contour characteristic of the mature state. 

 It usually divides the cell into two equal halves, though sometimes the 

 parts are unequal. The next stage is exhibited in c, in which a con- 

 striction has taken place in the middle, at the point where the new 

 wall is located. The subsequent stages are accomplished in two ways. 

 In the first, shown in d, e, /, the constriction deepens {d ), and this is 

 followed by a splitting of the division-wall into two halves, separated 

 by a thin clear line which does not take up the stain (e). This clear 

 line is composed of a thin mucilaginous layer, formed by a change of 



