Lecture VII. 55 



a tuft at one end or a single cilium. Similar ciliation may be 

 found in the corkscrew-shaped v&riones and spirilla. 



If non-motile bacteria are kept under continuous observation 

 it may be observed that the cells multiply by enlargement and 

 division. Under favourable conditions the interval between one 

 division and the succeeding one may not occupy more than 20 

 minutes. This appears a moderate rate of reproduction for so 

 small an organism. The very minuteness of the cells favours the 

 prolongation of favourable conditions of nutrition, and it has been 

 estimated, assuming a reproductive division at the end of every 

 20 minutes in each cell, that a single bacterium would give rise to 

 1 6 x io 18 individuals in 24 hours. Limited supply of material 

 and the accumulation of waste products naturally prevent the 

 realisation of such a result, as may be easily seen when the 

 weight of such a mass of individuals is worked out. 16 x io 18 

 individuals would weigh between 90 and 100 tons. The calcu- 

 lation has its value, however, in showing how quickly an organic 

 medium may become crowded with bacteria. 



The consistency of the protein cell-wall varies widely. In 

 some species the wall is a thin membrane which appears to be 

 comparatively hard and smooth. In these forms the products 

 of cell-division do not tend to cohere together, but the resulting 

 individuals easily part from one another either automatically or 

 as the result of external forces. When they retain their original 

 positions but are not bound together in any regular relation the 

 organisms forming such a group are spoken of as staphylococci. 

 In others the products of division cohere and often become 

 bound together by the outer layers of the cell-walls, which are 

 more or less viscid and gelatinous. When the succeeding divisions 

 are parallel to one another the cohering cells form long chains, 

 which, in the case of cocci, are called strepto-cocci. If cohesion 

 is temporary and only lasts till the inauguration of the second 

 subdivision, the cells of a growth or culture appear linked together 

 in pairs. Such forms are described as diplococci. When the 

 succeeding divisions follow at right angles to each other, small 

 cubic bundles or blocks of cells are formed which are called 

 sarcinae. Again, when divisions follow one another in regular or 

 irregular sequence while the whole culture remains bound to- 

 gether by its gelatinous cell-walls, a gelatinous skin or film is 

 formed which is often described as a zooglcea. 



On careful microscopic examination some bacteria show a com- 

 paratively large enclosed granule occupying a large or even the 

 greater part of the cavity of the cell. One or two of these bodies 



