FINER STRUCTURE OF THE BACTERIAL CELL n 



enclosing the cylindrical cells in such a way that they are freely movable 

 within (Figs. 2 and 5). At the period of reproduction the cells, them- 

 selves devoid of sheaths, become furnished with cilia, and, escaping from 

 the open end of the sheath, swim away and give rise to fresh colonies. 

 In this way whole forests of cladothrix are emptied of their cells and 

 there remain only the stiff sheaths which finally swell up and become 

 disintegrated. Occasionally the sheaths become fossilized as it were, by 

 the deposition upon them of oxide of iron. They are then very resistant 

 to decay, and accumulate sometimes in enormous quantities in chalybeate 

 springs and ferruginous marsh water (see Chap. VII). The term ' sheath ' 

 can only be applied when a distinct tube is formed in which the chain of 

 cells are enclosed ; colourless spaces in stained preparations of filamentous 

 bacteria do not necessarily imply the existence of a sheath, being often the 

 result of plasmolysis. 



