BACTERIA. 79 



certainty ; in sulphur bacteria, granules of sulphur occur as 

 brightly refractive globules. Other granules occur in plasma, 

 which contain either starch or glycogen, especially before the 

 formation of spores ; they are used up as the spores develop. 

 These granules are coloured either blue or reddish-brown on 

 treatment with iodine. 



Surrounding the protoplasmic body we find a cell- wall or 

 membrane. By treatment with a hygroscopic substance the 

 inner surface of the cell-wall stands out clearly, owing to the 

 contraction of the plasma. An examination by Loffler's 

 method of staining generally shows that its outer layers are 

 swollen up into a gelatinous mass, which becomes quite distinct 

 when large numbers of bacteria are massed together. From 

 a chemical standpoint it must be provisionally assumed that 

 the cell- wall is differently constituted in different species. 

 In some it recalls the cellulose of the higher plants, whilst in 

 others it appears rather to resemble the albuminoids in its 

 properties. 



In decomposing liquids, as well as in the slime-fermenta- 

 tions, bacteria are frequently observed, in which the outer 

 layers of the cell-wall have been converted into a viscous 

 condition, so that the mass of cells is embedded in a structure- 

 less slime. Such structures are called Zoogloea. When the 

 slime has a sharply defined edge, and doubtless a harder con- 

 sistency, it has been described as a capsule formation, as, 

 for instance, in the " frog-spawn " (Leuconostoc] of the sugar 

 factory. Occasionally the outer layers of the cell- wall are 

 surrounded by a sheath of firm consistency, which eventually 

 encloses a large number of independent cells (Crenothriz). 

 The enclosed cells multiply inside the sheath until at last 

 they force their way out, and the sheath survives for a time 

 as an empty husk. 



At this point we may note the remarkable phenomena 

 which Thaxter, Bauer, Quehl, and others have described. 

 Myxo-bacteria i.e., swarms of organisms occurring particu- 

 larly on excrement which multiply by division, exhibit a 

 slow crawling motion, and during their growth secrete a 

 colourless slime, in which they live, and which enables the 

 swarm to hold together. The rods are eventually transformed 



