THE BACTERIA 273 



is in each case due to the action of a definite species of 

 the Bacteria. On the same power of initiating far-reaching 

 decompositions in the bodies which they inhabit, depends 

 the fatal efficiency of the parasitic Bacteria in producing 

 disease. The whole subject of the fermentations set up 

 by these organisms has become in recent years of the 

 greatest possible practical importance in relation both to 

 medicine (as regards the parasitic forms), and to innumer- 

 able branches of industry, as regards the saprophytes. 

 A vast literature has grown up on these subjects, which 

 lie beyond the province of the present Introduction. 



TYPE XXIV. BACILLUS SUBTILIS 



This is one of the commonest and best known forms of 

 Bacteria. It occurs constantly in hay, and can be ob- 

 tained with certainty by soaking or boiling hay in water. 

 In the latter case the appearance of the Bacillus depends 

 on the extraordinary resistance to heat shown by its 

 spores, which can stand an hour's boiling with impunity. 

 After a little time the whole of the liquid simply swarms 

 with the cells of the Bacillus, which in its active vegeta- 

 tive condition is a strictly unicellular organism, the 

 isolated cells having the shape of short rods rather more 

 than roVoth of a millimetre in diameter and from 10 5 00 to 

 Tinnr nun. in length. The cells are thus far more minute 

 than those of any plant we have hitherto considered, if 

 we except spermatia or the oidial cells sometimes formed 

 by certain Fungi. The excessive smallness of the cells 

 has placed great difficulties in the way of their investiga- 

 tion, and the histology of Bacteria is still very little 

 understood. So far as we know at present, however, the 

 structure appears to be very simple. There is a definite 

 membrane which, however, does not consist of cellulose, 



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