Bacteria and Protozoa 



17 



cases for a just-divided cell to attain full size and 

 divide again. Thus in a few days time the number 

 of bacteria resulting from a single individual would 

 be inconceivable if they should all develop. 



Fortunately for us, however, they do not all 

 multiply so rapidly as this and besides there are 

 natural checks, not the least of which are the sub- 

 stances given off by the bacteria themselves in 

 their growth and development. Such excretions 

 often serve to inhibit further multiplication. Some- 

 times, though not often, they form spores which 

 not only provide for a more rapid multiplication, 

 but enable the organism to live under conditions 

 that would otherwise prove fatal to it. 



Bacteria may be conveniently grouped under 

 two heads: those that live upon dead organic mat- 

 ter, known as the saprophytic forms, and those 

 that are found in living plants or animals, the true 

 parasites. Such a grouping is not always entirely 

 satisfactory, for many of the kinds that live sapro- 

 phytically under normal conditions may become 

 parasitic if opportunity offers, and also many of 

 those that are usually regarded as parasitic may be 

 grown in cultures of agar or other media, under 

 which conditions they may be regarded as living 

 saprophytically. 



It is this power of easily adapting themselves 



