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spore-formation, but the conditions under which this 

 happens are not very clear, and at best we can hardly 

 be said to have yet emerged from the region of 

 speculation as to the reasons for this departure from 

 their normal habits. Sometimes fission and spore- 

 bearing can be seen to be in operation at the same 

 time. Multiplication bj^ fission is a rapid process and 

 as it takes place in consonance with the laws of 

 progression there are, as will be seen, some startling 

 results. It varies as to rate in different species. In 

 some it takes as long as half an hour, and in some as 

 short a time as ten minutes. The daughter cells 

 formed by this division proceed immediately to grow 

 and to divide again in like manner, with the effect 

 that a single bacterium may thus easily multiply itself 

 into sixteen millions in the short space of twenty four 

 hours ! 



Bacteria are in their turn subject to different 

 methods of classification and from different points of 

 view. For instance there are some that live only on 

 dead matter — animal or vegetable — and are hence 

 known as "saprophytic"; others that find their 

 means of existence in living tissues and, while doing 

 so, set up a condition of disease. These are therefore 

 termed "pathogenic" or disease producing, and some- 

 times "parasites" in a limited meaning. Then there 

 are those which in a general way are saprophytic, but 

 which under favouring conditions are able to become 

 parasites or pathogenic. These are then known as 

 " facultative parasites." A bacterium that requires 

 oxygen to maintain its life is called an " aerobe " ; 

 one that prefers no oxygen receives the name of 

 " anaerobe" ; and those which cannot live in oxygen 

 are said to be " obligate anaerobes." With this 

 explanation to guide them my readers can easil}^ 

 divine the meanings of yet two more cross-divisions, 

 viz. "facultative aerobes" and "facultative anaerobes." 



