CHAPTER XI. 



Systematic study of an organism — Points to be considered in identi- 

 fying an organism as a definite species. 



After isolating an organism by the plate method 

 considerable work is necessary in order to establish its 

 identity as a definite species. It must possess certain 

 morphological and cultural peculiarities, which must be 

 constant under constant conditions. Its form at dif- 

 ferent stages must always be the same. Its ability or 

 inability to produce spores must not vary under suitable 

 conditions. Its growth upon different media under 

 constant conditions of temperature and reaction must 

 always present the same outward appearances. The 

 changes brought about by it in the reaction of the 

 media in which it is growing must follow a fixed rule. 

 Its power to produce liquefaction of the gelatin, or to 

 grow upon it without bringing about this change, must 

 always be the same. Its motility or non-motility, 

 and, if motile, the approximate number and position of 

 its organs of locomotion, must be determined. Its pro- 

 duction of certain metabolic products must be established 

 by chemical analysis. Its behavior toward oxygen — 

 i. e., Does it require this gas for its growth ? Is this gas 

 an indifferent factor ? or. By its presence are the life- 

 processes of the organism checked? — must be decided. 

 Its behavior under varying conditions of temperature 

 and under the influence of different chemic'al bodies, as 

 well as its growth in media of different reactions, is 

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