CHAPTER IX. 



The study of colonies— Their naked-eye pecnliarities and their appearance 

 under different conditions— DiflFerences in the structure of colonies from 

 different species of bacteria— Stab-cultures — Slant-cultures. 



The plates of agar-agar which have been prepared 

 from a mixture of organisms and have been placed in 

 the incubator, and those of gelatin which have been 

 maintained at the ordinary temjjerature of the room, 

 are usually ready for examination after twenty-four to 

 forty-eight hours. They will be found marked here 

 and there by small points or little islands of more or 

 less opaque appearance. In some instances these will 

 be so transparent that it is with difficulty one can see 

 them with the naked eye. Again, they may be of a 

 dense, opaque appearance, at one time sharply circum- 

 scribed and round, again irregular in their outline; here 

 a point will present one color, there perhaps anotlier. 

 On gelatin some of the points will be seen to be lying 

 on the surface of the medium, others will have sunk 

 into little depressions, while at still other points the 

 clear gelatin will be marked by more or less saucer- 

 shaped pits containing opaque fluid. 



Place the plate containing these points upon the 

 stage of the microscope and examine them with a low- 

 power objective, and again differences will be observed. 

 Some of these minute points will be finely granular, 

 others coarsely so ; some will present a radiated appear- 

 ance, while a neighbor may be concentrically arranged; 



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