CHAPTER IX. 



The study of colonies— Their naked-eye peculiarities aud their appearance 

 under different conditions— Differences in the structure of colonies from 

 ditferent species of bacteria— Stab cultures— Slant cultures. 



The plates upon agar-agar wliicli have been prejmred 

 from a mixture of organisms and have been placed in 

 the incubator, and tliose of gelatin ^\'hi<•ll have been 

 maintained at the ordinary temperature of the room, air 

 usually ready for examination after twenty-four to 

 forty-eight hours. They will be found to be 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 

 another. 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- 

 shape pits containing opaque fluid. 



Place the plate containing these points upon the stage 

 of the microscope and examine them with the lowest 

 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. 



