CHAPTER IX. 



The study of colouies — Their nated-eye peculiarities and their appear- 

 ance under different conditions — Differences 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 temperature of the room, 

 are usually ready for examination after from 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 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- 

 shaped pits containing opaque fluid. 



Place the plate containing these points upon the 

 stage of a microscope and examine them with a low- 

 power objective, and again diiFerences will be observed. 

 Some of these minute points will be finely granular, 

 others coarsely so ; some \vill present a radiated appear- 

 ance, while a neighbor may be concentrically arranged ; 

 150 



