CHAPTER IX. 



The Study of Colonies— Their Naked-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 incuba- 

 tor, 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 diffi- 

 culty one can see them with the naked eye. Again, they 

 may be of a dense, opaque appearance; at one time sharply 

 circumscribed 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 fluids 



Place the plate containing these points upon the stage of 

 a microscope and examine them with a low-power objec- 

 tive, and again differences will be observed. Some of these 

 minute points will be finely granular, others coarsely so; 

 some will present a radiated appearance, while a neighbor 

 may be concentrically arranged; here nothing particularly 

 characteristic will present, there the point may resolve 

 itself into a mass having somewhat the appearance of a 

 (152) 



