GROWTH AND APPEARANCE OF COLONIES. 65 



Some are elevated, some depressed, and some, like cholera, cup- 

 shaped umbilicated. 



FIG. 35. 



Staphylococcus pyogenes aureus : colony two days old, seen upon an agar-agar plate; 

 X 40 (Heim). 



Then they are variously pigmented. Some liquefy the gela- 

 tine speedily, others not at all. The appearances of a few are 

 so characteristic as to be recognized at a glance. 



Microscopic. We use a low-power lens, with the abbe nearly 

 shut out, that is the narrowest blender. The stage of the 

 microscope should be of such size as to carry a culture plate 

 easily upon it. 



The second dilution or third plate is usually made use of, that 

 one containing the colonies sufficiently isolated. 



These isolated ones should be sought for, and their appearances 

 well noticed. 



There may be two or three forms from the same germ, the 

 difference due to the greater or less amount of oxygen that they 

 have received, or the greater or less amount of space that they 

 have had to develop in. 



The microscopic picture varies greatly ; now it is like the 

 gnarled roots of a tree, and now like bits of frosted glass ; the 

 pictures are very characteristic, and the majority of bacteria 

 can be told thereby. (Fig. 32.) 



