THE GROWTH AND APPEARANCES OF COLONIES 87 



CHAPTER XIII 



THE GROWTH AND APPEARANCES OF COLONIES 



Macroscopic. Depending greatly upon the temperature, 

 which should be about 65 F. (20 C.) for gelatin, and 40 C. 

 for agar, the colonies ordinarily develop so as to be visible to 

 the naked eye in two to four days. Some require ten to four- 

 teen days, and others grow rapidly, covering the third dilu- 

 tion in thirty-six hours. The plate should be looked at each 

 day. 



The colonies present various appearances 'from that of a 



Fig. 32. Staphylococcus pyogenes aureus: colony two days old, seen 

 upon an agar-agar plate (X4o) (Heim). 



small dot, like a fly-speck, to that resembling a small leaf. 

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

 cup-shaped umbilicated. 



Then they are variously pigmented. Some liquefy gelatin 

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

 so characteristic as to be recognized at a glance. Some 

 produce gas-bubbles. 



