CULTIVATION OF MICROORGANISMS 39 



tion of a mass of cells sufficiently large to be visible to the 

 unaided eye. Such masses are termed colonies. 



Each colony represents the progeny of a single cell or a 

 group of cells. The colonies of many kinds of bacteria ex- 



Fig. 1C. A Plate Culture 



Each of the spots or colonies on the plate has resulted from the growth of a 

 single cell or group of cells of the same kind in the substance with which the 

 plate was seeded. Plate cultures are used for determining the number of 

 bacteria in various substances and for the isolation of pure cultures of bacteria 



hibit more or less characteristic appearances, with reference 

 to the size of the colony, its shape, the nature of its edge, 

 and its interior structure. These are termed cultural char- 

 acteristics. The colonies resulting from cells that were on 

 the immediate surface of the medium or very close to it 

 give rise to surface colonies. These exhibit the more char- 

 acteristic appearances, since the growth can develop freely. 



