Methods of Studying Bacteria. 23 



To this melted gelatin a definite quantity of milk is 

 added. The medium is gently shaken, so as to thoroughly 

 mix the milk and gelatine, and the mixture then poured 

 into a sterile, flat, glass dish, and quickly covered, where 

 it is allowed to cool until the gelatin hardens. After 

 the culture plate has been left for twenty-four to thirty- 



FIG. 4. PLATE CULTURE. 



Each of the dots is a colony that has been formed by the 

 growth of an organism embedded in the solid culture-medi- 

 um. By counting the colonies, the number of living bacteria 

 in the amount of milk added to the culture is determined. 



six hours at the proper temperature, tiny spots will be- 

 gin to appear on the surface, or in the depth of the cul- 

 ture-medium. These spots are called colonies, and are 

 composed of an almost infinite number of individual 



