ESMARCH ROLL CULTURE. 71 



The Petri dishes are placed in the incubator for twenty- 

 four hours, when the surface of the medium is seen to be 

 studded with fine dots, the number depending on the degree 

 of dilution. These dots or specks are called colonies. Colo- 

 nies also develop, but more slowly, in the body of the medium. 

 Koch found that when bacteria are transplanted on a large flat 

 surface (Fig. 25) they invariably exhibit a tendency to isolate 

 themselves and form colonies which are characteristic of each 

 species of bacteria. The study of these colonies is the first 



FIG. 26. 



Demonstrating Booker's method of rolling Esmarch tubes on a block of ice. 

 (Abbott.) 



step in the differentiation of bacteria. Each colony repre- 

 sents a pure culture of the germ, and from it the species is 

 propagated for further study. The colonies are examined 

 first with the naked eye and then with a low power (-) 

 of the microscope, the Petri dish being placed upside down 

 on the stage of the microscope. An accurate note is made 

 of the appearance of the colony, its color, contour, size, etc. 

 The description should be accompanied by a sketch or photo- 

 graph. 



Esmarch roll culture (Fig. 26) : In this method the wall 

 of the test-tube takes the place of the plate or Petri dish. 

 Roll cultures are made easily, and possess the advantage that 



