CULTURE METHODS 63 



measuring the surface of the glass covered by the media, and multiply- 

 ing by the number of colonies found in a certain square. 



Gelatine plate or dish cultures are kept at ordinary room tempera- 

 ture or in the incubator at 22 C., and agar-agar plates at 37. In 

 twenty-four, forty-eight, or seventy-two hours they can be examined 

 with the naked eye, hand lens, or microscopically with a low power 

 lens. 



The various kinds of characteristic growths are noted, and the 

 number of colonies counted with the Wolffhligel apparatus (see 

 Fig. 13). 



FIG. 13. Wolffhiigel's Counting Apparatus. 



CXVII. METHOD OF OBTAINING A PURE CULTURE 

 FROM A COLONY ON A PLATE OR DISH. 



1. Examine the plate under the microscope with a low power, or 

 use a hand lens, and find the desired colony, always selecting an 

 isolated colony when possible. 



2. Take a tube of sterilized nutrient gelatine, sterilize a platinum 

 wire, and carefully remove a part of the desired colony on the point 

 of the wire. 



3. Remove the plug from the gelatine tube, holding the tube 

 perpendicular, mouth downwards, in the left hand, and make a stab 

 culture in the middle of the gelatine medium, extending almost to 

 the bottom of the tube, heat the neck of the tube in the flame, and 

 replace the plug ; with liquid media the tube is held slanting. 



METHOD OF KEEPING GROWING CULTURES PURE. 



1. Inoculate the cultures into fresh medium every fourteen or 

 twenty-one days. 



2. Place the tube containing the original culture between the 

 thumb and first finger of the left hand, and place the tube to be 

 inoculated between the first and second fingers of the same hand. 



