PLATE CULTIVATION 97 



" plated/ 5 so many organisms may be present in the film 

 that the colonies which develop will not be separate, but 

 will form a confluent growth. To make certain of obtain- 

 ing isolated colonies a second and a third dilution are 

 prepared. The second dilution is made by inoculating 

 the tube of melted gelatin No. 2 with one platinum loopful 

 from tube No. 1, and thoroughly mixing up ; and the 

 third dilution is prepared in the same manner by inocu- 

 lating the tube of melted gelatin No. 3 with two to four 

 platinum loopfuls from tube No. 2. The organisms intro- 

 duced are thoroughly distributed throughout the gelatin 

 by rolling and gently shaking each tube and the contents 

 of each tube are then poured 

 into a sterile Petri dish a 

 separate dish for each tube. 

 Before pouring, the mouth 

 of each tube should be 

 flamed so as to sterilise it, 

 but the glass must not be 



too hot or it will crack. FlG - 18. Petri dish for plate culti- 



The Petri dishes are shaUow vation ' 



glass dishes with drop-on lids (Fig. 18), 3 or 4 in. 

 in diameter ; they are previously sterilised in the 

 hot-air steriliser in suitable iron or copper boxes 

 holding a dozen or so, or each dish with cover may be 

 wrapped in paper. 1 The melted gelatin having been 

 poured in, the dish is tilted to and fro to diffuse the 

 gelatin over the bottom of the dish, placed on a level 

 surface for the gelatin to set, and then stored in the cool 

 incubator. The plates are examined daily, with a hand 

 lens if necessary, or with a low power of the microscope, 

 the dish being turned bottom upwards on the stage of the 

 microscope for this purpose. When the colonies have 



1 Formerly, the inoculated gelatine was poured on to a sterile glass plate 

 hence the name " plate culture," 



M.B. 7 



