MAKING PLATE CULTURES. 259 



inoculated material uniformly. It is necessary to avoid shak- 

 ing too vigorously, or otherwise bubbles will make their 

 appearance, which will interfere with the accuracy of the 

 test. The shaking should be thorough but not violent. 



FIG. 40. 



Petri dish for making " plate cultures." 



Six sterilized petri dishes (Fig. 40) should have been 

 placed upon a plate of glass which is held in as nearly a 

 level position as possible and cooled artificially. This can 

 be accomplished by taking a large, flat dish, filling it with 

 water and ice and then laying a large plate of glass upon 

 the top. The ice will rapidly cool the glass plate, and the 

 petri dishes placed upon the plate will also rapidly become 

 cooled. The contents of each of the test-tubes inoculated 

 with the diluted milk are now to be poured each into a petri 

 dish and the cover quickly replaced. The culture medium will 

 distribute itself in a thin layer over the bottom of the petri 

 dish and soon harden. The dishes are to be labelled and then 

 set aside in a proper place for growth. If desired to hasten 

 the analysis the dishes may be placed in a culture oven kept 

 at a temperature of 98. For ordinary study of milk bac- 

 teria it is usually most satisfactory to leave the petri dishes 

 at a room temperature of 70, allowing them to remain for 

 three or four days before the final study is made. 



The Study of the Plates. The solidified culture medium 

 fixes each bacterium at a single point. As the bacteria feed 



