68 CULTURES AND THEIR STUDY. 



with an accurately fitting lid. These dishes are washed and 

 sterilized by dry heat for one hour. The covers should not 

 be removed until the dishes are ready to be used. Take 

 tube No. 1, burn off the projecting end of the cotton plug in 

 the flame of an alcohol lamp or Bunsen burner ; then with 

 sterilized forceps push the plug down into the tube for about 

 an inch. Hold the end of the tube containing the plug in 

 the flame for a few minutes to sterilize it. An assistant now 



FIG. 23. 



Colonies in Petri dish. 



raises the lid of the Petri dish directly over the dish about 

 one inch (just high enough to admit the end of the tube), the 

 cotton plug is removed with sterile forceps, and the contents 

 of the tube (which are still liquid) poured into the dish. The 

 cover is replaced immediately and the dish moved gently to 

 and fro so that the medium will be spread evenly over the 

 bottom of the dish. The film of medium in the dish should 

 be sufficiently thin to be transparent, so as not to hinder 

 microscopic examination of the colonies of bacteria (Fig. 23). 



