1899] MICROSCOPICAL JOURNAL. 239 



or colony. For this purpose agar and gelatin are. used. 

 Originally, Koch employed a rectangular piece of glass 

 for holding the layer of medium and protected it from 

 contamination by patting it under a bell jar. Later Es- 

 march introduced the "roll culture" method which was 

 extensively followed, until the Petri dishes were intro- 

 duced. Since that time they have been largely used in 

 place of the Koch plate and Esmarch tube. On this ac- 

 count the plate cultures of to-day are usually made in 

 Petri dishes. The roll culture, however, is occasionallv 

 made. 



General Directions. — Make a series of 3 agar plates, 

 one of 3 gelatin plates, and a series of 3 gelatin roll cul- 

 tures (Esmarch rolls) from the bouillon culture of Bacil- 

 lus coli communis. Place the agar plates in the incubator 

 and the gelatin plates and rolls in a locker for that pur- 

 pose. 



Making Agar Plates. — Take three large tubes of agar, 

 stand them in a water bath and boil until the agar is 

 liquefied. Then cool by standing the tubes with a ther- 

 mometer in a cup of water at a temperature of about 50 

 deg. C. As the temperature rises add a little cold water. 

 When the temperature of the agar reaches that of the 

 water and the temperature of the whole has lowered to 

 45 deg. C. the agar is ready for use. For convenience in 

 labeling number the tubes 1, 2, and 3. 



Place 3 sterilized Petri dishes on the leveling tripod 

 and adjust it by means of a spirit level. With the wire 

 loop proceed by the same method as followed in making 

 bouillon cultures. Take one loopful of the bouillon cul- 

 ture and place it in an agar tube No. 1 and mix by care- 

 fully shaking it. Flame the wire and transfer two loop- 

 fuls of agar from tube i to tube 2 and mix as before. 

 Again flame the loop and transfer 3 loopfuls from tube 2 

 to tube 3 and mix as with tubes 1 and 2. After the tubes 

 are inoculated, pour the agar into the Petri dishes. In 



