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IX. 

 MODIFIED GELATIN PLATE CULTURES. 



Several modifications of the Koch plate method as 

 just described have been introduced whereby the same, if 

 not better results are obtained with less apparatus. In 

 the plate method contamination not infrequently results 

 from exposure to the air while on the ice apparatus, or 

 subsequently when kept in the large, moist chamber. An 

 examination of one plate necessitates the exposure of the 

 remaining plates to contamination with the organisms in 

 the air. Furthermore, it not infrequently happens that 

 the gelatin on an upper plate undergoes liquefaction and 

 then drips over the edges of the plates on those below it. 

 To overcome these difficulties Petri introduced the use of 

 shallow dishes which are about 10 cm. in diameter. 



Petri Dish Culture. Place the Petri dishes in a wire 

 basket and sterilize in the diy-heat oven by heating I 

 hour at a temperature of 150-175 C., then allow to cool. 

 Inoculate three gelatin tubes with the organism to be 

 plated in the same manner as for ordinary plates. Cut off 

 the projecting cotton, sterilize the lip of the tube as before, 

 then pour the contents of each tube into one of the cool, 

 sterilized Petri dishes, properly labeled. Replace the 

 cover and gently tilt the dish from side to side so as to 

 cause the gelatin to spread evenly over the bottom. Allow 

 the gelatin to solidify, then set the dishes aside for colo- 

 nies to develop. When the colonies develop examine on 

 the stage of the microscope and transplant as with ordi- 

 nary plates. 



In this method each dish constitutes a plate by itself. 

 It can be readily examined and the risk of contamination 

 is reduced to a minimum. In addition to that the use of 



