80 ESSENTIALS OF BACTERIOLOGY 



gelatin or agar; this is labeled first dilution (a colored pencil 

 is useful for such markings). From the first dilution three 

 drops are taken into a third tube, which becomes the second 

 dilution. 



The plugs of cotton must be replaced after each inocula- 

 tion, and while being held must be carefully protected from 

 contamination. 



Glass Plating. The larger the surface over which the 

 nutrient medium is spread, the more isolated will the colonies 

 be; window glass cut in rectangular plates 6x4 inches in 

 size was formerly used, but now Petri dishes consisting of 

 2 circular glass or porcelain dishes, one fitting over the other 

 as a cover, are universally employed (Fig. 24). They are 

 sterilized, the softened and inoculated agar or gelatin is 

 poured from the test-tube into the dish with as much speed 



Fig. 24. Petri dish for making plate cultures. 



as possible, and the lid replaced, avoiding contamination 

 from the air and surroundings. They are labeled or marked 

 with pencil, and placed in the incubator or kept at room 

 temperature for further development. 



This method is very useful for transportation, and does 

 away with the cooling apparatus and moist chamber for- 

 merly employed; the saucers can be viewed under micro- 

 scope similar to the glass plates, and have entirely super- 

 seded them. 



Esmarch's Tubes or Rolled Cultures. This method, 

 especially used in the culture of anaerobic germs, consists in 

 spreading the inoculated gelatin upon the inner walls of the 

 test-tube in which it is contained and allowing it to congeal. 



