2o8 



Cultures, and their Study 



can be melted and solidified at will can be used for plate cultures— viz., gelatin, 

 agar-agar, and glycerin agar -agar. Blood-serum and Loffler's mixture are en- 

 tirely inappropriate. 



The chief drawbacks to this excellent method are the cumber- 

 some apparatus required and the comparative impossibility of 

 making plate cultures, as is often desirable, in the clinic, at the bed- 

 side, or elsewhere than in the laboratory. The method therefore 

 soon underwent modifications, the most important being that of 

 Petri, who invented special dishes to be used instead of plates. 



Petri's Dishes.^ — These are glass dishes, about 4 inches in diameter 

 and y^ inch deep, with accurately fitting lids. They were iirst 



Fig. 46. — Petri dish for making plate cultures. 



recommended by Petri* and greatly simplify bacteriologic technic 

 by dispensing with the plates and plate-boxes, the moist chambers 

 and benches, and usually with the leveling apparatus of Koch, 

 though this is still employed in some laboratories, and must always 

 be employed when an even distribution of the colonies is necessary 

 in order that they can be accurately counted. 



The method of using the Petri dishes is very simple. They are 

 carefully cleaned, polished, closed and sterilized by hot air, care 



Fig. 47. — Petri dish forceps. 



being taken that they are placed in the hot-air closet right side up, 

 and after sterilization are kept covered and in that position. They 

 should be sterilized immediately before using, or if they must be 

 kept for a time should be wrapped in tissue paper and then sterilized. 

 ' The tissue paper protects the accidental entrance of dust between 

 dish and lid, keeps the dish closed, and need not be removed until 

 the last moment before using. 



Time can be saved by sterihzing the dish and cover in the direct 



* "Centralbl. f. Bakt. u. Parasitenk.," 1887, i. No. i, p. 279- 



