68 LABORATORY EXERCISES IN BACTERIOLOGY, 



to remain until they are to be used, in order to prevent accidental separation of the 

 covers from the dishes. Contamination of the interior by bacteria from the surround- 

 ing atmosphere is, as in case of the potato dishes (but to a less degree because of the 

 smaller size of the Petri dishes and the better adaptation of their covers), more likely to 

 take place than in the use of culture tubes ; but this can in some measure be obviated 

 by the application of a strip of paper or a rubber band, such as is used about bank books, 

 about the outside surface along the line of overlapping of the cover. 



4. Plates. Plates of glass, usually four by five inches in size, were formerly much 

 used in the separation of the bacteria of impure cultures to obtain pure cultures, the 

 inoculated medium being spread over the surface of such a plate so that the organisms 

 present might in development grow into well-separated colonies and thus favor the re- 

 moval of such as may be desired, by means of the needle, to fresh culture tubes. They 

 are occasionally used at present for this purpose, but have been largely superseded by 

 Petri dishes and by the so-called Esmarch tubes. In preparation for use, the plates are 

 cleaned in the same manner as indicated for tubes and are then dried and placed in a 

 metal box with close-fitting lid, in which they are baked in the dry-air oven in the usual 

 manner of fractional sterilization. The box is opened only when the plates are required 

 to be withdrawn for use (Fig. 20). In arranging plate cultures it is customary to place 

 these plates in the large culture dishes above mentioned, usually three in each dish, each 

 supported upon a glass platform and built into a "set," one over the other (Fig. 21). 

 The glass platforms are to be prepared for use in the same way as the plates, but when 

 sterilized should be wrapped separately in paper, which is kept about them for protec- 

 tion until they are to be arranged in the culture. 



In addition to the above apparatus there will be required a number of other forms 

 of glassware and other appliances for the preparation and distribution of the culture 

 media and for the prosecution of culture experiments, which may be described to best 

 advantage in connection with the processes in which they are employed. 



Exercise 22. Let the student at this time prepare, according to the 

 preceding instruction, half a gross of ordinary culture tubes, one dozen 

 potato tubes, ten Petri dishes, and clean all tubes, etc., which have been 

 used in previous exercises. 



