112 BACTERIOLOGY . 



and agar-agar, from the micro-organisms transferred to the cover- 

 glass before it is dried and stained, from any remnants of the colony 

 which was examined, or from other colonies bearing exactly similar 

 appearances. In this way pure cultivations are established, and 

 the macroscopical appearances of the growth in test-tubes can be 

 obtained. The plates should be replaced in the damp chamber 

 as soon as possible ; drying of the gelatine, or contamination with 

 micro-organisms gravitating from the air during their exposure, 



may spoil them for subsequent examination. 

 A much simpler method of plate-cultivation 



is to dispense with the levelling apparatus, and 



pour the liquefied ielly into shallow, flat dishes. 

 FIG. 41. PETRI'S 



j) ISH They take up much less room, and in many 



ways are more convenient (Fig. 41). 



Nutrient agar-agar can also be employed for the preparation 

 of plate-cultivations, but it is much more difficult to obtain satis- 

 factory results. The test-tubes of nutrient agar-agar must be 

 placed in a beaker with water and heated until the agar-agar 

 is completely liquefied. The gas is then turned down, and the 

 temperature of the water allowed to fall until the thermometer 

 stands just above 50 C. The water must be maintained at this 

 temperature, and the test-tubes must be in turn rapidly inoculated 

 and poured out upon the glass plates, or better still, into glass 

 dishes, as already described. 



A very much simpler plan is to liquefy the agar, pour it into 



FIG. 42. GLASS BENCHES AND SLIDES. 



a shallow dish, and allow it to solidify. The culture material is 

 thinned out in sterilised broth, and a few drops are spread out over 

 the surface of the agar. The dishes are then placed in the incubator 

 at 37 C. 



Glass plates may also be employed in a much simpler way. 

 The nutrient jelly is liquefied, poured out, and allowed to set. A 

 needle charged with the material to be inoculated is then drawn 

 in lines over the surface of the jelly. This method is of use for 

 inoculating different organisms side by side, and watching the effect 

 of one upon the other, or a micro-organism in this way may be 



