112 llACTEKIOLOGY. 



and agar-agar, from the micixi-oi'gaui.sms transferred to the euver- 

 u-lass 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 giuvitating from the air dui'iug 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 liquetied ielly into .shallow, flat dishes. 

 Fig. 41.^Petri's i^ '^ ■' r ' 



j)jgjj -Lhey take up much less I'oom, and m many 



ways are more convenient (Fig. 41). 



Nutiient agar-agar can also be employed for the preparation 

 of plate-cidtivatious, 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 Avater and heated until the agar-agar 

 is completelj- 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 

 tempei'rttui-e, and the test-tubes must be in tui-n rapidly inoculated 

 and poured out upon the glass plates, or bettei' still, into glass 

 dishes, as alieady described. 



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



I 



Fii;. 42.— Gl.\sm Benches and Sudes. 



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 tlien i>laceil in the incubator 

 at 37° 0. 



Glass plates may also l^e 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 sui-face of the jelly. This method is of use for 

 inoculating different organisms .side by side, and watching the effect 

 of one >ipon the other, or a. micro-organism in this Avay may be 



