Lecture VIII. 6 1 



At this point we may proceed to separate the individuals of this 

 growth from one another. Place the test tube between the palms 

 of the hands and incline the tube so that it is as nearly horizontal 

 as may be without letting the medium touch the cotton-wool plug. 

 In this position roll the tube to and fro between the hands. 

 The motion will distribute the bacteria through the liquid medium. 

 To secure uniform distribution a considerable amount of rolling 

 must be done. When it is judged that this distribution is effected 

 and the gelatine is still liquid we must transfer it into a petri 

 dish. This latter is a disk-shaped shallow glass dish covered by a 

 similar but slightly larger dish inverted over it. It has been previ- 

 ously sterilised in a hot air oven. Again burn the projecting 

 cotton-wool, pinch the plug of the test tube as before between 

 the right third and fourth fingers and withdraw it. Gently raise 

 the lid of the petri dish, using the finger and thumb of the right 

 hand, and introduce the neck of the test tube over the dish and 

 pour the liquid gelatine into it. Close the petri dish and by 

 suitably inclining it spread the medium uniformly over its surface. 

 Now leave it on a horizontal table to cool and set. 



It will be understood that all these operations must be con- 

 ducted in air as dust-free as possible, so that the chance of 

 contamination while the medium is exposed may be reduced to a 

 minimum. With this in view, draughts and hasty movements 

 which stir up dust should be avoided. 



When the gelatine has set in the dish, the latter may be placed 

 aside for a day or so, according to the temperature. If the dis- 

 tribution has been satisfactorily effected, single individuals will be 

 scattered through the layer of the medium in the dish. Conditions 

 being favourable, these cells grow and subdivide, and each becomes 

 the originator of a tiny colony. When multiplication has suffi- 

 ciently proceeded the colony becomes a visible speck. Where the 

 originating individuals are sufficiently apart each colony is formed 

 of the descendants of one individual and is in fact what may 

 be termed a pure culture. By direct observation we may often 

 notice striking differences between these colonies. The gelatine 

 in the immediate neighbourhood of some is liquid, while around 

 others it is unaltered. The cells of the former colony have pro- 

 duced an enzyme which liquefies gelatine, while the latter do not 

 possess this power. Some of the colonies are pale pink in colour, 

 others are cloudy white or yellow. Differences also in the out- 

 lines of the colonies will also be noticed. 



In order to examine further these different kinds of bacteria and 

 to keep them free from contamination, a special procedure must 



