TEST-TUBE, STAB- AND SMEAR-CULTURES. 135 



dinm can usually be detected. When deep down in the 

 medium, owing to surrounding pressure, they are quite 

 round, oval, or lozenge-shaped; whereas when they are 

 on the surface of the gelatin or agar they may take 

 quite a different form. This is purely a mechanical 

 effect due to the pressure of, or resistance offered by, 

 the medium surrounding them, and is always to be 

 borne in mind, otherwise errors are apt to arise. 



Pure Cultures. — If from one of these small col- 

 onies a bit be taken upon the point of a sterilized plati- 

 num needle and introduced into the tube of sterilized 

 gelatin or agar-agar, the growth that results will be 

 what is known as a "pure culture," the condition to 

 which all organisms must be brought before a system- 

 atic study of their many peculiarities is begun. Some- 

 times several series of plates are necessary before the 

 organism can be obtained pure, but by patiently follow- 

 ing this plan the results will ultimately be satisfactory. 



Test-tube Cultures; Stab-cultures ; Smear- 

 cultures. — After separating the organisms the one 

 from the other by the plate method just described, they 

 must be isolated from the plates as pure stab- or smear- 

 cultures. 



This is done in the following way: decide upon the 

 colony from which the pure culture is to be made. 

 Select preferably a small colony and one as widely sep- 

 arated from other colonies as possible. Sterilize in the 

 gas-flame a straight platinum-wire needle. The glass 

 handle of the needle should be drawn through the flame 

 as well as the needle itself, otherwise contamination from 

 this source may occur. When it is cool, which is in five 

 or ten seconds, take up carefully a portion of the colony. 

 Guard against touching anything but the colony. If 



