ASSIMILATION OF CARBON 59 



In the case of liquid media the dilution method described above is used to 

 separate the cells. With solid media, which are very valuable for the pro- 

 duction of pure cultures, Petri dishes are used for this purpose (Fig. 39). Each 

 dish consists of two shallow glass pans (9 or 10 cm. in diameter), one being a 

 little larger than the other and forming a cover for it. A trace of the mixed 

 culture is introduced into a flask containing, for instance, a mixture of bouUlon 

 and gelatine, at 3o°C., after which the flask is shaken, and the contents are then 

 poured into the dish and the latter is covered. After some time each bacterial 

 cell builds a colony around itself, which can be seen by the unaided eye or with 

 a magnifying glass. 



When a pure culture of a certain microorganism is finally obtained, then 

 any number of pure cultures of that form may be readUy prepared. Inoculations 

 of liquid nutrient media are effected by means of a glass rod, a platinum wire or 

 a pipette, with all the requisite precautions. Inoculations of solid media may 

 take the form of either stab or streak cultures. To make a stab culture a 

 platinum needle is dipped in the original culture and is then thrust upward into 

 the solid medium held in an inverted test-tube (Fig. 40). For a streak cul- 

 ture, a test-tube of solid medium with a slanting surface is prepared, and 

 the point of the inoculating needle is drawn across this surface. 



