ASSIMILATION OF CARBON 



6l 



ture — previously shaken — is introduced into a small flask of sterilized water. 

 From this is inoculated, by means of the tip of a platinum wire, another flask 

 containing beer-wort and gelatine, warmed to 45°C. The latter is vigorously 

 shaken and then a drop of the liquid is transferred to a circular cover glass (30 

 mm. in diameter), which has been marked off into numbered squares, and the 

 cover is laid over a glass ring to form a moist chamber or van Tieghem cell. The 

 yeast cells are held immovable in the hardened gelatine so that it may now be 

 noted in which squares single ones lie, and the development of colonies from 

 these may be readily followed. When the colonies become clearly visible to the 

 unaided eye, one of them is removed from the cover glass and placed in a flask of 

 nutrient solution. The colonv is lifted on the end of a bit of flame-sterilized 



Fig. 38. — Pasteur flask; a 

 slightly different form from 

 that of Fig. 32, p. 55. 



Fig. 39. — Petri dish. 



Fig. 40. — Showing insertion 

 of needle into solid medium in 

 inverted tube, to make stab 

 inoculation. 



platinum wire, held by means of forceps, and the wire, with its colony, is dropped 

 into the flask. During this operation the cover glass must be held with the drop 

 on its under side, to prevent infection from the air. If a large quantity of pure 

 culture is desired, a portion of a young culture a day old, obtained as just de- 

 scribed, is transferred with a pipette to a Pasteur flask (capacity about 200 cc.) 

 of sterilized beer- wort (Fig. 38). After a day the contents of this flask are 

 poured into second flask (capacity about 500 cc.) also filled with sterile beer- 

 wort. 



Solid as well as liquid nutrient media are used for pure cultures of bacteria. 

 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 bouillon 

 and gelatine, at 30°C, after which the flask is shaken, and the contents are then 



