6o 



PHYSIOLOGY OF NUTRITION 



this is only highly probable, however, and not definitely established. Hansen 

 employed this method in his work with yeasts. Flasks containing freshly 

 inoculated beer-wort are vigorously shaken and then allowed to stand. The 

 cells sink to the bottom and begin to multiply, so that, after a time, whitish 

 colonies of cells become visible with the unaided eye. If a flask shows but 

 one such colony it follows that only a single cell was introduced, since it is 

 highly improbable that two cells might have settled together after the shaking. 

 If, on the other hand, two or three cells have been introduced into the flask, 

 then two or three colonies, respectively, develop. 



In order to secure pure yeast cultures, solid substrata may also be em- 

 ployed, which make it possible to follow, under the microscope, the development 

 of a colony from a single cell. For this purpose a drop from a young yeast cul- 

 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 

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



Fig. 38. — Pasteur flask; 

 a slightly different form from 

 that of Fig. 32, p. 54. 



Fig. 39. — Petri dish. 



Fig. 40. — Showing insertion 

 of needle into solid medium in 

 inverted tube, to make stab 

 inoculation. 



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 un- 

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

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

 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- 



