40 THE AMERICAN MONTHLY [Feb., 



teriological and physiological text-books and laboratory 

 guides. 



FRACTIONAL CULTURES. 



There is more than one way of making pure cultures ; 

 none of these is absolutely exact save Hansen's method 

 described above. We see that Hansen's method is not 

 the same as Koch's plate method, and that the difference 

 is that we are by the former, able to trace the develop- 

 ment of individual cells. This, we cannot do by the lat- 

 ter ; in bacteriology we see the colonies before we see 

 the cells. 



A pure culture of yeast may be obtained more easily 

 by a fractional culture than by the absolute pure culture. 

 The number of cells in one cm-' of the well stirred sample 

 is ascertained by one of the many counting apparatus 

 existing, and this number we call n. A certain number 

 (s) of cm-' are then transferred from the sample by 

 means of a pipette, into a flask containing v cnr^ of ster- 

 ile water. When this emulsion has been stirred, each 

 cm^ of the water will contain "^-^ cells. The dilution 

 of the emulsion is carried further until 2-^-?- = J, or 



V 



there are e. g., 30 cells in 60 cnr^ of water. When this 

 has been done, 25 or 30 flasks with beer-wort are infected 

 each of them with one cm'^ of the diluted emulsion, and 

 thus we have the prospect that as far as can be judged 

 from calculations, half of the entire number of flasks con- 

 tain each one cell. As the method is, however, not abso- 

 lutely certain, we run the risk of finding in some flasks 

 no cells, while in others we may find two or more cells. 

 The flasks are placed in the thermostat by 25°C., having 

 been carefully stirred. Afterwards, stirring should be 

 entirely avoided, the cells will then descend to the bot- 

 tom. The cultures are now looked over daily for about 

 a week. It is evident that the cells have developed in 

 the places where they have been deposited, and thus each 

 cell is represented by a small, white, round spot or flock 



