LABORATORY AND TEACHING METHODS 62 1 



ing powers of two species. In the first two cases, it is required to determine tlie 

 actual number of cells which are to be seeded, and no attention is paid to the quantity 

 of liquid inoculated; in the last case, it is required only to know the relative number 

 of cells, but regard must be had to the quantity of liquid seeded. Finally, the follow- 

 ing must be remembered: If there is to be a definite volume in the flask after seed- 

 ing, then, in the case where the seeding is not to be made in water, or where the con- 

 centration of the liquid is of some account, no water must be used in shaking up the 

 yeast. In this case the same culture liquid must be employed. The same quantity 

 of culture liquid is then removed from the flask before seeding, as will be added when 

 seeding takes place. 



The procedure in the above three cases is as follows: (i) After shaking, a drop of 

 the water is placed in the hsmatimeter, or in the Thoma chamber, and the number 

 of cells is determined in the usual manner. On seeding a measured portion of the 

 water mixture is taken, and we thus know how many cells have been sown. 



2. As above. In counting we learn, for example, that a cells are present in a 

 certain volume. It is here necessary to know the quantity of culture liquid in the 

 flask to be inoculated; assume the amount to be p c.c. If it is desired to seed so 

 many cells that there will be ai cells per unit of volume, the number of cubic centi- 

 meters X of the water-yeast mixture, which must be added in order to arrive at this, 



a p '\- X 

 is found from the following equation: — = ■ or the number of cells in the 



water mixture (the seeding liquid) has the same proportion to the cells after seed- 

 ing as the whole amount of liquid after seeding has to the amount of seeding liquid. 

 The quantity of liquid in the flask after seeding has taken place is thus p + x. 



O-i p 



From the given equation, x = . Example: It is found that the seeding 



liquid contains 75 cells per unit of volume and the flask to be infected contains 70 

 c.c. of wort, and it is further desired to have 5 cells per unit of volume after inocula- 

 tion. Accordingly, x = — = 5 c.c. to be withdrawn from the seeding liquid. 



The result may be checked by another counting after seeding. If the result is in- 

 correct, either more liquid or more cells must be added. But in exact work this 

 contingency does not arise. 



Suppose it is wished to sow ai cells of a yeast species .4, and 61 cells of a species B 

 in a flask containing p c.c. of culture liquid, from two seeding liquids containing a 

 and b cells per unit of volume respectively. The number of cubic centimeters x 

 and y, to be sown from A and B respectively, is found from the following equations. 



a p + x + y b p + x + y 



— = and 7- = 



oi x bi y 



the quantity of liquid after infection being p + x + y; from this we find: 



aibp , abip 



X = T r r and y = -7 j ;-■ 



ab — OjO — OiOi ab — a\b — aibi 



Combinations of the above three cases may of course occur but from the explana- 

 tions given here it will not be difficult to solve them. 



