66 THE STRUGGLE FOR EXISTENCE 



same age were taken and a uniform suspension of yeast was made by- 

 shaking. One cm 3 of liquid was taken by a pipette from every tube 

 and poured into another clean tube, where the three cm 3 obtained 

 from three tubes were fixed by three cm 3 of 20 per cent solution of 

 H2SO4. Individual fluctuations of separate cultures were thus neu- 

 tralized, and a certain "average suspension" from three test tubes was 

 obtained. The material fixed was more or less diluted with water, 

 and then the number of cells per unit of volume was counted in the 

 Thoma chamber. Quite recently Richards ('32) in his interesting 

 paper describes in detail the methods of studying the growth of yeast, 

 where he points out that the counting of the number of yeast cells 

 is a very satisfactory method. As regards the possible sources of 

 error, he indicates the following: (1) the sample placed in the count- 

 ing chamber is not truly representative of the population sampled; 

 (2) the cells do not settle evenly in the counting chamber. To elimi- 

 nate these errors it is necessary to take several sample groups from 

 the "average suspension," and to count a great number of squares in 

 the chamber. In our experiments the fixed suspension was carefully 

 mixed before the taking of the sample, a few drops were taken with a 

 pipette, placed in the chamber, and ten squares were counted. Six 

 such sample groups were successively taken, and the total number of 

 counted squares amounted to sixty. Sometimes a lesser number of 

 squares sufficed. 



The average number of cells in one large square of a Thoma cham- 

 ber at the dilution corresponding to the material fixed (i.e., twice 

 thinner than the initial suspension) is given in our tables. It is 

 understood that the counts sometimes were made with considerably 

 stronger dilutions, and they were correspondingly reduced to the 

 accepted standard. A few words must be added concerning the 

 counting of cells in mixed cultures. After a certain amount of prac- 

 tice it is quite easy to distinguish the two species of yeast, as the cells 

 of Saccharomyces are much larger than those of Schizosaccharomyces 

 and their structure is different. 



(5) The numbers of yeast cells belonging to two different species 

 do not allow us to form an idea as to their masses. But it is just 

 the masses of the species that are of particular importance in the 

 processes of the struggle for life. This is because a unit of mass of a 

 given species is usually connected by definite relations with the 

 amount of food consumed or that of the waste-products excreted or, 



