1 98 
Brown. — Some Studies on Yeast. 
When such a suitable nutrient liquid, saturated with atmospheric 
oxygen, is seeded with a small amount of yeast anywhere in the neighbour- 
hood of 100 c.c. of pressed yeast per 100 cc. (an amount equivalent to about 
1 cell per unit volume of cubic mm. 1 ) and cell-counts are made in the 
haemocytometer at intervals of a few hours, the cell-reproduction expressed 
as a function of the time always progresses at a much slower rate than is 
demanded by the exponential law. This is due, as was first pointed out by 
Adrian Brown, to the reproduction being restricted and limited by the 
amount of oxygen dissolved in the nutrient liquid. 
Quite recently, however, it has been shown by A. Slator (Biochem. 
Journ., vii (1913)^. 197) that it is possible to realize the logarithmic rate of 
increase by inoculating the nutrient liquid with very small amounts of yeast, 
and slowly agitating the containing tubes in a thermostat. The seedings 
of yeast employed corresponded to from 1,360 to 90,100 cells per c.c., 
which are equivalent respectively to 0-0003 and 0-0225 cells per unit volume 
of wo- c.mm. 
These are conditions which might be expected to give the theoretical 
exponential rate of increase, since, owing to the wide distribution of the 
cells and the constant stirring, there must have been within the range of the 
experiments a practically unrestricted supply of oxygen, and consequently 
an absence of competition for this essential factor of cell-reproduction. 
The rate of reproduction of yeast-cells under the ordinary conditions of 
seeding a nutrient liquid in flasks with limited access of oxygen has been 
investigated by Adrian Brown (Trans. Laboratory Club, vol. iii (1890); and 
Journ. Chem. Soc., Trans., 1905, p. 1395). 
.In two series of experiments in which the nutrient liquid was on the 
one hand a solution of dextrose with yeast-water, and on the other a malt- 
wort of sp. gr. 1*053, flasks containing equal volumes of the solutions were 
seeded with yeast in amounts varying from 0-145 to 15*75 cells per unit 
volume of 40V0 c.mm., this initial rate of seeding being denoted by A. 
At the end of eighteen hours, the temperature remaining constant, a further 
count was made in each case ; this is denoted by B. And the c rate of cell- 
2 ^ 
reproduction’ 2 is given by It was found that ‘cell-reproduction 
proceeds at the maximum rate when the number of cells present is at 
a minimum, and that the rate falls with a decreasing velocity as the number 
of cells in the experiments is increased Later on the author suggests that 
1 One cell per unit volume of 4-^^ c.mm. is equivalent to 4 million cells per cubic centimetre. 
This rate of seeding is closely approximated when o*i gramme of ordinary brewers’ yeast (top fermenta- 
tion) which has been washed and pressed between folds of blotting-paper is mixed with 100 c.c. of 
liquid. 
J? 
2 The ‘ rate of cell reproduction ’, — , really represents the average number of cells derived from 
A 
a single cell during the experiment. 
