Brown.— Some Studies on Yeast \ 
201 
a sphere of 12*6 mm. diameter. The apparent volume of the magnified 
cell will be 1*06 c.c., or about o-x per cent, of the total volume of the 
liquid in which it is suspended. With this distribution the average distance 
apart of the cells of the yeast-seed would be about eight diameters, and even 
when the cells have increased to eight cells per unit volume their average 
distance apart will not be less than four diameters. 
When these facts are taken into consideration, and also that the Indivi- 
dual cells are bathed in a nutrient liquid of a concentration hundreds of 
times greater than that requisite to supply all their demands by the process 
of diffusion, It must be evident that distributions or packings of the cells of 
such orders of magnitude as we are now considering cannot be responsible 
for the great slowing down of the cell-increase which Is implied by the 
progression of such increases being an arithmetical and not a geometrical 
one. Were this the case there would be some indications of it in the earlier 
stages by an upward trend of the curve, whereas the more carefully the 
experiments are made the more strictly rectilinear is the ‘ curve ’ found to 
be, subject to a slight departure at the commencement, marking a short 
incubation period before active growth commences. 
We are therefore obliged to seek for an explanation of the linear 
progress of yeast reproduction in some other direction than that suggested 
by overcrowding and mutual interference of the cells in obtaining their 
food material from the nutrient liquid containing them. 
The two most potent factors in controlling the reproduction of yeast- 
cells are undoubtedly the amount of free oxygen at the disposal of the cell 
prior to the commencement of reproduction, and the limitation and partial 
inhibition of growth brought about by the gradually increasing amount of 
alcohol produced by the fermentative action of the yeast. These facts were 
thoroughly well established by Adrian Brown (loc. cit.), who also showed 
that neither the carbon dioxide 1 nor the non-volatile products of fermenta- 
tion had any appreciable effect. The final conclusions at which he arrived 
were that in a suitable nutrient liquid yeast reproduction is conditioned 
and governed mainly by the amount of oxygen originally at the disposal of 
the cell prior to the commencement of reproduction, and that it is the 
exhaustion of the stimulating influence of this oxygen which becomes the 
limiting factor in arresting the reproductive functions, and not the alcohol 
produced, since the sharp arrest of cell-growth is often noticed when the 
percentage of alcohol falls below that necessary to produce any appreciable 
effect. The finite amount of oxygen initially present in the liquid was also 
1 A. Slator (Biochem. Journ., vii (1913), p. 201) refers incidentally to the influence of carbon 
dioxide in retarding yeast-growth. I am unaware of any experiments which show this influence of 
C 0 2 apart from its effect of preventing access of oxygen. Even should it be shown that a. distinct 
retarding effect can be produced by C 0 2 , this would not have any material bearing on the experiments 
here recorded, since such effect would speedily attain a maximum when the liquid was saturated 
with C 0 2 , i. e. in the earliest stage of the experiment. 
