2 I 6 
Brown . — Some Studies on Yeast. 
limit of reproduction, as determined by cell-counting in a unit volume of 
liquid, should be the same in both cases. Again, the reduced number of 
subdivisions in C, due to the reduction of the oxygen charge on the individual 
cells to one quarter, is compensated by the fourfold number of seed-cells in 
this case. 
This fundamental idea of the dependence of reproduction on the intensity 
of the initial oxygen charge also gives a complete explanation of the rate of 
reproduction being linear instead of logarithmic, for at each cell-subdivision 
the oxygen charge of the mother-cell must be halved, consequently the time 
required for the next subdivision of the mother- and daughter-cell will be 
double that required for the previous subdivision. 
These considerations, taken in conjunction with the experimental data 
given in this paper, afford a quantitative proof of the correctness of the 
idea, first expressed by Pasteur, that the oxygen originally absorbed by the 
yeast from its surrounding medium is the primum movens which determines 
its subsequent power of reproduction. 
The question now arises as to the limits of reproductive power which 
are imposed on the cell by this initial absorption of oxygen. 
Although a yeast-cell can, under favourable conditions, continue to 
take in oxygen from the surrounding liquid if this is maintained saturated 
with air, the cell can only store up a limited amount of oxygen in a form 
available for subsequent reproduction. All attempts to increase this effective 
‘oxygen charge’ by previously submitting the seed-yeast to strong aeration, 
or even by passing pure oxygen through the liquid, were unsuccessful. 
The yeast under these circumstances continues to take in oxygen, but its 
potentiality of reproduction is not thereby increased. Whether the whole 
of the surplus oxygen absorbed, over and above that required for the repro- 
ductive ‘ charge ’, is used up in ordinary processes of respiration appears to 
be very doubtful, and much more work requires to be done before this 
question can be definitely answered. 
Meanwhile, the experiments which have been summarized in the curves 
of Fig. 4 give us some information about the actual values of an oxygen 
charge requisite to produce a given amount of cell-reproduction. In these 
experiments, in which the nutrient liquids contained known and graduated 
amounts of dissolved oxygen and were initially seeded at a uniform rate of 
one cell per unit volume, it is seen that a direct proportionality exists between 
oxygen supply and maximal cell-reproduction, up to a point within about 
60 to 65 per cent, of complete saturation of the liquid with atmospheric 
oxygen. This point corresponds to an initial oxygen content of the liquid 
of about o*6 x 0*559 — 0*335 c.c. of oxygen per 100 c.c. of the liquid. 
When the oxygen in the liquid is increased beyond this point up to full 
saturation with air, although further yeast reproduction takes place, the final 
cell-increase is no longer strictly proportional to the available oxygen 
