reached similar results and some have gone so far as to maintain that the
stimulation is not due to vitamine "B" at all. It is therefore evident
that until this controversy is settled the yeast test cannot be used for
the purpose proposed. Our own experiments at present make us still firm in
our belief that _one_ of the factors and perhaps the most important
factor in the stimulation effect is the vitamine but until we can devise a
basal medium that is comparable to that used in rat feeding experiments,
i.e., one that contains all the elements for optimum growth of yeasts
except vitamine "B" it will be unsafe to draw conclusions from the test as
to vitamine content. It may be possible to so treat our extracts as to
eliminate from them all other stimuli except the vitamine or to destroy
the vitamine in them and thus permit the comparison of an extract with the
vitamine destroyed against one in which it is present and thus arrive at
the result desired. At any rate all we can say at present is that the
yeast test is unreliable as a measure of vitamine content but that if it
can be made quantitative its advantages are so great that it is very much
worth while to continue work upon it until it is certain that it cannot be
made to produce the desired result.

[Illustration: FIG. 7. GROWTH RATE OF YEAST UNDER ALFALFA EXTRACT
STIMULATION

This chart shows the effect of varying concentrations of an alfalfa
extract on the growth rate of the yeast cell. The rate of growth was
determined after the Funk method by centrifuging the cells after seventy-
two hours incubation and measuring the volume in cubic centimeters. The
shape of the curve shows that this method will not give comparative
results unless the extracts tested are dilute enough for the
determinations to fall in the steep part of the curve.]