550 Hansen. — Experimental Studies on the 
organisms offer the most favourable conditions, everything 
. being here simpler, more easily seen through, and more easily 
mastered, than in the case of the higher plants. Further, 
their much stronger powers of multiplication here play an 
important part, as enabling us to arrive at a definite result 
within a reasonable time. 
As is known, Reess published in 1870 a work, important at 
that time, on alcoholic fermentation-fungi. Those yeast-cells 
which give endospore-formation are referred by him to a par- 
ticular genus, Saccharomyces , of which he establishes a series 
of species. He takes as the basis of his description the form 
and size of the cells. The large oval cells commonly found in 
beer-yeast are referred by him to the species Saccharomyces 
cerevisiae ; the cells of wine-fermentation, which are also oval, 
but generally smaller, he considers as constituting a different 
species, 5 . ellipsoideus ; the sausage-shaped cells are termed 
S. Pastorianus , and so forth. In 1883 and the following 
years I proved that the limits drawn by Reess do not exist 
in reality, and that we are able from each of the species as 
conceived by him to -develop the rest. If we therefore still 
use the designations of Reess, it can at most be as denomina- 
tions of groups of species, and from an essentially different 
stand-point from his. It is not , as he and his successors 
thought , the form and size of the cells per se, in which the 
distinguishing characters lie , but in the form and size con- 
tingent upon particular conditions of cultivation. Besides 
shape and size, we now make use of a whole series of other 
characteristics. When, therefore, we use the old appellations 
of Reess, it must be borne in mind that they have now very 
different significations from those in which he used them, and 
we must have a clear understanding that each of these groups 
of forms may transform itself pretty readily into each of the 
others. Several of the best and most vigorous yeast-fungi of 
wine may, morphologically, be referred as well to Saccharo- 
myces cerevisiae as to 5 . ellipsoideus , and in like manner there 
are beer-yeast fungi which, if we go by the system of Reess, 
may equally well be looked upon as belonging to 5 . ellip - 
