MODERN RESEARCHES INTO THE NATURE OF YEAST. 157 
therefore disposed of. The genus Saccharomyces is distin- 
guished by the typical yeast-like form of its cells, which are 
almost globular, separable from one another, and possess an 
unlimited power of propagation. M. Brefeld has carried on 
a series of researches to determine whether, under different 
conditions, as free access of air or growth in a very thin 
stratum of a perfectly neutral solution, true yeast will ever 
develop. into a filamentous form; but always with negative 
results. He was therefore unable to come to any other con- 
clusion than that there was no genetic connection between 
Saccharomyces and Mucor. Under ordinary conditions the 
cells of Saccharomyces divide in the most irregular manner 
and to an unlimited extent, the newly formed cells separating, 
and repeating the process indefinitely. If the separate cells 
come into contact with the air without being able to grow, 
each cell is transformed into a sporangium with two or four 
spores ; and this, if again immersed in a fluid, reproduces the 
ordinary yeast-cells. The cycles of development of Mucor 
and Saccharomyces present therefore many points of resem- 
blance. The filiform cells of the one correspond to the 
spherical cells of the other, the resting-cells of the former 
having no morphological importance ; both produce sporangia 
when removed from the nutrient fluid and exposed to air; 
those of the former being more complicated in structure than 
those of the latter. ‘These considerations led M. Brefeld 
to conclude that the two genera are nearly related, and 
that Saccharomyces is a simple form of the Mucorini. 
This view is opposed to the ordinarily accepted systematic 
position of Saccharomyces, that adopted by Reess, who looks 
on it as the simplest form of the Ascomycetes, and regards 
the yeast-cell as an ascus in which the spores are produced 
by free cell-formation. Brefeld maintains, however, that the 
term ascus can only rightly be applied to a sac which 
produces spores belonging to a sexual generation, the pro- 
duct of a fertilised ascogonium. ‘To this process there is 
no resemblance in Saccharomyces. The production of the 
spores in a yeast-cell resembles in every respect that in a 
sporangium of Mucor. There occurs in this group’ every 
intermediate form between free cell-formation and cell-divi- 
sion.  Brefeld believes that none of the lowest forms of 
vegetable life can fairly be referred to the Ascomycetes. 
There seems but little hope of finally determining the 
systematic position of Saccharomyces by the discovery of any 
sexual mode of propagation. Even in the Mucorini the 
similarity of the fertilising cells to one another indicates the 
form of this process to be as elementary as possible ; and it 
