55 
Variation of Yeast-cells. 
soidcus as to 5. cerevisiae ; both of these groups again run 
into 5. Pasiorianus. Large oval cells like those which are 
characteristic of S. cerevisiae , may, under favourable circum- 
stances of nutrition, be developed by wine-yeasts with small 
oval cells ; and, conversely, large typical cells of beer-yeast 
fungi may form smaller cells resembling wine-yeast. And, 
again, as regards the sausage-shaped cells the form of which 
distinguishes those species which, by a name of Reess’s, we 
call 5. Pasiorianus, it must be noted, that there are several 
ways in which we can produce 5. Pastorianus in the sense 
used by Reess, from either of the groups .S'. cerevisiae and 
S. ellipsoideus . These various relations deserve notice at the 
present moment, because there is now again a certain tendency 
to speak of cerevisiae , 5. ellipsoideus , &c., as if each of these 
names designated a well-defined species. 
If we ask the question, which of the various forms of yeast- 
cells is the primitive one, there are many reasons which make 
it probable that it must be the oval form, and that the species 
classed under the group 5. Pastorianus have developed from 
the groups cerevisiae and .S', ellipsoideus. Yeast- cells are, 
upon the whole, of a very plastic nature ; yet it is true of 
at least a large number of the species, that, if cultivated 
under the same particular circumstances, they appear in the 
same forms again and again. 
As regards the factors which determine the variation of the 
form of the cells, I have given information in previous papers. 
Among my former researches on the variations, some experi- 
ments are described on the action of the air on some species 
of beer-yeast, and also on the influence of certain conditions 
of nutrition with regard to spore-formation. I have shown 
that in the case of several species there appear suddenly, 
generally from unknown causes, varieties which have lost, 
more or less completely, their power of forming endogenous 
spores. The usual cultivation in a favourable nutritive liquid, 
as beer-wort, at favourable temperatures, in these cases avails 
nothing; but in the case of some species, I found that such 
cells are reduced to their normal state, if some dextrose is 
