560 Hansen . — Studies on Variation of Yeast-cells. 
As to the nutritive substratum and aeration my experiments 
have proved that only this much is required, that they must 
allow of a vigorous multiplication, and the range within which 
these factors may vary is a very large one. But this does not 
apply to temperature. One or two degrees too much or too 
little is enough to prevent the changes from taking place ; 
if the temperature be a little too low, the effect will not be 
sufficiently marked, whilst at a somewhat too high temperature 
the multiplication of the cells ceases too soon, the consequence 
being that the transformation, which has commenced, does not 
become fixed. In comparing the temperature with the other 
external factors we must, in the experiment under discussion, 
consider it the most influential. 
The above described phenomena of variation affect the 
yeast-cells in very different ways, influencing both morpho- 
logical and physiological characters. Some of the phenomena 
still remain isolated as if accidentally caused, others group 
themselves under certain rules. It is especially the latter, 
which invite to new investigations in order at last to find out 
the laws. To discuss what relation there may be between the 
observed facts and the hitherto established theories of variation 
would at present only be to waste words. When finishing 
my experiments I hope in that respect also to be able to give 
a survey. 
