It may be mentioned that yeast itself undergoes fermentation. Nearly all samples of yeast contain glycogen, the amount varying according to the age of the yeast. Glycogen is the reserve carbohydrate of the yeast-cell, and it is readily fermented within the cell, this "autofermentation" producing alcohol and carbon dioxide as in the fermentation of the ordinary sugars. It has been suggested, indeed, that the formation of glycerol also is due to a similar autolytic destruction of the yeast itself, and not to the decomposition of sugar by yeast. Such an autolysis, it is considered, might occur through the action of an enzyme on the protein material of the yeast-cells.4 This view is supported by the statement that when egg-albumin is added to sugar solutions which are undergoing fermentation by yeast, the proportion of glycerol produced is greatly increased. No further evidence, however, appears to have been adduced in favour of this suggestion, and the sugar is generally regarded as the most probable source of the glycerol.

1 Proc. Roy. Soc, 1904, 73, 516-26. 2 Trans. Chem. Soc., 1906, 89, 128.

3 Slator and Sand, ibid., 1910, 97, 922-927.

4 J. R. Carracido, Bevista Acad. Sci., Madrid, 1904, 1, 217.