LABORATORIES FOR SPECIAL PURPOSES 49 



the microscope presents the appearance of nearly spherical cells 

 containing partially granular substances. Immersed in a suit- 

 able fluid at the temperature of 60 to 70 Fah., some of these 

 cells will quickly proceed to multiply by budding, which is the 

 common mode of reproduction. The result is the formation of 

 long strings of cells resembling the parent cell. Under other cir- 

 cumstances yeast is formed by generating within itself minute 

 granules or spores which, escaping from the parent cell when the 

 latter bursts, grow into individual yeast cells of the ordinary 

 form. 



It has been found by modern researches that the cell wall of 

 the yeast organism is not necessary to the process, for the liquid 

 contents of the cells when added to a fermentable liquid are 

 capable of setting up true alcoholic fermentation without any 

 process of growth or multiplication. In fact the decomposition 

 of the sugar has been traced to the presence in the liquid of a 

 soluble unorganised nitrogenous substance of the class of com- 

 pounds called " enzymes " (q.v.), to which the name zymase has 

 been given. It appears, however, that this zymase alone is 

 incapable of producing fermentation of sugar, but is dependent 

 on the presence of another substance, the exact nature and 

 composition of which is unknown, but which differs from zymase 

 in not being rendered inactive by the temperature of boiling 

 water. Fermentation is also dependent, as was found out by 

 Pasteur, on the presence in the liquid of a small quantity of a 

 phosphate, which has more recently been found to associate 

 itself in a remarkable way with the sugar to form a compound, 

 from which both the sugar and the phosphate are again re- 

 generated. The process of fermentation is therefore essentially 

 a chemical process resulting from a succession of changes in 

 which the complex organic materials are supplied by the yeast cell, 

 and are not as yet producible by purely chemical means. 



Pasteur's classical researches, however, led to another very 

 important practical conclusion. The uninstructed reader may 

 perhaps be led to think of yeast as though it consisted of one 

 kind of substance or organism, and in well-conducted operations 

 in skilled hands the examination of a sample of yeast under the 

 microscope might substantially confirm that idea. But part of 

 Pasteur's work consisted in showing that there are many varieties 

 of yeast, and that each one has properties of its own, and further 

 that many other organisms, present in the air or wate or brew- 



