148 STUDIES ON FERMENTATION. 



from their first appearance till they become as large as the parent 

 cells. They continue to grow until they detach themselves ; 

 then they bud in their turn, so that the same figure may furnish 

 examples of cells of the first, second, and third generations. In the 

 right hdlf, the protoplasm contained in the cells exhibits circular 

 spots or vacuoles, which may be made to appear lighter or darker 

 than the rest of the cell by slight movements of the object-glass 

 of the microscope. These spots are due to a migration of the 

 protoplasm towards the sides ; they commonly occur in yeast 

 cells the vitality of which, from deficient nourishment, has 

 become suppressed — the shrivelled appearance which they then 

 assume being due to their being forced to live upon themselves, 

 so to speak. However, by introducing such cells into a nutritive 

 and aerated liquid the vacuoles quickly disappear. 



In the ordinary yeast, as met with in breweries, the 

 majority of cells show one or more of these vacuoles ; if, 

 however, we place a little of this yeast in an aerated wort, and 

 watch under the microscope the changes that occur in the 

 cells, we shall witness, often in the course of a few seconds, 

 a kind of turgescence, a greater tension of the cell- walls, which 

 seem to grow thinner, and a complete disappearance of the 

 vacuoles. At the same time the interior gelatinous matter 

 will become filled with fine granulations that are scarcely 

 visible, but which at a certain distance appear brilliant. At 

 the same time protuberances begin to show themselves, and 

 next day the budding will have already become very active. 

 The newly-formed cells will have such a delicacy of aspect and 

 contour as to be scarcely discernible in the field of the micro- 

 scope. There will also be a tendency to ramification in the 

 budding, which appearance will be more or less marked accord- 

 ing to the kind of alcoholic ferments present, as we shall see 

 presently, attaining its maximum in each case when the cells 

 have been revived after exhaustion by rest and want of food. 

 In the latter case, the process of rejuvenescence may be pro- 

 tracted ; but this is not the case with cells of commercial yeast, 

 which is always used within a few days of its formation. 



