STUDIES ON FERMENTATION. 245 



first appears in the production of a minute froth, which 

 gradually increases till it whitens the surface of the liquid, we 

 take, every twentj'-four hours, or at longer intervals, a trace of 

 the yeast deposited on the bottom of the vessel and use it 

 for starting fresh fermentations. Conducting these fermenta- 

 tions all under precisely the same conditions of temperature, 

 character, and volume of liquid, let us continue this for a 

 prolonged time, even after the original fermentation is finished. 

 We shall have no difilculty in seeing that the first signs of 

 action in each of our series of second fermentations appear 

 always later and later in proportion to the length of time 

 that has elapsed from the commencement of the original fer- 

 mentation. In other words, the time necessary for the develop- 

 ment of the germs and the production of that amount of yeast 

 sufficient to cause the first appearance of fermentation varies 

 with the state of the impregnating cells, and is longer in 

 proportion as the cells are further removed from the period of 

 their formation. It is essential, in experiments of this kind, 

 that the quantities of yeast successively taken should be as 

 nearly as possible equal in weight or volume, since, ceteris 

 paribus, fermentations manifest themselves more quickly the 

 larger the quantity of yeast employed in impregnation. 



If we compare under the microscope the appearance and 

 character of the successive quantities of yeast taken, we shall 

 see plainly that the structure of the cells undergoes a pro- 

 gressive change. The first sample which we take, quite at the 

 beginning of the original fermentation, generally gives us cells 

 rather larger than those later on, and possessing a remarkable 

 tenderness. Their walls are extremely thin, the consistency 

 and softness of their protoplasm is akin to fluidity, and their 

 granular contents appear in the form of scarcely visible spots. 

 The borders of the cells soon become more marked, a proof 

 that their walls undergo a thickening ; their protoplasm also 

 becomes denser, and the granulations more distinct. Cells of 

 the same organ, in the states of infancy and old age, should not 

 differ more than the cells of which we are speaking, taken 



