THEORY OF FERMENTATION 290 



character of the successive quantities of yeast taken, we 

 shall see plainly that the structure of the cells undergoes a 

 progressive 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 pos- 

 sessing a remarkable tenderness. Their walls are exceed- 

 ingly 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 in their extreme 

 states. The progressive changes in the cells, after they have 

 acquired their normal form and volume, clearly demon- 

 strate the existence of a chemical work of a remarkable in- 

 tensity, during which their weight increases, although in 

 volume they undergo no sensible change, a fact that we 

 have often characterized as " the continued life of cells al- 

 ready formed." We may call this work a process of matuja- 

 tion on the part of the cells, almost the same that we see 

 going on in the case of adult beings in general, which con- 

 tinue to live for a long time, even after they have become 

 incapable of reproduction, and long after their volume has 

 become permanently fixed. 



This being so, it is evident, we repeat, that, to multiply 

 in a fermentable medium, quite out of contact with oxygen, 

 the cells of yeast must be extremely young, full of life and 

 health, and still under the influence of the vital activity 

 which they owe to the free oxygen which has served to 

 form them, and which they have perhaps stored up for a 

 time. When older, they reproduce themselves with much 

 difficulty when deprived of air, and gradually become more 

 languid; and if they do multiply, it is in strange and mon- 

 strous forms. A little older still, they remain absolutely 

 inert in a medium deprived of free oxygen. This is not be- 

 cause they are dead; for in general they may be revived in 

 a marvellous manner in the same liquid if it has been first 

 aerated before they are sown. It would not surprise us to 



