4 LECTURE I. 



second experiment, a drop of yeast be added to a quantity 

 of pure distilled water, the liquid will not become turbid, 

 that is, there will be no indication of any increase in the 

 number of yeast-cells present ; and not only so, but it will 

 be found, after a time, that their protoplasmic contents have 

 perceptibly diminished, so that an actual loss of substance 

 has taken place. 



The inferences to be drawn from the observations are 

 (i) that the substances contained in the Pasteur's solution 

 are of such a nature as to supply the yeast-cells with the 

 materials necessary for the formation of protoplasm and of 

 cell-wall, that is, to serve as food to the plant ; (2) that the 

 yeast-cell is capable of absorbing these substances and of 

 elaborating from them protoplasm and cell-wall, and that 

 it does so to such an extent as not only to provide for its 

 own maintenance but also for the formation of new yeast- 

 cells ; and (3) that there are processes going on in the yeast- 

 cell which tend to diminish its substance. 



The protoplasm of the yeast-cell, and this is true of every 

 living organism, is the seat of active chemical processes which 

 are inseparably associated with the vital activity of the 

 protoplasm. These collectively may be termed the meta- 

 bolism of the organism. We may conveniently distinguish 

 as constructively metabolic those processes which tend to form 

 more and more complex compounds in the organism, and as 

 destructively metabolic those processes which tend to break 

 down the complex compounds with the formation of others 

 of simpler composition. Some of the products of destructive 

 metabolism are such that they cannot enter into the construc- 

 tive metabolism of the organism ; they are accordingly thrown 

 off, and so, just as constructive metabolism is connected with 

 the absorption of comparatively simple chemical compounds 

 which constitute the food of the organism, destructive metabo- 

 lism is connected with the elimination of comparatively simple 

 chemical compounds which are the excreta of the organism. 



The accumulation and dissipation of matter are not, 

 however, the only results of the metabolic activity of the 

 protoplasm of the yeast-cell. If a drop of yeast be dried and 



