THE MECHANISM OF LIFE 525 



a medium that contains food-stuffs, such as a bacterium in 

 a nutrient liquid. By means of osmosis and the chemism of the 

 cell the constituents of the nutrient liquid that are miscible with 

 the cell-contents and do not possess too large molecules 

 must pass through the cell-wall into the cell-substance and, 

 vice versa, the constituents of the cell-substance that are mis- 

 cible with the nutrient liquid and are able to pass through the 

 cell-wall, must go out of the cell into the nutrient liquid. This 

 exchange goes on in so far as the substances in question are not 

 held fast by other forces upon the one or the other side. The- 

 oretically, it will necessarily continue until a balance as regards 

 the substances capable of transportation is struck between the 

 cell-contents and the medium, when the metabolism will neces- 

 sarily cease. But in the living cell such a condition is never 

 reached, since compounds there exist which are continually decom- 

 posing and building themselves up anew. On the one hand, the 

 substances received by the cell from the medium are always con- 

 sumed at once and transformed into other compounds ; and, on the 

 other hand, those that the cell gives off to the medium are con- 

 stantly being formed. Hence the exchange between cell and 

 medium must continue as long as the cell is still capable of taking 

 up food-stuffs in sufficient quantity from the medium and of giving 

 off excretory substances in sufficient quantity to the medium. If, 

 therefore, the mass and character of the medium are fixed, and not 

 changed from the outside, after some time the cell must perish m t 

 this will occur either when the food-stuffs contained in the medium 

 are consumed, or when the latter is so saturated with excretory 

 substances, that the output of them by the cell has diminished or 

 ceased. It is very easy to produce both cases experimentally in 

 cultures of Bacteria. The bacteria die either from lack of food or 

 from the accumulation of the products of their own metabolism, 

 because the osmotic exchange of substance between bacterium-cell 

 and nutrient liquid gradually ceases through the gradual equalisa- 

 tion of the substances belonging to the two. 



In many cases the mechanism of exchange between cell and 

 medium is more complex. If, for example, the nutrient substances 

 in the surrounding medium are not present in a diffusible form, i.e., 

 if they are either solid or possess so large molecules that they are 

 unable to pass through the pores of the cell-wall, they must be 

 made soluble and diffusible. This is performed through the action 

 of ferments which the cell produces and in many cases gives off to 

 the outside. In contact with these ferments, the polymeric mole- 

 cules of proteid, of gelatine, of starch, etc., and solid masses of 

 these substances become split up and brought into solution, and 

 they are then able to diffuse into the interior of the cell. This 

 process may be followed very easily in bacterial cells. If a 

 bacterial cell be placed upon a glass plate covered with solid nutrient 



