98 



The Mechai^istic Conception of Life 



determined by the same mechanical forces which determine 

 growth in plants. An obstacle to such a conclusion seems to lie 

 in the fact that many plant-cells have solid walls, while this is 

 not the case in most animal cells. The solid cell-wall, however, 

 does not determine the peculiar character of growth. This 

 character is determined first, by chemical processes within the 

 cell, which result in a higher osmotic pressure, and, secondly, by 

 the osmotic qualities of the outer layer of protoplasm, which 



Fig. 33. — Curve representing the influence of diluted sea-water. The 

 abscissae represent the concentrations, the ordinates the corresponding growth 

 in the unit of time. The maximum growth is at a concentration between 2 and 3 

 per cent of salt, while the normal concentration is indicated by the vertical line 

 between 3 and 4. 



allows water to pass through freely, but does not allow all salts 

 dissolved in it to do the same. Both these qualities are inde- 

 pendent of the solid cell-wall, and I see no reason why the animal 

 cell should not agree in these two salient features with the 

 plant-cell. 



In order that the foregoing explanation of the mec hanism of 

 growth in the animal cell might be based only upon known pro- 

 cesses, it was necessary to find out whether, in case of growi:h, 

 chemical processes of such a character take place that substances 

 of higher osmotic pressure are formed than those from which 

 they originate. Everyone knows that by exercise our muscles 

 increase in size. No satisfactory explanation of this fact has 



