72 THE STORY OF THE LIVING MACHINE. 



it is possible to trace the fundamental properties of 

 all living things. Cells are endowed with the prop- 

 erties of irritability, contractibility, assimilation 

 and reproduction, and it is thus plainly to the 

 study of cells that we must look for an interpreta- 

 tion of life phenomena. If we can reach an intel- 

 ligible understanding of the activities of the cell 

 our problem is solved, for the activities of the 

 fully formed animal or plant, however complex, 

 are simply the application of mechanical and chem- 

 ical principles among the groups of such cells. 

 But wherein does this knowledge of cells help us ? 

 Are we any nearer to understanding how these 

 vital processes arise ? In answer to this question 

 we may first ask whether it is possible to determine 

 whether any one part of the cell is the seat of its 

 activities. 



The Cell Wall. The first suggestion which 

 arose was that the cell wall was the important 

 part of the cell, the others being secondary. This 

 was not an unnatural conclusion. The cell wall 

 is the most persistent part df the cell. It was the 

 part first discovered by the microscope and is the 

 part which remains after the other parts are gone. 

 Indeed, in many of the so-called cells the cell wall 

 is all that is seen, the cell contents having disap- 

 peared (Fig. 14). It w r as not strange, then, that 

 this should at first have been looked upon as the 

 primary part. The idea was that the cell wall in 

 some way changed the chemical character of the 

 substances in contact with its two sides, and thus 

 gave rise to vital activities which, as we have 

 seen, are fundamentally chemical. Thus the cell 

 wall was regarded as the most essential part of 

 the cell, since it controlled its activities. This 

 was the belief of Schwann, although he also re- 



