146 THE STORY OF LIFE'S MECHANISM. 



It is equally clear that the cell substance is the 

 seat of most of the destructive processes which 

 constitute vital action. The cell substance is 

 irritable, and is endowed with the power of con- 

 tractility. Cell fragments without nucleii are 

 sensitive enough, and can move around as readily 

 as normal cells. Moreover, the various fibres 

 which surround the centrosomes in cell division 

 and whose contractions and expansions, as we 

 have seen, pull the chromosomes apart in cell 

 division, are parts of the cell substance. All of 

 these are the results of destructive metabolism, 

 and we must, therefore, conclude that destructive 

 processes are seated in the cell substance. 



The centrosome is too problematical as yet for 

 much comment. It appears to be a piece of the 

 machinery for bringing about cell division, but 

 beyond this it is not safe to make any state- 

 ments. 



In brief, then, the cell body is a machine for 

 carrying on destructive chemical changes, and 

 liberating from the compounds thus broken to 

 pieces their inclosed energy, which is at once 

 converted into motion or heat or some other 

 form of active energy. This chemical destruc- 

 tion is, however, possible only after the chemical 

 compounds have become a part of the cell. The 

 cell, therefore, possesses a nucleus which has the 

 power of enabling it to assimilate its food that 

 is, to convert it into its own substance. The 

 nucleus further contains a marvellous material 

 chromatin which in some way exercises a con- 

 trolling influence in its life and is handed down 

 from one generation to another by continuous 



