THE CELL AND PROTOPLASM. 125 



tive metabolism, it can continue to move around 

 actively and behave like a complete cell, but 

 eventually it dies from starvation. On the other 

 hand, those fragments which retain a piece of the 

 nucleus, even though they have only a small por- 

 tion of the cell substance, feed, assimilate, and 

 grow ; in other words, they carry on riot only the 

 destructive but also the constructive changes. 

 Plainly, this means that the nucleus controls the 

 constructive processes, although it does not neces- 

 sarily mean that the cell substance has no share 

 in these constructive processes. Without the nu- 

 cleus the cell is unable to perform those processes, 

 while it is able to carry on the destructive pro- 

 cesses readily enough. The nucleus controls, 

 though it may not entirely carry on, the construc- 

 tive metabolism. 



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 irri- 

 table, and is endowed with the power of contrac- 

 tility. Cell fragments without nucleii are sensi- 

 tive 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 statements. 



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



