THE CELL AND PROTOPLASM. 147 



descent. Lastly, the cell has the centrosome, 

 which brings about cell division in such a manner 

 that this chromatin material is divided equally 

 among the subsequent descendants, and thus 

 insures that the daughter cells shall all be 

 equivalent to each other and to the mother cell. 



We must therefore look upon the organic cell 

 as a little engine with admirably adapted parts. 

 Within this engine chemical activity is excited. 

 The fuel supplied to the engine is combined by 

 chemical forces with the oxygen of the air. The 

 vigour of the oxidation is partly dependent upon 

 temperature, just as it is in any other oxidation 

 process, and is of course dependent upon the 

 presence of fuel to be oxidised, and air to furnish 

 the oxygen. Unless the fuel is supplied and the 

 air has free access to it, the machine stops, the 

 cell dies. The energy liberated in this machine 

 is converted into motion or some other form. 

 We do not indeed understand the construction of 

 the machine well enough to explain the exact 

 mechanism by which this conversion takes place, 

 but that there is such a mechanism can not be 

 doubted, and the structure of the cell is certainly 

 complex enough to give plenty of room for it. 

 The irritability of the cell is easily understood ; 

 for, since it is made of very unstable chemical 

 compounds, any slight disturbance or stimulation 

 on one part will tend to upset its chemical sta- 

 bility and produce reaction ; and this is what is 

 meant by irritability. 



Or, again, we may look upon the cell as a little 

 chemical laboratory, where chemical changes are 

 constantly occurring. These changes we do not 



