24 THE NATURE OF LIFE 



plant, it constitutes a complete new in- 

 dividual. 



In some cases the detached portion 

 is incomplete, as when a willow branch 

 is broken off by the wind, falls into soft 

 mud and begins to grow. It must pro- 

 duce roots in order to become a complete 

 individual. This process of completing 

 the individual is known as regeneration. 

 It is of common occurrence, but, as we 

 have seen, it is absent in the simplest 

 cases and is therefore not essential to 

 the idea of reproduction. 



The detachment of a portion of the 

 organism is often brought about by in- 

 ternal forces as seen in the process of cell 

 division by which a cell separates itself 

 spontaneously from the parent organism. 



In the simplest cells, where there is no 

 definite nucleus, the process of cell divi- 

 sion may consist in a simple constriction 

 which cuts the cell in two. This process 

 may be imitated in drops of oil and seems 

 to be due primarily to the forces which 

 act at the surface of every liquid. Under 



