696 TEXTBOOK OF ZOOLOGY 



because of profligate rates of increase. Man succeeds because he 

 avoids physical injury. These three dominant groups all have 

 limited regenerative capacity. On the other hand, it would be 

 foolish to maintain that extensive regenerative capacity is of no 

 value to those species possessing it. 



Summary 



All animals have some regenerative capacity. Some of the simpler 

 ones may regenerate a new organism from a small portion of an 

 older one, while in the most complex animals only relatively small 

 parts may be regenerated after loss. 



Regeneration consists of the assembly of physiologically young 

 cells at the site of injury, usually through migration of such cells, 

 but sometimes by dedifferentiation, a kind of physiological re- 

 juvenation, of local, differentiated cells. These physiologically 

 young cells proliferate and differentiate under the inductive influ- 

 ence of surrounding parts to reproduce the lost part in so far as 

 regenerative capacity is characteristic of the species. 



