ORIGIN OF THE INDIVIDUAL 221 



chiefly to the replacement of cells which especially suffer from 

 wear and tear, such as those forming the outer layer of the 

 skin. It will be recognized that regeneration is but one 

 phase of a fundamental property of protoplasm, namely 

 growth, whether it consists in restoring a part of a Parame- 

 cium, transforming a bit of a Flatworm into a complete 

 animal, or replacing half of an Earthworm, the head of a 



I 



FIG. 120. Regeneration and grafting in Hydra. A, an individual with 

 seven ' heads' as a result of lengthwise cuts. B, stages in the regeneration of a 

 complete individual from a small piece. C, Portions of two individuals grafted 

 together. (From Hegner; A, after Trembley; B, after Morgan; C, after King.) 



Snail, the claw of a Crayfish, or the leg of a Salamander. But 

 the experimental study of regeneration phenomena has 

 opened up a new vista of the regulatory powers of living 

 things from Protist to Vertebrate and from egg to adult, and 

 has afforded a means of approach to some fundamental bio- 

 logical problems. And withal it has a practical value. The 

 surgeon now knows more of the regeneration of tissues in 

 general and nerves in particular in wound healing, and the 

 oysterman knows or should know that his attempt to 

 destroy Starfish by tearing them up and throwing the pieces 



