100 HOW ANIMALS DEVELOP 



activation centre in the dragon-fly can be regarded 

 as an organization centre which evocates but does 

 not individuate, whereas the differentiation centre 

 does individuate. 



Regeneration 



In many animals individuation fields persist all 

 through life. Their effect is seen when such an 

 animal is badly wounded, loses a leg, for example. 

 The stump of the leg completes itself and builds up 

 a complete leg again by regenerating the part which 

 has been lost. In this regeneration the processes 

 which go on are often very similar to those con- 

 nected with organization centres in earlier embryonic 

 life. For instance, if an adult newt loses a leg, a little 

 cap of cells called the regeneration bud grows on the 

 end of the stump. The regeneration bud is simply a 

 little bit of competent tissue, and the individuation 

 field controls it so that the lost part of the leg 

 develops out of it. It is not only competent to form 

 legs but can turn into other things if necessary. If it 

 is taken off* and grafted on to the stump from which 

 part of the tail has been cut, the tail individuation 

 field now uses the bit of competent tissue to com- 

 plete itself, and the regeneration bud, instead of 

 becoming part of the leg, becomes part of the tail. 

 In many of the lower organisms, all the tissues 

 seem to be permanently in a competent state, but 

 they are normally held together as a complete indi- 

 vidual in an equilibrium which is determined with 

 respect to some particular part, which is usually the 



