162 REGENERATION 



and it is always directed towards the wound. The cells can move 

 both forwards and backwards, and sideways. The stimulus that 

 causes the onset of migration spreads throughout the body; it 

 disappears four to five days after the wound has been made. 

 Once started, the migration continues until a regeneration 

 blastema has formed in the wound area. Apart from the capacity 

 to form a regeneration-bud, the neoblasts possess the power 

 to repair other damage in the body. They are omnipotent, i.e. 

 they can form any other tissue. 



It is possible that in other cases the regeneration cells are 

 formed by the de-differentiation of tissue cells which thereby 

 acquire a more embryonic character, and an increased power 

 of division. 



Once the regeneration-bud has reached a certain size, an 

 outwardly visible differentiation begins to take place in this 

 material, and it becomes possible to see what will develop 

 from it. 



Now it has been found that in many annelid worms the 

 regeneration-bud which develops at a front edge will always 

 differentiate into a fixed number of segments, which agree 

 with those found in the rostral end of the body, the "head" of 

 a normal worm. The number of segments in the regenerate is 

 entirely independent of the place of the cut. In the polychaete 

 worm Sabella, for instance, three segments are always formed, 

 the foremost of which carries a crown of branchial palps, even 

 if the regenerating fragment belongs to the caudal end of a 

 normal worm (Fig. 59). This shows that here regeneration 

 does not lead to restitution of the missing part of the body, 

 but produces only a new apical end by an autonomous differ- 

 entiation. In contrast, the type of differentiation in regeneration 

 buds at a caudal edge depends upon the nature of the tissues 

 of the fragment, and here the lost part of the body is completely 

 replaced. 



The same applies to planarians. Here, again, regeneration at 

 a front edge, irrespective of its position, produces only a head, 

 whereas that at a hind edge leads to complete restitution of 

 the missing parts. 



Similar phenomena are found in the restitution processes 



