CHAPTER XI 



Regeneration 



Regeneration is the term applied to a process of restitution 

 which occurs after removal of a part of the body and which 

 results in complete or partial replacement of the lost part. 



If an animal of simple structure, e.g. a planarian, is cut 

 into two transversely, a bud-like outgrowth will within a few 

 days develop on the cut surface in each half. This is the so- 

 called regeneration-bud. It grows steadily, and replaces the lost 

 part of the body, i.e. a new fore-part is formed on the caudal 

 half, and a new hind-part on the anterior half (Fig. 56). The 

 position of the cut is of no, or at most of secondary importance. 

 Whether it is made in the middle, or in the front or hind part 

 of the worm, the result is always the same. This shows already 

 that the quality of the regenerate is not determined exclusively 

 by the nature of the regenerating material. The situation is not 

 such that, e.g., in the front half of the 

 body "tail-forming" cells or substances 

 are present, and "head-forming" mate- 

 rial in the posterior half. At each level 

 in the body, both a head and a tail can 

 be produced. What will in a given case 

 develop from the regeneration-bud, 

 depends only on whether the cut at 

 which this bud is formed faces forwards 

 backwards. In other words, the 



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or 



original polarity of the worm determines 

 the result of the regeneration. Evidently, 

 this polarity has remained intact in both 

 pieces formed by the transection of the 

 worm. This is true even if the worm is 



Fig. 56, Regeneration 

 of rostral and caudal 

 halves in the flat 

 worm Polycelis nigra, 

 after 0, 7, 9, and 

 14 days respectively. 

 After Dresden. 



