REGENERATION 309 



since we still do not know the full range of capacities of blastema cells, 

 but it seems to be tending in this direction. 



2. Field action in regeneration 



The process of regeneration usually restores exactly what is missing to 

 complete a normal individual. That means that the growth and differen- 

 tiation of the material is reV^ed in the first place to the stump or re- 

 maining part of the animal, am^in the second to the final complete form. 

 It was this situation, more than any other, which has tempted biologists 

 to employ the concept of 'fields', and regeneration provides the classical 

 context for a discussion of the validity and meaning of this notion. We 

 shall fmd that so long as it is not taken to provide a solution to the prob- 

 lems, but rather as an enhghtening way of formulating them, the field 

 concept can be very useful. Even those cases, known as 'heteromorphoses', 

 in which regeneration does not restore normality, can be usefully dis- 

 cussed in such terms. 



There is not space here to discuss regeneration fields in all the groups in 

 which they occur, and we shall limit our attention to certain aspects of 

 the process in coelenterates, platyhelminths and vertebrates. An intro- 

 duction to the literature, including that concerning other groups, may 

 be found in A. E. Needham (1952). 



{a) Coelenterates 



During the eighteenth century, the experiments of Trembley of 

 Geneva on the regeneration of Hydra made this topic, for a time, a fashion- 

 able diversion in the drawing-rooms of the elegant. Work has continued 

 on it ever since. In recent times, the elongated marine hydroids, such as 

 Obelia, Tubular ia, Corymorpha, etc., have been more commonly employed 

 as experimental material (General Reviews: Barth 1940, Child 1941). 



The power of regeneration of all these organisms is exceedingly great. 

 The animals consist of an apical region, the hydranth, which is provided 

 with tentacles surrounding the mouth; the main body, containing a 

 gastric cavity; and a foot or stolon region, by which the animal or 

 colony is attached to the substratum and which does not contain any 

 gastric space. The marine forms which have been mainly studied occur as 

 colonies, consisting of many individual polyps united by their gastric 

 portions. These regions are known as the coenosarc, and in many forms 

 are enclosed in a hard chitinous sheath, the perisarc. 



Any fragment of the gastric or coenosarc region, which is large enough 

 not to fall to pieces as a result of the wounds inflicted in isolating it, can 

 produce a new hydranth by regeneration; usually it also produces a new 



