CHAPTER XrV 



REGENERATION 



THE WORD 'regeneration' is used to refer to the processes by which an 

 animal restores, or tends to restore, any regions which may be removed. 

 It covers a wide range of phenomena (General Review: A. E. Needham 

 1952). At one extreme an adult mammal which has suffered the loss 

 of a small part, such as a fmger, or a larger part, such as a limb, can do no 

 more in the way of regeneration than merely repair the wound and close 

 the cut surface. At the other extreme a very small part of the normal body 

 of a coelenterate, a flat worm, or a starfish, can restore the whole large 

 region which is missing and become a complete individual. There are all 

 grades in between these two extremes. The power of regeneration is in 

 general greater in lower forms and less in more highly evolved ones, but 

 this rule is only very rough, and when one looks at the matter in more 

 detail it becomes apparent that the capacity for regeneration is distributed 

 in a rather arbitrary manner throughout the animal kingdom. Often even 

 closely related forms differ considerably in their regenerative powers. The 

 mass of information on the subject is very large and there would not be 

 space here to review it completely. It is, however, necessary to take a 

 glance at certain aspects of the subject, particularly because of the light it 

 throws on two general points of embryological theory. These are, firstly, 

 the reversibility of determination, and secondly, the field theory. 



Workers on regeneration (particularly Morgan in 1901), have distin- 

 guished two different modes in which the phenomenon can occur. If a 

 part is removed from some organism, what is left may remain unaltered 

 and regeneration be effected by the outgrowth of a new mass of tissue 

 which becomes modelled into the missing parts. Such a process was 

 called epimorphosis by Morgan. The word 'regeneration' is sometimes 

 used in a narrow sense to refer to it alone (e.g. by Child, who employs 

 'reconstitution' as the more general term). In the other mode of regenera- 

 tion, the part of the organism which is left after the amputation itself 

 becomes remodelled so as to be transformed from a part into a whole 

 organism. To this Morgan gave the name 'morphallaxis' which is still in 



common use. 



Epimorphosis can certainly occur with little or no sign of morphallaxis. 

 It does so, for instance, when a young newt regenerates an amputated 

 limb, or an earthworm an amputated head, but it is not quite so certain 



302 



