REGENERATION 3O3 



that morphallaxis ever occurs without some preHminary epimorphosis. 

 For instance, if a flat worm is cut transversely in half some distance behind 

 the pharynx the posterior half will eventually become transferred into a 

 complete worm and this involves the appearance of a new pharynx some 

 distance posterior to the cut. But it is probable that the first step is the 

 appearance of a small outgrowth from the cut edge and that it is only 

 after this has formed a new anterior part of the worm by epimorphosis 

 that morphallaxis begins to occur and to cause the remodelling of the 

 rest of the posterior portion. 



Regeneration is often incomplete, less being formed than is necessary 

 to replace what has been lost. For instance, a regenerated leg in the sala- 

 mander may lack one or two of the toes, or the regenerated tail in a 

 lizard show defects in its bony structure. There is perhaps nothing very 

 surprising in this. It is more astonishing that animals should be able to 

 regenerate at all than that they should sometimes fail to do so perfectly. 

 It is more unexpected to fmd that there are also cases of super-regeneration. 

 It is by no means uncommon for a missing part to be restored in duplicate. 

 In some ways very closely comiected with this is a phenomenon which 

 can be regarded as regeneration without any previous loss to account 

 for it. This is the process known as 'budding' in which a group of cells 

 at some point in a complete normal animal suddenly start prohferating 

 and develop into a new individual. The process occurs in groups such as 

 the coelenterates and ascidians, when it gives rise to colonies of individuals 

 which often remain closely attached to one another. In some worms, too, 

 a head begins to form in the posterior region of the body and eventually 

 the whole hind end breaks away and becomes a new individual. 



I. The origin of regeneration cells and their potentialities 



When a part of an animal is removed there are three sources from which 

 the cells which build up the regenerate may be derived: (i) the tissues of 

 the stump may grow out and form the new organ, retaining their original 

 histological character and altering only in so far as they become part of 

 the new region of the body (we shall use the word 'stump' in a general 

 sense, to mean the region in the immediate neighbourhood of the place 

 from which the organ has been removed) ; (2) the body may contain a 

 reserve of undifferentiated or embryonic cells which accumulate at the 

 wound and then differentiate into the tissues of the regenerate; (3) some 

 of the already differentiated tissues of the stump may lose their differ- 

 entiation and return to a more plastic condition from which they are 

 able to redifferentiate into the speciahsed tissues of the regenerate, suffer- 

 ing during the process a greater or lesser change in their histological 



