REGENERATION 3O7 



holds the retina in check, and regeneration begins as soon as this is re- 

 moved. Within the retina there is a gradient in readiness to undergo 

 regeneration, the dorsal region showing greatest capacity and the ventral 

 the least. 



The situation is not so clear in the more usual types of regeneration, 

 such as that of the limb or tail. It has been claimed by many authors (e.g.' 

 by Weiss 1930), that the blastema is at first quite undetermined and is 

 competent to develop into almost any part. The first part of this claim 

 seems to be justified. For instance, when early blastemas from regeneratino- 

 hmbs are transplanted into an indifferent situation, such as the body cavity 

 of an adult salamander, they form masses of undifferentiated cells, which 

 appear similar to malignant tumours (Waddington 1940^) ; it is probable, 

 however, that they do not become fully mahgnant but are eventually 

 encapsulated and keratinised. If a similar blastema is transplanted from 

 a Hmb to the cut surface of an amputated tail, Weiss originally clauned 

 that it would become determined by its new surroundings and thus 

 differentiate into a tail; but this interpretation has been challenged on the 

 grounds that it is difficult to be certain that the transplanted blastema was 

 not simply resorbed, the tails which eventually appeared being formed 

 not from grafted tissues but by the stump of the tail in the normal way. 

 It would seem that the question could be settled by using polyploid or 

 other specifically recognisable tissues to provide the transplant, but this 

 has not yet been successful ; May (1952), who tried it, found it impossible 

 to recognise the cells of a triploid transplant in the redifferentiating tissues 

 of the regenerate. Most authors who have reviewed the subject recently 

 (e.g. J. Needham 1942, A. E. Needham 1952) reach the conclusion that 

 the weight of the evidence is against the possibility of changing the fate 

 of the blastema from one organ to another. 



In contrast with this, there is a good deal of evidence that cells of the 

 blastema can differentiate into any of a number of different types of tissue. 

 Thus the power of regeneration can be removed from an amphibian tail 

 or limb by x-raying; then one particular type of tissue from a normal 

 Hmb can be grafted into it, and a few days later the limb amputated in the 

 region of the graft. Luther (1948) claims that under these conditions, leg 

 skin transplanted to an x-rayed tail gives rise to blastema cells which 

 form all the tissues of the appendage (muscle, bone, blood vessels, etc.) 

 but that they showed a tendency to form legs instead of tails. Trampusch 

 (195 1) has described similar evidence of changes of tissue specificity 

 following transplantation of healthy skin, muscle or skeletal tissues to 

 irradiated hmbs. A converse type of evidence was obtained at a much 

 earher date by Weiss (cf 1930), who showed that if one of the long bones 



