306 PRINCIPLES OF EMBRYOLOGY 



(cf. Brrnist 1950), chiefly because it produces chromosome rearrangements 

 which, when mitosis occurs, lead to unbalanced and inviable nuclear 

 constitutions. Thus x-radiation can knock out the reserve cells of a 

 planarian while leaving the animal as a whole capable of hving but no 

 lonc^er of regenerating. If only the anterior region of a worm is irradiated 

 and then a part of this region removed, regeneration is delayed for the 

 time that it is necessary for the reserve cells to move from the non- 

 irradiated region through to the cut surface (Wolff and Dubois 1948). 

 Experiments of this kind in newts have given no evidence of any extensive 

 movement of cells to form the regeneration bud in vertebrates. 



It appears therefore that a regenerated organ in a vertebrate is in the 

 main constructed out of dedifferentiated cells from the tissues close to the 

 cut surface. New cells accumulate on the surface in a densely packed group 

 to form a regeneration bud or blastema. Histological examination of the 

 process gives evidence that muscles cells, bone-forming cells and other 

 mesodermal elements contribute to the undifferentiated mass (cf. Manner 

 1953). Recent studies by Rose (1948^ also suggest that dedifferentiated 

 epidermal cells make a considerable contribution. In the blastema all the 

 cells lose their characteristic histological appearance. The important 

 question arises whether this is a true dedifferentiation, and the cells able 

 to develop again into something other than they were originally, or 

 whether it is a deceptive appearance similar to that which can be seen in 

 tissue culture, where cells from differentiated tissues lose their character- 

 istic appearance and make an impression of being dedifferentiated, but in 

 reality retain their specific nature and can develop again only into what 

 they were before. The evidence suggests that to some extent at least the 

 formation of a regeneration blastema involves a true dedifferentiation. 



Perhaps the most conclusive evidence for the occurrence of true meta- 

 plasia (a change of character from one differentiated type to another) 

 comes from the rather special case of the so-called Wolffian regeneration 

 of the lens (Reyer 1954, J. Needham 1942). If the lens is removed from the 

 eye of an amphibian the edge of the retina begins to grow and forms a 

 group of cells which differentiate into a new lens replacing the old one. 

 It seems quite clear here that differentiated retinal cells change their 

 character completely to give rise to the final lens. This very odd type of 

 regeneration has been known since the nineties of the last century when 

 it was discovered by G. Wolff, after whom it is named. The retina does 

 not need to suffer any wounding to become stimulated to start regenerat- 

 ing; this occurs as soon as the lens is removed even if the retina is quite 

 uninjured in the process. It has been clearly demonstrated that the 

 stimulus is chemical. The lens apparently gives off some substance which 



