174 REGENERATION 



that both motor nerves and sensory nerves play a role here. 

 A certain minimum number of nerve fibres per unit area of the 

 wound is required for the normal progress of regeneration. It 

 also appears that a considerable invasion of nerve fibres into 

 the epidermis of the regenerate takes place in the early stages 

 of regeneration. Finally, experiments by Butler and Schotte 

 (1949) have proved that the presence of nerves is necessary 

 for the first phases of regeneration, during which the blastema 

 is formed and determined, but that the later morphogenesis, 

 differentiation, and growth of the regenerate are independent 

 of the nervous system. 



The de-differentiation which supplies the material for the 

 regeneration blastema can be inhibited not only by the forma- 

 tion of this blastema, but also by a premature growth of skin 

 over the wound. This may be the explanation of the difference 

 in regenerative capacity between urodeles and anurans. In 

 urodeles, an amputated limb is completely regenerated, both in 

 larvae and in adults, whereas in anurans the capacity to re- 

 generate is permanently lost during metamorphosis. Now it 

 has been found in adult anurans that the skin soon overgrows 

 the amputation wound, and that the wound is covered by a 

 scar tissue consisting of coarse connective tissue fibers. No 

 regeneration blastema is then formed. It is of great practical 

 importance that de-differentiation has been successfully pro- 

 voked by mechanical (Polezajev, 1939-41) or chemical stimula- 

 tion (Rose, 1942-45; Polezajev, 1945-46). In adult anurans, 

 too, a certain degree of regeneration has been produced in this 

 way. Man and the mammals do not possess the capacity for 

 spontaneous regeneration, but these investigations have revealed 

 methods of stimulating the regeneration of lost parts of the 

 body which might bring regeneration in this group within the 

 realm of practical possibilities. 



We shall now discuss some details of the later development 

 of these regeneration-buds. Here all the missing parts are re- 

 placed by the regenerate, in contrast to what we have seen 

 earlier in the case of total regeneration at a front edge. The 

 differentiation in the regenerate adapts itself completely to 

 the remaining organs of the stump. Moreover, we find no trace 



