640 REGENERATION AND GROWTH 7 



(d) Nerve-supply and regeneration 



It has been clear for some time (Korschelt, 1927) that in most Metazoa inner- 

 vation of structures is necessary for their normal regeneration, and there is little 

 doubt that it is essential in phyla, previously problematical (Korschelt, 1927; 

 Abeloos, 1932), the Platyhelminths, Annelids and Arthropods, (Lender, 1954; 

 Brondsted, 1955; Janda, 1930; Avel, 1930; Zhinkin, 1936; Harms, 1944; Suster, 

 1933; Needham, 1945, 1946, 1953). Its importance is seen very clearly in some 

 nemertines (Dawydoff, 1909; Coe, 1929, 1934) of which a very small fragment, 

 containing the brain, regenerates a complete animal, whereas the whole of the 

 rest of the worm, lacking the brain, does not regenerate the missing part. 



Naturally Protozoa and sponges must regenerate without a nervous system, 

 though the ciliate Protozoa have a functional analogue in the kinetosome-system 

 (LwofT, 1949) and this plays an analogous role also in regeneration (see Weisz, 

 1954). The anterior h.di\^ oi Par ajneciiim, containing the "motorium", shows better 

 regeneration than the posterior half (Peebles, 1912). Reconstitution of hydroid 

 coelenterates from masses of dissociated cells (p. 589) also is independent of inner- 

 vation (Millot, 1931 ; Lehn, 1953). Even in animals so highly evolved as the Amphi- 

 bia it is believed that regeneration is possible without innervation, if the latter is 

 prevented initially, in ontogenesis (Yntema, 1949; Singer, 1952). Innervation 

 seems unnecessary for early ontogenetic development of limbs (Hamburger, 1928; 

 Stone, 1 931), though required (tropic action) for subsequent maintenance (Huxley 

 and De Beer, 1934) both of sense organs and of effectors (Lewis, 1937; Wyburn 

 Mason, 1950). Some organs persist for a long time after denervation, e.g. the sense- 

 organs of the lateral line of Anuran tadpoles for 140 days (Wright, 1947), but 

 eventually they undergo an atrophy. The local nerve-supply has been shown es- 

 sential also for wound-healing in mammals (Gurevitsch, 1 94 1 ) and any apparent ex- 

 ception, among animals possessing a nervous system, requires careful examination; 

 it is extremely difficult to prevent re-innervation (Butler and Schotte, 1941, 1944). 



Regenerating limbs of Amphibia become very richly innervated (Singer, 1952; 

 Thornton, 1954) within the first few days (van Stone, 1955) i.e. the nerves regener- 

 ate well ahead of other tissues. Their ability to do so depends on the normal 

 dedifferentiation and "opening up" of the dermis (Thornton, 1954). The subse- 

 quent rate of regeneration of the blastema is proportional to the number of nerve- 

 fibres per unit cross-sectional area of the limb (Singer, 1952; van Stone, 1955)- 

 Recently, Singer (1950) demonstrated that the normally non-regenerating limbs 

 of adult Anura show considerable power if their nerve supply is increased. Singer 

 (1952) concludes that the action depends on the quantity of nerve fibres and not 

 on their qualitative neurological properties [cf. Kamrin and Singer, 1955), and 

 it is perhaps equally significant that the action is only on the quantitative aspect 

 of regeneration, — on cell-proliferation, not on differentiation (Singer and Graven, 

 1948). The neurotropic action is not organ-specific (Singer, 1952) or even species- 

 specific (Lender, 1954; Harms, 1944). 



The sensory nerve supply at one time seemed the most important component but this 

 is probably because it is normally the largest component. In the limbs of newts one third to 

 one half of the normal number of nerve fibres is adequate for regeneration but in adult 

 Anura even the full number is inadequate. The number of nerve-fibres per unit cross-sec- 



