642 REGENERATION AND GROWTH 7 



the nervous system seems natural enough. Grafts of nerve tissue also promote cell- 

 division in neighbouring tissues (Overton, 1950; Weiss, 1950; Thornton, 1954); 

 grafts of other tissues have no such action (Overton, 1950). Regenerating nerves 

 stimulate cell-division among Schwann cells (Abercrombie and Johnson, 1946). 



Although the action of the nervous system on regeneration is essentially non- 

 specific and quantitative it appears to have some qualitative effects. Thus in Platy- 

 helminths and Annelids a particular cell is induced to develop as part of head- 

 er tail-regenerate according as transection is in front of it, or behind it (see Child, 

 1941). A piece of nerve cord implanted into the flank of an Oligochaete induces 

 the local tissues to regenerate a head or a tail depending on its orientation (Sayles, 

 1942). Similarly the nerve-supply probably determines polarity in Vertebrate 

 limbs and tail (Needham, 1952), and polarity-change in narrow transverse pieces 

 of Planarians is believed to depend on the contained nerves (Huxley and De Beer, 

 1934. P- 279)- 



It was believed that the homoeotic type of heteromorph in Arthropods, e.g. the regenera- 

 tion of an antenna in place of an eye (Herbst, 1896) was determined by the nervous system 

 but this now seems improbable. S. Holter et al. (1954) found that the nerve-cord deter- 

 mined the typical pattern of the mvisculature of a tail-regenerate, but it did not affect 

 limb-muscle and the apparent qualitative specificity needs further clarification. Experi- 

 mental evidence in general continues to deny any influence of nerves on quality, other 

 than polarity, in the regenerate (Needham, 1950a). The work of Sperry (1951) and others 

 indicates, on the contrary, that peripheral organs may determine the functional quality 

 of any nerves which come to innervate them. 



Recently Chandebois (1952) has shown that it is possible to inhibit differentially the two 

 sides of the body in the triclad, Procerodes, by an inhibitor applied to the whole transverse 

 section. It seems likely that such a sharp bilateral difference must depend on the bilaterally 

 arranged nerve cords. The result, — independent right and left heads, — is very similar to 

 those of Brondsted (1955) when grafting together right and left halves of a Planarian, with 

 their levels in the antero-posterior axis "staggered". It seems probable, therefore, that the 

 instrinsic regeneration-rate at any level depends on a property of the nervous system at 

 that level, a property graded through the body. In the normal body the combined activities 

 of the nerve-cords results in a single regenerate with maximal activity in the mid-line of 

 the body (Brondsted, 1955). 



The asymmetry observed by Chandebois recalls an occasional bilateral asymmetry in the 

 extent of morphallactic regeneration in Sabella (Gross and Huxley, 1934, 1935; Berrill and 

 Mees, 1936). It is difficult to explain this except as the work of the nervous system, and like- 

 wise the observed progressive extension backwards of the morphallactic change, in response 

 to repeated transections far away from the "front" of the change, or again an occasional 

 tendency of the change to "skip" one or more segments. Bilateral independence is seen also 

 in other Annelids (Berrill, 1952, p. 429). 



In the experiments of Chandebois the less inhibited side appeared to be dominating the 

 other since, if it was reamputated, the other then took the lead. This strikingly resembles 

 the phenomenon of chela-reversal in Crustacea and of operculum-reversal in Hydriodes, for 

 which nervous control has been suggested by some (Huxley, 1932) and denied by others 

 (Abeloos, 1953; Ludwig and Ludwig, 1954). It also seems to show once more (pp. 636-639) 

 both promotor- and inhibitor-actions, by the nervous system. 



Regeneration of the limbs of Arthropods depends only on the peripheral nerve 

 and the local segmental ganglion (Needham, 1953), and in the limbs of vertebrates 

 the control is similarly local (Singer, 1952). The higher centres indeed may inhibit 

 limb-regeneration (Jurand, 1954; Maron, 1954), during the early stages. The 



