VI EFFECT OF EXTRINSIC FACTORS 625 



more electro-positive in the R-phase and more electronegative in the P-phase. 

 Lund (1947) finds growing regions in general negative to non-growing tissues. Im- 

 posed electrical potentials naturally affect regeneration (Davenport, 1899; Child, 

 1941 ; Lund, 1947), and the dosage has usually been adequate to show this, not- 

 withstanding the reversal of intrinsic potential during the process, though this 

 may explain some of the paradoxes reported. Recent work (Barth, 1934; Marsh 

 and Beams, 1952; Dimmitt and Marsh, 1952; Moment, 1946-50) with physiolog- 

 ical dosage, carefully controlled, has yielded more intelligible results, though the 

 spontaneous intrinsic reversal of potential (Crane, 1950) is still often ignored. The 

 apparent sharp contrasts between closely related genera of hydroids (Barth, 1934; 

 Child, 1 941) possibly depends on this. 



The ends of the intact body of the earthworm, Eisenia, (Moment, 1953, 1954) 

 are electro-positive to the middle region, the posterior being more positive than 

 the anterior end, so that SH-concentration (Perkins, 1929; Maloeuf, 1936b) is 

 proportional to electronegativity. When either end is amputated the exposed 

 surface becomes negative to the middle of the body; as it returns to the normal 

 gradient, cell-poliferation ceases. There is no further change during hypertrophy 

 and differentiation (Moment, 1953, 1954). Crane's initial electropositive trend 

 may be brief in these animals. 



In Planarians and Hydroids the intrinsic gradient is probably simple, in both 

 intact animals and isolated pieces, the head-end being the most electropositive. 

 An imposed, adverse, direct current inhibits head-regeneration progressively, 

 and induces increasingly vigorous regeneration of a head at the opposite end 

 of an isolated piece (Marsh and Beams, 1952). Rate of regeneration and the nature 

 of the regenerate are both related to the intrinsic electrical field, in accordance 

 with the views of Child (1941). In Tubularia and Pennaria, again, an adverse exter- 

 nal field is inhibitory (Barth, 1934); these hydroids are about three times as re- 

 sistant as the Planarian Dugesia (Marsh and Beams, 1952) as measured by the cur- 

 rent (50-75 [J. amp/sq.mm) necessary to reverse polarity. 



A current increasing the intrinsic potential does not accelerate regeneration ap- 

 preciably, and indeed becomes inhibitory at about one tenth of the flow of an 

 adverse current (Barth, 1934). The adverse field retards regeneration but a con- 

 current field will accelerate the termination of the process (Moment, 1953). 

 Dimmitt and Marsh (1952) suggest that normal regeneration is possible only within 

 a narrow range of gradients in ionic potential. They showed that the static poten- 

 tial is more important than the flow of current, the morphogenetic result of a 

 particular potential-gradient being unchanged over a twelve-fold range in spe- 

 cific resistance and flow. This would seem to restrict effective changes to valency- 

 changes, i.e. to oxidation-reduction processes, which is consistent with the SH- 

 gradient observed in earthworms. 



{d) Extraneous chemicals 



The action even of such completely unphysiological substances as beryllium 

 salts (Needham, 1941; Thornton, 1949, 1951; Tuchmann-Duplessis, 1950, 1953) 

 may help in the analysis of the process of regeneration, and may be very specific 

 (Dubois et al., 1949; Chevremont and Firket, 1949; Aldridge, 1950). A compara- 



Literature p. 64g 



