624 REGENERATION AND GROWTH 7 



(b) Short-wave radiation 



Low dosage of ultra violet (U.V.), X-rays or y-rays, accelerates regeneration, 

 but the direct effect is inhibitory. Doses of 5,000 to lOjOOOy may inhibit completely 

 (Brunst, 1950). Irradiation causes general damage to living material but this is 

 reparable. Permanent inhibition of this, as of other types of growth, is due to in- 

 hibition of cell-division (Torraca, 19 14; Butler, 1933; Pucket, 1936; Horn, 1942; 

 Wolff and Dubois, 1948a). The effect is entirely local (Butler and O'Brien, 1942; 

 Wolff and Dubois, 1948a) and in some animals regeneration is eventually resumed 

 by immigrant neoblasts. An irradiated region, in fact, traps the neoblasts so that if 

 amputation is performed distally to an irradiated zone, no neoblasts reach the 

 site and there is no regeneration (Wolff and Dubois, 1948a). 



The effect of irradiation is proportional to the youth, rate of proliferation, and 

 lack of differentiation of the cell. Cells of older animals, and of older regenerates, 

 as proliferation-rate declines, become progressively less sensitive (Brunst, 1950), 

 whereas persistently proliferating cells of the gonads, bone marrow, lymphoid 

 tissues, perichondrium, mesenchyme and epidermis remain very sensitive. The 

 highly differentiated nerve-tissue is most resistant. The posterior growth-zone of 

 annelids is the most sensitive region of the body (O'Brien, 1942), while in plana- 

 rians the head-end is most active in growth and regeneration and most sensitive 

 to radiations (Hinrichs, 1924). 



Chemically the only consistent effect of radiations (Thomson et al., 1954) is 

 a reduction in concentration, and in rate of turnover, of DNA. The more oxidised 

 cytoplasmic form of nucleic acid, PNA, is less sensitive. SH-compounds, and other 

 reducing agents, protect against irradiation (Bacq, 1951) which accelerates 

 oxidation-processes and probably in this way inhibits the normal action of DNA in 

 nuclear division. 



Radioactive chemicals also inhibit regeneration ; radiophosphorus completely 

 inhibits that of the newt's leg at a dosage of 166 [x-curies per gram of body weight, 

 (Dent, 1949). Neutrons have the same effect as short-wave radiations, and are 

 probably more potent; 62 n causes complete inhibition in the urodele limb 

 (Horn, 1 94 1, 1942). The resemblance extends to a further effect, peculiar to regen- 

 erating tissues, — an excessive regression of the tissues of the stump. An established 

 blastema prevents the induction of this regression even though its further growth 

 may be completely inhibited (Butler, 1933). 



There is here a close similarity to the responses to denervation (p. 641) and to colchicine- 

 treatment (p. 627), the primary effect of which again is on cell-division. Regression is 

 facilitated by amputation and by other kinds of wound (Thornton, 1954), and only very 

 high dosage of the inhibitor of mitosis will cause regression without this aid. There may be 

 more than a simple quantitative summation of damage, therefore. A grafted blastema will 

 halt regression already "under way", a demonstration of the incompatibility of R- and 

 P-phase processes. 



(c) Electrical forces 



Electrical potential-differences are usually associated with activity of any kind, 

 in the body (Lund, 1947), and regeneration is characterised by an oscillation in 

 potential between active and uninjured regions (Crane, 1950). The former is 



