VII 



INTERNAL FACTORS 647 



tiation (De Giorgi and Guyenot, 1923). Promotor and inhibitor substances seem to 

 be as localised in action as in origin, notwithstanding their systemic transport. 

 Quantitative powers of regeneration also vary locally (Polezhayev and Favorina, 

 1935), sometimes in great detail. The differences are largely, though perhaps 

 not entirely, due to inherent local differences in the tissues. Thus the regeneration- 

 rate at a particular level of the Planarian body is independent of the size of the 

 piece isolated (Brondsted, 1954). Taxonomic differences in regenerative power 

 likewise are intrinsic to the tissues; Anuran limbs grafted on to Urodeles lose the 

 power to regenerate when the host metamorphoses, just as they do in situ, though 

 the newt's own limbs retain the power. In general local differences in power, and 

 species differences also, seem to be biologically adaptive (Needham, 1952), and are 

 scarcely amenable to any other comprehensive explanation. Like other physiolog- 

 ical "mechanisms", regenerative power is determined primarily by biological, 

 and not by physical laws. 



VIII. CONCLUSION 



In most essentials, regenerative growth resembles other types of growth and com- 

 petes with any occurring simultaneously in the same body. The nature of a regener- 

 ate depends more on local, slowly diffusing factors, and therefore there is little 

 qualitative interaction with such other morphogenetic processes. To some extent 

 the regenerate is an in vivo tissue-culture (Brondsted, 1955, p. 76). The absolute rate 

 of regeneration changes little during ontogenesis and has changed little during 

 phylogenesis, wherever the power has persisted at all. It increases to a maximum 

 soon after the onset of an act of regeneration and declines progressively as the 

 limiting size is approached. The maximal rate similarly varies with the amount 

 of material amputated. 



Regenerative growth differs from normal growth most in the necessity for an 

 initial regressive (R-) phase affecting tissue- and cell-structure, and metabolism, 

 both locally and systemically. This R-phase produces rejuvenated cells, and the 

 appropriate metabolic conditions for their subsequent proliferation, in the pro- 

 gressive (P-) phase. Some metabolic changes are bicyclic. Other peculiarities of 

 regeneration also are probably associated with the special metabolism of the 

 R-phase, for instance a squandering of protein, and other materials, and an in- 

 difference to the protein-sparing action of anterior pituitary (APGH) and other 

 hormones. Anti-mitotic agents, of very varied nature, including denervation, all, 

 with the aid of trauma, cause excessive regression ; there is no neutral state be- 

 tween R- and P-phase activities. The quantitative, tropic action of the nervous 

 system, therefore is more dramatically evident than in other types of growth. 



There is evidence for an R-P oscillation in metabolism also in the intermoult 

 growth-cycle of arthropods, in the metamorphosis of animals, in encystment and 

 related processes, in starvation- and atrophy-phenomena, in the general adapta- 

 tion-response to stresses in general, and in nerve-conduction and other work- 

 functions. 



A young regenerate liberates a regeneration-promoting substance, and later 



Literature p. 649 



