I INTRODUCTION 589 



part of a spontaneous act of morphogenesis. "Physiological regeneration", the replacement 

 of a relatively steady amount of "wear and tear", as among the cells of the mammalian 

 epidermis, blood, etc., is often regarded as a normal growth-process but it is largely con- 

 trolled by the extent of losses and is a true regeneration-process. Many Amphibia and Repti- 

 lia cast their superficial epidermis, en masse, periodically. Wear and tear is a recognised 

 phenomenon at the molecular level (Morel, 1941 ; Haldane, 1954) ; it is less rapid than the 

 total rate of metabolic turnover revealed by studies using "tracer" atoms. 



Like ontogenesis, regeneration involves not only growth but also the movements 

 of cells, the differentiation of cells and their collective organisation into an integral 

 structure. Indeed many sponges and coelenterates are able to regenerate from 

 dissociated cells which initially reconstitute purely by aggregation and redistribu- 

 tion of the individual cells (Brien, 1937; Beadle and Booth, 1938; Von Bertalanffy 

 and Rella, 1941). Growth occurs eventually (Brien, 1937), but only after the es- 

 sential processes of regeneration are complete. A number of animals regenerate 

 largely or partly by "morphallaxis" (Morgan, 1901), another process involving 

 relatively little growth, but mainly reorganisation within the parent portion of 

 the body. However, the nature of morphallaxis is far from clear, and it may 

 approximate to a disseminated epimorphosis of local blastemata throughout the 

 body (Van Asperen, 1946). Planarian worms regenerate mainly by epimorphosis 

 when well fed but by morphallaxis when starved (Chranova, 1938), and the sea 

 squirt, Clavellina, by morphallaxis when young and by epimorphosis when mature 

 (Millot, 1 93 1, p. 135), so that the distinction between the two methods is probably 

 facultative rather than fundamental. Like epimorphosis, morphallaxis probably 

 involves an initial process of dedifferentiation or regression (R-phase) followed by 

 the progressive processes (P-phase) of growth and differentiation (p. 595). 



This oscillation is probably characteristic of all types of true regeneration, following 

 acute accidents, and probably distinguishes them from physiological regeneration and 

 other types of response to chronic low-grade losses. The function of the R-phase is to provide 

 cells capable of proliferation and growth (p. 596). In the lower metazoa cells not previously 

 differentiated, "neoblasts" (Stolte, 1935; Schotte, 1939) contribute to regeneration and 

 the R-phase may be less extensive or less significant here. This method recalls a method of 

 spontaneous asexual reproduction in the Parazoa and lower Metazoa by means of "gem- 

 mules" or "statoblasts", — accumulations of "embryonic" cells. The "statoblasts" of some 

 Polyzoa and other animals, however, are aggregates of dedifferentiated cells. These do not 

 differ very fundamentally from the dedifferentiated remains of whole, starved animals of 

 some groups of animals, which are capable of regenerating when refed (J. Needham, 1942). 

 Starvation leads to increased fat-mobilisation (Frazer, 1954) as in the R-phase of regener- 

 ration (p. 619). 



For the present purpose interest centres on the characteristic R — P oscillation 

 in metabolism during regeneration. The Protozoa show it not only during regener- 

 ation but also in the course of asexual and sexual reproductions, and in encyst- 

 ment, and there is evidence of it in the metamorphosis of amphibia (J. Needham, 

 1942) and of insects (D. Needham, 1931; Wiggles worth, 1954). The control of 

 qualitative processes in regeneration will be considered only so far as it is related 

 to growth-problems. Regeneration is viewed therefore as a re-accelerated growth- 

 process (Przibram, 191 9; 1925; 193 1). 



For further general information on regeneration as an independent study the following 

 works may be consulted: Morgan (1901), Davenport (1908), Przibram (1909), Korschelt 



Literature p. 64^ 



