646 REGENERATION AND GROWTH 7 



of regeneration, and deeper tissues the later stages. A number of recent results 

 indicate that the whole skin, rather than the epidermis alone, controls the early 

 stages (Guyenot and Ponse, 1930; Hellmich, 1930; Kiortsis, 1953). 



The epidermis has usually been found to induce regeneration and the dermis 

 to determine its general nature. The general rule therefore is still valid that the 

 deeper the tissue the later its dominance. The nervous system, controlling mainly 

 cell-proliferation, might be said also to obey this rvile. Each tissue is in control at 

 the time of its own maximal activity (Needham, 1952). Epidermis is necessary 

 to induce dedifferentation (Polezhayev and Favorina, 1935; Gidge and Rose, 

 1944). Dedifferentation begins in the dermis and without this regeneration is 

 halted. In the Protozoa the ectoplasm probably plays a similar role to that of the 

 epidermis of metazoa (Weisz, 1954). The later, organising action of muscle 

 tissue has been demonstrated by Liosner and Voronzova (1938), Thornton (1942) 

 and by Trampusch (1951), that of cartilage and bone by Bischler (1926), Polezha- 

 yev (1949), Trampusch (1951), and that of notochord by Morgan and Davis 

 (1902), Marcucci (1925), Llischer (1946) and Santa (1951). The gut (endoderm) 

 is relevant only to whole-body regeneration. Evidence of some controlling action 

 by the gut is given by Krecker (1923) and Child (1941 ; p. 360). 



In the reconstitution of hydroid Coelenterates (Beadle and Booth, 1938; Von 

 Bertalanffy and Rella, 1941) and of sponges (Huxley, 1921; Brien, 1953) all or 

 most cell-types from the parent must be present, and in fairly normal propor- 

 tions, and this is true also of amphibian tail-regenerates (Liischer, 1946). Each cell- 

 type therefore has its unique, indispensable contribution. 



(g) Local factors 



In the lower Metazoa vagrant neoblasts contribute cells to the regenerate but 

 in Amphibia regeneration is inhibited by a very local inactivation of cells by radia- 

 tions. Even in the Protozoa (Tartar, 1941) regeneration may be an extremely 

 localised process. Not only the rate of regeneration but still more the quality of 

 the regenerate is controlled by this very local slice of parental tissue (De Giorgi 

 and Guyenot, 1923). Heteromorphic and more trivial abnormalities do occur, 

 though infrequently under normal conditions. Heteromorphs show that the fate 

 of regenerating tissues is susceptible to modification by the higher-grade, systemic 

 type of controls already considered, and plasticity is shown also in the ability of 

 tissues, at virtually all levels in the body of the lower Metazoa, to regenerate 

 either anterior or posterior portions of the body. In some of these animals the 

 parental tissues themselves are probably extensively modified by subsequent 

 morphallactic processes and by grafts (Miller, 1938; Santos, 193 1). Even in these 

 simple animals, however, the modified cells only very gradually change their 

 intrinsic character (Abeloos, 1930). There are certainly limits to the plasticity 

 of their tissues, and only a narrow antero-lateral region may be induced to regener- 

 ate eyes (Lender, 1954). Very young blastemata, even in Amphibia and Arthro- 

 poda, may be determined placewise, after transplantation (Needham, 1952, 

 p. 70), or at least may be changed in fate (Gebhardt, 1926), but they are normally 

 determined, irreversibly, early in the process. Many tissues regenerate in con- 

 tinuity with the homologous tissues of the stump which determine their differen- 



