Ill STAGES IN AN ACT OF REGENERATION 597 



of regenerative powers (Abeloos, 1930; Sivickis, 1933). The number and size of 

 cells decreases (Brondsted, 1955) and oxygen-consumption decreases (Hyman, 

 19 1 9, 1920) as in early regeneration. Repeated evocation of regeneration prolongs 

 the life of Planarians (Chranova, 1938). Rejuvenation in Protozoa has consider- 

 able similarities to the early phase of regeneration (Weisz, 1954). 



The regeneration of nerves has been interpreted rather commonly as a return 

 to an embryonic condition (Van Biervliet, 1900; May, 1930; J. Needham, 1942, 

 p. 554; Bodian, 1947; Mannell and Rossi ter, 1954). 



In some animals very extensively (Wolff and Dubois, 1947; 1948), and in most 

 to some extent (Wolff and Wey-Schue, 1953) immigrant neoblasts contribute to 

 the blastema. Cell-migration, whether local or over long distances, marks the 

 "turn of the tide" in regeneration. As in vitro (Medawar, 1940) migration is a 

 necessary prelude to cell-proliferation (Abercrombie and Johnson, 1942, 1946), 

 though Liischer (1946), and Chalkley (1954), saw active cell-proliferation in the 

 stump of the amputated limbs and tail of Amphibia, in the early stages, indicating 

 that the onset of proliferation may precede that of migration. The daughter cells 

 of this activity move into the "open" and later the maximal rate of proliferation 

 is in the blastema itself, and moves progressively distalwards (Table 4). The epi- 

 dermis behaves differently from the deeper tissues because of its role in healing 

 the wound and because its cells move less individually, though they do show con- 

 siderable shearing and jostling (Lehmann, personal communication). It is usually 

 claimed (Schaxel, 1921; Butler, 1933; von Levetzow, 1939; Waechter, 1949; 

 Paton, 1955) that there is no cell-division in the epidermis until after wound-clo- 

 sure, but again Manner (1953) saw mitosis in the stump-epidermis prior to cell- 

 movements. After closure there is rapid proliferation in the covering cells, which 

 become the epidermis of the regenerate, so that the epidermis probably behaves 

 as do the other tissues, but somewhat ahead of them in phase. 



Like the epidermis, the gut, blood vessels, and other epithelia regenerate in 

 continuity in the Amphibia (Korschelt, 1927) and so do the non-epithelial 

 tissues, — bone and muscle (Naville, 1922-24; Liischer, 1946; Waechter, 1949; 

 Chalkley, 1954). Only organs completely removed, and connective-tissue, are 

 regenerated from cells of uncertain, and possibly distant, origin. On the other 

 hand in some animals neoblasts may enter tissues otherwise regenerating in conti- 

 nuity, and proliferate there (Korschelt, 1927, p. 298; Brondsted, 1955; p. 75). 



At first the growth of the blastema is almost entirely by cell-proliferation or 

 "hyperplasia" (Litwiller, 1939; Polezhayev and Ginsburg, 1944; Singer and 

 Craven, 1948; Moment, 1953) and only later by cell-enlargement or "hyper- 

 trophy". There is no evident differentiation at this stage though the blastema- 

 tissues become "determined" qualitatively (Liischer, 1952; Needham, 1952). 

 Proliferation is maximal after one quarter of the total time of regeneration (Table 

 5), and corresponds to the steepest part of the curve of size/time. If proliferation 

 is multiplicative then five to six cell-generations, each fifteen days long, would 

 suffice for the complete regeneration of the limb of the newt studied by Litwiller 

 (1939). The larvae of some species of newt regenerate four to five times as rapidly 

 as this, however. Moreover the proximodistal progress of maximal activity implies 

 an approximation to linear, arithmetic, rather than to geometric progression, so 



Literature p. f>4g 



