596 REGENERATION AND GROWTH 7 



"flow" it seems much too sensitive to external factors (Butler and Schotte, 1941, 

 1944) to have any such precise quantitative significance and the production of 

 a qualitative condition of regression is probably the extent of its significance. 

 Dedifferentiation occurs also in regenerating Protozoa, so that it is not a reorga- 

 nisational phenomenon which evolved only at the tissue-grade of structure. Presum- 

 ably therefore it is a chemical rather than a cellular phenomenon, cytolytic in 

 "spirit", though histolytic in appearance in metazoa. Cytolysis yields materials 

 with potent inducing properties when injected into embryos (Holtfreter, 1948; 

 Weiss, 1950) and a similar mechanism of induction of regenerates seems probable; 

 the complete cytolysis during demolition-processes perhaps induces a milder cyto- 

 lytic change in intact cells which so become embryonic in character. The cells of 

 the blastema are large, with large nuclei and nucleoli, and relatively little, clear 

 cytoplasm (Horn, 1942) like the typical cells of a young embryo. Granules reap- 

 pear in the cytoplasm later (Thornton, 1938) probably after the phase of maximal 

 proliferation. 



It is evident enough that the blastema-cells, like cells in culture-media, and other 

 appropriate conditions, are embryonic in their dedifferentiation and in their high 

 powers of proliferation, but further evidence is necessary to prove that they return 

 to a generally embryonic condition. Normal periosteal tissue of mammals does 

 not readily proliferate in vitro but cells from a healing fracture do (Allgower and 

 Rosin, 1953), so that there is little doubt also of the similarity between regeneration 

 and culture in vitro. The movements of sodium and potassium between recently 

 explanted cells and their medium (Davies, 1954) resemble these movements in 

 regenerating tissues, and it would be valuable to know if a low K/Na ratio character- 

 ises also the cells of young embryos. An uptake of water certainly renders cells 

 more "embryonic" (J. Needham, 1931) and the glycolytic respiration of early 

 regenerating tissues compares with that of many young embryos. The amount of 

 the following substances normally diminishes with age, but increases again in 

 regenerating tissues: hexosamine (Boas and Foley, 1954); glucuronidase (Mills 

 et al., 1953) cathepsins (Sokolova, 1942) sulphydryl compounds, alkaline phos- 

 phatase (Junqueira, 1950). Yakovleva (1943) found that the condition, as well as 

 the amount, of pentosenucleic acids in regenerating tissues became more like 

 that of embryo-tissues. An increased digestibility of the tissues (Orechovitch and 

 Sokolova, 1940) is probably an embryonic property, though in general regener- 

 ating tissues are said to be less sensitive than embryo-tissues to chemical agents 

 (Singer, 1952). Cells of regenerating tissues stain less readily than do mature cells 

 (Singer, 1954) and this is probably true of the cells of early embryos and of the 

 cells of lower invertebrates. However embryonic and regenerating tissues both 

 are more sensitive than mature cells (Brunst, 1938) to irradiation. 



It is probably significant that the regenerative power in the limbs of Anuran 

 Amphibia, which normally fails at metamorphosis, may be preserved by the re- 

 peated evocation of regeneration throughout metamorphosis (Schotte, 1939; Moro- 

 sow, 1941; Polezhayev, 1946) and this has been seen in insects also (Przibram 

 1932). The "youth" of the cells is thereby preserved; metamorphosis itself is 

 delayed (Polezhayev, 1946). 



Starvation of Planarians causes degrowth, dedifferentiation and an increase 



