REGENERATION IN VERTEBRATES 283 



by gradual transformation. Epithelial cells have also frequently been 

 reported to be the source of the basal membrane of epidermis, includ- 

 ing its reticular fibers and ground substance ( reviewed by Singer and 

 Andrews, 1956). 



Another theory of epidermal action in amphibian regeneration is 

 that it calls forth histolysis of the wound tissues, a theory to which 

 Polezaiev and Faworina ( 1935 ) lent some weight. They noted the ex- 

 periments of Bromley that histolysis in tissues of the wound is low 

 when the wound is covered by whole skin but intense when there is no 

 intervening dermis. A fresh wound transecting stump tissues is another 

 precondition of histolysis, because regeneration fails when whole skin, 

 after being transplanted over a wound and allowed to heal in place, is 

 then peeled off, unless the underlying tissue is also freshly damaged. 

 Polezaiev and Faworina concluded that epidermis calls forth a strong 

 histolysis in tissues of the stiunp, resulting in dedifferentiation and the 

 appearance of cells of regeneration (see also Polezaiev, 1947; Need- 

 ham, 1952). How the epidermis participates in histolysis was not 

 stated, except that tissue breakdown fails without a covering epithelium 

 (see also Adova and Feldt, 1939). Histolysis in the regenerate under 

 the influence of the epithelium was studied morphologically and bio- 

 chemically by Orechowitsch and Bromley ( 1934 ) . 



The epidermis itself on occasion has been credited with histolytic 

 and phagocytic abilities. Taban ( 1955) observed that a blood clot was 

 liquefied by the wound epithelium in the salamander. He also observed 

 debris of various sorts within epidermal cells. Phagocytic activity of 

 regenerate epithelium was seen after amputation of the tadpole tail 

 (Ide-Rozas, 1936); as it grew over the wound, the epithelium picked 

 up red blood cells, cell fragments, and debris of various sorts. It 

 dipped down into the crevices of underlying tissues and was so dis- 

 rupted that in places it was difficult during the early days to distinguish 

 epithelial from mesenchymatous cells. 



Another scheme of the role of wound epidermis was advanced by 

 Goss ( 1956 ) , based on the view of Thornton that a nerve-epidermal 

 interaction is important for growth: The nerve causes the epidermis to 

 grow, and the expansion of the epidermis in turn provides enough 

 space for increase in the blastema. A number of earlier workers had 

 considered and rejected a possible mechanical role of epidermis, in- 

 cluding Tornier (1906), Taube (1921), Schaxel (1921), Godlewski 

 (1928), Jefimoff (1931), and Polezaiev and Faworina (1935). They 

 showed that mechanical pressure, whether of a piece of chamois or nor- 

 mal skin transplanted and secured in such a way as to cover the epi- 

 thelium of the regenerate, fails to suppress the expansion of the blas- 

 tema, and that the growing structure makes its own space, so to speak, 

 to which the epidermis adjusts itself continuously. 



