300 CELLS, TISSUES, AND ORGANISMS 



these cells (Figure 6). Concentrations of cellular debris and even 

 erythrocytes have been described within amphibian epidermis by Ide- 

 Rozas ( 1936 ) and Taban ( 1955 ) . Kamrin and Singer ( 1955 ) remarked 

 upon the disruption of the epithelium and the presence therein of 

 cellular products in denervated resorbing taste barbels of the catfish. 

 There is still other experimental evidence from our laboratory 

 (Riddiford, in press) on movement of substances from deep re- 

 gions into the regenerate epithelium. Tritiated thymidine, infused 

 into the early regenerate, was incorporated widely into dividing me- 

 senchymatous and epithelial cells. The incorporation, identified in 

 radioautography, was evident in the first few days after infusion but 

 then faded with continued cellular division and growth of the re- 

 generate. The opposite control regenerate was completely unlabeled 

 by the localized infusion. Riddiford exchanged the labeled epidermis 

 of one side for the unlabeled one of the opposite. During subsequent 

 days the unlabeled epithelium, now covering labeled blastema, gradu- 

 ally became labeled, presumably by movement of cells and cellular 

 debris into the epidermis from below. Tritiated thymidine, whether 

 released by death of mesenchymatous cells or otherwise, was carried 

 or swept into the epithelium and there incorporated into epidermal 

 cells. The blastema of the opposite side remained unlabeled, although 

 covered with labeled epithelium. Indeed, with the continuous dis- 

 placement of epidermal cells to the surface, the labeling was gradually 

 washed from the epithelium. She never observed the reverse move- 

 ment into the blastema from the epithelium. Her results affirm that 

 epithelium continuously receives cellular materials from below and 

 disposes of them. But they do not support the view of cellular move- 

 ment from epidermis into mesenchymatous tissue. 



Role of the epidermis in development of the new part 



It is now possible to define more clearly the role that epidermis 

 plays in regeneration of a body part. For one thing, the epidermis col- 

 lects a steady stream of stuff from the underlying developing blastema. 

 We have seen in our various experiments an apparent progression of 

 transfer of material from inside to outside via the epidermis. The stuff 

 gathered by epidermis consists of various orders of substances. There 

 are, first of all, living and dead cells, among which are active phago- 

 cytes. It is possible that the phagocytes enter to remove the debris 

 from the epidermis as they do from elsewhere (Ide-Rozas, 1936) and 

 then return to subepidermal regions. However, the progression of 

 substances outward through the epithelium leads us to the belief that 

 the epidermis serves as a "graveyard," so to speak, for phagocytes and 



