REGENERATION IN VERTEBRATES 289 



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Figure 3. Electron micrograph of part of basal epidermal cell of moderate 

 early regenerate twelve days after amputation, showing surface in contact 

 with blastemal ground substance (G). A large extrusion (£) of the cortical 

 cytoplasm (C) extends into the underlying region. The surface of the ex- 

 trusion is periodically broken. Note absence of adepidermal membrane. Note 

 abundant strands and vesicles of endoplasmic reticulum, oriented around the 

 nucleus (iV). Fixative: potassium permanganate. Magnification: approxi- 

 mately 12,300 X. 



processes of an occasional regenerate cell seem to contact the basal 

 layer directly, but the undersurface of the epithelium is in general 

 well delimited, and the cells seem to form a continuous line of separa- 

 tion. We have seen no cytological evidence for massive movement of 

 epithelial cells into the blastema as Rose ( 1948a ) has reported. 



Electron-microscope studies of the boundary region also show 

 that the ultramicroscopic barrier that exists under normal epidermis is 

 greatly altered or lacking under that of the early regenerate (Figure 2) 

 and under the very tip of the regenerate during subsequent stages of 

 development (Salpeter and Singer, 1960). In the normal skin of various 



