290 CELLS, TISSUES, AND ORGANISMS 



animals the epidermis is separated from underlying parts by a layer of 

 homogeneous substance (approximately 300 A°) and then by an unin- 

 terrupted membrane (also approximately 300 A°) (Ottoson et al., 

 1953; Selby, 1955; Odland, 1958; Salpeter and Singer, 1959). We have 

 called the homogeneous substance the "adepidermal space," and the 

 membrane, the "adepidermal membrane" (Salpeter and Singer, 1959). 

 The adepidermal membrane is not a continuous structure in much of 

 the early regenerate and in the distal tip of more advanced regener- 

 ates. It is imperfect, as if in the process of being formed, and there may 

 be a number of adjacent epidermal cells not underlain by even a frag- 

 ment ( Figures 2 and 3 )-. Therefore our studies reveal not only that the 

 basal membrane of light-microscope studies is absent but also that the 

 adepidermal membrane of ultramicroscopic dimensions does not exist 

 as an eflfective barrier to free exchange between blastema and epi- 

 dermis. Consequently there is direct communication between the epi- 

 dermal cells and the ground substance of the blastema. In regions of 

 the regenerate where growth is presumably most rapid this contact 

 and direct communication persist into late stages of regeneration. The 

 adepithelial structures apparently diflFerentiate gradually and are 

 completed in a proximal distal direction, starting near the adult skin 

 along the original wound edge. Individual sectors of the adepidermal 

 membrane and space appear to develop separately in relation to indi- 

 vidual epidermal cells. They secondarily unite, eventually to form a 

 continuous structure. 



The earliest sign of membrane formation is the appearance of a 

 coarse fuzzy or spongy precipitate at the site of the new membrane 

 (see Figure 3). This loosely organized deposition appears to be the 

 rudiment of the future adepidermal membrane. Then, it seems to be 

 organized under the influence of the epidermis, because the scattered 

 sectors of membrane that first develop conform in orientation with the 

 undersurface of the basal cells. What role the adepidermal membrane 

 normally plays in the activity of epidermis is not known, nor do we 

 know the real significance of its incomplete character during regen- 

 erative development. The lack of an unbroken membrane is a notable 

 feature of regeneration, and it may mean that a freer exchange of sub- 

 stances can now occur between epidermis and blastema. The free 

 movement of substances may be essential to the process of growth. In 

 this way epidermis may contribute to the chemical milieu of the 

 blastema and the blastema may pass materials in the other direction. 



In regions where the adepidermal membrane is lacking, the cyto- 

 plasm of the basal epidermal cell forms one or more surface protru- 

 sions. In light-microscope studies the protrusions give to the under- 

 surface of the basal cells a cobblestone appearance. In the electron 

 microscope the protrusions are more clearly visualized ( see Figures 3 



