88 KERATIN AND KERATINIZATION 



membrane of the cell about 70 A thick, the fine structure of which was 

 discussed above (p. 37); next we enter a lighter space (150-200 A wide) 

 which is continuous with the lighter space surrounding and separating the 

 cells of the overlying epidermis and is, presumably, of a similar nature. 

 Beneath this layer again we encounter another more dense, diffuse layer 



Fig. 39. The fine structure of the basal membrane and an hypothesis to 

 explain its formation. 



M represents the cell membrane of two contacting ectodermal 

 (epidermal) cells A and B. Their surfaces are covered with a sheet of 

 intercellular cement which functions as an adhesive (I). On the surfaces 

 facing the mesodermal space a diffuse, denser and thicker membrane 

 (BM) appears which defines the boundary between epidermis and 

 dermis and is named the basal membrane. 



It is supposed that this membrane results from an interaction 

 between the fixed ectodermal exudate forming the intercellular cement 

 (I) and a more diffusible component emanating from the mesodermal 

 cells. 



which is everywhere continuous (surrounding hair follicles and other 

 epidermal irregularities) and represents a quite definite morphological 

 surface separating the dermis and epidermis. We have reserved for this 

 particular layer the name basal membrane {BM, Fig. 39). 



Membranes of this special character, to judge from the limited electron- 

 microscopic data yet available (Policard and Collett, 1959), appear to form 

 whenever two populations of cells which have followed sufficiently different 

 embryonic pathways, and become differently differentiated, are brought 



