(rROWTH AND REGENERATION OF EPITHELIA 



39 



brane, which is a homogeneous hiyer of nou-ceUular substance 

 derived ])resumably from the su})jacent connective tissue with which 

 it is most intimately associates 1. The l)asement menil)rane does 

 not ap])ear to be ])resent in all cases. It is absent in the follicles of the 

 thyroid gland where cuboidal cells compose the epithelial mem- 

 branes. 



One of the characteristics of epithelial tissues is the fact that 

 they rest upon connective tissue. E\'en in those cases where the 

 epithelial membrane is much folded, connective tissue fills in the 



Fig. 16. — Photograph of a section through the stratified squamous epithelium of the 

 toe of Necturus, showing intercelhilar strands or intercellular bridges. 



spaces between the folds. This subjacent region of connective 

 tissue carries a network of blood and lymph vessels which come into 

 close contact with the base of every epithelial membrane, but do 

 not extend in among its cells. There is a plexus of nerve fibers in 

 the connecti\'e tissue below the basement membrane and some of 

 these fibers pass along the base of the cells while others extend 

 between the cells. 



GROWTH AND REGENERATION OF EPITHELIA. 



As one might ex])ect from the exposure of various epithelia to 

 mechanical and physiological wear, there is a varying quantitative 

 loss of cells that must be replaced. The continual wearing awa>' 

 of the superficial layers is characteristic of stratified squamous 



