EPITHELIUM. 45 



With further development the epithelial tissue may change 

 superficially, giving rise to such epidermal structures as hairs, 

 nails, claws, talons, the papillas filiformes of the tongue, etc. ; or 

 it may be modified and grow in the deeper layers and give rise 

 to glands. The superficial layers of a stratified epithelium 

 which are worn away by use are replaced by cells from the 

 deeper layers produced by mitotic division. 



At the place where the epithelium comes in contact with the 

 connective tissue, there is usually to be seen a bright refractive 

 line, which forms a boundary between the tissues. This fine 

 structureless membrane is called the basal membrane. It cannot 

 be said with certainty whether it is a product of the epithelial 

 cells or of the connective tissue. In certain cases when two 

 epithelial layers lie upon one another and are separated by a 

 refractive boundary line, there is no doubt that this basal 

 membrane is derived from, the epithelium. 



The flat epithelium which arises from the middle germinal 

 layer and clothes the joint spaces, the serous surfaces of the 

 pleural and peritoneal cavities, the tendon sheaths, and the 

 blood- and lymph-vessels, was for a long time considered as 

 belonging to a separate group of cells known as endothelium. 

 These cells were classed with connective tissue, because they 

 have a certain similarity to the flat cells which line small spaces 

 and lacunae in connective tissue; and also because connective 

 tissue is derived likewise from the middle germinal layer. In 

 order to make the classification definite, it is best to regard these 

 cells as epithelial cells of mesoblastic origin, so that there will be 

 no middle group" formed between epithelium and connective 

 tissue. The main reasons for classifying these cells with 

 epithelial tissues are the characteristic arrangement of the cells 

 to form membranes, the small quantity of intercellular substance, 

 and the absence of any properties which would stand in the 

 way of their being so grouped. At the same time it must be 

 noticed that often no sharp line can be drawn between connec- 

 tive-tissue cells arranged like epithelium and the simple flat 

 epithelium itself. 



