EPITHELIAL TISSUES. 8 1 



from neighboring cells, thus forming intercellular bridges. Between 

 these bridges are intercellular spaces filled with lymph-plasma for 

 the nourishment of the cells. Epithelia do not, as a rule, possess 

 processes of any length. However, it would appear that the base- 

 ment membranes, situated beneath the epithelia, consist chiefly of 

 processes from the basal portion of the cells. Some authors ascribe 

 to them a connective-tissue origin, a theory which conflicts with the 

 fact that such membranes are present in the embryo before 

 connective tissue, as such, has been developed (jnembrana prima, 

 Hensen, 76). 



The free surfaces of epithelia often support cuticular structures 

 which are to be regarded as the products of the cells. The cutic- 

 ulse of neighboring cells fuse to form a cuticular membrane or mar- 

 ginal zone which can be detached in pieces of considerable size 

 (cuticula). In longitudinal sections the cuticula show, in many 

 cases, a striation which would seem to indicate that they are com- 

 posed of a large number of rod-like processes cemented together by 

 a substance possessing a different refractive index. The cell-body is 

 also striated for more than half its length, corresponding to the rods 

 of the marginal zone. In the region of the nucleus at the basal por- 

 tion the striation disappears, the cell here consisting of granular pro- 

 toplasm of a more indifferent character. 



Since one surface of each epithelial layer lies free, and is conse- 

 quently exposed to other conditions than the inner surface, certain 

 differences are noticed between the two ends of each cell. The 

 cells may develop cuticular structures as above stated. In other 

 cases motile processes (cilia) are developed on their exposed surface, 

 which move in a definite direction in the medium surrounding 

 them, and by means of this motion sweep away foreign bodies. It 

 is not strange that the free surface of the epithelia, exposed as it is 

 to stimulation from without, should develop special structures for 

 the reception of sensations (sense cells). 



On the other hand, the inner or basal surfaces of the cells usually 

 retain a more indifferent character, and serve for the attachment of 

 the cells and the conveyance of their nourishment. For this reason 

 the nuclei of such cells are usually situated near the basal surface. 



From the above it is seen that the two ends of the epithelial cell 

 undergo varying processes of differentiation, the outer being adapted 

 more to the animal, the inner more to the vegetative functions. 

 This differentiation has recently been known as the polarity of the 

 cell. This polarity appears to be retained even when the cell loses 

 its epithelial character and assumes other functions (Rabl, 90). 



With few exceptions, blood- and lymph-vessels do not penetrate 

 into the epithelia, but the latter are richly supplied with nerves. 

 The finer morphology of the epithelia will be described in the chap- 

 ters on the different organs in Part II. 



Epithelia are classified according to the shape and relation of 

 the epithelial cells. 



