EPITHELIUM. 27 



the original nucleus, but in the reverse order viz., skein at first more 

 open and rosette-like (m), then closer (n), then a network (o, p, q); passing 

 finally into the typical reticular condition of a resting nucleus. 



The protoplasm of the cell divides soon after the formation of the 

 dyaster (m). During division fine lines are seen in the protoplasm, 

 radiating from the ends of the nucleus. Other lines produced by a 

 spindle-shaped system of achromatic fibres lie within the nucleus, diverg- 

 ing from the poles towards the equator (figs. 23, 24); they are far less easily 

 seen than the other or chromatic fibres, but are not less important, for 

 they are derived from the attraction-spheres, which, as we have seen, 

 always initiate the division of a cell. Moreover, the achromatic fibres 

 within the nucleus appear to form guides along which the chromosomes 

 or chromoplasmic filaments are conducted towards its poles. 



Classification of epithelia. Epithelia are classified according to the 

 shape and arrangement of the component cells. Thus we speak of 

 scaly or pavement, cubical, columnar, polyhedral, and spheroidal epithelium. 

 All these are simple epithelia, with the cells only one layer deep. If 

 forming several superposed layers, the epithelium is said to be stratified, 

 and then the shape of the cells differs in the different layers. Where 

 there are only three or four layers in a stratified epithelium, it is 

 termed transitional. 



Stratified epithelium covers the anterior surface of the cornea, 

 lines the mouth, pharynx (lower part), and gullet, and forms the epi- 



FIG. 25. SECTION OF THE STRATIFIED EPITHELIUM COVERING THE FRONT OF 

 THE CORNEA OF THE EYE. 



c, lowermost columnar cells ; p, polygonal cells above these ; fl, flattened cells near the surface. 

 Between the cells are seen intercellular channels bridged over by processes which pass 

 from cell to cell. 



dermis which covers the skin. In the female it lines the vagina and 

 part of the uterus. The cells nearest the surface are always flattened 

 and scale-like (fig. 25, fl\ fig. 26), whereas the deeper cells are more 

 rounded or polyhedral, and those of the deepest layer generally some- 

 what columnar in shape (fig. 25, c). Moreover, the deeper cells are soft 

 and protoplasmic, and are separated from one another by a system of 



