EPITHELIUM 



21 



formed into a sort of skein, formed apparently of one long convoluted 

 filament ; the nuclear membrane and the nucleoli disappear or are 

 merged into the skein (fig. 17, b, c, d). 



2. The skein becomes arranged in the form of a rosette, the fila- 

 ments looping in and out, to and from the centre (e). 



3. The outer loops of the rosette separate, so that the filament 

 breaks into a number of V-shaped fibres arranged like a star (aster, 



f, 9, h-) 



4. The V-shaped fibres separate into two groups, the ends of which 



for a time are interlocked (i, j, k). 



5. The two groups pass to the opposite poles of the now elongated 

 nucleus and form a star-shaped figure (/) at each pole (dyaster). Each 

 of the stars represents a daughter nucleus. 



6. 7, 8. Each star of the dyaster goes through the same changes as 

 the original nucleus, but in the reverse order viz. rosette (in), skein 

 (n), and network (o, p, q) ; passing finally into the condition of a typi- 

 cal resting nucleus. The protoplasm of the cell divides soon after the 

 formation of the dyaster (in). Sometimes fine lines may be seen in 

 the protoplasm, during the process of division, radiating from the poles 

 of the nucleus, and others uniting the two daughter nuclei. 



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 dif- 

 ferent 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- 

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



FIG. 18. 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. 



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

 and scale-like (fig. 18, fl; fig. 19), whereas the deeper cells are more 



