34 THE EPITHELIAL TISSUES 



of the intestines. Other cohunnar cells, composing the small glands 

 associated with the stomach and intestine, appear to be uniformly 

 distended by their secretory activity and the passage of the secre- 

 tion out of the cells is gradual and continuous, not abrupt as in the 

 case of the goblet cells. 



In a number of cases cells with a sensory function are derived 

 from embryonic ectoderm to form parts of sensory organs. Although 

 such cells may be classified as columnar they are commonly fusiform 

 in shape. Associations of such cells, called neuro-epithelium, occur 

 in the taste-buds of the tongue, the rods and cones of the retina 

 of the eye, and in special sensory bodies in the skin of fishes and 

 amphibians. 



STRATIFIED EPITHELIA. 



When considering stratified types several subdivisions are possible 

 on the basis of the shape and arrangement of the cells, with the em- 

 phasis on the type of cell forming the superficial layer. The basal 

 layer of cells is generally prismatic and appears as cuboidal or low 

 columnar in the sections. The cells between the base and the surface 

 appear as irregular polygonal figures, showing considerable variation 

 in size, regularity, and number of layers. In some cases the cells 

 flatten out as they near the surface, until those of the superficial 

 layer are squamous in form; in other instances the cells retain a 

 prismatic structure exen at the surface, save that their free bound- 

 aries are usually convex; but in other cases the surface cells are 

 elongated prisms appearing as columnar cells in sections, with 

 tapering bases passing among the underlying polygonal cells. Four 

 subdivisions of stratified epithelia are discussed in the following 

 paragraphs. 



Stratified Squamous Epithelium.^ This is a common stratified type 

 and the number of cell layers composing the membrane varies in 

 difl'erent parts of the same animal. In sections, the basal layer of 

 cells appears cuboidal or low columnar. Progressing ujiward 

 there are several layers of polyhedral cells showhig a gradual change 

 toward the surface where flattened squamous cells occur. (Fig. 14.) 

 The superficial scpiamous cells are constantly being worn away and 

 replaced by underlying cells. The cells of the basal layers divide 

 frequently and the plane of division parallels the base, so that 

 after each division one daughter cell remains in the same position 

 as the mother cell while the other is pushed upward into the next 

 laver. The newh' formed cells are constantly undergoing chemical 



