EPITHELIA. 



tory and genital tracts. A stratified epithelium is one which actually 

 consists of several layers of cells (Fig. 28). In descriptions of stratified 

 epithelia the number of layers should be recorded, especially if few. We 

 may say that it is 2-layered, 4-6-layered, or many layered, as the case may 

 be. The shapes of the cells in the basal, middle, and superficial strata 

 should be noted. The cells are formed in the basal layer, and as they are 

 pushed outward they become changed in shape and character. The 

 "superficial cells, for which the entire stratified epithelium is often named, 

 may be columnar, cuboidal, or flat. The flat ones are called squamous. 

 especially when they have become detached and are found in urine or 

 saliva (Fig. 29). (Transitional epithelium is an undesirable name for that 

 form of stratified epithelium found in the bladder, ureter, and pelvis of 

 the kidney. It will be described in connection with those organs.) 



PERIPHERAL DIFFERENTIATION. 



The differentiation of epithelial cells is chiefly along three lines, 

 first, the transformation of entire cells into cornified masses as in the 

 outer cells of the skin, in the nails, 

 and hair; second, the development 

 of various structures around the 

 borders of the cells, particularly 

 along the free surface; and third, 

 the elaboration of secretion within 

 the protoplasm. The last two forms 

 will be considered in detail. 



Cell walls in young epithelia 

 are generally lacking. In the early 

 embryonic skin and in its basal 

 layers in the adult, they are often 

 absent, so that the cells are in very 

 close contact. Later they become 

 separated from one another by "ce- 

 ment substance," probably fluid. This is true of mesothelial and endo- 

 thelial cells also. Since silver nitrate is precipitated by the intercellular 

 substance, their cell boundaries become very distinct after treatment with 

 this reagent. Lymph corpuscles and leucocytes may pass out from thin- 

 walled blood vessels, between the endothelial cells, into the mesenchymal 

 spaces. They may enter the intercellular substance between the columnar 

 cells of the intestinal epithelium. Here they are prevented from reaching 

 the free surface by terminal bars. The diagram, Fig. 30, illustrates 



Intercellu- 

 lar sub- 

 stance. 



FIG. 30. DIAGRAM OF THE NETWORK OF 

 The two cells 



lengthwise 

 e drawn as 



