304 



Basic Structure of Vertebrates 



Simple Epithelium. An epithelium only one cell in thickness is 

 termed simple. There is, however, great variation in the thickness of 

 simple epithelia. The cells, seen in sections perpendicular to the sur- 

 face, may be approximately square in outline. Such an epithelium is 

 called cuboidal (Fig. 243), but incorrectly, for the cells are usually 

 hexagonal prisms. A simple epithelium consisting of tall prismatic cells 

 (Figs. 244C, 245) is called columnar. At the extreme of thinness are 

 epithelia (flat or squamous) each of whose cells is a broad flat plate, 

 hexagonal in outline (Figs. 243, 244B). Endothelia of blood-vessels are 

 among the thinnest of epithelia. 



Terminal bar 



Top plate 



x~x 



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Fig. 243. (Top) Section of the amnion and allantois of a 60 mm. pig embryo 

 at a region where the mesodermal layers of the two membranes have coalesced. 

 The section is perpendicular to the surfaces of the allantois (above: cuboidal 

 epithelium) and the amnion (beneath: squamous epithelium). The "top plate" is a 

 superficial denser layer of the cuboidal cell. "Terminal bars" are thickenings of 

 intercellular substance just beneath the surface of the epithelium. 



(Bottom) Surface view of allantois. (Courtesy, Bremer-Weatherford : "Text- 

 Book of Histology," Philadelphia, The Blakiston Company.) 



