HISTOLOGY 131 



endothelium would interfere with easy release of these cells. Also in the 

 spleen and smaller blood glands — that is, glands whose spaces are occupied 

 by blood instead of lymph — there is more or less interruption of the 

 endothelial lining. 



Epithelia carry on functions of most diverse kinds. The diversity is 

 reflected in the structure of epitheha. Only a few of the more general 

 features of structure can be mentioned here. 



Simple Epithelium. Most epithelia are only one cell in thickness. 

 Such layers are termed simple. There is, however, great variation in the 



Term. bar. ^ Top plate. q^U ^^H_ 



^ (^ ^ r-s ^^V. 



^^ \ ^TS 6 3 "^ 



Conn, tissue. Squam. epith. 



Fig. 92. — At left, section of the allantois and amnion of a 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) and the amnion (below). At 

 right, surface view of allantois. The allantoic epithelium is cuboidal, the amnionic 

 epithelium is squamous. The "top plate" is a superficial denser layer of the cell; 

 "terminal bars" are thickenings of intercellular substance just beneath the surface 

 of the epithelium. (After Bremer, Text-book of Histology.) 



thickness of simple epithelia. The cells may be of such form that, seen in 

 sections perpendicular to the surface, their outline is approximately square. 

 Such an epithehum is commonly called cuboidal (Fig. 92), but unfor- 

 tunately so, for the cells are not cubes. They are prisms of usually hexa- 

 gonal form and with bases at opposite surfaces of the layer. Or a simple 

 epithelium may consist of greatly elongated cells with long axes extending 

 from base to free surface of the layer (Fig. 91C). Such a layer is called 

 colimmar — but incorrectly called cyhndrical for the cells are elongated 

 prisms, approximately hexagonal. At the extreme of thinness are epithelia 

 each of whose cells is a broad flat plate, hexagonal in outhne (Figs. 91-S 

 and 92), but so thin that, as seen in section perpendicular to the surface, 

 even under high power of a microscope it has scarcely perceptible thick- 

 ness except where the nucleus, itself much flattened, occasions a bulge in 

 the outline. Such a layer is called flat or squam.ous. 



Many columnar epithelia depart from being strictly "simple" in 

 having relatively small or short basal cells interposed among the basal ends 

 of the taller columnar cells (Fig. 93). 



Most of the ectodermal and endodermal epithelia are relatively thick, 

 ranging from taU to low columnar. The squamous form occurs commonly 



