HISTOLOGY 



87 



inner layer of the gastrula, giving rise to various organs such as the liver, 

 pancreas and lungs which grow outward from the enteron, otherwise 

 persists as the lining of the digestive tube, the digestive epithelium, 

 which is the innermost epithelium of the adult body. By far the greater 



Term. bar. 



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Top plat 







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Conn, tissue. Squam.epith. 



Fig 77. — 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. (From Bremer, "Text-book of Histology.") 



part of the massive adult has been inserted between the two primary 

 layers. 



The term endothelium is commonly applied to the lining layer of 

 blood-vessels and lymphatics. Mesothelium may be used for the peri- 

 toneal epithelium. Cells may 

 form a layer resembling an epithe- 

 lium but not abutting upon a 

 cavity. The tissues of some endo- 

 crine glands are of this nature. 

 To such tissues is applied the adjec- 

 tive epithelioid. 



Fig. 78. — Types of epithelia. B. simple squamous; C, simple columnar; D, strati- 

 fied columnar, ciliated a.t E; F, stratified polyhedral, upper cells squamous. (From 

 Kingsley.) 



Epithelia carry on functions of most diverse kinds. The diversity 

 is reflected in the structure of epithelia. Only a few of the more general 

 features of structure can be mentioned here. 



