EPITHELIUM 



81 



rived from a subdivision of the mesoderm which he designates the 

 mesothehal layer. 



Mesothehum and endothehum are similar in structure. Each 

 consists of thin flattened cells with clear or slightly granular proto- 



FiG. 28. — Mesothelium from Omentum of Dog Treated according to Technic 7, p. 

 S2. X350. Black wavy lines indicate the intercellular cement substance. The 

 mesothelial cells cover the strands of connective tissue, the fibres of the latter being 

 visible through the transparent cell bodies. 



plasm and bulging oval or spherical nuclei. The edges of the cells 

 are usually wavy or serrated. The cells are united by an extremely 

 small amount of intercellular "cement'' substance, which is usually 

 indistinguishable except by the use of a special technic 



Fig. 29. — The Endothelium of a Small Blood-vessel. Silver nitrate stain. X350. 



Endothelium forms the walls of the blood and lymph capillaries 

 and lines the entire blood-vessel and lymph-vessel systems. 



Mesothelium lines the body cavities — the pleura, the pericardium 

 and the peritoneum. 



Recent researches make it seem probable that the surface cells 



