NON-STKATIFIED EPITHELIA 



37 



I. NON-STRATIFIED EPITHELIA 



1. SIMPLE SQUAMOUS EPITHELIUM 

 (Pavement Epithelium) 



This variety of epithelium comprehends two main groups: (1) the 

 endotlielia, lining the vascular system, and (2) the mesothelia of the 

 serous membranes lining the large internal closed cavities pleura, peri- 

 cardium and peritoneum. This distinction is 

 somewhat arbitrary but nevertheless useful, and 

 derives justification in that endothelia arise in 

 the first instance from syncytial mesoderm (mes- 

 enchyme) and mesothelium from epithelial meso- 

 derm. 



But according to Bremer (Amer. Jour. Anat., 

 16, 4, 1914), at least some of the earliest blood- 

 vessels in man also arise from true mesothelial 

 cells. Mesothelium lines the extra-embryonic 

 body cavity and is reflected over the yolk-sac and 

 body-stalk. In the latter location Bremer de- 

 scribes ingrowths of mesothelium into the mesen- 

 chyme, from which endothelium and blood-cells 

 develop. 



This classification should include also another 

 group of closed cavities, namely, the tendon 

 sheaths, bursae, joint or synovial cavities, cham- 

 bers of the eye, and the scalse tympani and vesti- 

 buli of the internal ear. These cavities arise as 

 splits, or by the union of isolated spaces, in the 

 mesenchyma, the mesenchymal lining cells taking FlG ^. 

 onepithelioid characters and arranging themselves MATIC ILLUSTRATION 



in the form of a membrane. In their method of OF ENDOTHELIUM LIN- 

 T . , . ,, T, , , ING A LARGE AR- 



denvation these cells resemble more closely the TERY 



earliest endothelial anlages. The most satisfac- 

 tory disposition of this group of epithelia seems to be to classify them 

 as 'false' or 'mesenchymal' epithelia, as prroposed by F. T. Lewis. 



Such epithelia have been experimentally produced by the introduc- 

 tion of small sheets of celloidin and masses of paraffin into the subcuta- 



