NON-STRATIFIED Kl'ITIIELIA 



37 



I. NON-STRATIFIED EPITHELIA 



1. SiMi'u: SQUAMOUS 



(Pavement E [lilli cTnim} 



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

 endofhelia, lining the vascular system, and (2) the mesotliclia of the 

 serous memhranes lining the large internal closed cavities pleurae, peri- 

 cardium and peritoneum. This distinction is ,. 



somewhat arbitrary hut 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, bursa?, joint or synovial cavities, cham- 

 bers of the eye, and the sea la? 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 4 i._s EMIDIAGRAM . 

 onepithelioid characters and arranging themselves MATIC ILLUSTRATION 



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

 , . ,. ,, ING A LARGE AR- 



(lerivation these cells resemble more closely the TERY 



earliest endothelia 1 anlages. The most satisfac- 



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



as 'false' or 'mesenchymal' epithelia, as proposed 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- 



