EPITHELIUM 47 



the middle germ layer have so much in common, and from the time of their first ap- 

 pearance differ so materially from those derived from the two peripheral germ layers, 

 that it would be well to distinguish them by a special term either to contrast them, as 

 false epithelia, with the true, or to name them endothelia, thus expressing their relation 

 to the inner surfaces of the body. 



The name endothelium, etymologically absurd, has become generally 

 accepted for the lining of the blood vessels and lymphatic vessels. For 

 the other forms of epithelium which it was intended to include, special 

 names have been proposed. 



Minot (1890) introduced mesothelium to designate the layer of meso- 

 dermal cells which bounds the body cavity. Thus mesothelium does not 

 include the endothelium of the vessels, or the lining of the synovial cavi- 

 ties; but it does include the cells of the nephrotome, through which the 

 body cavity may extend, and also the epithelium which bounds the somites 

 in early stages. Professor Minot applies the term also to the thick epithe- 

 lium of the renal organs, which is derived from the cells of the nephrotome. 



As seen in Fig. 33, the epithelium lining the vessels closely resembles 

 that which lines the body cavities, and to a certain extent this justifies 

 the use of the term endothelium for 

 both layers as proposed by His. But 

 it has been shown embryologically 

 that the vessels and body cavity are 

 of different origin, and are distinct 

 even in the earliest stages. More- 

 over the linings of the synovial cavi- 

 ties, tendon sheaths, and the chambers 

 of the eye form a third separate group. 

 They arise relatively late in develop- A> Surface view of a ^3i uin from the mesen . 

 ment by the confluence of intercel- * e r r t y B> surface view of endothelium from an 

 lular spaces in the mesenchyma, and 

 they are therefore bounded by flattened mesenchymal cells. 



In accordance with these embryological facts, the following use of 

 terms is here proposed: 



Endothelium should be restricted to the lining of the blood vessels 

 and lymphatic vessels. 



Mesothelium, except in young embryos, should be restricted to the 

 lining of the body cavity and its subdivisions. 



Mesenchymal epithelium (or false epithelium) should be applied to 

 the lining of joint cavities and bursae. 



All of these forms of epithelium are primarily thin and are derived 

 from the mesoderm. The lining of the body cavity is, however, thickened 

 in places. Thick epithelium may be ectodermal, entodermal or meso- 

 dermal in origin. 



