ON THE TERM ENDOTHELIUM. 221 



quently endothelium means " that which is itiside a papilla ." 

 The extension of the phrase epithelium to the cellular covering 

 of such parts of the corium as are destitute of papillae may be 

 easily allowed, but it does seem a most daring defiance of all 

 meaning of words to apply the phi-ase " within the papilla " 

 to the cells coatinr/ surfaces of which one great characteristic 

 is that they are devoid of papilla? ! There seems to be some- 

 thing attractive about " thelium " that tempts writers to 

 make use of it. Already endothelium has given rise to a new 

 " ectothelium," and probably after a few years "thelium" 

 will become a sort of histological maid-of-all-work, Avith as 

 many prefixes as there are kinds of cells. 



In the second place, there are objections to the use of 

 endothelium not etymological in their nature. 



The peculiar views of His on the origin of the connective 

 tissues of the body would, if true, afford a strong argument 

 for the use of some special term to denote such kinds of 

 epithelium as were formed out of his parablast. Putting these 

 aside as mistaken, there still remains the question whether it 

 is not desirable to have some distinctive appellation to denote 

 the epithelium Avhich is formed out of the elements of the 

 middle of the three layers of the germ (the mesohlast of Mr. 

 Huxley and myselfj, the word epithelium itself being reserved 

 for the nether layer (or hypoblast). 



If so, the word endothelium cannot be employed with this 

 meaning, for it would then include structures still called epi- 

 thelium, and differing in no essential characters from the 

 epithelium derived directly from the hypoblast. 



The cells lining the Wolffian duct, and its derivative the 

 ureter, with their branches, would then come under the head- 

 ing endothelium. Whatever be the exact mode of the first 

 formation of the Wolffian duct, Avhether by the central solu- 

 tion of a solid ridge, or by an infolding of the lining of the 

 pleuroperitoneal cavity, it is lined by cells which are clearly 

 mesoblastic in origin, not hypoblastic nor, as Avas once sug- 

 gested, epiblastic. 



The case of Miiller's duct is still more clear. This un- 

 doubtedly arises by an infolding of the lining of the pleuro- 

 peritoneal cavity. Its epithelium is distinctly mesoblastic in 

 origin. The germinal epithelium which gives rise to the 

 ovaries is also essentially mesoblastic. 



If the word endothelium, then, be taken to denote an epi- 

 thelium derived from the mesohlast, it must be extended to 

 include the epithelium of the Wolffian and Miillerian ducts, 

 and of the parts which are formed ultimately out of those 

 structures. But if these be included, the phrase loses all its 



