343 
The cells lining the Wolffian duct and its derivative the 
ureter, with their branches, would then come under the heading 
endothelium. Whatever be the first formation of the Wolffian 
duct, whether by the central solution of a solid ridge, or by an 
infolding of the lining of the pleuro-peritoneal cavity, it is lined 
by cells which are clearly mesoblastic in origin, not hypoblastic, 
nor, as was once suggested, epiblastic. 
The case of Miiller’s duct is still more clear. This undoubt- 
edly 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 mesoblast, it must be extended to 
include the epithelium of the Wolffian and Miillerian ducts, 
and of the parts which are formed ultimately out of these 
structures. But if these be included, the phrase loses all its 
practical utility. If they are excluded, all the little meaning it 
ever had, vanishes. 
It may be urged that we need a word to denote the epithe- 
lium which is found in the vascular and lymphatic spaces. 
There does not however appear to be sufficient reason why the 
same term should be applied to the whole of this epithelium. 
As we have seen, its common mesoblastic origin will not justify 
this. From a structural point of view, three distinct varieties 
may be recognized in it, viz. the spindle-shaped ceils of the 
blood-vessels and larger lymphatic vessels, the smuous cells of 
the commencing lymphatics, and the polygonal cells of the 
large serous cavities. The fact that the epithelium of the peri- 
toneum is continuous with that of the lymphatics, affords no 
argument at all for classing them together. We find continuity 
everywhere. The epidermis is continuous with the alimentary 
epithelium, and with the urimary and generative epithelium ; 
-and the generative epithelium is in turn continuous with the 
