EPITHELIAL AND ENDOTHELIAL TISSUE. 313 



time, maintained that the epithelia were an offspring of the epi- 

 blast and the hypoblast, while the endothelia were formations of 

 the mesoblast, and closely allied to connective tissue. A perfect 

 distinction between epithelia and endothelia, however, cannot 

 be carried out for two reasons. First : The history of develop- 

 ment and the scheme of a single plastid demonstrates that both 

 epithelia and endothelia are originally flat layers of living matter, 

 which split up into plastids, the only difference between them 

 being that the epithelial layer is on the surface of the body, while 

 the endothelial layer lines closed cavities in the interior of the 

 body. Secondly : There is a direct communication between epi- 

 thelia and endothelia at the openings of the uterine tubes into 

 the peritoneal cavity ; the epithelial formations of the ovaries are 

 in no communication with the outer world ; the crystalline lens, 

 which is in its nature a formation completely epithelial, is covered 

 by the endothelium of the anterior and posterior chambers of the 

 eyeball ; the epithelia of the liver join the endothelia of the capil- 

 laries, and are in direct connection with them. Further, epithe- 

 lia and endothelia are identical in their intimate structure. Again, 

 there exist single epithelial layers in the body f. i., those of the 

 bile-ducts, the salivary glands, the uriniferous tubules; and, on the 

 other hand, ciliated endothelium is also found in the ventricles of 

 the brain and the central canal of the spinal cord, etc., etc. 



Epithelia and endothelia represent continuous investing layers 

 of living matter. The former are the earliest formations in a 

 developing body, after the stage of indifference introduced by the 

 segmentation of the impregnated germ is passed. The latter are 

 formations, starting with the appearance of vacuoles in the middle 

 layer of the germ, the future heart and vessels. Both epithelia and 

 endothelia are devoid of blood-vessels and lymphatics. 



Structure. The epithelial and endothelial layers are con- 

 structed of single polyhedral plastids, the formerly so-called 

 " epithelial and endothelial cells." Each plastid is separated 

 from its neighbors by a narrow cloak of a lifeless, horny cement- 

 substance, which is closely allied to the basis-substance of the 

 connective tissue, but is not glue-yielding. Concerning the chem- 

 ical constitution of these substances nothing is determined. Un- 

 der the microscope we can see only the lateral portions of the 

 envelope, which has the appearance of a pale seam around each 

 polyhedral body. (See Fig. 132.) 



The net-work of living matter within the plastids sends delicate 

 conical offshoots through the cement-substance, both in epithelia and 



