312 EPITHELIAL AND ENDOTHELIAL TISSUE. 



germ, indicative of the symmetrical halves of the future organ- 

 ism, three layers are recognized. The so-called mesoblast, the 

 layer which forms the main bulk of the germinal disk, is covered 

 on its outer surface by a thin layer of flattened plastids, termed 

 the epiblast ; the under surface of the mesoblast is also covered 

 by a similar layer, which is termed the hypoblast. In the meso- 

 blast the first-formed vacuole, the future heart, is lined by a thin, 

 continuous layer, which is connected with all surrounding plas- 

 tids by means of delicate filaments. According to our ideas, the 

 mesoblast must give rise to the principal part of the organism, 

 composed of connective tissue, of muscles, and of nerves. The 

 nerve-layer is closely attached to the epiblast, and this is the 

 reason that, since Remak's time, it has been considered a forma- 

 tion of the epiblast. Such a conception, however, cannot be 

 correct, as the nervous system is largely intermixed with con- 

 nective tissue, carrying blood-vessels, and these latter for- 

 mations never appear in the derivations of the epiblast and 

 hypoblast i. e., the epithelia. The thin layer around the first- 

 formed vacuole is the representative of all the future lining 

 investing layers of closed cavities, and furnishes in the devel- 

 oped organism th,e tissue termed endothelium. 



All formations in a highly developed animal body which are 

 analogous to the outer or covering layer of a single plastid that 

 is, those which cover the external surface of the body and line all 

 the inner cavities in direct or indirect connection with the outer 

 surface are termed epithelia. Formations, on the contrary, which 

 are analogous to the wall of a closed vacuole of a single plastid, 

 bear the name of endothelia. Epithelia are found : On the outer sur- 

 face of the body, the skin and its appendages : the hairs, nails, seba- 

 ceous, sudoriparous, and mammary glands j the crystalline lens 

 of the eye ; in the system known as the gastro-intestinal tract and 

 its prolongations, the mucous and salivary, pepsine and intestinal 

 glands, and the liver 5 in the cavity of the respiratory tract and its 

 mucous glands ; and in the cavities and canals of the genito-urinary 

 tract, including all its prolongations into the kidneys and the 

 genital glands. Endothelia line the closed cavities of the sJcull and 

 the spine, all its covering membranes, and all ventricles in the 

 brain and their prolongation into the spinal cord ; the cavities of 

 the chest, both pleural and pericardial; the cavity of the peri- 

 toneum ; all articulations, and all blood- and lymph-vessels, includ- 

 ing the cavities of the heart. 



This distinction was first established by His, who, at the same 



