EPITHELIAL AND ENDOTHELIAL TISSUE. 



321 



appear partly homogeneous or partly nucleated, with offshoots. 

 These indifferent corpuscles are evidently the starting material 

 for newly growing epithelia. The connection of these wedged 

 plastids in situ is accomplished by delicate filaments, which trav- 

 erse the surrounding rims. The filaments themselves often 

 increase in bulk and give rise to new epithelia. 



Ciliated endothelia are met with in the investment of the 

 ventricles of the brain and their continuation, the central canal 

 of the spinal cord, i. e., the so-called ependyma of Purkinje ; and 

 also on the surface of the plexus choroidei of the pia mater. In 

 children their presence is invariable, but in adults they are not 

 found in every case. In the central canal of the spinal cord the 



FIG. 138. CENTRAL CANAL OF THE SPINAL CORD OF A CHILD. 

 TRANSVERSE SECTION. 



E, wreath of ciliated endothelia, in connection with the subjacent connective-tissue ; N, 

 nerve-fibers of the anterior commissure ; W, white substance; f, floor of the anterior longi- 

 tudinal fissure, covered with flat endothelia. Magnified 30Q diameters. 



ciliated endothelia have a radiating arrangement, and in the 

 posterior wall of the canal exhibit a gradual decrease in size 

 toward the median line, where a slight fissure is observed. Here 

 the cement-substance between the endothelia and also their 

 nuclei are, as a rule, but slightly marked, while the cement-layer 

 at the base of the endothelial wreath is quite broad. (See Fig. 

 138.) 



Single epithelial layers may exhibit any of the above-named forms 

 (flat, cuboidal, columnar} ; in the uriniferous tubules, for instance, 

 21 



