THE EPITHELIAL TISSUES. 51 



in composition has led to the inference that the endothelium of the 

 capillary walls exercises an active function in determining what 

 shall pass through it ; that the lymph is a sort of endothelial secre- 

 tion. It is difficult, however, to reconcile this view with the fact 

 that the endothelial cells are so poor in cytoplasm. (See Lymph, 

 p. 120.) 



Endothelium is developed from the mesoderm. 



II. EPITHELIUM. 



General Characters. — (1) The cells are nearly always large and 

 rich in granular cytoplasm. They contain distinct round or oval, 

 vesicular nuclei, of which there is usually only one in each cell. 

 (2) The intercellular substance is very small in amount and is clear 

 and homogeneous. (3) The arrangement of the cells and their size 

 and shape all vary greatly, giving rise to a number of varieties of 

 epithelium, which are classified according to the shape and arrange- 

 ment of the cells. In pavement-epithelium the cells are thin and 

 arranged in a single layer, not unlike endothelium. In cubical 

 epithelium the cells are thicker and also usually arranged in but a 

 single layer. In columnar epithelium the cells are prismatic in form 

 and rest with their bases upon the surface of the tissues beneath. 

 They are usually separated at their bases by pyramidal cells, so 

 that the layer of epithelium cannot be said to consist strictly of but 

 one layer of cells, and in some situations there are several distinct 

 layers. In stratified epithelium the cells are superimposed upon 

 each other to form a layer of cells, the thickness of which is several 

 times the diameter of a single cell. The cells of the variety of epi- 

 thelium called ciliated epithelium differ from those of the other 

 varieties in possessing delicate, hair-like processes which project 

 from the free surface of the tissue. 



Epithelium resembles endothelium in being composed almost 

 exclusively of cells separated by a minimal amount of intercellular 

 substance. Like endothelium, it is nearly always found covering 

 other tissues and having one free surface. The two tissues differ 

 greatly in the character of their cells, with one notable exception. 

 This exception is found in the epithelial lining of the pulmonary 

 alveoli, where the pavement-epithelium contains cells that closely 

 resemble those of endothelium. These cells are, however, directly 

 exposed to the inspired air, while endothelium is only found in situa- 

 tions where it is protected from all contact with the exterior. 



1. Cubical Epithelium. — The cells of this variety of epithelium 



