EPITHELIA. 27 



ORIGIN*. 



Epithelia arise from all three of the germ layers as described in the 

 section on Histogenesis. The terms which may be applied to adult epithelia 

 indicating their origin are ectodermal, entodermal, mesodermal, mesothelial, 

 and mesenchymal. Mesothelium is a term applied sometimes to all meso- 

 dermal epithelia except the mesenchymal. There is a tendency, however, 

 which seems desirable, to limit its application in the adult to the pericardial, 

 pleural, and peritonaeal epithelia. Endothelium is from the "angioblast" 

 and lines the heart, the blood vessels and the lymphatic vessels only. 

 The loose but rather common application of this name to mesothelium and 

 mesenchymal epithelium is much to be regretted. Mesenchymal [or 

 false] epithelia are formed by flattened mesenchymal cells, developing 

 relatively late in embryonic life. They line the bursae, tendon sheaths, 

 joint cavities, the chambers of the eye, and the scalae tympani and vestibuli 

 of the ear. The table on page 26 indicates to which germ layer the 

 epithelia belong. 



SHAPES OF EPITHELIAL CELLS. 



Epithelial cells may be grouped, according to their shape, in three 

 classes,^/, cuboidal, and columnar. These names apply to the appearance 



"Me so. 



FIG. 24. AMNION OF PIG. (A FETAL MEMBRANE COVERING THE EMBRYO.) 



S. C. Epi., Simple cuboidal epithelium; Mesen., a mesenchymal tissue; Meso., mesothelium, a simple 



flat epithelium. 



of the cells when cut in a plane perpendicular to the free surface. On 

 surface view all three kinds are usually polygonal and often six sided. 

 If the epithelium consists of but a single layer of cells it is called simple. 

 Fig. 24 shows along its upper surface a simple cuboidal epithelium. The 

 sections of its cells are approximately square. On the lower surface is a 

 simple flat epithelium, which, being an extension of the layer lining the 

 coelom, is a mesothelium. A surface view of mesothelial cells on a smaller 

 scale is shown in Fig. 25, A. Endothelium, Fig. 25, B, is quite like meso- 

 thelium in appearance; its cells, however, are usually more elongated, 

 parallel with the course of the vessel which they line. It is a simple epi- 

 thelium, so flat that the thickest part of its cell is that which accommodates 



