EPITHELIUM 



43 



FIG. 47. Diagrammatic fig- 

 ure of two cells and the side 

 of a third, t.b., terminal bar 

 seen from a lateral view on 

 the uncut side of a cell; t.b. 2 , 

 cross section of another ter- 

 minal bar whose two parts 

 are somewhat separated. 



the surface, there is developed a peculiar "closing plate" or terminal bar 

 that serves to close the whole layer of cells into an impervious layer or 

 covering (Fig. 47). The terminal bar is prob- 

 ably impervious to all gases, fluids, and other 

 materials, and it remains for the cell to deter- 

 mine what shall and what shall not pass into 

 or out of the body. This bar is double in 

 section, as can be seen when the two parts 

 are separated by dissolving the cement sub- 

 stance that holds them together (see Fig. 47). 

 Seen from a surface view, the bar is rod- 

 shaped, but it is not altogether a continuous 

 structure, being rather a series of closely set 

 granules. These granules are specializations 

 of the desmochondria found on the surface of 

 most cells, and the cytoplasmic fibrils that 

 end in the desmochondria are also found 

 ending in the closing plates. These same 

 desmochondria are found all over the surface 

 of the stratified epithelial cells of the mammals. Here the individual 

 desmochondria that compose the bars are entirely separate and give 

 the surface of the cell the appearance that was formerly known as the 

 prickles on the prickle-cell. The desmochondria that compose the ter- 

 minal bars of most cells of assimilation are particularly easy to see 

 individually. In all simple epithelial cells the terminal bars, since they 

 lie between all cells, are united into a reticulum, the meshes of which 

 conform to the outlines of the cells. This appears in a surface view of 

 the epithelium. 



We must keep in mind that these various features of the epithelial 

 cell, polarity, shape together with the large differences in structure due 

 to the differences in function are, as is mentioned above, the result of 

 their position on an inner or outer surface of the body. This idea can be 

 excellently understood by studying the dividing oosperm of the frog or 

 other amphibian, which will clearly show the origin of epithelium and 

 some first causes of its differentiation from the other cells of the body. 



The oosperm is at first a single cell whose entire surface touches the 

 exterior (Fig. 48, A}. There are no inner surfaces at this time. A divi- 

 sion into two, then into four, and again into eight cells, makes every cell 

 in the developing body have an outer surface and three sides, but no inner 

 surface, as the three sides bring it to a point. All cells are to be considered 

 as epithelial cells at this time, although they perform all the functions of 

 the body (Fig. 48, B). 



The next few divisions of these cells result in the formation of some 



