54 



OUTLINES OF EVOLUTIONARY BIOLOGY 



The term epithelium is applied to any layer of cells covering, 

 a free surface. An epithelium is therefore primarily a protective 

 layer, but it frequently becomes modified for other purposes. It 

 may, for example, become glandular, certain of its cells taking on 

 the function of secretion, or it may become sensory, with cells 

 specially adapted for the reception of stimuli. It may consist of 

 a single layer of cells or of several layers one above the other. 



A good example of the single-layered type is found in the 

 peritoneal epithelium which covers the surface of the mesentery 

 or membrane supporting the intestines in the coelom or body 

 cavity. Fig. 16 represents a portion of such an epithelium in 



which the cell -outlines have 



^^^^^^fj^^^^Si been rendered very distinct ( 



by staining with silver 

 nitrate; the nuclei, how- 

 ever, are not shown by this 

 method. Each cell has the 

 form of a thin, flat, poly- 

 gonal plate, and they all 

 fit accurately together at 

 their edges. With this 

 Figure should be compared 

 Fig. 28, A, which represents 

 a single-layered epithelium 

 prepared in such a way as 

 to show both nuclei and cell- 

 outlines. 



If we gently scrape the inside of the cheek with some clean, 

 blunt instrument, and examine the milky-looking product unde, 

 the microscope, we shall find that it contains a number o 

 flattened, scale-like bodies (Fig. 17), either entirely separate* 

 from one another or still more or less connected together by thei 

 edges, and probably to some extent overlapping. These also ar< 

 epithelial cells, which have formed part of the special epithelium 

 known as the epidermis, which is derived from the epiblast or 

 external cell -layer of the embryo and covers the outer surface of 

 the body. If we examine our preparation more carefully we shall 

 find that the cells have an irregularly rounded contour and that 

 they measure about 0'08 mm. in diameter. There is a more or 

 less centrally placed nucleus (which appears dark in the figure 

 owing to the manner in which it has been stained) and the 



FIG. 17. Five isolated Epithelial Cells 

 from the inner Surf ace of the human 

 Cheek, X 420. (From a photograph.) 

 The nuclei are stained darkly. 



