8 4 



COMPARATIVE ANATOMY 



membrane is composed of very thin, flat, scale-like cells, which 

 may be polygonal, and fit together by their edges, like the tiles 

 in a pavement, but more usually have serrated or sinuous 

 edges which dovetail into one another, like the wooden puzzles 

 of children (fig. 14, B}. Each scale-like cell has a distinct 

 nucleus which bulges out its central portion. In order to see 



i^.t^-*Z*. 



Fig. 14. 



A, stratified epithelium from the oesophagus of the rabbit, seen in section. In the lower 

 part of the figure the connective tissue and muscular layers are shown. B, pavement 

 epithelium from the mesentery of the Frog, silver nitrate preparation ; r, 2, 

 goblet cells from the frog's mouth ; I)i, 1)2, isolated ciliated epithelium cells from 

 the frog's mouth ; Dj, an isolated ciliated cell from the gill of the mussel ; C, 

 columnar epithelium from the intestine of the frog. (From a drawing by Dr K. H. 

 Schuster.) 



the outlines of pavement-epithelial cells it is necessary to treat 

 the membrane with nitrate of silver and expose it to the light. 

 The intercellular substance between the cells has the property 

 of being able to reduce the nitrate in the sunlight, and is 

 accordingly filled with particles of metallic silver, which mark 

 out the outlines of the cells in black. 



