40 EPITHELIUM AND EXDOTHELIUM. 



or goblet-shape, containing in the part which is next the tissue 

 of the villas, a bit of protoplasm of variable size, refracting 

 light strongly; within this is included a compressed, nuclear 

 body. Amongst the isolated cells, also, we meet with nume- 

 rous goblet-shaped ones, which may be examined in various 

 positions. These cells are most numerous in the intestines of 

 the dog and cat, in which it often occurs in preparations which 

 have been kept in dilute chromic acid, or bichromate, that the 

 epithelium is almost entirely transformed into goblet cells. The 

 facts show that they are transformations of C3 7 lindrical epithe- 

 lial cells, and that they may either be produced spontaneously, 

 or, as more commonly happens, may be the product of certain 

 reagents. 



Pavement Epithelium. This variety is well known to 

 occur, chiefly as laminated . epithelium, in the conjunctiva 

 corneae, mucosa of mouth and pharynx of mammals, and in 

 the skin. In the urinary bladder of mammalia the epithelium 

 is not purely pavement, but is mixed with, and shades off into, 

 the cylindrical variety. We accordingly call it "transitional." 

 The epithelium of the frog's urinary bladder is a single la} r er 

 of pavement epithelium. That of the serous membranes, of 

 the membrana Descemeti, and of the iris, consists mostly of a 

 single layer of flat cells. 



Fresh specimens of the epithelium of the mouth may be pre- 

 pared either with indifferent reagents or with very dilute solu- 

 tion of bichrojnate of potash; but, if we wish to study the 

 relation of the various layers of the laminated epithelium to 

 each other, it is needful to make vertical sections through the 

 superficial layers of the mucous membrane. To study the 

 forms of the various cells of the separate layers, we may ob- 

 tain a thin shred from the surface of the tongue or gums of a 

 mammal by energetically scraping it with a scalpel. What is 

 removed is broken up with needles, and covered either in half 

 per cent, solution of common salt, or, what is quite as good, a 

 very weak solution of bichromate of potash. In the surface 

 layers of the epithelium, we find flat tablet-shaped cells, with 

 small, oblong, strongly refracting nuclei; the borders of these 

 cells are sharp and doubly-contoured. Their substance is 

 mostlj 7 clear, containing only a few granules, generally situated 

 in the immediate neighborhood of the nucleus. Their surface 

 is generally beset with irregular folds and furrows. If one of 

 these cells is seen edgewise it appears spindle-shaped, because 

 the thickness of the nucleus is greater than that of the cell. 

 Besides these we find smaller polyhedric pavement cells, which 

 consist of a nearly uniformly granular protoplasm, and possess 

 one, or very rarely two, roundish, clear, and sharply-defined 

 nuclei, with one or two large granules i. e., nucleoli within 

 them. Finally, if we have scraped very energetically with the 



