EPITHELIUM 



47 



the alligator, b.m., basement mem- 

 brane. 



secreting an oily substance for lubrication (Fig. 51). This last modifi- 

 cation is rare, as can be imagined when we consider that all the mate- 

 rials to be used in secretion by the outer 

 cells must be passed out to them by the 

 epithelial cells that lie between them and 

 the basement membrane. This calls into 

 play an extra, and what nature seems to 

 consider a needless, effort, and one that 

 is avoided wherever it can be. 



For such reasons the secretion of ma- 

 terials is nearly always confined to a 

 simple epithelium, which, to get the re- 



. , , , .. . . , FIG. 51. Pseudostratmed secreting 



quired body Of Cytoplasm, IS Composed epithelium from the conjunctiva of 



of much elongated cells, and thus be- 

 comes columnar. The secretion may be 

 delivered in the form of a fluid or of granules. It may change from 

 one to the other after delivery, and before its use, or it may be trans- 

 formed into gas at or about the time of delivery. 



The secretion is produced, sometimes at the expense of the cell, as 

 has been mentioned in the case of stratified epithelium. In mucous 

 and other columnar cells it is produced at the expense of the cell's distal 

 cytoplasm, which is later regenerated. 



The outer edges of simple epithelial cells show many modifications, 

 intended to be of service to them in their activities. In other forms the 



production and delivery is steady and con- 

 stant, and the cell is always in the same 

 condition physiologically and structurally. 



The cell-product is sometimes a hardened 

 edge or a continuous cuticle which is pro- 

 duced jointly by all the surface cells. In 

 other cases it is a striated border, which 

 may or may not be furnished with cilia. 

 Cilia are shown in figure 52, which repre- 

 sents the digestive epithelium of a small ple- 

 cypod mollusk, Cyclas. As can be seen here, 

 the cilia enter the cell and converge in a 

 course through the cytoplasm until they 

 arrive, as a single fiber, at the side of the 

 nucleus. 



Technic. The very simplest methods are 

 of the widest use in studying this tissue. 

 Two special methods should be mentioned. The use of a little 

 nitrate of silver by brushing it on the fresh tissue after washing with 



FIG. 52. Cells from the diges- 

 tive tract of a plecypod mol- 

 lusk, Cyclas. 



