CELLULAR DIFFERENTIATION. 



43 



materials, called secretions. In its simplest form the gland or secreting 

 surface may consist of a single cell, as the goblet or slime cells (Fig. 15,0). 



FIG. 14. 



,cu, 





FIG. 14. Various kinds of epithelial cells (semi-diagrammatic). A, columnar; B, 

 cuboidal; C, pavement; D and E, ciliate (sectional views). In F is shown the surface 

 view of pavement epithelium, cl., cilia; en., cuticula. 



Questions on the figure. For what different uses would you judge 

 these variously shaped epithelial cells to be suited? Under what circum- 

 stances and on what surfaces would you expect to find each type? Com- 

 pare with your reference texts and see if they are so found. Under what 

 circumstances is a cuticula to be expected? Where would it be a disad- 

 vantage? What are cilia? 



FIG. 15. 



,-a. 



FIG. 15. Glandular Epithelium, a, goblet or slime cells, unicellular glands; b, 

 similar cells which have become depressed below the surface, and empty their secretion 

 through a duct. 



Questions on the figure. Are the glandular cells modified epithelial 

 cells ? In what respects do they differ from the cells about them ? 



