THE STRUCTURE OF THE ELEMENTARY TISSUES. 23 



the lymphatic system. They should be distinguished from smaller and 

 more numerous apertures between the cells which are not lined by small 

 cells, although the surrounding cells seem to radiate from them, filled 

 up by intercellular substance or by processes of the cells underneath. 

 These are called pseudo-stomata (Fig. 16). 



In the neighborhood of the stomata, the cells often manifest indica- 

 tions of germinating. They may be either large with two or more nu- 

 clei, or about half the size of the generality of cells. Germinating cells 

 of this kind or of the kind above described, are generally very granular. 



2. Spheroidal epithelial cells are the active secreting agents in most 

 secreting glands, and hence are often termed glandular ; they are gene- 

 rally more or less rounded in outline : often polygonal from mutual 

 pressure. 



Excellent examples are to be found in the liver, in the secreting tubes 

 of the kidney, and in the salivary and gastric glands (Fig. 19). 



FIG. 19. Glandular epithelium. A, small lobule of a mucous gland of the tongue, showing 

 nucleated glandular spheroidal cells. B, Liver cells, x 200. CV. D. Harris.) 



3. Columnar epithelium (Fig. 21, a and b) as a single layer, lines 

 (a.) the mucous membrane of the stomach and intestines, from the car- 

 diac orifice of the stomach to the anus, and (b.) wholly or in part the 

 ducts of the glands opening on its free surface ; also (c.) many gland- 

 ducts in other regions of the body, e. g., mammary, salivary, etc. 



Columnar epithelium consists of cefls which are cylindrical or pris- 

 matic in form, and contain a large oval nucleus. They vary in size and 

 also in shape to a certain extent, the outline being often irregular from 

 pressure of neighboring cells, but speaking generally one end of the cell 

 is narrower than the other, and by this end the cell is attached to the 

 membrane beneath. The intercellular and internuclear network are 

 well developed. 



The columnar epithelial cells of the alimentary canal possess a struc- 

 tureless layer on their free surface : such a layer, appearing striated 



