STRUCTURAL CHARACTERS OF ANIMAL ORGANISMS. 37 



function performed by the epithelial tissues, namely, that of man- 

 ufacturing certain materials which, being collected by suitable 

 channels, appear as secretions. 



The active elements of glandular tissue are epithelial cells 

 whose nutrition seems to lead to the formation of specific chemical 



FIG. 10. 



Section of the epiderm of the prepuce, showing the superimposed layers 

 of cells of a stratified epithelium. (Cadiat.) a. Young proliferating cells. 

 b-d. Cells advancing towards surface, c. Flattened cell of horny layer. 

 /. Basement membrane, g. Connective tissue. 



products within their protoplasm. These products pass out com- 

 monly as fluids, and form various substances of great importance 

 in the economy. A gland then is simply a special arrangement 

 of epithelial cells, generally lining the sacs or tubes into which the 

 secretion is poured. 



The covering of epithelium is in various places found to be 



