THE CELL-DOCTRINE 



147 





/Irfcriole 



FIG. 21. Diagrammatic Vertical Section through Skin of Frog. Showing 

 cellular structure and parts. 



shaped cavity, with the neck opening as a pore on the outer 

 surface of the skin (Fig. 21). Lining the gland is a single 

 layer of cells. Outside is a network of capillaries, through 

 which passes a constant flow of blood. Each gland-cell 

 produces within itself a substance, which, if not the actual 



