32 



THE STEUCTUEAL UNITS OF LIVING MATTER 



scrapings resolve themselves into a collection of irregular 

 bodies whose most prominent structure is a round particle 

 imbedded in what seems to be a granular liquid. The 

 bodies are cells, the particle is a cell nucleus, and the gran- 

 ular liquid is called protoplasm. 



If we examine in the same way a little of the common 

 green pond scum (Spirogyra) we shall see that it also is 

 made up of oblong bodies which contain a nucleus and 

 protoplasm (see Ex. XXIII.). These plant cells differ 

 from the cheek cells, in having a distinct boundary wall, 

 and appear to be filled with long spiral bands of a green 

 color. 



Both the bodies mentioned above are typical cells, 



Fig. 9 — Diagram of a cell. [After Wilson.] Notice in this diagram of a typical 

 cell the cell protoplasm, composed of cell plasm and cell lymph, and containing 

 cell foods, C f. and cell sap C.s. The large spherical nucleus has a true nucle- 

 olus t.n., a false nucleolus /.»., a nuclear network N.n., and nuclear lymph N.l. 

 c is the centrosome. 



and all plants and animal tissues are built of such struc- 

 tures. For this reason the cell is called the structural unit 



