40 



APPLIED BIOLOGY 



FIG. 10. Upper fig- 

 ure shows that there 

 are many layers of 

 cells in epidermis, 

 those of Fig. 9 being 

 at the surface. The 



that the cells are closely joined together; in 

 fact, there is between them a cement substance, 

 which can be dissolved with some chemicals, 

 and the cells are then easily separated. 



Cells. We have chosen the frog's epi- 

 thelium as a convenient introduction to 

 cells. All other animal and plant tissues 

 which biologists have examined micro- 

 scopically have been found to have cells ; 

 and so the cells are regarded as the units 

 of which the bodies of organisms are com- 

 posed. Various forms of cells are shown 

 in Figs. 10, 14, 16, 18, and 20. The word 

 cell commonly means a cavity, and it was 



resent two cells in 

 surface and edge 

 views. 



lower diagrams rep- originally applied to plant cells (e.g. } in 

 cork and elder-pith which have cavities 

 in their substance) ; but it is now known 

 that most animal cells and many plant 

 cells do not have cavities. Nevertheless, the word cell has 

 become firmly fixed in biological language, and so we must 

 use it as the scientific word for 

 the elements or units of animal 

 or plant tissues. The spherical 

 structure seen near the center of 

 each cell of the frog's skin is the 

 most common form of nucleus; 

 but in some cells of other ani- 

 mals the nuclei (plural) are in 

 various forms ribbon-like, like 

 a string of beads, or even scat- 

 tered in fragments in the sub- 

 stance of the cell. Apparently FlG 1L Diagram of cells cty> 



every living Cell has a nucleus cell-wall; cb, cell-body; n, mem- 



of some one of these forms. bra ? e of nucle ^ s; black sp ts in 



nucleus are chromatm. (From 



The substance of a nucleus is Verwom.) 



