42 



APPLIED BIOLOGY 



creted by the cells. In an epithelial tissue, such as the frog's 

 outer skin, there is comparatively little inter-cellular sub- 

 stance. In some tissues there is a large proportion of inter- 

 cellular substance, and an example will now be examined. 



(D) With a sharp knife or razor cut very thin slices of cartilage 

 or "gristle" from the joint end of a bone procured at a butcher's 

 shop, or from a frog's bone. Mount these slices in a drop of water 

 on a slide. Examine with a microscope. Notice that the greater 

 part of the tissue is a bluish-white substance in which the cells lie. 

 The distance of one cell from another is often more than the diameter 

 of the cells (see Fig. 12). Was this so in the epithelium of the skin ? 

 It is evident that the greater bulk of cartilage is not cells, but the 

 substance between the cells (i.e., inter-cellular). 



A section of dry bone is similar to cartilage in that there are 

 numerous small cavities in which origi- 

 nally were located the cells that formed 

 the hard bony substance, which is en- 

 tirely inter-cellular (Fig. 13). 



If we were to 

 examine all the 

 tissues besides 

 bone and cartilage 

 and all the organs 

 of the frog's body, 

 everywhere we 

 should find cells 

 and inter-cellular 

 substance. We 

 may therefore 

 make the general 

 statement that 

 the body of the 

 frog is composed 



FIG. 14. Unstriated muscle f Cells and illter - 

 cells from frog's intestine. Cellular Substance. 



nit, nucleus. Compare with B t th } tt j formed or secreted 



m.c. in Fig. 19. (From 



Howes.) by the cells, and hence the cells are 



FIG. 15. Parts of 

 three striated 

 muscle cells from 

 a leg, showing 

 many nuclei (n) in 

 each cell, and 

 blood-capillaries 

 (black lines) be- 

 tween cells. 



