AN INTRODUCTION TO ANIMAL BIOLOGY 



39 



to be covered; cartilage wherever flexibility combined 

 with considerable rigidity (e.g., at ends of bones) is needed; 

 connective tissue wherever other tissues must be joined to- 

 gether; bones for supporting framework; and nervous 

 tissue in all places where nervous activity (feeling, sensation, 

 control, etc.) is needed. Each tissue has its peculiar purpose, 

 just as wood, bricks, iron, have their own purposes. Briefly, 

 the purposes or functions of the tissues are as follows : epithe- 

 lium for covering, connective tissue for uniting, bone for 

 rigid support, cartilage for flexible support, muscular tissue 

 for contraction and movement, and nervous tissue for feel- 

 ing and control. 



So far we have been studying the larger structure of the 

 frog as seen by the unaided eye. We have been able to 

 locate the various organs and to learn something about their 

 general form or position; but concerning the structure of 

 the organs themselves our unaided eyes have been able to 

 discover only the tissues. It is now necessary to make use of 

 the microscope in order to see the minute structure of the 

 tissues which we find in organs. 



(L) A lesson on use of the compound 

 microscope should be introduced at this 

 point. 



39. Cells. (D) Mount a small piece of 

 the outer skin (epithelium) of the frog in 

 a drop of water on a glass slide and cover 

 with a cover-glass. Examine this with a 

 compound microscope (magnification 50 

 to 100). The epithelium is seen to be 

 composed of small (usually hexagonal) 

 blocks, called cells, set side by side like 

 bricks in a wall or pavement (see Fig. 9). 

 A small spherical mass (called nucleus) 

 may be seen near the center of each cell, 

 and in most of the cells the nuclei may be 

 brightly stained by dipping a piece of epithelium into a dye such as 

 eosin solution (red ink), then into water, and then mounting on a 

 glass slide for microscopic examination. In the same slide notice 



FIG. 9. Group of cells 

 from surface epithelium 

 of frog, s, opening of 

 a skin-gland (From 

 Holmes.) 



