PROBLEM *J How Are the Cells Arranged in Animals 



and Plants? 



How animal cells differ from one another. 



It has been known for a long time that 

 the body of an animal consists of cells 

 and that these cells are not all alike. 

 Blood cells are very different from skin 

 cells; muscle cells are different from 

 either of the other two. Cells vary in 

 shape or structure, in their position in 

 the body, and in the work they do. This 

 is not surprising, ^ut it may surprise 

 you to learn that cells in corresponding 

 parts of mice and elephants are much 

 alike in shape, activities, general appear- 

 ance, and even in size. You can, as a mat- 

 ter of fact, learn much about cells in 

 your own body by studying the corre- 

 sponding cells in the bodies of cats or 

 white rats or frogs or other animals. See 

 Figure 154. 



The different kinds of cells are found 

 in groups. Examination of the arm of man 

 shows that cells are arranged in groups. 

 The outside of the arm is made up of 

 flattened cells called epithelial (ep-e- 

 thee'lee-al) cells lying together in a 

 group. Under these are groups of cells 

 {gland cells) that differ from the flat- 

 tened cells; substances such as sweat and 

 oil come from these cells. Still deeper 

 in the arm are masses of fat cells. All of 

 these together make up what we call by 

 the simple name "skin." Under the skin 

 is \\hat you sometimes call the "flesh." 



This is composed of muscle cells in 

 groups. Among the muscle cells are nerve 

 cells, blood cells in the blood, and some 

 other types of cells. At the very inside 

 of the arm is the bone. The bone con- 

 tains a huge number of bone cells which 

 have a very characteristic appearance. 

 Among the bone cells there are also 

 nerve cells, blood cells, snd still other 

 kinds of cells. See Figures 157-159. The 

 important fact is that the different kinds 

 of cells are found in groups or masses. 

 Such a group of cells which are similar 

 in structure and which do much the 

 same kind of work in the body is called 

 a tissue. 



Sponge 



Fish 



Frog 



Cat 



Cow 



Fig. 154 Muscle cells of the sponge, fish, frog, 

 cat, and cow. Compare these with muscle cells 

 of man. Figure ifj, page 1^2. 



