8 PLANT AND ANIMAL LIFE 



Differentiation of cells. In animals thus made up 

 of many cells, the cells have fixed positions in the body 

 and are crowded by one another into such permanent 

 shape as best adapts them to their position and work. 

 Unlike the single cell of the ameba, no one cell has the 

 power of doing all the kinds of work required by the 

 body, as moving the limbs, digesting food and breathing, 

 but each group of cells constitutes an organ which does 

 a special part of the body's work. The outer cells, for 

 example, do the work of protecting the inner ones and 

 for this purpose become tough and resistant, forming 

 the skin. The inner ones in turn carry on the spe- 

 cial functions of the body, as motion, digestion and 

 respiration. 



Individuality of cells. Although the cells have thus 

 lost the power of doing more than one kind of work for 

 the body as a whole, they nevertheless resemble the single 

 cell of the ameba in that they can do the various kinds 

 of work necessary to maintain their own existence. For 

 example, each one absorbs and uses the nourishment 

 which it needs to do its work and maintain its health 

 and strength. It takes in the oxygen necessary for the 

 production of energy 1 and throws out carbon dioxide 

 and other waste products. 



Forms of cells. The cells have further lost the 

 ameba 's power of radically changing their shape and 

 they cannot perpetuate life by mere division of them- 

 selves. Each cell is limited to the shape best adapted 

 to its service in the organism. If, for example, it forms 

 part of the deep layers of the skin, it is small and block- 

 shaped. If it forms a part of the connection between 

 the muscle and its bony attachment, it is long and 

 slender like a silk fibre. If it is a part of the muscle, 

 1 See Appendix, p. 316. 



