DIVISION OF LABOR 



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sional men, so among plants 

 and animals, wherever col- 

 lections of cells live together 

 to form an organism, there 

 is division of labor, some 

 cells being fitted to do 

 one kind of work, while 

 others are fitted to do work 

 of another sort. This 



Different forms of tissue cells. 

 C, bone making cells ; E, epi- 

 thelial cells; F, fat cells; L, liver 

 cells ; M , muscle cell ; i, invol- 

 untary; v, voluntary; N, nerve 

 cell; CB, cell body; N.F., nerve 

 fiber ; T.B., nerve endings ; 

 W, colorless blood cells. 



Enlarged lengthwise section of the hydra, a 

 very simple animal which shows slight 

 division of labor. ba, base ; 6, bud ; 

 ra, mouth; ov, ovary; sp, spermary. 



is called physiological division of 

 labor. 



As we have seen, the higher plants 

 are made up of a vast number of cells 

 of many kinds. Collections of cells 

 alike in structure and performing the 

 same function we have called a tissue. 

 Examples of animal tissues are the 

 highly contractile cells set apart for 

 movement, muscles; those which 

 cover the body or line the inner parts 

 of organs, the skin, or epithelium; the 

 cells which form secretions or glands 

 and the sensitive cells forming the 

 nervous tissues. 



Frequently several tissues have cer- 



