214 ANIMALS 



magnesium, potassium, and others. From these the simpler 

 organic compounds, such as the carbohydrates, are formed, also 

 more complex nitrogenous substances like aleurone and finally 

 protoplasm itself. Animals, however, lack the power of build- 

 ing up protoplasm from its inorganic constituents. They 

 require food containing organic nitrogenous compounds like 

 aleurone, albumen and protoplasm. These may be supple- 

 mented by the simpler carbon compounds, like the carbo- 

 hydrates, fats and oils. The nitrogenous substances are 

 necessary wherever growth or repair are taking place, i. e., 

 wherever protoplasm is being formed. The carbon compounds 

 may be used as well as the nitrogenous where there is merely an 

 evolution of energy demanded, as in locomotion and the produc- 

 tion of heat. The details of the processes which take place in 

 the cell are not known. But w^hen foods are assimilated, 

 growth takes place and the cell becomes energized so that it is 

 capable of performing the functions peculiar to it. 



485. The results of the activities of the cell may be briefly 

 summarized: 



486. Growth is the most general result of assimilation, but 

 need not be further discussed here. 



487. In glandular cells, activity results in the formation of 

 the special secretions which are characteristic of the gland. 

 These may be used in the building up of permanent structures 

 of the organism, such as bone or cartilage, or the secretions 

 may have only a temporary value, and after they have served 

 their purpose, be eliminated from the body as slime and oil 

 from the glands of the skin. With this class may be included 

 those cells w^hich produce substances by the transformation of 

 protoplasm, although in the true glandular cell the secretions 

 have probably not reached the complexity of structure of proto- 

 plasm. Cuticular and epidermal structures are of the trans- 

 formed protoplasm type. 



488. The activity of muscle is manifested primarily by a 



