GROWTH, AGE, AND DEATH 291 



growth of differentiated tissues is a more or less continuous process, 

 for as maximum size is reached the animal divides. Thus in the 

 planarian worm fission occurs when growth slows and instead of 

 one old animal there are two young animals entering a period of 

 more rapid growth. 



Growth rate varies with a number of circumstances. In the first 

 place, rapidity of growth fluctuates normally during the develop- 

 mental period. Secondly, the various organs of the animal body do 

 not all grow at the same rate; some, for example the human brain, 

 attain their final size earlier than others. Moreover, some tissues 

 continue to grow indefinitely after the growth of others has ceased. 

 For example, the surface of the human skin that is being con- 

 stantly worn off is replaced by the continuous growth and cell 

 divisions occurring in the lower layers of the epidermis. Growth of 

 this sort in the adult is essentially the replacement of lost cells; the 

 total tissue so produced does not increase the size of the animal. 



Third, size is an inherited character; hence because of inherited 

 qualities some individuals in a species grow more rapidly than 

 others. Fourth, the supply and quality of food and the general 

 vigor of the organism are important not only in determining 

 rate of growth but also in regulating the final size. 



The basis of all organic growth is the holophytic metabolism of 

 green plants, converting inorganic substances into forms that may 

 be used in constructing plant and animal protoplasm. All growth 

 is then initiated by plant metabolism; all living forms are thus 

 linked together, from the minute nitrogen-fixing bacteria to the 

 most advanced animal, as a consequence of the complete depend- 

 ence of animals on plants for the materials and energy for growth. 

 Animal growth is dependent upon a food supply containing com- 

 plex compounds, proteins, fats, and carbohydrates. In the develop- 

 ment of the embryo these are supplied by materials stored in the 

 tgg, or by the maternal blood. With the development of the organs 

 of nutrition in the oviparous animals and after birth in the vivi- 



