CHAPTER IV 

 GROWTH 



IN all normal plants and animals, anabolism 

 nearly always tends to exceed katabolism, with a 

 consequent increase in the bulk of the living sub- 

 stance. When this increase in volume is permanent, 

 we call it growth. Such changes are to be distin- 

 guished from temporary changes in size or form due 

 to the rapid imbibition of water or the evolution of 

 gases. They are also to be distinguished from dif- 

 ferentiating changes such as occur in development. 

 The latter may or may not be accompanied by the 

 increase in mass called growth. In plants, growth 

 is a phenomenon which generally continues as 

 long as the organism lives. In animals, it is a special 

 feature of the earlier period of the organism's life, 

 and then usually comes to an end. At that time 

 a balance of metabolism is struck, after which the 

 energies of the organism are directed, not toward 

 increase in size, but toward reproduction. For this 

 reason the period of greatest growth is coincident 

 with immaturity. The enormous disproportion in 

 the amount of growth in the earliest stages of exist- 

 ence is illustrated by some calculations of Professor 

 Hertwig. He estimates the volume J of the human 

 ovum at .004 cubic millimeter, whereas that of the 



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