Chapter VIII 

 GROWTH, AGE, AND DEATH 



Age and the Passage of Time. We watch the animals with 

 which we are famiHar in everyday Hfe grow to maturity, even- 

 tually reach a stage of decreased activity which we call old, and 

 finally, even if they have escaped accident and disease, die. Conse- 

 quently there is a general interest in what constitutes growth, what 

 is the significance of age or senescence, and why does death ensue. 

 One's interest is the more acute because the human body goes 

 through exactly these experiences. 



Recent investigations on the development of some invertebrates 

 have indicated that within a very brief time after an animal egg 

 is fertilized it begins to consume oxygen very rapidly. After soon 

 reaching a maximum, the rate of respiration in proportion to mass 

 starts to decline, rapidly at first, then more and more slowly until 

 in adult life the decline, if it continues at all, is very slow indeed. 

 The deceleration in rate of respiration is not steady but fluctuates 

 with temporary events in the life of the animal; the general trend 

 is downward, however. The rate at which oxygen is being utilized 

 by protoplasm is an index of the rate of energy transformation; 

 hence more energy is being utilized by the developing egg in pro- 

 portion to mass of living protoplasm than at any other time of 

 life. If we think of age not in terms of lapsed time but in terms 

 of ability to transform energy, an animal grows old much more 

 rapidly as an embryo than as an adult. 



Growth. Similar rules hold true with rate of growth. Growth, 



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