CHAPTER X 



GROWTH AND REPRODUCTION 



Whenever during the hfetime of an animal assimilation exceeds 

 dissimilation, there results an increase in the actual amount of protoplasm 

 in the body; this increase is termed growth. When the reverse is true and 

 dissimilation exceeds assimilation, the body shrinks in size; this process 

 is known as emaciation. Many animals continue to grow throughout 

 their Hfetimes, although growth is more rapid at the beginning and slows 

 up more and more with advancing age. This is true of many cold- 

 blooded vertebrates, in the case of which size is a fairly clear index of 

 age, other conditions being equal. Of course, in this case care must be 

 taken to judge of the amount of available food, for in an environment in 



Birth 



Age 21 



Age 45 



Age 60 Death 



Fig. 14. — Diagram illustrating the growth cycle in man. This is intended to be typical, 

 but individual growth cycles vary greatly, both as to the span of the whole and the propor- 

 tionate lengths of different periods. 



which food is hmited a Hmit is also set to the size of the animal, and no 

 matter how old it may grow it will never equal in size an animal hving 

 under more advantageous conditions. 



70. Growth Cycles. — The life cycle of an animal comprises the whole 

 series of phenomena from the time development begins to the death of the 

 organism. Among the various aspects in which this can be studied is 

 that which involves the growth cycle. This varies greatly with different 

 animals. As has just been stated, some animals never cease to grow; 

 others grow only during the early parts of their lives. The latter is true 

 of insects, none of which ever grows at all after the adult condition is 

 reached. The higher vertebrates, however, including man, have a 

 regular growth cycle involving youth, maturity, and old age (Fig. 14). 

 Growth is most rapid in this case at the beginning of life and remains 

 still rapid until the end of the period of youth, when the individual has 

 practically attained full stature. A very gradual growth still continues, 



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