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In every higher animal we recognize certain more or less definite periods of physiologic activity, and we roughly divide the span of life into three stages, which are in no way sharply outlined. These we call the stages of youth, adolescence, and old age. Youth, characterized by a high degree of vitality, is the period of rapid cell multiplication and growth; organs are formed and perfected, functions are unimpaired and active, and the body is a perfect living thing. The second period is characterized by functional and sexual maturity; the' multiplication of tissue cells is less rapid; the organs strengthen and their functions are more perfectly correlated; growth comes to an end. In the perfected animal it is a period for perpetuation of the race, and in conformity with this great function sexual differentiation is fully established. The third period, old age, brings a marked change, the potential of vitality
LITTERER W. THE NON-OVIPAROUS FEMALE HOOKWORM. JAMA. 1910;55(16):1378–1379. doi:10.1001/jama.1910.04330160046017
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