904 THE PHYSIOLOGY OF REPRODUCTION. 



fertilization, while the others give rise to males. In other cases 

 all the spermatozoa exhibit the same number of chromosomes, 

 but one-half of them contain a large " idiochromosome, " the other 

 half a small one. The former give rise on fertilization to females, 

 the latter to males. If, on the basis of such facts, we assume the 

 existence of male and female eggs and male and female spermatozoa, 

 the question of the sex of the offspring would seem to depend upon 

 which of the sex-determining structures in the two cells predominates 

 after union. The problem still remains unsolved, although reduced 

 to a narrower field for observation and experiment. 



Growth and Senescence. The body increases rapidly after 

 birth in size and weight. It is the popular idea that the rate of 

 growth increases up to maturity and then declines as old age ad- 

 vances. As a matter of fact, careful examination of the facts shows 

 that the rate of growth decreases from birth to old age, although not 

 uniformly. At the pubertal period and at other times its downward 

 tendency may be arrested for a time. But, speaking generally, the 

 maximum rate of growth is reached some time during the intra- 

 uterine period, and after birth the curve falls steadily. Senescence 

 has begun to appear at the time we are born.* Thus, according to 

 the statistics of Quetelet, the average male child weighs at birth 6J 

 pounds. At the end of the first year it weighs 18J pounds, a gain of 

 12 pounds. At the end of the second year it weighs 23 pounds, a 

 gain of only 4J pounds, and so on, the rate of increase falling rap- 

 idly with advancing years. The actual statistics of growth have 

 been collected and tabulated with great care by a number of ob- 

 servers; for this country especially by Bowditch, Porter, and Beyer.f 

 An interesting feature of the records collected by Bowditch is the 

 proof that the prepubertal acceleration of growth comes earlier 

 in girls than in boys, so that between the ages of twelve and fifteen 

 the average girl is heavier and taller than the boy. Later, the boy's 

 growth is accelerated and his stature and weight increase beyond 

 that of the girl. It appears from the examinations made upon 

 school children by Porter and by Beyer that a high degree of 

 physical development is usually associated with a corresponding 

 pre-eminence in mental ability. The signs of old age may be de- 

 tected in other ways than by observations upon the rate of growth. 

 Changes take place in the composition of the tissues; these changes, 

 at first scarcely noticeable, become gradually more obvious as old 

 age advances. The bones become more brittle from an increase in 

 their inorganic salts, the cartilages become more rigid and calca- 



* See Minot, 'Journal of Physiology," 12, 97. 



f See Bowditch, "Report of State Board of Health of Massachusetts," 

 1877, 1879, and 1891; Porter, "Transactions, Academy of Science," St. 

 Louis, 1893-94; Beyer, "Proceedings, United States Naval Institute," 21, 

 297, 1895. 



