Prenatal Growth of the Human Body. 131 



half of the fetal period, the extremities continue to increase in relative 

 size, but more slowly; the increase of both counterbalancing the con- 

 tinued decrease in the relative size of the head, the trunk remaining 

 nearly unchanged. At birth, the upper extremities foi*m about 10 

 per cent of the whole body, the lower about 20 per cent. In the 

 adult (judging from the data of Meeh and Harless), the upper ex- 

 tremities have increased but slightly, if at all; while the lower ex- 

 tremities have increased to about 35 per cent, or nearly twice the 

 relative size at birth. 



In general, it may be said that the period of maximum relative 

 growth passes somewhat wave-like over the body from the head toward 

 the foot. The head, as we have seen, reaches its maximum relative 

 size in the 2d month. In the trunk, the upper portion, including 

 the thorax and the upper abdominal viscera, is relatively largest 

 during the earlier half of the fetal life. The lower part of the ab- 

 domen becomes more prominent toward the end of the fetal period, 

 due chiefly to the rapid expansion of the intestines at this time. The 

 pelvis and lower extremities do not reach their greatest relative size 

 until early adult life, although the upper extremities have reached 

 their maximum relative size at birth. 



It may also be noted that the organs lying dorsal to the body axis 

 (brain and spinal cord) grow at first far more rapidly than the organs 

 ventral to the body axis. The volume of the brain and spinal cord 

 together at the beginning of the 2d month is nearly 3 times as great 

 as the combined volume of the organs lying ventral to the body axis. 

 At birth, they are about equal. In the adult, the ventral organs are 

 () times as large as the brain and spinal cord. The significance of 

 the rapid growth of the brain and spinal cord in determining the 

 marked fiexure of the body in the early embryo has already been 

 pointed out by Merkel (32) and by Keibel (E"ormentafel zur Ent- 

 wicklungsgeschichte des Schweines, 189Y). 



3. Growth of the Individual Organs. 



As the growth rate of the whole body is the resultant of the growth 

 rate of the various parts, so the grov^th of the various parts depends 



