THE NATURAL HISTORY OF MAN 63 



dence that the latter reproduce, in many respects, a compara- 

 tively recent phase in the history of human evolution. 



POSTNATAL DEVELOPMENT 



In the final analysis, growth in any living organism is the 

 result of the division of a cell into two daughter cells each of 

 which, when first formed, is half the size of the original cell. 

 The daughter cells very soon, by the process of intussuscep- 

 tion which undoubtedly involves complex physico-chemical 

 processes, grow to the size of the original cell and then the 

 process can be repeated. In prenatal life, the embryonic cells, 

 in general, divide comparatively rapidly and, as a rule, are not 

 as large as in postnatal development. The increase in size 

 after birth is believed to be due primarily to a general increase 

 in the size of the cells of the body rather than to the formation, 

 by cell division, of additional cells. Development is therefore 

 accomplished by cell division, cell enlargement, and cell differ- 

 entiation. 



The ultimate size of an animal depends both upon the rate 

 and the duration of growth. Minot has shown that man 

 becomes larger than the rabbit, not because of a more rapid 

 growth, but because he grows for a longer period of time. 

 The rabbit, on the other hand, becomes larger than the guinea 

 pig because of a more rapid rate of growth. Large animals, 

 with few exceptions, continue to grow during a longer period 

 and live longer than do small animals. Statistical studies show 

 that the rate of growth is most rapid in early prenatal life and 

 steadily diminishes until birth. During the prenatal period 

 of the human embryo the weight increases more than 5,000,- 

 ooo times, while after birth the increase is only 20.6 

 times. As the rate of growth is a constantly decreasing 

 one, its cessation seems to be the final term of a diminishing 

 series. During the most rapid period of growth the cells have 

 large nuclei but, with advancing age, the cytoplasm relatively 



