NORMAL AND PATHOLOGICAL GROWTH 147 



and embryonic nervous system is one of the first differentiations 

 to occur. It quickly attains a mass representing a high per cent 

 of the total volume of tissue in the embryo. The rate of growth 

 of the brain tissue and the degree of differentiation in growth 

 of nerve cells is enormously accelerated in the human during 

 the first nine months of embryonic life. In fact it is the general 

 belief at the present time that few if any new nerve cells are 

 formed after birth. The nerve cells continue to grow in volume 

 and length of processes throughout childhood and adolescence, 

 but they do not form new cellular units. 



Another internal organ offers a striking illustration in this 

 respect, namely, the liver. The liver originates as an outgrowth 

 of the embryonic alimentary tube. It is not recognizable until a 

 somewhat advanced stage in embryonic differentiation. How- 

 ever, the embryonic liver tissue grows so rapidly that it repre- 

 sents a large proportion of the total body mass at a late embry- 

 onic and prenatal stage. In postnatal development the growth 

 of the liver is ever at a relatively much slower rate. Compared 

 with the heart the liver is much later to appear in the cycle of 

 embryonic development. It reaches a maximum rate of growth 

 before birth, and grows at a slower and less prominent rate 

 during childhood and early adult life. 



It is obvious that certain outstanding physiological factors are 

 operating in the human body and in the body of animals to con- 

 trol or regulate, or at least to influence the rate of growth, and 

 that these influences do not affect all parts of the body in the 

 same degree at the same time. 



Furthermore, it must be recognized that the growth-influ- 

 encing factors operate more and more on individual organs and 

 tissues and each influence is intensified at different times in the 

 developmental cycle. At times the algebraic result is a physical 

 body quite out of all proportion to the average size and body 

 form which we ordinarily recognize as normal. Such individuals 

 are expressions of growth abnormalities and yet they are only 

 extreme variants. 



