THE NATURAL HISTORY OF MAN 53 



Reproductive System. The gonads, in which the germinal 

 cells form, are developed in the body cavity just at the lower 

 end of the kidneys, probably from mesodermal cells, and early 

 begin to move downward in the abdomen. In the female they 

 go as far as the pelvic cavity where they remain. In the male 

 they pass through a canal in the lower part of the abdominal 

 wall, called the inguinal canal, to the exterior of the body. 

 Similar migrations of the gonads occur in most mammals. The 

 reasons for these migrations are not known, but the inguinal 

 canal remains as a weak spot in the abdominal wall and under 

 special stress may permit loops of the intestines to pass 

 through, thus forming a hernia. This is the penalty man is 

 still paying for the lack of structural adjustment to the upright 

 position. 



The external genital organs develop in a similar way in 

 both sexes and no structural distinction is to be observed until 

 near the beginning of the third month of fetal life. At this 

 time the indifferent sex condition begins to develop into one 

 sex or the other by the greater growth of some parts and the 

 partial suppression of others, depending upon the sex to be 

 formed. In rare cases the indifferent condition of the sex 

 organs may persist permanently in either sex, a condition 

 known as spurious hermaphroditism, which has particular in- 

 terest to criminologists and sociologists. The urinary system 

 develops in close relationship to the genital system and, before 

 the permanent adult condition is reached, passes through a 

 remarkable series of changes, many of which are typical of the 

 permanent adult condition in the lower vertebrates. 



The Muscles. As noted above, the mesoderm, which lies 

 along each side of the developing spinal cord, becomes trans- 

 versely divided into a linear series of segments (Fig. 14). 

 This process begins when the embryo is about two weeks old 

 and continues until a total of thirty-eight segments are formed 

 which extend the entire length of the embryo. From these 



