REPRODUCTION 



543 



Fig. 21-10. A human embryo (about six weel<s old) floating in the amnionic cavity. The spherical-shaped body in the 

 lower left is the yolk sac. The feathery nature of the chorion is shown. The part nearest the viewer has been cut 

 away. 



digestive system, excretory system — and all 

 except the circulatory system have never 

 functioned but must be ready to do so 

 within a matter of minutes after birth. All 

 of its nourishment and oxygen have come 

 to the embryo through the umbilical vein 

 from the placenta, and all of its waste prod- 

 ucts (carbon dioxide and urea) have been 

 delivered through the same organ. Its lungs 

 have never breathed, its kidneys have never 

 excreted, and its digestive tract has never 

 functioned, yet within a few minutes after 

 it is born all of these organs begin perform- 

 ing their jobs and, almost without excep- 

 tion, they function perfectly from the very 



start. The most dramatic change occurs in 



o 



the circulation. 



The blood path through the fetus is quite 

 different from that in the adult (Fig. Si- 

 ll). Obviously, circulation of blood must 

 start very early in the developing fetus be- 

 cause it is only through the circulation that 

 nourishment and the elimination of wastes 

 can take place. The blood must be pumped 

 to and from the placenta at a rapid rate 

 during development, hence the circulatory 

 mechanism is one of the first organs that 

 is well developed in the fetus. The beating 

 fetal heart is easily detected with a stetho- 

 scope long before the child is born. Since 



