Chap. 19 



DEVELOPMENT 



385 



B. Postnatal 



Fig. 19.19. General scheme of human circulation before (fetal) and after birth 

 (postnatal). Before birth. The outstandingly important organ is the placenta 

 through which the embryo receives oxygen, food and other substances from the 

 maternal blood, all of it passing through membranes. Before birth blood passes 

 freely from the right to the left auricles (or atria). 



Supplies from the mother's blood are carried to the embryo via the placenta, the 

 umbilical vein and the vena cava (on left side) to the right auricle (atrium) of the 

 heart. Carbon dioxide and substances to be eliminated from the embryo are 

 brought to the placenta via the aorta (right side) and the umbilical artery. After 

 birth. At birth the vessels in the umbilical cord shrink and close and the placental 

 blood stream is abruptly cut oif. The circulation to the lungs is immediately and 

 completely underway. The passage (ductus arteriosus) between the two auricles is 

 soon closed. (Courtesy, Patten: Human Embryology, ed. 2. New York, The 

 Blakiston Co., 1953.) 



placenta and amniotic sac for each one and there are two afterbirths (Fig. 

 19.20). 



Identical twins come from a single fertilized egg that divides after fertiliza- 

 tion, begins to grow, splits in half and develops into two individuals. Each 

 one has the same inheritance as the other and since sex is inherited they are 

 always of the same sex. They share the same placenta and there is only one 

 afterbirth (Fig. 19.20). Siamese twins are identical twins only partly sep- 



