REPRODUCTION. 241 



nutritive materials of the maternal blood pass through the intervening 

 membrane into the fetal blood on the one hand, while waste products pass 

 in the reverse direction into the maternal blood on the other hand. Inas- 

 much as oxygen is absorbed and carbon dioxid exhaled by the same structures, 

 the placenta is to be regarded as both a digestive and a respiratory organ. 

 So long as these exchanges are permitted to take place in a normal manner 

 the nutrition of the embryo is secured. 



The Nutrition of the Embryo. As the ovum passes down the oviduct 

 it imbibes nutritive materials from the mucosa. As it lodges in the uterus it 

 Is nourished as first in the same way. The first circulation developed is the 

 vitelline, but as the amount of nutritive material is very small in mammals 

 its activity is limited. In the oviparous forms, however, where the nutritive 

 material is large in amount this circulation is important. The allantoic cir- 

 culation is likewise of importance in the oviparous forms and constitutes 

 their last fetal circulation In mammals the allantoic circulation is merely 

 a transitional stage in the formation of the placental circulation. 



Circulation of Blood in the Fetus. The blood returning from the 

 placenta, after having received oxygen and being freed from carbonic acid, 

 is carried by the umbilical vein to the under surface of the liver; here a portion 

 of it, about one-half, passes through the ductus venosus into the ascending 

 vena cava, while the remainder flows through the liver and passes into the 

 inferior vena cava by the hepatic veins. When the blood is emptied into the 

 right auricle, it is directed by the Eustachian valve through the foramen 

 ovale, into the left auricle, thence into the left ventricle, and so into the aorta 

 and to all parts of the system. The venous blood returning from the head 

 and upper extremities is emptied, by the superior vena cava, into the right 

 auricle, from which it passes into the right ventricle, and thence into the 

 pulmonary artery. Owing to the condition of the lung only a small portion 

 flows through the pulmonary capillaries, the greater part passing through 

 the ductus arteriosus, which opens into the aorta at a point below the origin of 

 the carotid and subclavian arteries. The mixed blood now passes down the 

 aorta to supply the lower extremities, but a portion of it is directed, by the 

 hypogastric arteries, to the placenta, to be again oxygenated. 



At birth, the placental circulation gives way to the circulation of the adult. 

 As soon as the child begins to breathe, the lungs expand, blood flows freely 

 through the pulmonary capillaries, and the ductus arteriosus begins to con- 

 tract. The foramen ovale closes about the tenth day. The umbilical vein, 

 the ductus venosus, and the hypogastric arteries become impervious in 

 several days as far as the bladder. Their distal ends ultimately form 

 rounded cords. 

 16 



