356 REPRODUCTION 



forms the auricle. At this time- the heart consists of a single 

 ventricle and a single auricle. Later the ventricular and auric- 

 ular septa are formed. The latter appears after the former and 

 is incomplete ; the opening left between the auricles is the fora- 

 men ovale. 



2. Placental Circulation. As the allantois is developed and 

 the vitelline circulation is abolished, the hypogastric arteries 

 are given off first from the aorta, but later (with the develop- 

 ment of the vessels of the lower extremities) they are pushed 

 down, as it were, so that they take origin from the internal iliacs. 

 They pass to the umbilicus and thence to the placenta by the 

 cord. Blood is at first returned from the placenta by two um- 

 bilicial veins, but these soon fuse into one. 



Object of Placental Circulation. Since the activity of the 

 respiratory and alimentary tracts has not been established, their 

 functions must be performed by those of the mother and the 

 necessary materials supplied from her blood. Consequently 

 there must be a continual passage of fetal blood to and from the 

 placenta to discharge effete matter and to absorb nutriment. 

 Certain modifications of the circulatory apparatus, not requisite 

 after birth, are necessary to bring this about. 



Course of Fetal. Circulation. The umbilical vein containing 

 blood enriched with oxygen and other materials enters the body 

 at the umbilicus and passes to the under surface of the liver. 

 Here it divides into two branches. The larger joins the portal 

 vein and enters the liver; the smaller is the ductus venosus, which 

 enters the ascending vena cava. 



The ascending vena cava, when it enters the right auricle, 

 therefore, contains blood from the lower extremities, blood 

 which has come from the placenta directly through the ductus 

 venosus, and blood which has come from the placenta indirectly 

 through the liver. Considering that blood from the body of the 

 fetus is venous and that blood directly from the placenta is 

 arterial, the contents of the ascending vena cava are mixed when 



