858 DEVELOPMENT [GIL LIX. 



the stream is divided, a part of the blood passing straight on to the 

 inferior vena cava, through a venous canal called the ductus venosus, 

 while the remainder passes into the portal vein, and reaches the 

 inferior vena cava after circulating through the liver. Whether, 

 however, by the direct route through the ductus venosus or by the 

 roundabout way through the liver, all the blood which is returned 

 from the placenta by the umbilical vein reaches the inferior vena 

 cava at last, and is carried by it to the right auricle of the heart, into 

 which cavity is also pouring the blood that has circulated in the head 

 and neck and arms, and has been brought to the auricle by the 

 superior vena cava. It might be naturally expected that the two 

 streams of blood would be mingled in the right auricle, but such is 

 not the case, or only to a slight extent. The blood from the superior 

 vena cava the less pure fluid of the two passes almost exclusively 

 into the right ventricle, through the auriculo- ventricular opening, just 

 as it does in the adult ; while the blood of the inferior vena cava is 

 directed by the fold of the lining membrane of the heart, called the 

 Eustachian valve, through the foramen ovale into the left auricle, 

 whence it passes into the left ventricle, and out of this into the aorta, 

 and thence to all the body, but chiefly to the head and neck. The 

 blood of the superior vena cava, which, as before said, passes into the 

 right ventricle, is sent out thence in small amount through the 

 pulmonary artery to the lungs, and thence to the left auricle, by the 

 pulmonary veins, as in the adult. The greater part, however, does 

 not go to the lungs, but instead, passes through a canal, the ductus 

 arteriosus, leading from the pulmonary artery into the aorta just below 

 the origin of the three great vessels which supply the upper parts of 

 the body ; and there meeting that part of the blood of the inferior 

 vena cava which has not gone into these large vessels, it is distributed 

 with it to the trunk and other parts a portion passing out by way 

 of the two umbilical arteries to the placenta. From the placenta it 

 is returned by the umbilical vein to the under surface of the liver, 

 from which the description started. 



Changes after Birth. Immediately after birth the foramen ovale 

 begins to close, and so do the ductus arteriosus and ductus venosus, 

 as well as the umbilical vessels ; the closure is completed in a few 

 days, so that the circulation then takes the course it traverses for the 

 rest of life. 



Development of the Nervous System. 



The nervous system originates from the thickened walls of the 

 medullary groove, which by the meeting of the dorsal ridges is con- 

 verted into the medullary canal. These walls are composed entirely 

 of epiblast. The anterior part of this mass becomes the brain, the 



