LATER DEVELOPMENT OF VASCULAR SYSTEM 377 



insignificant vessels. The veins of the yolk-sac likewise disap- 

 pear. The ductus arteriosus (Botalli) is obliterated on both 

 sides, and becomes a solid cord uniting the pulmonary arteries 

 and arch of the aorta. Thus the blood from the right ventricle 

 is driven into the lungs, and the pulmonary artery enlarges. 

 The foramina in the septum atriorum gradually close, and so a 

 complete double circulation is established. The right auricle 

 receives all the systemic (venous blood), which is then driven 

 through the lungs by way of the pulmonary artery, and returned 

 in an oxygenated condition through the pulmonary veins to 

 the left auricle; thence to the left ventricle and out through the 

 aorta into the systemic circulation again. 



