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 atriorium 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. 
