DEVELOPMENT OF FOURTH DAY 535 



to the head and fore-limbs, while the blood from the anterior venae cavae 

 passes out through the right ventricle through the pulmonary trunk and 

 thus through the fifth aortic arches into the dorsal aorta, from where the 

 blood goes to the body and hind-limbs of the embryo. A small portion, 

 however, is carried out along the omphalomesenteric arteries to the yolk- 

 sac and through the allantoic arteries to the allantois to take up nutriment 

 and oxygen. In the early embryo, a much greater portion of this pul- 

 monary circulation goes to yolk-sac and allantois. 



It is assumed that the vastly greater proportion of blood supply to 

 the anterior region, as contrasted with the smaller quantity to the pos- 

 terior portions, accounts for the greater and more rapid development of 

 the head region, which it will be remembered is the first part of the chick 

 to develop. 



The disproportionate development of the head may be realized when 

 it is known that the human child at birth has a head about one-fourth the 

 length of its entire body. 



At about the time of hatching the ductus Botalli (which it will be 

 remembered is that portion of the fifth aortic arch lying between the 

 dorsal aorta and the point of origin of the vessel that runs to the lung) 

 (Fig. 309) closes up entirely, so that the blood from the right ven- 

 tricle must pass through the pulmonary veins back to the left auricle. 



The lungs now become functional, and the true pulmonary circulation 

 is established. The allantoic circulation, being no longer needed, ceases, 

 while the allantoic arteries and veins disappear, as do also the omphalo- 

 mesenteric arteries and veins when the yolk-sac has finished its work, and 

 the hatched chick can take in its own food. 



It is at this time also that the entire supply of blood which goes to 

 the liver passes through the mesenteric vein, which is now called the 

 hepatic portal vein. The ductus venosus has closed, and so all blood 

 brought to the liver must pass through the hepatic capillaries before 

 reaching the heart. 



The foramen ovale does not close immediately after hatching, but 

 does so in a few days ; but, as soon as it does, all blood returned to the 

 heart by the three venae cavae is emptied into the right auricle from 

 which it is then forced into the right ventricle, thence through the pul- 

 monary artery to the lungs, and back through the pulmonary veins to the 

 left auricle, from which it is forced into the left ventricle, and finally 

 through the systemic trunk. Such an entire separation of venous and 

 arterial blood is called a double circulation. 



