184 EMBRYOLOGY 



is short-circuited from the lungs into the dorsal aortae by the ductus arterio- 

 sus. As a result, almost all the blood from the right ventricle of the heart 

 finally flows into the dorsal aorta and is then diverted to the allantois and 

 the embryo. The blood going to the allantois is oxygenated there, while 

 that circulating through the embryo loses its oxygen to the developing 

 tissues. 



Shortly after hatching, a number of important changes take place. Cir- 

 culation through the yolk sac and the allantois stops. The ductus arteriosus 

 and the openings between the two auricles close. The septum between the 

 two ventricles is completed and blood begins to flow through the lungs. 

 With these changes carbon-dioxide-laden blood from the embryo passes 

 directly through the right side of the heart, through the pulmonary arteries, 

 and through the lungs into the left side of the heart. Thus, the adult cir- 

 culation is established. 



Frequently the opening between the right and left auricles of the human 

 heart fails to close or closes incompletely. If the opening is too large and a 

 considerable mixing of arterial and venous blood occurs, the blood in the 

 arteries becomes reddish-blue, and the baby looks blue and may not survive, 

 owing to lack of oxygen. If the opening between the two halves of the heart 

 is small, there is no injurious effect. 



In looking for the factors which control the changes in the circulatory 

 system, we see certain correlative changes in kidney and blood vessels. Which 

 of these changes is primary? Do the changes in circulation cause the changes 

 in the kidney system or vice versa? Since the parts of the kidney will develop 

 in tissue culture without a circulatory system, it seems clear that the kidney 

 controls in some way the changes in the circulatory system. The differentiation 

 of the circulatory system and the outgrowths of blood vessels may conform 

 to the same principles which we outlined for the differentiation of the 

 nervous system and the outgrowth of nerve fibers. 



Once blood vessels make contact with an organ, the pathways within the 

 organ are determined by the structure of the organ itself. A foreign blood 

 vessel forms the same branches within an organ as does the normal blood 

 vessel. This principle of structural determination of pathways within an organ 

 applies also to nerve fibers. A foreign nerve entering a limb breaks up into 

 the same branches to the muscles as does a normal nerve. 



