182 



EMBRYOLOGY 



embryo from the yolk sac by means of the vitelline veins drains directly 

 into the ductus venosus. With development of the liver around the ductus 

 venosus, the union between the vitelline vein and the ductus venosus becomes 

 more and more constricted and other branches form within the tissue of 

 the liver. Finally the blood from the vitelline vein empties directly into 

 the spaces in the liver and is picked up by the hepatic veins. This type of 

 circulation is termed a hepatic portal circulation. The tissues of the liver 

 actually transport the blood. The enzymes of liver cells catalyze many im- 

 portant changes in the composition of incoming foods — deamination, glucose 

 regulation, detoxification. The substances in the blood come in direct con- 

 tact with the liver cells and the enzymes are able to function. 



Changes in circulation with hatching 



A number of changes occur at the time of hatching. During the later 

 part of incubation the chick possesses a four-chambered heart, but the 

 two auricles and the two ventricles communicate with each other through 

 openings (Fig. 110). These foramina permit mixing of bloods from the 

 two sides of the heart. As Figure 110 indicates, oxygen-containing blood 

 from the allantois mixes with carbon-dioxide-laden blood from the embryo. 

 The mixture passes into the right auricle. From the right auricle the mixed 



HEART 



Fig. 109. The development of a 

 hepatic portal system. In the early 

 embryo blood from the yolk sac, 

 lower right, flows directly through 

 / into the ductus venosus and to the 

 heart. With the differentiation of 

 the liver the connection / gradually 

 becomes closed and the blood is 

 shunted into the spaces in the liver. 

 It is then picked up by the hepatic 

 veins at K. 



