THE DEVELOPMENT OF THE VASCULAR SYSTEM 



261 



that is brought in from the placenta by the umbilical veins may pass through 

 the liver. Then the portion of each umbilical between the anastomosis and 

 the duct of Cuvier atrophies and disappears (Fig. 238). The remaining 

 portion of the left umbilical, which was originally the smaller, gradually 

 increases in size and finally carries all the blood from the placenta. The 

 right umbilical, on the other hand, loses its connection with the liver and 

 persists only as a small vein in the body wall, which opens into the left 

 umbilical vein near the umbilical cord (Fig. 239). Thus there is the peculiar 

 phenomenon of a vessel carrying blood in different directions at different 

 periods of its history. During the course of development of the septum 



CEsophagus 



Ant. cardinal 



Post, cardinal 



Liver 

 Right umbilical 



Venous ring 

 Venous ring 



Duct of Cuvier 

 Left umbilical 

 Ductus venosus 



Left umbilical 



Om phalomesenteric 

 Intestine 



FIG. 238. Veins in the liver region of a human embryo of 4 mm. His, Kollmann's Atlas. 



transversum and diaphragm the left umbilical is withdrawn from the body 

 wall and passes directly from the umbilicus to the ventral side of the liver. 

 During foetal life it conveys all the blood from the placenta to the liver. A 

 part of the blood is distributed in the liver, a part is carried directly to the 

 inferior vena cava by the ductus venosus (Fig. 240). After birth the 

 placental blood is cut off and the umbilical vein degenerates to form 

 the round ligament of the liver. 



The venous rings around the intestine also undergo marked changes. 

 The right side of the most caudal and the left side of the most cranial dis- 

 appear; the remaining vessel finally loses its connection with the ductus 

 venosus and becomes the portal vein (Figs. 237, 238, 239 and 240). The 



