MORPHOGENESIS OF CIRCULATORY SYSTEM 745 



veins, representing branches of the hepatic portal vein. The latter brings blood 

 from the stomach and intestinal areas to the liver. 



During the fifth to seventh days of incubation, the afferent and efferent 

 sets of hepatic veins develop profuse branchings, and venous sinusoids are 

 formed within the liver substance between these two sets of veins. Meanwhile, 

 the meatus venosus within the liver atrophies and a complete hepatic portal 

 system is established between afferent and efferent hepatic veins during the 

 seventh and eighth days of incubation as shown in figure 337E. 



While the above changes in the liver are emerging, changes in the omphalo- 

 mesenteric veins, posterior to the liver substance, are produced as shown in 

 figure 337A-E. By the fifth day, a new vein, the mesenteric vein, is formed 

 (fig. 337D), which begins to drain blood from the developing midgut and 

 hindgut areas. By the eighth day, the mesenteric vein is a prominent structure 

 (fig. 337E). At this time, the blood from the yolk sac, via the omphalo- 

 mesenteric veins, and that from the mesenteric vein must pass through the 

 liver sinusoids en route to the efferent hepatic veins (fig. 337E). 



b) Allantoic Veins. The two allantoic or lateral veins begin to develop 

 during the third day of incubation, and, by the end of this day, two delicate 

 blood vessels extend along the lateral body wall, reaching back toward the 

 hindgut area (figs. 336D; 337B). During the fourth day (fig. 337C), the 

 caudal ends of the two allantoic veins begin to ramify within the walls of 

 the allantois. A secondary attachment to the hepatic veins within the liver 

 is established also at this time (fig. 337C). During the late fourth day and 

 the fifth day of incubation, the right allantoic vein degenerates, and the 

 proximal portion of the left allantoic vein loses its connection with the common 

 cardinal vein (fig. 337D), During the seventh and eighth days (and until 

 the time of hatching), the passage of blood from the allantois through the 

 liver to the vena cava inferior is as indicated in figure 337E. The portion of 

 the allantoic vein extending anteriorly from the umbilical area to the liver 

 persists after hatching and drains blood from the midventral portion of the 

 body wall. It is called the epigastric vtMs (fig. 3371). 



2) Formation of the Inferior Vena Cava. The formation of the inferior 

 vena cava of the chick is shown in figure 337F-I and needs no other ex- 

 planation. It is to be observed that, following the degeneration of the meso- 

 nephric kidneys and the ascendancy of the metanephric kidney, the passage 

 of blood by way of the renal portal system through the mesonephric kidney 

 is abated. In the newly hatched chick, a much-weakened, renal portal system 

 is established via the renal portal vein (fig. 3371: However, most of the 

 blood through this vein passes directly into the common iliac vein and not 

 through the kidney substance. 



3) Development of the Precaval Veins. The precaval veins are the direct 

 descendants of the anterior cardinal and common cardinal veins as indicated 

 in figure 337F-1. In figure 3371, it is to be observed that the caudal ends of 



