310 THE CHICK. 



plexus of the liver substance ; from this plexus, efferent hepatic 

 vessels (Fig. 128, VH) arise, which open into the meatus venosus 

 shortly before this emerges from the anterior end of the liver. 

 Thus, while the main stream of blood, entering the liver from 

 the vitelline veins, passes along the meatus venosus direct to 

 the heart, a small part of it is diverted through the capillary 

 system of the liver, joining the meatus venosus again further 

 on in its course. 



The part of the meatus venosus in the substance of the liver, 

 between the openings of the afferent and efferent hepatic vessels, 

 is usually spoken of as the ductus venosus. 



The vitelline veins return to the embryo the blood from the 

 capillary network of the vascular area of the blastoderm (cf. Fig. 

 99, AV). From the greater part of the area vasculosa the blood 

 is returned directly by the right or left vitelline veins ; but from 

 the more peripheral part of the area the blood is collected by a 

 circular marginal or terminal vein, which runs round the outer 

 edge of the area vasculosa, and from which anterior and poste- 

 rior veins carry the blood to the vitelline veins. 



Of the two vitelline veins, the left is, almost from the first, 

 larger than the right. After the embryo has turned so as to lie 

 on its left side, the difference between the two veins becomes 

 more pronounced, and ultimately the right one disappears. 



The allantoic veins. As soon as the allantois is definitely 

 formed, on the fourth day, two allantoic veins are developed, re- 

 turning the blood from it to the embryo. These unite on entering 

 the embryo to form a single allantoic vein, which runs forwards in 

 the splanchnopleure of the left side to join the left vitelline vein. 

 With growth of the allantois, the allantoic vessels increase greatly 

 in size, but their relations remain practically the same. In the 

 earlier stages (Fig. 128), the vitelline vein is much larger than 

 the allantoic ; but later on, as the yolk becomes smaller through 

 absorption of its contents, while the allantois continues to in- 

 crease, the proportions are reversed, and the allantoic vein 

 becomes the larger of the two. 



The mesenteric vein is formed by the union of the veins 

 which return the blood from the intestine of the embryo. It 



