532 EMBRYOLOGY OF THE CHICK 



The posterior cardinals remain large as long as the Wolffian body 

 is functional, but as the permanent kidneys develop, these veins become 

 smaller and smaller and ultimately disappear. 



The ducts of Cuvier persist in the adult chick as the anterior venae 

 cavae. 



The posterior or inferior vena cava develops from the meatus veno- 

 sus, which was formed by the union of the two omphalomesenteric 

 veins. To understand the evolving process by which the posterior vena 

 cava comes into existence, it is necessary to follow carefully the devel- 

 opment of the surrounding organs. 



The liver forms as a diverticulum from the digestive tract. This 

 diverticulum then grows around the meatus venosus until it completely 

 surrounds the meatus. Blood-vessels form in the liver, extending toward 

 the meatus venosus, into which they open by the fifth day. 



At the posterior edge of the liver, there are a number of afferent 

 hepatic vessels coming from the meatus venosus through which some 

 of the blood coming to the heart from the vascular area may enter the 

 capillaries formed in the liver substance. 



At the anterior edge of the liver, where the meatus venosus might 

 be said to be leaving the liver, there is a collection of efferent hepatic 

 vessels whose distal ends are in direct connection with the capillaries 

 of the afferent hepatic vessels. 



The blood passing through the liver has two courses it may take. 

 Most of it passes through the large meatus venosus into the heart, but 

 some of it passes through the afferent hepatic vessels into the liver sub- 

 stance where it is collected by the efferent hepatic vessels and carried 

 to the meatus venosus. 



That part of the meatus venosus lying between the afferent and 

 efferent hepatic vessels is often called the ductus venosus. 



The two allantoic veins already described unite on entering the 

 body to form a single vein emptying into the left (persistent) omphalo- 

 tnesenteric vein. It is well to remember that as the yolk-sac decreases 

 in size, the allantois increases, and so, too, the relative size of omphalo- 

 mesenteric veins and allantoic veins changes ; the omphalomesenteric 

 becomes smaller and the allantoic becomes larger, so that it almost 

 seems as though the omphalomesenteric were a branch of the allantoic. 

 Both of these veins disappear at the time of hatching. 



The superior mesenteric artery was formed by the closure of the 

 ventral body-wall so as to bring the paired omphalomesenteric veins 

 together, to form a single vessel running from the aorta to the yolk- 

 stalk. As the yolk-sac atrophies, the proximal portion of the omphalo- 

 mesenteric artery becomes the superior mesenteric artery. 



The mesenteric vein is formed by a union of the veins from the walls 

 of the hinder part of the digestive tract, which there form a single vein. 

 This vein is at first quite small, and empties into the omphalomesenteric 



