MORPHOGENESIS OF CIRCULATORY SYSTEM 743 



(3) two delicate capillaries, the future ventral aortae, course anteriad from 

 the heart below the foregut. As the ventral aortae approach the anterior 

 limits of the foregut they diverge and travel dorsad as the mandibular 

 aortal arches, one on either side of the gut tube, to the dorsal region. 

 In the dorsal area of the foregut the mandibular aortal arches become 

 continuous with 



(4) the dorsal aortae. These two delicate vessels lie upon the foregut on 

 either side of the notochord, and extend caudalward into the region 

 of the developing midgut. 



During the period of 40 to 50 hours of incubation the following changes 

 occur in the above system (fig. 336B and B'): 



( 1 ) The rudimentary vitelline arteries extend outward over the yolk-sac 

 area from the dorsal aortae, forming many small capillaries. 



(2) The anterior and posterior cardinal veins and connecting interseg- 

 mental veins are established and unite with the sinus venosus by means 

 of the common cardinal vein (fig. 336B'). 



(3) The vitelline veins extend outward over the blastoderm and continue 

 anteriorly around the head area as the anterior vitelline veins. The 

 latter veins unite with the circumferential blood sinus. A complete 

 circulation through the embryo and out over the yolk-sac area is thus 

 effected. 



During the early part of the third day of incubation the right and left vitelline 

 veins begin to fuse in the area just posterior to the heart. This fusion forms 

 a single vein, the ductus venosus (fig. 337A). The latter structure joins the 

 sinus venosus of the heart. Posteriorly, the vitelline veins make a secondary 

 connection with the developing posterior vitelline or omphalomesenteric veins 

 which extend backward along the sides of the midgut to the area where the 

 vitelline arteries leave the dorsal aortae. At this point each omphalomesen- 

 teric vein turns sharply laterad and courses along the pathway of a vitelline 

 artery (fig. 336C). 



At the end of the third day of incubation the ductus venosus is present as 

 an elongated structure lying between the anterior intestinal portal and the 

 heart. A posterior vitelline vein continues posteriad from the ductus venosus 

 around each side of the anterior intestinal portal (fig. 336D). As observed 

 in Chapter 13, during the third and fourth days of incubation the liver rudiment 

 begins to form. In doing so, the trabeculae of the liver surround the ductus 

 venosus. The immediate segment of the ductus venosus which becomes sur- 

 rounded by the forming liver substance forms the meatus venosus. As devel- 

 opment of the liver proceeds, two main groups of veins develop in the liver 

 substance (fig. 337B, D): (1) An anterior efferent group of hepatic veins 

 which drain blood from the liver and (2) a posterior afferent set of hepatic 



