738 THE CIRCULATORY SYSTEM 



Proceeding forward from the heart region, the two primitive subintestinal 

 blood vessels continue forward below the rudiment of the foregut where they 

 form the rudiments of the ventral aorta. They diverge and extend dorsad 

 around the foregut to the dorsal area of the foregut. These vessels which 

 thus pass around the foregut represent the third pair of aortal arches, i.e., 

 the first pair of branchial aortal arches (fig. 335A). The first branchial aortal 

 arches join the forming dorsal aortae. The dorsal aortae form first as irregular 

 blood spaces, extending along the primitive gut from below the forming brain 

 posteriad to the midgut area. Here they diverge to give origin to the vitelline 

 arteries which ramify over the yolk substance of the midgut and there anas- 

 tomose with branches of the vitelline veins. 



About the time of hatching, the two vitelline veins become enmeshed in 

 the substance of the developing liver, and the vitelline veins gradually become 

 divided into three groups (fig. 335B): 



(a) a right and left vitelline vein between the liver and the sinus venosus 

 of the heart, 



(b) the veins within the liver which form an irregular meshwork, and 



(c) the two vitelline veins, posterior to the liver substance. 



The left vitelline vein, anterior to the liver, soon atrophies and becomes 

 fused with the right vitelline vein as indicated in figure 335C and D. The right 

 vitelline vein thus receives the hepatic veins. Within the liver substance, the 

 two vitelline veins break up into smaller veins to form ultimately the sinusoids 

 of the liver (fig. 335C). Posterior to the liver, the vitelline veins form the 

 hepatic portal and intestinal veins (fig. 335C). 



2) Lateral (Ventral Abdominal) Veins. The lateral veins form first as two 

 minute veins, which extend posteriad from the lateral ends of the sinus venosus 

 of the heart. Eventually they unite with the iliac veins as shown in figure 335D. 



Fig. 334. The developing venous system in Squalus acanthias. (Modified from Hoch- 

 stetter, '06.) (A) An early stage in the development of the venous system. The two 

 primitive vitelline veins only are present. (B) Later stage in development of vitelline 

 veins. (C) Early stage in development of hepatic portal system. A venous ring is 

 formed around the duodenum. Anterior and posterior cardinal veins are evident. (D) 

 Later stage in the hepatic-portal system development. Left segment of duodenal collar 

 has disappeared. Observe that the efferent hepatic veins (V. hepaticae revehentes) repre- 

 sent the right and left vitelline veins between the liver lobes and the sinus venosus, 

 whereas the afferent hepatic veins (V. hepaticae advehentes) are the vitelline veins just 

 posterior to the liver. (E) Lateral veins make their appearance. Posterior cardinal veins 

 join veins around the cloacal area and thus assume responsibility for venous drainage 

 of the tail region, and the intestinal vein in consequence loses its connection with the 

 caudal vein of the tail. (F) Subcardinal veins appear between the kidneys. (G) Sub- 

 cardinal veins make connection with posterior cardinal veins. Posterior cardinal veins 

 regress anterior to the mesonephric kidneys where the posterior cardinal and subcardinal 

 veins anastomose. (H) Mature plan of the venous system showing the converging veins 

 of the heart. Hepatic portal vein omitted. 



