The sinus venosus at this time also receives two large veins, transversely directed, called the ducts of Cuvier, which are formed near the heart by the union of the anterior cardinal (primitive jugular) and posterior cardinal veins, draining respectively the head end of the embryo, and the body walls and Wolffian bodies. The vitelline veins are placed on each side of the primitive small intestine, and become connected with each other by a broad anastomotic branch (Fig. 249). When the hepatic outgrowth buds from the duodenum the vitelline veins send out branches which break up into a wide-meshed capillary network in the mesodermic tissue enveloping the hepatic cylinders. Hence at this period the circulation in the vitelline veins is made up of three districts: (_a_) Distal segment of veins, coursing along duodenum, and joined by a transverse anastomosis, before reaching the liver bud (subintestinal veins). (_b_) Middle segment, from which capillary vessels are derived, ramifying upon and between the developing hepatic cylinders. (_c_) Proximal segment, formed by the continuation of the proximal part of the vitelline veins into the sinus venosus of the heart. [Illustration: FIG. 250.--Schema of primitive portal circulation.] =3. Formation of Portal Circulation. A.=--With the further development of the liver the direct connection of the distal segment of the vitelline veins with the sinus venosus becomes lost, the intermediate segment being entirely broken up into an intrahepatic network (Fig. 250). Hence all the blood brought to the liver by the vitelline veins (venae hepaticae advehentes) passes through the hepatic capillary circulation, before it is carried by the proximal segment of the vitelline veins (venae hepaticae revehentes) into the sinus venosus. The amount of this blood increases with new connections which the vitelline veins make with the venous radicles developing in the intestinal tract and its appendages. In proportion as, with the development of the placenta and reduction of the yolk-sac, the original significance of the vitelline veins as nutritive and respiratory vessels disappears, this secondary connection of the vitelline veins with the veins of the alimentary tract becomes more and more important, until finally the original vitelline veins, now properly called omphalo-mesenteric veins, return the blood from the intestinal tube, pancreas and spleen to the liver. The venae hepaticae advehentes, becoming connected in this way with the developing intestine, pancreas and spleen, form the rudiments of the future portal system, while the venae hepaticae revehentes are prototypes of the hepatic veins of the adult circulation.