It will be noticed, in reference to the _direction_ of the blood current, that at birth a sudden reversal takes place in the right terminal branch of the umbilical vein at the transverse fissure (Figs. 260 and 261). Before birth the blood current of the umbilical vein divides into three streams, right, left and central. The latter enters the ductus venosus. The left enters the liver directly, the right traverses, from left to right, the segment between the termination of the umbilical and the bifurcation of the portal vein. This segment in the adult carries blood from right to left, as left branch of the portal vein. In the foetus, however, the blood traverses this segment from left to right, in passing from the umbilical to the right branch of the portal vein. The blood entering the liver through the portal vein passes chiefly into the right division of that vessel (Fig. 260). After birth all the venous blood entering the liver passes through the portal vein. In the right division the direction of the current is the same as in the foetus. On the left side, however, the current is now from right to left, from the bifurcation of the portal into the channels of the left lobe formerly connected with the umbilical vein (Fig. 261). Hence the direction of the current in this segment is reversed at birth. SUMMARY OF HEPATIC CIRCULATION. The foregoing consideration of the development shows us that the hepatic circulation presents successively three main stages: =1. Omphalo-mesenteric or Vitelline Stage=, which results in the laying down of the primary capillary circulation of the liver and in the establishment of its connection with the developing veins of the alimentary tract (primitive portal channels). =2. Umbilical or Placental Stage=, in which the greater part of the blood circulating through the liver is oxygenated blood returned from the placenta by the umbilical vein, accounting for the rapid growth and relatively large size of the organ during foetal life. The placental blood uses the preformed capillary channels of the vitelline or primitive portal system in the liver, and the same rapidly extend and enlarge with the accelerated growth of the gland. During this stage venous blood is also returned from the alimentary tract to the liver by the portal vein, produced by fusion of the distal segments of the primitive vitelline veins and their secondary connection with the mesenteric, splenic and pancreatic veins (omphalo-mesenteric development of primitive vitelline veins). =3. Adult or Portal Stage.=--With the interruption of the placental circulation the portal vein assumes again its original position as the only vein carrying blood to the liver. With the establishment of intestinal digestion and absorption this vessel grows rapidly in size. COMPARATIVE ANATOMY OF THE HEPATIC VENOUS CIRCULATION.