=B. Development of the Portal Vein.=--The distal subintestinal segments of the vitelline veins are early united by a transverse anastomotic branch. The section of the veins above this anastomosis is seen already in Fig. 250 to have assumed an annular shape, while the veins below the primary anastomosis are approaching each other to form a second ring-like junction. In Fig. 251 the subintestinal segments of the two vitelline veins are seen to have communicated with each other by transverse anastomotic branches around the duodenum, two of these branches being situated ventrad and one dorsad of the intestinal tube. These branches, and the portions of the primitive vitelline veins between their points of derivation, form two vascular loops or rings, encircling the primitive duodenum (Fig. 251). [Illustration: FIG. 251.--Schema of further development of portal circulation and connection of same with umbilical veins in early stages.] The distal portions of the vitelline veins, before reaching the caudal annular duodenal anastomosis, next fuse into a single longitudinal vessel which also receives the veins from the stomach, intestine, spleen, and pancreas, and forms the beginning of the portal vein. By atrophy of the right half of the lower, and of the left half of the upper duodenal venous ring (Figs. 252 and 253), the proximal portion of the portal vein is formed as a single vessel, taking a spiral course around the duodenum (Fig. 256). Hence in the adult the portal vein and its principal branch (the superior mesenteric vein) crosses over the ventral surface of the duodenum (third portion), turns along the mesal side of the second portion, and then continues to the liver along the dorsal aspect of the first portion (Fig. 254). _Note_--In comparing Fig. 254 with the schematic figures it should be noted that the same presents the parts in the _ventral_ view, while the schemata offer the _dorsal_ aspect. [Illustration: FIG. 252.--Second stage in development of circulation through portal and umbilical veins. The proximal segment of the main portal vein is formed by the persistence of the left half of the distal and right half of the proximal periduodenal vascular ring of the omphalo-mesenteric veins. The distal segment of the main portal vein is the product of the fusion of the omphalo-mesenteric veins, and becomes connected with the veins of the intestinal canal, pancreas, and spleen. The proximal terminal segment of both umbilical veins becomes included in the system of the venae hepaticae revehentes.] [Illustration: FIG. 253.--Third stage in development of portal and umbilical veins during the placental period.] [Illustration: FIG. 254.--Corrosion preparation showing course of portal vein and tributaries in relation to duodenum. (Columbia University Museum, No. 1857.)]