All veins in the developing embryo ultimately drain into the sinus venosus, which drains into the primitive atrium of the developing heart.  In early embryonic development, the body's veins fall into 3 main groups: umbilical, vitelline, and cardinal veins.  The umbilical vein degenerates, the vitelline veins form the portal system, and the cardinal veins form constituents of the systemic venous circulation.