Re: When the umbilical cord enters the foetus, which vein does it enter?

During its time in the uterus, the eutherian (placental mammalian) fetus receives nourishment, expels waste, and exchanges blood gases through the intimate contact of the placenta with the uterine wall. Connecting the placenta to the fetus is the umbilical cord, which consists of two umbilical arteries, an umbilical vein, and the vestiges of the yolk and allantoic stalks.

In reptiles and birds, the allantois serves the same functions as the placenta, even allowing gas exchange through the shell, and it is the allantoic circulation of the early eutherian embryo that develops into the umbilical arteries and vein. The paired umbilical arteries arise from the iliac arteries, which supply blood to the legs. Similarly, two umbilical (allantoic) veins arise from the hepatic portal system, which takes blood from the gastrointestinal tract to the liver. The right umbilical vein is lost as the embryo grows, leaving the left to act as the single umbilical vein. Once the umbilical circulation is set, the umbilical vein empties directly into the liver, where it joins the hepatic portal vein entering the liver capillary beds. After birth, the lack of circulation through the umbilical system reduces the umbilical arteries to internal iliac arteries, and forms the vestiges of the umbilical vein into the round ligament which connects the liver to the naval in adults.

Patten, BM, and Carlson, BM, Foundations of Embryology, McGraw-Hill, Inc., New York.