Umbilical cord from and contains remnants of the yolk sac and allantois. It forms by the fifth week of development, replacing the yolk sac as the source of nutrients for the embryo. The cord is not directly connected to the mother's circulatory system, but instead joins the placenta, which transfers materials to and from the maternal blood without allowing direct mixing. The length of the umbilical cord is approximately equal to the crown-rump length of the fetus throughout pregnancy. The umbilical cord in a full term neonate is usually about 50 centimeters (20 in) long and about 2 centimeters (0.75 in) in
Studies of the Fetus in the Womb imbricato" ("Ask Biagino Crivelli's wife how the capon rears and hatches the eggs of hens when he is unplucked"). Da Vinci theorized that the umbilical cord was responsible for taking the fetus’ urine outside of the uterus. The studies were initially bequeathed to Francesco Melzi. In c. 1582–90 they were bought from his heirs by Pompeo Leoni and by 1630 they belonged to Thomas Howard, 2nd Earl of Arundel. Since 1690 the studies have been housed in the Royal Collection, United Kingdom. Studies of the Fetus in the Womb Studies of the Fetus in the Womb are two colored annotated