Answer:
Umbilical cord
Step-by-step explanation:
Allantois is a hollow sac like structure. Its primary role is to eliminate liquid waste from the fetus and help in exchange of gases. Amniote is the group of organisms which contain allantois, amnion and chorion. Reptiles, birds and mammals are part of this group.
In mammals, allantois develops into umbilical cord. Umbilical cord is present between the fetus and the placenta. Human umbilical cord has two arteries and one vein. The vein brings nutrient rich, oxygenated blood from the placenta to fetus. The arteries brings back nutrient depleted, deoxygenated blood to placenta.