Umbilical Cords point to biology of creatures.

We get to see something interesting when we look at the umbilical cords in the movie. We see two of them. One is connected to Shaw when she initiates a self-directed cesarean section. It is connected from her body to the amniotic sac containing baby alien creature. This is correct biology (science) for mammals in which the cord provides the nutrient and waste exchange between the growing embryo and the mother. Second umbilical cord is seen when the Deacon xeno breaks free of his host Engineer body and slides onto the floor. He has a umbilical cord that is connected to a large yellow sac similar to a egg yolk. This would imply that the host body acts as a egg would while the fetus develops. Birds lay eggs which contain a nutrient yolk sac and the allantois that stores wastes and allows for improved gas exchange. There is no umbilical cord. This is incorrect biology (fiction). What does this suggest? Just a thought. I welcome yours :)