As the fertilized egg or zygote moves down the oviduct, cell divisions begin, and the zygote becomes an embryo. Before implantation in the side wall of the uterus, which occurs between eight to ten days after fertilization, the egg's limited yolk nourishes the embryo. After implantation, the embryonic membrane forms the umbilical chord. Blood vessels contributed by the allantois run through the chord to the placenta. The placenta, formed by both embryonic membrane (primarily the chorion) and uterine tissue, contains both blood vessels of the embryo and the mother. These lie very closely together, but the blood of embryo and mother do not mix. In the placenta, nutrients and oxygen diffuse from mother's blood to embryo's blood while urinary wastes and carbon dioxide diffuse from embryo to mother. The embryonic membrane that forms the placenta also secretes gonadotrophic hormone which preserves the corpus luteum. Thus, progesterone levels are sustained, the uterine lining is not shed, and pregnancy is maintained.
