The Growing Embryo
The growing embryo is kept alive by the placenta, an organ formed from the embryonic tissue which connects it to the mother's uterus. There are five stages in the development of the placenta, as illustrated here. During the development of the placenta the embryo, protected by fluid in the amniotic sac, gains nourishment for the first few weeks from the yolk sac. Later the placenta, formed by projections of an embryonic membrane, the chorion, which penetrates the uterine wall, provides the embryo with food and oxygen. The blood vessels of the allantoic sac later become part of the umbilical cord.
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