The following is true of the placenta:
a. It is formed from trophoblast cells at the site of implantation.
b. Maternal and fetal blood mix in nutrient exchange.
c. The placental barrier protects the fetus from viral infections.
d. It begins functioning at the 6th week of em-bryonic development.
A
Trophoblast (the outer layer of blastocyst cells) is part of the structure that forms the placenta, while the inner cluster of blastocyst cells develops into the embryo. The maternal portion circulates nutrients to the embryo and receives wastes from the fetus. Viruses can cross the placental barrier. The placenta begins functioning around the 3rd week after implantation.
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