| The fetus obtains oxygen, antibodies, and nutrients from the maternal blood supply, through the placenta. The placenta begins development when the fertilized egg, now called a blastocyst, implants itself into the uterine wall, soon after fertilization. The blastocyst rapidly divides and replicates itself, while the outer cell mass, called the trophoblastic cells, divides into 2 parts: an anchoring structure for the connection between the embryoblast and trophoblast cells and a multinucleated and differentiated mass of trophoblastic cells that expand outward onto the parenchyma of the placenta. The expanding edges of this mass are known as the trophoblastic cords; and blood capillaries grow into these cords from the blood vessels of the developing fetus. By day 16, the trophoblastic cords have pulsatile blood flow. | | | | |