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The zygote begins to divide very soon by mitosis to form a ball of cells called an embryo. As it divides, it moves down the oviduct to the uterus. Once the embryo reaches the uterus it will implant in to the soft lining. Some of the cells of the embryo grow in to villi which fix to the wall of the uterus. Villi also develop from the wall of the uterus. The two sets of villi grow closely together to form the placenta. Other cells form a bag called the amniotic sac. The remaining cells continue to divide to produce a foetus. Inside the villi own the foetus's side of the placenta are millions of tiny blood capillaries. Blood is returned to the foetus along the umbilical vein and leaves the foetus from the umbilical artery which are both contained in the umbilical cord.

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