During the three to four day passage through the fallopian tube following fertilization, three of the days floating in the intrauterine fluid, the zygote undergoes a series of divisions giving rise to a ball of cells with the beginnings of structure. This ball of cells is called the blastocyst, consisting of an outer layer of cells, the trophoblast, and an inner cell mass as well as a fluid filled cavity. The inner cell mass gives rise to the embryo. Upon implantation, the trophoblast develops into structures involved in secretory and nutritive support of the developing embryo/fetus. The outer layer of the trophoblast is called the chorion, which gives rise to chorionic villi, which integrate with maternal endometrial tissue in the placenta. Although there is extensive exchange of materials, there is no actual mingling of fetal and maternal blood. Further structures that form include the amniotic cavity, which forms between the inner cell mass and the trophoblast. The amniotic cavity is lined with the amnion, which eventually fuses with the chorion.
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