The placenta is the only body that brings oxygen to the fetus, nutrition, removes its waste and produce its hormones. It is therefore the most important organ of the fetus.
Once the multicellular and gastrula embryo is inserted into the endometrium, begins to occur the interrelationship between 2 parts are distinguished in that multicellular and cavitated embryo: the trophoblast, which is extracorporeal tissue, and body. What relates to the mother is the trophoblast, the outer layer of the embryo.
The trophoblast rapidly dividing cell into 2 groups: the cytotrophoblast, which has cells that are dividing like most tissues, and the syncytiotrophoblast, in which cell divisions are not as orderly.
The trophoblast has enzymes that destroy the decidua, invading. A level of decidual vessels are starting to get that form lagoons (of maternal origin) within the space fetal maternal blood begins to circulate in the syncytiotrophoblast. Begin to appear septations fetal side which are the primary villi.
Finally, gaps in maternal circulation permanently bathed in fetal placental villi, covered by syncytium and cytotrophoblast, and in has capillaries. At no time are mixed maternal and fetal blood.
This type of placental circulation in humans is type hemocorial: namely that the maternal blood (heme) is in contact with the chorion with fetal trophoblast. As membrane separating the maternal blood and fetal blood is found only in fetal tissue.