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* The placenta consists of two parts: a larger fetal part derived from the villous chorion and a smaller maternal part developed from the decidua basalis. The two parts are held together by stem chorionic villi that attach to the cytotrophoblastic shell surrounding the chorionic sac, which attaches the sac to the decidua basalis.
* The principal activities of the placenta are metabolism (synthesis of glycogen, cholesterol, and fatty acids), respiratory gas exchange (oxygen, carbon dioxide, and carbon monoxide), transfer of nutrients (vitamins, hormones, and antibodies; elimination of waste products), and endocrine secretion (e.g., hCG) for maintenance of pregnancy
* The fetal circulation is separated from the maternal circulation by a thin layer of extrafetal tissues-the placental membrane. It is a permeable membrane that allows water, oxygen, nutritive substances, hormones, and noxious agents to pass from the mother to the embryo or fetus. Excretory products pass through the placental membrane from the fetus to the mother.
* The fetal membranes and placenta(s) in multiple pregnancies vary considerably, depending on the derivation of the embryos and the time when division of embryonic cells occurs. The common type of twins is DZ twins, with two amnions, two chorions, and two placentas that may or may not be fused.
* MZ twins, the less common type, represent approximately one third of all twins; they are derived from one zygote. MZ twins commonly have one chorion, two amnions, and one placenta. Twins with one amnion, one chorion, and one placenta are always monozygotic, and their umbilical cords are often entangled. Other types of multiple birth (triplets, etc.) may be derived from one or more zygotes.
* The umbilical vesicle and allantois are vestigial structures; however, their presence is essential to normal embryonic development. Both are early sites of blood formation and both are partly incorporated into the embryo. Primordial germ cells also originate in the wall of the umbilical vesicle (yolk sac).
* The amnion forms an amniotic sac for amniotic fluid and provides a covering for the umbilical cord. The amniotic fluid has three main functions: to provide a protective buffer for the embryo or fetus, to allow room for fetal movements, and to assist in the regulation of fetal body temperature.

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