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There are two vessel-rich features in the amniote, the yolk sac and the allantois. When the chorion fuses with the former, the result is a choriovitelline placenta. When it fuses with the latter, the result is a chorioallantoic placenta. Most mammals first form a temporaty choriovitelline placenta, then the chorioallantoic placenta takes over.
[1] [2] They share the placenta, but have two separate umbilical cords. Monoamniotic twins develop when an embryo does not split until after formation of the amniotic sac, [1] at about 9–13 days after fertilization. [3] Monoamniotic triplets or other monoamniotic multiples [4] are possible, but extremely rare. [1]
The placenta (pl.: placentas or placentae) is a temporary embryonic and later fetal organ that begins developing from the blastocyst shortly after implantation.It plays critical roles in facilitating nutrient, gas and waste exchange between the physically separate maternal and fetal circulations, and is an important endocrine organ, producing hormones that regulate both maternal and fetal ...
On the outer side, the amniotic sac is connected to the yolk sac, the allantois, and via the umbilical cord, the placenta. [ 4 ] The yolk sac, amnion, chorion, and allantois are the four extraembryonic membranes that lie outside of the embryo and are involved in providing nutrients and protection to the developing embryo. [ 5 ]
The fluid then collects into a single lumen in a process akin to Ostwald ripening to form the blastocoel, which determines the first axis of symmetry of the mammalian embryo. [19] The side of the blastocyst where the inner cell mass forms is called the embryonic pole, and the opposite side is the abembryonic pole.
Identical twins have identical genomes in the immediate aftermath of twinning. About two-thirds of monozygotic twins share the same placenta, arising by cleavage before the fourth day of development; the other third have separate placentas because cleavage has taken place after the fourth day after choriogenesis has begun.
The embryo, the extra-embryonic membranes, and the placenta are collectively referred to as a conceptus, or the "products of conception". Rapid growth occurs and the embryo's main features begin to take form. This process is called differentiation, which produces the varied cell types (such as blood cells, kidney cells, and nerve cells).
Placenta shown with attached fetal membranes. The fetal membranes surround the developing embryo and form the fetal-maternal interface. [3] The fetal membranes are derived from the trophoblast layer (outer layer of cells) of the implanting blastocyst. [3]