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The amnion and the chorion are the chorioamniotic membranes that make up the amniotic sac which surrounds and protects the embryo. [2] The fetal membranes are four of six accessory organs developed by the conceptus that are not part of the embryo itself, the other two are the placenta , and the umbilical cord .
The part of the chorion that is in contact with the decidua capsularis undergoes atrophy, so that by the fourth month scarcely a trace of the villi is left. This part of the chorion becomes smooth, [2] and is named the chorion laeve (from the Latin word levis, meaning smooth). As it takes no share in the formation of the placenta, this is also ...
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 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 ...
The embryo is joined to the trophoblastic shell by a narrow connecting stalk that develops into the umbilical cord to attach the placenta to the embryo. [11] [15] Arteries in the decidua are remodelled to increase the maternal blood flow into the intervillous spaces of the placenta, allowing gas exchange and the transfer of nutrients to the ...
The gestational sac is spherical in shape, and is usually located in the upper part (fundus) of the uterus.By approximately nine weeks of gestational age, due to folding of the trilaminar germ disc, the amniotic sac expands and occupy the majority of the volume of the gestational sac, eventually reducing the extraembryonic coelom (the gestational sac or the chorionic cavity) to a thin layer ...
Various types of chorionicity and amniosity (how the baby's sac looks) in monozygotic (identical) twins as a result of when the fertilized egg divides. Monochorionic twins are monozygotic (identical) twins that share the same placenta. If the placenta is shared by more than two twins (see multiple birth), these are monochorionic multiples.
Due to the fact that placental mammals and marsupials nourish their developing embryos via the placenta, the ovum in these species does not contain significant amounts of yolk, and the yolk sac in the embryo is relatively small in size, in comparison with both the size of the embryo itself and the size of yolk sac in embryos of comparable developmental age from lower chordates.