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Innermost of the extraembryonic membranes enveloping the embryo in utero and containing the amniotic fluid; it consists of an internal embryonic layer with its ectodermal component, and an external somatic mesodermal component; in the later stages of pregnancy the a. expands to come in contact with and partially fuse to the inner wall of the chorionic vesicle; derived from the trophoblast cells. SYN: amnionic sac. [G. the membrane around the fetus, fr. amnios, lamb]
- a. nodosum nodules in the a. that consist of typical stratified squamous epithelium. SYN: squamous metaplasia of a..
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am·ni·on 'am-nē-.än, -ən n, pl amnions or am·nia -nē-ə a thin membrane forming a closed sac about the embryos and fetuses of reptiles, birds, and mammals and containing the amniotic fluid called also amniotic sac
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n.
the membrane that forms initially over the dorsal part of the embryo but soon expands to enclose it completely within the amniotic cavity; it is connected to the embryo at the umbilical cord. It expands outwards and fuses with the chorion, obliterating virtually all the intervening cavity. The double membrane (amniochorion) normally ruptures at birth.
• amniotic adj.
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am·ni·on (amґne-on) [Gr. “bowlâ€; “membrane enveloping the fetusâ€] the thin but tough extraembryonic membrane of reptiles, birds, and mammals that lines the chorion and contains the embryo and later the fetus, with the amniotic fluid around it; in mammals it is derived from trophoblast by folding or splitting. See also amniotic sac, under sac.
Amnion and other extraembryonic membranes surrounding a human embryo at 10 weeks; the amnion has enveloped the umbilical cord.
Medical dictionary. 2011.