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The amnion (: amnions or amnia) is a membrane that closely covers human and various other when they first form. It fills with amniotic fluid, which causes the amnion to expand and become the amniotic sac that provides a protective environment for the developing embryo. The amnion, along with the chorion, the yolk sac and the allantois protect the embryo. In , and , the protective sac is enclosed in a shell. In and Placentalia, it is enclosed in a uterus.
The amnion is a feature of the vertebrate clade amniote, which includes reptiles, birds, and . and fish lack the amnion and thus are anamniotes (non-amniotes). The amnion stems from the extra-embryonic somatic mesoderm on the outer side and the extra-embryonic ectoderm or trophoblast on the inner side.
- "[...] it has been suggested that ἄμνιος derives from Ἀμνιάς, epithet of Eileithyia, a goddess of childbirth, which may reflect her supposed birthplace, Ἀμνισός, the port of Knossos in Crete."
When first formed, the amnion is in contact with the body of the embryo, but about the fourth or fifth week amniotic fluid (also called liquor amnii) begins to accumulate within it. This fluid increases in quantity and causes the amnion to expand and ultimately to adhere to the chorion's inner surface, so that the extra-embryonic part of the coelom is obliterated. The amniotic fluid increases in quantity up to the sixth or seventh month of pregnancy, after which it diminishes somewhat; at the end of pregnancy it amounts to about one liter.
The amniotic fluid allows the free movements of the fetus during the later stages of pregnancy, and also protects it by diminishing the risk of injury from without. It contains less than two percent solids, consisting of urea and other extractives, inorganic salts, a small amount of protein, and frequently a trace of sugar. That some of the liquor amnii is swallowed by the fetus is proved by the fact that epidermal debris and hairs have been found among the contents of the fetal alimentary canal.
At the point of constriction where the primitive digestive tube of the embryo joins the yolk sac a reflection or folding upward of the somatopleure takes place.
This, the amniotic fold, first makes its appearance at the cephalic extremity, and subsequently at the caudal end and sides of the embryo, and gradually rising, its different parts meet and fuse over the dorsal aspect of the embryo, and enclose a cavity, the amniotic cavity. This kind of amnion is known as pleuroamnion (formed by folding), as opposed to schyzoamnion (formed by delamination).
After the fusion of the edges of the amniotic fold, the two layers of the fold become completely separated, the inner forming the amnion, the outer the false amnion or Serous membrane.
The space between the amnion and the serosa constitutes the extra-embryonic celom, and for a time communicates with the embryonic celom.
Kitten and puppy are born inside of the amnion; the mother cuts it open and eats it.
In , "The amnion is continued from the base of the umbilical cord upon the allantois, which is of considerable size, and is so interposed between the chorion and amnios, as to prevent any part of the amnios attaining the inner surface of the placenta. The amnios consists of two layers:one is the granular layer, continued upon the inner or foetal surface of the allantois, and thence upon the umbilical cord; the other is the smooth outer layer, continued upon the outer or chorional surface of the allantois, and thence upon the inner surface of the chorion."
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