Amniotic membrane - Good

The amnion is the embryonic membrane that directly encases the embryo during embryonic development in some invertebrates such as insects, and in vertebrates such as reptiles, birds, and mammals. The gap between the amnion and the embryo is called the amniotic cavity, which is filled with amniotic fluid and is thought to protect the embryo from mechanical shock. The amnion has the same origin as the chorion, which is one of the embryonic membranes. In vertebrates, the ectoderm just outside the embryo rises in folds with the mesoderm that lines it during early embryonic development, enveloping the embryo in a double layer from the front, back, and both sides, and then joining and fusing in the middle. The side of this fold that faces the embryo becomes the amnion, and the outer side becomes the chorion. The gap between the amnion and the chorion becomes the extraembryonic coelom, into which the allantois later enters. Vertebrates that form an amniotic membrane during the embryonic stage are sometimes called amniotes, while vertebrates that do not (fish, amphibians) are called anamniotes. Unlike vertebrate amniotic membranes, insect amniotes are not lined with mesoderm. After cleavage, a germinal band is formed, where the embryonic body is formed, and the remaining extraembryonic cell layers move relative to the embryonic body to become the amniotic membrane and chorion.

[Shigeo Takeuchi]

Source: Shogakukan Encyclopedia Nipponica About Encyclopedia Nipponica Information | Legend

昆虫など無脊椎(むせきつい)動物の一部と、脊椎動物のうち爬虫(はちゅう)類、鳥類、哺乳(ほにゅう)類の胚(はい)発生の際、胚体を直接包む胚膜をいう。羊膜と胚体の間の間隙(かんげき)は羊膜腔(こう)とよばれ、羊水で満たされていて、胚体を機械的衝撃から保護していると考えられている。羊膜は、胚膜の一つである漿膜(しょうまく)と同じ起源で、脊椎動物では胚発生の早い時期に、胚体のすぐ外側の外胚葉が裏打ちをしている中胚葉とともにひだとなって盛り上がり、胚体の前後、両側から胚体を二重に包み中央で合して融合する。このひだの胚体に面している側が羊膜となり、外側が漿膜となる。羊膜と漿膜の間隙は胚体外体腔となり、のちに尿嚢(のう)が入り込んでくる。胚期に羊膜をつくる脊椎動物を羊膜類、つくらない脊椎動物（魚類、両生類）を無羊膜類と分けることがある。昆虫の羊膜は脊椎動物のものと異なり、中胚葉の裏打ちがない。表割後になると胚帯ができ、そこで胚体の形成がおこるが、残された胚体外の細胞層が胚体との相対的な位置の移動により羊膜と漿膜になる。

［竹内重夫］

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