Embryonic membrane

A cellular membrane that spreads out to encase the embryo during animal embryo development. It is also called the fetal membrane or embryonic membrane. It is found in the embryos of some invertebrates such as insects, and vertebrates such as reptiles, birds, and mammals. The chorioallantoic membrane, which is the outermost membrane of the embryo, the amnion that encases the embryo body, and the allantois that forms the allantois are basic embryonic membranes. Other examples of embryonic membranes include the chorioallantoic membrane, which is a combination of the chorioallantoic membrane and the fetal placenta (mammals), which is a modified chorioallantoic membrane, and the yolk sac wall that encases the yolk. Embryonic membranes also perform functions other than protecting the embryo body. The large surface area of ​​the embryonic membrane is effectively utilized for gas exchange in the chorioallantoic membrane of birds, nutrient absorption and waste excretion in the fetal placenta, nutrient intake in the yolk sac, and material movement within the embryo.

[Shigeo Takeuchi]

Source: Shogakukan Encyclopedia Nipponica About Encyclopedia Nipponica Information | Legend

動物の胚発生時、胚を包むように広がる細胞性の膜をいう。胎膜、胚付属膜ともよばれる。昆虫など一部の無脊椎(むせきつい)動物や、爬虫(はちゅう)類、鳥類、哺乳(ほにゅう)類などの脊椎動物の胚にみられる。胚の最外側に位置する漿膜(しょうまく)、胚体を包む羊膜、尿嚢(のう)をつくる尿膜などは基本的な胚膜で、漿膜と尿膜とが合した漿尿膜、漿尿膜の変化した胎児性胎盤（哺乳類）、あるいは卵黄を包む卵黄嚢壁なども胚膜に含められる。胚膜は胚体の保護以外の働きも兼ねる。鳥類の漿尿膜でのガス交換、胎児性胎盤での養分の吸収と老廃物の排出、卵黄嚢での養分摂取、あるいは胚内での物質の移動など、胚膜の表面積の大きさが有効に利用されている。

［竹内重夫］

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