Allantois - Nyomaku

It is one of the embryonic membranes of vertebrate reptiles, birds, and mammals, and forms the allantois. Early in the embryonic stage, a part of the intestine bulges out into a spherical shape from the ventral side near the end of the primitive gut tube, and then rapidly expands into the extraembryonic cavity between the amnion and choriospore to become the allantois. The inside of the allantois is made up of endodermal cells, and the outside is made up of mesodermal cells. In birds and reptiles, the allantois combines with the choriospore that is in close contact with the eggshell membrane to become the chorioallantoic membrane, which is lined with numerous blood vessels. From here, the embryo releases carbon dioxide and takes in oxygen through the eggshell. In mammals, the chorioallantoic membrane becomes the fetal placenta, which combines with the maternal placenta in the uterine wall to form the placenta, which exchanges various substances.

[Shigeo Takeuchi]

Source: Shogakukan Encyclopedia Nipponica About Encyclopedia Nipponica Information | Legend

脊椎(せきつい)動物の爬虫(はちゅう)類、鳥類、哺乳(ほにゅう)類の胚(はい)膜の一つで、尿嚢(のう)を形成する。胚の早い時期に、原腸管末端近くの腹側から球形に膨れ出した腸の一部が、羊膜と漿(しょう)膜の間の胚体外体腔(たいこう)に速やかに拡大し尿嚢となる。尿嚢の内側は内胚葉細胞、外側は中胚葉細胞からなる。鳥類や爬虫類では尿膜は卵殻膜に密着している漿膜と合して漿尿膜となり、多数の血管が配される。胚はここから卵殻を通して二酸化炭素を放出し酸素を取り込む。哺乳動物では漿尿膜は胎児性胎盤となり、子宮壁の母性胎盤と合し胎盤となって諸物質の交換にあたる。

［竹内重夫］

出典　小学館　日本大百科全書(ニッポニカ)日本大百科全書(ニッポニカ)について　情報 | 凡例