yolk sac
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[生] Yolk sac (난황낭) (卵黃囊)
placenta : an organ characteristic of true mammals during pregnancy, joining mother and
offspring, providing endocrine secretion and selective exchange of soluble, but not
particulate, blood-borne substances through an apposition of uterine and trophoblastic
vascularized parts. According to species, the area of vascular apposition may be diffuse,
cotyledonary, zonary or discoid; the nature of apposition may be labyrinthine or villous;
the intimacy of apposition may vary according to what layers are lost of those
originally interposed between maternal and fetal blood (maternal endothelium, uterine
connective tissue, uterine epithelium, chorion, extraembryonic mesoderm, and endothelium
of villous capillary). The chorion may be joined by and receive blood vessels from either
the yolk sac or the allantois, and the uterine lining may be largely shed with the
chorion at birth (deciduate) or may separate from the chorion and remain (nondeciduate).
The human placenta is discoid, villous, haemochorial, chorioallantoic, and deciduate. After
birth, it weighs about 600 gm. and is about 16 cm. in diameter and 2 cm. thick,
discounting a principal functional part, the maternal blood in the intervillous space
(which leaks out at birth) into which the chorionic villi dip. The villi are grouped into
adjoining cotyledons making about 20 velvety bumps on the side of the placenta facing
outward to the uterus; the inner side of the placenta facing the fetus is smooth, being
covered with amnion, a thin avascular layer that continues past the edges of the
placenta to line the entire hollow sphere of chorion except where it is reflected to cover
the umbilical cord, which joins fetus and placenta. The cord usually joins the placenta
near the centre but may insert at the edge, on the nonplacental chorion, or on an
accessory placenta.
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