Updated by: Arabinda Naik
It is formed above the embryo. It consists of outer mesoderm and inner ectoderm. The amnion and the fluid filled amniotic cavity encloses, enlarge, and nearly surrounds the embryo. Amniotic fluid secreted by both embryo and amnion. At the beginning of the second week, the cells of the inner cell mass form into a two-layered disc of embryonic cells, and a space—the amniotic cavity—opens up between it and the trophoblast. Cells from the upper layer of the disc (the epiblast) extend around the amniotic cavity, creating a membranous sac that forms into the amnion by the end of the second week. The amnion fills with amniotic fluid and eventually grows to surround the embryo. Early in development, amniotic fluid consists almost entirely of a filtrate of maternal plasma, but as the kidneys of the fetus begin to function at approximately the eighth week, they add urine to the volume of amniotic fluid. Floating within the amniotic fluid, the embryo—and later, the fetus—is protected from trauma and rapid temperature changes. It can move freely within the fluid and can prepare for swallowing and breathing out of the uterus.
(i) The amniotic fluid cushions the embryo.
(ii) It protects the embryo from jerks, injury, and shocks.
(iii) It prevents desiccation of the embryo.
(iv) It maintains constant temperature and pressure.
(v) It prevents bacterial infections.
(vi) It permits fetal movement.