<P> The amniotic sac, commonly called the bag of waters, sometimes the membranes, is the sac in which the fetus develops in amniotes . It is a thin but tough transparent pair of membranes that hold a developing embryo (and later fetus) until shortly before birth . The inner of these fetal membranes, the amnion, encloses the amniotic cavity, containing the amniotic fluid and the fetus . The outer membrane, the chorion, contains the amnion and is part of the placenta . On the outer side, the amniotic sac is connected to the yolk sac, the allantois and, via the umbilical cord, to the placenta . </P> <P> Amniocentesis is a medical procedure where fluid from the sac is sampled to be used in prenatal diagnosis of chromosomal abnormalities and fetal infections . </P> <P> The amniotic cavity is the closed sac between the embryo and the amnion, containing the amniotic fluid . The amniotic cavity is formed by the fusion of the parts of the amniotic fold, which first makes its appearance at the cephalic extremity, and subsequently at the caudal end and sides of the embryo . As the amniotic fold rises and fuses over the dorsal aspect of the embryo, the amniotic cavity is formed . </P> <P> At the beginning of the second week, a cavity appears within the inner cell mass and when it enlarges it becomes the amniotic cavity . The floor of the amniotic cavity is formed by the epiblast . Epiblast migrates between the epiblastic disc and trophoblast . In this way the epiblastic cells migrate between the embryoblast and trophoblast . The floor is formed by the epiblast which later on transforms to ectoderm while the remaining cells which are present between the embryoblast and trophoblast are called amnioblasts (flattened cells). These cells are also derived from epiblast which is transformed into ectoderm . </P>

When does the sac attached to the uterus