"The amnion can be visualised in most pregnancies before the 12th week of gestation and appears as a thin membrane separating the amniotic cavity, which contains the fetus, from the extra-embryonic coelom and the secondary yolk sac 1."
""Amnion" derives from the Greek word "άμνίον", a bowl in which the sacrificial blood of sheep was caught, which in turn derives from "άμνός", meaning "lamb". This is thought to be because fetal membranes were first described in sheep. The term was originally used for fetal membranes in general by Empedocles, whereas J Pollux was the first to use it to specifically refer to the amniotic membrane 5."
"By 14 weeks gestation, the amnion and chorion become closely apposed and are not seen as separate structures on ultrasound, often referred to as "chorioamniotic fusion" 6,7; however, the amnion and chorion remain separated by a thin layer of connective tissue and never undergo histological fusion 8."