The developing foetus soon becomes enclosed by four foetal membranes, amnion, chorion, allantois and yolk sac. Amnion and chorion are formed by the fusion of the amniotic folds arising from the two ends of the foetus whereas allantois is produced from emoryonic hind gut.
1. Amnion is formed of mesoderm on outside and ectoderm inside. It has no blood vessels. Space between amnion and foetus is called amniotic cavity and it contains amniotic fluid. Amnion protects the foetus from mechanical shock.
2. Chorion is formed of ectoderm externally and mesoderm inside. Along with the allantois it participates in the formation of placenta. Space between amnion and chorion is externally embryonic coelom. It completely surrounds the embryo and protects it by providing nourishment and removing waste.
3. Allantois consists of mesoderm on outside, and endoderm internally. It extends to fuse with chorion and forms a structure rich in blood vessels called the allanto-chorion which gives rise to foetal part of placenta. It helps in respiration, nutrition and excretion.
4. Yolk sac is a membranous sac attached to embryo near allantois; having yolk in egg laying animals. In mammals the yolk sac functions as the site of blood cell formation until about the 6th week, when the liver takes over this role. Thereafter the yolk sac starts to shrink.