1. Embryonic and Foetal Membranes

2. Need for Foetal Membranes An aquatic embryo is surrounded by water, which protects the embryo, keeps it most, removes wastes and permits gas exchange. An aquatic embryo is surrounded by water, which protects the embryo, keeps it most, removes wastes and permits gas exchange. In land vertebrates (reptiles, birds and mammals) these functions are taken over by the extra embryonic membranes. In land vertebrates (reptiles, birds and mammals) these functions are taken over by the extra embryonic membranes. Some of which take part in the formation of placenta in mammals. Some of which take part in the formation of placenta in mammals. The placenta develops before the embryo because the embryo depends heavily on the mother’s body for its vital requirements. The placenta develops before the embryo because the embryo depends heavily on the mother’s body for its vital requirements. Four foetal membranes are formed: Four foetal membranes are formed: Yolk Sac Yolk Sac Amnion Amnion Chorion and Chorion and Allantois Allantois

3. 1. Yolk Sac It is formed below the embryo. It is formed below the embryo. It contains fluid not yolk. It contains fluid not yolk. The yolk sac is a vestigial organ inherited from the oviparous reptiles ancestors. The yolk sac is a vestigial organ inherited from the oviparous reptiles ancestors. Its wall first consist of trophoblast and ectoderm. At this stage it is called primary yolk sac. Its wall first consist of trophoblast and ectoderm. At this stage it is called primary yolk sac. Later the wall of yolk sac comes to consists of endoderm and mesoderm. It is called now secondary yolk sac. Later the wall of yolk sac comes to consists of endoderm and mesoderm. It is called now secondary yolk sac. The yolk sac functions as the site of blood cell formation until about the 6 th week, when the liver takes over this role. The yolk sac functions as the site of blood cell formation until about the 6 th week, when the liver takes over this role. Thereafter the yolk starts to shrink. Thereafter the yolk starts to shrink.

4. 2. Amnion It is formed above the embryo. It is formed above the embryo. It consists trophoblas inside and mesoderm outside. It consists trophoblas inside and mesoderm outside. It encloses a fluid-filled space, the amniotic cavity. It encloses a fluid-filled space, the amniotic cavity. The amnion and the amniotic cavity enlarge and nearly surround the embryo. The amnion and the amniotic cavity enlarge and nearly surround the embryo. The embryo is suspended in the amniotic cavity by an umbilical cord. The embryo is suspended in the amniotic cavity by an umbilical cord. This cord is formed of the stalks of the yolk sac and allantois. This cord is formed of the stalks of the yolk sac and allantois. The main blood vessels from the placenta reach the foetus through the umbilical cord. The main blood vessels from the placenta reach the foetus through the umbilical cord. The amniotic fluid is derived from the foetal kidneys and bladder. (i.e. it is foetal urine). The amniotic fluid is derived from the foetal kidneys and bladder. (i.e. it is foetal urine). It is completely replaced very 24 hours. It is completely replaced very 24 hours. It serves many functions. It serves many functions.

5. It cushions the embryo, protecting it against bumps and against pressure of maternal organs. It cushions the embryo, protecting it against bumps and against pressure of maternal organs. Permits foetal movements. Permits foetal movements. Prevents bacterial infections Prevents bacterial infections Maintains a constant temperature and pressure. Maintains a constant temperature and pressure. Protects the embryo from injury if the mother falls Protects the embryo from injury if the mother falls Allows development to proceed unhampered by the force of gravity. Allows development to proceed unhampered by the force of gravity. The amniotic fluid absorbs shocks and prevents desiccation of the embryo. The amniotic fluid absorbs shocks and prevents desiccation of the embryo.