What are foetal membranes. Give their names. Mention the function of each membrane.

Explanation

Embryonic or Foetal Membranes. Need for Foetal Membranes. An aquatic embryo is surrounded by water, which protects the Extra embryo, keeps it moist, removes wastes, and permits gas exchange. In land vertebrates (reptiles, birds and mammals), these functions are taken over by the extraembryonic membranes, some of which take Placental 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. Four foetal membranes are formed : amnion, chorion allantois and yolk sac.

(i) Amnion. It is formed above the embryo. It Allantois consists of trophoblast inside and mesoderm outside. It encloses a fluid-filled space, the amniotic cavity. 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 latter 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 amniotic fluid is derived from the foetal kidneys and bladder, i.e., it is foetal urine. It is completely replaced every 24 hours. It serves many functions. (i) It cushions the embryo, protecting it against bumps and against pressure of maternal organs, (ii) permits foetal movement, (iii) prevents bacterial infections, (iv) maintains a constant temperature and pressure, (v) protects the embryo from injury if the mother falls, (vi) allows development to proceed unhampered by the force of gravity, and (vii) the amniotic fluid absorbs shocks and prevents desiccation of the embryo.
Amniocentesis. The amniotic fluid contains foetal cells. These cells are the basis of prenatal test, called amniocentesis, for the sex of the foetus and for checking chromosomal defects in it. For this purpose, it is drawn out with a hypodermic needle and examined.
(ii) Chorion. It completely surrounds the embryo. It is composed of trophoblast outside and mesoderm inside. It protects the embryo and forms placenta for metabolic exchange between the foetus and the mother. In the later stages of pregnancy, the amnion expands to come in contact and partially fuse with the chorion.
(iii) Yolk Sac. It is formed below the embryo. It contains fluid, not yolk. The yolk sac is a vestigial organ inherited from the oviparous reptilian ancestors. Its wall first consists of trophoblast and endoderm. At this stage, it is called primary yolk sac (Fig. ???). Later, the wall of yolk sac comes to consist of endoderm and mesoderm. It is now called secondary yolk sac (Fig. ???). 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.
(iv) Allantois. It is a small sac that pushes out from the gut of the embryo near the yolk sac by the third week. It remains small and does not reach the chorion. However, it forms umbilical arteries and vein which grow up to the chorion to vascularise it. The allantois also forms blood cells. It consists of endoderm inside and mesoderm outside.