Short notes on the structure of foetal membrane (amnion, chorion, yolk sac and allantois)

These are the membranes that develop outside the embryo but in close association with it and they carry out certain specific functions. In human beings foetal membranes are amnion, chorion, yolk sac and allantois.

1. Amnion:

This is formed above the embryo. This consits of a cavity (amniotic cavity) and encloses a fluid called amniotic fluid. The embryo is suspended into the amniotic cavity by the umbilical cord. The aminotic fluid provides a shock absorbing effect to the embryo against bumps in­fections etc.


The watery fluid around the embryo helps in maintaining constant temperature and pressure and protects the embryo in case the mother has a fall. The aminotic fluid is derived from the mother’s blood and contains foetal cells. This is made use of in the prenatal sex test for the foetus- known as aminocentesis. In aminocentesis the amniotic fluid is drawn out wtih a syringe and the cells are tested for the presence of the sex chromosomes.

2. Chorion:

The chorion completely surrounds the embryo and has small projections all around it during early stages of development. The chorion is composed of trophoblast on the outside and mesoderm on the inside. Chorion protects the embryo and forms placenta for metabolic exchange between the mother and the foetus.

3. Yolk sac:


This is formed below the embryo. In human beings this con­tains a fluid but no yolk. It is vestigial organ. Its wall is made up of tropho- blast and endoderm. The yolk sac functions as the region of formation of blood cells upto about 6th week of development when the liver of the foetus takes up this function.

4. Allantois:

This is a small bag like structure that develops from the gut of the embryo and near the yolk sac. This membrane develops around the third week of development. Gradually the allantois shrinks in size and gets enclosed in the umbilical cord. Allantois helps in the formation of umbilical arteries and veins. The allantois also forms blood cells.