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In addition to the embryo proper, the blastoderm gives rise to certain other structures which lie outside the embryo. These are called as foetal membranes or extra embryonic membranes.
These are essential for the complete development of the embryo. These include chorion, amnion, allantois and yolk sac.
(i) Chorion:
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It completely surrounds the embryo and protects it. It consists of ectoderm outside and mesoderm inside. It takes part in the formation of placenta for the exchange of materials between the foetus and the mother.
(ii) Amnion:
It is formed just above the embryo. It consists of ectoderm inside and mesoderm outside. The space between the embryo and the amnion is called as amniotic cavity. It is filled with a clear, watery fluid secreted by both the embryo and the membrane.
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The amniotic fluid performs many important functions:
(i) Prevents drying of the embryo.
(ii) It cushions the embryo, protecting it against bumps and pressure of the maternal organs.
(iii) It maintains constant temperature and pressure.
(iv) It prevents bacterial infections.
(v) It permits foetal movement.
(iii) Allantois:
It is a sac-like structure which arises from the gut of the embryo near the yolk sac. Allantois is formed of endoderm inside and mesoderm outside. In man the allantois is small and forms blood vessels of placenta and blood cells.
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(iv) Yolk sac:
It is a relatively larger sac-like structure formed below the embryo. It contains fluid, but not yolk. Its wall consists of endoderm inside and mesoderm outside. The yolk sac is non functional in human beings. The yolk sac is responsible for forming blood cells until about 6th week.
