Category : 12th Class
These membranes are formed outside the embryo from the trophoblast only in amniotes and perform specific function. Some of these membranes take part in the formation of placenta in mammals.
(1) Yolk sac : It is formed below the embryo. It contains fluid, not yolk. The yolk sac is a vestigeal organ inherited from the oviparous reptilian ancestors. Yolk sac encloses by outer mesoderm and inner endodermal layer.
Function : In human beings, it is vestigial. In human embryo it act as the site of blood cell formation until about the 6th week, when the liver takes over this role.
(2) Amnion : It is formed above the embryo. It consist of outer mesoderm and inner ectoderm. The amnion and the fluid filled amniotic cavity it encloses, enlarge and nearly surround the embryo. Amniotic fluid secreted by both embryo and amnion.
(i) The amniotic fluid cushions the embryo.
(ii) It protects the embryo from jerk, injury and shocks.
(iii) It prevents desiccation of the embryo.
(3) Allantois : It is a fold of splanchnopleur developed from the hind gut of the embryo. It consist of outer mesoderm and inner endoderm.
(i) The cavity of the allantois serves as a urinary bladder. It stores the protein breakdown product in the form of water-insoluble crystals of uric acid and inside the egg upto the time of hatching.
(ii) The vascular “chorioallantoic membrane” lies in a close proximity to the inner surface of the porous shell. It acts as an extraembryonic lung by supplying the embryo with oxygen.
(4) Chorion : It is outermost fold of somatopleur (outer ectoderm and somatic mesoderm) and surrounds the embryo. In reptiles, birds and prototherians, allantochorion act as extra embryonic lungs help in exchange of gases. But in primates including human beings, only chorion forms the placenta (chorionic placenta).
Function : It protects the embryo and forms placenta for metabolic exchange between the foetus and the mother.
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