Question

What are the four embryonic layers ?

Answer 1

Mammals and birds (and even reptiles) produce four different kinds of extraembryonic membranes to protect the embryo: amnion, yolk sac, allantois, and chorion.

1. Yolk sac:
It is formed of splanchnopleur (inner endoderm and outer mesoderm) and is well developed in reptiles, birds and prototherians having poly lecithal egg. It is mainly digestive in function so acts as extra embryonic gut. It also absorbs the dissolved yolk and passes it to developing embryo. In human beings, it is vestigial.
2. Amnion:
It is innermost fold of somatopleur (inner ectoderm and outer mesoderm) above the embryo. Between the amnion and embryo, there is amniotic cavity filled with amniotic fluid secreted by both embryo and amnion. Amnion protects the embryo while amniotic fluid acts as shock absorber and also prevents dessication of embryo.
3. Allantois:
It is a fold of splanchnopleur developed from the hind gut of the embryo. It is well developed in amniotes with polylecithal egg (e.g., reptiles, birds and prototherians) and stores the nitrogenous wastes of the embryo so acts as extra embryonic kidney. In most of eutherian, it combines with chorion to form allantochorion which takes part in placenta formation (Allantoic placenta). It is reduced in human beings.
4. Chorion:
It is outermost fold of somatopleur and surrounds the embryo. In reptiles, birds and prototherians, allantochorion acts as extra embryonic lung and helps in exchange of gases. But in primates including human beings, only chorion forms the placenta (chorionic placenta) while in other eutherian, allantochorion forms allantoic placenta.