Question

Describe the structure of different extraembryonic membranes and their functions in birds​

Answer 1

Answer:

The extraembryonic membranes consist of the chorion (the combination of trophoblast plus underlying extraembryonic mesoderm), amnion, yolk sac, and allantois.

The amnion, a thin ectodermal membrane lined with mesoderm, grows to enclose the embryo like a balloon

Some of the important types of extra embryonic membranes are: 1. Yolk sac 2. Amnion 3. Allantois and 4. Chorion!

These membranes are formed outside the embryo from the trophoblast only in amniotes (reptiles, birds and mammals) and perform specific functions.

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.

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