What is structure function and importance of extraembryonic membranes?

Extra-embryonic membranes include Chorion , Amnion , Allontois and Yolk sac . They are formed in Amniotes including Reptiles , Birds and Mammals . They are formed only during development and are of essential for development of Amniotes .
1 : Chorion : It is outermost extra-embryonic membrane . It is formed by ectoderm and mesoderm . Its major function is protection in reptiles and birds . In mammals it helps in formation of Placenta and umbilical cord along with allontois .
2 : Amnion: It is made up of mesoderm and ectoderm . It contains amniotic fluid for protection which acts as shock absorber . It has evolutionary significance .
3 : Allontois; It is made up of endoderm and ectoderm .In reptiles and birds it helps in respiration and excretion . In mammals it helps in formation of umbilical cord .
4 Yolk Sac : It is made up of endoderm and ectoderm . It is more important in reptiles and birds as it protects yolk . In mammals it is not important
Placenta in mammals help in nutrition respiration and excretion .