EXTRAEMBRYONIC MEMBRANES

When the embryo has been implanted, it produces four extraembryonic membranes (membranes outside the body of the embryo namely: amnion, chorion, allantois and yolk sac). Since these membranes are located outside the body of the embryo, they are referred to as extraembryonic membranes.

 

A) Amnion: Amnion is a thin inner most membrane that covers the embryo. The amnion lines the cavity that surrounds the embryo. The space between the amnion and the embryo is called amniotic cavity. The space is filled with watery fluid called amniotic fluid.

 

Functions of amnion

1. The amniotic fluid protects the embryo from shock and mechanical injury.
2. The amniotic fluid provides an even temperature for the embryo and allows the embryo to move freely.
3. The amniotic fluid keeps the embryo moist.
4. The amniotic fluid is slippery and therefore helps for an easy passage of the foetus during child birth.

 

B) Chorion: The chorion is the outermost membrane that surrounds the amnion.

 

Functions of chorion

1. Protects the embryo outside.
2. It helps in the formation of placenta. After the full formation of placenta, chorion and amnion, the embryo is now called foetus. This happens after about eight weeks.

 

C) Allantois: Allantois grows out from the embryonic gut and fuses with the chorionic villi in the uterine wall.

 

Functions of Allantois 

It is a part of umbilical cord that carries foetal blood vessels to and from the chorionic villi, thus serving for respiration, nutrition and excretion of the foetus.

 

D) Yolk sac: It has no function in mammals. It is hidden in placenta.