There are some extra-embryonic membranes develop during embryo development also require for the nourishment of embryo.
They remain outside the embryo and also protect it.
There are four membranes:
It develops at the time of 14 days of embryo development.
It develops from the proximal region of the embryonic disc and completely envelops, first the embryo and then the fetus.
In amnion, amniotic fluid occurs that protects the fetus against temperature changes and mechanical shocks.
B. Yolk sac:
It develops in a 21 day embryo.
In human it is present only namesake as the fetus receives nourishment from mother’s body. So it has no importance in human body.
As with yolk sac, allantois also develops in a 21 day embryo.
It plays a constitutional role in placenta with chorion.
It is a vascularized tubular structure that forms umbilical cord.
By the 5th day of embryo development, morula (16-celled structure in uterus) is converted into a blastocyst that is a fluid-filled bag like structure.
On the outer side of blastocyte, a thin single layer of cell occurs, known as trophoblast that secretes fluid in blastocyte.
The mass of cell present in the trophoblast is known as inner cell mass that is responsible for the formation of the embryo.
Later the trophoblast associate with the layer of mesoderm and form an extra-embryonic membrane called “CHORION”.
Here both the allantois and chorion take part in the formation of placenta called as chorioallantoic-placenta. It develops at the time of embryo implantation.
On the one side of placenta, mother’s uterus tissues are located and on the other side of placenta, tissues of chorion-allantois occur.
There is presence of chorion villi on the placenta that are covered by the blood sinuses of maternal tissues, which is called as “hemochorial placenta”.
Due to this arrangement, blood of fetus and mother do not mix with each other.
The fetus is nourished by the nutrients, oxygen and antibodies from the mother body and also excretes the wastes into the same.
At the stage of further development umbilical cord develops and the distance between placenta and fetus increases. Now the materials are exchanged through the cord, so it is also known as “Life line” of the fetus.