FunWhat are the extra-embryonic membranes and what is their...

What are the extra-embryonic membranes and what is their function

What are the extra-embryonic membranes and what is their function . As their name suggests, these are structures attached to the body of the embryo that ensure the initial development of the animals . Without these membranes, the survival of the new being would not be possible, since their vital functions would be affected, since they could not breathe, nourish themselves and excrete toxic substances.

What are the extra-embryonic membranes and what is their function

Reptiles , birds and mammals are protected before birth thanks to the extra-embryonic membranes that are divided into four main types: amnion, chorion, yolk sac and allantois .

It should be noted that some authors consider the umbilical cord and the placenta as extra-embryonic membranes and they do so because the umbilical cord contains allantois and its vessels and because the placenta has a fetal component (chorionic villi).


It is absent in fish and amphibians, being observed only in reptiles, birds and mammals, that is, in the group of amniotes. It is characterized by being a resistant membrane that forms a bag that surrounds the entire embryo . The amniotic sac contains amniotic fluid , which acts by preventing dehydration, maintaining temperature and protecting the body from mechanical blows and adhesions that may arise, such as between the embryo and the egg shell.

The amniotic egg receives this name due to the presence of the amnion. It was undoubtedly an important evolutionary novelty for life in the terrestrial environment, since it ensured that tetrapods reduced their dependence on the aquatic environment. In addition to the amnion, the amniotic egg has other embryonic junctions: the chorion, the yolk sac, and the allantois.


The chorion is present in reptiles, birds, and mammals, but does not occur in fish and amphibians. In reptiles and birds, the chorion is the most external accessory, being smooth and resistant. In mammals, this union is modified to form part of the placenta , forming extensions called chorionic villi, which are responsible for increasing the contact surface with maternal tissues. The chorion has different functions, one of them is to ensure the gas exchange of the embryo.

The yolk sac

The yolk sac is an embryonic accessory present in fish, reptiles, birds and mammals. This structure is related to the nutrition of the embryo, being a storage place for nutrients . This accessory is an endodermal derivative that is externally coated with mesoderm and grows around the calf. The yolk sac connects to the intestine through the yolk pedicle. In viviparous mammals, the yolk sac is reduced and is not related to the nutrition of the embryo, this function being performed by the placenta.


The allantoic is an embryonic accessory present in reptiles, birds and mammals, being absent in fish and amphibians. It is related to two main functions: the storage of products of the excretion of the embryo and the performance of gas exchange .

Umbilical cord

The umbilical cord develops from the involvement of the amnion around the yolk sac and the allantoic pedicle. This cord works by ensuring that nutrients are transported from the mother’s body to the fetus and that catabolites are removed in the opposite direction.


The placenta is a transitory organ present in eutherian mammals that guarantees the adhesion of the maternal organism to the embryo. The placenta performs different functions, being related to respiration, nutrition, excretion and secretion.

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