Embryonic Annexes: Adaptation to the Terrestrial Environment

Embryonic attachments are structures that derive from the germinal leaflets of the embryo but are not part of the embryo's body.

The embryonic attachments are: yolk sac (yolk sac), amnion (or amniotic sac), chorion and allantoid.

Vitelline gallbladder

During the evolution of the group of animals, the first vertebrates that emerged were fish, a group that has as its only embryonic annex the vitelline vesicle.

Differentiating the mesoderm and the neural tube, part of the germ leaflets develop forming a membrane that surrounds the entire bud, constituting (membrane + bud) the vitelline bag an embryonic attachment, which remains attached to the embryo's intestine. As it develops, there is the consumption of the calf and, consequently, the yolk sac decreases until it disappears. It is well developed not only in fish but also in reptiles and birds. The mammals have a reduced yolk bladder, because in these animals, eggs are poor in calf. The yolk bladder therefore has no meaning in the nutrition process of most mammals.

In amphibians, although eggs are rich in calf, the typical yolk bladder is lacking. In these animals the calf is found in large cells (macromers) not surrounded by the proper vitelline membrane.

Ammonium and Corium

O amnio It is a membrane that completely surrounds the embryo, delimiting a cavity called the amniotic cavity. This cavity contains the amniotic fluid, whose functions are to protect the embryo against mechanical shock and desiccation. At the end of the development of reptiles and birds, all the amniotic cavity fluid was absorbed by the animal.

O chorionic or serous It is a membrane that surrounds the embryo and all other embryonic attachments. It is the most external embryonic attachment to the body of the embryo. In reptiles and bird eggs, for example, this membrane is under the shell. In these animals, the chorion, along with the allantoid, participates in the gas exchange processes between the embryo and the external environment.


The allantoid is an attachment that derives from the posterior portion of the embryo's intestine. The function of the allantoid in reptiles and birds is to transfer to the embryo the proteins present in the egg white, to transfer part of the calcium salts present in the shell to the embryo, which will use these salts in the formation of its skeleton, to participate in the gas exchange. the o2 passes from the air chamber to the allantoid and from the embryo to the embryo, while CO2 produced travels the opposite way, and store nitrogen excreta. The nitrogenous excreta eliminated by embryos from these animals is water-insoluble and non-toxic uric acid, which can be stored inside the egg without contaminating the embryo.