Around two-thirds of monozygotic pregnancies see the embryos share a placenta, but develop their own amnion, a membrane that serves as further protection for the embryo.
The allantois forms as an outpocketing of the embryo's hindgut and grows outward into the space between the amnion and chorion.
In reptiles and birds, the amnion, chorion, and allantois are further protected by a hard or leathery calcareous shell.
Absence or abnormality of whole limbs is less common and includes, besides clubfoot, the so-called congenital amputations once thought to be caused by the strangulation of a limb by a fold of embryonic membrane (amnion).
Vertebrates have three embryonic membranes: the amnion, the chorion, and the allantois.
About the time that the cervix becomes fully dilated, the amnion breaks, and the force of the involuntary uterine contractions may be augmented by voluntary bearing-down efforts of the mother.
By this combined action each contraction of the uterus not only forces the amnion and fetus downward against the dilating neck of the uterus but also pulls the resisting walls of the latter upward over the advancing amnion, presenting part of the child.
The inner lining of the fold becomes the amnion, and the outer becomes the chorion, which ultimately surrounds the entire embryo.
The challenge and power of such separations is demonstrated in the two-dimensional gel electrophoretic separation of sulfur-35 methionine-labeled polypeptides, or proteins, from transformed epithelial amnion cells (AMA).
In reptiles and birds it expands greatly between two other membranes, the amnion and chorion, to serve as a temporary respiratory organ while its cavity stores fetal excretions.
Lying in the chorionic cavity (a thin liquid-filled space) between two membranous envelopes (chorion and amnion) is a small balloon-like sac, yolk sac, or vitelline sac, attached by a delicate strand of tissue to the region where the umbilical cord (the structure connecting the fetus with the placenta) leaves the amnion.
