The placenta and embryo at 8 weeks of pregnancy. Image credit: Merck.
A chorion is one of the two membranes that surrounds the embryo. The chorion lies outside the amnion and is formed from trophoblast lined with mesoderm and becomes closely associated with the allantois. The blood vessels (supplied by the allantois) are concentrated in the region of the chorion that is attached to the wall of the uterus and develops into the placenta.
A chorion is found in the embryonic system of mammals, birds, and reptiles,
and the insect egg. In birds it is the moist lining between the shell and
the allantois (the organ for gaseous exchange).
Chorionic tissue may be removed for prenatal or antenatal diagnosis. See chorionic villus sampling.