In all vertebrates, the chorion is a membranous structure which completely encloses a developing embryo. After birth, this chorion is no longer needed, and is discarded.

In birds and reptiles, the chorion lies up against the interior of the eggshell, enclosing the entire contents of the egg. At one point, the chorion and another membrane, the allantois, unite to form the chorioallantoic membrane. Vessels pass through this area, delivering oxygen and water to the developing embryo, and allowing carbon dioxide to be released.

In mammals, the chorion is an important tissue which is produced very early in a pregnancy. Ultimately, it becomes part of the placenta. The placenta is an organ which connects the developing embryo with the mother's uterus, allowing the exchange of nutrients, oxygen, and waste between the embryo and its mother.

Soon after an egg is fertilized by a sperm, the resulting single-celled zygote divides repeatedly, to make up the multicelled blastocyst. This blastocyst is composed of inner cells which will ultimately become the embryo, and outer cells which will ultimately form the chorion.

In mammals, the chorion is involved in the development of finger-like projections which push into the lining of the uterus. These chorionic villi will harbor blood vessels responsible for the delivery of oxygen and nutrients, and the removal of carbon dioxide and other wastes.

Human chorionic gonadotropin (hCG) is a hormone produced by the human chorion. This is the hormone which is measured to reveal the presence of a pregnancy in simple urine home pregnancy tests.