Which of the following sentence is true for the chorion?

The correct option is: A membrane developing around an embryo, contributing to the formation of the placenta

Explanation:
The chorion is the outermost extraembryonic membrane that surrounds the developing embryo in mammals, birds, and reptiles. In placental mammals, it plays a critical role in fetal development by growing chorionic villi, which invade the uterine wall. These villi facilitate the transfer of nutrients, gases, and wastes between the maternal blood and the fetal blood. Consequently, the chorion directly contributes to the formation of the fetal portion of the placenta.

Let's look at why the other options are incorrect:
1. The sac or pouch-like organ within uterus containing the amniotic fluid surrounding the fetus: This describes the amnion, not the chorion. The amnion is the inner membrane that forms the fluid-filled amniotic sac to cushion and protect the fetus.
2. The organ in the uterus connected to the fetus by the umbilical cord: This describes the entire placenta as a fully formed organ, rather than the chorion membrane specifically.