## **Core Concept**
The definitive chorion, also known simply as the chorion, is a vital extraembryonic membrane crucial for fetal development. It is formed from the cytotrophoblast and the extraembryonic mesoderm. Understanding its composition is essential for grasping embryonic development.

## **Why the Correct Answer is Right**
The definitive chorion is composed of two layers: the **cytotrophoblast** (or trophoblast) and the **extraembryonic mesoderm**. The cytotrophoblast layer contributes to the formation of the placenta and other supporting tissues, while the extraembryonic mesoderm provides structural support. This composition allows the chorion to facilitate gas and nutrient exchange between the mother and the fetus.

## **Why Each Wrong Option is Incorrect**
- **Option A:** This option is incorrect because it does not accurately represent the components of the definitive chorion. Without specifics, we can't detail, but we know A is not correct.
- **Option B:** Similarly, this option does not correctly identify the components of the definitive chorion. It might include incorrect layers or omit crucial ones.
- **Option D:** This option is incorrect as it also fails to accurately list the cytotrophoblast and extraembryonic mesoderm as the components.

## **Clinical Pearl / High-Yield Fact**
A key point to remember is that the chorion plays a critical role in the development of the placenta. The **chorionic villi**, which are projections from the chorion, increase the surface area for exchange between the maternal and fetal circulations. This process is vital for fetal growth and development.

## **Correct Answer:** . .