Which of the following extra-embryonic membrane plays an important rote in the formation of the placenta?

Options

A. Yolk sac
B. Amnion
C. Chorion
D. Allantois

Answer

To determine which extra-embryonic membrane plays an important role in the formation of the placenta, we can analyze each of the options provided:

1. **Yolk Sac**:
- The yolk sac is involved in early embryonic development, primarily serving as a site for blood cell formation until about the sixth week of gestation. However, in humans, the yolk sac does not contribute to the formation of the placenta since human eggs lack significant yolk content.

2. **Amnion**:
- The amnion is a membrane that surrounds the embryo and forms the amniotic cavity, where the embryo is suspended. Its primary function is to protect the embryo and provide a cushioning environment, but it does not play a direct role in placenta formation.

3. **Chorion**:
- The chorion is a critical extra-embryonic membrane that contributes to the formation of the placenta. It is involved in the exchange of gases and nutrients between the mother and the developing fetus. The chorion develops into the outer layer of the placenta, making it essential for placental function.

4. **Allantois**:
- The allantois is a small structure that has limited function in humans. It is primarily involved in the development of blood vessels that will connect to the placenta but does not directly contribute to the formation of the placenta itself.

Based on this analysis, the extra-embryonic membrane that plays an important role in the formation of the placenta is the **chorion**.

**Final Answer**: Chorion