In mammals the chorion and the allantois together form

Options

A. Placenta
B. Endometrium
C. Uterus
D. Yolk sac

Explanation

**Step-by-Step Solution:**

1. **Understanding the Structures**: The question asks about the chorion and allantois in mammals. These are two important extra-embryonic membranes that develop during embryonic development.

2. **Identifying the Functions**:
- The **chorion** is primarily involved in the protection of the embryo and plays a crucial role in the formation of the placenta.
- The **allantois** is involved in the excretion of waste and gas exchange, and it also contributes to the formation of the umbilical cord.

3. **Combining the Functions**: Together, the chorion and allantois contribute to the formation of the placenta, which is essential for the exchange of nutrients, gases, and waste between the mother and the developing embryo.

4. **Evaluating the Options**: The options provided are:
- A) Placenta
- B) Endometrium
- C) Uterus
- D) Yolk sac

Among these, the placenta is formed by the combination of the chorion and allantois.

5. **Conclusion**: Therefore, the correct answer to the question is that the chorion and allantois together form the **placenta**.

**Final Answer**: A) Placenta

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