Which is formed first ?

Options

A. Yolk sac
B. E mbryonic disc
C. Trophoblaset
D. Inner cell mass.

Answer

To determine which structure is formed first during embryonic development, we can analyze the options provided:

1. **Inner Cell Mass (ICM)**: This is a group of cells that forms within the blastocyst. It is crucial for the development of the embryo.

2. **Yolk Sac**: This structure is formed from the hypoblast layer of the inner cell mass and is involved in early nutrient transfer.

3. **Embryonic Disc**: This structure develops from the inner cell mass and eventually differentiates into the bilaminar and then trilaminar disc.

4. **Trophoblast**: This is the outer layer of cells surrounding the blastocyst that eventually contributes to the placenta.

### Step-by-Step Solution:

1. **Identify the Initial Structure**: The first structure to form in the early stages of embryonic development is the inner cell mass (ICM) within the blastocyst.

2. **Formation of Other Structures**:
- The **yolk sac** is derived from the hypoblast of the inner cell mass.
- The **embryonic disc** develops from the inner cell mass as it differentiates into bilaminar and trilaminar forms.
- The **trophoblast** is formed after the inner cell mass and surrounds the blastocyst.

3. **Conclusion**: Since the inner cell mass is the first structure to form, it is the correct answer to the question.

### Final Answer:
The structure that is formed first is **D. Inner Cell Mass**.

---