Which layer of embryo is formed first

Options

A. Ectoderm
B. Mesoderm
C. Endoderm
D. Both B and C.

Answer

To answer the question "Which layer of the embryo is formed first?" we can follow these steps:

### Step-by-Step Solution:

1. **Understanding Germ Layers**:
- There are two types of organisms based on the number of germ layers: diploblastic (two layers) and triploblastic (three layers). The three germ layers are ectoderm, mesoderm, and endoderm.

2. **Identifying Germ Layers**:
- Ectoderm: The outermost layer.
- Mesoderm: The middle layer.
- Endoderm: The innermost layer.

3. **Formation Sequence**:
- In the development of an embryo, the first germ layer to be formed is the endoderm. This layer serves as the foundation for the other layers.
- After the endoderm is formed, the ectoderm is developed as the outer layer.
- Finally, the mesoderm is formed as the middle layer.

4. **Conclusion**:
- Therefore, the first layer of the embryo that is formed is the endoderm.

### Final Answer:
The first layer of the embryo that is formed is the **endoderm**.

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