The organ not formed by ectoderm is

Options

A. Brain
B. Spinal cord
C. Internal ear
D. gonads.

Answer

To determine which organ is not formed by the ectoderm, we need to understand the three germ layers formed during embryonic development: ectoderm, mesoderm, and endoderm. Each layer gives rise to different tissues and organs in the body.

**Step-by-Step Solution:**

1. **Identify the Germ Layers**:
- The three primary germ layers are ectoderm, mesoderm, and endoderm. Each layer has specific derivatives.

2. **Ectoderm Derivatives**:
- The ectoderm primarily develops into structures related to the nervous system and skin. Key derivatives include:
- Brain
- Spinal cord
- Internal ear
- Hair and nails

3. **Mesoderm Derivatives**:
- The mesoderm gives rise to structures such as:
- Heart
- Kidneys
- Gonads (ovaries and testes)
- Muscles and bones

4. **Endoderm Derivatives**:
- The endoderm forms internal organs such as:
- Lining of the gastrointestinal tract
- Liver
- Pancreas
- Respiratory tract

5. **Analyzing the Options**:
- A. Spinal cord - formed by ectoderm
- B. Brain - formed by ectoderm
- C. Internal ear - formed by ectoderm
- D. Gonads - formed by mesoderm

6. **Conclusion**:
- The organ that is not formed by the ectoderm is **D. Gonads**.

**Final Answer**: D. Gonads

---