Notochord occurs in the embryonic stage in

Options

A. Some chordates
B. All chordates
C. All vertebrates
D. Some vertebrates

Answer

### Step-by-Step Solution:

1. **Understanding Notochord**:
- The notochord is a flexible rod-like structure that is a defining characteristic of all chordates during their embryonic development.

2. **Presence in Chordates**:
- All chordates possess a notochord at some stage in their life cycle. This includes both protochordates (urochordates and cephalochordates) and vertebrates.

3. **Notochord in Vertebrates**:
- In vertebrates, the notochord is present during the embryonic stage but is typically replaced by the vertebral column (spine) in adults. The vertebral column can be either bony or cartilaginous.

4. **Notochord in Protochordates**:
- Protochordates are divided into two groups: urochordates and cephalochordates.
- In urochordates, the notochord is present only in the larval (embryonic) stage and is not retained in the adult form.
- In cephalochordates, the notochord is persistent throughout the organism's life, meaning it is present in both the embryonic and adult stages.

5. **Analyzing the Options**:
- The question asks if the notochord occurs in the embryonic stage in all chordates.
- Option 1: "Some chordates" - Incorrect, as all chordates have a notochord in the embryonic stage.
- Option 2: "All chordates" - Correct, as all chordates exhibit a notochord during embryonic development.
- Option 3: "All vertebrates" - While true, it is not the most inclusive answer since it excludes protochordates.
- Option 4: "None of the above" - Incorrect, as option 2 is correct.

6. **Conclusion**:
- The correct answer is **Option 2: All chordates**.