[A]: Emboly involves the invagination and involution through bottling process by the mesentoblast cells.
[R]: Emboly is exhibited by the formation, elongation and ultimate recession of the primitive streak.

Options

A. If both A and R are true and R is the correct explanation of A
B. If both A and R are true but R is not the correct explanation of A
C. If A is true and R is false
D. If both A and R are false

Answer

To solve the question regarding emboly and its relation to the primitive streak, we will break down the assertion and reason step by step.

### Step-by-Step Solution:

1. **Understanding the Assertion**:
- The assertion states that "Emboly involves the invagination and involution through a bottling process by the mesentoblast cells."
- **Explanation**: Emboly refers to a specific process during embryonic development where certain cells undergo invagination (folding inward) and involution (cells moving inward) to form different germ layers. Mesentoblast cells are involved in this process.

2. **Understanding the Reason**:
- The reason states that "Emboly is exhibited by the formation, elongation, and ultimate recession of the primitive streak."
- **Explanation**: The primitive streak is a structure that forms during the early stages of embryonic development. It is crucial for the process of gastrulation, where the three germ layers (ectoderm, mesoderm, and endoderm) are established. The primitive streak undergoes changes, including elongation and eventual recession, which are characteristic of the emboly process.

3. **Evaluating the Truth of Both Statements**:
- Both the assertion and the reason are true.
- The assertion accurately describes the process of emboly, and the reason correctly identifies the role of the primitive streak in this process.

4. **Determining the Relationship**:
- The reason provides a correct explanation of the assertion. The formation and changes in the primitive streak are integral to the process of emboly, as they facilitate the invagination and involution of cells.

5. **Conclusion**:
- Since both the assertion and reason are true, and the reason is the correct explanation of the assertion, the correct answer is that both the assertion and reason are true, and the reason is the correct explanation of the assertion.

### Final Answer:
**Option A**: Both assertion and reason are true, and reason is the correct explanation of assertion.