[A] : Emboly involves the invagination and involution.
[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, we need to analyze both the assertion and the reason provided.

### Step-by-Step Solution:

**Step 1: Understand the Assertion**
- The assertion states that "Emboly involves invagination and involution."
- Emboly refers to the process of gastrulation, which is the formation of the gastrula from the blastula. This process involves the invagination (inward folding) and involution (inward rolling) of the epithelial layers to form the three germ layers: ectoderm, mesoderm, and endoderm.

**Step 2: Understand the Reason**
- The reason states that "Emboly is exhibited by the formation, elongation, and ultimate recession of the primitive streak."
- The primitive streak is a linear band of thickened epiblast that forms during gastrulation. It is crucial for the organization of the embryo and the formation of the three germ layers. The primitive streak appears at the caudal end and elongates toward the cranial end, facilitating the movement of cells that will form the germ layers.

**Step 3: Analyze the Relationship**
- The assertion is correct because emboly indeed involves invagination and involution during gastrulation.
- The reason is also correct as the formation and regression of the primitive streak are key events in the process of gastrulation (emboly).

**Step 4: Determine the Correct Option**
- Since both the assertion and reason are true, and the reason correctly explains the assertion, the correct choice is:
- **Option A: Both assertion and reason are true, and reason is the correct explanation of assertion.**