**Core Concept**
The formation of the three primary germ layers (ectoderm, mesoderm, and endoderm) is a critical event in embryonic development. These layers will eventually give rise to all tissues and organs in the body.

**Why the Correct Answer is Right**
At approximately 21 days post-fertilization, the blastula undergoes gastrulation, a complex process of cellular differentiation and migration. During gastrulation, the blastula is reorganized into the three primary germ layers: ectoderm (outermost layer), mesoderm (middle layer), and endoderm (innermost layer). This event marks a crucial milestone in embryonic development, as these layers will eventually give rise to all tissues and organs in the body. The ectoderm will form the central nervous system, skin, and other external structures, while the mesoderm will form muscles, bones, and connective tissue. The endoderm will form the lining of the digestive tract, respiratory system, and other internal organs.

**Why Each Wrong Option is Incorrect**
**Option A:** 8 days is too early for the formation of the three primary germ layers. At this stage, the embryo is still in the blastocyst stage, and gastrulation has not yet occurred.
**Option B:** 12 days is still too early for gastrulation to complete. While the embryo is beginning to undergo gastrulation, the three primary germ layers have not yet fully formed.
**Option C:** 16 days is closer to the correct answer, but gastrulation typically completes at around 21 days post-fertilization.

**Clinical Pearl / High-Yield Fact**
The formation of the three primary germ layers is a critical event in embryonic development, and any disruptions to this process can lead to severe congenital anomalies or developmental disorders.

✓ Correct Answer: D. 21 days