**Core Concept**
Gastrulation is a critical phase in embryonic development, marking the transition from the blastula to the gastrula stage. During this process, the blastula undergoes a series of complex cellular movements and reorganizations, ultimately giving rise to the three primary germ layers: ectoderm, endoderm, and mesoderm.

**Why the Correct Answer is Right**
Gastrulation typically occurs around 3 weeks of gestation, during which the primitive streak forms in the epiblast layer of the embryo. This streak serves as a site of cell migration and reorganization, allowing the ectoderm, endoderm, and mesoderm to differentiate from a single layer of cells. The process of gastrulation is essential for the proper formation of the embryo's body plan and the development of its various organs and tissues.

**Why Each Wrong Option is Incorrect**
**Option A:** This is incorrect because gastrulation occurs before organogenesis, which typically begins around 4-5 weeks of gestation.
**Option B:** This is incorrect because implantation occurs around 1-2 weeks of gestation, well before gastrulation.
**Option C:** This is incorrect because neurulation, the process of neural tube formation, occurs after gastrulation, typically around 3-4 weeks of gestation.

**Clinical Pearl / High-Yield Fact**
It's essential to remember that gastrulation is a critical period in embryonic development, and any disruptions during this time can lead to severe congenital anomalies or even embryonic death. The proper formation of the germ layers during gastrulation lays the foundation for the development of the embryo's various tissues and organs.

**Correct Answer: C. 3 weeks of gestation.**