**Core Concept**
Gastrulation is a critical process in early embryonic development where the three primary germ layers – ectoderm, endoderm, and mesoderm – begin to form and differentiate from a single layer of cells. This process is essential for the establishment of the basic body plan and the formation of the three germ layers.

**Why the Correct Answer is Right**
Gastrulation occurs after the morula stage, around the 8th day of gestation, and involves a series of complex cellular movements and transformations. The process is initiated by the formation of the primitive streak, a linear structure that eventually gives rise to the mesoderm. As the primitive streak elongates, cells from the epiblast layer migrate towards it, forming the ectoderm and endoderm. The three germ layers then separate and begin to differentiate, eventually giving rise to all tissues and organs in the body.

**Why Each Wrong Option is Incorrect**
**Option A:** Ovulation refers to the release of a mature ovum from the ovarian follicle, which occurs in the female reproductive cycle, not in embryonic development. It is not related to the formation of the germ layers.
**Option B:** Neurilation is the process of neural tube formation, which occurs later in embryonic development, around the 23rd day of gestation. It is not directly related to the separation of the three germ layers.

**Clinical Pearl / High-Yield Fact**
Gastrulation is a critical period in embryonic development, and any disruptions to this process can lead to severe congenital abnormalities, such as neural tube defects or cardiac anomalies.

**✓ Correct Answer: C. Gastrulation**