**Core Concept**
Gastrulation is a critical phase in embryonic development, marking the transition from a single-layered blastula to a multi-layered gastrula. It involves the reorganization of cells to form the three primary germ layers: ectoderm, mesoderm, and endoderm. This process is essential for the formation of the embryo's basic body plan and organogenesis.

**Why the Correct Answer is Right**
The beginning of gastrulation is characterized by the formation of the primitive streak in avian and mammalian embryos, or the blastopore in amphibian embryos. The primitive streak is a linear groove that forms on the surface of the epiblast, marking the site of cell migration and germ layer formation. As cells migrate through the primitive streak, they undergo epithelial-to-mesenchymal transition (EMT), allowing them to move and eventually form the three primary germ layers.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because it does not specify a particular structure or event associated with the beginning of gastrulation. Gastrulation is a complex process that involves multiple cellular and molecular events, making it difficult to pinpoint a single event or structure.
**Option B:** This option is incorrect because the formation of the notochord, while an important event in embryonic development, occurs later in gastrulation, after the formation of the primitive streak. The notochord is a rod-like structure that provides axial support and patterning to the embryo.
**Option C:** This option is incorrect because the formation of the neural plate, while a critical event in embryonic development, occurs later in gastrulation, after the formation of the primitive streak. The neural plate eventually gives rise to the neural tube, which forms the brain and spinal cord.

**Clinical Pearl / High-Yield Fact**
A key feature of gastrulation is the reorganization of cells through epithelial-to-mesenchymal transition (EMT). EMT is a critical process that allows cells to acquire a mesenchymal phenotype, enabling them to migrate and eventually form the three primary germ layers.

**Correct Answer: C. The primitive streak.**