Final answer:
Gastrulation is initiated by the formation of the primitive streak on the dorsal surface of the embryonic disc, ultimately leading to the creation of three germ layers and setting the stage for further embryonic development like neurulation(option c).
Step-by-step explanation:
Gastrulation begins when a groove with raised edges called the c) primitive streak appears on the dorsal surface of the embryonic disc.
This process transforms the two-layer embryonic disc into a three-layered disc by creating a third middle layer called the mesoderm and an indentation that establishes the embryonic gut. The primitive streak serves as the entry point for cells to migrate and form the different embryonic layers. As these cells develop, the notochord forms underneath the neural tube, which will give rise to the central nervous system. It is the formation of the primitive streak that sets the stage for all subsequent embryological development, including neurulation, where the neural plate transitions into the neural tube and eventually the spinal cord and brain structures.