Final answer:
The dorsal surface cells of the inner cell mass form the primitive streak. This occurs during the third week of embryogenesis, when the embryonic disc transforms into a three-layered structure through the process of gastrulation.
Step-by-step explanation:
The dorsal surface cells of the inner cell mass form the primitive streak. During embryogenesis, specifically in the third week of development, the two-layered disc of cells known as the embryonic disc undergoes gastrulation to become a three-layered structure. This process starts when an indentation known as the primitive streak forms on the dorsal surface of the epiblast. Cells migrate through this streak, creating the endoderm and mesoderm, with the remaining cells forming the ectoderm.