Final answer:
Gyri and sulci develop during embryonic development as the telencephalon expands within the skull, resulting in the folding of the cortex and the formation of landmarks on the surface of the brain.
Step-by-step explanation:
The gyri and sulci of the brain develop during embryonic development. As the telencephalon expands within the skull, the brain goes through a regular course of growth, resulting in the development of gyri and sulci. These structures form important landmarks that allow us to separate the brain into functional centers. The folding of the cortex maximizes the amount of gray matter, and the surface of the brain can be mapped based on the locations of the gyri and sulci.