Final answer:
The neural tube is the earliest embryonic structure that develops into the entire nervous system, formed during the process of neurulation in the fourth week of development.
Step-by-step explanation:
The earliest discrete embryonic structure from which the entire nervous system develops is the neural tube. The process of neural tube formation is known as neurulation. Initially, during embryonic development, there is a thickening of specialized neuroectodermal tissues along the embryo called the neural plate. As the embryonic development progresses, this plate begins to fold, forming the neural groove. Eventually, the neural groove edges come together to form the neural tube. The anterior portion of the neural tube will then differentiate into the brain structures, while the posterior portion gives rise to the spinal cord. This happens around the fourth week of development, following the foundational stage of neurulation.