Final answer:
The most development of the nervous system takes place during embryonic neurulation, starting around the third week after fertilization, resulting in the formation of the neural tube, which gives rise to the brain and spinal cord.
Step-by-step explanation:
Most development of the nervous system occurs in the embryo during a stage called neurulation, which begins around the third week after fertilization. During this critical phase, the embryo forms a structure known as the neural tube, that eventually develops into the spinal cord and brain. This process is fundamental for the correct formation of the central nervous system (CNS).
By the fourth week of development, the early nervous system starts as a hollow tube and progresses into more complex structures. The anterior portion of the neural tube differentiates into the brain, while the posterior portion becomes the spinal cord. These stages lay the groundwork for the intricate network of the adult nervous system. It is also during this time that an adjacent structure, the neural crest, begins to form what will become the peripheral nerves.
Therefore, understanding the embryonic development of the nervous system is essential for grasping the complexity of its adult form, as well as the underlying order of its structure and function.