Final answer:
Neurulation begins in the third week after fertilization, with the formation of the neural plate from neuroectodermal tissues. It progresses to the fourth week, where the neural folds arise and eventually converge to form the neural tube, which becomes the central nervous system. Subsequent weeks involve further differentiation into the brain and spinal cord as well as the surrounding skeletal and muscle structures.
Step-by-step explanation:
Neurulation begins in the third week after fertilization, following the process of gastrulation. During this critical phase of embryonic development, specialized neuroectodermal tissues thicken into what is known as the neural plate. As the process continues into the fourth week, the tissues on either side of the neural plate rise to form neural folds.
The neural tube lies atop a rod-shaped, mesoderm-derived notochord, which is key in the development of the nucleus pulposus of intervertebral discs. Alongside the neural tube, block-like structures called somites form, which will differentiate into the axial skeleton, skeletal muscles, and dermis. During the fourth and fifth weeks, the anterior portion of the neural tube begins to dilate and subdivide, forming vesicles that will develop into different structures of the brain.