Final answer:
By the end of the third week, the neural folds grow and fuse above the midline to form the neural tube, which later develops into the brain and spinal cord, establishing the central nervous system.
Step-by-step explanation:
By the end of the third week, what happens to the neural folds? They grow over the midline and fuse to form the neural tube. This tube will eventually give rise to both the spinal cord and the brain. Initially, neurulation involves the thickening of ectodermal tissue to form the neural plate, which subsequently begins to fold inwards, creating a neural groove flanked by neural folds.
As the edges of these folds meet in the midline above the neural groove, they merge to construct the neural tube. The anterior end of the neural tube will develop into the brain, while the posterior part becomes the spinal cord. This essential process shapes the foundation of the central nervous system throughout the fourth and fifth weeks of embryonic development.