Final answer:
Neurulation begins in the third week post-fertilization simultaneously with the ongoing process of gastrulation, leading to the formation of the central nervous system and the establishment of the three primary germ layers.
Step-by-step explanation:
Neurulation and Gastrulation
Neurulation begins during the third week after fertilization, concurrently as gastrulation is still progressing. This sequential development illustrates how the embryonic structures for the central nervous system begin to form while the primary germ layers are still being established. Neurulation is characterized by the formation of the neural plate from the ectoderm, which then folds inward, eventually converging to create the neural tube—the precursor to the spinal cord and brain. Meanwhile, gastrulation forms the three germ layers: endoderm, mesoderm, and ectoderm, each giving rise to different cells and organ systems within the body. During this crucial phase, both processes overlap, signaling a complex coordination in embryonic development.
The neural tube not only eventually becomes the spinal cord but also develops a bulge at one end that will differentiate into the brain. This happens as the gastrulation is still leading to the differentiation of cells into the embryonic disc and establishing the three primary layers which will give rise to various organ systems through organogenesis.