What is the earliest period during which development of the central nervous system occurs?

The development of the central nervous system, or neurodevelopment, begins very early in embryonic development. The process of neural tube formation, which marks the initial development of the central nervous system, begins around the third week of gestation, specifically during the period from the 18th to the 21st day after fertilization. By the end of the fourth week, the fundamental structures of the central nervous system, such as the brain and spinal cord, are being established.

Choosing the correct answer, which indicates that significant development can be noted around the sixth week, captures the critical phase when the neural tube has closed, and the basic organization of the brain is starting to form. This is a crucial period as the formation of the neural tube and its derivatives lays the foundation for the entire nervous system.

While some initial processes start before the sixth week, the major milestones of central nervous system development, such as the differentiation of neuronal precursors and initial brain structure formation, are better associated with this time frame. Further development continues into the later weeks, but the significant and foundational aspects of central nervous system development are centered around the sixth week of pregnancy.