Central nervous system development, illustration. The developing baby is known as a foetus from around 8 weeks until birth, which occurs around week 39 to 41. One of the final stages of the development of the central nervous system is the closing up of the backbone and associated tissues around the backbone (left), and the formation of the skull around the brain (upper left). This process is shown here, with a normal baby's head at upper right. At lower right, the process has gone wrong, resulting in conditions such as spina bifida and encephalocele.