Primary and secondary neurulation

At S8, in the region of the median plane and cranial to the primitive node, ectodermal cells become elongated. They form a pseudo-stratified epithelium called the neural plate. The neural plate folds inwards on both sides, causing a deepening in the median plane. This is the neural furrow, which enlarges to form the neural groove. At S10, the neural folds come together and fuse to form the neural tube at the level of the cervical somites. From there, the fusion extends both cranially and caudally. The ends of the neural tube are still open and are called rostral neuropore (neuroporus cranialis) and caudal neuropore (neuroporus caudalis). The rostral neuropore closes at S11, and the caudal neuropore at S12. The remaining surface ectoderm grows over the closed neural tube.
Most of the central nervous system arises through this form of neurulation (primary neurulation). A secondary neurulation occurs in the region of the caudal eminence once the caudal neuropore has closed. In this region, nervous tissue develops in the form of a mesodermal cell cord, consisting of presumptive nerve cells (S12, S13). The cavity of the neural tube situated just cranially expands into this cell cord. Through this process, the central canal of the neural tube also extends into the region of the secondary neurulation.