Neurulation refers to the folding process in vertebrate embryos, which includes the transformation of the neural plate into the neural tube. The embryo at this stage is termed the neurula.
The process begins when the notochord induces the formation of the central nervous system (CNS) by signaling the ectoderm germ layer above it to form the thick and flat neural plate. The neural plate folds in upon itself to form the neural tube, which will later differentiate into the spinal cord and the brain, eventually forming the central nervous system.
Different portions of the neural tube form by two different processes, called primary and secondary neurulation, in different species.
— In primary neurulation, the neural plate creases inward until the edges come in contact and fuse.
— In secondary neurulation, the tube forms by hollowing out of the interior of a solid precursor.