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. Computer simulations found that cell wedging and differential proliferation are sufficient for mammalian neurulation. 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. Primary neurulation Primary neural induction The concept of induction originated in work by Pandor in 1817. The first experiments proving induction were attributed by Viktor Hamburger to independent discoveries of both Hans Spemann of Germany in 1901 and Warren Lewis of the USA in 1904. It was Hans Spemann who first popularized the term “primary neural induction” in reference to the first differentiation of ectoderm into neural tissue during neurulation. It was called "primary" because it was thought to be the first induction event in embryogenesis. The Nobel prize-winning experiment was done by his student Hilda Mangold. Ectoderm from the region of the dorsal lip of the blastopore of a developing salamander embryo was transplanted into another embryo and this "organizer" tissue “induced” the formation of a full secondary axis changing surrounding tissue in the original embryo from ectodermal to neural tissue. The tissue from the donor embryo was therefore referred to as the inducer because it induced the change. It is important to note that while the organizer is the dorsal lip of the blastopore, this is not one set of cells but rather is a constantly changing group of cells that are migrating over the dorsal lip of the blastopore by forming apically constricted bottle cells. At any given time during gastrulation there will be different cells that make up the organizer. Subsequent work on inducers by scientists over the 20th Century demonstrated that not only could the dorsal lip of the blastopore