Neurulation in Amphibians
The first step in the neurulation process is the flattening and thickening of dorsal ectoderm to form neural plate. The plate of cells are different from the rest of ectoderm cells in that they have changed shape and appear more columnar. The edges of the neural plate after that rise above along the rest of the neural plate to form neural folds. The neural folds are now found together the two flanks of a central depression called neural groove. The neural groove extends along the entire middorsal line of the embryo. Eventually the neural folds meet in the middle above the deepening neural groove and fuse to make the neural tube. At the time of rolling of neural plate to make the neural tube, the neuroepithelial cells at the lateral folds of the neural plate develop constrictions at their apical edges.
This alterations the shape of columnar epithelial cells into pyramidal cones. As the surface area of apical end of neural epithelial cells turns into smaller relative to the basal surface, the rolling up of the neural plate and formation of neural tube occurs. Along with the fusion of neural folds along the middorsal line, the neural ectoderm separates from epidermal ectoderm that now covers the neural tube and completely surrounds the embryo. The change in the shape of cells in the neural tube results in the regionalisation of neural tube. In the cephalic end, i.e. the end which will differentiate into brain, the wall of the tube is broad and thick and a series of swellings and constrictions develop which define the several divisions of brain. Caudally, towards the posterior end, the tube is simple and narrow extending into the tail.