The cells at the dorsalmost portion of the neural tube become the neural crest cells. The neural folds migrate toward the midline of the embryo, eventually fusing to form the neural tube beneath the overlying ectoderm. Shortly after the neural plate has formed, its edges thicken and move upward to form the neural folds, while a U-shaped neural groove appears in the center of the plate, dividing the future right and left sides of the embryo (see Figures 12.2C and 12.3). The process of primary neurulation appears to be similar in amphibians, reptiles, birds, and mammals ( Gallera 1971).
The serial sections can be displayed either as a continuum in movie format or individually, along with labels and color-coding that designates germ layers.
Both whole mounts and a complete set of serial cross sections through a 33-hour chick embryo are included in this segment so that you can see this amazing event. By 33 hours of incubation, neurulation in the chick embryo is well underway. In mice (and probably humans, too), secondary neurulation begins at or around the level of somite 35 ( Schoenwolf 1984 Nievelstein et al.
In amphibians, such as Xenopus, most of the tadpole neural tube is made by primary neurulation, but the tail neural tube is derived from secondary neurulation ( Gont et al. In birds, the anterior portions of the neural tube are constructed by primary neurulation, while the neural tube caudal to the twenty-seventh somite pair (i.e., everything posterior to the hindlimbs) is made by secondary neurulation ( Pasteels 1937 Catala et al. Neurulation in fishes is exclusively secondary. The extent to which these modes of construction are used varies among vertebrate classes. In secondary neurulation, the neural tube arises from a solid cord of cells that sinks into the embryo and subsequently hollows out (cavitates) to form a hollow tube. In primary neurulation, the cells surrounding the neural plate direct the neural plate cells to proliferate, invaginate, and pinch off from the surface to form a hollow tube. There are two major ways of forming a neural tube.