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.
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