In the formation of the neural tube, which event occurs second?

Are you missing information? How do we know the baseline of the events that occur?

According to one source:

There are two major ways of forming a neural 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. 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. The extent to which these modes of construction are used varies among vertebrate classes. Neurulation in fishes is exclusively secondary. 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. 1996). 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. 1993). In mice (and probably humans, too), secondary neurulation begins at or around the level of somite 35 (Schoenwolf 1984; Nievelstein et al. 1993).