Movie shows gastrulation and neurulation viewed from the vegetal pole of a Xenopus embryo, the future dorsal side at the top. Note the dramatic involution of the Marginal Zone, which forms a ring of cells surrounding the large central disc of vegetal endodermal cells at the center of the vegetal pole, the blastopore (look at Figure 7.28 in Principles of Development 6th edition for reference). The marginal zone rolls over the blastoporal lip and disappears inside; subsequently this involution proceeds laterally, on both sides, and finally at the midventral line. As the marginal zone involutes, it also extends posteriorly and converges around the circumference of the blastopore, but does so inside, out of sight. The emergence of the neural tube can be seen in the formation of the neural plate, followed by the elevation of the neural folds and the closure of the tube (see Figure 7.38 as a reference). Note that the posterior neural tissue likewise converges and extends in the same fashion, on the outside; together these convergent extension movements squeeze the blastopore shut and simultaneously elongate the anterior-posterior axis of the embryo, pushing the future tail away from the head. Note that the converging and extending neural plate simultaneously rolls up to form a neural tube.

Movie courtesy David Shook.