Epiboly continues, and in addition, the morphogenetic cell movements of involution, convergence, and extension occur, producing the primary germ layers and the embryonic axis. Just as there was no blastocoele during the blastula period, there is no archenteron in the gastrula.
Involution produces the germ ring by folding the blastoderm back upon itself.
Hence, within the germ ring there are two germ layers, the upper, the epiblast, continues to feed cells into the lower, the hypoblast, throughout gastrulation. Note that these terms, epiblast and hypoblast, are also used to describe layers of the avian embryonic blastoderm, but the layers so-named seem to be altogether different in these two kinds of vertebrate embryos. Cells in the two layers are streaming in different directions. The cells remaining in the epiblast when gastrulation ends correspond to the definitive ectoderm and will give rise to such tissues as epidermis, the central nervous system, neural crest, and sensory placodes. Both names emphasize the absense of three separate germ layers at any stage during the gastrula period, only two are present.