The developmental process in which the cells of the blastula are reorganized by folding and migration into the blastocoele, resulting in an embryo with three distinct layers of cells (see figure below). These primary germ layers consist of the ectoderm (outer), mesoderm (middle), and entoderm (inner). The developmental biologist Lewis Wolpert penned a pithy sentence about the centrality of gastrulation: “it is not birth, marriage or death, but gastrulation, which is truly the most important time in your life.” In: Egg to embryo: determinative events in early development. Cambridge; Cambridge University Press, 1986.
Gastrulation occurs after the formation of the blastula, and its function is to establish a second layer of cells beneath the first ( viz., the entoderm) that are one hemisphere of the blastula. The second layer forms by folding inwards (invagination) that never breaks the surface of the blastula . Approximately at the margin between the two hemispheres, cells on the surface of the blastula bend inward and start moving into the blastocoel, alining themselves along the underside of the sphere’s surface.
See Archenteron, Blastomere, Blastopore, Blastula, Cleavage, Ectoderm, Entoderm (or endoderm), Gastrula, Germinal (or germ) layers, Mesoderm, Neural crest, Neural plate, Neural tube, Neurulation, Notochord, Zygote