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Gastrulation is a phase in the embryonic development of multicellular organisms, characterized by the formation of the gastrula stage. During this phase, the initially single-layered blastula undergoes a complex series of cell movements and changes, which ultimately give rise to the formation of the three primary germ layers: ectoderm, endoderm, and mesoderm.

Gastrulation typically occurs in the early cleavage stage of embryonic development, after the blastocyst has formed and before the onset of organogenesis. During gastrulation, the blastula's outer layer, the trophectoderm, invaginates and begins to envelop the inner cell mass, which eventually differentiates into the germ layers. This process involves the coordinated movement of cells, often through a series of convergent and divergent extensions, as well as the activation of gene expression regulating programs.

The outcome of gastrulation is the definitive acquisition of the spatial relationship between the germ layers, which provides the foundation for organogenesis and tissue patterning. Disruptions in gastrulation can result in various birth defects, such as neural tube defects and intestinal malformations. The mechanisms underlying gastrulation are highly conserved across species, although the timing and specific morphological features may vary.

Gastrulation is a critical process in vertebrate development, and has been the subject of extensive research in developmental biology. Its study has shed light on the complex interplay of genetic and epigenetic factors that regulate this essential developmental transition.