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The gastrula is the structure resulting from the phase of early embryonic development known as gastrulation. Gastrulation is the process whereby the blastula - which is a hollow "ball" made up of a single layer of cells - folds inward and enlarges forming the elongated structure (open at one end) pictured here. The gastrula is composed of three (germ) cell layers: ectoderm, mesoderm, and endoderm. The opening, known as the blastopore, opens to the archenteron. As the embryo developes, the blastopore becomes the anus while the archenteron becomes the gut.

Gastrulation is followed by organogenesis (when individual organs develop from the germ layers). Each germ layer gives rise to specific tissues and organs in the developing embryo. Thus, from the ectoderm is formed the epidermis (skin) and the nervous system (brain, spinal cord); the mesoderm - the middle germ layer (between the ectoderm and the endoderm) - forms the musculoskeletal system (muscles, bones) and the circulatory system (blood, blood vessels); and the endoderm gives rise to organs of the digestive system (stomach, liver, intestines) and the respiratory system (lungs).