Gastrulation is a crucial phase in embryonic development that occurs after the blastula stage. During this process, the single-layered blastula reorganizes into a multi-layered structure called the gastrula, forming three primary germ layers: ectoderm, mesoderm, and endoderm. These germ layers give rise to various tissues and organs in the developing organism. Gastrulation is essential for establishing body plan and organizing the future development of the embryo.