Early embryogenesis proceeds through four defined stages in which a fertilized zygote is progressively subdivided and specified: cleavage, a series of mitotic divisions without cellular growth that yields the morula; blastulation, in which compaction and differentiation separate outer trophoblasts from an inner cell mass of embryoblasts and a fluid cavity (blastocoel) forms, producing the blastocyst and subsequently the bilaminar disk of epiblast and hypoblast; gastrulation, in which epiblast cells migrate through the primitive streak to convert the bilaminar disk into a trilaminar disk of the three germ layers, ectoderm, mesoderm, and endoderm; and neurulation, in which mesoderm-derived notochord induces overlying ectoderm to thicken into the neural plate, which invaginates and closes into the neural tube while neural crest cells delaminate. The organizing principles are cell division without growth, compaction, progressive differentiation into distinct cell populations, morphogenetic cell migration, and inductive signaling between tissue layers. The concept belongs to developmental biology and embryology, supplying the framework from which all adult tissue lineages are traced back to germ-layer origin.
Early embryogenesis proceeds through four defined stages in which a fertilized zygote is progressively subdivided and specified: cleavage, a series of mitotic divisions without cellular growth that y…