Development in humans begins as a fertilized egg undergoes cell division and differentiation through defined stages. The embryonic period involves cleavage of the zygote, formation of the blastula and gastrula stages. Gastrulation restructures the embryo's morphology through cell migration, producing the three germ layers - ectoderm, endoderm and mesoderm. Organogenesis follows, as the germ layers differentiate into specific tissues and organs through both cell movements and differentiation. Growth continues post-embryonically until maturity is reached.