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The formation of an embryo from a fertilized ovum, or zygote. Development begins when the zygote, originating from the fusion of male and female gametes, enters a period of cellular proliferation, or cleavage. Cells of the embryo subsequently give rise to the tissues and organs of the body in a temporal and spatial pattern that creates a functional, multicellular organism.
Following cleavage, the cells of the animal embryo rearrange into three germ layers: an outer ectoderm, a middle mesoderm, and an inner endoderm. Cells, responding to intrinsic and extrinsic factors, eventually segregate from the germ layers and organize into the rudiments of the tissues and organs of the body. These rudiments alter the size and the shape of the embryo, endowing the body with its axial symmetry. Cellular growth and differentiation are the principal processes that transform the rudiments into functional tissues and organs. Once the organs and organ systems are formed, further development consists primarily of growth. See Germ layers
Major features of embryogenesis in flowering plants include the formation of root and shoot apical meristems; differentiation of primary vascular tissue; the transition from a heterotrophic zygote to an embryo capable of independent growth and development; and preparations for desiccation, dormancy, and germination. See Apical meristem, Cell differentiation, Developmental biology, Embryology, Embryonic differentiation, Embryonic induction