After fertilization, the egg is transformed into the zygote, which embarks upon one of the most critical periods in its development as it is partitioned into cells that ultimately make up the body of the embryo. What makes the zygote so unique a cell is that it is the product of fusion of two gametes, the sperm contributing the paternal genome and the egg providing the maternal counterpart. The point in time when the egg and sperm fuse together to be woven into a new sporophyte marks the beginning of the ontogeny of the species. The phase of ontogeny concerned with progressive division of the zygote to form the embryo is known as embryogenesis, or as zygotic embryogenesis (to avoid semantic confusion with somatic embryogenesis and pollen embryogenesis). Classical histological analyses of embryo development in a large number of plants have generated an invaluable guide to the division patterns of the zygote and its immediate derivatives, and contemporary studies are beginning to provide some much needed insight into the associated cellular and molecular changes.

The history of the study of the development and physiology of embryos can be considered to have evolved in three distinct phases. Much of the early work was focused on the first few rounds of division of the zygote and on the subsequent morphogenesis of cells to give rise to the embryo.