that division of anatomy which treats of the growth of individual organisms. The growth may start from an egg, a bud or a germ cell. The causes of development are unknown, though all forms are primarily due to the same cause. In one-celled animals, the cell increases in size without subdividing; in animals of more than one cell, cell division takes place. In the egg, the yolk serves as nutriment for the developing germ, which must be fertilized before growth commences.