The mechanism of adventive embryogenesis was studied at ultrastructural level. ThreeCitruscultivars were used as material, two poly-embryonic and one mono-embryonic. The adventive embryos inCitrusare initiated in the nucellar tissue adjacent to the embryo sac in the micropylar half and incidentally from the chalazal end. The nucellar embryoids derive from initial cells. Initial cells are present already at the time of flowering in the poly-embryonic cultivars: in the mono-embryonic cultivar initial cells are absent. At the mature embryo sac stage the initial cells have a « meristematic » character. Before division they develop new cell walls and become « zygote-resembling » cells. During this development they are isolated and their original cell walls are disintegrated. Simultaneously the surrounding nucellar cells degenerate and disappear and the initial cells become embedded in the endosperm.