A morphologically, anatomically and physiologically unique callus has been developed from unfertilized ovules of Shamouti orange. The callus, which is suspected to be of nueellar origin, is not made up of the normal unorganized parenchymatous tissue, but solely of numerous proembryoids which vary between 0.1 and 1.0 mm in diameter. Adventive embryogenesis in this tissue is autonomous and is in fact depressed by the inclusion of growth regulators in the medium, despite having been in culture for more than 2 years. Embryogenesis occurs in single cells on the periphery and within existing proembryoids. Cells destined to form new proembryoids are surrounded by greatly thickened cell walls which lack plasmodesmata. Cell divisions occur within the thickened walls to give rise to globular proembryoids which are freed from encasing thick walls as these degenerate. Proembryoids may enlarge into spherical pseudobulbils up to 4 mm in diameter with an epidermal cell layer but no vascularization. Such structures rarely develop into plantlets but do form further proembryoids from surface cells. Alternatively, proembryoids may develop into heart-shaped, torpedo, and cotyledonary embryoids, and thence into plantlets with varying degrees of organ fasciation. Since plantlets are derived from single, usually surface cells, this system holds great promise for the production of solid genetic mutants by irradiation.