Immature embryos ofZea maysL. were cultured on N6 medium to obtain embryogenie callus. Cultured tissue fragments with various developmental stages of friable callus were sampled and prepared for light microscopy and transmission electron microscopy. Pieces of compact callus were also prepared for light microscopy to compare the structural organization of both callus types. Friable callus develops from a thin layer of abaxial scutellum cells, including the epidermis. During further development the callus cells dissociate, owing to the breakdown of the middle lamellae, while older vacuolated cells degenerate. This results into long cell aggregates separated by large intercellular spaces, giving the callus its friable appearance. The microscopical sections showed a striking difference between friable and compact callus. Vascular elements were not found in the friable callus. On the contrary, vascular bundles were prominent in compact callus. Friable callus is therefore correlated with a less differentiated state than compact callus. The embryogenic potential of friable callus is situated in embryogenic units. These are aggregates of small isodiametric cells containing a central nucleus, an electron-dense cytoplasm, and many organelles. Proliferation was only observed in these cells, which are therefore presumed to generate new embryogenic units, somatic embryos, and vacuolated callus cells. The results further indicate that discrete masses of embryogenic cells, possibly early embryoids, have a unicellular origin.Key words:in vitroculture, callus, somatic embryogenesis, ultrastructure,Zea mays.