Embryonic pattern formation has been studied extensively in many organisms. In Drosophila, the powerful combination of genetics cytoplasm transplantation experiments, as well as recent molecular data, have helped to elucidate the mechanisms responsible for the establishment of embryonic polarity. A small number of genes, most of them maternally expressed, are involved in this process and participate in four independent systems--three for the antero-posterior axis (A/P) and one for dorsoventral axis (D/V)--which define various embryonic territories by specifically localized cues. This review concerns the definition of the dorsoventral polarity responsible for the establishment of the germ layers of the embryo. Dorsoventral development is regulated by a single group of maternally expressed genes: the "dorsal group" of genes. It includes 11 genes, the loss of function of any of which results in a dorsalized development, whereas mutation of the 12th gene, cactus, results in a ventralized development. These genes are arranged according to a functional hierarchy, and have been shown to cooperate in the formation of a graded nuclear concentration of the dorsal gene product. The dorsal product corresponds to the dorsoventral morphogen and is homologous to the transcription factor NF-kappa B. Among the 11 genes of the dorsal group, 3 are required in the somatic line. This suggests the existence of inductive signals originating during oogenesis from the follicle cells that surround the developing oocyte. This somatically expressed spatial information probably controls dorsoventral development by defining the polarity of a signal transducing pathway that specifically activates the nuclear uptake of the dorsal product. This model, highlights the importance of the polarity of the egg chamber, and suggests that it is the oocyte nucleus due to its asymmetrical localization, that determines the dorsoventral pattern formation of the embryo.