We have tried to obtain new insight into the development of the medulla oblongata by using the quail-to-chick chimera system. Five types of isotopic and isochronic grafts were carried out, between quail and chick embryos, at the 10- to 12-somite stage: exchanges of (I) the entire myelencephalon, (II) the dorsal half of the myelencephalon, (III) the ventral half of the myelencephalon, (IV) the right half of the myelencephalon and (V) the dorsal quarter of the myelencephalon. Before analyzing the chimeric embryos, we studied the ontogeny of the various nuclei in the medulla oblongata of normal birds. The first appearance of nuclei in quail embryos preceded in many cases that of their chick counterpart by 12 to 24 h. The adult pattern of the nuclei was established by E8 in quail and E9 in chick. Similarly, during early development of chimeras, the migration of quail cells began earlier than that of chick cells. This shows that the species specific temporal sequence of proliferation and migration is not significantly altered by transplantation into the host. The possibility of grafting selectively the ventral or dorsal half of the neural tube allowed us to distinguish the fate of the cells belonging respectively to the alar and the basal plate. The nuclei with a total or partial motor function, such as the nucleus nervi abducentis, the nucleus nervi facialis, the nucleus nervi glossopharyngei and the nucleus motorius dorsalis nervi vagi, have either an exclusive or predominant origin from the basal plate. In contrast, the nuclei with essentially or exclusively sensory components (i.e., nucleus angularis, nucleus laminaris, nucleus magnocellularis) arise from the alar plate. The reticular formation such as the nucleus reticularis gigantocellularis and the nucleus reticularis subtrigeminalis was strikingly mixed, with both alar and basal plate origin of neurons. Active dorsoventral migrations of cells originating migrations from the dorsal neural tube, the "rhombic lip", contribute the ventral nuclei (i.e., nuclei pontis medialis, lateralis and olivaris inferior), whose functions are essentially associative. This study shows different types of cell migration. Dorsoventral and ventrodorsal movements are essentially active from E5 to E8. In the medulla oblongata, the dorsoventral stream is highly predominant. From E8 to E9, cells belonging to the marginal stream cross the midline laterally in both directions. Beyond E12, longitudinal migrations occur ventrally in both rostrocaudal and caudorostral directions. The immunohistochemical analyses carried out on chimeras generated in experiment V revealed the existence of fibers in marginal zones prior to the onset of the migration of cell bodies.(ABSTRACT TRUNCATED AT 400 WORDS)