Neurogenesis, cell migration and early histogenesis of the isthmic nuclear complex in chick embryos were investigated in autoradiographic and Golgi material. The aim of the experimental observations was to detect whether the apparent origin of different grisea of this complex at separate matrix territories (neuromeres) was accompanied by peculiar generation patterns, consistent with predictions of neuromeric theory. Differential birthday patterns were indeed obtained for a) n. semilunaris--born in the rh1 a rhombomere, b) n. isthmi principalis pars parvocellularis, nn. lemnisci lateralis dorsalis and ventralis, and n. isthmi ventralis--born in the isthmic rhombomere, and c) n. isthmi principalis pars magnocellularis--born at the m1 mesomere. Only the nuclear group at (b) shows a clear-cut gradient of generation. The morphological analysis aimed to describe isthmic neuroblast cell form before, during and immediately after migration into the mesencephalic optic lobe. Golgi data indicate that isthmic neuroblasts emerge as free cells from the matrix and aggregate into a dense superficial mantle layer. Between stages HH26 and 30, the whole mass of cells translocates tangentially in a rostrolateroventral direction, invading the m2 mesomere. The individual migrating neuroblasts have a leading axonal process which rapidly grows into the tectum in advance of the cell body, which follows at a slower pace. As the migration runs to an end the neuroblasts start to differentiate, sprouting dendritic processes. A joint origin in the isthmic mantle primordium is proposed for the nuclear group at (b) (above), whereas n. isthmi principalis pars magnocellularis is formed separatedly from the rest, and shows no tangential migratory behaviour of its neuroblasts. The complex histogenetic and morphogenetic processes at the isthmo-mesencephalic boundary may be explained on the basis of these new data, but this requires a tridimensional viewpoint that is exposed in the Discussion.