An autoradiographic analysis of the time and sites of origin, and the migration and settling patterns of neurons was made in the spinal cord of the mouse. The neurons originated on days 10--14 of gestation with temporal gradients along the ventrodorsal and rostrocaudal axes. The motor neurons originated on days 10 and 11 of gestation; the neurons in the intermediate gray region originated on days 11--14 of gestation; the neurons of the head of the dorsal horn originated on days 12--14 of gestation. The neurons that originated on days 10 and 11 originated and migrated primarily from the basal plate, and they settled in the adjacent regions of the intermediate zone; those neurons formed on days 12--14 originated and migrated primarily from the alar plate, and it was concluded that these neuroblasts similarly settled in the adjacent regions of the intermediate zone. Extraventricular proliferation, which presumably signaled the initial stages of gliogenesis, was first observed on day 12 of gestation. This study supports the classical idea of the mosaic pattern of neurogenesis in the embryonic spinal cord.