The stratification of principal fiber systems affiliated with the developing neocortex has been analyzed by means of HRP tracing methods, monoamine histofluorescence and silver impregnations in mouse embryos ranging from the 15th to 16th embryonic day (E15/16) to the end of gestation (E19 = the day of birth). As early as E15/16 a fiber stratum divides the subplate and marks the inferior boundary of the developing cortex. Axons coursing in this fiber plane, termed the external sagittal stratum (ESS), include components of at least 5 identifiable systems: thalamocortical, corticothalamic, ipsilateral corticocortical, callosal and monoaminergic. The neocortical afferents of extrinsic origin, i.e., the thalamocortical, callosal and monoaminergic systems, cross the intermediate zone from their separate directions and converge upon the ESS. After a variable course through this stratum, single fibers ascend from their parent fascicles to ramify densely in the cortical subplate (CSB). Fibers of each of the extrinsic afferent systems mingle with each other and with locally arising axons within the CSB. Axons of the monoaminergic projection as well as fibers of the thalamic projection cross the cortical plate to ramify in the marginal zone. Other axons apparently of local intracortical origin course tangentially through the cortical plate. Otherwise, the cortical plate is devoid of proliferating axons at this early developmental stage. The set of observations illustrates the existence of sharply defined boundaries between axon-rich and axon-poor strata of the developing neocortex. These boundaries also compartmentalize postmigratory neurons with respect to their state of differentiation.