The time of neuron origin has been determined in the basal ganglia and related basal forebrain structures of the Chinese hamster with the aid of3H-thymidine autoradiography. Large-celled structures like the globus pallidus, nucleus of the horizontal limb of the diagonal band of Broca as well as large cells in the rostral part of the substantia innominata, in the caudate-putamen-complex and in the olfactory tubercle arise early (E12–E16), whereas medium-sized and small cells in the basal forebrain have a persistent origin over a much longer period. Neuron formation in the basal forebrain persists decrementally until P4. A clear caudorostral spatiotemporal gradient as well as a distinct ‘outside-in’ gradient have been observed in the caudate-putamen-complex. Medium-sized neurons in the neostriatum and in the nucleus accumbens, generated simultaneously, are usually arranged in scattered clusters. The present data on time of neuron origin strongly support other evidence which points to the conclusion that the nucleus accumbens can be considered as a ventromedial extension of the caudate-putamen-co
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