The central nervous system of murine intraspecies chimeras generally consists of an apparently random mixture of neurons derived from each of the parental genotypes. In this study, interspecies mouse chimeras were examined, and an analysis of the parental genotype donation to the chimeric CNS revealed large clusters of like-genotype neurons in small regions as well as in major subdivisions of the CNS. This coherent clustering of like-genotype neurons is proposed to be due to an autonomous developmental clock that is sufficiently mismatched between species to create preferential cell allotment in the chimeric brain.