Transgenic mice harboring thelacZreporter gene on one X chromosome have been used to mark 50% of all retinal progenitors. The distribution of clones arising from this population of marked progenitors reveals a conspicuous columnar segregation of clonally related cells, indicating that most retinal neuroblasts migrate exclusively radially. Against this columnar background of the transgenic retina, single cone, horizontal, amacrine and ganglion cells are observed to transgress clonal borders, mixing freely with cells derived from different precursors. This tangential dispersion is due to the lateral movement of postmitotic neuroblasts around the time of their differentiation, rather than to the dispersion of a proliferative sibling at the time of cell birth. Tangential dispersion is suggested to play a significant role in creating the functional architecture of the mature retina, being the means by which the orderly spacing, or regularity, of retinal mosaics is established during development.