In order to gain insight into the mechanisms involved in the formation of groupings of functionally similar cells in the developing nervous system, we have studied the formation of cell layers in the developing dorsal lateral geniculate nucleus (dLGN). To examine the possibility that a higher affinity or adhesion between cells in individual layers may play a role in laminar segregation, we studied cellular interrelationships in the dLGN of tree shrews before (P0), during (P4 and P7), and just after (P15) laminar segregation has taken place. We compared our observations at these stages of development with similar observations in the adult. In none of the cases do we see evidence of gap junctions either between adjacent neurons or between neurons and processes in the surrounding neuropil. However, we frequently observe the presence of puncta adherentes between adjacent neurons at all stages of development. These profiles are also present between neurons and cellular processes in the neuropil. We also see subsurface cisternae in all of our cases, although these are more pronounced before and during interlaminar space formation. As with the puncta adherentes, these are found both between adjacent neurons as well as between neurons and other elements in the neuropil. We also see some evidence of what appear to be cytoplasmic bridges between adjacent neurons; these are quite rare but appear to be present only before and during laminar segregation. Finally, we frequently see cytoplasmic processes interdigitated between otherwise immediately adjacent cells. These processes also are often found oriented along other portions of the neuronal plasmalemma. Whether these processes are portions of neuronal growth cones or glial processes is impossible to determine at this time. Because of the potential role glial processes may play in the formation and maintenance of laminar cell groupings during layer formation, we have also made a preliminary survey of whether glial cells can be distinguished ultrastructurally at the stages we have studied.