The primary olfactory cortex of mouse embryos varying in age from the 13th to the 15th day of gestation is investigated with the aid of several techniques: Nissl and Golgi preparations, autoradiography and electron microscopy. It is observed that the earliest cells to complete their migrations are diffusely scattered at all levels of the emerging cortical plate. The dendrites of these cells are predominantly tangentially aligned. Subsequently these early formed cells are subdivided into two groups. One group remains superficial in close relation to the lateral olfactory tract while the second is displaced to the deepest level of the cortex. The dendrites of many cells which remain at superficial cortical levels become realigned in a radially upward direction. "Maculae adherens diminutae" are abundant at points of apposition of membranes of cells of all classes at superficial cortical levels. It is suggested that these serve to stabilize cell position and that the upward dendritic realignment is an expression of radially directed stresses. Synapses appear relatively late and probably play no role as determinants of cell position.