Freshly dissociated cells from embryonic chick neural retinas grow in characteristic patterns on flat cells or on chick embryo mesodermal cells. A striking difference between the two patterns is that the cells grown on flat cells are interconnected by a complex network of fibers, whereas those grown on mesodermal cells are aggregated into clusters that remain relatively isolated within the mesodermal monolayer. Analysis by time-lapse video microscopy indicates that two processes produce the fibers. (1) Fibers grow out by the extension of growth cones from cells within aggregates. (2) Neuronal cell aggregates that attach to two flat cells are pulled apart by the movement of the cells beneath them. As the aggregate is pulled apart, portions of the cells remain attached to the two halves, and their cytoplasm is drawn into thin fibers. The lack of fibers on a mesodermal substrate is due to two factors: (1) Aggregates are widely spaced on the substrate surface and do not come into contact often. (2) On those occasions when they do come into contact, the movement of the monolayer is so vigorous that emerging fibers are torn.