Early phases of aggregation of dispersed unincubated chick blastoderm cells were investigated. Dispersed cells, when maintained in appropriate cultures, began adhering to one another almost immediately and eventually formed spherical aggregates without any recognizable structures except for occasional blood islands, after 24 hr of incubation. The number and size of aggregates derived from dispersed cells depended largely upon the speed of rotation and initial cell density. Treatment with colchicine (4 microgram/ml) had no apparent effect on adhesion of cells, although it yielded smaller aggregates than controls, especially after 14 hr of incubation. Direct observation on mitotic activity, estimations of cell number and nucleic acid content, and pulse labeling experiments with 3H thymidine clearly showed that aggregates were derived entirely from adhesion of free cells and/or fusion of smaller aggregates during the first 12 to 13 hr of incubation. In addition, all cell types present in the unincubated blastoderm contributed to the formation of aggregates, not one particular cell type.