The formation of a segmentation cavity or blastocoele within a mass of cleaving blastomeres and rearrangement of blastomeres around this cavity in such a way as to form the type of definitive blastula characteristic of each species. The blastocoele originates as an intercellular space which sometimes arises as early as the four- or eight-cell stage. Thus blastulation is initiated during early cleavage stages, and formation of the definitive blastula is thought to terminate cleavage and to initiate gastrulation. Accordingly, cleavage and blastulation are simultaneous events which follow activation of the egg and precede the next major step in development, namely, gastrulation. Initially the diameter of the blastula is no greater than that of the activated egg; subsequently it increases.