part of part of immediately preceded by immediately preceded by occurrent embryogenesis phase cleavage period blastula period The period that immediately follows cleavage and precedes gastrulation. It is characterized by the progressive maturation of the embryo into a hollow sphere, which is one cell layer thick and encompasses a central, fluid-filled cavity (the blastocoel). This period is associated with a gradual increase in cell cycle length and a loss of mitotic synchrony. During mitosis, cells round up and the wall of the blastula becomes irregular, while during interphase the cells flatten and the wall of the blastula appears smooth. During this period, the blastocoel gradually expands and becomes lined by a thin basal lamina. Cells in the wall of the blastula thereby become polarized; the inner (basal) surface of each cell contacts the basal lamina while the outer (apical) surface contacts the hyaline layer, an extracellular matrix that surrounds the embryo. Each cell further forms a single apical cilium. The onset of ciliary motility causes the blastula to move within the fertilization envelope and, after hatching, coordinated ciliary beating allows the embryo to swim freely. The blastula period culminates in the formation of the vegetal plate, a placode of elongated, presumptive endomesoderm cells. UBERON:0000307 ecao_developmental_stage