A late blastula characterized by a single layer of cells surrounding the central hollow area – the blastocoel (B). The blastomeres at the vegetal pole (VP) are taller than those at the animal pole (AP), making the vegetal pole appear slightly thicker. Photo: University of Guelph.

A blastula is a stage of embryonic development of animals near the end of cleavage (cell division), but before gastrulation. In animals where cleavage involves the whole egg, the blastula usually consists of a hollow ball of cells (blastomeres) surrounding a fluid-filled central cavity, the blastocoel. In mammals a similar cluster of cells, the blastocyt is formed, with an inner cell mass and a spherical envelope that develops into the placenta.