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.
|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