blastula The stage of development of an animal embryo that results from cleavage of a fertilized egg. This stage generally resembles a hollow ball with the dividing cells (blastomeres) of the embryo forming a layer (blastoderm) around a central cavity (blastocoel). In vertebrates the blastula forms a disc (blastodisc) on the surface of the yolk. In mammals the blastula stage is known as a blastocyst. See also gastrula.

blas·tu·la / ˈblaschələ/ • n. (pl. -las or -lae / -ˌlē/ ) Embryology an animal embryo at the early stage of development when it is a hollow ball of cells.

blastula An early embryonic stage of an animal, around the time of cleavage, that usually consists of a hollow sphere of cells. It is formed immediately prior to gastrulation.

blastula Stage in the development of the embryo in animals. The blastula consists of a hollow cavity (blastocoel) surrounded by one or more spherical layers of cells. Commonly called the ‘hollow ball of cells’ stage, it occurs at or near the end of cleavage and precedes the gastrulation stage.

blastula (blast-yoo-lă) n. an early stage of the embryonic development of many animals. The equivalent stage in mammals (including humans) is the blastocyst.