An early form (indeed a small sprout) in the development of an embryo, consisting of a spherical layer of cells filled with fluid; in the stages of embryological development, the blastua develops after the morula but before the blastocyst. The blastula is subtly different from the blastocyst in that a blastocyst is considered to have two distinct cell types: an inner cell mass (which will later become the embryo) and the trophoblast cells (which will later become the placenta).