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) forming a layer (blastoderm) around a central cavity (blastocoel). In vertebrates the blastula forms a disc (blastodisc) on the surface of the yolk. In reptiles, birds, and mammals the upper (dorsal) cell layer of the blastodisc is called the epiblast; it gives rise to the embryo. In placental mammals it also contributes to the formation of the amnion, one of the membranes enclosing the embryo. The lower (ventral) cell layer of the blastodisc, called the hypoblast, gives rise to extraembryonic membranes that will enclose the developing embryo. In mammals the blastula stage develops from the morula and is known as a blastocyst, comprising an inner cell mass, blastocoel, and enveloping outer cells of the trophoblast. See also gastrula.