Blastulation is the stage in early animal embryonic development that produces the blastula. The blastula (from Greek "" ( meaning "sprout") is a hollow sphere of cells (blastomeres) surrounding an inner fluid-filled cavity (the blastocoel). Embryonic development begins with a sperm fertilizing an egg cell to become a zygote, which undergoes many cleavages to develop into a ball of cells called a "morula". Only when the blastocoel is formed does the early embryo become a blastula. The blastula precedes the formation of the gastrula in which the germ layers of the embryo form.