The
blastula (from Greek (blastos), meaning "sprout") is a hollow sphere of
cells, referred to as
blastomeres, surrounding an inner fluid-filled cavity called the
blastocoele formed during an early stage of
embryonic development in
animals.
Embryo development begins with a sperm fertilizing an egg to become a
zygote which undergoes many
cleavages to develop into a ball of cells called a
morula. Only when the blastocoele 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.