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Blastulation: 1 - morula, 2 - blastula.
The blastula (from Greek βλαστός (blastos), meaning "sprout") is a hollow sphere of cells formed during an early stage of embryonic development in animals . The blastula is created when the zygote undergoes the cell division process known as cleavage. The blastula is preceded by the morula and is followed by the gastrula in the developmental sequence.
A common feature of a vertebrate blastula is that it consists of a layer of blastomeres, known as the blastoderm, which surrounds an interior central cavity known as the blastocoel.  In mammals, blastulation leads to the formation of the blastocyst, which must not be confused with the blastula. The blastocyst contains an embryoblast, which is homologous to the blastula. However, it also includes the trophoblast, which goes on to form the extraembryonic tissues.