<Tr> <Th> MeSH </Th> <Td> D036703 </Td> </Tr> <Tr> <Td_colspan="2"> Anatomical terminology (edit on Wikidata) </Td> </Tr> <P> The blastula (from Greek βλαστός (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 . </P> <P> A common feature of a vertebrate blastula is that it consists of a layer of blastomeres, known as the blastoderm, which surrounds the blastocoele . In mammals the blastula is referred to as a blastocyst . The blastocyst contains an embryoblast (or inner cell mass) that will eventually give rise to the definitive structures of the fetus, and the trophoblast, which goes on to form the extra-embryonic tissues . </P>

The development of a zygote into a blastula is called
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