: an early metazoan embryo typically having the form of a hollow fluid-filled rounded cavity bounded by a single layer of cells — compare gastrula, morula
Illustration of blastula
section of blastula: c blastocoel, ma macromere, mi micromere, a animal pole, v vegetal pole
borrowed from New Latin (in a German context), from Greek blastós "shoot, bud, embryo" + New Latin -ula (as in gastrula, planula) — more at -blast
Note:
Term apparently introduced by the Russian zoologist and embryologist Vladimir Vladimirovič Zalenskij ("W. Salensky," 1847-1918) in "Bemerkungen über Haeckel's Gastraea-Theorie," Archiv für Naturgeschichte, 40. Jahrgang, 1. Band (1874), p. 162.
: an early metazoan embryo typically having the form of a hollow fluid-filled rounded cavity bounded by a single layer of cells — compare gastrula, morula