n. In embryology: A hollow sphere (vesicular morula) composed of a single layer of blastomeres or derivative cells, inclosing a central cavity or blastocœle.n. By Haeckel restricted to the germ-vesicle, vesicular embryo, or blastodermic vesicle of the Mammalia, which follows after gastrulation, and is called by him a gastrocystis, or intestinal germ-vesicle. Also called blastula.