A stage in the development of the embryo prior to the blastula in which a cluster of complex cells form from the solid mass of blastomere cells after the zygote splits or cleaves, resulting in 12-15 cells and looking like a mulberry (hence its name; see figure below). Human embryos implant at this stage in the uterine wall. This stage is followed by the formation of a cavity in the cellular mass (the blastoceol which indicates the formation of the blastocyst).
