During the first few divisions after fertilization, the cells that are formed are called blastomeres. These are the initial cells created during the cleavage stage of embryonic development. Cleavage leads to a series of rapid mitotic divisions, resulting in a multicellular structure without overall growth in size. The blastomeres will eventually differentiate and give rise to the various cell types that form the embryo.
As the divisions continue, the blastomeres may reorganize to form a structure called the blastocyst in mammals, which consists of an inner cell mass (that will develop into the embryo) and an outer layer of cells called the trophoblast (that will contribute to forming the placenta).
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