Cleavage begins 30-45 minutes after formation of the zygote. The zygote divides by cleavage to form micromere and macromere cells. In the process of blastulation, these cells become blastomeres. Blastomere cells are arranged in a certain layer. During gastrulation, blastomere cells are arranged in three layers. Ectoderm, Mesoderm and Endoderm.