What defines the stages of a cleavage stage embryo?

The stages of a cleavage stage embryo are primarily defined by the formation of blastomeres. After fertilization, the zygote undergoes a series of rapid mitotic divisions, leading to the formation of these smaller cells called blastomeres. Each division produces two cells from a single cell, and this process continues, resulting in an increase in cell number without a corresponding increase in overall size of the embryo. This period is crucial for the initial stages of embryonic development, setting the stage for subsequent processes, such as the formation of the blastula, where those blastomeres eventually reorganize. The focus on blastomeres helps to underscore the importance of early cellular differentiation and organization in the development of a multicellular organism.