The zygote now begins to cleave, with each division occurring into two cells called blastomeres. The zygote's first cell division begins a series of divisions, with each division occurring approximately every twenty hours. Each blastomere within the zona pellucida becomes smaller and smaller with each subsequent division.

When cell division generates about sixteen cells, the zygote becomes a morula (mulberry shaped). It leaves the fallopian tube and enters the uterine cavity three to four days after fertilization.