Zygotes are single cell that is generated after the egg has been fertilized by the sperm. The zygote then merges chromosomes from both of the gametes. After which, several divisions will take place for the production of embryonic cells at roughly four days after fertilization. The zygote stage lasts for only about 4 days in total. On and around the 5th day, the cell mass of become known as blastocysts.