Question: when a zygote is formed?
Answer: Zygotic stage: The zygote is formed when the male gamete (sperm) and female gamete (egg) fuse. Blastocyst stage: The single-celled zygote begins to divide into a solid ball of cells. Then, it becomes a hollow ball of cells called a blastocyst, attaching to the lining of the mother's uterus.