Here's the breakdown:
* Fertilization: When a sperm cell fertilizes an egg cell, a single-celled zygote is formed.
* First Cell Division: The zygote then undergoes its first cell division, called cleavage, around 24-36 hours after fertilization. This results in two identical cells.
* Continued Division: These cells continue dividing rapidly, eventually forming a ball of cells called a morula.
* Blastocyst Formation: The morula develops into a blastocyst, which implants in the uterine lining.
* Fetus Development: After implantation, the blastocyst continues to develop into an embryo, and eventually a fetus, over a period of months.
So, while a fetus is the developing stage of a human being, the initial cell division that creates two cells happens at the very beginning of the process, with the zygote.
