Through the transformation of fertilization and development of a baby, many different changes occur at a cellular level. First, once the sperm and egg meet, a zygote forms. It is 2 cells, with 23 male and 23 female chromosomes. It then through mitosis begins to go through cell division. Once divided into 16 cells, a blastocyst begins to forms. This ball of cells attaches itself to the uterine wall, where it begins to gain all of its vital nutrients from. This is known as plantation. Once this process occurs, the embryonic stage begins and continues up until the beginning of week 9. During the embryonic stage the eyes, nose, ears and mouth begin to develop, followed by arms and legs. Once week 9 is reached, the baby transforms into a fetus. Once in the fetal stage, the baby will start to develop its sex organs, bone and muscle formations, reflexes, and movement.