During fertilization in multicellular organisms, a sperm cell fuses with an egg cell to form a zygote. then, the zygote begins to divide. first, one cell becomes two, then two cells become four, four cells become eight and so on and so forth until a large ball of identical cells is formed. at this point, the ball begins to change. a space forms in the middle, and the cells are pushed inward to form layers. if cell differentiation begins after this phase, what most likely triggers this process?