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?