Final answer:
Growth factors induce neural progenitor proliferation by signaling some ectodermal cells to become epidermal cells and the remaining cells to form the neural plate.
Step-by-step explanation:
In vertebrates, including mammals, the proliferation of neural progenitors is induced by growth factors that signal some cells in the ectoderm to form epidermal cells while the remaining cells form the neural plate. These growth factors play a crucial role in the formation and development of the neural system. If the signaling by growth factors is disrupted, the entire ectoderm would differentiate into neural tissue.