Think of the human brain for a moment. It is an incredibly complex mechanism. It consists of a range of structures, including hemispheres, lobes, and even layers. Each structure is comprised of various types of cells, including nerve cells, blood cells, and cells that make up blood vessels. Where do you think all these cells came from? Consider what you know about mitosis and meiosis. Have the cells that make up your brain existed since fertilization? If so, how does human growth occur? If not, can you trace their origins back to fertilization?