How do cells that are initially on the outer portion of that developing embryo eventually form structures found on the inside of that embryo, on the inside of that fetus? Well, as it turns out, this process of embryological development is actually a complicated process and we not only have cell differentiation and cell growth and cell proliferation but all these cells also move around many times and rearrange themselves inside that developing embryo. In fact, as we discussed in the process of neuralization when we produce the nervous system it's the ectodermal cells that eventually invaginate and move into that embryo and form the neural crests and the neural tube. Now it's the neural tube that gives rise to the central nervous system that makes up the brain and a spinal cord and it's the neural crest, the cell of the ectoderm that end up inside that embryo that eventually gives rise to the peripheral nervous system. All the nerve cells down on the outside of the central nervous system the cells we call the ganglia. So the entire nervous system and the entire or essentially the entire integumentary system is formed by the ectoderm.