mesoderm and endoderm – the ectoderm forms shortly after fertilization, after which rapid cell division begins. The position of the ectoderm relative to the other germ layers of the embryo is governed by "selective affinity", meaning that the inner surface of the ectoderm has a strong (positive) affinity for the mesoderm, and a weak (negative) affinity for the endoderm layer. This selective affinity changes during different stages of development. The strength of the attraction between two surfaces of two germ layers is determined by the amount and type of cadherin molecules present on the cells' surface. For example, the expression of N-cadherin is crucial to maintaining separation of precursor neural cells from precursor epithelial cells. Likewise, while the surface ectoderm becomes the epidermis, the neuroectoderm is induced along the neural pathway by the notochord, which is typically positioned above it. Gastrulation During the process of gastrulation, bottle cells invaginate on the dorsal surface of the blastula to form the blastopore. The cells continue to extend inward and migrate along the inner wall of the blastula to form a fluid-filled cavity called the blastocoel. The once superficial cells of the animal pole are destined to become the cells of the middle germ layer called the mesoderm. Through the process of radial extension, cells of the animal pole that were once several layers thick divide to form a thin layer. At the same time, when this thin layer of dividing cells reaches the dorsal lip of the blastopore, another process occurs termed convergent extension. During convergent extension, cells that approach the lip intercalate mediolaterally, in such a way that cells are pulled over the lip and inside the embryo. These two processes allow for the prospective mesoderm cells to be placed between the ectoderm and the endoderm. Once convergent extension and radial intercalation are underway, the rest of the vegetal pole, which will become endoderm cells, is completely engulfed by the prospective ectoderm, as these top cells undergo epiboly, where the ectoderm cells divide in a way to form one layer. This creates a uniform embryo composed of the three germ layers in their respective positions. Later development Once the three germ layers have been established, cellular differentiation can occur. The first major process here is neurulation, wherein the ectoderm differentiates to form the neural tube, neural crest cells and the epidermis. Each of these three components will give rise to a