It is not necessary to describe in detail the beginnings of all the various structures which arise from that important layer of cells in the embryo which is termed the ectoderm; but since it gives rise to that part of the embryo, which eminently places man in the first place in the world of animals, we may select it for a little further description. We may leave out of account the beginnings of the skin and the glands, and some other parts, and look for the moment at the origin of the nervous system, which includes the brain, the spinal cord, and the whole nervous mechanism of the individual. Since man's prominence depends upon the wonderful capacities in his nervous system, it is all the more interesting to note from what small and simple beginnings it has arisen. As we have already seen, at a very early stage in the development of the embryo, a folding of its cells takes place, so that the upper embryonic area assumes the character of a groove. We may confine our attention to this groove for the moment, leaving out of account the other two layers of the embryo--namely, the mesoderm and the entoderm. It is this groove, which thus early makes its appearance, which is subsequently to play such a tremendous part in the formation of the most important structures. It is called the "medullary groove." As growth proceeds and the cells continue to multiply and increase in numbers, the two edges or lips of the groove gradually approximate, and ultimately fuse together. Obviously the effect of this is to transform what was the groove into a closed cavity or canal, which is therefore now termed the medullary canal. Arising in this simple manner, this equally simple structure is destined to become the central canal of the spinal cord, and the cavities in the brain, known as the ventricles. The walls of this canal, be it remembered, are composed of cells of the layer of ectoderm, and it is these cells which, as we saw, appeared very early in the development of the embryo that are now to proceed to develop into the brain, spinal cord, and, in fact, the whole central nervous system. At first the cells appear all similar, but, as development goes on, they begin to differentiate themselves into different kinds, some forming the actual nervous cells of the brain and spinal cord, others developing into protective structures.