The hinder or posterior part of this medullary groove and canal is narrower than the anterior portion. This posterior narrower part is that which gives rise to the spinal cord. It very soon changes its character by the appearance of a number of constrictions at intervals running along its whole length. It becomes, as it is termed, segmented. A little later these successive segments are seen to correspond to the pairs of spinal nerves which arise from the cord. For the first part of embryonic life the developing spinal cord is of the same length as the canal, but as time goes on the canal grows longer than the cord. This involves the nerves coming from the hinder portion growing longer than others. It is the front or anterior portion of this medullary canal which is concerned in the development of the brain itself, and here, at an early stage, two very obvious constrictions appear in the region of what is to be the brain, and these constrictions divide that brain area into three distinct parts, or vesicles. Part of the posterior vesicle ultimately develops into the _cerebellum_, or little brain. Another part forms the _medulla oblongata_, that important hind brain in which lie so many of the vital centres of nervous energy. The central cavity formed by these constrictions is of comparatively less importance, forming ultimately what is known as the _mid-brain_. The foremost or anterior vesicle, however, is of the very greatest importance, and its subsequent changes are more marked than either of the other two. From it is developed the great mass of the cerebrum itself, together with various outgrowths from it which have most important functions. Thus two of these outgrowths appear projecting from the lower part of the sides of the walls, and ultimately coming to reach the outer ectoderm. These two projections, or pouches, ultimately form the optic vesicle. Still later in development the whole of the anterior vesicle is again constricted, thus forming two distinct parts, the foremost of which, growing rapidly in two halves on either side of the middle line, ultimately give rise to the two cerebral hemispheres. These two cerebral hemispheres, therefore, arise, in the first place, as lateral enlargements of the anterior part of one of the primitive constrictions of the medullary canal. In their outer layers cells continue to make their appearance with great rapidity, and thus is formed the cerebral cortex; and the remarkable thing about this all-important part of the brain itself is that all the cells of this cerebral cortex appear to be produced during the life of the embryo; there being in all probability none added after birth has occurred. That is to say, the possibilities of the actual physical growth of brain tissue in any given embryo are fixed from the beginning. Brain tissue, in other words, is born, not made. It is the manner in which it is treated afterwards upon which depends whether that given quantity of