Why should the tube take this peculiar shape at its first formation? No explanation is given or suggested in any text-book of embryology, and yet it is so natural, so simple: it is simply the shape of the invertebrate alimentary canal with its cephalic stomach and straight intestine ending in the anus. Again embryology indicates most unmistakably the past history of the race. How are the nervous elements grouped round this tube when it is first formed? Here embryology shows that a striking difference exists between the part of the tube which forms the spinal cord and the dilated cephalic part. Fig. 21, A (2), represents the relation between the nervous masses and the epithelial tube in the first instance. At this stage the nervous material in the spinal cord lies laterally and ventrally to this tube, and at a very early stage the white anterior commissure is formed, joining together these two lateral masses; as yet there is no sign of any posterior fissure, the tube with its open lumen extends right to the dorsal surface. {44}The interpretation of this stage is that in the invertebrate ancestor the nerve-masses were situated laterally and ventrally to the epithelial tube, and were connected together by commissures on the ventral side of the tube (Fig. 21, A (1)); in other words, the chain of ventral ganglia and their transverse commissures lying just ventrally to the intestine, which are so characteristic of the arthropod nervous system, is represented at this stage. [Illustration: FIG. 21.--A, METHOD OF FORMATION OF THE VERTEBRATE SPINAL CORD FROM THE VENTRAL CHAIN OF GANGLIA AND THE INTESTINE OF AN ARTHROPOD, REPRESENTED IN 1; B, METHOD OF FORMATION OF THE VERTEBRATE MEDULLA OBLONGATA FROM THE INFRA-OESOPHAGEAL GANGLIA AND THE CEPHALIC STOMACH OF AN ARTHROPOD.] Subsequently, by the growth dorsalwards of nervous material to form the posterior columns, the original epithelial tube is compressed dorsally and laterally to such an extent that those parts lose all signs of lumen, the one becoming the posterior fissure and the others the _substantia gelatinosa Rolandi_ on each side. The original tube is thus reduced to a small canal formed by its ventral portion only (Fig. 21, A (3)). In this way the spinal cord is formed, and the walls of the original epithelial tube are finally visible only as the lining of the central canal (Fig. 21, A (4)).