This is exactly what is found in the vertebrate, for the infundibular tube passes into the third ventricle of the brain, which forms, with the other ventricles of the brain, the large dilated cephalic portion of the so-called nerve tube, and at the junction of the medulla oblongata and spinal cord, this dilated anterior part passes into the small, straight, central canal of the spinal cord, which in the embryo terminates in the anus by way of the neurenteric canal. If the animal is regarded as not having been turned over, then the conclusion that the infundibulum was the original oesophagus leads immediately to the further conclusion that the ventricles of the vertebrate brain represent the original cephalic stomach, and the central canal of the spinal cord the straight intestine of the arthropod ancestor. {38}For the first time a logical, straightforward explanation is thus given of the peculiarities of the tube of the central nervous system, with its extraordinary termination in the anus in the embryo, its smallness in the spinal cord, its largeness in the brain region, and its offshoot to the ventral side of the brain as the infundibular channel. It is so clear that, if the infundibular tube be looked on as the old oesophagus, then its lining epithelium is the lining of that oesophagus; and the fact that this lining epithelium is continuous with that of the third ventricle, and so with the lining of the whole nerve-tube, must be taken into account and not entirely ignored as has hitherto been the case. If, then, we look at the central nervous system of the vertebrate in the light of the central nervous system of the arthropod without turning the animal over, we are led immediately to the conclusion that what has hitherto been called the vertebrate nervous system is in reality composed of two parts, viz. a nervous part comparable in all respects with that of the arthropod ancestor, which has grown over and included into itself, to a greater or less extent, a tubular part comparable in all respects with the alimentary canal of the aforesaid ancestor. If this conclusion is correct, it is entirely wrong to speak of the vertebrate central nervous system as being tubular, for the tube does not belong to the nervous system, but was originally a simple epithelial tube, such as characterizes the oesophagus, cephalic stomach, and straight intestine of the arthropod. Here, then, is the crux of the position--either the so-called nervous tube of the vertebrate is composed of two separate factors, consisting of a true non-tubular nervous system and a non-nervous epithelial tube, these two elements having become closely connected together; or it is composed of one factor, an epithelial tube which constitutes the nervous system, its elements being all nervous elements.