This investigation forcibly impressed upon my mind the uniformity in the arrangement of the central nervous system as far as the centres of origin of all the segmental nerves are concerned, {4}both cranial and spinal, and also the original segmental character of this part of the nervous system. I could not, therefore, help being struck by the force of the comparison between the central nervous systems of Vertebrata and Appendiculata as put forward again and again by the past generation of comparative anatomists, and wondered why it had been discredited. There in the infundibulum was the old oesophagus, there in the cranial segmental nerves the infraoesophageal ganglia, there in the cerebral hemispheres and optic and olfactory nerves the supraoesophageal ganglia, there in the spinal cord the ventral chain of ganglia. But if the infundibulum was the old oesophagus, what then? The old oesophagus was continuous with and led into the cephalic stomach. What about the infundibulum? It was continuous with and led into the ventricles of the brain, and the whole thing became clear. The ventricles of the brain were the old cephalic stomach, and the canal of the spinal cord the long straight intestine which led originally to the anus, and still in the vertebrate embryo opens out into the anus. Not having been educated in a morphological laboratory and taught that the one organ which is homologous throughout the animal kingdom is the gut, and that therefore the gut of the invertebrate ancestor must continue on as the gut of the vertebrate, the conception that the central nervous system has grown round and enclosed the original ancestral gut, and that the vertebrate has formed a new gut did not seem to me so impossible as to prevent my taking it as a working hypothesis, and seeing to what it would lead. This theory that the so-called central nervous system of the vertebrate is in reality composed of two separate parts, of which the one, the segmented part, corresponds to the central nervous system of the highest invertebrates, while the other, the unsegmented tube, was originally the alimentary canal of that same invertebrate, came into my mind in the year 1887. The following year, on June 23, 1888, I read a paper on the subject before the Anatomical Society at Cambridge, which was published in the _Journal of Anatomy and Physiology_, vol. 23, and more fully in the _Journal of Physiology_, vol. 10. Since that time I have been engaged in testing the theory in every possible way, and have published the results of my investigations in a series of papers in different journals, a list of which I append at the end of this introductory chapter.