First, take the embryological evidence. On the ground that the individual development reproduces to a certain extent the phylogenetic development, the peculiarities of the formation of the central nervous system in the vertebrate embryo ought to receive an appropriate explanation in any theory of phylogenetic development. Hitherto such explanation has been totally lacking; any suggestion of the manner in which a tubular nervous system may have been formed takes no account whatever of the differences between different parts of the tube; its dilated cephalic end with its infundibular projection ventrally, its small straight spinal part, and its termination in the anus. My theory, on the other hand, is in perfect harmony with the embryological history, and explains it point by point. From the very first origin of the central nervous system there is evidence of two structures--the one nervous, and the other an epithelial surface-layer which ultimately forms a tube; this was first described by Scott in Petromyzon, and later by Assheton in the frog. In the latter case the external epithelial layer is pigmented, while the underlying nervous layer contains no pigment; a marked {43}and conspicuous demarcation exists, therefore, between the two layers from the very beginning, and it is easy to trace the subsequent fate of the two layers owing to this difference of pigmentation. The pigmented cells form the lining cells of the central canal, and becoming elongated, stretch out between the cells of the nervous layer; while the latter, on their side, invade and press between the pigmented cells. In this case, owing to the pigmentation of the epithelial layer, embryology points out in the clearest possible manner how the central nervous system of the vertebrate is composed of two structures--an epithelial non-nervous tube, on the outside of which the central nervous system was originally grouped; how, as development proceeds, the elements of these two structures invade each other, until at last they become so involved together as to give rise to the conception that we are dealing with one single nerve tube. It is impossible for embryology to give a clearer clue to the past history than it does in this case, for it actually shows, step by step, how the amalgamation between the central nervous system and the old alimentary canal took place. Further, consider the shape of the tube when it is first formed, how extraordinary and significant that is. It consists of a simple dilated anterior end leading into a straight tube, the lumen of which is much larger than that of the ultimate spinal canal, and terminates by way of the neurenteric canal in the anus.