This communication between the neural and alimentary canals, which is coupled, as will be seen in the sequel, with the atrophy of a posterior segment of the alimentary canal, is a feature of great interest which ought to throw considerable light upon the meaning of the neural canal. So far as I know, no suggestion as to the origin of it has yet been made. It is by no means confined to Elasmobranchii, but is present in all the vertebrates whose embryos are situated at the centre and not at the periphery of the blastoderm. It has been described by Goette[184] in Amphibians and by Kowalevsky, Owsjannikow and Wagner[185] in the Sturgeon (Acipenser). The same arrangement is also stated by Kowalevsky[186] to exist in Osseous Fishes and Amphioxus. The same investigator has shewn that the alimentary and neural canals communicate in larval Ascidians, and we may feel almost sure that they do so in the Marsipobranchii. Footnote 184: _Entwicklungsgeschichte der Unke._ Footnote 185: _Mélanges Biologiques de l'Académie Pétersbourg_, Tome VII. Footnote 186: _Archiv. f. mikros. Anat._ Vol. VII. p. 114. In the passage on this point Kowalevsky states that in Elasmobranchii the neural and alimentary canals communicate. This I believe to be the first notice published of this peculiar arrangement. The Reptilia, Aves, and Mammalia have usually been distinguished from other vertebrates by the possession of a well-developed allantois and amnion. I think that we may further say that the lower vertebrates, Pisces and Amphibia, are to be distinguished from the three above-mentioned groups of higher vertebrates, by the positive embryonic character that their neural and alimentary canals at first communicate posteriorly. The presence or absence of this arrangement depends on the different positions of the embryo in the blastoderm. In Reptiles, Birds and Mammals, the embryo occupies a central position in the blastoderm, and not, as in Pisces and Amphibia, a peripheral one at its edge. We can, in fact, only compare the blastoderm of the Bird and the Elasmobranch, by supposing that in the blastoderm of the Bird there has occurred an abbreviation of the processes, by which the embryo Elasmobranch is eventually placed in the centre of the blastoderm: as a result of this abbreviation the embryo Bird occupies _from the first a_ central position in the blastoderm[187].