the lining epithelium ; it is, however, easily distinguished from the choroid plexus, to which it is attached above, and which has the usual relations. It seems most likely that the vertical septum, if it ever existed in the European form, has simply been withdrawn upwards in the course of development, to take part in the formation of the extensive tela choroidea which forms the roof of the mid-brain.)
The subject can hardly be dismissed without some attempt to explain the presence of the ciliated grooves, and it is by no means difficult to assign to them a probable function. Gaskell, as is well known, has endeavoured to prove that the tubular nature of the central nervous system of vertebrates may be accounted for by its derivation from the alimentary canal of an invertebrate ancestor. He asks,* " On any of the other theories, why is there a nerve tube found with a dilated anterior extremity ? "Why are the cerebral vesicles formed 1 Why the choroid plexuses ? Why does that tube terminate in the anus ? Why is there an infundibular prolongation ?" Without attempting to answer all these questions, I venture to hope that the facts recorded in this communication may throw some light upon the function of the cavities of the central nervous system. It has already been suggested that these may aid in the process of gaseous interchange. Thus, H. Blanc "believes that the paraphysis is not a degenerating organ, but is of great importance in connection with the development of the choroid plexus. Like the last-named, it has probably to do with the gaseous interchanges which are carried on in the cavities of the ventricles."! Now gaseous interchange, or respiration in the widest sense of the term, must obviously be a very necessary function for the nerve-cells. These cells in the Ammocoete lie, as is well-known, in the inner portion of the brain substance, which forms a thick layer, separated from the fluid in the brain-cavity by the lining epithelium. Their relative positions are such that gaseous interchange may take place freely between the fluid in the brain-cavity on the one hand, and the grey matter of the brain on the other ; but in order that this interchange may continue effectually two other conditions must be fulfilled : (1) Means must be provided for removing from the fluid in the brain-cavity the waste matters which it takes up from the brain- substance, and in return providing it with fresh supplies of oxygen and perhaps also of dissolved nutriment ; and (2) Means must also be provided for maintaining a circulation of the fluid in the brain-cavity itself. The first of these conditions is apparently fulfilled by the vascular -choroid plexus, while the second is, I believe, fulfilled in the Animoccete to a large extent by the ciliated grooves described above, probably
- ' Journal of Anatomy and Physiology,' vol. 32, p. 545.
t H. Blanc, " Epipliysis and Paraphysis in Salamandra atra," ' Arch. Soi. Phys. Nat,,' vol. 10 (1900), pp. 571-2. Abstracted in ' Journal of Koyal Micro- scopical Society,' June, 1901, p. 257.
