lii the ///v//'// of the Ainmoccete. 401 



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 existed in the European form, has simply been withdrawn upwards 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 witli 

 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. 



