516 



they are I think better defined than in the New Zealand specimen, 

 and the left one can be traced a good deal further forwards than the 

 right, owing apparently to the great development of the right ganglion 

 habenulae, which projects into the brain-cavity and more or less blocks 

 up the angle between roof and side wall. Posteriorly (fig. (J) they 

 appear to terminate — as grooves — at about the level of the hinder 

 margin of the posterior commissure, but a band of long columnar cells 

 appears to be continued backwards for a short distance after their 

 groove-like character has disappeared, so that we find such a band on 

 either side, bordering the narrow opening between the iter below and 

 the cavity of the choroid plexus of the mid-brain above. 



The most striking difference as compared with the New Zealand 

 Ammocoete concerns the arrangement of the choroid plexus of the 

 mid-brain, which no longer dips down into the iter in the form of a 

 deep, continuous, vertical septum (compare figs. 3 and 6). Such a 

 septum is, however, simulated in the sections by a mass of granular 

 material containing what look like nuclei irregularly scattered through 

 it. This appearance, which is not represented in the figures, suggests 

 that the septum has undergone degeneration, or possibly the granular 

 mass is simply a coagulum containing the remains of cells shed from 

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 existed in the European form, has simply been withdrawn upwards 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

 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, 



3 Journal of Anatomy and Physiology, Vol. 32. p. 545. 



