517 



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 Ammocoete 

 to a large extent by the ciliated grooves described above, probably 

 acting in conjunction with the longer cilia which line other parts of 

 the brain-cavity. The occurrence of cilia in some part or other of the 

 central nervous system of vertebrates has been so often demonstrated^ 

 that it is hardly necessary to discuss other cases here, except to point 

 out that their function is probably in all cases to promote the circula- 

 tion of the brain-fluid. 



As to the choroid plexuses of the vertebrate brain there can, I 

 venture to think, be little doubt as to their importance in promoting 

 the oxygenation of the brain-fluid. Especially suggestive from this 

 point of view is the manner in which these plexuses sometimes project 

 into the cavities of the ventricles. A good example of this is afforded 

 by the vertical septum of the choroid plexus of the mid-brain in the 

 New Zealand Ammocoete, as described above. This septum is highly 

 vascular, and it is extremely significant that the ciliated grooves are 

 so arranged as probably to direct a stream of brain-fluid along each 



4 H.Blanc, Epiphysis and Paraphysis in Salamandra atra, Arch. Sci. 

 Phys. Nat. , Vol. 10 (1900), p. 571—572. Abstracted in Journal of Royal Micro- 

 scopical Society, June. 1901, p. 257. 



5 See for example Klein, Quart Journ. Microsc. Sci., Vol. 20. N. S. p. 47G; 

 Retzius, Biologische Untersuchungen , Neue Folge, Vol.6, p. 59; Beard, Ana- 

 tomischer Anzeiger", Vol. 3. p. 902. 



