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 Ammoccete, as described above. This septum is highly vascular, and it is extremely significant that the ciliated grooves ate so arranged as probably to direct a stream of brain-fluid along each side of it. Whether the direction of this stream is forwards or back- wards, it is of course impossible to say without direct experiment. Personally, I am inclined to think that it will be found to flow forwards, and that the ciliated grooves may be looked upon as special organs for conveying an abundant supply of oxygenated fluid to the forepart of the brain (especially, perhaps, to the right ganglion habenulae, which, as is well known, is enormously developed in the Lamprey, and apparently fulfils some important function in connection with the parietal organs). In the young Ammoccete the first choroid plexus, which may be supposed to be especially concerned in the respiration of the fore-brain, is not yet developed ; the second and third choroid plexuses, belonging respectively to the mid- and hind-brain, are, on the other hand, already extensive. We need not, therefore, be surprised to find that the fore-brain at this stage is dependent to a large extent for its means of respiration, and perhaps also for its nutrition, upon the choroid plexus of the mid-brain, and that a special apparatus is developed for securing a forward flow of the necessary fluid in the brain-cavity. With regard to this fluid itself it is interesting to note in this connection the recently expressed opinion of Minot :f " The pineal region develops a series of structures, which, from their anatomical characteristics, appear to be directly concerned in the formation of the fluid in the cavities of the brain. We may assume that the choroid plexus supplies the main bulk of the fluid, but the gland-like organisation of the epiphysis and of the paraphysis indicates that they supply by secretion special chemical substances to the encephalic fluid."
- See for example Klein, 'Quart. Journ. Micros. Sci.,' vol. 20, N.S., p. 476 ;
Eetzius, ' Biologische Untersuchungen,' Neue Folge, vol. 6, p. 59; Beard, ' Aiiatomischer Anzeiger,' vol. 3, p. 902.
t ' American Journal of Anatomy,' vol. 1 , No. 1, p. 96.
