518 



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 Ammocoete 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-lnain, 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*»: "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." 



Considering how much attention has of late years been devoted to 

 the study of the Lamprey's brain, it seems almost incredible that such 

 definite structures as the ciliated grooves described above should have 

 hitherto escaped observation. I have, however, been unable to find 

 any reference to them in the littérature available. It remains to be 

 seen whether or not they occur in the adult I am inclined to think 

 that they probably disappear when the development of the first choroid 

 plexus renders their presence no longer necessary, and such circulation 

 of the fluid in the cavity of the brain as is requisite may be safely left 

 to the cilia lining other parts. Allibo rn ^ figures a transverse section 

 of the brain of Petromyzon Planeri taken in the region of the posterior 

 commissure, but makes no mention of the structures in question. 



f' American Journal of Anatomy, Vol. ]. No. 1. p. 96. 



"^ Ahlborn, Untersuchungen über das Gehirn der Petromyzonten, Zeit. 

 Wiss. Zool.. Vol, 39. p. 191. Plate 14. fi^r. 20. 



