PARIETAL SENSE-ORGANS OF GEOTRfA. 15 



ular) commissure, the pineal organ itself is connected with 

 the posterior commissure. Studuicka makes use of this 

 apparent discrepancy as an argument against the theory of 

 the paired origin of the parietal sense organs. We shall have 

 occasion to discuss this question somewhat more in detail at 

 a later stage. 



According to the observations recorded above the con- 

 nection of the piueal nerve with the posterior commissure, 

 about tlie existence of which there can be very little doubt, 

 may be due simply to the fact that some of the nerve fibres 

 traverse this commissure in order to reach the epithelium of 

 the ependymal groove. Curiously enough, the existence of 

 this remarkable structure — the ependymal groove — appears 

 to have been hitherto ignored by those authors who have 

 investigated the pineal organs, and, conversely, those who 

 have dealt with the ependymal groove have entirely neglected 

 its relations to the pineal nerve. 



In my memoir on the subject, published in 1902 (5), I 

 described a pair of these grooves in the Ammoccetes both of 

 Geotria and Petromyzon, and, believing that I had de- 

 tected cilia on the long columnar cells with which they are 

 lined, I termed them " ciliated grooves," and suggested that 

 they might serve to promote the circulation of the fluid in the 

 brain cavities, especially in relation to a highly vascular 

 vertical fold of the choroid plexus, which in the Ammoccete 

 hangs down, gill-like, into the brain cavity in the immediate 

 neighbourhood of the grooves in question. Sargent (6) in his 

 remarkable memoir on the optic reflex apparatus of verte- 

 brates, criticises this view, maintaining that what I had inter- 

 preted as cilia are really constituent fibrils of Reissner's fibre, 

 and that the ependymal groove functions merely as an attach- 

 ment plate for these fibrils, which supports them as they leave 

 the brain on their Avay to join the main fibre lying freely in 

 the brain cavity. It does not seem to me that these two views 

 are incompatible with one another, and I find it difiicult to 

 believe that such a remarkable and well-developed structure 

 as the ependymal groove should be required solely for the 



