SAKGENT : THE OPTIC EEFLEX APPARATUS OF YERTEBRATES. 141 



encephalic cavities and their contained fluid, the recently expressed 

 opinion of Minot (:01, p. 96) is of interest and importance: "The 

 pineal region develops a series of structures, which, from their anatomi- 

 cal characteristics, appear to be directly concerned in tlie formation of 

 the fluid in the cavities of the brain. "We may assume tliat the 

 choroid plexus supplies the main bulk of the fluid, but the gland-like 

 organization of the epiphysis and of the paraphysis indicates that they 

 supply by secretion special chemical substances to the encephalic fluid." 

 Dendy (:02, p. 492) has ventured the suggestion that the choroid 

 plexus is of " importance in promoting the oxygenation of the brain- 

 fluid," and has made some conjectures as to the means by which 

 the fluid is circulated. The fact that the composition of the cerebro- 

 spinal fluid varies with the state of fatigue of the nervous system also 

 points to an important function. 



2. Cilia. 



The occurrence of cilia projecting into the fluid from the walls of the 

 ventricles and canal has long been known (see Kolliker, '96, p. 1-44) ; 

 but the distribution of these ciliated cells has not yet been worked out. 

 From my incomplete observations, I believe that they have a definite 

 but not uniform distribution. As a rule, the cilia on the roof of the 

 canal and ventricles are larger and more numerous than elsewhere. 

 The cilia in a larval Amia 15 mm. long are 5 micra in length, equal- 

 ling one fourth the diameter of the lumen of the canal. The floor of 

 the canal and ventricles seems to be destitute of cilia. Probably the cilia 

 perform some function in keeping the cerebro-spinal fluid in circulation. 



Johnston (:03') has recently observed the movements of the cerebro- 

 spinal fluid in Cryptobranchus. " There is a general current which flows 

 backward on the floor and the lower part of the side walls of the brain, 

 and forward along the roof and upper part of the side walls." These 

 currents are kept up, he believes, by the cilia. 



In this connection it would be of interest to know more of the ciliated 

 groove in the floor of the central canal, somewhat vaguely described by 

 Beard ('88, p. 902), in the early development of the neural tube. 



3. Ependtmal Groove. 



A characteristic ependymal structure lies in the roof of the dien- 

 cephalon, extending from beneath the posterior commissure cephalad 

 to the base of the ganglia habenulae. In gnathostomes this forms a 

 median groove, generally somewhat horseshoe-shaped in transverse 



