18 ARTHUR DENDY. 



the ceutral nervous system is^ to say the least of it, extremely 

 improbable. This difficulty may be overcome, however, by 

 supposing that the " reflex cells " required are situate in the 

 great ganglionic swelling which surrounds the atrium of the 

 pineal eye, and which, of course, is actually developed as an 

 outgrowth of the central nervous system. 



I do not wish to press this suggestion too far, however, in 

 the present state of our knowledge, nor is it necessary to do 

 so in order to link up the pineal eye with the optic reflex 

 apparatus, for Sargent has shown that some of the con- 

 stituents of Reissner's fibre issue from the base of the right 

 habenular ganglion. Now this ganglion is undoubtedly 

 connected with the pineal eye through the pineal nerve, as I 

 have already indicated, and it is extremely probable that we 

 have here reflex cells which transmit stimuli received through 

 the pineal nerve to Reissner's fibre. 



From the region of the posterior commissure it is quite 

 easy to trace Reissner's fibre backwards through the iter and 

 the fourth ventricle, into the canalis centralis of the 

 spinal cord, as shown in fig. 2. It is worth noticing that, as 

 it passes beneath the cerebellum, it does not become imbedded 

 in the roof of the brain as takes place in adult Petromyzon, 

 but remains free throughout its course. This free condition 

 is also found in the young Petromyzon, so that it is not 

 unlikely that in Greotria also Reissner's fibre may become 

 imbedded in the growing tissue of the cerebellum with 

 advancing age. I have not followed it backwards beyond 

 the commencement of the spinal cord. 



(e) The Parapineal Okgan (Left Paeietal Eye) and its 

 Relations to the Brain. 



The parapineal organ, or left parietal eye (figs. 2, 8, L.P.E.), 

 is, as already pointed out by Studnicka for Petromyzon, 

 essentially similar in structure to its larger and more perfectly 

 developed fellow of the primitive right side. Its position, in 

 front of and a little to the left of the "pineal organ," has already 



