404 THE PINEAL ORGAN 



cumstance that only his earlier works have been quoted in many of the 

 published references to the literature of the pineal system, and these 

 authors have most probably been unaware of the existence of his later 

 publications. Dendy, as is well known from his earlier communications, 

 regarded the pineal eye of Sphenodon as the left pineal organ and the 

 pineal sac (epiphysis) as the right pineal organ of a paired system which 

 included the pineal eye and pineal sac, the pineal stalk, and the associated 

 vessels and nerves. He also considered that the primarily right and left 

 components of this system have, in the course of evolution, become dis- 

 placed towards or into the median plane, so that the left organ has become 

 anterior and the right posterior. The latter or pineal sac of Sphenodon 

 has also become more degenerate than the left, which is separated off as 

 the parietal organ and presumably has retained, to a much greater extent, 

 the original structure of the ancestral pineal eye. In his later memoir, 

 published in 191 1, Dendy clearly demonstrated that the wall of the pineal 

 sac of Sphenodon has a nervous structure which is essentially similar to 

 that of the pineal eye, and that although less highly differentiated it shows, 

 as in the pineal eye, internal and external limiting membranes, radial 

 supporting or glial fibres, which are comparable to Miiller's fibres in the 

 retina of the lateral eyes of vertebrates, also neuro-epithelial cells which 

 he regarded as sensory in nature, ganglion cells, nerve fibres, and at its 

 distal extremity pigment cells. In one of his specimens this apical part 

 was partially constricted off from the main diverticulum so as to form a 

 thin- walled sac, containing pigment cells in the wall of the main sac. 

 He regarded this sac as being comparable to the accessory parietal organs 

 described by Leydig and Studnicka, and as supporting his view that " the 

 structure of the pineal sac is fundamentally identical with that of the 

 pineal eye." 



In cyclostomes (Studnicka, Gaskell, Dendy) there is the same funda- 

 mental similarity in structure of the pineal eye and the parapineal organ 

 as is met with in the pineal eye and the pineal sac or epiphysis of reptiles. 

 Both in fishes and reptiles there are sometimes two outgrowths, one of 

 which, Epiphysis I, is anterior, while the other, Epiphysis II, is posterior. 

 In both fishes and reptiles the anterior epiphysis is usually to the left of the 

 median plane. In fishes, however, the posterior organ is the more highly 

 evolved and in cyclostomes it forms the pineal eye, whereas in reptiles the 

 anterior organ is the more highly evolved and forms the pineal eye or 

 parietal organ. 



In Geotria Professor Dendy demonstrated non-medullated nerve- 

 fibres which apparently arose from the ganglion cells of the retina of the right 

 or posterior pineal eye ; these converged towards the optic stalk and then, 

 forming a nerve bundle in the stalk, coursed backwards to end in the right 



