THE PINEAL EYES OF CYCLOSTOMES 



191 



by Retzius many of the cells which are blackened in the retina are sup- 

 porting ependymal cells which show typical sole-like thickenings, where 

 their outer or proximal ends are attached to the external limiting membrane, 

 whereas the inner or distal ends of the columnar retinal cells which 

 contain the pigment remain unstained. On the other hand the nerve- 

 fibre layer and the continuity of many of the nerve-fibres with the fibres 

 of the pineal-nerve ; and also of the retinal cells of the parapineal organ 

 and the connections of these with the anterior (left) habenular ganglion 

 and the nerve-fibres of the pineal tract were well brought out by the 

 impregnation process. 



The columnar cells containing the " white pigment " (Fig. 45, B) are 

 variously regarded as being : (1) ependymal cells, which function as 

 supporting cells and are comparable to the " fibres of Miiller " in the 

 retina of the lateral eyes (Studnicka) ; (2) " pigment " cells, which 



>.f. 



/-^.Ol ap 



\0 v° rb 



A B C D 



Fig. 135. — Diagram to show the Different Shapes of Head-shield, due to 



the Forward Growth of the Somatic Musculature. (After Gaskell.) 



A : Didymaspis ; B : Auchenaspis ; C : Cephalaspis ; D : Ammocoetes. 



Ol. ap. : olfactory aperture ; Orb. : orbital cavity ; pJ. : parietal foramen. 



contain a crystalline deposit of calcium phosphate, guanin, and a small 

 quantity only of melanin. The deposit of phosphate of lime was regarded 

 by both Mayer and Gaskell as an indication of degeneration and com- 

 parable with the " brain sand " found in the pineal body of mammalia. 



The Parietal Foramen of Ammocoetes 



The position of the narial and pineal openings in the muco-cartilage 

 which forms the temporary head shield of an Ammocoete was studied by 

 Gaskell, who compared the dorsal head-shield of the larval lamprey with 

 the very similar head-shields of certain extinct fishes, belonging to the 

 class Ostracodermata, and more particularly Didymaspis, Auchenaspis, 

 and Cephalaspis (Fig. 135) ; in all of which the parietal foramen is placed 

 behind the single nasal aperture and between the two orbital cavities 

 for the lateral eyes. He also compared the microscopical structure of 

 the head-shields of the Ostracodermata, as represented by Rohon, with 



