MEDIAN EYES 713 



the three arches of the roof of the diencephalon — a pineal organ or 

 EPIPHYSIS ^ lying more posteriorly and a parietal or parapineal organ 

 lying more anteriorly, sometimes arising in association with the pineal body, 

 but sometimes independently of it. The former is connected with the 

 posterior commissure ; the latter with the superior (habenular) commissure ; 

 their development in the embryo of the lizard {Lacerta) is seen in Figs. 

 853-5). The pineal body is connected nervously with the right habenular 

 ganglion, the parietal with the left, suggesting that originally they may have 

 been right and left members of a pair. 



The highest development of a median eye is seen in the most primitive 

 Vertebrates, the cyclostomes (Fig. 856). The lamprey {Petromyzon) is 

 provided with both a pineal and a parietal organ having the structure of an 

 eye with a considerable degree of retinal differentiation (Fig. 864) ; but in 

 Myxinoids no trace of either is seen. The presence of an impression in the 

 mid-line of the roof of the cranial cavity in fossil remains of the closely- 

 related Agnatha {Pteraspis, Cephalasjns) — the oldest known Vertebrates — 

 is an indication of the occurrence of a pineal organ in these very primitive 

 types, and since the impression is often duplicated the presumption is that 

 the median eye at this stage in evolution was paired (Gaskell, 1908 ; Wood- 

 ward, 1922 ; Heintz, 1932 ; Hills, 1933). 



Among FISHES certain old-fashioned ganoid types retain a relatively 

 well-developed median eye somewhat resembling the parietal eye of 

 Cyclostomes. In the sturgeon, Acipenser, in addition to supporting cells of 

 ependymal character, the vesicle contains many cells of a sensory type with 

 ganglion cells and efferent nerve fibres ; the structure thus resembles the 

 parietal sense-organ rather than a secretory gland. A somewhat similar 

 organ is seen in the primitive fish, Polypterus, found in African rivers, and 

 the Holostean, Amia (Hill, 1894 ; and others). In Selachians (skate, 

 shark, dogfish, etc.) the pineal body is set on a long stalk and often per- 

 forates the skull through a pineal foramen to appear beneath the skin as 

 a closed vesicle (Fig. 857) ; alternatively it may lie \\ithin the skull in 

 a recess in its cartilaginous roof (Holocephali). In these fishes the eye- 

 structure has disappeared, the vesicle is small and consists of ependymal 

 cells, and the tendency is probably towards glandular formation (Cattie, 

 1882 ; Locy, 1894). In Teleosteans (trout, salmon, pike, herring, etc.) 

 the pineal apparatus is not so well developed, and is somewhat variable ; 

 in contrast to "ganoid" and cartilaginous fishes, the vesicle tends to be large 

 and the stalk short. In these bony fishes it rarely reaches the under- 

 surface of the skull, and although it contains cells of neural and glial 

 character among the ej^endymal cells, it never shows a developed ocular 

 structure. It is interesting, however, that in this class of fishes the superficial 



^ Galen (c. a.d. 130-200) used the non-committal, topographical Greek term — eTri, upon, 

 (f>vai^, growth ; the Latin term is descri^Jtive of the shape — pinus, a fir-cone. The term 

 " epiphysis " is usually applied to the deeply situated glandular organ seen in Mammals in 

 contrast to the sensory " pineal eye " of the lamprey or Sphenodon. 



