59© COMPARATIVE ANATOMY 



The remaining parts of the adult eye are derived from mesenchyma. 

 The cellular elements of the aqueous humor, and possibly also some of 

 those of the vitreous body are mesenchyme cells which enter the optic 

 cup by way of the optic fissure. But the fact that fibers connect the lens 

 and retina has led some investigators to infer that the vitreous body is 

 derived from the retina and therefore is ectodermal. On the outside of the 

 eye, the chorioid and fibrous tunics are added from the surrounding mesen- 

 chyma. The eye muscles of man have a like origin, although in lower 

 vertebrates they arise from the walls of the head "cavities." Folds of 

 skin form the eyelids, which unite temporarily but in man separate again 

 before birth. 



Unpaired Eyes 



Two kinds of median eyes are recognized, the parietal and the pineal. 

 Each, when present, arises from the roof of the diencephalon, and lies 

 beneath an unpigmented "apical spot" on the upper surface of the head. 



That parietal and pineal eyes are light-recipient organs is indicated by 

 the presence of a retina with photoreceptors, pigment, ganglion cells, 

 nervous connexions with the brain, and sometimes a lens. Experimental 

 evidence is somewhat conflicting. But lizards show a muscular response 

 to photic stimulation of the parietal organ, and teleosts react to stimulation 

 of the pineal organ. 



The history of the median eyes of vertebrates is one of reduction and of 

 functional change. Among the possible factors in the degeneration of these 

 organs may be the increased importance of the lateral eyes and the great 

 enlargement of the cerebral hemisphere, which in mammals overlie the 

 diencephalon. 



Cyclostomes have both parietal and pineal organs, in the form of 

 epithelial vesicles immediately below an unpigmented "apical spot" 

 between the lateral eyes. The pineal vesicle, which lies above the parietal, 

 is considerably larger, and its upper epithelium is thin and translucent, 

 while its lower layer is thickened, and consists of columnar epithelial 

 cells, some of which contain pigment. For this reason, the lower layer is 

 regarded as a retina. This inference is supported by the fact that the 

 photoreceptors of the retina are supplied with nerve fibers from adjacent 

 ganglion cells. Neurites of these ganglion cells unite to form a pineal 

 nerve, the fibers of which may be traced by way of the posterior commis- 

 sure to the mesencephalon. 



The lower or parietal vesicle resembles the pineal vesicle in essentials, 

 but rests upon a segregated portion of the left habenular ganglion. The 

 nerve fibers of the parietal nerve connect with the left habenular ganglion. 

 Whether parietal and pineal organs are serially homologous, one anterior 

 and the other posterior, or whether they are members of a pair which 



