592 



COMPARATIVE ANATOMY 



the same cross section. On the whole, these facts tend to suggest that 

 cyclostomes have a right pineal eye and a left parietal eye. In myxine, 

 however, the pineal organ is wanting. 



The median eyes of fishes are degenerate. The parietal organ is 

 lacking, and the pineal does not perforate the skull. A tendency for the 

 organ to become glandular is shown by the folding of the wall of the vesicle. 

 In the elasmobranch embryos, the anlage of the parietal organ appears 

 but later degenerates. 



Among the amphibia, the anura have a translucent "apical" spot like 

 that of cyclostomes. Beneath this spot, lies the vesicle of the pineal eye, 

 connected by a nerve with the brain. The basal layer of the vesicle forms 



Fig. 490. — Parietal eye of Anguis fragilis. ct, connective-tissue cells around nerve; 

 gc, ganglion cells; I, lens; n, nerve fibers; pn, parietal nerve; pc, pigment cells; r, retinal 

 cells; vb, vitreous body. (From Kingsley's "Comparative Anatomy of Vertebrates" 

 after Nowikofl.) 



a thickened retina. A parietal organ is wanting. In the fossil stego- 

 cephala, the roof of the cranium is perforate between the two parietal 

 bones and from this a functional pineal eye in these animals is inferred. 



In reptiles the tendency of the epiphysis to become glandular is evident. 

 But in crocodiles, the pineal organ wholly disappears. The parietal organ 

 of this group assumes even more markedly than in the lower vertebrates 

 the characteristics of the vesicular eye of invertebrates. In sphenodon 

 and many lizards, the upper layer of cells of the vesicle forms a lens-like 

 thickening, which focusses light upon the retina. The skull is perforate. 



Among the reptiles the parietal eye of anguis fragilis is most highly 

 differentiated. The terminal vesicle in this animal Hes immediately 

 below a translucent apical spot, and is connected with the left habenular 

 ganglion by a ganglionated parietal nerve. The vesicle is lined by 

 columnar epithelial cells, which become locally thickened on the upper 



