STRUCTURE OF PARIETAL ORGAN OF SPHENODON 257 



5. An eye without visible doubling of any part may lack an optic nerve. 



6. An apparently single eye, in which the lens is found to be com- 

 pounded of two halves. 



The possibility of the apparently single median or " third " eye of 

 certain fishes and reptiles being compounded of elements of parts of two 

 eyes growing in contact with each other is thus evident, and appearances 

 such as those shown in Fig. 176, p. 247, and Fig. 182, p. 255, are readily 

 explained. It is quite possible that the rounded pigment cells in the 

 centre of the lens represent a vestige of interposed median parts of retinae 

 which have become included between the two halves of the compound 

 lens ; and the presence of a median notch on the under surface of 

 the lens or a seam on the superficial surface affords additional evidence 

 of the fusion, moreover, the deep brownish-black colour of the pigment 

 and its deposit in large, rounded cells are typical of retinal pigment rather 

 than a mere degeneration product ; although the element of degener- 

 ation is undoubtedly present, the localization of the pigment in the 

 centre of the proximal half of the lens is strongly in favour of the cells 

 in which it is deposited being retinal in origin. A reference to Fig. 175, 

 p. 245, in which a " parietal organ " and an " accessory organ " are seen 

 lying close together, in a sagittal section through the parietal foramen 

 of Phrynosoma coronatum will show how if these two vesicles were fused, 

 the opposed portions of the retinae between the two lenses would almost 

 certainly be included in the centre of the compound lens. It may also 

 be noted that the two vesicles are surrounded by a common fibrous 

 capsule, and that pigment is less in the more degenerate accessory organ 

 than in the more highly differentiated parietal organ. 



Structure of the Lens. — The lens of the parietal organ is formed of 

 transparent elongated cells lying parallel to each other, and extending 

 from its superficial to its deep surface. The cells are longer in the 

 centre of the lens than at its periphery. Their form and general arrange- 

 ment resembles that of the cells forming the posterior wall of the embryonic 

 lenses of the lateral eyes ; there is, however, at no period of development 

 any cavity or thin anterior lamina, as in the lateral eye, the lens of the 

 parietal eye being solid from the first — it having been developed from a 

 segment of the superficial wall of the vesicle, which was primarily directly 

 continuous with the retinal segment, the two parts passing insensibly into 

 each other, and a definite limiting membrane only being formed at a later 

 stage. 



The lenticular cells are cylindrical or prismatic in type, and show no 

 special thickening in the situation of the nucleus. The nuclei are some- 

 times arranged in an approximately single plane, but in young embryonic 

 specimens four or five nuclei may be present between the two surfaces, 

 17 



