PARIETAL EYE AND ADJAOKNT ORGANS IN SPHENODON. 129 



3. Discussion or Results. 



(fl) Structure of the Parietal Eye. 



The Retina, — Certainly the most remarkable feature about 

 the retina of the parietal eye in Sphenodon is its differentia- 

 tion at Stage R into two very distinct layers^ as shown in figs. 

 14 — 17. It was this feature^ which I first observed in an 

 Australian skink embryo (Hi null a, sp.), which led me to 

 undertake the present investigation. The parietal eye at a 

 certain stage thus acquires an extraordinarily close resemblance 

 to the ordinary paired Vertebrate eye, consisting of a two- 

 layered optic cup grasping a cellular lens within its margin. 

 The developmental history shows, however, that this result is 

 arrived at in a totally different way in the two cases, for in 

 the parietal eye there can be no doubt that the lens is formed 

 simply by thickening of the front wall of an originally single- 

 layered vesicle. It will also be observed that the situation of 

 the pigment is different in the two cases, being in the inner 

 layer, next to the cavity of the cup, in the parietal eye, and 

 in the outer layer, away from the cavity, in the paired eye. 

 Baldwin Spencer (33) has also shown that the rods lie next to 

 the cavity in the parietal eye, instead of being formed from 

 the outermost part of the inner layer as in the paired eye. 



It appears to me, from what Baldwin Spencer says of the 

 structure of the retina, that even in the adult parietal eye of 

 Sphenodon we can still trace the two embryonic layers. 



This author describes no less than six series of histological 

 elements in the retina of the adult eye. His '^molecular 

 layer, consisting of fine punctated material, which takes the 

 stain (heematoxylin) with difficulty,'^ evidently marks the 

 embryonic separation of the wall of the optic cup into two 

 primary layers, as I have noticed previously. In the adult 

 the inner of the two primary layers is still much thicker, and 

 appears to have become differentiated into (1) a layer of rod- 

 like bodies enveloped in deep pigment, evidently formed from 

 the innermost layer of columnar cells which I have described 

 in the embryo, with the pigment granules between them ; 



