THE PATH OF DEGENERATION IN ANIMALS 183 



(2) This little sac elongates, changes its direction, 



FIG. 62. Diagram indicating the progressive evolution and the degeneration of 

 the pineal eye. 



A. Perfect pineal eye, as found in the slow-worm before birth, or in the adult 

 Sphenodon (Hatteria) ; c, lens; r, retina; ra, optic nerve; rf, divertialum of the 

 thalamencephalon. B. Pineal eye in first stage of degeneration as it exists in 

 Chamceleo and as it was in the slow-worm before stage A. The lens (c), and 

 the retina (r), are not differentiated. C. Pineal eye in the degenerate form 

 found in Calotes and Leiodera; c, lens; r, retina; n, optic nerve in fatty de- 

 generation. D. Very degenerate pineal eye as in Cyclodus and like the earliest 

 stage in the slow- worm ; there is no differentiation of the divertialum from the 

 thalamencephalon. E, F, G. Other modes of degeneration of the pineal eye. 

 The eye lies within the skull and there is no parietal foramen ; cr, cranial 

 membranes; E. Ceratophora. F. Birds; g, mammals. (After Baldwin 

 Spencer.) 



and becomes divided into a proximal and 

 distal portion. The cells lining the distal 



