186 THE PATH OF DEGENERATIVE EVOLUTION 



so degenerate : the retina has not a complex 

 structure, but both retina and lens are present. 



In the type Cydodus (fig. 62, D), the degeneration 

 is still greater : the epiphysis is a vesicle attached 

 to the thalamencephalon. The walls of this 

 vesicle show only the smallest symptom of primi- 

 tive differentiation into lens and retina, The 

 proximal part remains hollow, and shows no trace 

 of differentiation at all. 



This, however, is not the only fashion in which 

 degeneration of the pineal eye proceeds. In 

 another series of creatures it retains its connec- 

 tion with the thalamencephalon, but remains inside 

 the skull. The parietal foramen closes, thus 

 completely shutting off the eye from the light; 

 the eye becomes useless, degenerates, and the 

 optic nerve loses its function as a conducting 

 channel, Ceratophora (fig. 62, E). The pineal 

 organ then becomes a degenerate structure in 

 which it is exceedingly difficult to see traces of 

 its original condition, and which is usually marked 

 by an abundance of blood-vessels : Birds (fig. 62, F), 

 Mammals (fig. 62, G). 



Thus, the degeneration of the pineal eye shows 

 that the optic nerve, the last organ to be completed, 

 is the first to disappear. 1 



In studying the degeneration of the pineal eye, 



1 The same happens in the case of the ordinary paired eyes. In 

 the degenerate eyes of the mole, the optic nerve is more reduced 

 than are the other structures. 



