184 THE PATH OF DEGENERATIVE EVOLUTION 



part, that farthest from the brain, become 

 differentiated into the cells which will 

 form the lens, and the cells which will 

 form the retina. 



(3) The distal part becomes specialized, the lens, 



the retina, and the stalk of the optic nerve 

 are mapped out. 



(4) The lens, the retina, and the optic nerve 



become fully formed (fig. 62, A). 



At this stage the third eye has reached its limit 

 of development. 



There is a well-formed retina connected with the 

 brain by a special optic nerve. The organ projects 

 strongly from the surface of the head, but from 

 this point, owing to the development of the 

 cerebral hemispheres, degeneration' begins. The 

 nerve (fig. 52, c), becomes broken and fatty, and 

 pigmentary degeneration occurs in it. At _the 

 same time, the pineal eye having become useless 

 or even harmful to the animal possessed of it, 

 before the power of receiving perceptions of light 

 has been lost, and before the organ has been far 

 reduced by phylogenetic destruction, a veil of 

 black pigment is formed over it, completely shut- 

 ting it off from the outer light. The nerve 

 disappears completely before birth, its degenerate 

 cells becoming lost in the mesoblastic skeletal 

 tissue of that region. At the time of birth 

 the whole eye is enclosed in a thick membrane 

 which isolates it. The deposition of pigment has 



