OUR FACE FROM FISH TO MAN 



preting as a repetition of long past adult stages 

 such arrangements or conditions as may be merely 

 adaptations of the growing embryo to its own 

 physiological needs. 



Studnicka (quoted by Plate), basing his theory 

 chiefly on the embryology of the lampreys and 

 their relatives (which may represent the degenerate 

 descendants of the ostracoderms) , holds that 

 originally there were two pairs of paired eyes in the 

 pre-chordates, one pair dorsal, on the top of the 

 head, consisting of the pineal and parapineal 

 organs, the second pair low down on the sides of 

 the head, the eyes of later vertebrates. Both 

 pairs were derived from patches of cells sensitive 

 to light, located in the broad sensitive tract that 

 later folded up to become the brain tube. Up to 

 this time both sets of eyes had served merely to 

 orientate the animal with reference to the direction 

 of light. When as a result of its growing mass the 

 primitive nerve tract swelled outward, its crests 

 grew upward and curved over toward the mid- 

 line, carrying the primary optic depressions on to 

 its inner side, so that the future "rods" would 

 now point away from the light, and their nerve 



fibers, formerly beneath them, would now be bent 



186 



