124 



THE GENESIS OF THE VERTEBRATE EYE 



Fig. 51 — Illustrating the placode theory. From Walls, after Beraneck. 



A vesicular eye derived from a lateral-line organ loses its photosensitivity and becomes the 

 definitive lens, while its ganglion becomes photosensory and is converted into the definitive 

 retina. 



St 



fs 



/-^^/ 



Fig. 52 — Hesse's organs of the 'spinal cord' of Amphioxus. From Walls, after Hesse. 



a, a single organ, consisting of a pigment cell and a photosensory ganglion cell, whose 

 'stiftchensaum', st, was believed by Boveri to be a cuticular struaure comparable with the 

 outer segment of a rod or cone. 



b, cross section of nerve cord, showing various orientation of the organs (enabling the animal 

 to respond to the direction of light). 



