STRUCTURE OF THE VERTEBRATES 305 



change in the organ itself. The evolutionary changes have 

 been in position, the optic nerves, and the protective structures 

 around the eye. 



Embryology. The eyes begin their development as lateral 

 pouches from the forebrain before the posterior cord is closed. 

 As each evagination continues to grow laterally the distal end 

 enlarges into an optic vesicle which soon begins to inpocket and 



^ptic vesicle 



Retina, 



Optic cup Ophl 



Lens 



Fig. 166. Development of the Eye. (A) Section through the developing 

 optic vesicles; (B) the invagination of the optic cup, and the epidermis to 

 form the lens; (C) a later stage showing the lens; and (D) the 



embryonic eye. 



