200 



Basic Structure of Vertebrates 



The paired eyes work on the principle of the photographic camera. 

 In the camera a lens, or system of lenses, focuses light to form an 

 image on a chemically sensitized plate or film. In the eye a lens, to- 

 gether with other refracting mediums, forms an image on a nervously 

 sensitive retina. In both camera and eye, the amount of light admitted 

 is regulated by an iris-diaphragm whose aperture can be increased or 

 diminished. 



The essential nervous parts of the eye are derived from the neural 

 tube. At an early embryonic stage, a pair of globular primary optic 

 vesicles bulge out ventrolateral^ on either side of the most anterior 

 of the three primary divisions of the brain (Figs. 141, 172). Later the 

 outer wall of each primary optic vesicle is concaved to such an extent 

 that the vesicle becomes a more nearly hemispheric thing with a double 

 wall, the secondary optic vesicle or "optic cup" (Fig. 172D). The 

 basal region of the vesicle becomes constricted to form an optic stalk 

 which connects the "cup" with the floor of the prospective dien- 

 cephalon. The inner layer of the double wall differentiates into the 

 complex nervous retina. The outer layer appears in the adult eye as 



PHOTORECEPTOR CELLS 



Fig. 172. Diagrams illustrating Boveri's theory that the paired eyes of verte- 

 brates have evolved from lateral outgrowths of the brain-wall. The theory accords 

 well with embryologic evidence. (Courtesy, Neal and Band: "Chordate Anatomy," 

 Philadelphia, The Blakiston Company.) 



