306 STRUCTURE OF THE VERTEBRATES 



form a double layered optic cup. The cell layer lining the cavity 

 of the optic cup (which will be spoken of as the inner layer) 

 develops into the retina which contains the light sensitive cells. 

 The outer, surrounding, layer of cells is thinner than the retinal 

 layer, and develops into a pigmented coat protecting the retina. 



Synchronously with the development of the optic cup, the 

 covering ectoderm of the embryo thickens over this region, 

 pushes inward, and becomes the lens of the eye, lying within 

 the circular lip of the cup. As the lens pushes inward, the skin 

 ectoderm closes over to form a continuous layer over the eye. 

 In the completed structure this thin layer forms the epithelium 

 of (1) the conjunctiva which surrounds the eye as folds of 

 tissue, and (2) the cornea, the transparent part of the mesoderm 

 which completely surrounds the eye ball. The continuity of this 

 epithelium and the epidermis is demonstrated whenever a reptile 

 sheds its skin, for the eye covering is moulted as an integral 

 part of the shed epidermis. 



The inner layer of the cup (that which receives light rays as 

 they pass through the cornea and lens) becomes rapidly modi- 

 fied into sensory retinal, cells and association nerve cells. The 

 axones of the latter grow centrally toward the optic stalk which 

 connects the eye with the brain, and through the stalk to the 

 visual centers of the central nervous system. In this way the 

 stalk is filled and transformed into the optic nerves, which 

 become a nerve tract connecting the retinal ganglion with the 

 central nuclei. 



The original optic cup is surrounded by the cartilaginous 

 optic capsule of the chondrocranium. This never fuses to the 

 skull as do the other capsules, and thus permits free movement 

 of the eye. The larger proximal portion of the capsule becomes 

 thickened into the sclerotic protective membranes of the eye. 

 The circular region over the lens forms the transparent cornea, 

 covered externally by the ectodermal epithelium. 



Structure of the Eye. The eye consists of (1) ectodermal 

 retina, pigmented layer, lens, and outer epithelium; and (2) 

 mesodermal iris which is pigmented, sclerotic protective mem- 

 branes, and cornea. As the lens is attached along its equator 

 to the eye membranes, the spherical hollow of the eye is divided 

 into two cavities which are filled with a heavy fluid. 



