EYE 



441 



shown in the figure that the optic nerve at its origin interrupts the retinal 

 layers, producing a "blind spot." The part of the nerve which forms the 

 blind spot, with the vessels in its center, is called the papilla o1 the optic 

 nerve. 



The lens (Fig. 451, D) loses its central cavity by the elongation of the 

 cells in its posterior layer. These become the fibers of the lens. The 

 anterior layer remains throughout life as a simple epithelium, called the 

 epithelium oj the lens. The lens becomes covered by an elastic capsula 



i.f. 



FIG. 451. SECTIONS OF RABBIT EMBRYOS TO SHOW THE DEVELOPMENT OF THE EYE. A, 9} days. 3.0 mm. 

 B, 10} days, 5.4 mm.; C, n days, 5.0 mm.; D, 14 days, 18 hours, 12.0 mm.; E, 20 days, 29 mm. 



a c. r., Arteria centralis retinae; c., conjea; c. a., anterior chamber; conj., conjunctiva; c. p., posterior 

 chamber; c. v., corpus vitreum; e. 1., eyelid; f. b., fore-brain; 1., lens; 1. e., lens epithelium; 1. f., lens 

 fibers; o. c., optic cup; o. n., optic nerve; o. v., optic vesicle; r. p., pigmented layer of the retina; r. v., 

 visual layer of the retina. 



lentis, and in embryonic life it possesses a -vascular capsule (Fig. 451, E) 

 containing branches of the central artery. The vascular layer covering 

 the anterior surface of the lens is designated the pupillary membrane, and 

 it disappears shortly before birth. Its occasional persistence interferes 

 with vision. 



Between the lens and the retina there is a peculiar tissue, mucoid in 

 appearance and resembling mesenchyma in form. Since processes from 



