848 



THE NERVOUS SYSTEM 



SCLEROTIC 



UPPER 

 VITREOUS BO 

 TIN4L LAYER 

 GMENTED COiT 



ANTERIOR CHAMBE 

 l:S CORNE 



LENS EPITHELIUM 

 NS RUDIMENT 

 LENS FIBERS 

 ELOPING CORNEA 



LOWER LIO 

 RS CAECA 

 RS OPTICA 



PECT 



96 00 HOURS 



C INNI 



MARGINAL 



L MfiNT E 



MARGIN OF IRIS 



LENS EPITHELI 



P GMENTED LAYER OF 

 OPT C CUP BECOMES 

 P GMENTED LAYER OF 



RET NAL LAYER OF 

 OPT c CUP BECOMES 

 SENSORY LAYER OF 

 RETINA 



-AR CAPSULE 

 LENS FORMING FRON 

 BRANCHES OF THE 

 H ALOID ARTERY 



RODS AND f 



CONES ^1 



OUTER [U4ljllliiMlllilli, 



NUCLEAR -J OO'^oOj^?*^ 



NUCLEAR Ho"9/«jl/Oi0( 



For legend see p. 846. 



lumen of the optic stalk and the central artery and vein of the retina. The 

 lumen of the optic stalk thus becomes converted into the optic nerve. 



Turning now to the chick embryo, we observe that the choroid fissure has 

 an added significance. In this embryo, as in the mammal, the presence of the 

 choroid fissure permits mesenchyme and blood vessels to enter the vitreous 

 chamber (optic-cup chamber) of the eye, and the optic nerve fibers travel 

 toward the brain along the lumen of the optic stalk. However, as the fissure 

 closes in the region of the optic cup, the ectodermal edges of the cup fold 

 inward in the region where the optic cup joins the optic stalk and this optic- 

 cup fold comes to enclose the inward migrating mesenchyme and blood 



