ONTOGENY 



257 



o.v. 



o.c. 



I 



tympanic membrane to unite with a small cartilage plugging 

 the foramen ovale. 



Eye. — The optic vesicles are paired evaginations of the ventro- 

 lateral walls of the forebrain which grow out toward the ectoderm. 

 Each optic vesicle becomes constricted at its point of attach- 

 ment to the brain to form the optic stalk. As the vesicle 

 approaches the ectoderm, its outer surface invaginates to form a 

 double-walled optic cup (Fig. 160). The invagination is contin- 

 ued along the ventral side of the 

 cup to form the choroid fissure. 

 In the meanwhile a thickening in 

 the ectoderm opposite the mouth 

 of the cup is pinched off the inner 

 surface of the ectoderm to form 

 the lens. The choroid fissure now 

 closes down to the optic stalk 

 where an opening persists, through 

 which blood vessels and nerve 

 fibers enter the optic cup. When 

 the choroid fissure is closed, the 

 optic cup is a double-walled hemi- 

 sphere whose cavity is largely 



occupied by the lens (Fig. 160). Fig. 160.— Cross sections illustrat- 

 The ectoderm Overlying the lens ing four stages in the development 

 . . , j. ,r of the eye of Rana sylvatica. 1, 



becomes transparent to form the optic vesicle stage . 2 optic cup 



OUter layer of the cornea. The stage; 3, formation of the lens; 4, 

 , - . , , . . . c , separation of lens from ectoderm. 



inner layer of the latter is formed e> cornea . h lens; . c ., 0P tic cup; 

 of mesodermal cells (mesenchyme) . o.s., optic stalk; o.v., optic vesicle; 



„,, . . , , - , r, retina (inner layer of optic cup). 



The ins develops from mesoderm {After PoUister and Moore.) 

 at the edge of the optic cup and 



also perhaps from ectoderm derived from the cup. The inner 

 layer of the optic cup becomes thicker than the outer layer early 

 in its formation. It gives rise to the sensory part of the retina in 

 which the rods and cones develop. The outer layer of the optic 

 cup develops into the pigmented layer of the retina. The 

 choroid and sclerotic coats of the eye are derived from mesoderm 

 formed over the pigmented layer of the retina. The fibers 

 forming the optic nerve arise from cells lying in the inner surface 

 of the retinal layer of the optic cup. This passes out of the eye 



