588 EYE CHAP. 



against the optic vesicle which becomes invaginated 

 (JB\ the single-layered optic vesicle thus being con- 

 verted into a two-layered optic cup (opt. <:, opt. c\ its cavity, 

 continuous with the ventricle of the fore-brain, becoming 

 obliterated. Between the edge of the cup and the lens, on 

 the ventral side, is a small space which gradually extends 

 towards the stalk of the cup, and thus gives rise to a slit 



in the wall of the latter : this 

 choroid fissure (Fig. i6i,aus), 

 as it is called, soon becomes 

 closed by the union of its 

 edges. The outer layer of 

 the optic cup gives rise to the 

 pigment-layer of the retina 

 (p. 183) from its inner layer 

 the rest of that membrane 



FIG. xoi. Plastic representation of the 



optic cup and lens. including the rods and cones 



ab. outer wall of optic cup ; tins. 



choroid fissure ; gl. cavity of optic is formed. The Stalk of the 

 cup ; h. space between the two 



walls, which afterwards disappears ; Optic CUp OCCUpieS, in the 

 ib. inner wall of optic cup ; /. lens ; . 



Sn. stalk of optic cup (rudiment of embryonic eye, the place of 



optic nerve), (After Hertwig.) 



the optic nerve, but the actual 



fibres of the nerve are formed from the nerve-cells of the 

 retina and grow inwards to the brain. 



During the formation of the lens, mesoderm extends in 

 between the ingrowth "from which it arises and the external 

 ectoderm ; from this the main substance of the cornea and 

 its inner or posterior epithelium are formed, the adjacent 

 ectoderm becoming the external epithelium, i.e. that of 

 the conjunctiva (p. 182). Mesoderm also makes its way 

 into the optic cup, through the choroid fissure, and gives 

 rise to the vitreous humour. Lastly, the mesoderm imme- 

 diately surrounding the optic cup is differentiated to form 

 the choroid, the iris, and the sclerotic. 



