57° 



EYE 



small cavity and thick walls (compare Fig. 64, I, e). This 

 body is the rudiment of the lens : as it enlarges it pushes 

 against the optic vesicle and causes it to become in- 

 vaginated (B), the single-layered optic vesicle thus be- 

 comes converted into a two-layered opiu cup (opt. c, opt. c), 

 its cavity, originally continuous with the diacoele, 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 {Y\g. 1^0, aus), 

 as it is called, soon becomes 

 closed by the union of its 

 edges. The outer layer of 

 the optic cup becomes the 

 pigment-layer of the retina 

 (p. 183) ; from its inner layer 

 the rest of that membrane — 

 including the rods and cones 

 — is formed. The stalk of the 

 optic cup occupies, in the 

 embryonic eye, the place of 

 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- 



F1G.5150. — Plastic representation of the 

 optic cup and lens. 

 a&. outer wall of optic cup ; at/s. 

 choroid fissure ; g-/. cavity of optic 

 cup ; /i. space between the two 

 walls, which afterwards disappears ; 

 t6. inner wall of optic cup ; /. lens ; 

 Sn. stalk of optic cup (rudiment of 

 optic nerve), (After Hertwig.) 



