xiu EYE . 593 



At an early stage of development a hollow outgrowth 

 the optic vesicle (Fig. 157, A, op. v)is given off from 

 either side of the fore-brain and extends towards the 

 side of the head, where it meets with an in-pushing of 

 the ectoderm (/), which becomes thickened, and finally, 

 separating from the ectoderm, forms a closed, spherical 

 sac (B, /) with a very small cavity and thick walls 

 (compare Figs. 64, L, e, and 158). This body is the 

 rudiment of the lens : as it enlarges it pushes against 

 the optic vesicle which becomes invaginated, the single- 

 layered optic vesicle thus being converted into a two- 

 layered optic . cup (Fig. 157, B, oc', oc"), its cavity, 

 originally continuous with the diaccele, becoming 

 obliterated. Between the edge of the cup and the 

 lens, on the ventral side, is a small space which gradu- 

 ally extends towards the stalk of the cup, and thus gives 

 rise to a slit in the wall of the latter : this choroid fissure 

 (C, 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 the retina 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 immediately surrounding the optic cup 



