THE EYE. 279 



cells arranged side by side, and vertically to the surface. From 

 the fourth day onwards it increases rapidly in thickness, and by a 

 series of histological changes which have not yet been determined 

 very accurately, it becomes converted into the several layers of 

 the retina. The inner layers of the retina are the first to be 

 established, and the last elements formed are the rods and 

 cones ; these latter growing outwards as processes from the 

 outer nuclear layer, which until the appearance of the rods and 

 cones is the outermost layer of the retina. 



The outer wall of the optic cup (Fig. 122, od) is, from the 

 first, much thinner than the inner wall. By the middle of the 

 fourth day it is reduced to a single layer of flattened cells, which 

 soon become pigmented, and ultimately give rise to the layer of 

 special pigmented cells which lie in close contact with the outer 

 ends of the rods and cones. 



It appears, therefore, that_yia_wJiole_of_the sensory part of 

 the retina is derived from the inner layer of the optic cup. It 

 is worthy of notice that the rods and cones, the only elements 

 of the retina directly sensitive to light, are the las_tparts to be 

 formed. 



The optic nerve is very commonly said to be formed from the 

 optic stalk ; but it is more probable that it arises in the chick, 

 as it is known to do in the frog (p. 138), by the formation of 

 processes from cells in the retina, which grow inwards along the 

 optic stalk to the brain, and become the fibres of the optic nerve. 



The iris. The marginal part of the optic cup, nearest to the 

 lens, does not become converted into the retina, but undergoes 

 changes of a different character, the boundary between the 

 retinal and non-retinal parts being indicated by the ora serrata. 

 The inner and outer walls of this marginal, or non-retinal, part 

 of the cup coalesce completely, and become pigmented through- 

 out their whole thickness. They become closely connected with 

 the choroid coat, on their outer surface ; and the combined choroid 

 and retina grow forwards, in front of the lens, to form the iris, 

 which reduces the mouth of the optic cup to a comparatively 

 narrow aperture, the pupil. 



The pecten arises on the fifth day as a lamellar process of 

 mesoblast, which grows into the cavity of the optic cup through 

 the choroidal fissure, close to the optic nerve. It early becomes 

 very vascular; about the tenth day it becomes folded in the 



