Development of the Third Day 181 



layer of pigmented cells that lies close to the 

 outer ends of the rods and cones. The whole 

 of the sensory part of the retina is, therefore, 

 derived from the inner layer or wall of the 

 optic cup. 



It is sometimes stated that the optic nerve is 

 formed by the hollow stalk of the optic cup ; 

 but it is probable that it is formed by an out- 

 growth of cells from the retina, this outgrowth 

 extending along the optic stalk to the brain, 

 and forming the fibres of the optic nerve. 

 The growth of these fibres may have, as has 

 been mentioned, something to do with the 

 formation of the choroid fissure. 



The choroid and sclerotic coats are formed 

 from a layer of condensed mesoblast that col- 

 lects around the optic cup : and an ingrowth 

 of mesoblast, through the choroid fissure, is 

 converted into the vitreous humor. 



The retina does not, of course, cover the 

 entire inner surface of the optic cup. The 

 edges of the optic cup, beyond the limits of 

 the retina, form a part of the iris. In this re- 

 gion the two layers of the cup completely fuse, 

 and their cells become deeply pigmented. 

 Fusion now takes place between this layer of 

 pigmented cells and the choroid layer that has 



