n EYE 135 



stricted • off and finally completely separated from the outer 

 ectoderm (Fig. 74, (J and D). 



In the meantime a marked difference becomes apparent between 

 the two layers forming the wall of the optic cup. The layer next 

 the cavity of the cup becomes greatly thickened its cells becoming 

 tall and columnar : it forms the rudiment of the visual layer or 

 retina in the strict sense. The outer layer of the cup -wall on 

 the other hand degenerates, it becomes thinner and later it de- 

 posits melanin pigment in its cells. It forms the pigment-layer of 

 the retina. 



The invagination of one wall of the optic outgrowth within the 

 other is not confined to that portion of the outgrowth in proximity 

 to the external ectoderm as might be supposed from the description 

 so far. The invagination involves also the ventral wall of the 

 rudiment towards its outer end and for some distance along the 

 optic stalk. The result is that the wall of the optic cup is 

 interrupted by a gap ventrally the choroid fissure x — and that the 

 optic stalk for some distance from the, optic cup has a deep groove 

 along its ventral side. 



The cavity of the optic cup, as is the case with cavities 

 generally in the embryonic body, becomes filled with clear fluid 

 secreted into it by the surrounding cells. This fluid becomes jelly- 

 like later on and forms the basis of the vitreous .body. 



As development goes on the eye increases greatly in size and 

 assumes a spherical shape, the lens blocking up its opening towards 

 the skin and the choroid fissure becoming obliterated by its lips 

 coming together. The site of the fissure remains apparent for some 

 time owing to the formation of pigment in the pigment-layer being 

 delayed in its immediate neighbourhood. 



As the eye increases in size the Eetina for a time grows more 

 actively than the rest so as to be thrown into wrinkles (Fig. 74, E). 

 The lens which was a hollow vesicle becomes solid its cavity being 

 filled up by a great thickening of its deep wall, the cells of which 

 grow out into a tall columnar form (Fig. 74, D and E). 



The essential parts of the eye as an optical instrument have now- 

 been laid down — the lens for the production of an image, the 

 retinal wall of the optic cup for the reception of that image and the 

 conversion of its light waves into nerve impulses, and the optic 

 stalk which will become the optic nerve for the transmission of 

 these impulses to the brain. To these essential parts there are 

 added various accessory structures developed from mesenchymatous 

 cells which accumulate round the parts of the eye already formed. 

 In particular there is formed a protective capsule of tough con- 

 nective tissue — the sclerotic with its transparent portion the 

 cornea, covered externally by the ectoderm forming the corneal 



1 The term "choroid" fissure is in reality misleading, having been adopted when 

 the fissure was interpreted as a cleft in the choroid, in the days before the existence 

 of the pigment-layer of the retina was recognized. 



