THE ORGANS OF SPECIAL SENSE. 567 



imagination of the optic vesicle is carried over along the posterior surface of the 

 optic stalk forming the chorioidal fissure (Fig. 502, see also p. 575). 



The lens area is thicker at its center than at its periphery and when the 

 center of the lens area becomes the bottom of the lens depression and later 

 the posterior wall of the lens vesicle this greater thickness is maintained. In 

 fact, the posterior wall of the vesicle becomes still thicker so that it projects into 

 the cavity of the lens vesicle as an eminence (Fig. 503, g.) . In the chick the lens 

 vesicle is hollow. In man and in Mammals generally it is more or less filled 

 with cells. These, however, degenerate and take no part in the formation of the 



Pigmented layer of retina Nervous layer of retina 



(outer layer of optic cup) (inner layer of optic cup) 



Cavity of 

 optic vesicle ^ ^ ___*_ 



Rim of optic cup. 



Optic furrow 



Lens 



Hyaloid artery I Optic furrow 



Hyaloid artery entering 

 cavity of vitreous 



FIG. 502. Model showing lens and formation of optic cup. A piece has been removed from the 

 upper part of cup to show the cavity of the optic vesicle and the position of the inner layer 

 of the cup (nervous layer of retina) . Bonnet. 



permanent lens. Comparing the posterior with the anterior wall of the lens at this 

 stage, the latter is seen to be composed of a single layer of cuboidal cells, the an- 

 lage of the anterior epithelium of the lens (Figs. 501 , 503 , g, h, i) . This layer passes 

 over rather abruptly into the posterior wall which consists of a single layer of 

 greatly elongated lens cells, the anlagen of the lens fibers. The lens fibers con- 

 tinue to elongate until by the end of the second month they touch the anterior 

 epithelium, thus completely obliterating the cavity of the lens vesicle (Fig. 505). 

 A small cleft containing a few drops of fluid, the liquor Morgagni, may remain 

 between the anterior epithelium and the lens fibers. 



When the lens fibers are first formed, the longest fibers are in the center and 

 the fibers gradually get shorter toward the periphery of the lens where they pass 

 over into the anterior epithelium (Fig. 503), As the lens develops, the periph- 



