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around toward the outer surface. Meanwhile the 

 dorsal pair near the front edge of the brain tract 

 were not turned over, so that their retina remained 

 on the outer side of their nerve layer. As the brain 

 swelling increased it pressed the future optic cups 

 against the epithelium on the surface of the head; 

 the epithelium sank inward, folded up into a lens, 

 and the lens in turn increasing rapidly, conditioned 

 the insinking of the optic swelling, which thus 

 became the optic cup. The optic stalk or nerve 

 is simply the constricted part between the brain 

 and the cup. By this time the lateral paired eyes 

 were becoming true organs of vision, while the 

 dorsal pair gradually degenerated and their nerve 

 stalks finally became the pineal and parapineal 

 organs of the brain. It is important to remember 

 that the retina apparently represents an inverted 

 patch of epithelium and that the layer of nerve 

 fibrils now covering it represents the former 

 underside of the patch. Also that the optic cup 

 was pushed in from the outside so that its primary 

 cavity was squeezed out of existence. 



The lens is at first connected with its parent epi- 

 thelium by a slender stalk, which is soon lost. The 



lens thus finds itself protruding into the hollow side of 



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