MORPHOLOGY OF ANIMALS 

 superficial, resemblance to the vertebrate eye. A rudimentary 

 eye of this type is present in some vertebrates as the pineal 

 eye. This is an unpaired structure on the dorsal side of the 

 'tween brain, and in certain reptiles it is plainly a vesicular 

 eye with direct retina (Fig. 28, B). 



The paired eyes of vertebrates are also vesicular, but in 

 them the retina is inverse^ that is, the free ends of the retinal 

 cells are directed away from the cavity of the vesicle, while 

 the ends which bear the fibres are directed towards it. The 

 explanation of this remarkable condition is found in the 

 study of the development of these eyes. They arise as lateral 

 evaginations of the walls of the embryonic fore-brain, are 

 then constricted from the brain, and become vesicles con- 

 nected with the fore-brain by only a stalk. At this stage the 

 vertebrate eye is like the invertebrate one save only that it 

 has arisen from the neural instead of the superficial epi- 

 thelium. All the cells which form the vesicle have their free 

 ends directed toward its cavity, while their basal ends are di- 

 rected away from it (Fig. 28, C). The outer wall of this optic 

 vesicle is then infolded until it comes into contact with the 

 inner wall, thus forming a cup open toward the skin. The 

 ectoderm over the opening of the optic cup is then infolded 

 to form the lens^ which completely separates from the skin 

 and lies in the mouth of the cup. The infolded wall of the 

 cup alone forms the retina, and therefore the free ends of 

 the retinal cells are directed away from the lens and from 

 the cavity of the cup (Fig. 28, D). The lens and optic cup 

 are then surrounded by fibrous and vascular coats, the scle- 

 rotic and choroid', a chamber is formed in front of the lens 

 which is filled with aqueous humor, while one behind the 



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