SENSE ORGANS. 



171 



(3) Meanwhile the infolding from the epidermal sur- 

 face has formed the lens. (4) Then the optic vesicle 

 becomes folded back upon itself like a double nightcap, 

 so as to leave a large space between it and the lens. 

 The anterior or back-folded layer of the double night- 

 cap becomes the retina, and the posterior layer the 

 choroid (Fig. 108, C). (5) The two folds come in con- 



FlG. 108. — Diagram representing different stages in the development of the 

 vertebrate eye : C V, cerebral vesicle ; O V, optic vesicle ; r, retina ; ck, 

 choroid ; b, bacillary layer ; /", fibrous layer ; /, lens. 



tact and the vesicle is obliterated. The space between 

 the concave retina and the lens is filled and forms the 

 vitreous humor, the whole becomes encysted by the 

 sclerotic, and the eye is finished. Now observe that 

 the cerebral vesicle and the optic vesicle are lined 

 with epithelium. When this is folded back the pos- 



