452 COMPARATIVE ANATOMY AND PHYSIOLOGY. 



thus formed grows out at its anterior end into three 

 brain vesicles ; from the foremost of these an out- 

 growth is given off on either side which forms the 



D 



Fig. 192. Diagrams to show (A, B) the relative position of the Kods 

 and Cones (r) and the Nerve Fibres (n) in an Invertebrate (A) 

 and a chordate Eye (B). c shows the inpushing of the Cells from 

 which r and n have their origin, c 1 the final result in a chordate 

 Spinal Chord ; D, the outgrowing Optic Vesicle from the Brain ; E, 

 the formation of the Optic Cup. 



optic vesicle (Fig. 191 D) ; the outer wall of this 

 vesicle next gets pushed in (E), but the layers retain 

 the relations to one another that they had in the optic 



