214 



ANIMAL BIOLOGY 



a — 



—e 



Fig. 149. — Diagrams illustrating the method of formation of the eye of an 

 Invertebrate (A) and a Vertebrate (B, C, D, E, — successive stages). Note that 

 the opposite surface of the retinal cells is exposed to the light rays in the Verte- 

 brates as compared with the Invertebrate eye. a, ectoderm; b, retinal area; c, 

 future position of optic nerve; d, cavity of the diencephalon ; e, optic vesicle; 

 /, stalk of optic vesicle, later replaced by the optic nerve; g, vitreous chamber 

 within optic cup; h, developing lens. 



