200 



FOUNDATIONS OF BIOLOGY 



a 



h 



c 



I) 



FIG. 111. 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 with- 

 in optic cup; h, developing lens. 



