50 



EVOLUTION AND GENETICS 



outer ectoderm as a hollow vesicle. The optic nerve grows back 

 to the brain from the inner layer of the optic cup, and the entire 



eye is enveloped by the 

 hard sclerotic coat de- 

 rived from the meso- 

 dermal layer. Later 

 on, in all forms which 

 have eyelids, the skin 

 in front of the eye 

 forms two folds which 

 grow until they meet 

 and fuse, later on to 

 separate again as the 

 eyelids. In some ani- 

 mals the lids remain 

 fused until after birth, 

 hence the young are 

 said to be born blind. 

 The skin continues 

 around the edges of 

 these folds and joins 

 the delicate conjunc- 

 tiva which covers their 

 inner surfaces and ex- 

 tends across the ex- 

 posed portion of the 

 eyeball. In this proc- 

 ess the eyes of all 

 vertebrates correspond 

 in so far as their adult 

 structures correspond. 

 Accompanying these 

 _ and many other points 



Fig. 30.— Diagrams illustrating the formation of of similarity in the 



the eye in an invertebrate (A) and a vertebrate parts of vertebrate 



(B, C, D, E, successive stages), a ectoderm; gj^^ryog ^ striking 

 b, retinal layer; c, future position of the optic ^ ■ i ui 



nerve; d, cavity of the brain; e, optic vesicle; superhcial resemblance 



/, optic stalk, later replaced by the optic nerve; has been noted in the 



g, cavity of the optic cup, later the post^erior ^^^^^ embryos of the 



h 



chamber of the eye; /;, developing lens. (From 

 Woodruff.) 



five highest classes of 



