4o8 



CHORDATE ANATOMY 



On the other hand, we are still painfully ignorant as to the factors 

 which started the growth of the eyes towards the skin and converted the 

 skin into a lens. Possibly the invagination of the optic cup was primarily 

 determined by the enlargement and ingrowth of the lens (Fig. 358). Yet 

 experiment shows that lens formation in the embryo is stimulated by the 

 optic vesicle, in the absence of which no lens develops, and that if the 

 optic vesicle is removed from the brain and grafted under the skin of 

 the trunk, it will cause a lens to develop there. 



PHOTORECEPTOR CELLS 



HOTORECEPTDR 

 CELLS 



\\l OPTIC 



CUP 



Fig. 358. — Diagrams illustrating Boveri's theory that the paired eyes of vertebrates 

 have evolved from lateral outgrowths of the brain wall. The theory accords well with 

 embryological evidence. 



As the eye became a two-layered cup enclosing a lens and surrounded 

 by a connective-tissue capsule, the sclerotic, it acquired nerve connexion 

 with other parts of the brain. The eye muscles became attached to the 

 eyeball and were preserved while other pre-otic muscles disappeared. 

 That the lens is a modified lateral-line organ has been plausibly urged 

 both from its position of origin and from its mode of development. 



The phylogenesis of the paired eyes of vertebrates may be briefly 

 summarized. First these were open beaker eyes, like those of such inverte- 

 brates as Nautilus, with photoreceptors towards the source of light and 

 with pigment, but without lens. Later, by invagination of the neural plate, 

 the eyes were carried into the fore-brain, so that the retina became inverted 

 and each eye grew in the form of a hollow ball towards the skin and the 

 source of light. The photoreceptors, however, came to lie on the side of 



