THE SENSE ORGANS 



583 



ably the enlargement of the head and the recession of the brain from the 

 surface. Less light, consequently, would reach the photoreceptors in the 

 brain wall. When the skin became pigmented, eyes in the brain wall 

 would become useless. 



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

 which would start the growth of the eyes towards the skin and convert 

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

 primarily determined by the enlargement and ingrowth of the lens. Yet 



NEURAL 

 TUBE 



PHOTORECEPTOR CELLS' 



Fig. 480. — 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. 



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. 



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 the eyes were 



