STRUCTURE AND DEVELOPMENT OF EYE 



551 



Stapes of Mammals has a double origin, being formed from the 

 upper part of hyoid arch+an ossification from the wall of the 

 ear capsule= (wholly ?) columella of Birds, Reptiles, and 

 Amphibians. 



The eye. — There is no eye in Amphioxus, it is rarely 

 more than larval in Tunicates, it is rudimentary in Myxine 



op.v. 



l.^ 



Fig. 313. — Development of the eye. — After Balfour and 



Hertwig. 



I. Section through first embryonic vesicle, showing outgrowth of 



optic vesicles {op.v.) to meet the skin ; f.h., thalamencephalon ; 



G., the gut. 

 2-4. Sections illustrating the formation of the lens (/.) from the skin, 



and the modification of the optic vesicle into an optic cup ; 



R., retina ; v.h., vitreous humour. 

 5. External aspect of embryonic eye ; /., lens. 



and in the young lamprey. In higher forms the eye is 

 always present, though occasionally degenerate, e.g. in 

 fishes from caves or from the deep sea. It is hidden under 

 the skin in Proteus, an amphibian cave-dweller, and in the 

 subterranean amphibians like Coecilia, very small in a few 

 snakes and Uzards, and its nerves are abortive in the 

 mole. 



