SIGHT 47 



optic nerve. Since the brain itself is of ectodermic origin (see 



p. 20), it is clear that the parts of the eye so far mentioned are all 



derived from ectoderm. The rest of the eyeball, including its two 



outer coats and refracting contents (see vol. i, p. 57), are formed 



from the middle embryonic layer (mesoderm). This curious kind 



of development clearly suggests that in the remote ancestors of 



Vertebrates the eyes were internal projections from the brain, and 



received their light through the transparent tissues external to 



them, as is still the case in the single eye of the tadpole of a Sea- 



Squirt. The free ends of the visual cells (rods and cones) were 



directed towards the cavity of the brain. 



As in the course of evolution the brain 



became more and more complex, an 



opaque skull was developed for its pro- 



tection, and the brain -eyes, having their 



supply of light thus cut off, were obliged, 



so to speak, to grow outwards. Subse- 



quently they were improved into camera 



eyes by the development of a lens. 



Further improvements consisted in the 



evolution of eye - muscles, eyelids, and 



complex focussing arrangements. The F i g . I0 6i. - section through the 



visual cells (rods and cones) of the Ver- 



o.n. 



tebrate eye present the remarkable pecu- lens; r -. r -> retina: * 



o.n., optic nerve. 



liarity of pointing away from the light, 



one result of the manner in which the retina is developed. 



In Vertebrates, such as Fishes, which have to see under water, 

 the lens of the eye is spheroidal, and one mark of the aquatic 

 ancestry of the Amphibia is the possession of a lens of similar 

 shape. But thoroughgoing land Vertebrates have lost this primi- 

 tive character, for in them the lens is more or less flattened and 

 biconvex, as an adaptation to seeing in air. 



An extremely interesting and remarkable arrangement is found 

 in certain bony fishes known as Double- Eyes (Anableps), native 

 to the coasts and estuaries of tropical America. The name has 

 been given because either eye, as seen from the exterior, is marked 

 off into upper and lower halves by a dark transverse band. Dis- 

 section shows that the upper half of the lens is biconvex, and the 

 lower half spheroidal. And since these fishes habitually swim at 

 the surface, with only the lower part of the eye immersed, we can 



