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 =) Li 
complex focussing arrangements. The fig. :06r.—Section through the 
visual ‘cells (rods and cones) of the Ver- Ei" s Saee eae 
tebrate eye present the remarkable pecu-_ Jens 7.7» retina: 4, blood-vessel 
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 (Azad/eps), 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 
