Travel Reference
In-Depth Information
around, with their clumsy focusing rings and motors that move the rigid
glass lenses back and forth.
Our eyes and the eyes of the cuttlef ish can both be traced back to
a simple eyespot possessed by our common ancestor. This ancestral
eye consisted of a few photosensitive cells, perhaps overlaid by a layer
of transparent cells that protected them and concentrated the light. The
evolutionary path that eventually led to the fancy capabilities of cuttle-
f ish eyes diverged from the one that led to the equally fancy capabilities
of our own eyes.
The cuttlefi sh eyes are better than ours in at least one important respect.
They can form crisp images of their surroundings in full color across the
entire span of their light-sensitive retinas. We have to be content with a fuzzy
image that has a little clear spot in the center.
Why are the cuttlefi sh eyes better? The dif erence can be traced to
how our eyes and those of cuttlefi sh develop. During embryogenesis our
light-sensitive retina begins as a hollow ball of cells called an optic vesicle
at the end of a stalk of brain tissue. The back region of this ball dif erentiates
into pigmented light-sensitive cells and the front region becomes the nerve
cells that will pick up the retinal signals. The two regions then collapse and
fuse into a single cup-like structure. The result is a retina in which the nerve
cells lie on top of the retinal cells. The nerve cells interfere with image forma-
tion, which is why most of our vision is blurred. The only clear part of our
visual fi eld is the fovea, a small region in which the nerve cells fan away from
the underlying retinal cells so that they do not interfere with the image. You
are looking at these words through your fovea.
The cuttlefi sh eye develops dif erently. An optic vesicle develops from
its brain as well, but the ball does not dimple inwards and form two layers
of cells. Instead it forms a single layer of nerve cells. Meanwhile, part of the
outer layer of the embryo's developing head moves in to bond with this layer.
It is this piece of ectoderm, rather than the optic vesicle tissue, that dif eren-
tiates into the light-sensitive pigment cells. The result is a retina that gets it
right. The nerves that transmit visual signals to the brain form a layer behind
the retinal tissue rather than in front of it, so that they do not interfere with
the image.
