Prenatal development is a 
gargantuan subject; I know 
little about it and certainly will 
not attempt to describe it in any 
detail here. One of the more 
interesting but baffling topics 
it deals with is the question of 
how the individual nerve fibers 
in a huge bundle find their 
proper destinations. For 
example, the eye, the 
geniculate, and the cortex are 
all formed independently of 
each other: as one of them 
matures, the axons that grow 
out must make many decisions. 
An optic-nerve fiber must grow 
across the retina to the optic 
disc, then along the optic nerve 
to the chiasm, deciding there 
whether to cross or not; it must 
then proceed to the lateral 
geniculate body on the side it 
has selected, go to the right 
layer or to the region that will 
become the right layer, go to 
just the right part of that layer 
so that the resulting topography 
becomes properly ordered, and 
finally it must branch and the 
branches must go to the correct 
parts of the geniculate cells--
cell body or dendrite. The same 
requirements apply to a fiber 
growing from the lateral 
geniculate body to area 17 or 
from area 17 to area 18. 
Although this general aspect of 
neurodevelopment is today 
receiving intense study in 
many laboratories, we still do 
not know how fibers seek out 
their targets. It is hard even to 
guess the winner out of the 
several major possibilities, 
mechanical guidance, 
following chemical gradients, 
or homing in on some 
complementary molecule in a 
manner analogous to what 
happens in immune systems. 
Much present-day research 
seems to point to many 
mechanisms, not just to one.

    This chapter deals mainly 
with the postnatal development 
of the mammalian visual 
system, in particular with the 
degree to which the system can 
be affected by the environment. 
In the first few stages of the cat 
and monkey visual path--the 
retina, geniculate, and perhaps 
the striate, or primary visual, 
cortex--an obvious question is 
whether any plasticity should 
be expected after birth. I will 
begin by describing a simple 
experiment. By about 1962 some 
of the main facts about the 
visual cortex of the adult cat 
were known: orientation 
selectivity had been 
discovered, simple and complex 
cells had been distinguished, 
and many cortical cells were 
known to be binocular and to 
show varying degrees of eye 
preference. We knew enough 
about the adult animal that we 
could ask direct questions 
aimed at learning whether the 
visual system was malleable. So 
Torsten Wiesel and I took a 
kitten a week old, when the 
eyes were just about to open, 
and sewed shut the lids of one 
eye. The procedure sounds 
harsh, but it was done under 
anesthesia and the kitten 
showed no signs of discomfort 
or distress when it woke up, 
back with its mother and 
littermates. After ten weeks we 
reopened the eye surgically, 
again under an anesthetic, and 
recorded from the kitten's 
cortex to learn whether the eye 
closure had had any effect on 
the eye or on the visual path.