Here the other hemisphere is 
cut so that the knife grazes the 
buried part of the striate cortex. 
We can now see hints of stripes 
in the upper part of 4C. (These 
stripes are in a subdivision 
related to the magnocellular 
geniculate layers. The deeper 
part, ß, forms a continuous ring 
around a and so presumably is 
later in segregating.)

Only when we sliced the cortex 
parallel to its surface was it 
possible to see a faint ripple at 
half-millimeter intervals, as 
shown in the autoradiograph at 
left. Evidently, fibers from the 
geniculate that grow into the 
cortex do not immediately go to 
and branch in separate left-eye 
and right-eye regions. They 
first send branches everywhere 
over a radius of a few 
millimeters, and only later, 
around the time of birth, do 
they retract and adopt their 
final distributions. The faint 
ripples in the newborn make it 
clear that the retraction has 
already begun before birth; in 
fact, by injecting the eyes of 
fetal monkeys (a difficult feat) 
Pasko Rakic has shown that it 
begins a few weeks before birth. 
By injecting one eye of monkeys 
at various ages after birth we 
could easily show that in the 
first two or three weeks a steady 
retraction of fiber terminals 
takes place in layer 4, so that by 
the fourth week the formation 
of the stripes is complete.

    We easily confirmed the idea 
of postnatal retraction of 
terminals by making records 
from layer 4C in monkeys soon 
after birth. As the electrode 
traveled along the layer 
parallel to the surface, we 
could evoke activity from the 
two eyes at all points along the 
electrode track, instead of the 
crisp eye-alternation seen in 
adults. Carla Shatz has shown 
that an analogous process of 
development occurs in the cat 
geniculate: in fetal cats, many 
geniculate cells temporarily 
receive input from both eyes, 
but they lose one of the inputs 
as the layering becomes 
established.