52
The Body Electric
FORMING A TWO-HEADED PLANARIAN
each tissue of the stump can vote to be represented in the blastema, and
some of them can even cast absentee ballots."
Any such field must be able to stimulate cells to switch various genes
on and off, that is, to change their specialization. A large body of re-
search on embryonic development has identified various chemical induc-
ers, compounds that stimulate neighboring cells to differentiate in a
certain fashion, producing the next type of cells needed. But these sub-
stances act only on the basis of simple diffusion; nothing in the way they
operate can account for the way the process is controlled to express the
overall pattern.
Another classic experiment helps clarify the problem. A salamander's
hand can be amputated and the wrist stump sewn to its body. The wrist
grows into the body, and nerves and blood vessels link up through the
new connection. The limb now makes a U shape, connected to the body
at both ends. It's then amputated at the shoulder to make a reversed
limb, attached to the body at the wrist and ending with a shoulder
joint. The limb then regenerates as though it had simply been cut off at
the shoulder. The resulting limb looks like this: from the body sprouts
the original wrist, forearm, elbow, upper arm, and shoulder, followed
by a new upper arm, elbow, forearm, wrist, and hand. Why doesn't the
regenerate conform to the sequence already established in this limb in-
stead of following as closely as possible the body's pattern as a whole?
Again, what is the control factor?
Information, and a monumental amount of it, is clearly passed from
the body to the blastema. Our best method of information processing at