48
The Body Electric
because of the overriding importance of organization, and each part of
the pattern somehow contains the information as to what it is in relation
to the whole. The ability of this pattern to maintain itself reaches its
height in the newts, mud puppies, and other amphibians collectively
called salamanders.
The salamander, directly descended from the evolutionary prototype
of all land vertebrates, is a marvelously complex animal, almost as com-
plicated as a human. Its forelimb is basically the same as ours. Yet all its
interrelated parts grow back in the proper order—the same interlocking
bones and muscles, all the delicate wrist bones, the coordinated fin-
gers—and they're wired together with the proper nerve and blood vessel
connections.
The same day the limb is cut off, debris from dead cells is carried
away in the bloodstream. Then some of the intact tissue begins to die
back a short distance from the wound. During the first two or three
days, cells of the epidermis—the outer layer of skin—begin to prolifer-
ate and migrate inward, covering the wound surface. The epidermis then
thickens over the apex of the stump into a transparent tissue called the
apical cap. This stage is finished in about a week.
By then, the blastema, the little ball of undifferentiated cells de-
scribed by Morgan, has started to appear beneath the apical cap. This is
the "organ" of regeneration, forming on the wound like a miniature
embryo and very similar to the embryonic limb bud that gave rise to the
leg in the first place. Its cells are totipotent, able to develop into all the
different kinds of cells needed to reconstitute the limb.
The blastema is ready in about two weeks. Even as it's forming, the
cells at its outer edge start dividing rapidly, changing the blastema's
shape to a cone and providing a steady source of raw material—new
cells—for growth. After about three weeks, the blastema cells at the
inner edge begin to differentiate into specialized types and arrange
themselves into tissues, beginning with a cartilage collar around the old
bone shaft. Other tissues then form, and the new limb—beginning with
a characteristic paddle shape that will become the hand—appears as
though out of a mist. The elbow and long parts of the limb coalesce
behind the hand, and the regrowth is complete (except for some slight
enlargement) when the four digits reappear after about eight weeks.
This process, exquisitely beautiful and seemingly simple,
is full of
problems for biology. What organizes the growth? What is the control
factor? How does the blastema "know" that it must make a foreleg in-
stead of a bind leg? (The salamander never makes a mistake.) How does
all the information about the missing parts get to these undifferentiated