50
The Body Electric
put into a hind-limb territory naturally became a hind limb. This was
an attractive theory, but unfounded. Exactly what did this territory con-
sist of? No one knew. To make matters worse, it was then found that
transplantation of a slightly older blastema from a foreleg stump to a
hind-limb area produced a foreleg. The young blastema knew where it
was; the older one knew where it had been! Somehow this pinhead of
primitive cells with absolutely no
distinguishing characteristics con-
tained enough information to build a complete foreleg, no matter where
it was placed. How? We still don't know.
One attempt at an answer was the idea of a morphogenetic field,
advanced by Paul Weiss in the 1930s and developed by H. V. Bronsted
in the 1950s. Morphogenesis means "origin of form," and the field idea
was simply an attempt to get closer to the control factor by reformulat-
ing the problem.
Bronsted, a Danish biologist working on regeneration in the common
flatworms known as planarians, found that two complete heads would
form when he cut a strip from the center of a worm's front end, leaving
two side pieces of the original head. Conversely, when he grafted two
worms together side by side, their heads fused. Br0nsted saw an analogy
with a match flame, which could be split by cutting the match, then
rejoined by putting the two halves side by side, and he suggested that
part of the essence of life might be the creation of some such flamelike
field. It would be like the field around a magnet except that it reflected
the magnet's internal structure and held its shape even when part of the
magnet was missing.
The idea grew out of earlier experiments by Weiss, an American em-
bryologist, who stymied much creative research through his dogmatism
yet still made some important contributions. Regrowth clearly wasn't a
simple matter of a truncated muscle or bone growing outward to resume
its original shape. Structures that were missing entirely—the hand,
wrist, and bones of the salamander's lower forelimb, for example—also
reappeared. Weiss found that redundant parts could be inserted, but the
essential ones couldn't easily be eliminated. If an extra bone was im-
planted in the limb and the cut made through the two, the regenerate
contained both. However, if a bone was completely removed and the
incision allowed to heal, and the limb was then amputated through what
would have been the middle of the missing bone, the regenerate pro-
duced that bone's lower half, like a ghost regaining its substance Weiss
suggested that other tissues besides bone could somehow project a field
that included the arrangement of the bones.
As a later student of re-
generation, Richard Goss of Brown
University, observed, "Apparently