Good News for Mammals 151
Smith implanted his wires along the bone remnant, with one end
bent over into the marrow cavity. The limbs with the positive silver
electrode at the cut showed no growth, and in some cases tissue actually
disintegrated. The negative platinum ends, however, started regenera-
tion; the new limbs all stopped growing at about the same distance from
the device, suggesting that regeneration might have been complete if
the batteries had been able to follow along. In 1974 Smith made a
device that could do just that, and achieved full regrowth.
Despite Smith's success, there was
no reason to suppose that his
method would work in mammals. One researcher had recently noted
some regeneration in the hind legs of newborn opossums, but, since
marsupials are born very immature and develop in the mother's pouch to
a second birth, we suspected that this was merely a case of embryonic
regrowth. Most fetal tissues were known to have some regenerative abil-
ity while they weren't yet fully differentiated. Richard Goss had shown
that the yearly regrowth of deer and elk antlers was true multitissue
regeneration, but this feat seemed too specialized to make us confident
about restitution in other mammals or other parts of the body.
Many thought all such attempts were doomed, because the process of
encephalization had progressed much further in mammals than amphib-
ians. All vertebrates were known to have roughly the same ratio of nerve
tissue to other kinds of tissue, but in mammals most of the limited
nerve supply went into the ever more complex brain, until, as Singer
had shown in a recent study, the proportion of nerve to other tissue in
rat legs was 80 percent less than in salamander legs. This was well below
the critical
mass needed for normal regeneration, and we thought it
might be impossible to make up the difference artificially.