. Thus, for instance, Smithers in
the development of his ideas.
If the crystalline lens of a salamander-eye is removed, part of the
iris de-differentiates, forms a vesicle, enters the cavity through the
pupil, re-differentiates, and forms a normal lens -- whereas in embryonic
development the lens is formed by the epidermis overlaying the eye-cup,
without participation of the iris. Thus the morphogenetic skill of making
a lens can make use of either of two different materials; the code is
again invariant, the strategy adaptable.*
On the other hand, the factors which, in a higher organism, determine
whether a given trauma will lead to regenerative or pathological changes
are of an extremely delicate nature. Thus Smithers [3] writes:
The type of structure regenerated, or the kind of neoplasm formed,
will depend on the level of the controlling field-gradient against
which it is exerting itself, and the steepness of the gradient it
can itself establish and promote as shown by its tendency towards
undifferentiated cell-reproduction. The part which is physiologically
isolated then produces an imperfect portion of a new whole, giving rise
to whatever tissues it is capable of forming under the circumstances
pertaining. This may result in malformations of all degrees, from
simple overgrowth of adult tissues, through irregular mixtures of
recognizable, well-differentiated cells, to the most rapidly growing,
undifferentiated tumors.
Pathogenic regulatory responses are reactions to stimulations which are
'outside the standardized range of normal experience of the species
during its developmental peak period. They do not differ from the normal
regulatory responses, however, in any fundamental particular. . . . Tissue
overgrowth as a response to a long-continued external irritant, is of the
same order as heat regulation, wound-healing or lactation. . . . Useless
or harmful regulating mechanisms and tissue responses to isolation,
injury, or stimulation are not fundamentally different in kind from those
favourable ones which have become incorporated into the inheritance of the
species because they promoted survival through the period of reproductive
activity. . . . The tissues most often called on for regeneration and
repair, or most liable to recurrent stimulation into specialized activity,
are those most prone to tumour formation.' [4]
'Routine Regenerations'
The last sentence that I have quoted leads into the borderland between
regenerative and 'normal' processes: namely routine replacements. They
range from the periodic moulting of feathers and shedding of the antlers,
to the replacement of the whole human epidermis about once a month
owing to wear and tear, and the replacement of red blood cells at the
rate of 3 x 10^11 per day; not to mention the metabolic turnover on the
molecular level which consumes about thirty per cent of our total protein
intake. This type of routine (or so-called 'physiological') regeneration
which goes on all the time is sometimes described as a constant 'renewal'
or 'rejuvenation' of the body. It is often impossible to make a clear
distinction between 'wear' and 'tear' -- for instance in minor abrasions
of the skin. The differential factor is obviously the degree of stress,
which, past a critical threshold, will bring general alarm reactions and
'adaptations of the second order' into play.
Reorganizations of Function
The transplanted salamander limb which functions normally in spite
of its randomized nervous connections can be regarded as an example
of both regeneration of structure and reorganization of function. The
pathways leading into the limb all seem to be equipotential in their
capacity as conductors of the excitation-clang. Without entering the old
controversy about equipotentiality versus localization of functions in
nervous tissues, it seems to be safe to say that in repetitive routines
and local reflexes, equipotentiality has 'frozen up' into fixed local
arrangements; whereas in case of injury to the pathways in question,
the equipotentiality (or rather, multi-potentiality) of alternative
'canal-systems' is revived, and they take over the function of the injured
system. To quote Lashley: 'The results indicate that when habitually
used motor organs are rendered non-functional by removal or paralysis,
there is an immediate, spontaneous use of other motor systems which had
not previously been associated with, or used in, the performance of the
activity.' [5] Nearly a century earlier Pflüger had shown that even
the spinal reflexes of a frog are capable of 'crisis adaptations'. If a
drop of acid is placed on the back of the left front limb of a decapitated
frog, it will attempt to wipe it away with the left hind limb; but if
prevented from doing so it will use the right hind limb -- which it
normally never does in the exercise of the wiping reflex.