III. REGENERATION OF VERTEBRATE APPENDAGES
133
particular, it is important whether or not the skin of the limb remains
intact. If half of the lower arm of a newt is completely removed (e.g.,
ulna, associated musculature and posterior half of the skin), the
remaining portion of the limb (with major nerves and blood vessels left
undisturbed) proceeds to regenerate but produces an incomplete hand.
Instead of four digits, such regenerates possess an average of approximately two fingers (Figs. 2 and 5d). The posterior halves of such hands
are missing, this being an accurate reflection of the original defects in
the stumps. In this case it would appear that half of a limb has only
half a limb field. However, if the same operation is performed except
that the posterior half of the limb skin is not removed with the underlying mesodermal tissues, the resulting regenerates are normal in size
and structure (Fig. 5c).
Since the wholeness of regenerates produced from half limbs parallels
the degree of completeness of the skin on the stump, it can only be
concluded that in these cases the skin exerts a morphogenetic influence
on the developing blastema. In corroboration of this, it has been noted
(Polezhaev, 1936) that limbs containing minced tissues enveloped by
intact skin are capable of giving rise to normal regenerates. Again the
conclusion is inescapable that morphogenetic normality must have been
conferred on the blastema by the only remaining intact tissue in the
stump, namely, the skin.
As previously noted (Section V, A, 4), this effect is most probably
attributable to the dermal, rather than the epidermal, component of the
skin. The above conclusion does not necessarily signify that the dermis
is the only histological component of the limb capable of exerting a
morphogenetic effect, but when the dermis is the only tissue of the limb
that is intact, it can be responsible for the completeness of the regenerate.
An incomplete regenerate is produced only when all tissue components
of the limb are deficient.
C. Partially Irradiated Limbs
X-rays, at the proper intensity, can inhibit regeneration (Butler,
1933). This is largely the result of the deleterious effects of X-rays on
the mitotic activity of cells participating in regeneration. All developing
systems, therefore, which depend upon mitotic activity can be inhibited
by X-irradiation. Thus, the regeneration of limbs, jaws, tails, fins, taste
barbels (Goss, 1955) and lenses (Politzer, 1952) can be prevented or
retarded by exposure to mitosis-arresting doses of X-rays. Other kinds
of growth, such as the constructive changes attending amphibian
metamorphosis (Puckett, 1937) and fracture healing of bones (Cooley
and Goss, 1958) are likewise precluded by irradiation. Other growth
133
particular, it is important whether or not the skin of the limb remains
intact. If half of the lower arm of a newt is completely removed (e.g.,
ulna, associated musculature and posterior half of the skin), the
remaining portion of the limb (with major nerves and blood vessels left
undisturbed) proceeds to regenerate but produces an incomplete hand.
Instead of four digits, such regenerates possess an average of approximately two fingers (Figs. 2 and 5d). The posterior halves of such hands
are missing, this being an accurate reflection of the original defects in
the stumps. In this case it would appear that half of a limb has only
half a limb field. However, if the same operation is performed except
that the posterior half of the limb skin is not removed with the underlying mesodermal tissues, the resulting regenerates are normal in size
and structure (Fig. 5c).
Since the wholeness of regenerates produced from half limbs parallels
the degree of completeness of the skin on the stump, it can only be
concluded that in these cases the skin exerts a morphogenetic influence
on the developing blastema. In corroboration of this, it has been noted
(Polezhaev, 1936) that limbs containing minced tissues enveloped by
intact skin are capable of giving rise to normal regenerates. Again the
conclusion is inescapable that morphogenetic normality must have been
conferred on the blastema by the only remaining intact tissue in the
stump, namely, the skin.
As previously noted (Section V, A, 4), this effect is most probably
attributable to the dermal, rather than the epidermal, component of the
skin. The above conclusion does not necessarily signify that the dermis
is the only histological component of the limb capable of exerting a
morphogenetic effect, but when the dermis is the only tissue of the limb
that is intact, it can be responsible for the completeness of the regenerate.
An incomplete regenerate is produced only when all tissue components
of the limb are deficient.
C. Partially Irradiated Limbs
X-rays, at the proper intensity, can inhibit regeneration (Butler,
1933). This is largely the result of the deleterious effects of X-rays on
the mitotic activity of cells participating in regeneration. All developing
systems, therefore, which depend upon mitotic activity can be inhibited
by X-irradiation. Thus, the regeneration of limbs, jaws, tails, fins, taste
barbels (Goss, 1955) and lenses (Politzer, 1952) can be prevented or
retarded by exposure to mitosis-arresting doses of X-rays. Other kinds
of growth, such as the constructive changes attending amphibian
metamorphosis (Puckett, 1937) and fracture healing of bones (Cooley
and Goss, 1958) are likewise precluded by irradiation. Other growth