134
R. J.
GOSS
changes, on the contrary, which do not depend upon mitosis, are usually
unaffected by X-rays. Histolytic changes, as exemplified by the
destructive alterations of amphibian metamorphosis, are not inhibited
by X-rays, nor is epidermal wormal healing, which is essentially an
instance of cellular migration. Transformations of the noncellular parts
of connective tissues, including scale regeneration in teleost fishes
and tendon regeneration in mammals, are likewise unaffected by Xirradiation. Even cellular differentiation proceeds normally despite
exposure to mitosis-inhibiting doses of X-rays: the cartilaginous callus
of a fractured bone, irradiated after the initial proliferative phase of its
growth, can continue to become an ossified callus in a normal manner;
transected nerves can regenerate new fibres even after exposure to high
doses of X-rays. These examples testify to the fact that X-ray inhibition
of growth is mostly due to interferences in mitotic mechanisms.
X-irradiation, therefore, can be a useful technique for analysing the
morphogenetic potentialities of limbs. Thus, it is possible to inactivate
parts of limbs without actually removing them; just as the roles of
bones, muscle and skin in regeneration were studied with the aid of
selective irradiation methods (Section V, A), so also the regeneration of
double and half limbs can be further analysed by partially irradiating
the extremities. With reference to the regeneration of double limbs,
experiments were performed in which one arm, prior to grafting, was
exposed to a growth inhibiting dose of X-rays (5000 r). When grafted
to the opposite unirradiated limb and later amputated, regeneration
occurred only from the untreated side. The X-rayed arm neither
participated in regeneration nor exerted any effect on the growth of the
grafted partner limb. In another series of experiments, one limb was
X-rayed and the other completely denervated. Though neither arm
alone would have been able to regenerate, when joined together the
denervated side exhibited some regenerative potential by producing
abnormal outgrowths, usually comprised of single digits (Fig. 3).
Apparently this result is correctly explained by the fact that the X-rayed
arm provided regenerating nerve fibres which crossed over to the
denervated side thus initiating the regenerative response. The denervated limb, however, failed to reciprocate by providing what the
irradiated side lacked, namely, healthy cells.
The effects of partial irradiation on the morphogenesis of hind leg
regenerates has also been investigated (Goss, 1957b). Such studies have
revealed that if the posterior half of the lower leg is irradiated with
2000 r, which normally precludes appendage regeneration, while the
anterior half is shielded with lead, subsequent amputation through the
treated region is followed by regeneration only from the unexposed side
(Figs. 4 and 5g). Regenerates of this kind consist only of the anterior
R. J.
GOSS
changes, on the contrary, which do not depend upon mitosis, are usually
unaffected by X-rays. Histolytic changes, as exemplified by the
destructive alterations of amphibian metamorphosis, are not inhibited
by X-rays, nor is epidermal wormal healing, which is essentially an
instance of cellular migration. Transformations of the noncellular parts
of connective tissues, including scale regeneration in teleost fishes
and tendon regeneration in mammals, are likewise unaffected by Xirradiation. Even cellular differentiation proceeds normally despite
exposure to mitosis-inhibiting doses of X-rays: the cartilaginous callus
of a fractured bone, irradiated after the initial proliferative phase of its
growth, can continue to become an ossified callus in a normal manner;
transected nerves can regenerate new fibres even after exposure to high
doses of X-rays. These examples testify to the fact that X-ray inhibition
of growth is mostly due to interferences in mitotic mechanisms.
X-irradiation, therefore, can be a useful technique for analysing the
morphogenetic potentialities of limbs. Thus, it is possible to inactivate
parts of limbs without actually removing them; just as the roles of
bones, muscle and skin in regeneration were studied with the aid of
selective irradiation methods (Section V, A), so also the regeneration of
double and half limbs can be further analysed by partially irradiating
the extremities. With reference to the regeneration of double limbs,
experiments were performed in which one arm, prior to grafting, was
exposed to a growth inhibiting dose of X-rays (5000 r). When grafted
to the opposite unirradiated limb and later amputated, regeneration
occurred only from the untreated side. The X-rayed arm neither
participated in regeneration nor exerted any effect on the growth of the
grafted partner limb. In another series of experiments, one limb was
X-rayed and the other completely denervated. Though neither arm
alone would have been able to regenerate, when joined together the
denervated side exhibited some regenerative potential by producing
abnormal outgrowths, usually comprised of single digits (Fig. 3).
Apparently this result is correctly explained by the fact that the X-rayed
arm provided regenerating nerve fibres which crossed over to the
denervated side thus initiating the regenerative response. The denervated limb, however, failed to reciprocate by providing what the
irradiated side lacked, namely, healthy cells.
The effects of partial irradiation on the morphogenesis of hind leg
regenerates has also been investigated (Goss, 1957b). Such studies have
revealed that if the posterior half of the lower leg is irradiated with
2000 r, which normally precludes appendage regeneration, while the
anterior half is shielded with lead, subsequent amputation through the
treated region is followed by regeneration only from the unexposed side
(Figs. 4 and 5g). Regenerates of this kind consist only of the anterior