120
R. J.
GOSS
formation. If denervation operations are performed after these stages of
regeneration, growth and differentiation can continue. Since differentiation and morphogenesis normally occur after nerves are no longer
required for growth, it is reasonable to conclude that they are independent of nervous influences. Indeed, there is reason to believe that the
pattern of innervation is determined largely by the arrangements of
surrounding tissues, not vice versa. The rearrangement of nerve fibres in
regenerating structures has yielded further evidence that nerves do not
act as morphogenetic factors; at best they can determine the site of
regeneration. If the pattern of nerves within a limb is disturbed,
subsequent regeneration is normal so long as the requisite number of
nerve fibres remains. Moreover, deviation of nerves from a limb to
another nearby region within the regenerative territory results in the
formation of supernumerary regenerates. However, the kind of regenerate thus produced, as well as its orientation, is unrelated to the kind of
nerves providing the initial stimulus or their orientation. These experiments argue in favour of the hypothesis that what is regenerated is
determined solely by the non-nervous participating tissues. The important role of nerve fibres is stimulatory, but not directive.
Notwithstanding the above conclusion regarding the influences of
nerve fibres, there is reason to believe that in urodele tail regeneration
the spinal cord's role is more than stimulatory. Holtzer (1954, 1956) has
shown that the motor region of the spinal cord in Ambystoma larvae is
capable of inducing chondrogenesis in the nearby blastema cells of
regenerating tails. Furthermore, the axis of tail regeneration, as well as
the arrangement of vertebrae and myotomes, may also be at least
indirectly influenced by the spinal cord. Thus, the cord itself may have
effects that individual nerve fibres lack. The relation between the
inductive influence of the spinal cord and the stimulatory effect of nerve
fibres remains to be clarified.
2. Skeleton
The importance of skeletal tissues in regeneration and morphogenesis
varies considerably from one kind of appendage to another. In some
structures, e.g., tails of larval amphibians, no skeleton except the
notochord is present, yet regeneration can occur in these structures.
Obviously, the form of such regenerates is unaffected by skeletal tissues,
even though cartilaginous elements differentiate in regenerates of larval
urodele tails which originally possessed no skeleton except the notochord
(Holtzer, et al., 1955). At the other extreme is the catfish taste barbel,
which, when deprived of the central cartilaginous rod and its investing
perichondrium, lacks not only morphogenetic effects but fails to
regenerate altogether (Goss, 1954). The opposite type of experiment,
R. J.
GOSS
formation. If denervation operations are performed after these stages of
regeneration, growth and differentiation can continue. Since differentiation and morphogenesis normally occur after nerves are no longer
required for growth, it is reasonable to conclude that they are independent of nervous influences. Indeed, there is reason to believe that the
pattern of innervation is determined largely by the arrangements of
surrounding tissues, not vice versa. The rearrangement of nerve fibres in
regenerating structures has yielded further evidence that nerves do not
act as morphogenetic factors; at best they can determine the site of
regeneration. If the pattern of nerves within a limb is disturbed,
subsequent regeneration is normal so long as the requisite number of
nerve fibres remains. Moreover, deviation of nerves from a limb to
another nearby region within the regenerative territory results in the
formation of supernumerary regenerates. However, the kind of regenerate thus produced, as well as its orientation, is unrelated to the kind of
nerves providing the initial stimulus or their orientation. These experiments argue in favour of the hypothesis that what is regenerated is
determined solely by the non-nervous participating tissues. The important role of nerve fibres is stimulatory, but not directive.
Notwithstanding the above conclusion regarding the influences of
nerve fibres, there is reason to believe that in urodele tail regeneration
the spinal cord's role is more than stimulatory. Holtzer (1954, 1956) has
shown that the motor region of the spinal cord in Ambystoma larvae is
capable of inducing chondrogenesis in the nearby blastema cells of
regenerating tails. Furthermore, the axis of tail regeneration, as well as
the arrangement of vertebrae and myotomes, may also be at least
indirectly influenced by the spinal cord. Thus, the cord itself may have
effects that individual nerve fibres lack. The relation between the
inductive influence of the spinal cord and the stimulatory effect of nerve
fibres remains to be clarified.
2. Skeleton
The importance of skeletal tissues in regeneration and morphogenesis
varies considerably from one kind of appendage to another. In some
structures, e.g., tails of larval amphibians, no skeleton except the
notochord is present, yet regeneration can occur in these structures.
Obviously, the form of such regenerates is unaffected by skeletal tissues,
even though cartilaginous elements differentiate in regenerates of larval
urodele tails which originally possessed no skeleton except the notochord
(Holtzer, et al., 1955). At the other extreme is the catfish taste barbel,
which, when deprived of the central cartilaginous rod and its investing
perichondrium, lacks not only morphogenetic effects but fails to
regenerate altogether (Goss, 1954). The opposite type of experiment,