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R. J. GOSS
depends upon the cause of the original defect. Occasionally, malformed
limbs are encountered in nature. Rostand (1952) studied a population
of Rana esculenta larvae exhibiting polydactylous and supernumerary
legs. When amputated, these limbs gave rise to normal regenerates.
If it is assumed that genes controlling morphogenesis exert identical
effects during embryonic and regenerative development, the cause of
the original abnormality was not genetic.
Other investigations have involved appendages with morphological
aberrations of known etiologies. Harrison (1918), for example, produced
abnormal limbs from grafted embryonic limb buds. When amputated,
these limbs gave rise to normal regenerates in some cases and abnormal
ones in other instances. Repeating Harrison's experiments, Swett (1924)
obtained variously abnormal forelimbs in larval Ambystoma from
transplanted limb buds. Upon amputation, deficient limbs regenerated
correspondingly imperfect structures, while polydactylous and double
limbs tended to produce single regenerates in most cases. Whether such
regenerates were truly single or were fused double limbs might have
been revealed had descriptions of the skeletal structures been presented.
Unfortunately, the internal anatomy of these limbs at their levels of
amputation must remain a matter of conjecture. Takaya (1941) likewise
obtained duplicate limbs from grafted limb discs, which, after amputation proximal to the bifurcation, most often regenerated single structures,
but occasionally reproduced the original malformation.
These investigations repeatedly demonstrate the capacity of abnormal
limbs to improve upon their original misdirected ontogeneses when
compelled to regenerate. This possibility has been more thoroughly
explored by the recent experiments of Newth (1958b). By implanting
nasal placodes in the flanks of Triturus helveticus embryos, accessory
limbs exhibiting various degrees of abnormalities were induced. These
were then subjected to two successive basal amputations in order to
determine to what extent the regenerates would resemble or differ
from the primary limbs. Although there were some notable exceptions,
the results revealed a tendency for abnormal limbs to give rise to more
nearly normal regenerates.
Experiments in the same vein have been carried out on amphibian
tails rendered abnormal by interferences in their embryogenesis.
Vogt (1931) produced larval urodele tails lacking ventral fins by removing the ectoderm next to the blastopores of neurulae. These tails
regenerated similarly defective structures when amputated through the
finless regions. Regeneration in Rana temporaria tadpoles of tails made
abnormal by injuring the embryonic tail buds was studied by Woronzowa
and Liosner (1947). Large numbers of animals were produced which had
tails lacking parts of the fins. These were amputated either immediately
R. J. GOSS
depends upon the cause of the original defect. Occasionally, malformed
limbs are encountered in nature. Rostand (1952) studied a population
of Rana esculenta larvae exhibiting polydactylous and supernumerary
legs. When amputated, these limbs gave rise to normal regenerates.
If it is assumed that genes controlling morphogenesis exert identical
effects during embryonic and regenerative development, the cause of
the original abnormality was not genetic.
Other investigations have involved appendages with morphological
aberrations of known etiologies. Harrison (1918), for example, produced
abnormal limbs from grafted embryonic limb buds. When amputated,
these limbs gave rise to normal regenerates in some cases and abnormal
ones in other instances. Repeating Harrison's experiments, Swett (1924)
obtained variously abnormal forelimbs in larval Ambystoma from
transplanted limb buds. Upon amputation, deficient limbs regenerated
correspondingly imperfect structures, while polydactylous and double
limbs tended to produce single regenerates in most cases. Whether such
regenerates were truly single or were fused double limbs might have
been revealed had descriptions of the skeletal structures been presented.
Unfortunately, the internal anatomy of these limbs at their levels of
amputation must remain a matter of conjecture. Takaya (1941) likewise
obtained duplicate limbs from grafted limb discs, which, after amputation proximal to the bifurcation, most often regenerated single structures,
but occasionally reproduced the original malformation.
These investigations repeatedly demonstrate the capacity of abnormal
limbs to improve upon their original misdirected ontogeneses when
compelled to regenerate. This possibility has been more thoroughly
explored by the recent experiments of Newth (1958b). By implanting
nasal placodes in the flanks of Triturus helveticus embryos, accessory
limbs exhibiting various degrees of abnormalities were induced. These
were then subjected to two successive basal amputations in order to
determine to what extent the regenerates would resemble or differ
from the primary limbs. Although there were some notable exceptions,
the results revealed a tendency for abnormal limbs to give rise to more
nearly normal regenerates.
Experiments in the same vein have been carried out on amphibian
tails rendered abnormal by interferences in their embryogenesis.
Vogt (1931) produced larval urodele tails lacking ventral fins by removing the ectoderm next to the blastopores of neurulae. These tails
regenerated similarly defective structures when amputated through the
finless regions. Regeneration in Rana temporaria tadpoles of tails made
abnormal by injuring the embryonic tail buds was studied by Woronzowa
and Liosner (1947). Large numbers of animals were produced which had
tails lacking parts of the fins. These were amputated either immediately