III. REGENERATION OF VERTEBRATE APPENDAGES
145
occurs from the distal sides of holes or transverse cuts made in fins
(Nabrit, 1929). As in other appendages already mentioned, these
regenerates are distal structures growing in reversed direction, and are
mirror images of the parts of the fins from which they are produced.
Even individual fin rays are capable of regenerating proximally from
distal stumps, though this occurs less readily than in the opposite
direction because of usually inadequate innervation (Goss and Stagg,
1957). However, the polarity of such fin ray regenerates has not been
diagnosed because of the absence of branching.
Skeletal structures in amphibians are in some instances capable of
regenerating missing parts. Delia Valle (1913) observed that bones of
Triturus could regenerate from distal surfaces to replace missing proximal
parts. Flat-Hauser and Przibram (1930) and Chen (1933) noted that
proximally directed regeneration from distal remnants of bones resulted
in the production of mirror images of the distal parts. Finally, in the
catfish taste barbel, extirpation of the central cartilaginous rod results
in the proximal regeneration of a new rod starting from the terminal
remnant of the original one (Goss, 1956b). Despite the basal direction
in which this structure grows, its tapered configuration resembling that
of distal parts of cartilaginous rods suggests its identity with the latter.
There is evidence, therefore, to support the contention that skeletal
parts, like entire appendages, can regenerate distal structures that are
proximally oriented.
Research on the regeneration of appendages of reversed polarity thus
leads inescapably to the conclusion that cells at any given level can give
rise only to parts normally situated distal to that level. However, in
doing so, cellular participants in regeneration are capable of migrating
either distally or proximally in order to form a blastema. It would
appear that the direction of cellular migration is determined by the
location of the wound surface. Once a blastema is formed its differentiation and morphogenesis are dictated by the stump tissues, which, in
the case of a reversed appendage, cause the formation of a mirror image
regenerate. To expect a true distoproximal regeneration from structures
with reversed polarities would be contrary to all that is known of
vertebrate appendage regeneration, especially to the fact that regenerative processes simulate very closely ontogenetic development. Vertebrate
appendages normally develop in a proximodistal direction; indeed, it is
not implausible that the differentiation of more distally situated parts
is induced by the previously formed structures lying in more proximal
regions. In order for regeneration to proceed in a distoproximal direction
the normal sequence of developmental events would have to be reversed.
To essay the reversal of such a complicated biological process would be
but a specious aspiration without precedent.
145
occurs from the distal sides of holes or transverse cuts made in fins
(Nabrit, 1929). As in other appendages already mentioned, these
regenerates are distal structures growing in reversed direction, and are
mirror images of the parts of the fins from which they are produced.
Even individual fin rays are capable of regenerating proximally from
distal stumps, though this occurs less readily than in the opposite
direction because of usually inadequate innervation (Goss and Stagg,
1957). However, the polarity of such fin ray regenerates has not been
diagnosed because of the absence of branching.
Skeletal structures in amphibians are in some instances capable of
regenerating missing parts. Delia Valle (1913) observed that bones of
Triturus could regenerate from distal surfaces to replace missing proximal
parts. Flat-Hauser and Przibram (1930) and Chen (1933) noted that
proximally directed regeneration from distal remnants of bones resulted
in the production of mirror images of the distal parts. Finally, in the
catfish taste barbel, extirpation of the central cartilaginous rod results
in the proximal regeneration of a new rod starting from the terminal
remnant of the original one (Goss, 1956b). Despite the basal direction
in which this structure grows, its tapered configuration resembling that
of distal parts of cartilaginous rods suggests its identity with the latter.
There is evidence, therefore, to support the contention that skeletal
parts, like entire appendages, can regenerate distal structures that are
proximally oriented.
Research on the regeneration of appendages of reversed polarity thus
leads inescapably to the conclusion that cells at any given level can give
rise only to parts normally situated distal to that level. However, in
doing so, cellular participants in regeneration are capable of migrating
either distally or proximally in order to form a blastema. It would
appear that the direction of cellular migration is determined by the
location of the wound surface. Once a blastema is formed its differentiation and morphogenesis are dictated by the stump tissues, which, in
the case of a reversed appendage, cause the formation of a mirror image
regenerate. To expect a true distoproximal regeneration from structures
with reversed polarities would be contrary to all that is known of
vertebrate appendage regeneration, especially to the fact that regenerative processes simulate very closely ontogenetic development. Vertebrate
appendages normally develop in a proximodistal direction; indeed, it is
not implausible that the differentiation of more distally situated parts
is induced by the previously formed structures lying in more proximal
regions. In order for regeneration to proceed in a distoproximal direction
the normal sequence of developmental events would have to be reversed.
To essay the reversal of such a complicated biological process would be
but a specious aspiration without precedent.