III. REGENERATION
OF VERTEBRATE
APPENDAGES
107
from which they protrude nor the snout can regenerate. Teleost scales,
though not really classified as appendages, have the capacity to regrow
if plucked. Many other bodily protuberances (e.g., the lures of the angler
fishes, the diverse appendages of deep-sea fishes) have never been tested
for regenerative ability.
2. Amphibians
Largely for reasons of convenience, most experimental studies of
regeneration have been carried out on the urodele limb. However, there
are a number of other amphibian structures equally well endowed with
the capacity to grow new parts, not the least of which is the tail. The
anuran tail, present only in larval forms, possesses a notochord but has
no true skeletal parts. When amputated it regenerates the same kind of
structure as that which was lost. The dorsal and ventral tail fins,
consisting of skin and loose connective tissue (lophioderm), also fill in
holes or notches cut in them. Like the anuran tail, the young larval
urodele tail is notochordal and lacks other supporting tissues. However,
when it regenerates it replaces the notochord with segmented cartilaginous vertebrae, thus exhibiting a precocious tendency to produce
structures more typical of the adult. The adult urodele tail has bony
vertebrae, and will grow typical new parts after transection.
Many amphibians are also capable of regenerating jaws and snouts,
although this capacity seems to be more typical of urodeles than anurans.
Amputated urodele snouts will regenerate if not too much tissue has
been removed. In so doing, the anterior parts of the brain are also
replaced, as are the nasal passages and teeth. The lower jaws of newts
likewise replace amputated parts, provided that the basal portion of the
mandibles remain intact. The mandibles are reconstituted, and new
teeth appear along the dorsal margin of the new jaw. The tongue is not
replaced, but new musculature develops to a limited extent. If the floor
of the mouth is excised from the lower jaw, leaving the mandible intact,
new tissues derived from the inner margin of the mandibles grow inward
to fill the intermandibular region. Thus, the original hole becomes
gradually reduced by the centripetal regeneration of tissues, and is
eventually obliterated. In this manner a new membrane is reconstituted
which contains muscle, and often cartilage and rudimentary salivary
glands.
Other amphibian appendages have been less extensively studied, but
nevertheless are known to be able to regenerate. There are, for example,
certain transitory structures present only in larval forms. Gills are
usually capable of some regeneration, and the paired tentacles of
Xenopus laevis larvae also regenerate. Balancers, however, which are
typical of certain urodele embryos, will not regenerate once they are
OF VERTEBRATE
APPENDAGES
107
from which they protrude nor the snout can regenerate. Teleost scales,
though not really classified as appendages, have the capacity to regrow
if plucked. Many other bodily protuberances (e.g., the lures of the angler
fishes, the diverse appendages of deep-sea fishes) have never been tested
for regenerative ability.
2. Amphibians
Largely for reasons of convenience, most experimental studies of
regeneration have been carried out on the urodele limb. However, there
are a number of other amphibian structures equally well endowed with
the capacity to grow new parts, not the least of which is the tail. The
anuran tail, present only in larval forms, possesses a notochord but has
no true skeletal parts. When amputated it regenerates the same kind of
structure as that which was lost. The dorsal and ventral tail fins,
consisting of skin and loose connective tissue (lophioderm), also fill in
holes or notches cut in them. Like the anuran tail, the young larval
urodele tail is notochordal and lacks other supporting tissues. However,
when it regenerates it replaces the notochord with segmented cartilaginous vertebrae, thus exhibiting a precocious tendency to produce
structures more typical of the adult. The adult urodele tail has bony
vertebrae, and will grow typical new parts after transection.
Many amphibians are also capable of regenerating jaws and snouts,
although this capacity seems to be more typical of urodeles than anurans.
Amputated urodele snouts will regenerate if not too much tissue has
been removed. In so doing, the anterior parts of the brain are also
replaced, as are the nasal passages and teeth. The lower jaws of newts
likewise replace amputated parts, provided that the basal portion of the
mandibles remain intact. The mandibles are reconstituted, and new
teeth appear along the dorsal margin of the new jaw. The tongue is not
replaced, but new musculature develops to a limited extent. If the floor
of the mouth is excised from the lower jaw, leaving the mandible intact,
new tissues derived from the inner margin of the mandibles grow inward
to fill the intermandibular region. Thus, the original hole becomes
gradually reduced by the centripetal regeneration of tissues, and is
eventually obliterated. In this manner a new membrane is reconstituted
which contains muscle, and often cartilage and rudimentary salivary
glands.
Other amphibian appendages have been less extensively studied, but
nevertheless are known to be able to regenerate. There are, for example,
certain transitory structures present only in larval forms. Gills are
usually capable of some regeneration, and the paired tentacles of
Xenopus laevis larvae also regenerate. Balancers, however, which are
typical of certain urodele embryos, will not regenerate once they are