106
Β.
J.
GOSS
Thus, the examination of different kinds of appendages reveals that the
role of the skeleton, for example, is relative to the type of structure
being considered.
Yet, such differences between appendages can be of advantage from
the experimental viewpoint. Due to the manifold anatomical variations
among fish, amphibian and reptilian appendages, the experimenter can
select the structure best suited for a certain type of study. An extremely
difficult operation in one kind of extremity might prove very easy in
another. Indeed, in some instances one finds that nature has, in a sense,
already performed certain experiments. Thus, to determine if muscle is
essential to the process of regeneration in general, it is not necessary to
attempt the almost impossible task of extirpating all muscle from an
amphibian limb or tail. Rather, one need only note that the teleost fin
and taste barbel are, except in their most proximal regions, devoid of
all striated musculature. Since these appendages can regenerate without
involving muscle fibres, there is reason to suspect that limb regeneration
could proceed rather normally even if all muscle were previously
removed from the stump. With reference to the trophic effect of nerves
on regeneration, it is of interest to learn if this influence depends upon
sensory or motor nerves. Although direct experimentation is the
preferred method of obtaining the answer to this question (cf. Singer,
1943), the fact that some fish structures (e.g., fins and barbels) are
innervated almost exclusively by sensory nerves would tend to implicate
these, at least, as a definite source of the trophic influence. Although
this kind of analogy may be slightly dangerous, the information it yields
can often provide the investigator with a valuable insight and a truer
perspective with which to approach a given problem of development.
A. Vertebrate Structures Capable of Regeneration
1. Fishes
Because of their greater diversity of forms, fishes possess a greater
variety of appendages than any other class of vertebrates. Many, but
not all of these structures are capable of regenerating lost parts. The fins
of teleosts readily regenerate after amputation as do the component
bony fin rays following their partial ablation. The fins of the lungfish,
Protopterus, likewise replace lost parts, but elasmobranchs are unable to
regenerate their fins. Some fins, modified as gonopodia in certain teleost
fishes, are also able to regenerate. Adipose fins, however, do not grow
back. Although the caudal fin regenerates well, the vertebral portion
of the tail does not. Gill filaments can regenerate, but gill arches and
the operculum cannot. Taste barbels, characteristic of the catfishes,
reconstitute themselves following injury, but neither the lower jaw
Β.
J.
GOSS
Thus, the examination of different kinds of appendages reveals that the
role of the skeleton, for example, is relative to the type of structure
being considered.
Yet, such differences between appendages can be of advantage from
the experimental viewpoint. Due to the manifold anatomical variations
among fish, amphibian and reptilian appendages, the experimenter can
select the structure best suited for a certain type of study. An extremely
difficult operation in one kind of extremity might prove very easy in
another. Indeed, in some instances one finds that nature has, in a sense,
already performed certain experiments. Thus, to determine if muscle is
essential to the process of regeneration in general, it is not necessary to
attempt the almost impossible task of extirpating all muscle from an
amphibian limb or tail. Rather, one need only note that the teleost fin
and taste barbel are, except in their most proximal regions, devoid of
all striated musculature. Since these appendages can regenerate without
involving muscle fibres, there is reason to suspect that limb regeneration
could proceed rather normally even if all muscle were previously
removed from the stump. With reference to the trophic effect of nerves
on regeneration, it is of interest to learn if this influence depends upon
sensory or motor nerves. Although direct experimentation is the
preferred method of obtaining the answer to this question (cf. Singer,
1943), the fact that some fish structures (e.g., fins and barbels) are
innervated almost exclusively by sensory nerves would tend to implicate
these, at least, as a definite source of the trophic influence. Although
this kind of analogy may be slightly dangerous, the information it yields
can often provide the investigator with a valuable insight and a truer
perspective with which to approach a given problem of development.
A. Vertebrate Structures Capable of Regeneration
1. Fishes
Because of their greater diversity of forms, fishes possess a greater
variety of appendages than any other class of vertebrates. Many, but
not all of these structures are capable of regenerating lost parts. The fins
of teleosts readily regenerate after amputation as do the component
bony fin rays following their partial ablation. The fins of the lungfish,
Protopterus, likewise replace lost parts, but elasmobranchs are unable to
regenerate their fins. Some fins, modified as gonopodia in certain teleost
fishes, are also able to regenerate. Adipose fins, however, do not grow
back. Although the caudal fin regenerates well, the vertebral portion
of the tail does not. Gill filaments can regenerate, but gill arches and
the operculum cannot. Taste barbels, characteristic of the catfishes,
reconstitute themselves following injury, but neither the lower jaw